Substituted butenylindium generated by transmetalation of cyclopropylmethylstannane with indium iodide: Synthesis and characterization of monobutenylindium

Article abstract of DOI:10.1021/om200094m

Transmetalation between substituted cyclopropylmethylstannanes and indium iodide provided the corresponding mono- and dibutenylindium species. The bulky substituent on a cyclopropyl ring selectively afforded the monobutenylindium species, which allowed th

Full text of DOI:10.1021/om200094m

ARTICLE
pubs.acs.org/Organometallics
Substituted Butenylindium Generated by Transmetalation of
Cyclopropylmethylstannane with Indium Iodide: Synthesis and
Characterization of Monobutenylindium
Kensuke Kiyokawa, Makoto Yasuda, and Akio Baba*
Department of Applied Chemistry, Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
S Supporting Information
b
ABSTRACT: Transmetalation between substituted cyclopropyl-
methylstannanes and indium iodide provided the corresponding
mono- and dibutenylindium species. The bulky substituent on a
cyclopropyl ring selectively afforded the monobutenylindium
species, which allowed the isolation and X-ray structural analysis
of the monobutenylindium complex. In an investigation of the
generated substituted butenylindium species, we found that indium
halide interacted with the less sterically hindered carbonÀcarbon bond of the cyclopropyl ring during transmetalation. In addition,
we examined the radical coupling reactions of substituted butenylindium species with an R-iodoester. The distribution of the
cyclopropylmethylated and alkene products was evidence that the reactions were remarkably affected by the steric effect of the
substituents of the butenylindium species, which retarded the cyclization of the radical intermediate.
’ INTRODUCTION
’ RESULTS AND DISCUSSION
Organoindium compounds are recognized as an important
class of organometallic reagents in organic synthesis because of
their characteristics: ease of handling, moisture stability, excellent
functional group tolerance, etc.1 In particular, allylic indiums
have been widely used for carbonÀcarbon bond formations such
as the allylation of carbonyl compounds.1,2 In addition, various
types of other organoindium compounds (alkenyl, alkynyl, aryl,
etc.) are also applicable to transition-metal-catalyzed cross-
coupling reactions.3 Although the synthetic applications of
organoindium compounds have been extensively studied, the
structure of the active species is virtually unknown, and the
nature of most organoindiums is still undefined.2f,4 To solve
these problems, our group has recently studied organoindium
compounds such as alkenylindium5 (2-carbon unit) and
allylindium6 (3-carbon unit). In addition, the higher homologue,
the butenylindium (4-carbon unit) species, was investigated for
radical and ionic reactivity, particularly dibutenylindium.7,8 How-
ever, the more basic monobutenyl derivative has not been
investigated.9 Herein, we focus on the synthesis of the mono-
butenylindium species. Fortunately, the employment of substi-
tuted cyclopropylmethylstannanes selectively afforded the
monobutenylindium species, as determined by NMR spectros-
copy and X-ray structural analysis (Scheme 1). A monobuteny-
lindium species had not previously been isolated, while our group
has previously reported the isolation of monobutenylgallium.8
Furthermore, the radical coupling between the substituted
butenylindium species with an R-iodoester elucidated the im-
portance of steric hindrance and the β-effect of indium.
Initially, the reaction of 2,2-dimethylcyclopropylmethyltribu-
tylstannane (1a) and phenyl 2-iodoacetate (2) was conducted to
investigate the effect of indium sources and solvents, in which the
generation of the butenylindium species was followed by a
reaction with 2 (Table 1). The treatment of InCl3 in toluene
under open air conditions resulted in a low yield, and unreacted
stannane 1a was recovered (entry 1). This result indicates that
the transmetalation between InCl3 and 1a was not effective.
When using InBr3, stannane 1a was completely consumed, and
the yield of the desired product 3a was improved by as much as
56% (entry 2). Finally, InI3 was found to be the best choice to
afford 3a (entry 3).10 In the presence of a radical inhibitor, the
coupling reaction was inhibited, indicating that the reaction
proceeds in a radical manner (entry 4). The reaction performed
in hexane gave a slightly lower yield than that in toluene (entries
3 and 5). Coordinating solvents significantly suppressed the
reactions (entries 6À8). Notably, no transmetalation proceeded
in THF. Gratifyingly, the addition of a catalytic amount of Et3B as
a radical initiator drastically improved the yield to 94% (entry 9).
The reaction in the absence of InI3 did not give the coupling
product (entry 10).
