Exhaustively Trichlorosilylated C1 and C2 Building Blocks: Beyond the Müller-Rochow Direct Process

Article abstract of DOI:10.1021/jacs.8b05950

The Cl^--induced heterolysis of the Si-Si bond in Si₂Cl₆ generates an [SiCl₃]^- ion as reactive intermediate. When carried out in the presence of CCl₄ or Cl₂C=CCl₂ (CH₂Cl₂ solutions, room temperature or below), the reaction furnishes the monocarbanion [C(SiCl₃)₃]^- ([A]^- 92%) or the vicinal dianion [(Cl₃Si)₂C-C(SiCl₃)₂]^2- ([B]^2- 85%) in excellent yields. Starting from [B]^2-, the tetrasilylethane (Cl₃Si)₂(H)C-C(H)(SiCl₃)₂ (H₂B) and the tetrasilylethene (Cl₃Si)₂C=C(SiCl₃)₂ (B; 96%) are readily available through protonation (CF₃SO₃H) or oxidation (CuCl₂), respectively. Equimolar mixtures of H₂B/[B]^2- or B/[B]^2- quantitatively produce 2 equiv of the monoanion [HB]^- or the blue radical anion [B^?]^-, respectively. Treatment of B with Cl^- ions in the presence of CuCl₂ furnishes the disilylethyne Cl₃SiC≡CSiCl₃ (C; 80%); in the presence of [HMe₃N]Cl, the trisilylethene (Cl₃Si)₂C=C(H)SiCl₃ (D; 72%) is obtained. Alkyne C undergoes a [4+2]-cycloaddition reaction with 2,3-dimethyl-1,3-butadiene (CH₂Cl₂, 50 °C, 3d) and thus provides access to 1,2-bis(trichlorosilyl)-4,5-dimethylbenzene (E1; 80%) after oxidation with DDQ. The corresponding 1,2-bis(trichlorosilyl)-3,4,5,6-tetraphenylbenzene (E2; 83%) was prepared from C and 2,3,4,5-tetraphenyl-2,4-cyclopentadien-1-one under CO extrusion at elevated temperatures (CH₂Cl₂, 180 °C, 4 d). All closed-shell products were characterized by ¹H, ¹³C{¹H}, and ²⁹Si NMR spectroscopy; an EPR spectrum of [nBu₄N][B^?] was recorded. The molecular structures of [nBu₄N][A], [nBu₄N]₂[B], B, E1, and E2 were further confirmed by single-crystal X-ray diffraction. On the basis of detailed experimental investigations, augmented by quantum-chemical calculations, plausible reaction mechanisms for the formation of [A]^-, [B]^2-, C, and D are postulated.

Full text of DOI:10.1021/jacs.8b05950

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Article
1
2
Exhaustively Trichlorosilylated C and C Building
Blocks: Beyond the Müller-Rochow Direct Process
Isabelle Georg, Julian Teichmann, Markus Bursch, Jan Tillmann, Burkhard Endeward,
Michael Bolte, Hans-Wolfram Lerner, Stefan Grimme, and Matthias Wagner
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05950 • Publication Date (Web): 09 Jul 2018
Downloaded from http://pubs.acs.org on July 9, 2018
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Exhaustively Trichlorosilylated C1 and C2 Building Blocks: Beyond
the Müller-Rochow Direct Process
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Isabelle Georga,†, Julian Teichmann a,†, Markus Bursch‡, Jan Tillmann†, Burkhard Endeward∥, Michael
Bolte†, HansꢀWolfram Lerner†, Stefan Grimme‡, and Matthias Wagner*,†
†
Institut für Anorganische Chemie, GoetheꢀUniversität Frankfurt am Main, MaxꢀvonꢀLaueꢀStraße 7, 60438 Frankfurt am
Main, Germany
‡
Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn,
Beringstraße 4, 53115 Bonn, Germany
∥
Institut für Physikalische und Theoretische Chemie, GoetheꢀUniversität Frankfurt, MaxꢀvonꢀLaueꢀStr. 7, 60438 Frankfurt
am Main, Germany
ABSTRACT: The Cl‒ꢀinduced heterolysis of the Si‒Si bond in Si2Cl6 generates an [SiCl3]– ion as reactive intermediate. When
carried out in the presence of CCl4 or Cl2C=CCl2 (CH2Cl2 solutions, room temperature or below), the reaction furnishes the monoꢀ
carbanion [C(SiCl3)3]– ([A]–; 92%) or the vicinal dianion [(Cl3Si)2C‒C(SiCl3)2]2– ([B]2–; 85%) in excellent yields. Starting from
[B]2–, the tetrasilylethane (Cl3Si)2(H)C‒C(H)(SiCl3)2 (H2B) and the tetrasilylethene (Cl3Si)2C=C(SiCl3)2 (B; 96%) are readily availꢀ
able through protonation (CF3SO3H) or oxidation (CuCl2), respectively. Equimolar mixtures of H2B/[B]2– or B/[B]2– quantitatively
produce 2 eq of the monoanion [HB]– or the blue radical anion [B•]‒, respectively. Treatment of B with Cl‒ ions in the presence of
CuCl2 furnishes the disilylethyne Cl3SiC≡CSiCl3 (C; 80%); in the presence of [HMe3N]Cl, the trisilylethene (Cl3Si)2C=C(H)SiCl3
(D; 72%) is obtained. Alkyne C undergoes a [4+2]ꢀcycloaddition reaction with 2,3ꢀdimethylꢀ1,3ꢀbutadiene (CH2Cl2, 50 °C, 3d) and
thus provides access to 1,2ꢀbis(trichlorosilyl)ꢀ4,5ꢀdimethylbenzene (E1; 80%) after oxidation with DDQ. The corresponding 1,2ꢀ
bis(trichlorosilyl)ꢀ3,4,5,6ꢀtetraphenylbenzene (E2; 83%) was prepared from C and 2,3,4,5ꢀtetraphenylꢀ2,4ꢀcyclopentadienꢀ1ꢀone
1
under CO extrusion at elevated temperatures (CH2Cl2, 180 °C, 4d). All closedꢀshell products were characterized by H, 13C{1H},
and 29Si NMR spectroscopy; an EPR spectrum of [nBu4N][B•] was recorded. The molecular structures of [nBu4N][A], [nBu4N]2[B],
B, E1, and E2 were further confirmed by singleꢀcrystal Xꢀray diffraction. Based on detailed experimental investigations, augmentꢀ
ed by quantumꢀchemical calculations, plausible reaction mechanisms for the formation of [A]–, [B]2–, C, and D are postulated.
Lewis bases L. The first type of interaction results in a partial
INTRODUCTION
E=SiX3 doubleꢀbond character and is also responsible for the
marked C–H acidity at the α position to a silyl group (E = C–).
The second type facilitates the formation of fiveꢀ and sixꢀ
Functionalized organosilanes are versatile reagents for organic
synthesis and indispensable building blocks for materials science.
Their outstanding performance results from a number of unique
electronic properties that distinguish silicon from its lighter group
homologue carbon: (a) The lower electronegativity of Si (1.7)1
compared to C (2.5)1 renders the ionicity of an E–Si bond distinctꢀ
ly different from that of the corresponding E–C bond (E = chemiꢀ
cal element). (b) Fragments C–SiR3 carrying electronꢀreleasing
substituents R possess polarizable, energetically highꢀlying C–
Si σ orbitals from which charge density can be transferred to
properly aligned empty orbitals. A classic example for such hyꢀ
perconjugation is seen in the stabilization of carbenium ions by
geminal SiR3 groups (βꢀsilicon effect; Figure 1).2 (c) Fragments
E–SiX3 carrying electronegative substituents X possess energetiꢀ
cally accessible antibonding Si–X σ* orbitals, which can accept
electron density from πꢀdonor atoms E, but also from external
coordinate
adducts
(Figure
1).3
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intermediates seem to be prerequisites for a successful haloꢀ
Figure 1. Simplified orbital interactions underlying the βꢀsilicon
effect (left), the αꢀsilicon effect (middle), or dative L‒Si σ interꢀ
actions (right; L = Lewis base).
gen/SiCl3 exchange.37,45,46 The limitation of the Benkeser
reagent to activated alkyl halide starting materials can partly
be overcome by employing the HSiCl3/[R4P]Cl pair, which
allows trichlorosilylations also of simple primary alkyl chloꢀ
rides.47,48 In terms of the reaction mechanism, already
Benkeser postulated the initial formation of [SiCl3]–/[HR3N]+
ion pairs.37,49 Depending on the reaction conditions, the chiꢀ
meric [SiCl3]– ion behaves either as a silanide nucleophile or
as a dichlorosilylene source: i.e., [SiCl3]–/[HR3N]+ vs.
