Generation of carbanions through stibine-metal and bismuthine-metal exchange reactions and its applications to precision synthesis of ω-end-functionalized polymers

Article abstract of DOI:10.1002/chem.201100265

Generation of carbanions from organostibines and organobismuthines through heteroatom-metal exchange reactions was examined from synthetic and mechanistic viewpoints. The exchange reaction proceeded spontaneously upon treatment with various organometallic reagents, such as alkyl lithiums, tetraalkyl zincates, and alkyl magnesium halides to afford the corresponding carbanions quantitatively. Due to the high reactivity of these heteroatom compounds, the exchange reactions took place exclusively even in the presence of various polar functional groups, which potentially react with organometallic species. The advantage of this method was exemplified by the end-group transformation of living polymers that bear these heteroatom species at the ω-polymer end, prepared by using organostibine and bismuthine-mediated living radical polymerizations. Various polymers that bear polar functional groups and acidic hydrogen-for example, poly(methyl methacrylate), poly(butyl acrylate), poly(N-isopropyl acrylamide), and poly(2-hydroxyethyl methacrylate)-could be used in the exchange reactions, and subsequent trapping with electrophiles afforded the corresponding polymers with controlled molecular weights, molecular weight distributions, and end-group functionalities. Competition experiments showed that organostibines and organobismuthines were among the most reactive heteroatom compounds towards organometallic reagents and that their high reactivity was responsible for the high chemoselectivity in the exchange reaction. All's well that ends well: The generation of carbanions from organostibine and -bismuthine compounds was achieved thorough a heteroatom-metal exchange reaction (see scheme). The highly chemoselective exchange reaction could be applied to precision synthesis of varieties of ω-end- functionalized polymers that possess a polar functional group.

Full text of DOI:10.1002/chem.201100265

DOI: 10.1002/chem.201100265
Generation of Carbanions through Stibine–Metal and Bismuthine–Metal
Exchange Reactions and Its Applications to Precision Synthesis of w-End-
Functionalized Polymers
Eiichi Kayahara,[a] Hiroto Yamada,[a] and Shigeru Yamago*[a, b]
Abstract: Generation of carbanions
from organostibines and organobis-
ACHTUNGTRENNUNGmuthines through heteroatom–metal
react with organometallic species. The
advantage of this method was exempli-
fied by the end-group transformation
of living polymers that bear these het-
eroatom species at the w-polymer end,
prepared by using organostibine and
bismuthine-mediated living radical
polymerizations. Various polymers that
bear polar functional groups and acidic
hydrogen—for example, poly(methyl
methacrylate), poly(butyl acrylate),
poly(N-isopropyl acrylamide), and
poly(2-hydroxyethyl methacrylate)—
could be used in the exchange reac-
tions, and subsequent trapping with
electrophiles afforded the correspond-
ing polymers with controlled molecular
weights, molecular weight distributions,
and end-group functionalities. Compe-
tition experiments showed that organo-
stibines and organobismuthines were
among the most reactive heteroatom
compounds towards organometallic re-
agents and that their high reactivity
was responsible for the high chemo-
exchange reactions was examined from
synthetic and mechanistic viewpoints.
The exchange reaction proceeded
spontaneously upon treatment with
various organometallic reagents, such
as alkyl lithiums, tetraalkyl zincates,
and alkyl magnesium halides to afford
the corresponding carbanions quantita-
tively. Due to the high reactivity of
these heteroatom compounds, the ex-
change reactions took place exclusively
even in the presence of various polar
functional groups, which potentially
Keywords: antimony
· bismuth ·
carbanions · metalation · polymeri-
zation
AHCTUNGTRENNsGUN electivity in the exchange reaction.
Introduction
Carbanions occupy a central position in organic synthesis.[1,2]
Many methods have been developed for their generation,
and their subsequent reactions with electrophiles are one of
the most reliable methods of forming new carbon–carbon
and carbon–heteroatom bonds. Heteroatom–metal exchange
reactions, which are also called transmetalation reactions,
have been widely used for carbanion generation because of
the ease of availability of starting heteroatom compounds
and their high efficiency in generating carbanions
(Scheme 1).[3–12] Heavier organoheteroatom compounds of
group 14, 16, and 17 elements in the periodic table (i.e.,
organostannanes, -selenides, -tellurides, -bromides, and -io-
dides) have been widely employed as precursors, and their
exchange reactions with various organometallic reagents
(e.g., organolithium, -magnesium, -aluminium, -zinc, and
-copper reagents) give the corresponding organometallic
species. On the other hand, although heteroatom–lithium
Scheme 1. Generation of carbanions by means of heteroatom–metal ex-
change reactions and subsequent reactions with electrophiles. R=alkyl,
aryl, alkynyl, or vinyl; R’=alkyl; X=heteroatom; M=metal; El+ =elec-
trophiles.
exchange reactions of heavier group 15 elements (i.e., orga-
nostibines and -bismuthines) have been known for over 70
years,[13–18] their reactivities and synthetic efficiencies remain
elusive.
In addition, despite their numerous synthetic applications,
little is known about the differences in reactivity of the het-
eroatom species towards generating carbanion species.