To confirm the generation of the butenylindium species, the
transmetalation between 1a and InI3 was monitored by 1H NMR
spectroscopy (Figure 1). The mixture of 1a and InI3 (1a/InI3 =
1:1) in toluene-d8 immediately produced a single product (4a)
with a doublet of doublet signal at δ 5.74 for the internal olefin
Received: February 2, 2011
Published: March 16, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Monobutenylindium Species by
Transmetalation between Substituted Cyclopropylmethyl-
stannanes and InI3
Scheme 2. Plausible Mechanism for Transmetalation be-
tween 1a and Indium Iodide
Table 1. Effect of Indium Sources and Solventsa
Figure 2. Substituted cyclopropylmethylstannanes.
entry
InX3
solvent
yield/%b
recovery of 1a/%b
Scheme 3. Isolation of 2-Cyclohexen-1-ylmethylindium
DiiodideÀPPh3 Complex 10
1
InCl3
InBr3
InI3
toluene
toluene
toluene
toluene
hexane
Et2O
25
56
64
0
44
0
2
3
4c
0
InI3
6
5
InI3
54
14
0
0
6
InI3
12
40
95
0
7
InI3
MeCN
THF
8
InI3
0
9d
10d
InI3
toluene
toluene
94
0
proton, two doublets at δ4.94 and 4.82 for terminal olefinprotons, a
singlet at δ 1.91 for methylene protons, and a singlet at δ 0.98 for
methyl protons (Figure 1i). 4a was most certainly a monobuteny-
lindium diiodide, because using two equivalents of 1a provided two
types of butenylindium species, perhaps mono-4a and dibutenylin-
dium 5a, with a small amount of starting material 1a remaining
(Figure 1ii). After the reaction mixture was stirred for a long enough
period (13 h), stannane 1a was almost consumed, and dibuteny-
lindium 5a was preferentially observed (Figure 1iii). These results
show that the first transmetalation between 1a and InI3, giving
monobutenylindium 4a, was quite fast, while the second transme-
talation, giving dibutenylindium 5a, was relatively slow probably
because of the steric hindrance between butenyl substituents on the
indium atom. This is the reason for the selective synthesis of
monobutenylindium 4a from equimolar amounts of 1a and InI3.
This tendency is quite different from the transmetalation of non-
substituted cyclopropylmethylstannane and InBr3, in which the
dibutenylindium was readily generated even before the complete
consumption of the starting stannane.7
none
89
a Using 1.5 mmol of 1a, 1.0 mmol of 2, and 0.75 mmol of indium halide.
b Determined by 1H NMR. c Galvinoxyl (0.1 mmol) was added. d Et3B
(0.1 mmol) was added, and the reaction was carried out under N2.
An investigation into the structure of the generated buteny-
lindium 4a (5a) provided some insight into the mechanism of
transmetalation. For efficient transmetalation, the π-Lewis acid-
ity of indium halide provides an important interaction with the
carbonÀcarbon bond of the cyclopropyl ring, which has a much
higher p character than a normal carbonÀcarbon σ bond.11 Since
butenylindium 4a (5a) has two methyl groups at the β-position,
the selective ring cleavage takes place at the less hindered car-
bonÀcarbon bond of 1a (6 vs 7) to produce butenylindium 9,
rather than 8, as shown in Scheme 2. This proposed mechanism is
Figure 1. 1H NMR spectra of the reaction mixture of 1a and InI3 with
(i) 1:1 (5 min), (ii) 2:1 (5 min), and (iii) 2:1 (13 h) ratios in toluene-d8.
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supported by the fact that InI3, which is a softer Lewis acid by
comparison with either InCl3 or InBr3, gave the best results (see
Table 1). In addition, suppression of the transmetalation in
coordinating solvents by lowering the Lewis acidity of indium
halide can be explained.
various combinations of stannanes (1aÀd) (Figure 2) and
phosphine ligands.7,8 Although some stable complexes were
isolated, only the combination of stannane 1d and PPh3 gave a
colorless single crystal that was suitable for X-ray structural
analysis (Scheme 3).12,13
The ORTEP drawing of 2-cyclohexen-1-ylmethylindium diio-
dideÀPPh3 complex 10 is shown in Figure 3.14 As far as can be
ascertained, this is the first example of the X-ray structural
analysis of a monobutenylindium species.15 The coordination
of one PPh3 constructed a distorted tetrahedral structure with a
four-coordinated indium center. In this complex, there was no
intermolecular interaction through bridging by halogen atoms.
The In1ÀC1 length at 2.165(9) Å was slightly shorter than
the sum of the individual covalent radii (dInÀC = 2.18 Å).16
Next, to confirm the generation of a monobutenylindium
species, isolation by complex formation was attempted using
Scheme 4. Plausible Reaction Mechanism
Figure 3. ORTEP drawing of 2-cyclohexen-1-ylmethylindium diiodi-
deÀPPh3 complex 10 (30% thermal ellipsoids; all hydrogen atoms are
omitted for clarity). Selected bond lengths (Å) and angles (deg):
In1ÀC1 = 2.165(9), In1ÀI1 = 2.7189(8), In1ÀI2 = 2.7252(8), In1ÀP1
= 2.6341(14), C1ÀC2 = 1.484(11), C2ÀC3 = 1.51(10), C3ÀC4 =
1.33(2); I1ÀIn1ÀI2
=
107.00(3), I1ÀIn1ÀC1
= 124.8(3),
I2ÀIn1ÀC1 = 116.5(3), P1ÀIn1ÀC1 = 106.03(18).
