Ir(I)/HCl catalyzed head-to-tail homocoupling reactions of vinylsilanes

Article abstract of DOI:10.1021/ol300203w

Novel homocoupling reactions of vinylsilanes, catalyzed by a mixture of Ir(I) and HCl, were developed. This process leads to exclusive formation of head-to-tail vinylsilane dimers in high yields at room temperature. Synthetic attributes of transformations

Full text of DOI:10.1021/ol300203w

ORGANIC
LETTERS
2012
Vol. 14, No. 6
1468–1471
Ir(I)/HCl Catalyzed Head-to-Tail
Homocoupling Reactions of Vinylsilanes
Jung-Woo Park, Sung Joon Park, and Chul-Ho Jun*
Department of Chemistry and Center for Bioactive Molecular Hybrid (CBMH), Yonsei
University, 50 Yonseiro, Shin-chon dong, Seodaemun-gu, Seoul 120-749, Korea
junch@yonsei.ac.kr
Received January 26, 2012
ABSTRACT
Novel homocoupling reactions of vinylsilanes, catalyzed by a mixture of Ir(I) and HCl, were developed. This process leads to exclusive formation of
head-to-tail vinylsilane dimers in high yields at room temperature. Synthetic attributes of transformations of the resulting head-to-tail vinylsilane
dimers and polymerization of bis(vinylsilane) were investigated.
Transformations of chemical feedstocks, such as ethy-
lene, styrene, and R-olefins, are of interest in the area of
organometallic chemistry.1 Particularly significant are
homocoupling reactions of unsaturated substances, and
as a result, these processes have attracted attention because
they serve as model systems for oligomerization and
polymerization of alkenes and as industrially important
processes for thepreparation ofhigher alkenes from simple
alkenes.2 Although a wide variety of homocoupling reac-
tions of unsaturated substances have been reported,3 only
a few exist that involve vinylsilanes.4,5 These examples
show low levels of regio- and chemoselectivity. In the
course of recent studies on transition metal/HCl-catalyzed
O-silylation reactions of vinylsilanes with alcohols,6 we
observed a new catalytic process in which vinylsilane
homocoupling took place under mild conditions (Scheme 1).
In this report, we describe an efficient homocoupling re-
action of vinylsilane derivatives at ambient temperatures
with excellent levels of regioselectivity.
n-Hexyldimethylvinylsilane (1a) was chosen as a model
substrate to explore various catalytic systems for this
homocoupling process (Table 1). Reaction of this sub-
stance in the presence of catalyst mixture of [(COE)2IrCl]2
(2, COE: cyclooctene) and HCl in 1,4-dioxane (3) at room
temperature for 30 min led to exclusive formation of the
head-to-tail homocoupling product 4a in 97% yield
(determined by GC analysis, Table 1, entry 1).7 However,
(1) (a) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc.
1980, 102, 5610–5618. (b) Pillai, S. M.; Ravindranathan, M.; Sivaram, S.
Chem. Rev. 1986, 86, 353–399. (c) Skupin’ska, J. Chem. Rev. 1991, 91,
613–648. (d) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E.
Synlett 1990, 74–84. (e) Ho, C.-Y.; Ohmiya, H.; Jamison, T. F. Angew.
Chem., Int. Ed. 2008, 47, 1893–1895.
(2) (a) Christoffers, J.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118,
4715–4716. (b) Janiak, C. Coord. Chem. Rev. 2006, 250, 66–94.
(3) (a) Kondo, T.; Takagi, D.; Tsujita, H.; Ura, Y.; Wada, K.; Mitsudo,
T.-a. Angew. Chem., Int. Ed. 2007, 46, 5958–5961. (b) Tobisu, M.; Hyodo, I.;
Onoe, M.; Chatani, N. Chem. Commun. 2008, 45, 6013–6015. (c) Ho, C.-Y.;
He, L. Angew. Chem., Int. Ed. 2010, 49, 9182–9186. (d) Ez-Zoubir, M.;
d’Herouville, F. L. B.; Brown, J. A.; Ratovelomanana-Vidal, V.Michelet,
V. Chem. Commun. 2010, 46, 6332–6334. (e) Barlow, M. G.; Bryant,
M. J.; Haszeldine, R. N.; Mackie, A. G. J. Organomet. Chem. 1970, 21,
215–226. (f) Dawans, F. Tetrahedron Lett. 1971, 12, 1943–1946. (g) Sen, A.;
Lai, T.-W. Organometallics 1983, 2, 1059–1060. (h) Grenouillet, P.;
Neibecker, D.; Tkatchenko, I. Organometallics 1984, 3, 1130–1132. (i)
Wu, G.; Rheingold, L. A.; Heck, R. F. Organometallics 1987, 6, 2386–
2391. (j) Jiang, Z.; Sen, A. J. Am. Chem. Soc. 1990, 112, 9655–9657. (k)
Tsuchimoto, T.; Kamiyama, S.; Negoro, R.; Shirakawa, E.; Kawakami,
Y. Chem. Commun. 2003, 40, 852–853. (l) Kabalka, G. W.; Dong, G.;
Venkataiah, B. Tetrahedron Lett. 2004, 45, 2775–2777. (m) Peng, J.; Li,
J.; Qiu, H.; Jiang, J.; Jiang, K.; Mao, J.; Lai, G. J. Mol. Catal. A: Chem.
2006, 255, 16–18.
(5) (a) For a reference about desilylative homocoupling reaction,
see:Yue, Y.; Yamamoto, H.; Yamane, M. Synlett 2009, 2831–2835. For
references about silylative coupling reaction and detailed mechanism,
see: (b) Marciniec, B. Acc. Chem. Res. 2007, 40, 943–952. (c) Marciniec,
ꢀ
B.; Walczuk-Gusciora, E.; Pietraszuk, C. Organometallics 2001, 20,
3423–3428. (d) Marciniec, B.; Kownacki, I.; Kubicki, M. Organometal-
lics 2002, 21, 3263–3270.
(4) (a) Yusupova, F. G.; Gailyunas, G. A.; Furley, I. I.; Panasenko,
A. A.; Sheludyakov, V. D.; Tolstikov, G. A.; Yurjev, V. P. J. Organomet.
Chem. 1978, 155, 15–23. (b) Cros, P.; Triantaphylides, C.; Buono, G.
J. Org. Chem. 1988, 53, 185–187. (c) Kretschmer, W. P.; Troyanov, S. I.;
Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 1998, 17, 284–
286. (d) Ho, C.-Y.; He, L. Chem. Commun. 2012, 48, 1481–1483.
(6) Park, J.-W.; Jun, C.-H. Org. Lett. 2007, 9, 4073–4076.
(7) Other iridium complexes such as [(COD)IrCl]2 (COD: 1,5-
cyclooctadiene), (Ph3P)2(CO)IrCl, [Cp*IrCl2]2, and IrCl3 xH2O did
3
not show the catalytic activity of this reaction.
r
10.1021/ol300203w
Published on Web 02/24/2012
2012 American Chemical Society