[SiCl2]/[HR3N]Cl.50–53 Thus, both nucleophilic pathways and
silylene insertion channels have to be regarded as plausible
reaction scenarios. Recently, the Si2Cl6/[R4E]Cl combination
has been identified as an alternative source of [SiCl3]– ions (E
= N, P; R = Et, nꢀBu, Ph):54 Nucleophilic attack of chloride on
the disilane results in the heterolytic cleavage of the Si–Si
bond with concomitant formation of SiCl4 and [SiCl3]–
(Scheme 1).55,56 In the presence of residual Si2Cl6, the thereby
liberated [SiCl3]– induces the assembly of oligosilane scafꢀ
folds, such as perchlorinated cyclohexasilanes or Si20 dodecaꢀ
hedranes (ˈsilafulleranesˈ).55,57,58
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The aptitude of moieties C–SiX3 to expand their coordination
spheres and the more polarized C–Si bonds in adducts [C–
SiX3L]n– (n = 0, 1) are widely exploited in protecting group
chemistry,4 Tamao oxidations,5–10 and Hiyamaꢀtype C–Cꢀ
coupling reactions.11–17 The Peterson olefination benefits from
both the α acidity and the Lewis acidity of alkylsilanes.18
Common to all these examples is that the silyl groups do not
appear in the final products, but are split off in the course of
the transformations. On the contrary, a number of important
functional materials are gaining their useful properties only
because the Si atoms remain as essential parts in the molecular
scaffolds. Examples are luminescent organosilanes, such as
hexaphenylsilole,19 and silicones, the most abundant class of
polymers with inorganic backbones.3 Hyperconjugative
nO→σSiX* interactions (cf. the α effect) are primarily responꢀ
sible for the increased torsional flexibility and diminished
basicity of silicones and thus contribute strongly to their broad
application range.20 Last but not least, organic trichlorosilyl
and trialkoxysilyl derivatives represent key starting materials
for the preparation of silsesquioxanes and other hybrid organꢀ
icꢀinorganic macromolecules, which find applications as surꢀ
face coatings, catalyst supports, xerogels, ceramic precursors,
and optoelectronic materials.21–28
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Scheme 1. Generation of [SiCl3]− and/or [SiCl2] from Si2Cl6
in the presence of chloride ions.
The MüllerꢀRochow Direct Process still constitutes the
dominant technology for the preparation of orꢀ
gano(chloro)silanes on an industrial scale. At its most comꢀ
mon level, the reaction involves CH3Cl and Si(Cu) as starting
materials and gives (CH3)2SiCl2 as the major product; the next
most abundant species are CH3SiCl3 and (CH3)3SiCl.3 For the
synthesis of more sophisticated (functionalized) organosilanes,
Ptꢀcatalyzed hydrosilylations of olefins and alkynes are extenꢀ
sively applied.29–33 A third prominent synthesis approach relies
on the transformation of SiCl4 with organolithium or Grignard
reagents.3 However, this method has its limits in terms of
functionalꢀgroup tolerance and selectivity (e.g., multiple vs.
single substitutions).34 Problems also occur in the case of
multiply silylated compounds, because the required multiply
metallated organic intermediates are often hard to obtain,
extremely sensitive, and poorly soluble.35,36 Alternatively, a
polarityꢀinverted pathway starting from organic halides and
nucleophilic silanides is conceivable.
Herein, we are expanding the reaction scope of the
Si2Cl6/[nBu4N]Cl combination to the synthesis of key orgaꢀ
nosilane reagents via convenient oneꢀpot protocols ([A]–– E;
Figure 2). The monoanion [A]– and vicinal dianion [B]2– are
archetypal examples of the stabilizing effect of silyl groups on
α carbanions. Moreover, potentially nucleophilic sites coexist
with strongly electrophilic sites. Compounds HA and H2B, the
conjugate acids of [A]– and [B]2–, are potentially relevant to
the preparation of organicꢀinorganic hybrid materials such as
silsesquioxanes and xerogels. The electronꢀpoor unsaturated
derivatives B, C, and D should readily undergo substitution
reactions at the Si atoms and/or cycloaddition reactions at the
unsaturated organic backbones. This provides a broad range of
oligoꢀfunctionalized precursors to be employed in, e.g., Hiyaꢀ
maꢀtype C–Cꢀcoupling protocols (cf. E).59–61
The only systematically investigated silanideꢀrelated sysꢀ
tems for the preparation of organo(chloro)silanes currently
remain the HSiCl3/NR3 combination (ˈBenkeser reagentˈ; NR3
= tertiary amine, typically N(nPr)3 or N(nBu)3, in stoichioꢀ
metric amounts) and the HSiCl3/[R4P]Cl pair ([R4P]+ = quaterꢀ
nary phosphonium salt, typically [nBu4P]+, in catalytic
amounts). The Benkeser reagent37 has been employed to proꢀ
duce organotrichlorosilanes through reductions of carboxyl or
carbonyl derivatives,38–41 hydrosilylations of alkenes or alꢀ
kynes,42–44 and the silylation of alkyl halides.45,46 With respect
to the latter transformation, it is important to note that nonꢀ
activated alkyl halides fail to yield the respective alkyl triꢀ
chlorosilanes. Suitable substrates are benzyl and allyl halides
as well as geminal polyhalogenated hydrocarbons. Thus, meꢀ
someric or inductive electron withdrawal from the reaction
center and the efficient stabilization of putative carbanionic
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photochlorination of H2C(SiCl3)2 (Scheme 3).65 The correꢀ
Figure 2. Target compounds of this work: the persilylated monoꢀ
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carbanion [A]–, the dicarbanion [B]2–, and their corresponding
acids HA/H2B; the (per)silylated ethenes B/D, ethyne C, and
benzenes E.
sponding alkene, (Cl3Si)2C=C(SiCl3)2 (B), and alkyne,
Cl3SiC≡CSiCl3 (C), are accessible through dechlorinative
coupling of Cl2C(SiCl3)2 with Cu metal in a stirred bed reactor
as parts of a multiꢀcomponent mixture (Scheme 3, bottom).66,67
Alternatively, CCl4 gives some C (18%) in the Müllerꢀ
Rochow Direct Process.66,68 Catalytic hydrosilylation of C has
been employed to prepare the 1,1,2ꢀtrisilylethene D (88%).69
RESULTS AND DISCUSSION
Some of the target compounds compiled in Figure 2 have
previously been mentioned in the literature. However, as will
be shown below, most of the respective synthesis protocols are
at best inconvenient and at worst not practical on a laboratory
scale.
Scheme 3. Selected literatureꢀknown synthesis routes to
[nBu4N][A], HA, H2B, B, C, D, and related compounds.
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In the following, we will first provide a brief literature surꢀ
vey on compounds [A]–– E. Second, we will disclose efficient
means to prepare the anions [A]– and [B]2– and show that all
the other target compounds are readily accessible from these
two starting materials. Analytical details and reactivity patꢀ
terns of [A]–– E will be discussed in the third section. The
final paragraph of this publication is devoted to mechanistic
considerations.
Previous Mentions of [A]–– E.
We first observed the tris(trichlorosilyl)methanide salt
[nBu4N][A] in the thermolysate of [nBu4N]2[1,1ꢀ
(Cl3Si)2Si6Cl10·2Cl], the chloride diadduct of the substituted
cyclohexasilane 1,1ꢀ(Cl3Si)2Si6Cl10 (CH2Cl2, 85 °C; Scheme
2).55,62 Under the conditions applied, stripping of both SiCl3
substituents
occurred
to
give
the
diadduct
[nBu4N]2[Si6Cl12·2Cl] of parent Si6Cl12.
Scheme 2. Thermolysis of [nBu4N]2[1,1ꢀ(Cl3Si)2Si6Cl10·2Cl]
in CH2Cl2 to furnish [nBu4N]2[Si6Cl12·2Cl] and [nBu4N][A]
as part of a complex product mixture.
Cation = [nBu4N]+ omitted for clarity. (i) CH2Cl2, 85 °C in a
sealed vial, 4 h.