Reich and co-workers reported that phenyl iodide is slightly
more reactive than diphenyl telluride when phenyl or p-tolyl
lithium was employed.[19,20] Although organostibines have
been thought to be more reactive than organoiodides, no
quantitative data have been obtained.[20] Sonoda and co-
workers have performed competition experiments on the ex-
change reactions of phenyl-substituted heteroatom com-
pounds with butyl lithium to generate phenyl lithium.[21] The
results showed that phenyl iodide is the most reactive, fol-
lowed by butyl phenyl telluride, and then tributyl phenyl
stannane in a ratio of 76:23:1 among heteroatom com-
[a] E. Kayahara, H. Yamada, Prof. Dr. S. Yamago
Institute for Chemical Research, Kyoto University
Uji, Kyoto 611-0011 (Japan)
Fax : (+81)774-38-3067
E-mail: yamago@scl.kyoto-u.ac.jp
[b] Prof. Dr. S. Yamago
CREST (Japan) Science and Technology Agency (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100265.
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Chem. Eur. J. 2011, 17, 5272 – 5280

FULL PAPER
pounds comprised of fifth-row elements in the periodic
table. Heteroatom compounds in the fourth row, such as
butyl phenyl selenide and phenyl bromide, are approximate-
ly 1000 times less reactive than phenyl iodide.
change reactions. The carbanions are generated very rapidly
and efficiently when stabilized carbanions, namely, benzilic
anions and enolate derivatives, are liberated. Since the reac-
tivities of organostibines and organobismuthines are ex-
tremely high, the exchange reaction occurred exclusively at
the polymer end even in the presence of polar functional
groups and acidic hydrogens in the polymer main chains.
Subsequent reactions with electrophiles gave w-end-func-
tionalized polymers with highly controlled and defined
structures in terms of molecular weights, molecular-weight
distributions, and end-group functionalities. In addition, the
reactivities of the heteroatom compounds towards the ex-
change reaction were examined. Competition experiments
showed organostibines and organobismuthines were among
the most reactive heteroatom compounds.
The exchange reaction proceeds exclusively through ate
complex 1 when heteroatom compounds comprised of
fourth- and fifth-row elements are used,[22–25] whereas when
heteroatom compounds with lighter atoms are used, minor
pathways—such as a single-electron transfer pathway, an
SN2 process, and a four-center process[26]—also compete. Be-
cause the reactivity of the exchange reaction should be cor-
related to the ease of formation of an ate complex, it is un-
derstandable that the heteroatom compounds with lighter
elements are less reactive than those of heavier heteroatom
compounds.[27] On the other hand, Reich and co-workers
have reported that organoleads and organobismuthines,
which are sixth-row elements, are less reactive than organo-
stannanes and organostibines in the exchange reaction.[20]
Therefore, there are no clear explanations or distinct evi-
dence to explain the differences in reactivity among hetero-
Results and Discussion
Generation of carbanions by stibine–metal and bismuthine–
metal exchange reactions: We initially examined stibine–
metal exchange reactions by using ethyl 2-dimethylstibanyl-
2-methylpropionate (2a) as a model substrate of polymeth-
ACHTUNGTRENNUNG
lized organostibine and organobismuthine compounds,[28–34]
we became interested in the possibility of generating car-
banions by means of heteroatom–metal exchange reactions
from organostibanyl and organobismuthanyl groups at the
polymer ends (Scheme 2). The resulting carbanions would
AHCTUNGTREGaNNUN crylate polymer end groups (Table 1). Compound 2a was
treated with nBuLi (1.1 equiv) in THF at À788C for 0.5 h,
and the generated anion, namely, the lithium enolate of
ethyl 2-methylpropionate, was treated with benzaldehyde
(1.1 equiv) for 0.5 h at À788C to room temperature. Workup
provided the desired alcohol 2A in 97% yield (Table 1,
entry 1). The exchange reaction occurred exclusively, and
side products, such as an alcohol derived from the direct ad-
dition of nBuLi to the ester, were not detected, thus indicat-
ing the high reactivity organostibanyl groups. Me4ZnLi2,[58–60]
tBu4ZnLi2,[61–63] and iPrMgCl·LiCl,[64,65] which exhibit high
functional-group compatibility, were also effective for the
exchange reaction, and 2A was selectively obtained in
nearly quantitative yield after the reaction with benz-
Scheme 2. Synthesis of w-functionalized polyACTHNUTRGNEU(GN meth)acrylate through poly-
mer-end anionic species.
be synthetically useful to introduce various functional
groups to the polymer ends, which cannot be made by other
methods, such as radical- or carbocation-mediated transfor-
mation reactions.[35,36] Such functionalization of the polymer
end groups would add new functionalities and properties to
produce new functional polymeric materials with new or im-
proved properties.[37–41] We have already reported that a
methyltellanyl w-end group in a polystyrene prepared by or-
ganotellurium-mediated living radical polymerization[28,42–49]
can be transferred to styryl lithium by a tellurium–lithium
exchange reaction,[50–56] and the reaction of the anion with
electrophiles, such as carbon dioxide, gave w-functionalized
polystyrene.[57] However, it is unclear whether this protocol
is applicable to the end group of living polymers that pos-
sess polar functional groups. Since the high compatibility of
polar functional groups is one of the most characteristic fea-
tures of living radical polymerization, direct end-group func-
tionalization of the functionalized polymers through carban-
ions is a major challenge in synthetic polymer chemistry.
