Development of highly chemoselective bulky zincate complex, tBu 4ZnLi2: Design, structure, and practical applications in small-/macromolecular synthesis

Article abstract of DOI:10.1002/chem.200800536

We present full details of the unique reactivities of the newly developed dianion-type bulky zincate, dilithium tetra-ferf-butylzincate (tBu ₄ZnLi₂). With this reagent, halogen-zinc exchange reaction of variously functionalized haloaromatics and anionic polymerization of N-isopropylacrylamide (NIPAm)/styrene with excellent chemoselectivity were realized. Halogen-zinc exchange reaction followed by electrophilic trapping with propargyl bromide provided a convenient route to functionalized phenylallenes, particularly those with electrophilic functional groups (such as cyano, amide and halogens). Spectral and computational studies of the structure in the gas and liquid phases indicated extraordinary stabilization of this dianion-type zincate by its bulky ligands.

Full text of DOI:10.1002/chem.200800536

DOI: 10.1002/chem.200800536
Development of Highly Chemoselective Bulky Zincate Complex, tBu4ZnLi2:
Design, Structure, and Practical Applications in Small-/Macromolecular
Synthesis
Taniyuki Furuyama,[a, b] Mitsuhiro Yonehara,[a, b] Sho Arimoto,[b] Minoru Kobayashi,[c]
Yotaro Matsumoto,[b] and Masanobu Uchiyama*[a, b]
Abstract: We present full details of the
unique reactivities of the newly devel-
oped dianion-type bulky zincate, di-
cellent chemoselectivity were realized.
Halogen–zinc exchange reaction fol-
lowed by electrophilic trapping with
propargyl bromide provided a conven-
ient route to functionalized phenylal-
lenes, particularly those with electro-
philic functional groups (such as cyano,
amide and halogens). Spectral and
computational studies of the structure
in the gas and liquid phases indicated
extraordinary stabilization of this dia-
nion-type zincate by its bulky ligands.
lithium
tetra-tert-butylzincate
(tBu4ZnLi2). With this reagent, halo-
gen–zinc exchange reaction of various-
ly functionalized haloaromatics and
anionic polymerization of N-isopropyl-
acrylamide (NIPAm)/styrene with ex-
Keywords: chemoselective metala-
tion · halogen–metal exchange ·
nucleophilic
substitution
·
polymerization · zinc
Introduction
stance, in 1,4-conjugated additions,[2] iodine–zinc exchange
reactions of organic halides,[3] and directed ortho-zincation
reactions.[4] In 1996, we reported dianion-type zincates
(R4Zn2À·2Li+) as a new type of zinc–ate complexes, which
added a new dimension to organozincate reagents: they can
promote bromine (or tellurium)–zinc exchange, and carbo-
(or silyl-)zincation reactions.[5] Various dianion-type zincates
have subsequently been developed, focusing mainly on high
aptitude for the transfer of ligands.
In this article, we present a new type of dianion-type zin-
cate, dilithium tetra-tert-butylzincate[6] (tBu4ZnLi2), which
we designed as a highly crowded, bulky zincate by utilizing
the feature that dianion-type zincates have four ligands.
tBu4ZnLi2 turned out to have a range of attractive features,
such as remarkable low basicity, excellent chemoselectivity,
high halogen–zinc exchange ability, and wide anionic poly-
merization ability. Furthermore, it showed excellent SN2’ se-
lectivity in the reaction with propargyl bromide due to its
steric bulkiness, which led to a simple one-pot synthesis of
various kinds of phenylallene derivatives from functional-
ized halobenzenes via successive chemoselective metalation/
regioselective SN2’ reaction with propargyl bromide. In
order to understand the origin of the unique reactivity/selec-
tivity of tBu4ZnLi2, we also examined the structural features
of the bulky ligand–zincates by means of multidimensional
NMR, ESI-MS, and DFT studies.
The use of organozinc compounds has opened up new ave-
nues in organic and organometallic chemistry. Diorganozinc
reagents such as Reformatsky-type (RZnX) and dialkylzinc
(RZnR’) compounds have widely been used for various che-
moselective reactions in organic synthesis, but these re-
agents are often plagued by poor reactivity in the absence
of transition-metal catalysts or activating ligands, such as
aminoalcohols.[1] Organozinc ate complexes (R3ZnÀM+) are
another important class of organozinc reagents, showing
higher reactivities than the diorganozinc reagents, for in-
[a] T. Furuyama, M. Yonehara, Prof. Dr. M. Uchiyama
Advanced Elements Chemistry Laboratory
Institute of Physical and Chemical Research, RIKEN
2-1 Hirosawa, Wako-shi, Saitama 351-0198 (Japan)
Fax : (+81)48-467-2879
E-mail: uchiyama@mol.f.u-tokyo.ac.jp
[b] T. Furuyama, M. Yonehara, S. Arimoto, Y. Matsumoto,
Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo, 7-3-1 Hongo
Bunkyo-ku, Tokyo 113-0033 (Japan)
[c] Dr. M. Kobayashi
PRESTO, Japan Science and Technology Corporation (JST)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800536.