The cation = [nBu4N]+ is omitted for clarity. The yields of H2B,
B, and C refer to H2C(SiCl3)2. (i) CH2Cl2, 85 °C in a sealed vial,
24 h; (ii) CH3CN, ‒40 °C to 65 °C, 16 h; (iii) gas phase, highꢀ
pressure Hg lamp, 30 h; (iv) Si(Cu), 300ꢀ320 °C in a fixed bed
reactor, 16 h; (v) CCl4, reflux temperature, 64 h; (vi) Cu, room
temperature to 380 °C, 2.5 h. (vii) Speier catalyst in iPrOH, 90
°C, sealed glass ampoule, 6 d.
Dichlorosilylenes, formally liberated in the course of the
stripping reaction, are found reꢀinserted into C‒Cl and C‒H
bonds of the solvent, thereby ultimately furnishing
[nBu4N][A] and H2C(SiCl3)2, which was detected as a side
product. When Kroke et al. reproduced the synthesis of
[nBu4N]2[Si6Cl12·2Cl], they succeeded in the isolation of
[nBu4N][A] with 17% yield (Scheme 3, top); they also menꢀ
tioned that [nBu4N][A] was formed from a mixture of
Si2Cl6/[nBu4N]Cl in CHCl3 (no yield given).63 In contrast to
[nBu4N][A], its dianionic homologue [nBu4N]2[B] remained
elusive until toꢀdate.
Syntheses of [nBu4N][A]– E.
We first revisited and expanded the previous studies by our
and Kroke’s group and systematically screened the behavior of
Si2Cl6/[nBu4N]Cl toward the entire series of chloromethanes.
In brief, C‒Si coupling took place with all four substrate molꢀ
ecules: From a solution of CH3Cl in CH2Cl2, we obtained
CH3SiCl3. CH2Cl2 and CHCl3, employed both as reagents and
solvents, furnished [nBu4N][A] in yields of 34% (sealed glass
ampoule, 120 °C, 30 h) and 78% (reflux temperature, 2 h),
respectively (cf. the SI for more details). Almost quantitative
yields of [nBu4N][A] were harvested from 4:1:1.5 mixtures of
Si2Cl6, [nBu4N]Cl, and CCl4 in CH2Cl2 (Scheme 4, top).70 This
new protocol opens a timeꢀ and costꢀefficient access route to
Corriu et al. synthesized preparatively useful amounts of
HA by treatment of CHCl3 with excess Benkeser reagent. The
chlorosilane HA was not isolated, but routinely transformed to
HC(Si(OEt)3)3 via ethanolysis (30%; Scheme 3, top).27,64 Some
tetrasilylethane H2B forms upon reaction of Cl(H)C(SiCl3)2
with Si(Cu) at 300ꢀ320 °C in a fixed bed reactor; the synthesis
of the precursor molecule, Cl(H)C(SiCl3)2, requires gasꢀphase
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nonꢀpolar solvent CCl4, the ionic species [nBu4N][Cl3C–SiCl4]
preparatively useful quantities of [nBu4N][A], it does not
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require elevated temperatures, the exothermic reaction occurs
instantaneously, and workup is facile.
and [nBu4N][Cl3C–SiCl3–CCl3] precipitate immediately and
can thus no longer participate in further reactions. The chemiꢀ
cal constitution of [nBu4N][Cl3C–SiCl4] was confirmed by Xꢀ
ray crystallography, however, the poor crystal quality preꢀ
cludes publication of the data (cf. the SI for more inforꢀ
mation). We finally emphasize a conceptual relationship beꢀ
tween [Cl3C‒SiCl3‒CCl3]‒ and the previously published anion
[Cl3Si‒SiCl3‒SiCl3]‒, which has been interpreted as the
[SiCl3]‒ adduct of Si2Cl6.55,62
Scheme 4. Reactions of CCl4 and C2Cl4 with
Si2Cl6/[nBu4N]Cl, resulting in the formation of C‒Siꢀ
bonded
species.
9
The fact that the reaction of CCl4 with Si2Cl6/[nBu4N]Cl
made three important organosilane building blocks readily
available, prompted us to study next the synthetic potential of
the higher homologue C2Cl6. Treatment of this compound with
1 eq of Si2Cl6 and catalytic amounts of [nBu4N]Cl at room
temperature in CD2Cl2 led to clean dechlorination with forꢀ
mation of tetrachloroethene (C2Cl4; see the SI for more details
and Ref[37] for a related transformation). C2Cl4 was therefore
employed as starting material in all further reactions with
Si2Cl6/[nBu4N]Cl (Scheme 4, bottom), which always furnished
at least some [nBu4N]2[B], irrespective of the stoichiometric
ratios employed. The optimized protocol for the synthesis of
[nBu4N]2[B] requires Si2Cl6 (4 eq), which is added at ‒10 °C
to a solution of [nBu4N]Cl (2 eq) and C2Cl4 (1 eq) in CH2Cl2.70
After warming to room temperature, colorless crystals of
[nBu4N]2[B] can be isolated from the deep blue reaction mixꢀ
ture in yields of 85%.
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As first reactivity tests, we treated [nBu4N][A] and
[nBu4N]2[B] with anhydrous MeOH/NMe2Et and obtained
HAOMe64 and H2BOMe in yields of 59% and 88%, respecꢀ
tively. By the same token, protonation with CF3SO3H selecꢀ
tively furnished HA71 and H2B,65 respectively. Both protonaꢀ
tion reactions can also be conducted with ethereal HCl, but
care must be taken to avoid partial desilylation by nucleophilic
attack of Cl‒. The monoprotonated species [nBu4N][HB] is
accessible by adjusting the amount of acid employed or by
simple mixing of [nBu4N]2[B] and H2B in equimolar quantities
(Scheme 4, bottom).
The cation = [nBu4N]+ is omitted for clarity. Synthesis of
HA/HAOMe or H2B/H2BOMe via protonation/methanolysis of
[nBu4N][A] or [nBu4N]2[B]; synthesis of [nBu4N][HB] through
proton transfer between H2B and [nBu4N]2[B]. (i) CH2Cl2, 0 °C to
room temperature, 1 h; (ii) CCl4, room temperature, 24 h; (iii)
CCl4, room temperature, 1 h; (iv) CH2Cl2, room temperature, 1 h;
(v) CH2Cl2, ‒10 °C to room temperature, 12 h.
Equimolar mixtures of [nBu4N]2[B] and B undergo a comꢀ
proportionation reaction to furnish the blueꢀcolored monoradiꢀ
cal [nBu4N][B•]. The twoꢀelectron oxidation of [nBu4N]2[B]
was carried out with 2 eq of CuCl2 under heterogeneous condiꢀ
tions and affords the tetrasilylethene B in yields of 96%
(Scheme 5).
CCl4 possesses the highest reactivity among the four
chloromethanes. Competing solvent activation does therefore
not occur even under conditions of high dilution, which is of
crucial importance in the present context: Contrary to the
cases of CH2Cl2 and CHCl3, no [nBu4N][A] formed when
Si2Cl6 was added to [nBu4N]Cl in excess neat CCl4 (Scheme 4,
middle). Depending on the relative amounts of the chloride
salt, we either obtained the perchlorinated methylsilane Cl3C‒
SiCl3 (catalytic [nBu4N]Cl; 95% conversion, 37% yield) or its
[CCl3]‒ adduct [nBu4N][Cl3C‒SiCl3‒CCl3] after recrystallizaꢀ
tion from CH2Cl2 (1 eq [nBu4N]Cl, 96% yield; relative to
Si2Cl6). Cl3C‒SiCl3 is usually prepared via the potentially
hazardous photochlorination of H3C‒SiCl3,69 whereas
[nBu4N][Cl3C‒SiCl3‒CCl3] is so far unknown. Our current
working hypothesis assumes that a primary Cl‒ adduct
[nBu4N][Cl3C–SiCl4] acts as a Cl3Cꢀtransfer reagent along the
way from Cl3C‒SiCl3 to [nBu4N][Cl3C‒SiCl3‒CCl3]. In the
The targeted cleavage of C–Si bonds by halide ions (mainly
F‒ and Cl‒) is broadly exploited in organic synthesis, particuꢀ
larly in protecting group chemistry.4 We now found that B
formally eliminates Si2Cl6 upon addition of [nBu4N]Cl (1 eq)
to provide the bis(trichlorosilyl)ethyne C (Scheme 5). During
workup,
C (41%) was separated from the byproduct
[nBu4N]2[B] (43%) through extraction into nꢀpentane. Based
on the observation that the byproduct [nBu4N]2[B] can be
recycled through in situ oxidation with CuCl2, we developed
an optimized oneꢀpot protocol to prepare C from Si2Cl6 (4 eq),
[nBu4N]Cl (4 eq), C2Cl4 (1 eq), and CuCl2 (4 eq) in a yield of
80%. The alkyne content of the nꢀpentane stock solution was
determined by quenching an aliquot with MeOH and titration
of the produced HCl. Even though fractional distillation of the
extract affords C as a colorless oil, the effort is lowered and
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Page 6 of 17
yield losses are avoided, if the nꢀpentane extract is used directꢀ published synthesis of 1,2ꢀbis(trichlorosilyl)benzene starts
ly for further transformations.