We report here the generation of carbanions from organo-
stibines and organobismuthines by heteroatom–metal ex-
AHCTUNGTREGaNNNU ldehyde in all cases (entries 2–4). Although a methyl anion
can be generated upon the exchange reaction with 2a
À
through Sb CH3 bond cleavage, no such product was detect-
ed. In other words, only the most stable anion is liberated
from 2a.
Methyl 2-dimethylbismuthanyl-2-methylpropionate (3b)
and 2-diphenylbismuthanyl-2-methylpropionitrile (4c) react-
ed with nBuLi, Me4ZnLi2, tBu4ZnLi2, and iPrMgCl·LiCl to
afford 3A and 4A, respectively, in excellent yield in all
cases after treatment of the generated carbanions with benz-
AHCTUNGTREGaNNNU ldehyde (Table 1, entries 5–12). The ester and cyano groups
remained intact in all cases, thereby revealing the high reac-
tivity of organostibanyl and organobismuthanyl groups to-
wards organometallic reagents.
The exchange reaction was further investigated with 1-di-
methylstibanyl-1-phenylethane (5a), which mimics the poly-
styrene end group. After exchange reactions with nBuLi,
Me4ZnLi2, tBu4ZnLi2, and iPrMgCl·LiCl, the resulting
anionic species were treated with benzophenone to give 5B
in excellent yields (Table 1, entries 13–16). Exchange reac-
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5273

S. Yamago et al.
Table 1. Stibine–metal and bismuthine–metal exchange reactions.[a]
are summarized in Table S1 in the Supporting Information.
The zincate enolate generated from 2a and 3b with
Me4ZnLi2 reacted with benzophenone (1.1 equiv) to give b-
lactone 7 in moderate yield (Table 2, entries 1 and 2). No b-
hydroxy ester products were detected. Since many metal
enolates are inert toward ketones, this is an advantage of
the zincate species. The reaction of the zincate enolate gen-
erated from 2a and Me4ZnLi2 or tBu4ZnLi2 with benzoyl
chloride and carbon dioxide took place efficiently to give a-
keto and a-carboxylic esters in nearly quantitative yields in
all cases (entries 3, 4, 6, and 7). The enolate generated from
3b and tBu4ZnLi2 showed the same reactivities and gave de-
sired products in excellent yields (entries 5 and 8).
À
Entry Substrate R’ M
Electrophile Product Yield[b] [%]
1
2
3
4
5
6
7
8
2a
2a
2a
2a
3b
3b
3b
3b
4c
4c
4c
4c
5a
5a
5a
5a
6d
6d
6d
6d
6e
6e
6e
6e
nBuLi
PhCHO
PhCHO
PhCHO
2A
2A
2A
2A
3A
3A
3A
3A
4A
4A
4A
4A
5B
5B
5B
5B
6A
6A
6A
6A
6A
6A
6A
6A
97
99
95
93
95
97
98
89
88
97
98
82
82
98
95
84
75
75
81
86
74
79
80
84
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl[c] PhCHO
nBuLi
PhCHO
PhCHO
PhCHO
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl[c] PhCHO
Zincate enolates were also effective for allylation reac-
tions when allyl iodide was used (Table 2, entries 9–11). Ar-
ylation of the zincate enolate with phenyl iodide in the pres-
9
nBuLi
PhCHO
PhCHO
PhCHO
PhCHO
PhCOPh
PhCOPh
PhCOPh
PhCOPh
PhCHO
PhCHO
PhCHO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl
nBuLi
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl
nBuLi
ence of [PdACTHNUGTRNEUNG(PPh3)4] gave the a-phenyl ester but in moderate
yields (entries 12 and 13), and the yields did not improve
when other metal catalysts and/or ligands were employed.[67]
Since arylation of the lithium enolate generated from 2a
and 3b also gave similar results (Table S1 in the Supporting
Information), the low yields may be due to the existence of
dimethylbutylstibine and -bisumuthine generated by the ex-
change reaction, which may influence the activity of the pal-
ladium catalysts.
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl[c] PhCHO
nBuLi
PhCHO
PhCHO
PhCHO
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl[c] PhCHO
Generation of carboanions at w-polymer end groups and
their application toward the synthesis of w-functionalized
polymers: The synthetic utility of the exchange reaction was
further examined in the transformation of organostibine and
[a] Unless otherwise noted, the exchange reaction was carried out using
À
R’ M (1.1 equiv) and substrate (1.0 equiv) in THF at À788C for 0.5 h,
and the resulting metalated intermediate was treated with electrophiles
(1.1 equiv) at À788C to RT for 0.5 h. [b] Determined by 1H NMR spec-
troscopy with pyrazine as an internal standard. [c] The exchange reaction
was carried out at 08C for 1 h, and the resulting metalated intermediate
was treated with electrophile (1.1 equiv) at 08C to RT for 0.5 h.
-bismuthine polymer end groups of polyACTHNUTRGNE(NUG meth)acrylate de-
rivatives. Poly(methyl methacrylate) (PMMA) 8a with a di-
methylstibanyl group at the w-end of the polymer (Mn =
3000, Mw/Mn =1.21, for which Mn and Mw are the number-
average and weight-average molecular weights, respectively,
and Mw/Mn is the molecular-weight distribution) was pre-
tions of aryl-substituted substrates, such as tris
ACHTUNGTRENUN(NG p-tolyl)-
ACHTUNGTRENNUNG
magnesium reagents proceeded
smoothly to give the desired
product 6A in high yields (en-
tries 17–24).