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FULL PAPER
Results and Discussion
affording 2a in quantitative yield. Stepwise treatments of
1a-I with alkyllithiums, followed by addition of zinc re-
agents, were examined, but all were ineffective for the for-
mation of the arylzincates. These observations suggest that
the precomplexation of tBu4ZnLi2 is essential for successful
metalation. (See Supporting Information, Table S1, for de-
tails.)
The arylzincate intermediate (1a-Zn) can be utilized as
an aryl anion equivalent. We next tested whether the aryl-
zincate intermediate (1a-Zn) can be utilized as an aryl
Unique reactivities of tBu4ZnLi2 (I): Application as a highly
chemoselective halogen-zinc exchange reagent: Traditional
organometallic reagents, including alkyllithiums or Grignard
À
À
À
reagents, are useful in procedures for C C, C X, and C
metal bond formation in organic synthesis.[7] However, the
application of these organometallic reagents for metalation
of highly functionalized substrates is generally limited by
low functional group tolerance, significant formation of by-
products, and severe restrictions regarding reaction condi-
tions, particularly the need for extremely low temperature,
strict exclusion of acidic protons, and hence tedious protec-
tion/deprotection processes typically required for such reac-
tions. These limitations arise because general organometallic
reagents have not only halogen–metal exchange ability, but
also high nucleophilicity and basicity, and these features
have been regarded as difficult or impossible to separate.
Therefore, the development of highly chemoselective metal-
ating reagents/reactions in which the nucleophilicity and ba-
sicity are masked is still highly desirable.
À
anion equivalent for the C C bond-forming reaction with
various electrophiles without any protection of the OH
moiety. The intermediate 1a-Zn, generated by the exchange
reaction of 1a-I with tBu4ZnLi2 could be trapped with vari-
ous electrophiles, such as DCl/D2O, allyl bromide and ben-
zaldehyde, to give the desired adducts without any O-func-
tionalized side-products (Scheme 1). The intermediate 1a-
À
Zn also undergoes copper- and palladium-catalyzed C C
bond-forming reactions, such as phenylation (5a) and alky-
lation (6a) in modest to good yields and with high chemose-
lectivity. Importantly, neither extremely low temperature
nor protection/deprotection processes for the OH moiety
are required in any of these transformations.
Development of chemoselective halogen–zinc exchange re-
action compatible with acidic proton: Beak et al. have re-
ported that in the lithiation reaction of o-bromo-(N-deuter-
io)benzamide using nBuLi, o-deuterated benzamide was ob-
tained as the main product [Eq. (1)], which clearly shows
that the halogen–metal exchange reaction occurred faster
than the deprotonation reaction of the amide N-H proton
with nBuLi, but due to the strong basicity of an intermedi-
ary C-lithiated species, the intermediate was quenched
smoothly with inter-/intramolecular acidic amide N-H
proton to give the thermodynamically more stable N-lithiat-
ed product.[8]
Scheme 1. Trapping of the intermediate with various electrophiles.
Scope and limitations of the halogen–zinc exchange reac-
tion using tBu4ZnLi2: With optimized conditions in hand,
we studied the scope of this unique metalation reaction for
various functionalized organic iodides (Table 1). Not only
suitably protected alcohols and terminal alkyne (entries 1–
2), but also unprotected acidic protons (amide N-H, phenol-
ic O-H, glycerol C2 proton) were compatible with the reac-
tion (entries 3–5). No epimerization at all was observed with
1 f-I, although epimerization to give racemic mixtures often
occurs in base-catalyzed reactions of glycerol derivatives.
Substrates containing electrophilic functional groups, such
as ester and amide groups, can be utilized for the zincation
(entries 5–7). Although a methyl ester is generally inconsis-
tent with the halogen–metal exchange reaction, essentially
no side reaction was observed under our reaction conditions,
due to the high chemoselectivity and mild conditions of this
interconversion reaction[5c] (entry 6). Nitrogen-containing
substrates such as aniline derivatives and p-deficient hetero-
Therefore, our approach capitalizes on the stability of in-
termediary aromatic zincates (relative to their lithium ana-
logues), which is well-known to minimize subsequent rear-
rangement or side reactions.[9] We hypothesized that a bulky
zincate might improve the stability (or isolability) of the ar-
omatic C-metal intermediate and prevent successive side re-
actions.