1
from 1,2ꢀdichlorobenzene and HSiCl3 and requires irradiation
of the reaction mixture with γ rays at temperatures of 300 °C.74
An exploratory NMR experiment was conducted with C and
an approximate twofold excess of 2,3ꢀdimethylꢀ1,3ꢀbutadiene
in CD2Cl2. After heating to 50 °C for 3 d in a flameꢀsealed
tube, NMR spectroscopy and GC/MS confirmed the quantitaꢀ
tive consumption of C and the concomitant formation of 1,2ꢀ
bis(trichlorosilyl)ꢀ4,5ꢀdimethylcyclohexaꢀ1,4ꢀdiene F as the
exclusive product (Scheme 6). Repeating the synthesis of F on
a preparative scale with subsequent oxidation using 2,3ꢀ
dichloroꢀ5,6ꢀdicyanoꢀ1,4ꢀbenzoquinone (DDQ) at room temꢀ
perature resulted in the quantitative conversion to the 1,2ꢀ
disilylbenzene derivative E1 (80% yield with respect to C; for
more information see the SI). In this way we achieved the
synthesis of E1 in an overall yield of 52%.75 Treatment of 1,2ꢀ
disilylbenzene E1 with 6 eq of MeOH in the presence of
NMe2Et resulted in the formation of the permethoxylated
derivative E1OMe (62% yield). To examine the scope of the
DielsꢀAlder reactions of C, also 2,3,4,5ꢀtetraphenylꢀ2,4ꢀ
cyclopentadienꢀ1ꢀone was used as diene: Heating a CH2Cl2
solution of both compounds at 180 °C for 4 d in a sealed glass
ampoule furnished E2 as the only reaction product (83 %
yield; Scheme 6). NMR spectroscopic monitoring of the reacꢀ
tion progress showed no resonances assignable to a cyclohexꢀ
adiene intermediate. Thus, the electrocyclic reaction between
C and the ketone appears to be rate limiting, while CO liberaꢀ
tion with formation of the aromatic system occurs instantaneꢀ
ously under the conditions applied. Importantly, unwanted
desilylation reactions are not an issue, in spite of the high
temperatures required for the cycloaddition reactions and the
pronounced steric repulsion between the two SiCl3 substituents
in E1 and E2.
2
3
4
5
6
7
8
9
Scheme 5: Transformations of [nBu4N]2[B] to give H2B,
[nBu4N][B•], B, or C.
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The cation = [nBu4N]+ is omitted for clarity. (i) CD2Cl2, room
temperature, 1 h; (ii) C6D6, 120ꢀ150 °C in a sealed NMR tube, 8
d; (iii) 2 eq CuCl2, CH2Cl2, room temperature, 1 h; (iv) 1 eq
[HMe3N]Cl, 10 mol% [nBu4N]Cl, CH2Cl2, room temperature, 3 d.
Scheme 6. Synthesis of cyclohexaꢀ1,4ꢀdiene F from C and
2,3ꢀdimethylꢀ1,3ꢀbutadiene; subsequent oxidation of F with
DDQ to furnish the benzene derivative E1. Synthesis of the
benzene derivative E2 from C and 2,3,4,5ꢀtetraphenylꢀ2,4ꢀ
cyclopentadienꢀ1ꢀone.
A
monoꢀdesilylation of
B
to afford the 1,1,2ꢀ
tris(trichlorosilyl)ethene D (72%) is achievable at room temꢀ
perature in CH2Cl2 using [HMe3N]Cl (1 eq) as a mild H+
source and a catalytic amount of [nBu4N]Cl.72
The unsaturated compounds B–D constitute highly promisꢀ
ing reagents for the construction of oligosilylated organic
scaffolds, but their limited availability has largely prevented
such developments in the past. As two of very few examples,
C can readily be permethylated and permethoxylated;68
D
enters into a stereoꢀ and regioselective [2σ + 2σ + 2π]ꢀ
cycloaddition with quadricyclane to give a corresponding
norbornene derivative, which is a useful monomer for the
synthesis of membrane polymers with high gas permeability.61
It is important to note in this context that the reactivities of a
number of silylated olefins increase in the order
Cl3Si(H)C=CH2 < transꢀCl3Si(H)C=C(H)SiCl3 < D and deꢀ
crease once electronꢀwithdrawing chlorine atoms are replaced
by electronꢀdonating methyl groups. Upon reaction with cyꢀ
clopentadiene, D forms exoꢀ and endoꢀnorbornenes, albeit
only in a dynamic equilibrium with the starting materials.61
(i) CH2Cl2, 50°C, 3 d, sealed glass ampoule; (ii) CH2Cl2,
room temperature, 12 h; (iii) CH2Cl2, MeOH/NMe2Et (6 eq),
0 °C to room temperature, 1 h; (iv) CH2Cl2, 180 °C, 4 d, sealed
glass ampoule
Alkyne
C
and
cyclopentadiene
provide
2,3ꢀ
bis(trichlorosilyl)norbornadieneꢀ2,5 in yields of 83%.73 To
push the synthetic utility of C to its limits, we considered the
preparation of 1,2ꢀbis(trichlorosilyl)benzenes for the following
reasons: (a) Already 1,2ꢀbis(trimethylsilyl)benzenes are not
trivial to make, mainly due to steric congestion.35 (b) The only
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Journal of the American Chemical Society
Further insight into the electronic structures of [B]2‒ and B
Characterization of the New Organotrichlorosilanes.
1
NMR Spectroscopy. Plots of the 1H, 13C{1H}, and 29Si NMR
spectra of all organosilanes covered in this publication, toꢀ
gether with a discussion of key NMR features, are provided in
the SI. Whenever literature data are available, our results are
in full agreement with the previously reported chemical shift
values.
was gained from quantumꢀchemical calculations. First, we
note a pleasingly good agreement between the computed and
crystallographically determined geometric parameters of both
compounds (Table 1).
2
3
4
5
6
7
8
9
Xꢀray Crystallography and Corresponding Quantumꢀ
Chemical Calculations. Crystals of [nBu4N][A] and
[nBu4N]2[B] suitable for Xꢀray analysis were grown from
CH2Cl2 (Figure 3a,b).76 The monoanion [A]‒ possesses a C3
axis, [B]2‒ has no symmetry element. Both compounds contain
essentially planar carbanion centers (bondꢀangle sums =
358.5°ꢀ359.9°). The two halves of [B]2‒ are mutually orthogoꢀ
nal, thereby minimizing the electrostatic repulsion between the
vicinal, carbonꢀbased loneꢀpair electrons (dihedral angle
Si(1)C(1)Si(2)//Si(3)C(2)Si(4) = 85.85(6)°). Nevertheless, the
C(1)‒C(2) bond is elongated from 1.47 Å, the expected value
for a single bond between sp2ꢀhybridized carbon atoms,77 to
1.532(5) Å, a value typically observed for single bonds beꢀ
tween sp3ꢀhybridized carbon atoms. The C‒Si bond length of
[A]‒ amounts to 1.768(1) Å and the corresponding bonds of
[B]2‒ are even slightly shorter (1.744(4) Å ꢀ 1.764(4) Å).