Table 2. Reaction of “zincate enolate” with electrophiles.[a]
Application of the exchange
reaction to carbon–carbon bond
formation reactions: We next
Entry
Substrate
Zincate
Electrophile [equiv]
t [h]
Product
Yield[a] [%]
investigated
carbon–carbon
1
2
3
4
5
6
7
8
2a
3b
2a
2a
3b
2a
2a
3b
2a
2a
3b
2a
3b
Me4ZnLi2
Me4ZnLi2
Me4ZnLi2
tBu4ZnLi2
tBu4ZnLi2
Me4ZnLi2
PhCOPh (1.1)
PhCOPh (1.1)
PhCOCl (5.0)
PhCOCl (5.0)
PhCOCl (5.0)
CO2 (excess)
CO2 (excess)
CO2 (excess)
allyl iodide (5.0)
allyl iodide (5.0)
allyl iodide (5.0)
PhI[b] (2.0)
3.0
3.0
1.0
1.0
1.0
2.0
2.0
2.0
5.0
3.0
3.0
24
7
7
61 (58)
61
97 (97)
99
99 (95)
92 (91)
90
95 (91)
85 (82)
94
bond formation between the
anionic species generated from
2a and 3b and various electro-
philes (Table 2). We focused on
the reaction of zincate deriva-
tives because, to the best of our
knowledge, there are no reports
on reactions that involve zin-
cate enolates.[66] The reactions
of lithium enolates generated
from 2a and 3b were examined
2C
2C
3C
2D
2D
3D
2E
2E
3E
2F
3F
tBu ZnLi2
4
tBu4ZnLi2
Me4ZnLi2
tBu4ZnLi2
tBu4ZnLi2
Me4ZnLi2
Me4ZnLi2
9
10
11
12
13
92 (92)
59 (58)
53 (48)
PhI[b] (2.0)
24
[a] Crude yield, which was determined by 1H NMR spectroscopy with pyrazine as an internal standard. The
(PPh3)4] (5 mol%) was added as a metal catalyst.
as references, and the results number in parentheses is the isolated yield. [b] [Pd
ACHTUNGTRENNUNG
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Carbanion Generation
FULL PAPER
pared in 97% yield by heating 3a and MMA (30 equiv) in
the presence of dimethyl 2,2’-azobis(2-methylpropionate)
(V-601) at 608C for 3 h.[30] It was dissolved in THF and
treated with nBuLi (1.1 equiv) at À788C for 0.5 h. The reac-
tion was quenched by adding DCl (2.0 equiv) in D2O to give
the w-end-deuterated PMMA 9F with Mn =3000 and Mw/
Mn =1.21 (Table 3, entry 1). The similarity of Mn and Mw/Mn
addition of CO2 produced w-carboxylic acid PMMA 9D,
which was converted to the corresponding methyl ester 9G
(Mn =3100 Mw/Mn =1.19) by treatment with trimethylsilyl
1
diazomethane (Table 3, entry 6). The H NMR spectroscopic
and MALDI-TOF MS analyses indicated a nearly quantita-
tive conversion for 8a to 9G. The lithium species was also
trapped with benzoyl chloride (5 equiv) and allyl iodide
(5 equiv) to give the corre-
sponding w-end-functionalized
PMMA 9C (Mn =3100, Mw/
Table 3. Synthesis of w-end-functionalized PMMA.
Mn =1.20) and 9E (Mn =3100,
Mw/Mn =1.20), respectively (en-
tries 8 and 10). The desired
products formed almost quanti-
tatively on the bases of
1H NMR spectroscopic and
MALDI-TOF MS analyses.
When benzaldehyde (5 equiv)
was used as an electrophile,
À
nBuLi
Entry
Substrate
R’ M
Electrophile [equiv]
Product
Mn/Mw/Mn
Incorp.[a] [%]
1
2
3
4
5
6
7
8
8a
8b
8b
8b
8b
8a
8b
8a
8b
8a
8b
8a
8b
DCl/D2O (2.0)
DCl/D2O (2.0)
DCl/D2O (2.0)
DCl/D2O (2.0)
DCl/D2O (2.0)
CO2 (excess)[b]
CO2 (excess)[b]
PhCOCl (5.0)
PhCOCl (5.0)
allyl iodide (5.0)
allyl iodide (5.0)
PhCHO (5.0)
PhCHO (5.0)
9F
9F
9F
9F
9F
9D
9D
9C
9C
9E
9E
10
3000/1.21
3400/1.13
3300/1.12
3200/1.11
3300/1.11
3100/1.19
3500/1.13
3100/1.20
3500/1.11
3100/1.20
3500/1.12
3100/1.20
3400/1.13
>99
>99
>99
>99
nBuLi
Me4ZnLi2
tBu4ZnLi2
iPrMgCl·LiCl
nBuLi
nBuLi
nBuLi
nBuLi
nBuLi
nBuLi
nBuLi
PMMA 10 with
a d-lactone
>99
formed as the major product
(entry 12) through intramolecu-
lar cyclization of the initially
formed lithium alkoxide 11
with the adjacent ester group
(Scheme 3). MALDI-TOF MS
analysis indicated the formation
of two minor products, which
were assigned to b-hydroxyl
ester 9A and end-protonated
PMMA 9F in a ratio of 10/9A/
9F=90:9:1. Alcohol 9A was
cleanly converted to 10 by
>99 (97)
>99 (97)
99 (96)
99 (98)
98 (98)
97 (95)
90[c]
9
10
11
12
13
nBuLi
10
89[c]
[a] Rate of end-group incorporation determined by MALDI-TOF MS. The number in parentheses is the data
derived from 1H NMR spectroscopic analysis. [b] Carboxylic acid on the w-end was converted to methyl ester
before gas-phase chromatography (GPC) analysis with trimethylsilyldiazomethane (2.0 equiv). [c] The minor
alcohol product 12 was quantitatively transformed into 10 by treatment with trifluoroacetic acid (2 equiv) in
CH2Cl2 at room temperature.
