Pinacol boronates by direct arene borylation with borenium cations

Article abstract of DOI:10.1002/anie.201006196

Borenium does it direct: A boron analogue of the Friedel-Crafts reaction uses inexpensive catecholatoborenium cations that are sufficiently electrophilic to borylate arenes by electrophilic aromatic substitution. The direct arene borylation proceeds with high regioselectivity for a range of anilines, N-heterocycles, and thiophenes. Subsequent one-pot transesterification provides the synthetically useful pinacol boronate esters (see scheme).

Full text of DOI:10.1002/anie.201006196

Communications
DOI: 10.1002/anie.201006196
Borylation
Pinacol Boronates by Direct Arene Borylation with Borenium
Cations**
Alessandro Del Grosso, Paul J. Singleton, Christopher A. Muryn, and Michael J. Ingleson*
Aryl boronate esters are essential synthetic building blocks,
electrophilic to intermolecularly borylate a range of arenes.
This is a boron analogue of classic Friedel–Crafts chemistry,
and represents an inexpensive methodology for the formation
of synthetically desirable pinacol boronate esters under
electronic control direct from the parent arene.
B-chlorocatecholborane (CatBCl) does not react with
AlCl3 at 208C.[16] In contrast, the addition of 1 equivalent of
AlCl3 to the amine adducts CatBCl(L) resulted in a rapid
reaction. Multinuclear NMR spectroscopy suggested the
formation of [CatB(L)][AlCl4] (L = Et3N (1) and L = N,N-
dimethyl-p-toluidine (2); Scheme 1) with 11B chemical shifts
their ubiquity arising from ease of handling combined with
[1]
ꢀ
high efficacy in C X (X = C, N, O) bond-forming reactions.
Aryl boronate esters are commonly synthesized from arenes
by a multistep process involving haloarene intermediates and
either stoichiometric hard organometallic reagents or metal-
catalyzed cross-coupling (e.g., Cu, Ni, Co, or Pd).[1–7] Recent
alternative approaches avoiding haloarenes include: [4+2]
cycloadditions,[8] aryl-CH deprotonation,[9,10] diazonium
ions,[11] and direct arene borylation with sterically hindered
iridium catalysts.[12–14] The latter represents a considerable
advance, being highly generic and proceeding in one step
from the parent aryl CH. It is however still subject to a
number of restrictions: 1) borylation is dominated by steric
factors;[4,15] 2) in low steric environments regioselectivities are
poor, e.g., anisole reacts with 74% meta and 25% para
borylation;[4] 3) iridium catalysts are required making larger
scale syntheses uneconomical.
The development of a generic direct arene borylation
route by electrophilic aromatic substitution is highly desir-
able, with the potential to provide complementary selectiv-
ities to iridium catalysis (electronic compared to steric
control) and enhanced reaction efficiency (to traditional
multistep borylation). In contrast to Friedel–Crafts chemistry,
direct intermolecular arene borylation by electrophilic sub-
stitution is rare,[16–19] requiring directing groups to pre-
coordinate boron (making the process intramolecular) and
forcing conditions.[20–23] We recently reported intermolecular
arene borylation using borocations,[16] with borylation regio-
selectivity controlled by arene electronic effects. However,
the active boron electrophile was poorly defined due to its
high reactivity. A related intramolecular arene borylation was
shown to proceed through a three-coordinate borocation
intermediate,[24] termed a borenium cation.[25,26] Borenium
Scheme 1. Equilibria involved in the formation of borenium cations by
halide abstraction with AlCl3.
of d = 27.9 and 28.1 ppm, respectively, comparable to the
related borenium cations [PinB(NMe2Ph)]+ and [CatB-
(PtBu3)]+ (d = 26.4 and 29.9 ppm).[29,30] A sharp 27Al reso-
nance at d = 104 ppm confirmed the formation of [AlCl4]ꢀ.