In the initial screening of suitable zincates, p-iodobenzyl
alcohol (1a-I) was selected as a model substrate having an
alcoholic proton on the iodobenzene ring. When alkyllithi-
ums, dialkylzinc, or monoanion-type zincates were used as
metalating reagents, the desired C-allylated product was not
obtained at all. On the other hand, when dianion-type zin-
cates were used, the C-allylated product was obtained pref-
erentially. Among them, tBu4ZnLi2 metalated 1a-I smoothly,
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10349

M. Uchiyama et al.
aromatic moieties also caused no problem (entries 8–9).
Aryl triflate was inert to metalation with tBu4ZnLi2 (not
shown), and the metalation reaction turned out to proceed
with complete selectivity in tri-
come to occupy a very important position in recent organic
synthesis as building blocks and pharmaceuticals.[13] Several
methods have been reported to date for the preparation of
Table 1. Metalation of various functionalized organic iodides.[a]
flate
mixed
substrates
Entry
Substate
Product
Yield[b] [%]
86
Electrophile
allyl bromide
(entry 10).
Besides p-iodobenzene deriv-
atives, aliphatic iodides includ-
1
ing CACHTUNGTRENNUNG ACHTUNGTRENNUNG
(sp2)- and C(sp3)-I deriva-
tives can be utilized as sub-
strates in this exchange reaction
2
94
allyl bromide
(entries 11–12).
meta-
and
ortho-Substituted
compounds
were also good substrates (en-
tries 13–14). ortho-Halo (includ-
ing pseudo-halogens) metallo-
benzenes, which are well-
known as good precursors for
3[c]
62
79
allyl bromide
allyl bromide
4[c,d]
benzynes,[10]
also
worked
smoothly when a low equiva-
lent amount of tBu4ZnLi2 was
used (entry 14). In the reaction
of a multi-iodo substrate, the
ortho position of iodide to the
methoxy group was metalated
5
74
allyl bromide
6
7
8
90
79
78
allyl bromide
allyl bromide
allyl bromide
with
high
regioselectivity,
mainly due to the coordination
of DMG to the cation moiety
of zincate[11] (entry 15).
Generally, the halogen-metal
exchange ability of aryl bro-
mides is lower than that of io-
dides.[12] We next turned our at-
tention to the bromine-zinc ex-
change reaction of aryl bro-
mides with tBu4ZnLi2 (Table 2).
9
77
allyl bromide
10
11
87
74
allyl bromide
PhCHO
Although
a
higher reaction
temperature was required, the
exchange reactions proceeded
smoothly (entry 1) and were
compatible with various func-
tional groups, such as TMS-pro-
tected acetylene, amides, p-de-
ficient heteroaromatic moiety,
and triflates (entries 2–5). How-
ever, in the case of using p-bro-
mobenzyl alcohol (1a-Br) as a
substrate, the deprotonation re-
action took place prior to bro-
mine-zinc exchange, and the de-
12
13
62
87
PhCHO
allyl bromide
14[e,f]
60
69
PhCHO
À
sired C C bond formation was
15[f]
allyl bromide
not observed at all (entry 6).
Unique
reactivities
of
[a] Unless otherwise noted, the metalation was carried out using tBu4ZnLi2 (1.1 equiv) and substrate
(1.0 equiv) in THF at 08C for 2 h, and the resultant metalated intermediate was treated with an electrophile
(allyl bromide or benzaldehyde). [b] Isolated yield. [c] Zincate (2.2 equiv) was used. [d] Metalated at 408C.
[e] Zincate (0.55 equiv) was used. [f] Metalated at À408C.
tBu4ZnLi2 (II): Application for
highly regioselective SN2’ reac-
tion: Allenic compounds have
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A Highly Chemoselective Bulky Zincate Complex
FULL PAPER
Table 2. Metalation of various functionalized aromatic bromides.[a]
tivities were unsatisfactory regardless of the procedure or
the amount of the cuprate (entries 1–2). In contrast, the use
of tBu4ZnLi2 turned out to improve drastically the SN2’ se-
lectivity, and the desired 4-ethoxycarbonyl phenyl allene
was obtained in a quantitative yield (entry 3). Addition of a
catalytic amount of a cuprate in this zincate reaction result-
ed in a decrease of the SN2’ selectivity, resulting in a lower
yield of the allenic product (entry 4).