before and after the exchange reaction indicated that de-
composition did not occur during the exchange reaction.
Matrix-assisted laser-desorption ionization time-of-flight
mass spectroscopy (MALDI-TOF MS) showed only a series
of peaks that possess the molecular-ion masses of 9F
treatment with trifluoroacetic acid in CH2Cl2.
2
(Figure 1). In the H NMR spectrum for 9F, a broad singlet
was observed at d=2.46 ppm, which corresponds to the a-
ester deuterium. Treatment of PMMA 8b, which has a di-
methylbismuthanyl group (Mn =3300, Mw/Mn =1.13), with
nBuLi under identical conditions as those used for 8a quan-
titatively transformed to 9F (Table 3, entry 2). The results
clearly demonstrate that the exchange reaction that involves
organostibine and -bismuthine polymer end groups and
BuLi is highly chemoselective despite the excess number of
ester groups in PMMA 8. The zincate and magnesium eno-
lates generated from 8b with Me4ZnLi2, tBu4ZnLi2, and
iPrMgCl·LiCl also afforded 9F quantitatively in all cases
after treatment with DCl/D2O (entries 3–5). The results
clearly indicated the high reactivity of these heteroatom
groups towards the exchange reaction.
Scheme 3. Formation of 10 and 9A and transformation from 9A to 10.
a) 1) BuLi (1.1 equiv), THF, À788C, 0.5 h; 2) PhCHO (5 equiv), À788C
to RT, 3 h. b) Trifluoroacetic acid (2.0 equiv), CH2Cl2, reflux, 3 h.
The reaction that used dimethylbismuthanyl-group-substi-
tuted PMMA 8b gave virtually identical results under the
same conditions as those for 8a, and the desired w-function-
alized PMMAs with nearly quantitative end-group transfor-
mation were obtained in all cases (Table 3, entries 7, 9, 11,
and 13). These results clearly demonstrated the successful
synthesis of highly controlled PMMAs in terms of molecular
The exchange reaction was successfully applied to the syn-
thesis of w-end-functionalized PMMA by the reaction of the
anionic species with various electrophiles. For example, the
exchange reaction of PMMA 8a with nBuLi followed by the
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5275

S. Yamago et al.
sibility was ruled out on the
basis of labeling experiments
using 13C-labeled benzoyl chlor-
AHCTUNGTRENNUNG
ide, Ph13COCl. In the 13C NMR
spectra of 13C-labeled PBA
13-13C, the signals that corre-
sponded to the carbonyl carbon
of the benzoyl group in the
range of d=193–194 ppm were
greatly enhanced than those of
13, and the chemical shifts were
similar to that for the benzoyl
group in methyl 2-benzoylpro-
pionate (d=196 ppm).[71] The
peak for the ketone carbon of
15, which has
a quaternary
carbon adjacent to the benzoyl
group, should appear lower
field by dꢀ10 ppm from that of
13. However, no such signals
could be detected. The results
unambiguously prove the struc-
ture of the product and thus the
stability of the polymer-end
anionic species.
The exchange reactions with
zincate were compatible with
free hydroxyl groups and amide
protons (Scheme 5). The ex-
change reaction of poly(N-iso-
propyl acrylamide) (PNIPAM)
16, which has a dimethylstiban-
yl w-end group (Mn =3400, Mw/
Figure 1. a) Full and b) enlarged TOF MS spectra of w-end-deuterated PMMA 9F (Table 3, entry 1). The mo- Mn =1.09),
with
tBuZnLi2
lecular ions were observed as sodium ion adducts; m/z: [M+Na+].
(1.1 equiv) selectively took
weights, molecular-weight distributions, and the end groups
by successive living radical polymerization and end-group
transformation.
The scope of the exchange reaction was further investigat-
ed by employing various functionalized polymers with acidic
hydrogen in the main chain. For example, the reaction of
poly(butyl acrylate) (PBA) 12, which has a w-dimethyl-
ACHTUNGTRENNUNGstibanyl group (Mn =4000, Mw/Mn =1.08), with nBuLi fol-
lowed by the addition of benzoyl chloride (5 equiv) gave the
desired w-benzoylated PBA 13 together with cyclic b-ke-
toester 14 and a w-end-protonated product in a ratio of
57:36:7 on the basis of MALDI-TOF MS (Scheme 4). The
formation of 14 must be due to intramolecular condensation
of the initially formed lithium species with the nearby ester
group.[68–70] In contrast, the exchange reaction of 12 (Mn =
4100, Mw/Mn =1.09) with tBu4ZnLi2 instead of nBuLi selec-
tively gave 13 (Mn =4000, Mw/Mn =1.10) with nearly quanti-
tative incorporation of the benzoyl group (97%).