The amine proton resonances in 1 were well defined, but in 2
they were significantly broadened at 208C indicating fluxion-
ality. On cooling to ꢀ208C new 11B resonances were resolved;
the two major products corresponded to CatBCl and the
borenium cation [CatB(Me2NTol)]+, whilst a minor reso-
nance was attributed to [CatBCl(Me2NTol)].[31] Attempts to
reach the slow exchange regime for 1 failed (to ꢀ408C,
CD2Cl2), but reactivity studies support the presence of
analogous equilibria, that is, addition of CatBCl to preformed
Et3N-AlCl3 resulted in approximately 20% conversion to 1
after 15 h. Furthermore, the addition of 1 equivalent of Et3N
to 1 produced the neutral adducts CatBCl(Et3N) and Et3N-
AlCl3 and not the boronium cation, [CatB(Et3N)2]+, expected
in the absence of reversible halide transfer. These equilibria
(Scheme 1) are analogous to the reaction of (9-BBN)BCl
(BBN = borabicyclo[3.3.1]nonane) and BCl3 with pyridines
and strong Lewis acids.[32–34] Borenium cations 1 and 2 are
therefore highly electrophilic, with a Lewis acidity towards
chloride comparable to AlCl3, important for applications in
electrophilic aromatic substitution where a strong boron
electrophile is essential.
cations are well documented to be strong electrophiles,[27]
a
key prerequisite for electrophilic aromatic substitution.[17] It is
plausible that borenium cations will be active electrophiles in
intermolecular borylation, though there have been no defin-
itive examples published to date.[28] Herein we report new
catecholato-ligated borenium cations that are sufficiently
[*] A. Del Grosso, P. J. Singleton, Dr. C. A. Muryn, Dr. M. J. Ingleson
Department of Chemistry, University of Manchester
Manchester, M13 9PL (UK)
Fax: (+44)161-275-4616
E-mail: michael.ingleson@manchester.ac.uk
[**] This work was supported by the Royal Society (University Research
Fellowship to M.J.I.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006196.
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Whilst attempts to crystallize compounds 1 and 2 were
unsuccessful, their formulation as borenium cations was
supported by X-ray diffraction on a related compound
derived from tetrachlorocatecholato (Cl4-Cat) [Cl4-CatB-
(NEt3)][AlCl4] (3; Figure 1). Crystallization of 3 is facilitated
sufficiently electrophilic for electrophilic aromatic substitu-
tion of N,N-dimethylaniline, which releases the base, Et3N,
necessary for proton scavenging. Electrophilic borylation of
N,N-dimethylaniline occurs exclusively in the para position
due to synergic electronic (meta deactivating) and steric
effects (ortho deactivating). Borenium cation borylation
therefore provides complimentary selectivity to iridium-
catalyzed borylation, which for mono-substituted arenes
gives predominantly meta-borylated products.[4]
This complementary behavior is further emphasized by
the electrophilic borylation of 3-bromo-N,N-dimethylaniline
that proceeds under electronic control to provide only the
1,3,4- borylated isomer, in high yield. In contrast, iridium-
catalyzed borylation of 1,3-disubstituted arenes gives the
1,3,5-trisubstituted isomer.[15] Whilst the borylation of 3-
bromo-N,N-dimethylaniline was extremely slow with 1 (only
ca. 50% after seven days), borylation proceeded to comple-
tion within 32 h using 3. This reactivity disparity can be
attributed to the greater electrophilicity of 3 relative to 1,
engendered by the inferior electron-donor ability of the
tetrachlorocatecholato ligand.
Figure 1. ORTEP drawing of compound 3 (thermal ellipsoids at 50%
probability and hydrogen atoms omitted for clarity). Selected bond
ꢀ
ꢀ
ꢀ
lengths [ꢀ]: B1 N1 1.505(3), B1 O1 1.364(3), B1 O2 1.370(3).
The wider borylation substrate scope using 1 was sub-
sequently investigated. At 208C in CH2Cl2 N-methylindole
was readily borylated exclusively at the 3-position, consistent
with the electrophilic aromatic substitution of indoles.[39]
However, attempts to isolate the catechol boronate ester
were hampered by protodeboronation to N-methylindole and
B-hydroxycatecholborane. This susceptibility to protodeboro-
nation can be attributed to the low steric environment and the
electron deficiency of boron in catechol boronate esters,
which allows coordination of protic species (e.g., H2O) to
boron, the first step in protodeboronation in weakly acidic
media.[40] The bulkier and more electron-donating diol,
pinacol, forms analogous pinacol boronate esters that are
significantly more resistant to protodeboronation in the
presence of H2O. Therefore, in situ transesterification of
catechol for pinacol was performed, eliminating the undesir-
able protodeboronation and enabling product isolation in
high yield. The use of pinacolato-ligated borenium cations
would conceptually enable the single-step synthesis of pinacol
boronate esters. This approach was initially complicated by
the instability of PinBCl,[41] and then precluded by the failure
of [PinB(amine)]+, synthesized from PinBH (amine = N,N-
dimethylaniline or 2,6-lutidine), to borylate N,N-dimethylani-
line and N-methylpyrrole.[31] This is attributable to the
reduced electrophilicity of boron on replacing catechol for
the more electron-donating pinacol.