Representative results from the exchange reactions of var-
ious functionalized aromatic iodides with tBu4ZnLi2, fol-
lowed by SN2/SN2’ reaction with propargyl bromide are sum-
marized in Table 4. Aryl tri(tert-butyl)zincate intermediates
prepared from the exchange reaction of various iodoben-
zene derivatives with tBu4ZnLi2 reacted smoothly with prop-
argyl bromide at room temperature in a SN2’ fashion to give
corresponding allenic compounds in good to excellent yields
(entries 1–9). Substrates bearing an OH group and a variety
of polar functional groups, including TMS-acetylene, amide,
triflate, halogens, cyano, and ester, caused no problems in
terms of yields and chemo- and regioselectivities. A naph-
thalene derivative (entry 10) could also be utilized in this
transformation.
Entry Substate
Product
Yield[b] Electrophile
[%]
1
74
PhCHO
allyl
bromide
2
98
87
77
87
0
allyl
bromide
3
allyl
bromide
4[c]
allyl
bromide
5
allyl
bromide
6[d]
In summary, this is an exceedingly practical method for
chemoselective arylallene preparation and offers an attrac-
tive alternative to more traditional procedures.
[a] Unless otherwise noted, the metalation was carried out using
tBu4ZnLi2 (1.1 equiv) and substrate (1.0 equiv) in THF at 608C for 2 h,
and the resultant metalated intermediate was treated with an electrophile
(allyl bromide or benzaldehyde). [b] Isolated yield. [c] Metalated at re-
fluxing temperature. [d] Benzyl alcohol was obtained as a major product.
Unique reactivities of tBu4ZnLi2
ACHTUNGTNER(NUNG III): Application for che-
moselective anionic polymerization
allenic compounds,[14] such as the Doering–Moore–Skattebçl
reaction,[15] the homologation of 1-alkynes,[16] the stereose-
lective reduction of alkynes,[17] and cross-coupling reactions
with allenyl–metal species.[18] Among them, one of the sim-
plest and widely used methods is the SN2’-type reaction of
organocuprate reagents with propargyl halides. However, its
application to the preparation of aryl allenes has generally
been limited by low availability of various functionalized
aryl copper compounds and unsatisfactory SN2’ selectivity. A
highly chemoselective halogen–copper exchange reaction
developed by Kondo et al.[19] and the introduction of trans-
metalation via an aryl Grignard intermediate prepared by
Knochelꢁs method[20] were major advances in the prepara-
tion of functionalized arylcuprates, but the SN2’ selectivity
remains an issue. We hypothesized that the steric bulkiness
of multiple tBu ligands on the
Polymerization of N-isopropylacrylamide (NIPAm) in
aqueous media: We have described the high chemoselectivi-
ty of the newly developed bulky zincate, tBu4ZnLi2 in previ-
ous sections. On the basis of its soft nucleophilicity and
weak basicity, we next focussed on the development of a
new organozincate-mediated anionic polymerization using
tBu4ZnLi2 as an initiator. Acrylic acid derivatives, which
behave as activated olefins owing to the conjugated carbon-
yl group, were chosen as the first target monomer.
Poly(N-isopropylacrylamide) (PNIPAm, P8) exhibits
unique temperature-sensitive properties.[21] It has been ap-
plied in various ways in organic chemistry,[22] materials scien-
ces,[23] and biotechnology.[24] However, since NIPAm (8) is a
multifunctionalized monomer, especially possessing an
acidic amide-NH proton, the polymerization of unprotected
Zn atom might favor SN2’-selec-
tive ligand transfer.
Table 3. Comparison of SN2’ selectivities.
In order to compare the reac-
tivities and chemo- and SN2’ se-
lectivities of intermediary aryl-
cuprate and arylzincate, we car-
ried out comparative reactions
using p-iodo ethylbenzoate as a
functionalized aryl halide and
propargyl bromide as an allenyl
precursor (Table 3). With orga-
Entry Condition
T [8C] Yield[a] [%] Ratio[b] (7r/7r’)
1
2
3
4
iPrMgBr (1.1 equiv), THF, À408C, 1 h; then CuCN (10 mol%) À40
76
79:21
63:37
98:2
Me2CuCNLi2 (1.1 equiv), THF, 08C, 2 h
RT
RT
RT
54[c]
100
89
tBu4ZnLi2 (1.1 equiv), THF, 08C, 2 h
tBu4ZnLi2 (1.1 equiv), THF, 08C, 2 h; then CuCN (10 mol%)
93:7
1
[a] Isolated yield of a mixture of SN2’ adduct 7r and SN2 adduct 7r’. [b] The ratio was determined by H NMR.
nocuprate, the yields and selec- [c] Biphenyl-4,4’-dicarboxylic acid diethyl ester (17%) was also formed.
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M. Uchiyama et al.
Table 4. One-pot synthesis of phenylallene derivatives by using
tBu4ZnLi2.