Scheme 4. Functionalization of poly(butyl acrylate). Reaction conditions:
a) 1) tBu4ZnLi2 (1.1 equiv), THF, À788C, 0.5 h; 2) PhCOCl (5 equiv),
À788C to RT, 3 h. b) 1) tBu4ZnLi2 (1.1 equiv), THF, À788C, 0.5 h;
2) Ph13COCl (5 equiv), À788C to RT, 3 h.
place in DMF at À608C without protection of the amide
protons. Subsequent addition of benzoyl chloride (5.0 equiv)
gave the desired product 17 (Mn =3500, Mw/Mn =1.10) with
quantitative incorporation (>99%) of the benzoyl group
(Scheme 5a). The exchange reaction in DMSO at room tem-
perature also gave 17 quantitatively after addition of benzo-
yl chloride. Although the exchange reaction in THF,
MeOH, and H2O proceeded efficiently, subsequent reactions
Introduction of an electrophile to the mid-chain of the
polymer to give 15 might occur through anion migration fol-
lowed by trapping with benzoyl chloride. However, this pos-
5276
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Carbanion Generation
FULL PAPER
Table 4. Competition experiments of alkylheteroatom compounds.
Entry
X
Yield[a] [%]
3A/2A
Reactivity
ACHTUNGTRENNUNG
[X/I][b]
3A
2A
21
1
2
3
4
BiMe2 (3b)
SbMe2 (3a)
TeMe (3g)
I (3 f)
54.5
43.0
25.5
14.5
9.0
35.0
32.5
26.5
31.0
6.1
3.5
2.3
1.3
4.7
2.7
1.8
–
Scheme 5. Functionalization of PNIPAM and poly(hydroxyethyl meth-
12.5
11.0
11.0
A
0.5 h; 2) PhCOCl (5 equiv), À608C to RT, 3 h. b) 1) tBu4ZnLi2
(1.1 equiv), DMSO, RT, 0.5 h; 2) allyl iodide (5 equiv), RT, 3 h.
[a] Determined by gas chromatography with tetradecane as an internal
standard. [b] Reactivity of heteroatom group after compensation of the
effect of the ester group.
with electrophiles gave w-protonated PNIPAM as the major
product. A polymer-end anion could be generated from
poly(2-hydroxyethyl methACHTUNGRTNEaUNG crylate) 18 (Mn =3800, Mw/Mn =
1.28) by treatment with tBuZnLi2 in DMSO, and subsequent
reaction with allyl iodide (5 equiv) gave desired allylated
product 19 (Mn =4000, Mw/Mn =1.28) quantitatively
(Scheme 5b).
On the basis of the high chemoselectivity, the current
method was applied to the synthesis of an w-biotinylated
polymer (Scheme 6). Treatment of PNIPAM 16 with tBuZn-
À788C. After stirring for 15 min at this temperature, the re-
action was quenched by adding AcOH/MeOH. After
workup, the yields of alcohols 3A, 2A, and 1-phenyl-1-pen-
tanol (21) were determined by GC analysis with tetradecane
as an internal standard. The same reaction was repeated six
times, and the yields of four experiments excluding the high-
est and lowest yields were averaged (see Table 4, entry 1).
The same experiments were carried out with a combination
of organostibine 3a and 2 f and organotellurium 3g and 2 f
(entries 2 and 3). In addition, the effects of the ester moiety
on the exchange reaction were examined by using 2 f and 2-
iodo-2-methylpropionate (3 f) (entry 4), and the reactivity of
each heteroACTHNUGRTNEUNGatom compound was corrected in relation to this
data. These results clearly show that the organobismuthine
has the highest reactivity, followed by the organostibine, and
then the organotelluride. The organoiodide is the least reac-
tive. In other words, the relative reactivity increased in the
order of iodine, telluride, stibine, and bismuthine in a ratio
of 1.0:1.8:2.7:4.7 in THF at À788C.
Scheme 6. Synthesis of w-biotinylated PNIPAM 20. Reaction conditions:
a) 1) tBu4ZnLi2 (1.1 equiv), DMF, À608C, 0.5 h; 2) biotinyl chloride
(5 equiv), À608C to RT, 3 h.