silylethyl group in 7 undergoes migratory insertion into the
vinylsilane ligand through a carbometallation process to
give 2,4-disilylbutyliridium(III) complex 8. β-Hydrogen
elimination in 8 then takes place to generate dimer 4 along
with regeneration of the hydridoiridium(III) complex 6.
Scheme 1. Catalytic Reactions of Vinylsilane 1
Scheme 2. Proposed Mechanism for the Homocoupling Process
no reaction occurs when either 2 or 3 are not present in the
mixture. Interestingly, the reaction promoted by the cor-
responding rhodium catalytic system [(COE)2RhCl]2/HCl
is also sluggish (Table 1, entry 3). Hydrogen bromide and
trifluoroacetic acid (TFA) can be employed in placeofHCl
for this process, but formation of 4aunder these conditions
takes place more slowly (Table 1, entries 4 and 5). Finally,
the homocoupling reaction of 1a catalyzed by 2/3 at room
temperature is highly sensitive to solvent. Noncoordinat-
ing solvents such as CH2Cl2 and toluene are effective for
the reaction, while coordinating solvents such as MeCN
and DMA are ineffective (Table 1, entries 1, 6ꢀ8).
Table 2. Homocoupling Reactions of Vinylsilanes 1 in the
Presence of 2 (Ir) and 3 (HCl)
Table 1. Homocoupling Reaction of n-Hexyldimethylvinylsi-
lane (1a) in the Presence of Various Catalysts
entry metal catalyst
acid
solvent yield (%)a
1
2
3
4
5
6
7
8
[(COE)2IrCl]2 (2) HCl in 1,4-dioxane (3) CH2Cl2
97
0
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
DMAb
[(COE)2RhCl]2
3
7
2
2
2
2
2
HBr, 48% in H2O
82
64
0
TFA
3
3
3
0
toluene
97
a GC yield. b DMA: N,N0-dimethylacetamide.
On the basis of the results presented above, we pro-
posed the mechanism displayed in Scheme 2 for the
vinylsilane homocoupling reaction.8 Initially, cyclooctene
in the iridium(I) complex 2 undergoes exchange with
vinylsilane to generate the corresponding vinylsilane-
coordinated iridium(I) complex 5, which then is protonated
by HCl to produce the β-silylethyliridium(III) complex 7 via
the intermediacy of hydridoiridium(III) complex 6.9 The
a Isolated yield. GC yields are given in parentheses. b Reactions were
complete within 30 min. c CH2Cl2 was used as a solvent. d Determined by
1H NMR based on naphthalene as an internal standard. e 4% of siloxane
was also observed.
(8) A similar type of heterocoupling reaction mechanism with vinyl-
silane was explained in ref 4d.
(9) (a) James, B. R.; Morris, R. H. Can. J. Chem. 1986, 64, 897–903.
(b) A heterocoupling reaction with vinylsilane involving hydridoiridium(III)
intermediates was reported. See ref 5d.
Org. Lett., Vol. 14, No. 6, 2012
1469