After extensive experimentation, we found an interesting
solvent effect in this polymerization, and P8 was obtained in
good yield in a protic solvent (MeOH or H2O) in only 3 h
(entries 3–4). To evaluate the solvent effects in more detail,
we monitored the time/yield profiles. In H2O, the yield
reached the maximum (92%) within 15 min (see Supporting
Information, Figure S1 for details). On the other hand, no
polymerization reaction took place with tBuLi, ZnCl2, LiCl,
or LiOH. This rules out the promotion of this polymeri-
zation by these species.
Entry
1
Substrate
Yield[a] [%]
77[c]
Ratio[b] (7/7’)
99<:1
2
98
99<:1
This polymerization in water proved to be applicable for
the polymerization of acryl acid derivatives, such as N,N-di-
methylacrylamide (DMA, 74%), acrylamide (AM, 84%),
and 2-hydroxyethylmethacrylate (HEMA, 92%).
3
4
5
6
7
78
79
70
71
90
99<:1
99<:1
99<:1
99<:1
99<:1
Polymerization of styrene: Styrene derivatives are not acti-
vated by a conjugate carbonyl group, in contrast to acryl
acid derivatives, and therefore it is of interest to know
whether the present anionic polymerization method is appli-
cable to polystyrene synthesis[25] (Table 6). Polystyrene (P9)
is among the most important synthetic polymers, being en-
countered ubiquitously in daily life. The control of its poly-
merization is of great commercial significance. Whereas con-
trolled thermal polymerization produces the highest molecu-
lar weight product in radical initiated synthesis of polystyr-
ene, undesirable spontaneous polymerizations can clog sty-
rene production facilities.
8
9
83
99<:6
100
98:2
10
95
99<:1
[a] Isolated yield of a mixture of SN2’ and SN2 adducts. [b] The ratio was
determined by H NMR. [c] Recovered 5% of 1a-I.
1
Table 6. Anionic polymerization of styrene by using tBu4ZnLi2.
8 has been reported only by a radical process, and anionic
polymerization of unprotected 8 presents a challenge.
We first evaluated the ability of tBu4ZnLi2 to promote
chemoselective anionic polymerization using 8 as a mono-
mer (Table 5). When a mixture of tBu4ZnLi2, and 8 (1:50
molar ratio) in THF was stirred at room temperature for
24 h, P8 was obtained, though in only 8% yield; this is the
first successful anionic polymerization of unprotected 8 (en-
tries 1–2). On the other hand, the polymerization reactions
with the use of Me ligand zincates, Me3ZnLi, or Me4ZnLi2,
as initiators did not proceed at all, but instead undesired re-
actions occurred. This confirms the unique character of tBu
ligand(s) in zincates.
[a]
[a]
Entry
Solvent
Yield [%]
Mn
Mw/Mn
1
2
3
4
H2O
MeOH
Et2O
Bulk (no solvent)
3.9
0.2
76
5500
–
3500
4200
1.58
–
1.02
1.13
92
[a] Determined by GPC based on polystyrene standards using DMF/LiBr
as a solvent. –: Not determined.
In the polymerization of styrene with tBu4ZnLi2, when
protic solvents were used, deprotonation reaction of solvents
took place prior to carbozincation (polymerization) reaction
between styrene (9) and tBu4ZnLi2, and very small amounts
of polystyrenes were obtained (entries 1–2). On the other
hand, in an aprotic solvent, such as Et2O, or under bulk
polymerization conditions,[26] polymerization proceeded
smoothly at room temperature to give polystyrene with low
polydispersity in quantitative yield (entries 3–4). The
1H NMR data of the obtained polystyrene were consistent
with linear structures, strongly supporting an anionic mode
of polymerization.
Table 5. Solvent effects in anionic polymerization of NIPAm by using
tBu4ZnLi2.
[b]
[b]
Entry
Solvent
t [h]
Yield[a] [%]
Mn
Mw/Mn
1
2
3
4
THF
THF
MeOH
H2O
24
7 d
3
8
7000
7000
18000
27000
1.50
2.17
1.65
2.72
33
76
92
To understand this polymerization process, and to obtain
spectroscopic evidence of the “active terminus”, we moni-
tored the polymerization of styrene using in situ FT-Raman
3
[a] Hot water-insoluble part. [b] Determined by GPC based on polystyr-
ene standards with DMF/LiBr as a solvent.
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A Highly Chemoselective Bulky Zincate Complex
FULL PAPER
spectrometry[27] (Figure 1). The polymerization process was
found to be very efficient, and Raman bands at n 1425,
1620, and 1645 cmÀ1 in the spectrum of styrene diminished
an initiator gave a single sharp peak (Mn =5100, Mw/Mn =
1.15) on GPC, which strongly suggests that the polymer ter-
minal zincate is sufficiently stable at room temperature, re-
sulting in a narrow molecular dispersion. This is in good
agreement with the above Raman analysis.