Li2 in DMF followed by the addition of biotinyl chloride
(5 equiv) exclusively afforded w-biotinylated PNIPAM 20
(Mn =3900, Mw/Mn =1.11). The structure of 20 was unambig-
uously confirmed by using 1H NMR spectroscopic and
MALDI-TOF MS analyses (Figure 2). PNIPAM is a ther-
moresponsive polymer and possesses lower critical solution
temperature in water. On the other hand, biotin is a good
ligand for proteins such as avidin and streptavidin. There-
fore, this polymer and its analogues would make it possible
to thermally modulate the properties of biomaterials for bio-
logical applications.[72]
It is worth noting that the exchange reaction of 3b and 3a
to give 3A took place much faster than the direct addition
of nBuLi to benzaldehyde to give 21. This result is consis-
tent with the selective exchange reaction observed for the
PMMA polymer-end organobismuthine and organostibine
species as mentioned above. Moreover, because a threefold
excess amount of benzaldehyde over the heteroACHTNUTRGNEaUNG tom species
was employed in the experiments, the exchange reaction be-
tween 3g and 2 f also took place considerably faster than
the addition of BuLi to benzaldehyde. The result suggests
that a similar polymer end-group transformation using
(meth)acrylate derivatives is possible for organotellurium
and organoiodine polymer ends.[73]
We next carried out the competition experiments using
aryl-substituted heteroatom compounds with phenyl iodide
as a reference (Table 5). The competition between triACHTUNGTRENNUNG(p-tol-
Relative reactivities of heteroatom compounds towards
exchange reactions: To clarify the reactivity of organosti-
bines and -bismuthines towards the exchange reactions, we
carried out competition experiments using organoiodides as
references. In addition, we examined the reactivity of orga-
notellurium compounds in conjunction with living polymeri-
zation methods that we developed. We first compared the
yl)bismuthine (6e) and phenyl iodide in the presence of
benzaldehyde gave 6A, diphenylmethanol (22), and 1-phe-
nylbutanol (23) in 0.1, 7.5, and 23% yields, respectively
reactivity of a-heteroatom-substituted esters
3 and 2 f
(Table 4). nBuLi (1.0 equiv) was added to a mixture of orga-
nobismuthine 3b (5.0 equiv), ethyl 2-iodo-2-methylpropio-
nate (2 f, 5.0 equiv), and benzaldehyde (15 equiv) in THF at
(Table 5, entry 1). The competitions between tri
bine (6d) and phenyl iodide and between di(p-tolyl) tellur-
ide (6h) and phenyl iodide were also examined (entries 2
ACHTUNGTREN(NUNG p-tolyl)sti-
AHCTUNGTRENNUNG
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5277

S. Yamago et al.
sults using 2 and 3, the reactivi-
ty of aryl-substituted hetero-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
bines>triarylbismuthines in a
ratio of 1:0.47:0.12:0.025 in
THF. However, since the mass
balance of the reaction was low,
especially when 6e and 6d
were employed, we concluded
that this ratio did not reflect
the kinetic reactivity of the het-
eroatom species for the follow-
ing reason.
The exchange reaction takes
place through ate complex 1
(Scheme 1), which is inert to
carbonyl compounds.[74] There-
fore, the formation of alcohols
2A, 3A, 6A, and 22 is affected
by both the kinetic reactivity in
forming the ate complex and
the ease of generating a free
anion from the ate complex.
The pKa vales of the conjugate
acid of lithium enolate, methyl
isobutyrate, and BuLi, butane,
are around 25 and 50, respec-
tively, thereby suggesting that
the formation of lithium eno-
late is highly exothermic.
Therefore, the liberation of the
enolate from the ate complex
should take place spontaneous-
ly, and the reactivity ratio using
3 and 2 f strongly reflects the
kinetic reactivities of the
hetero
atom
compounds
in
forming the ate complex. On
the other hand, the stability of
aryl lithiums (the pKa of ben-
zene is 43) is close to butyl lith-
ium, and the ate complexes
become more stable when aryl
groups are present.[20,22] There-
fore, the liberation of a reac-
tive, free anion from the ate
complex becomes slow in the
case of aryl-substituted hetero-
Figure 2. a) 1H NMR spectra and b) full and c) expanded TOF-MS spectra of w-biotinylated PNIPAM 20. In
the TOF-MS spectra, the molecular ions were observed as sodium ion adducts; m/z: [M+Na+].
AHCTUNGTRENNUNG
and 3). Despite the low yields of 6A and low mass balances
in all cases, the relative reactivity index for each heteroatom
compound was determined after compensating for the reac-
tivity difference between phenyl iodide and p-tolyl iodide
(entry 4). The results suggest that, in contradiction to the re-
explained by the rapid formation of the ate complex, from
which the liberation of the free anion is very slow. In other
words, the low yields of 6A reflect the rapid formation of
stable ate complexes and indicate the high reactivity of
these heteroatom compounds.
5278
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Chem. Eur. J. 2011, 17, 5272 – 5280

Carbanion Generation
FULL PAPER
Table 5. Competition experiments of arylheteroatom compounds.
treated with nBuLi (71.4 mL, 1.54m solution in hexane, 0.11 mmol) at
À788C. After stirring for 0.5 h at this temperature, benzaldehyde
(10.6 mL, 0.11 mmol) was added. The resulting mixture was slowly
warmed to room temperature over 0.5 h. The reaction mixture was
quenched with saturated aqueous NH4Cl solution (0.5 mL) and was ex-
tracted with ethyl acetate (0.5 mLꢁ 3). The organic layer was washed
with saturated aqueous NaCl solution, dried over MgSO4, filtered, and
concentrated under reduced pressure to give a crude mixture. The yield
1
(97%) of 2A was determined by H NMR spectroscopy using pyrazine as
Entry
X
Yield[a] [%]
6A/22
Reactivity
ACHTUNGTRENNUNG
[X/I][b]
an internal standard. Purification by silica gel chromatography afforded
2A in 92% yield (20.4 mg) as a colorless solid. Rf =0.32 (ethyl acetate/
hexane 25:75); m.p. 39.8–41.68C; 1H NMR (400 MHz, CDCl3): d=7.36–
À
6A
22
23
1
2
3
4
Bi
Sb
Te
I
G
0.1
0.8
1.9
4.6
7.5
12.5
8.0
23.0
42.0
50.5
54.5
0.013
0.064
0.24
0.025
0.12
0.47
–
7.25 (m, 5H; Ar H), 4.91 (s, 1H; CH), 4.18 (q, 3J
N
G
3
ACHTUNGTRENNUNG
CH2), 3.57 (s, 1H; OH), 1.27 (t, J
N
CH3), 1.10 ppm (s, 3H; CH3); 13C NMR (100 MHz, CDCl3): d=178.1,
140.1, 127.9, 127.8, 78.9, 61.1, 47.7, 23.3, 19.1, 14.3 ppm; IR (KBr): n˜ =
710, 1049, 1153, 1267, 1705, 2980, 3499 cmÀ1; HRMS (EI): m/z: calcd for
C13H18O3: 222.1256 [M+]; found: 222.1254.