by intermolecular CCl···H2CN hydrogen bonding.[35] The
boron center in 3 is trigonal planar (angles sum to 3608),
with the closest B···ClAlCl3 contact long at 3.444 ꢀ, consistent
ꢀ
with an ionic species. The short B N bond (1.505(3) ꢀ) is
comparable to the borenium cations [(aryl)2B(DMAP)]+
ꢀ
(e.g., B N 1.500(6) ꢀ; DMAP = 4-dimethylaminopyri-
dine).[36] The B O bonds in 3 (1.364(3) ꢀ and 1.370(3) ꢀ)
ꢀ
are short, shorter than in CatBCl (both 1.381(2) ꢀ) and
comparable
with
[CatB(PtBu3)]+
(1.369(6) ꢀ
and
1.373(5) ꢀ), indicative of significant O!B p donation in
3.[30,37] Despite this p donation from the catecholato moiety
compounds 1–3 are still highly electrophilic, presumably a
direct consequence of their cationic character.
For arene borylation studies 1 and 3 were chosen as the
boron electrophiles due to the neutral adducts, (e.g., CatBCl-
(Et3N)) not undergoing complicating disproportionation
reactions in contrast to 2.[37] Initial confirmation that 1 is
viable for arene borylation came on addition of 1 equivalent
of the activated arene N,N-dimethylaniline in CD2Cl2 at 208C.
This rapidly and quantitatively (by NMR spectroscopy) led to
regioselective arene borylation in the para position of N,N-
dimethylaniline, simultaneously producing 1 equivalent of
[Et3NH][AlCl4]. The sequestering of the proton by-product
from electrophilic aromatic substitution by Et3N prevents
formation of Brønsted superacidic H[AlCl4] which would
otherwise lead to competitive protodeboronation of the
boronate ester,[18] as observed in the analogous electrophilic
arene silylation.[38] In the previously reported intermolecular
arene borylation using BCl3/AlCl3 mixtures, activated alumi-
nium metal was required to consume the H[AlCl4] by-product
and prevent protodeboronation.[18,19] Therefore high-yielding
arene borylation by electrophilic aromatic substitution
requires both a strongly electrophilic boron source and a
proton scavenger that does not deactivate the electrophile.
These requirements are fulfilled by 1 where, despite ligation
by two p-donating alkoxy groups, the boron center is
A broad range of electron-rich N-heterocycles[42] were
amenable to borylation using this methodology (Table 1),
which proceeded effectively quantitatively (by in situ 1H and
11B NMR spectroscopy). Borylation is highly regioselective
and subsequent transesterification to the synthetically desir-
able pinacol derivatives is facile allowing for isolation of
pinacol boronate esters in high yield. Longer reaction times
were required for borylation of TIPS-protected indole,
compared to the alkyl-protected analogue, due to the
weaker electron-donating ability of trialkylsilyl groups, rela-
+
tive to alkyl groups (as indicated by sp values of ꢀ0.09 and
ꢀ0.31 for Me3Si and Me, respectively).[43] Borylation of
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Table 1: One-pot, direct arene borylation with borenium cations.[a]
by 1. It is particularly noteworthy that there was minimal
ether cleavage observed during borylation of the latter, in
contrast to the reactivity of other strong boron electrophiles
with ethers (e.g. BBr3). Whilst attempts to borylate 5-Cl-N-
TIPS-indole failed with 1, due to reduced arene nucleophi-
licity, borylation was successful using 3, proceeding in
excellent yield. Compound 3 also borylated N-methylcarba-
zole regioselectively in the 3-position in good yield, again in
contrast to 1 where no arene borylation was observed (N-
methylcarbazole is a poorer nucleophile than N-methylin-
dole). One-pot transesterification of tetrachlorocatechol for
pinacol is also facile, producing compounds 10b and 11b in
good isolated yields. Compound 1 is sufficiently reactive to
borylate N-benzylindoline, with borylation occurring exclu-
sively at the 5-position of indoline (12b), with no benzyl
borylation observed. This highlights the high degree of
electronic discrimination between inequivalent arene rings
achievable with borenium cation arene borylation: only one
Substrate
Product
Cation
t
Yield
[h] [%][b]
4a
5a
4b
5b
1
3
1
85
93
32
6a
7a
6b
7b
1
4
92
1
3
48
4
78
>95[c]
ꢀ
position out of seven inequivalent aryl C H sites in 12a is
borylated.