Structural study of tBu4ZnLi2: Direct effect of steric bulki-
ness of tBu ligand on the zincate structure: Finally, NMR,
ESI-MS, and density functional theory (DFT) studies were
performed in order to examine the origin of the unique se-
lectivity and reactivity of the tBu4ZnLi2-mediated reactions.
Structural information on tBu4ZnLi2 would aid rational
design and development of even more efficient metalating
reagents. The aims of our structural study were as follows:
1) characterization of the structure of tBu4ZnLi2 in the
liquid/gas phases, 2) analysis and comparison of the stability
of mono- and dianion type zincates with bulky ligands.[28]
The 13C NMR spectra of a ꢀ1m THF solution of tBu2Zn
or tBu4ZnLi2 showed only singlet peaks for a-carbon of the
tBu groups (22.7 (tBu2Zn) or 24.4 ppm (tBu4ZnLi2)) and for
b-carbon of the tBu groups (34.5 (tBu2Zn) or 36.8 ppm
(tBu4ZnLi2)) (Figure 4 and Supporting Information). When
one equivalent of tBuLi was added to a THF solution of
tBu4ZnLi2, only one set of new peaks (17.0 (aC) and
40.6 ppm (bC)), corresponding to tBuLi, was clearly ob-
served. The ESI-MS of a ꢀ1m THF solution of tBu4ZnLi2
was then measured, and a peak ([M]À =299.2) attributed to
[tBu4ZnLi]À (calcd for C16H27ZnLi: 299.2) was observed
(Figure 3). Based on these observations, tBu4ZnLi2 exists in
THF solution as a single, highly symmetrical species (dia-
nion-type zincate), not as a mixture of tBu3ZnLi and tBuLi.
Figure 1. Raman monitoring of polymerization of styrene. Further sty-
rene was added after 1 and 3 h.
in intensity as the reaction proceeded. The bands at n 1590
and 1615 cmÀ1, assigned to the vibrations of polystyrene, in-
creased in intensity. At 1 and 3 h after the first addition of
styrene monomer, further monomer was added twice, and
the monomer peaks again disappeared rapidly in both cases.
The bands assigned to the vibrations of polystyrene in-
creased in intensity, and the finally obtained polystyrene
showed low molecular dispersion (Mn =45000, Mw/Mn =
1.05). These observations strongly suggest that the polymer
terminal in this polymerization is the “active” zincate com-
plex.
Moreover, we carried out postpolymerization experiments
(Figure 2). A mixture of 9 and 2 mol% of initiator (tBuLi or
tBu4ZnLi2) was stirred for 12 h at room temperature, then
the same amount of 9 was added to the mixture and poly-
merized. The resulting polymers were analyzed with GPC.
When tBuLi was used as an initiator, the GPC curve of the
obtained polystyrene was rather broad, with two peaks
(Mn =21000 and 13000). This means the polymer terminal
was unstable at room temperature and gradual quenching
occurred before the 2nd addition of 9 to give a low-molecu-
lar-weight polystyrene. In contrast, the use of tBu4ZnLi2 as
Figure 2. GPC curves of postpolymerized polystyrene using a) tBuLi as
the initiator. b) tBu4ZnLi2 as the initiator.
Figure 3. a) Observed ESI-MS Spectra of tBu4ZnLi2 in THF. b) Calculat-
ed isotropic pattern of [tBu4ZnLi]À.
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M. Uchiyama et al.
À
ed Zn C bond lengths of
[tBu3Zn]ÀLi+ (2.160 ꢂ) is in
good agreement with the exper-
imental data for [Me3Zn]ÀK+.[31]
We could not find other ener-
getically plausible structures,
and confirmed that the planar
geometry is the global mini-
mum. Upon further addition of
tBuLi to tBu3ZnLi, the tetrahe-
dral
dianion-type
zincate
tBu4ZnLi2 forms with 11.7 kcal
molÀ1 stabilization. Among
other possible structures of
tBu4ZnLi2, we found one more
stable structure (10), which con-
sists of a complex of tBu2Zn–
tBuLi dimer. However, solva-
tion of the lithium atom of 10
with one molecule of Me2O led
to the collapse of 10. The feasi-
bility of the disproportionation
of tBu3ZnLi to tBu2Zn and
Figure 4. 13C NMR spectra of tBu ligand organozinc reagent: A capillary tube containing [D8]THF was used as
1
the 13C chemical shift reference and lock solvent. H and 13C NMR spectra were referenced to a solvent signal.