Table 2. Reactions of Cyclopropylmethylstannanes 1 with Iodoester 2a
a Using 1.5 mmol of 1, 1.0 mmol of 2, 0.75 mmol of InI3, and 0.1 mmol of Et3B. b Determined by 1H NMR. c Isolated yields (combined yields of 3 and
14). Values in parentheses are NMR-determined yields. d Et3B was not added. e Open air.
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Scheme 5. Plausible Mechanism for Alkene Formation
H-shift in intermediate 12 (Scheme 4) is perhaps the reason
there was no alkene formation from dimethyl-substituted stan-
nane 1a (entry 1). Unfortunately, the reaction of cyclic buteny-
lindium 13d did not proceed due to steric hindrance at the
reaction site (entry 5).
’ CONCLUSION
In conclusion, we have reported the facile preparation of
substituted butenylindium species from substituted cyclopropyl-
methylstannanes and InI3. The selective generation of mono-
butenylindium species was also confirmed by NMR spectroscopy
and X-ray structural analysis. In transmetalation, the π-Lewis
acidity of indium halide and the steric hindrance of the cyclo-
propyl ring are important factors for the effective and selective
synthesis of monobutenylindium species. Substituted butenylin-
dium species easily coupled with an iodoester to give the
corresponding cyclopropane products and alkenes. The results
of this radical coupling revealed a dependence on the substituent
for the change in reactivity of the butenylindium species.
The InÀC bond is comparable to a previously reported one in
the dibutenylindium complex.7 The C3ÀC4 length of 1.33(2) Å
indicates a double bond. Bond lengths of In1ÀI1 (2.7189(8) Å),
In1ÀI2 (2.7252(8) Å), and In1ÀP1 (2.6341(14) Å) and bond
angles between substituents at the indium atom were reasonable
and were comparable to those reported for the InI3ÀPPh3
complex.17
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, charac-
b
terization data, and CIF of 10. This material is available free of
charge via the Internet at http://pubs.acs.org.
Scheme 4 shows a plausible mechanism for the radical coupling
reaction of a butenylindium species with iodoester 2. First, mono-
butenylindium 4a is generated from the transmetalation between
stannane 1a and InI3, and further transmetalation partly provides
dibutenylindium 5a. The radical initiation step may be different
from the case of an unsubstituted butenylindium species,7
because the present case needs the addition of Et3B (see
Table 1). Therefore, two possibilities are proposed: (i) radical
species 11 is generated from 2 assisted by Et3B with O2; or (ii)
butenylindium 4a or 5a works as a radical initiator in the presence
of a small amount of O2 (or O2/Et3B), and the resultant radical
species abstracts the iodo radical from 2 to produce the corre-
sponding radical 11. The trap of 11 by the butenylindium species
is followed by the cyclization of 12 into cyclopropyl product 3a
along with an indium radical (other butenyl group and/or ligands
are omitted on In). Finally, the generated indium radical abstracts
the iodine from 2 to regenerate 11.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: baba@chem.eng.osaka-u.ac.jp.
’ ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (No. 22106527, “Organic Synth-
esis Based on Reaction Integration. Development of New
Methods and Creation of New Substances”) and Scientific
Research (No. 21350074) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We thank Dr.
Nobuko Kanehisa (Osaka University) for the valuable advice
regarding X-ray crystallography. K.K. thanks the Global COE
Program “Global Education and Research Center for Bio-
Environmental Chemistry” of Osaka University.
In order to investigate the reactivity of the substituted
butenylindium species for radical coupling, the reactions of
various cyclopropylmethylstannanes 1 and iodoester 2 mediated
by InI3 were conducted, as shown in Table 2. The corresponding
cyclopropylmethylated product 3a was afforded in high yield
with no byproduct (entry 1) when using 2,2-dimethylbutenylin-
dium 13a from 1a. Other stannanes 1b, 1c, and 1e gave varying
amounts of olefins. Among them, 1b, which generates 3-methyl-
butenylindium by transmetalation, gave the alkene product 14b
predominantly (3b/14b = 32/68) (entry 2).18 This is probably
because the cyclization of intermediate 15 is disturbed by the
steric hindrance of the tertiary radical (Scheme 5). In addition,
the β-effect of indium stabilizes radical intermediate 16 to accelerate
the isomerization from 15 to 16 through H-shift to give alkene
14b.8,19 Scheme 5 would also be a reasonable explanation for why
mono- and nonsubstituted cyclopropylmethylstannanes 1c and 1e
gave moderate (20%) and small (7%) selectivities of alkenes 14c
and 14e along with major products of desired cyclopropyls 3c
and 3e, respectively (entries 3 and 4).20 No β-hydrogen for an
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3
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