Table 1. Selected structural parameters of compounds B,
[B•]‒, and [B]2‒. The final calculated geometries were obꢀ
tained at the PBE0ꢀD3(BJ)(CPCM(CH2Cl2))/maꢀdef2ꢀ
TZVP level of theory.80–85
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C(1)‒C(2)
[Å]
C‒Si (av)
[Å]
Si(1)C(1)Si(2)//Si(3
)C(2)Si(4) [°]
comꢀ
Xꢀray
calcd
Xꢀray
calcd
Xꢀray
calcd
pound
B
1.368(4)
ꢀ/ꢀ
1.355
1.438
1.502
1.914
ꢀ/ꢀ
1.905
1.824
1.757
28.1(2)
ꢀ/ꢀ
27.8
44.9
83.4
[B•]‒
[B]2‒
1.532(5)
1.753
85.85(6)
Natural Bonding Orbital (NBO)86 analyses and Wiberg bond
indices (WBI)87 reveal a wellꢀdefined C=C double bond for B
(WBI = 1.90) and a C‒C single bond for [B]2‒ (WBI = 1.00)
with two equally occupied (occ = 1.61) pꢀtype electron lone
pairs located at the carbon atoms. The lowered occupation of
the lone pairs of [B]2‒, a reduced p character of the hybrid
orbital at silicon (Si: sp1.45, C: sp2.13), and the Natural Populaꢀ
tion Analysis (NPA)86 charges (12×Cl = ‒4.77 e, 4×Si = +5.80
e, 2×C = ‒3.02 e) indicate efficient delocalization of electron
density toward the SiCl3 groups via C‒Si π interactions (cf. the
α effect; Figure 1). This charge delocalization likely contribꢀ
utes to the stability of [nBu4N]2[B], which shows no tendency
for polymerization despite the presence of formally nucleoꢀ
philic carbanions and electrophilic SiCl3 moieties in the same
molecule (similar arguments hold for [nBu4N][A]).
For Eꢀtype compounds, two possibilities exist to reduce unꢀ
favorable interactions between the adjacent SiCl3 groups: The
molecule could widen the bond angles Si(1)‒C(1)‒C(2) and
Si(2)‒C(2)‒C(1) and/or bend both substituents in opposite
directions out of the benzene plane. In the solidꢀstate structure
of E1 (Figure 3c), the Si‒C‒C bond angles are expanded to
127.6(1)° and 128.5(1)°, while the Si(1)‒C(1)‒C(2)‒Si(2)
torsion remains negligible (‒1.1(2)°). Bondꢀangle expansion is
not an option for E2 (Figure 3d), because the SiCl3 groups
would inevitably collide with the orthoꢀphenyl rings. Thus,
Si(1)‒C(1)‒C(2) = 121.5(1)° and Si(2)‒C(2)‒C(1) = 122.7(1)°
are still close to 120°, while the Si‒C‒C‒Si torsion angle
assumes an appreciable value of 46.6(2)°. Similar features as
in the cases of E1 and E2 are evident in the solidꢀstate strucꢀ
tures of 1,2ꢀbis(trimethylsilyl)ꢀ4,5ꢀdichlorobenzene35 and
hexakis(trimethylsilyl)benzene88, respectively (cf. the SI for
more details).
Figure 3. Molecular structures of (a) [nBu4N][A], (b)
[nBu4N]2[B], (c) E1, and (d) E2 in the solid state; [nBu4N]+ ions
and H atoms omitted for clarity.
In comparison, the neutral olefin B features C‒Si bond
lengths in the range 1.910(3) Å ꢀ 1.917(3) Å.78 The significantꢀ
ly smaller values found for [A]‒ and [B]2‒ are partly due to
Coulomb attraction between the anionic carbon atoms and the
positively polarized Si atoms, but also symptomatic of partial
C=Si doubleꢀbond character as a result of π donation from the
carbanion lone pairs into energetically lowꢀlying substituent
group orbitals (cf. the α effect; a structural comparison of [B]2‒
with its SiMe3ꢀsubstituted congener is provided in the SI).79
Redox Properties of B and Corresponding Quantumꢀ
Chemical Calculations. Our syntheses of the radical species
[nBu4N][B•] and the dianion salt [nBu4N]2[B] prove that B is
capable of accepting not only one but even two electrons
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Journal of the American Chemical Society
without decomposition. Accordingly, the cyclic voltammoꢀ
gram (CV) of B in CH2Cl2 with [nBu4N][B(C6F5)4] as supportꢀ
ing electrolyte revealed two reversible redox transitions at half
Page 8 of 17
TDꢀDFT calculations show that the intense blue color of
[B•]‒ in solution arises from a π → π* transition into the singly
occupied molecular orbital of [B•]‒.
1
2
3
4
5
6
7
8
wave potentials of
E1/2 = –0.73 V and –0.98 V (vs.
EPR spectra were recorded at room temperature on soluꢀ
tions of [nBu4N][B•] in CH2Cl2 (0.75 mM). The radical gives
rise to a multiꢀline signal with partly overlapping 35/37Cl couꢀ
plings and obscured 29Si coupling. A simulation of the experꢀ
imentally obtained spectrum with hyperfine coupling constants
of a(29Si) = 4.4 G and a(35Cl) = 1.27 G gave an excellent
match (Figure 5). Moreover, the values obtained are in good
agreement with calculated coupling constants obtained at the
B3LYP(CPCM(CH2Cl2))/IGLOꢀIII//PBE0ꢀ
FcH/FcH+).89 We finally note that B is not inert toward
[nBu4N][PF6] in THF or CH2Cl2, but rather abstracts F‒ ions
from the commonly used supporting electrolyte, which underꢀ
scores a strong Lewis acidity of the alkene (cf. the SI for more
details).
The neutral compound B and its doubly reduced form
[nBu4N]2[B] are colorless in solution and the solid state. Soluꢀ
tions of the monoradical [nBu4N][B•] in CH2Cl2 show a deep
blue color and a concomitant broad absorption band in the
UV/vis spectrum (λmax = 650 nm). For comparison, the methꢀ
ylated monoanion radical [(Me3Si)2C‒C(SiMe3)2]•‒ gives a
dark green solution (THF; λmax = 697 nm).90
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D3(BJ)(CPCM(CH2Cl2))/maꢀdef2ꢀTZVP86,91 level of theory
(a(29Si) = ‒3.3 G and a(35Cl) = 1.29 G; cf. the SI for more
information). For comparison, the SiMe3ꢀsubstituted radical
[(Me3Si)2C‒C(SiMe3)2]•‒ is characterized by an a(29Si) value
of 4.65 G.92 The significant spin density ρs on each Si atom of
[nBu4N][B•] (ρs(Si) = 8%, ρs,tot(4Si) = 32%; ρs(C) = 30%,
The colors of the chlorinated vs. methylated species likely
depend on electronic substituent effects and/or the torsion
about the central C‒C bond. To evaluate the relative weights
of both influence factors, we performed quantumꢀchemical
ρ
s,tot(2C) = 60%) indicates a delocalization of the odd electron
over all four C‒Si bonds. The a(29Si) value thus seems to be
governed mainly by electronic substituent effects, which influꢀ
ence the C‒Si bonding interactions.93
calculations
on
the
neutral
olefins
B
and
(Me3Si)2C=C(SiMe3)2, because here, contrary to the cases of
the corresponding reduced forms, ion pairing does not have to
be taken into account as a third important parameter. We come
to the conclusions that twisting of the molecular scaffolds
influences
the
absorption
bands
of
B
and
(Me3Si)2C=C(SiMe3)2 to similar extents such that the differꢀ
c
ence between their calculated λmax values is largely independꢀ
ent of the C‒Cꢀtorsion angle. At any given torsion angle, difꢀ
ferences in the absorption maxima of both olefins result from
electronic substituent effects, such as the different πꢀacceptor
strengths of SiCl3 and SiMe3 groups (cf. the SI for more inꢀ
formation).
With a C‒C bond length of 1.438 Å and a dihedral angle of
44.9°, the calculated key structural parameters of the monoradꢀ
ical anion [B•]‒ are intermediate between those of B and [B]2‒
(Table 1). The progressively more pronounced twist helps to
avoid an unfavorable orbital overlap in the course of the stepꢀ
wise population of the antiꢀbonding π* molecular orbital
(LUMO) of B (Figure 4).
Figure 5. Optimized EPR spectrum of [nBu4N][B•] (blue;
0.75 m
M in CH2Cl2, room temperature); fit results: g value =
2.0026, a(29Si) = 4.4 G and a(35Cl) = 1.27 G, line width(G) =
0.118 G and line width(L) = 0.188 G.
Mechanistic Considerations.