Next, the monoanion-type tBu zincate (tBu3ZnLi), pre-
pared by mixing tBu2Zn and tBuLi (1 equiv) in THF,[3c] was
investigated. Unexpectedly, the 13C NMR spectrum of
tBu3ZnLi at room temperature gave two broad singlets at d
tBu4ZnLi2 was then examined. The gas-phase calculations
indicated that this reaction was thermodynamically favored
(DErel =À6.9 kcalmolÀ1
; B3LYP/631SVPs). These results
1
36.2 and 24.3 ppm, and H NMR spectrum also showed two
imply that tBu4ZnLi2 is highly stable, and tBu3ZnLi does not
exist in THF. Although tBu3ZnLi has been reported to be a
good metalating reagent for organic halides, this observation
implies that the real active species is tBu4ZnLi2, not
tBu3ZnLi.
singlet peaks (0.82 and 0.78 ppm). Interestingly, these singlet
peaks were split and became sharper with decreasing tem-
perature,[29] and were completely separated into two sharp
signals at À1008C. These signals were assigned to a mixture
of tBu2Zn and tBu4ZnLi2 on the basis of DEPT, HMQC,
and HMBC experiments. These results strongly suggest that
tBu3ZnLi does not exist in THF solution as a single species,
but disproportionates to tBu2Zn and tBu4ZnLi2. These ob-
servations are in sharp contrast to the Me-coordinated zin-
cates, and are in agreement with the experimental results
(see Scheme S1 in the Supporting Information for details).
On the basis of NMR and
Conclusion
A new type of dianion-type zincate, tBu4ZnLi2, a highly
crowded, bulky zincate, has been developed. This reagent
turned out to be effective for a protecting-group-free halo-
ESI-MS results, we compared
the thermodynamic stability of
tBu-ligand zincate complexes
by means of a DFT study, em-
ploying Me2O as a model for
THF (Figure 5). tBuLi and
tBu2Zn react to form a trigonal
planar complex [tBu3Zn]ÀLi+
with
small
exothermicity
; B3LYP/
(À4.8 kcalmolÀ1
631SVPs[30]), and dissociation of
tBu3ZnLi into an ion pair,
[tBu3Zn]À and Li+, occurs with
143.4 kcalmolÀ1 endothermicity.
This structure, [tBu3Zn]ÀLi+, is
essentially similar to the crystal-
lographic structure of Me3ZnK,
Figure 5. a) Optimized structures at B3LYP/631SVPs for tBu ligand zincate compounds. Solvent (Me2O) is ab-
breviated as S. The number of imaginary frequencies is 0 for all structures. Relative energies Erel are given in
and the average of the calculat- kcalmolÀ1. b) Energy diagram of tBu3ZnLi disproportionation reaction.
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Chem. Eur. J. 2008, 14, 10348 – 10356

A Highly Chemoselective Bulky Zincate Complex
FULL PAPER
luted 2n HCl and extracted with Et2O (2ꢃ30 mL). The organic layer was
washed with brine, dried over MgSO4, filtered and concentrated in vacuo.
The residue was purified by silica gel column chromatography (n-hexane/
AcOEt 19:1) and the mixture of the titled compound and 4-propargyl
ethylbenzoate was given as a yellow oil (188 mg, 100%). The ratio of the
products was determined by H NMR. H NMR (300 MHz, CDCl3/TMS):
d = 7.98 (d, 2H, J=8.1 Hz), 7.34 (d, 2H, J=8.1 Hz), 6.20 (t, 1H, J=
6.9 Hz), 5.21 (d, 2H, J=6.9 Hz), 4.37 (q, 2H, J=7.2 Hz), 1.39 ppm (t,
3H, J=7.2 Hz); HRMS (EI+): m/z: calcd for C12H12O: 188.0837; found:
188.0821 [M]+.
gen–metal exchange reaction. It is consistent with a wide va-
riety of functional groups, including esters, amides, and
acidic protons (C-H, N-H or O-H) (i.e., protection/deprotec-
tion processes for these electrophilic moieties are not re-
quired). We also highlighted the effectiveness of the steric
bulkiness of this reagent in promoting regioselective SN2’ re-
action with propargyl bromide. The method provides a
simple and direct route for the synthesis of various function-
alized phenylallene derivatives. We have also developed a
new organozincate-mediated anionic polymerization using
tBu4ZnLi2 as an initiator. This method possesses great versa-
tility and high chemoselectivity, and various monomers, such
as styrene, NIPAm, DMA, AM, and HEMA, could be poly-
merized in high yields. A combined spectral/theoretical in-
vestigation on the structure of tBu4ZnLi2 strongly supports
the idea that tBu4ZnLi2 exists as a remarkably stable single
species with tetrahedral dianion-type structure both in THF
solution and in the gas phase. Surprisingly, in contrast to
this, tBu3ZnLi is less stable and turned out to disproportion-
ate to tBu2Zn and tBu4ZnLi2 in THF solution. The stoichi-
ometry of ligands at the central atom in an ate complex has
generally been considered as the most important factor de-
termining the coordination number of the ate complex.