8a
8b
1
1
3
24
30
4
86
88
84
Pyrroles were also effectively borylated. Borylation of
TIPS-protected pyrrole with 1 proceeded in excellent yield in
72 h, generating only the 3-substituted product, 13b, due to
steric deactivation of the 2-position.[44] The use of the more
electrophilic cation 3 significantly reduced the overall reac-
tion time to 3 h for full borylation (by 11B and 1H NMR
spectroscopy). Thus, borenium cation electrophilicity is not
only important for controlling substrate scope but also for
ensuring reasonable rates of reaction. This is further empha-
sized by the borylation of N-TIPS-indole being complete
within 4 h using 3 whilst requiring 48 h when borylated with 1.
Borylation of pyrroles containing less bulky nitrogen sub-
stitutents, e.g., N-methylpyrrole, produced mixtures of 2 and
3-borylated regioisomers as expected,[39] along with trace
amounts of the 2,4-diborylated product (15b). The latter
could be synthesized in good yield by use of 2.1 equivalents of
1 and N-methylpyrrole, albeit requiring extended reaction
times due to the reduced arene nucleophilicity of the mono-
borylated intermediates, 14b.
Thiophenes are less aromatic and nucleophilic than their
N-heterocyclic analogues, but they are also amenable to
electrophilic aromatic substitution with borenium cations 1
and 3. 2-Piperidyl- and 2-methyl-substituted thiophenes are
borylated and subsequently transesterified to give the
expected 2,5-substituted products, 16b and 17b, respectively,
with no observable borylation at the 3-position, due to
combined electronic and steric deactivation. Whilst boryla-
tion of both 16a and 17a is essentially quantitative (by in situ
11B NMR spectroscopy) the yields are somewhat lower as
these arenes are extremely sensitivity to protodeboronation
on exposure to protic oxo species necessary to effect
transesterification. Attempts to borylate furans, the least
aromatic of the common five-membered heterocycles, with 1
or 3 failed (for furan, 2-methylfuran, and benzofuran).
Instead insoluble materials were formed, presumably from
Lewis acid initiated polymerization, consistent with the
instability of furans towards AlCl3.[39]
9a
9b
10a
10b
11a
12a
11b
12b
3
1
24
1
71
96
1
3
72
3
89
13a
14a
15a
13b
14b
>95[c]
1
20
91[d]
73[f]
15b 1[e]
192
16a
17a
16b
17b
1
3
0.5
72
62
69
[a] Borenium cations made in situ in CH2Cl2 from 1 equiv CatBCl (or
Cl4CatBCl), 1.05 equiv Et3N, and 1.1 equiv AlCl3. 1 equiv of arene
substrate is then added. Reaction time refers to consumption of all
borenium cation. TIPS=triisopropylsilyl. [b] Yield of isolated products.
[c] Yield >95% by in situ 11B and 1H NMR spectroscopy. [d] This is a
mixture of the 2- and 3-regioisomers, with individual yields at 39% and
52%, respectively. [e] 2.1 equiv of borenium cation used. [f] Other
products are regioisomers of 14b.
indoles is effective with both electron-donating and electron-
withdrawing groups on the phenyl ring. 5-Methyl-N-TIPS-
indole and 5-MeO-N-TIPS-indole were efficiently borylated
In conclusion, we have demonstrated that catecholato-
ligated borenium cations are strong Lewis acids, possessing
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reaction mixture and the mixture stirred vigorously until all AlCl3 had
dissolved. The desired arene (1 equiv) was then added to the mixture
and stirring continued until the borylation reaction was complete (by
11B and 1H NMR spectroscopy). Step 2: On completion of borylation,
excess Et3N (ca. 15 equiv) followed by pinacol (3 equiv) were added
to the reaction mixture and stirred for 1 h. (Caution, this is a strongly
exothermic reaction.) Volatiles were removed under vacuum and the
product was extracted with 3 ꢁ 10 mL of hexane and filtered through a
short plug of silica. Removal of the solvent yielded the desired
product analytically pure.
sufficient electrophilicity to react with arenes in a boron
analogue of Friedel–Crafts chemistry. The new borenium
cations can be readily synthesized using inexpensive, com-
mercially available reagents and subsequently used in situ.