Our mechanistic proposals are based on the view that the
chlorideꢀinduced disproportionation of Si2Cl6 generates
[SiCl3]‒ ions as the primary reactive species (Scheme 1). The
[SiCl3]‒ intermediate can principally react further in three
different ways: (1) A redox reaction may occur during which a
single electron is transferred from the anion to the organic
substrate R‒Cl. (2) [SiCl3]‒ may dissociate to liberate [SiCl2],
which can subsequently insert into the R‒Cl bond. (3) If
[SiCl3]‒ reacts as a nucleophile, it could attack either at the
carbon atoms or at the chlorine atoms with concomitant forꢀ
mation of SiCl4 and [R]‒. The carbanion would subsequently
Figure 4. Selected KohnꢀSham molecular orbitals and their energy
eigenvalues [eV] of compounds B, [B•]‒ and [B]2‒ at the PBE0ꢀ
D3(BJ)(CPCM(CH2Cl2))/maꢀdef2ꢀTZVP level of theory. Isosurꢀ
face value = 0.05 eꢁbohr‒3.
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Journal of the American Chemical Society
RS(CH2Cl2)/def2ꢀQZVP//PBE0ꢀD3(BJ)(COSMO(CH2Cl2))/def2ꢀ
have to bite back into SiCl4 (or Si2Cl6) to establish a C‒SiCl3
bond.46
Reaction of [SiCl3]‒ with CCl4. With regard to the reaction
1
TZVPD level of theory. 81–84,94–98
2
3
4
5
6
7
8
9
Yet, for the following reasons our current working hypotheꢀ
sis assumes an initial nucleophilic attack of [SiCl3]‒ on one of
the Cl atoms of CCl4, followed by a rebound reaction of the
thus generated [CCl3]‒ ion:46,99,100 (1) The carbon center of
CCl4 is sterically more shielded than that of CH2Cl2. (2)
[CCl3]‒ ions are good leaving groups compared to correspondꢀ
ing [H2CCl]‒ ions, as evidenced by the haloform reaction101
and the fact that Me3SiCCl3 is readily desilylated with
[nBu4N]F, whereas Me3SiCH2Cl is not.102 (3) Within the series
of chloromethanes CClnH4‒n (n = 1ꢀ4), the NBO charges at the
Cl atoms continuously become less negative as n increases; for
n > 2 the Cl sites are even positivated.103 CCl4 was recognized
as a typical halogenꢀbond donor due to positive surface potenꢀ
tials of up to 17 kcal mol‒1 at the end regions of the Cl atoms
(σꢀholes).104,105 (4) In neat CCl4, the Si2Cl6/[nBu4N]Cl system
forms the [CCl3]‒ adduct [nBu4N][Cl3C‒SiCl3‒CCl3] (see
above). (5) Phosphines are isoelectronic to the [SiCl3]‒ ion and
nucleophilic displacement on halogen atoms of halocarbons is
a central concept of phosphine chemistry (cf. the Appel and
CoreyꢀFuchs reactions).45,106,107 Moreover, our group recently
succeeded in the activation of CX4 (X = Cl, Br) by the geminal
frustrated Lewis pair (Fxyl)2B‒CH2‒PtBu2 to furnish
(Fxyl)2B(CX3)‒CH2‒P(X)tBu2 (Fxyl = 3,5ꢀ(CF3)2C6H3).108 To
further substantiate our working hypothesis, we calculated the
transition states of the carbophilic (ꢀG# = 40.0 kcal/mol) vs.
halophilic (ꢀG# = 11.3 kcal/mol) attack of [SiCl3]‒ on CCl4
and found the latter to be energetically favorable by 28.7
kcal/mol (cf. Scheme 7, bottom).
of Si2Cl6/[nBu4N]Cl with CH2Cl2, Kroke et al. suggested a
sequential nucleophilic substitution reaction at the carbon
atom (but did not exclude the possibility of a dichlorosilylene
pathway). The resulting H2C(SiCl3)2 undergoes in situ deproꢀ
tonation by
a
third molecule of [SiCl3]‒ to furnish
[HC(SiCl3)2]‒. This ionic intermediate subsequently attacks
SiCl4 (or residual Si2Cl6) and forms HC(SiCl3)3. A second
deprotonation step finally produces [nBu4N][A] (Scheme 7
top).63 The reaction of Si2Cl6/[nBu4N]Cl with CCl4 in CH2Cl2
proceeds so fast that the detection of intermediates by in situ
NMR monitoring becomes impossible. The conversion is also
highly selective, and we did not observe any side products
(only the byproduct SiCl4) that could shed light on mechanistic
details.
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Scheme 7. Proposed reaction mechanisms for the forꢀ
mation of [nBu4N][A] from [SiCl3]‒ and CH2Cl2 (Ref[63])
or CCl4 (this work, top). Comparison of the transition
states of the carbophilic vs. halophilic attack of [SiCl3]‒ on
CCl4 (bottom).
In contrast to the mechanism that is likely operative in the
formation of [nBu4N][A] from Si2Cl6/[nBu4N]Cl and CH2Cl2,
we thus propose the following scenario for CCl4 as the carbon
source (Scheme 7, top): [SiCl3]‒ abstracts a Cl+ cation from
CCl4 to form SiCl4 and [CCl3]‒, which then react further to
furnish Cl3C‒SiCl3 and Cl‒ (nucleophilic attack of [CCl3]‒ on
residual Si2Cl6 would provide the same product, together with
[SiCl3]‒). If this sequence is repeated twice, we obtain the
trisilylmethane ClC(SiCl3)3, which, upon losing a last Cl+
cation to [SiCl3]‒, is transformed to [A]‒. In line with the reꢀ
quired stoichiometry, all steps are catalytic in [nBu4N]Cl,
apart from the final one, which consumes one equivalent of
Cl‒.70 The carbon centers of the neutral intermediates become
sterically less and less accessible as the reaction proceeds,
while the charge of the carbanionic intermediates becomes
more and more delocalized due to the α effects of an increasꢀ
ing number of SiCl3 substituents (Figure 1). Even with some
excess of CCl4, we never observed partially silylated chloroꢀ
methanes and therefore conclude that the first Clꢀatom substiꢀ
tution is rate determining. For steric reasons, also this observaꢀ
tion points toward [SiCl3]‒ attack on Cl and not the central C
atom.
All four involved reaction steps are mechanistically related
to each other, which renders the proposed scenario simple and
conceptually appealing. Nevertheless, the involvement of
[SiCl2] cannot be excluded, but the nonꢀdonor solvent
(CH2Cl2) and the nonꢀcoordinating [nBu4N]+ counter ion emꢀ
ployed are not expected to promote [SiCl3]‒ dissociation. By
the same token, the putative intermediate [nBu4N][CCl3]
should have a longer lifetime than alkali metal carbenoids
Cation = [nBu4N]+ omitted for clarity. Calculated transition
states for the carbophilic (left) and halophilic (right) attack of
[SiCl3]‒ on CCl4 at the B2ꢀPLYPꢀD3(BJ,ATM)+COSMOꢀ
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M[CCl3] (M+ = alkali metal cation), because the thermodyꢀ
Page 10 of 17
rather than C2Cl6. Any further considerations will thus be
1
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5
6
7
8
namic driving force for MCl elimination with concomitant
[CCl2] formation is lacking.102,109,110 We therefore consider not
only [SiCl2]ꢀ but also [CCl2]ꢀbased pathways less likely than
the alternative ionic mechanisms. In view of the proven sensiꢀ
tivity of Cl3C‒SiCl3 toward Cl‒ ions (Scheme 4), we finally
note that fiveꢀcoordinate anions such as [Cl3C‒SiCl4]‒ and
[Cl3C‒Cl3Si‒CCl3]‒ not only occur as transient intermediates,
but can also serve as [CCl3]‒ꢀtransfer reagents (cf. Scheme 4).
focused on C2Cl4 as starting material.
It is possible to construct reasonable mechanistic scenarios
by combining the limited set of substance classes and elemenꢀ
tary reactions outlined in Scheme 8.