However, the present work underlines the steric factor of li-
gands as another tuneable functionality in the development
of highly coordinated (dianion-type) ate complexes. Further
studies to expand the scope and synthetic utility of highly
coordinated bulky zincates, including tBu4ZnLi2, are under
way, together with mechanistic and structural investigations
to find the reason for the unique selectivity and reactivity of
tBu4ZnLi2-mediated reactions.
1
1
Anionic polymerization of NIPAm (8) by using tBu4ZnLi2: A solution of
tBu4ZnLi2 (0.10 mmol) in THF was added to an aqueous solution of N-
isopropylacrylamide (0.57 g, 5.0 mmol; 25 mL), and the reaction mixture
was stirred at room temperature. After 3 h, the polymerization was
quenched with HCl (1 mL, 2.0m Et2O solution). For the isolation of the
polymer, the reaction mixture was heated over 808C, and the precipitate
was collected by filtration and centrifugation. The product was dissolved
in THF and poured into an excess amount of Et2O. The white precipitate
formed was collected and dried in vacuo at 1008C and yielded poly-N-
isopropylacrylamide as
a
white powder (0.53 g, 92%). 1H NMR
(400 MHz, [D6]DMSO): d = 6.51 (s, 1H), 3.86 (m, 1H), 2.05 (s, 1H),
1.8–1.2 (m, 2H), 1.11 ppm (s, 6H).
Acknowledgements
We gratefully acknowledge financial support from Hoansha and KA-
KENHI (Young Scientist (A), Houga, and Priority Area No. 452 and
459) (to M.U.), and a JSPS Research Fellowship for Young Scientists (to
T.F.). The calculations were performed on the RIKEN Super Combined
Cluster (RSCC). We thank Prof. H. Naka (Tohoku University) for ob-
taining FT-Raman spectroscopic data and Prof. T. Ohwada (The Univer-
sity of Tokyo) for his valuable comments.
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Experimental Section
Preparation of tBu4ZnLi2·THF solution (1.1mmol): ZnCl2 (2.2 mL, 0.5m
THF solution, 1.1 mmol) was added to THF (5 mL). Then, tBuLi
(3.17 mL, 1.39m n-pentane solution, 4.4 mmol) was added dropwise to
the solution at À788C and it was stirred for 30 min at 08C to give a pale
yellow tBu4ZnLi2·THF solution (1.1 mmol).
4-Allylbenzylalcohol (2a):
A solution of the 4-iodobenzylalcohol
(242.5 mg, 1.01 mmol) in dry THF (5 mL) was added dropwise to a solu-
tion of tBu4ZnLi2 (1.1 mmol) in THF at À788C, warmed to RT and
stirred for 2 h. Then, allyl bromide (0.43 mL, 5 mmol) added to the solu-
tion and it was warmed to RT and stirred for 12 h. The reaction mixture
was quenched with sat. NH4Cl aq (2 mL), diluted 2n HCl (10 mL) and
extracted with Et2O (3ꢃ10 mL). The organic layer was washed with
brine and dried over Na2SO4, filtered and concentrated in vacuo. Purifi-
cation by silica gel open column chromatography (n-hexane/ethyl acetate
1
2:1) yielded title compound as a colorless oil (149.5 mg, 100%). H NMR
(400 MHz, CDCl3/TMS): d = 7.29 (d, 2H, J=8.0 Hz), 7.18 (d, 2H, J=
8.0 Hz), 6.01–5.91 (m, 1H), 5.11–5.05 (m, 2H), 4.65 (s, 2H), 3.39 ppm (d,
2H, J=6.7 Hz); 13C NMR (100 MHz, CDCl3): d = 139.4, 138.5, 137.2,
128.7, 127.1, 115.7, 65.2, 40.0 ppm; HRMS (FAB+): m/z: calcd for
C10H12O: 148.0888; found 148.0838 [M]+.
4-Allenyl ethylbenzoate (7r): 4-Iodoethylbenzoate (276 mg, 1.0 mmol)
was added dropwise at 08C by a syringe to a stirred solution of tBu4ZnLi2
(1.1 mmol) in THF, and the solution was stirred for 2 h. Then, propargyl
bromide (0.38 mL, 5.0 mmol) was added at À788C. The solution was
slightly warmed to room temperature and stirred for 16 h. The reaction
mixture was quenched with sat. NH4Cl aq (2.0 mL) and a few drops of di-
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Received: March 24, 2008
Published online: September 24, 2008
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Chem. Eur. J. 2008, 14, 10348 – 10356