They are effective for the direct borylation of a range of
anilines, N-heterocycles, and thiophenes at room temper-
ature. Subsequent same-pot transesterification provides the
synthetically useful, and more robust to protodeboronation,
pinacol boronate esters in excellent yield. The direct boryla-
tion proceeds with outstanding regioselectivity, generating
products controlled by arene electronic effects. Overall this
work represents a new, inexpensive one-pot direct arene
borylation methodology for producing pinacol boronate
esters. Furthermore, it eliminates undesirable haloarene
intermediates and offers complimentary selectivities to iri-
dium-catalyzed direct borylation, which operates predom-
inantly under steric control or at the carbon ortho to the
heteroatom in five-membered heterocycles (e.g., indoles are
borylated at the 2-position under iridium-catalyzed boryla-
tion).[45,46] Borenium cation-based borylation also displays
remarkable functional-group tolerance for a boron based
Received: October 3, 2010
Revised: November 10, 2010
Published online: January 5, 2011
Keywords: borenium ions · borocations · boronate esters ·
.
borylation · electrophilic substitution
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Experimental Section
1: In a Schlenk tube equipped with a Young tap, Et3N (18 mL, 1.3 ꢁ
10ꢀ4 mol) was dissolved in 1 mL of CD2Cl2. CatBCl (20 mg, 1.3 ꢁ
10ꢀ4 mol) was added to the solution and the reaction mixture was
stirred for 5 min. Then powdered AlCl3 (18 mg, 1.3 ꢁ 10ꢀ4 mol) was
added and stirred until all AlCl3 had dissolved. Trace quantities of
CatBOH, [Et3NH][AlCl4] (combined < 5%), and AlCl3–NEt3 were
present. The first two derive from the reaction of borenium with trace
water, the presence of the latter is due to the equilibria. These
equilibria and the repeated inability to obtain crystallized 1 has
prevented accurate elemental microanalysis. 1H NMR (CD2Cl2,
400 MHz): d = 7.43–7.54 (m, 2H), 7.31–7.41 (m, 2H), 3.74 (q, J =
7.3 Hz, 6H), 1.43 ppm (t, J = 7.3 Hz, 9H). 13C NMR (CD2Cl2,
100 MHz): d = 146.9, 126.0, 114.6, 52.9, 9.4 ppm. 11B NMR (CD2Cl2,
128 MHz): d = 27.9 ppm (brs, pwhh = 131 Hz). 27Al NMR (CD2Cl2,
104 MHz): d = 103.9 ppm (sharp s).
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3: In a Schlenk tube equipped with a Young tap, Et3N (24 mL,
1.7 ꢁ 10ꢀ4 mol) was dissolved in CH2Cl2 (ca. 6 mL). Cl4CatBCl (50 mg,
1.7 ꢁ 10ꢀ4 mol) was added to the solution and the mixture was stirred,
followed by addition of powdered AlCl3 (23 mg, 1.7 ꢁ 10ꢀ4 mol). The
reaction mixture was stirred for 2 h and filtered. The volume was
reduced (to ca. 3 mL) and layered with pentane. Slow diffusion of the
layers yielded colorless crystals of [Cl4CatB(NEt3)][AlCl4] (3) (76 mg,
84%), that were of sufficient quality for single-crystal X-ray
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1
diffraction analysis. H NMR (CD2Cl2): d = 3.81 (q, J = 7.3 Hz, 6H),
1.49 ppm (t, J = 7.3 Hz, 9H). 13C NMR (CD2Cl2): d = 143.2, 131.1,
119.0, 48.6, 9.6 ppm. 11B NMR (CDCl3): d = 28.1 ppm (brs). 27Al
NMR (CD2Cl2): d = 103.9 ppm (sharp s). Analysis calculated for
C6H15AlBCl8NO2: C 27.37, H 2.87, N 2.66; found C 26.7, H 3.32, N
2.09.
General borylation procedure. Step 1: In a Schlenk tube equipped
with a Young tap under inert atmosphere, Et3N (1.05 equiv) was
dissolved in CH2Cl2, followed by slow addition of CatBCl (or
Cl4CatBCl, 1 equiv). Powdered AlCl3 (1.1 equiv) was added to the
Angew. Chem. Int. Ed. 2011, 50, 2102 –2106
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