Additionꢀelimination, (1)→(5): Carbophilic attack of
[SiCl3]‒ on chloroethenes will produce alkyl anions,111 which
eliminate Cl‒ to generate (trichlorosilyl)ethenes. A correspondꢀ
ing sequence has been postulated for the stereospecific reacꢀ
tion of C2Cl4 with sodium pꢀtoluenethiolate (NaSTol), resultꢀ
ing in the formation of TolS(Cl)C=CCl2 and ultimately transꢀ
TolS(Cl)C=C(Cl)STol.112
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Eliminationꢀaddition, (2)→(6), or eliminationꢀelimination,
(2)→(7): Halophilic attack of [SiCl3]‒ on chloroethenes will
produce vinyl anions, which can bite back into SiCl4 to generꢀ
ate (trichlorosilyl)ethenes. Provided that the vinyl anion bears
at least one chlorine substituent, an alternative sequence leads
to alkynes via release of Cl‒.113 Cl atoms in transꢀβ (highest
rates), cisꢀβ, or even α positions to the electron lone pair are
suitable leaving groups.114–116 Reaction scenarios related to
(2)→(6) and (2)→(7) have been investigated in detail using
Li(Cl)C=CCl2 as the example. At ‒110 °C, the halfꢀlife time of
the compound is t1/2 = 70ꢀ80 min.115 In our case, Cl‒ eliminaꢀ
tion from [nBu4N][(Cl)C=CCl2] should require longer times,
similar to the example of [nBu4N][CCl3] vs. M[CCl3] disꢀ
cussed above.102,117,118
Scheme 8: Conceivable elementary reactions underlying
the formation of [nBu4N]2[B].
Additionꢀaddition, (3)→(6), or additionꢀelimination,
(3)→(7): Carbophilic attack of [SiCl3]‒ on alkynes formed via
path (2)→(7) will again generate intermediate vinyl anions,
which can undergo the same followꢀup reactions as discussed
above. Comparable reactions have been described in the conꢀ
text of frustrated Lewisꢀpair chemistry. As an example, PPh3
and B(C6F5)3 add to PhC≡CH to afford the zwitterionic species
Ph(Ph3P)C=C(H)(B(C6F5)3).119 [SiCl3]‒ should be a stronger
nucleophile than PPh3 and the Lewis acidic SiCl4 may take the
role of B(C6F5)3 in C≡C tripleꢀbond activation.
Eliminationꢀaddition, (4)→(8): Similar to the cases of chloꢀ
roethenes, [SiCl3]‒ can accept Cl+ cations from chloroethynes
to generate acetylides, which will reꢀattack SiCl4 to afford
(trichlorosilyl)ethynes. As a related example, disilylethynes
are readily available upon treatment of R3SiCl with a mixture
of trichloroethene and nꢀBuLi (3 eq) in THF at ‒78 °C.120–122
The reaction proceeds via LiCl elimination with intermediate
formation of dichloroethyne, which undergoes Cl/Liꢀexchange
to form dilithioacetylide.123
Note: In the cases of paths (2)→(7) and (3)→(7), at least one of
the substituents R in the starting material and the vinylꢀanion
intermediate must be Cl; in the cases of paths (3)→(6) and
(3)→(7), at least one of the substituents R in the vinylꢀanion
intermediate and the products must be SiCl3. Residual Si2Cl6
should be capable of taking the role of SiCl4 as a source of SiCl3
substituents by releasing [SiCl3]‒ in place of Cl‒; the overall balꢀ
ance would remain the same. Cations = [nBu4N]+ omitted for
clarity.
One should note that all these elementary steps are competꢀ
ing with each other and that their relative importance can
change as the reaction proceeds and the nature of the substituꢀ
ents R changes (i.e. Cl vs. SiCl3). For example, an alkylꢀanion
intermediate (cf. (1)→(5)) will become increasingly more
stable as Cl atoms are replaced by SiCl3 groups (α effect;
Figure 1). The following insights provided by the literature
and by our own control experiments are helpful to arrive at a
more focused picture:
a) The dianion salt [nBu4N]2[B] also forms cleanly if the Cl‒
ꢀinduced disproportionation of Si2Cl6 is carried out in C2Cl4 as
the solvent (containing a small volume of CH2Cl2 to dissolve
[nBu4N]Cl). This indicates that the first step of the reaction
cascade is rate determining.
Reactions of [SiCl3]‒ with C2Cl6 and C2Cl4. As mentioned
above, treatment of C2Cl6 with 1 eq of Si2Cl6 and catalytic
amounts of [nBu4N]Cl quantitatively generates tetrachloroeꢀ
thene, C2Cl4. For similar reasons as outlined above, C2Cl6, the
higher homologue of CCl4, likely reacts with [SiCl3]‒ through
initial Cl+ꢀcation abstraction. The resulting [C2Cl5]‒ anion
eliminates Cl‒ to furnish C2Cl4.37 We have also confirmed that
the synthesis of [nBu4N]2[B] can equally well (and more costꢀ
efficiently) be performed using Si2Cl6/[nBu4N]Cl and C2Cl4
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b) The phosphorus congener of [SiCl3]‒, Li[PPh2], and C2Cl4
when we treat olefin B with 1 eq of [nBu4N]Cl in the absence
of CuCl2, [Gclosed]‒ likely also constitutes the key intermediate
in the conversion of B to C. This assumption is further supꢀ
ported by the fact that the corresponding protonation product,
D, quantitatively forms upon addition of [nBu4N]Cl to B in the
presence of [HMe3N]Cl as a mild proton source (red path)
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5
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7
furnish the tetrasubstituted ethene (Ph2P)2C=C(PPh2)2 at ‒78
°C, whereas under ambient conditions the ethyne
Ph2PC≡CPPh2 is obtained.124 It has been suggested that initial
substitution leads to the intermediate Cl2C=C(Cl)PPh2, which
upon warming in the presence of excess Li[PPh2] provides
Ph2PC≡CPPh2, LiCl, and ClPPh2 (product of a halophilic
attack).
c) The elementary reactions compiled in Scheme 8 lead eiꢀ
ther to (trichlorosilyl)ethenes or ꢀethynes. We therefore selectꢀ
ed transꢀCl3Si(Cl)C=C(Cl)SiCl3,69 and Cl3SiC≡CSiCl3 as
archetypal members of both compound classes and treated
them with Si2Cl6/Cl‒ (3 eq; CD2Cl2). In both cases, we obꢀ
tained [nBu4N]2[B] as the major product. In this respect, the
overall behavior is similar to that of the Cl2C=CCl2/Si2Cl6/Cl‒
8
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mixture.
However,
the
reaction
of
transꢀ
Cl3Si(Cl)C=C(Cl)SiCl3 is significantly slower than the reacꢀ
tions of Cl3SiC≡CSiCl3 or Cl2C=CCl2 (cf. the SI for more
information).
Only
in
the
case
of
transꢀ
Scheme 9: Proposed mechanism for the reactions underlyꢀ
ing the formation of all C2ꢀspecies covered in this work.
Cl3Si(Cl)C=C(Cl)SiCl3, the 29Si NMRꢀspectroscopic monitorꢀ
ing of the reaction mixture revealed significant amounts of
cyclohexasilaneꢀchloride
diadducts,
such
as
[1,1ꢀ
(Cl3Si)2Si6Cl10·2Cl]2‒ (cf. Scheme 2). These species are comꢀ
mon products of the Si2Cl6/Cl‒ system if other reaction partꢀ
ners are absent, which underlines the reluctance of transꢀ
Cl3Si(Cl)C=C(Cl)SiCl3 to take part in further transformations
under the conditions applied. Moreover, small quantities of
Cl3SiC≡CSiCl3 were detectable in the same sample, thereby
indicating that transꢀCl3Si(Cl)C=C(Cl)SiCl3 is slowly dechloꢀ
rinated to Cl3SiC≡CSiCl3 and that the observed formation of
[nBu4N]2[B] may ultimately start from the formed ethyne.
Based on these observations, transꢀCl3Si(Cl)C=C(Cl)SiCl3
cannot be excluded as a reaction intermediate, but the major
pathways likely proceed via ethyne species. It is important to
note in this context that the reactivity of ClC≡CCl toward
Si2Cl6/Cl‒ is not directly assessable, because this highly unstaꢀ
ble ethyne can only be handled in ethereal solvents, which are
not compatible with Si2Cl6.113
Black path: Formation of [B]2‒ from Si2Cl6/[nBu4N]Cl/C2Cl4
mixtures; green path: formation of C through treatment of B with
[nBu4N]Cl; red path: formation of D through treatment of B with
[nBu4N]Cl in the presence of [HMe3N]Cl; cations = [nBu4N]+
omitted for clarity.