Catalytic homocoupling reactions of vinylsilanes 1 bear-
ing various functional groups were performed to probe the
generality of the process (Table 2). In the presence of a
mixture of 2 and 3 at room temperature, trimethylvinylsi-
lane (1b) is highly reactive, giving the corresponding dimer
in >99% yield (GC), even when 0.5 mol % 2 with 1 mol %
3 are employed (Table 2, entry 2). Vinylsilanes containing
common alkyl Si-substituents were found to display simi-
lar reactivities (Table 2, entries 2ꢀ4, 7, and 8). However,
substrates with aromatic substituents on silicon are less
reactive than those with aliphatic substituents (Table 2,
entries5ꢀ6). The homocouplingreactivityprofileindicates
that reactivities of vinylsilanes are dependent on steric
effects (Table 2, entry 9).
Scheme 4. Polymerization of Bis-vinylsilanes 12
we attempted to carry out polymerization reactions of
bis-vinylsilanes bearing linkers between the two vinylsilyl
groups. For this purpose, reaction of 1,2-bis(dimethyl-
(vinyl)silyl)ethane (12a) was performed using the 2/3 co-
catalyst system in tolueneat room temperature (Scheme 4).
This process produced the head-to-tail polymer 13a. The
degree of polymerization was observed to reach 12 within
a 30 min period, as determined by a comparison of the
intensities of proton resonances in the 1H NMR spectrum
associated with the terminal vinyl groups with those of the
internal methylene groups. The data obtained by using
size-exclusion chromatography were consistent with this
assignment (Mn = 2.1 kDa, PD = 1.7). Analyses of the
13C and 29Si NMR spectra of 13a showed that only head-
to-tail type homocoupling had taken place in the polymer-
ization process (Figure 1).
Scheme 3. Synthetic Utilities of Dimer 4b
To probe the synthetic significance of the homocoupling
process, several transformations of the dimer product 4b
were investigated (Scheme 3). Shinokubo and Oshima,
et al. have demonstrated earlier that oxidative cleavage
of the SiꢀC bond in vinylsilane moieties occurs to form
R-functionalized ketones, indicating that vinylsilanes can
be regarded as synthetic equivalents of 2-oxyalkyl radical
acceptors.10 We have observed that dimer 4b reacts with
perfluorohexyl iodide in the presence of BEt3, followed by
treatment with H3PO2, pyridine, and BEt3, to produce the
R-perfluorohexyl ketone 9 in a high isolated yield. Inter-
estingly, reaction of 4b with NBS and an equimolecular
amount of TEMPO gives rise to the dibromination pro-
duct 10. Hydroboration-oxidation of 4b takes place to
afford the β-silyl alcohol 11 in a 77% yield.
Guided by the results arising in our study of the Ir(I)/
HCl-catalyzed homocoupling reactions of vinylsilanes,
Figure 1. Characteristic peaks of (a) 13C and (b) 29Si NMR
spectra of 13a.
(10) Kondo, J.; Shinokubo, H.; Oshima, K. Angew. Chem., Int. Ed.
2003, 42, 825–827.
(11) Compounds 19 were identified by comparison with authentic
Polymer 13a (n = 2) was degraded by using perfluoro-
hexyl iodide/BEt3-promoted iodine atom transfer radical
addition followed by atom transfer cleavage with BEt3/
H3PO2/pyridine as described above9 (Scheme 5a). This
reaction leads to selective carbonꢀsilicon bond cleavage to
give R-perfluoroalkyl ketone 14 and the unstable silanol
15, which dimerizes to form siloxane 16. To obtain clear
information about the nature of cleavage products 15 and
samples. See the Supporting Information.
(12) The x/y ratio in Scheme 5b was calculated by using the molar
ratio of 1/2 (19a) and 20a because the oxidative cleavage reaction in
regularly ordered part (x) gives rise to a 2-fold amount of 19a. Bis-
(fluorosilane) 21 is disregarded in this calculation because 21 is difficult
to isolate because of its high volatility.
(13) It is important to note that this reaction does not occur with
dimethyldivinylsilane as the starting material. The reason might be that
dimethyldivinylsilane is strongly coordinated to iridium in a bidendate
fashion.
16, the product mixture was treated with BF3 OEt2 to
3
Org. Lett., Vol. 14, No. 6, 2012
1470