9.0
0.52
[a] Determined by gas chromatography using tetradecane as an internal
standard. [b] Reactivity of heteroatom group after compensation of the
effect of the aryl group.
General procedure for synthesis of w-deuterated PMMA 9F (Table 3
entry 1):
A
solution of 8a (Mn =3000, Mw/Mn =1.21, 186.3 mg,
Our reactivity index between organotellurium and orga-
noiodide is opposite to the one reported by Sonoda and co-
workers.[75] We believe that the results must be due to the
substituent on the tellurium atom. Sonoda and co-workers
used butyl phenyl telluride, and we used 3g and 6h. In the
case of 3g, the formation of stable enolate species decreases
the activation energy for the formation of the ate complex.
In the case of 6h, the presence of two aryl groups in the
molecule stabilizes the ate complex and thus decreases the
activation energy for the formation of the ate complex. The
results suggest that the substituent on the heteroatom also
influences the reactivity of the exchange reaction.
0.062 mmol) in THF (1.2 mL) was treated with nBuLi (45.0 mL , 1.51m
solution in hexane, 0.068 mmol) at À788C. After stirring for 0.5 h at this
temperature, DCl (18.5 mL, 6.49m in D2O, 0.12 mmol) was added. The re-
sulting mixture was stirred for 1 h at this temperature, and was slowly
warmed to room temperature over 2 h. The reaction mixture was
quenched with saturated aqueous NaHCO3 solution (0.5 mL), and ex-
tracted with CHCl3 (0.5 mLꢁ3). The organic layer was washed with satu-
rated aqueous NaCl solution, dried over MgSO4, filtered, and concentrat-
ed under reduced pressure. The residue was dissolved in CHCl3 (1 mL)
and poured into vigorously stirred hexane (50 mL). The precipitated
polymer was collected by suction and was dried under vacuum at 408C to
give PMMA in 99% (185.8 mg) with Mn =3000 and Mw/Mn =1.21. Incor-
poration of w-deuterium was confirmed by 2H NMR spectroscopy (Fig-
ure S3 in the Supporting Information). MALDI-TOF MS analysis also in-
dicated the formation of the desired w-end-deuterated PMMA with Mn =
2700 and Mw/Mn =1.04 (Figure 1).
General procedure for competition experiments between 3 and 2 f: A so-
lution of 3b (22.4 mL, 0.2 mmol), 2 f (22.4 mL, 0.2 mmol), and benzalde-
hyde (61.0 mL, 0.60 mmol) in THF (1.0 mL) was cooled to À788C, and
nBuLi (26.3 mL, 0.04 mmol) was added to this mixture. After 15 min stir-
ring at this temperature, the reaction was quenched by the addition of
AcOH/MeOH. After extractive workup, the yields of alcohols 3A, 2A,
and 21 were determined by GC analysis with tetradecane as an internal
standard. The same reaction was repeated six times (Table S3 in the Sup-
porting Information), and the yields of four experiments excluding the
highest and lowest yields were averaged (Table 4).
Conclusion
Generation of carbanions from organostibine and organobis-
muthine compounds through stibine–metal and bismuthine–
metal exchange reactions was investigated. The exchange re-
actions proceeded quantitatively to generate the corre-
sponding carbanions with several organometallic reagents.
The exchange reactions occurred with high chemoselectivity,
and a variety of polar functional groups could be tolerated.
The resulting highly functionalized carbanions were utilized
for carbon–carbon bond formation with various electro-
philes. The advantage of this method was exemplified in the
selective functionalization of polymer ends, prepared by or-
ganostibine- and organobismuthine-mediated living radical
polymerization to give a variety of structurally well-defined
w-functionalized polymers. In addition to the synthetic utili-
ty, this work provides insight into the relative reactivities of
heavier organoheteroatom compounds in heteroatom–metal
exchange reactions. We believe that this work opens new
possibilities for synthetic application of organostibines and
organobismuthines in carbanion chemistry.
Acknowledgements
This work was partly supported by the Nagase Science Foundation, a
Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and the Core Research
for Evolution Science and Technology (CREST), Japan, Science and
Technology Agency. E.K. acknowledges the receipt of a Japan Society for
the Promotion of Science (JSPS) Fellowship for Young Scientists.
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Received: January 24, 2011
Published online: April 8, 2011
5280
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