Before this background, we can combine the limited set of
substrate classes and elementary reactions outlined in Scheme
8111 and arrive at a plausible mechanistic scenario, which
straightforwardly accounts for all the reactions and C2ꢀ
products discussed so far (Scheme 9): The rateꢀdetermining
first step likely involves a halophilic attack of [SiCl3]‒ on
C2Cl4, followed by Cl‒ elimination to give small local concenꢀ
trations of ClC≡CCl (cf. path (2)→(7) in Scheme 8). The
highly reactive alkyne is immediately transformed further to
afford the more stable disilylalkyne Cl3SiC≡CSiCl3 (via path
(3)→(7) and/or (4)→(8)). A carbophilic attack of [SiCl3]‒ on
the electronꢀpoor intermediate Cl3SiC≡CSiCl3 leads to the
vinyl anion [Cl3SiC=C(SiCl3)2]‒ ([Gopen]‒), which stabilizes
itself through intramolecular adduct formation resulting in the
SiC2ꢀring compound [Gclosed]‒ with pentacoordinate Si atom. If
[SiCl3]‒ ions are in sufficient supply, attack on [Gclosed]‒ inducꢀ
es ring opening with formation of the dianion [B]2‒ (black
path). If [SiCl3]‒ ions are lacking, one [G]‒ anion can transfer
an [SiCl3]‒ fragment to another [G]‒ anion, which is equivalent
to a disproportionation reaction and affords equimolar quantiꢀ
ties of C and [B]2‒ (green path). Since this is precisely the
product distribution we observe (together with 1 eq of SiCl4)
The vinyl anion [G]‒ is a crucial species in our mechanistic
scenario and the crossing point of several reaction paths. Yet,
despite many attempts, we have so far not succeeded in its
isolation, let alone structural characterization. We therefore
investigated the structure and reactivity of [G]‒ by means of
quantumꢀchemical
calculations
at
B2ꢀPLYPꢀD3(BJ,
ATM)+COSMOꢀRS(CH2Cl2)/def2ꢀQZVP//PBE0ꢀ
D3(BJ)(COSMO(CH2Cl2))/def2ꢀTZVPD level of theory.81–
84,94–98
We indeed found two minimum structures, the openꢀ
chain structure [Gopen]‒ and the cyclic structure [Gclosed]‒, the
former being less stable by ꢀG = 4.2 kcal/mol (Figure 6).
Given the low energy barriers of the transition state TS1 in
both directions, [Gopen]‒ and [Gclosed]‒ should be interconvertiꢀ
ble at room temperature. In line with that, the lengths of the
C=C double bonds in [Gopen]‒ (1.336 Å) and [Gclosed]‒ (1.349
Å) are very similar; the CꢀCꢀSi bond angle about the carbanion
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Page 12 of 17
[Gopen]‒ amounts to 143.8° and is thus significantly wider than
Cl2C=CCl2, a stable, nonꢀflammable compound, so cheap that
the ideal angle of 120° for an sp2ꢀhybridized C atom.
it is widely used in dry cleaning:127 Upon reaction with the
Si2Cl6/Cl‒ system, Cl2C=CCl2 furnishes the vicinal dianion
[(Cl3Si)2C‒C(SiCl3)2]2– ([B]2–) in excellent yields. Oneꢀstep
transformations of [B]2– lead to the tetrasilylethane
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The proposed attack of [SiCl3]‒ on [Gclosed]‒ requires only
little activation energy (TS2 = 10.5 kcal/mol), even though the
electrostatic repulsion of two negative charges has to be overꢀ
come. As to be expected from the high yields of [B]2‒, its
formation is strongly exergonic (ꢀG = ‒27.1 kcal/mol).
(Cl3Si)2(H)C‒C(H)(SiCl3)2
(Cl3Si)2C=C(SiCl3)2
(Cl3Si)2C=C(H)(SiCl3)
(H2B),
(B),
(D),
the
the
tetrasilylethene
trisilylethene
disilylethyne
or
the
Cl3SiC≡CSiCl3 (C). By virtue of their SiCl3 groups, all these
products are useful building blocks for silicones and silsesquiꢀ
oxanes. Moreover, the unsaturated C=C and C≡C bonds of
B/D and C offer ample opportunities for further functionalizaꢀ
tion, e.g., through hydrosilylation or cycloaddition reactions.
We have already used the latter, followed by a final aromatizaꢀ
tion step, to prepare previously unknown 1,2ꢀ
bis(trichlorosilyl)benzenes (E) from C and a selection of butaꢀ
dienes. The potential of Eꢀtype compounds as ditopic, chelatꢀ
ing Lewis acids or Hiyamaꢀcoupling reagents is currently
being explored in our laboratories.
The reaction cascade leading to the key product [B]2– likely
starts with the halophilic attack of [SiCl3]– on Cl2C=CCl2 and
continues with the intermediate formation of C, which is then
subject to a carbophilic attack of [SiCl3]– to give the vinyl
anion [(Cl3Si)2C=CSiCl3]– ([G]–). According to quantumꢀ
chemical calculations, [G]– readily adds a second [SiCl3]– ion
to afford [B]2–. The vinyl anion [G]– also forms during the
synthesis of C via the Cl‒ꢀinduced elimination of SiCl4 from
B. In the absence of other reaction partners, [G]– disproporꢀ
tionates and thereby furnishes equimolar amounts of C and
[B]2–. Added CuCl2 oxidizes [B]2– in situ and continuously
regenerates the starting material B, which substantially imꢀ
proves the yields of C. This last example underscores the
benefit gained from our mechanistic insights, even though the
full scenario is probably more complex and includes [SiCl2]
chemistry as well as singleꢀelectronꢀtransfer processes (cf. the
appearance of the blue radical [B•]‒ during the synthesis of
[B]2–). Fortunately, most of these pathways are apparently
funneling into the same channel such that [B]2– is formed with
remarkably high selectivity.
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Figure 6. Relative Gibbs free energy diagram for the formation of
‒
[B]2‒ from [Gclosed
]
at the B2ꢀPLYPꢀD3(BJ, ATM)+COSMOꢀ
RS(CH2Cl2)/def2ꢀQZVP//PBE0ꢀD3(BJ)(COSMO(CH2Cl2))/def2ꢀ
TZVPD level of theory.81–84,94–98 All energies are given in
kcal/mol relative to [Gclosed]‒.
Schemes 8 and 9 provide plausible explanations of how the
unique vicinal dianion [B]2‒ may form through the Cl‒ꢀinduced
disproportionation of Si2Cl6 in the presence of C2Cl4. Howevꢀ
er, they do not account for all available experimental evidence
and the detailed picture is likely more complex:
(a) The reproducible appearance of the blue radical [B•]‒ in
almost all reaction mixtures indicates that singleꢀelectronꢀ
transfer (SET) processes are operative in addition to the proꢀ
posed closedꢀshell pathways.
ASSOCIATED CONTENT
Supporting Information. Experimental details and characterizaꢀ
tion data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(b) The nature of [SiCl3]‒ as Cl‒ adduct of [SiCl2] (Scheme
1) opens a possibility for silyleneꢀtype reactions. For example,
[2+1] cycloaddition between the disilylethyne C and [SiCl2]
would lead to a silacyclopropene,125 which could subsequently
AUTHOR INFORMATION
Corresponding Author
engage in a Cl‒ association/dissociation equilibrium with
‒ 126
[G
] .
closed
*
matthias.wagner@chemie.uniꢀfrankfurt.de
(c) The tetrasilylethene B is readily reduced to [B]2‒ by
means of Si2Cl6/Cl‒ in CH2Cl2 and can thus not be ruled out as
a potential synthesis intermediate of [B]2‒.
Author Contributions
aThese authors contributed equally to this work.
Fortunately, “all roads lead to Rome” and all the conceivaꢀ
ble reaction pathways ultimately join to provide excellent
yields of [B]2‒.
ACKNOWLEDGEMENT
J.Te. and M.W. are grateful to the Evonik Resource Efficiency
GmbH, Rheinfelden (Germany) for financial funding and the
generous donation of Si2Cl6. I.G. wishes to thank the Evonik
Foundation for a PhD grant. This work was further supported by
the DFG in the framework of the "Gottfried Wilhelm Leibnizꢀ
Preis" to S.G. The authors acknowledge Prof. Dr. Ullrich Englert
CONCLUSION
We have developed convenient protocols for the synthesis
of multiply SiCl3ꢀsubstituted organic scaffolds, starting from
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Journal of the American Chemical Society
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