while diketones 20a and 21a are formed from the alter-
nately ordered part y. The x/y ratio was determined to be
33/67 on the basis of the relative amounts of 19a and 20a
Scheme 5. Oxidative Cleavage of Polymers 13a
1
using GCꢀMS and H NMR analysis.12 Other linked
vinylsilanes 12b (n = 3) and 12c (n = 5) were trans-
formed to the corresponding polymers 13b and 13c, which
were produced in respective x/y ratios of 41/59
and 56/44.13
In the studies described above, we have developed a
novel homocoupling reaction of vinylsilanes that is pro-
moted by a catalyst mixture comprised of Ir(I) and HCl.
This reaction affords exclusive head-to-tail dimers of the
vinylsilanes in high yields at room temperature. The
synthetic potential of secondary transformations of the
head-to-tail dimer derived from vinylsilane was investi-
gated. Further applications of this process to the polymer-
ization of linked bis-vinylsilanes were explored. These
reactions generate new head-to-tail polymers, in which
the regularity of polymerization is well-defined.
Acknowledgment. This work was supported by a grant
from the National Research Foundation of Korea (NRF)
(Grant 2011-0016830), WCU (World Class University)
program through the Korea Science and Engineering
Foundation, the Ministry of Education, Science and Tech-
nology (R32-2008-000-10217-0), and CBMH (2011-0001129).
J.-W.P. and S.J.P acknowledge fellowships from the BK21
program of the Ministry of Education and Human Resources
Development.
a (a) Overall oxidative cleavage reaction and (b) determination of the
x/y ratios by using oxidative cleavage reactions.
Supporting Information Available. Experimental de-
1
tails and H and 13C NMR spectra of new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
convert the silanol and siloxane groups to fluorosi-
lanes 18.11 The oxidative cleavage of 13a produces three
different segments, 19a, 20a, and 21a (Scheme 5b). Fluor-
osilane 19a is derived from the regularly ordered part x,
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 6, 2012
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