Platinum-catalyzed nucleophilic addition of vinylsilanes at the β-position

Article abstract of DOI:10.1021/ol902060r

In the presence of catalytic amounts of PtCl₂ and metal Iodides, β-substituted vlnylsllanes reacted with aldehydes at the β-position to give allyl silyl ethers. The Pt-catalyzed addition to aromatic aldehydes proceeded efficiently in the presence of Lil. The combined use of PtCl ₂ and Mnl₂ was found to be effective In addition to aliphatic aldehydes.

Full text of DOI:10.1021/ol902060r

ORGANIC
LETTERS
2009
Vol. 11, No. 21
5066-5069
Platinum-Catalyzed Nucleophilic
Addition of Vinylsilanes at the
ꢀ-Position
Katsukiyo Miura,*,† Gen Inoue,‡ Hisashi Sasagawa,‡ Hidenori Kinoshita,†
Junji Ichikawa,‡ and Akira Hosomi‡
Department of Applied Chemistry, Graduate School of Science and Engineering,
Saitama UniVersity, Sakura-ku, Saitama 338-8570, Japan, and Department of
Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba,
Tsukuba, Ibaraki 305-8571, Japan
kmiura@apc.saitama-u.ac.jp
Received September 5, 2009
ABSTRACT
In the presence of catalytic amounts of PtCl2 and metal iodides, ꢀ-substituted vinylsilanes reacted with aldehydes at the ꢀ-position to give
allyl silyl ethers. The Pt-catalyzed addition to aromatic aldehydes proceeded efficiently in the presence of LiI. The combined use of PtCl2 and
MnI2 was found to be effective in addition to aliphatic aldehydes.
Vinylsilanes have frequently been used as stable vinyl anion
equivalents for efficient, stereospecific carbon-carbon bond-
forming reactions promoted by Lewis acids and transition-
metal catalysts.1,2 They react with carbon electrophiles
usually at the R-position, although there are a few excep-
tions.3,4 We report here that Pt catalysis enables the addition
of vinylsilanes to aldehydes at the position ꢀ to silicon.5
We have previously reported the Pt(II)-catalyzed annula-
tion of hydroxyalkyl-substituted vinylsilanes with aldehydes,
which involves alkene migration to the corresponding
allylsilanes and subsequent Prins-type annulation with al-
dehydes.6 On the basis of the previous work, our interest
was focused on the Pt(II)-catalyzed intermolecular allylation
of aldehydes with vinylsilanes. Initially, the reaction of
benzaldehyde (1a) with vinylsilane 2a was carried out in
the presence of PtCl2 (Scheme 1). As a result, the desired
allylation product was not obtained. However, it was
fortunate that we found the formation of a small quantity of
allyl TMS ether 3aa. The vinylation product indicates that
2a reacted at the ꢀ-position. Thus, our interest was switched
to this novel type of nucleophilic addition of vinylsilanes.
Screening of some Pt salts and complexes revealed that
PtI2 works as an effective catalyst of the addition of 2a
(Scheme 1).7 Inspired by this result, we investigated the
effects of alkali iodides (LiI, NaI, and KI) on the catalysis
† Saitama University.
‡ University of Tsukuba.
(1) (a) Miura, K.; Hosomi, A. In Main Group Metals in Organic
Synthesis; Yamamoto, H., Oshima, K., Eds.; Wiley-VCH: Weinheim, 2004;
Vol. 2, p 409. (b) Brook, M. A. Silicon in Organic, Organometallic, and
Polymer Chemistry; Wiley: New York, 2000
.
(2) (a) Denmark, S. E.; Sweis, R. F. In Metal-Catalyzed Cross-Coupling
Reactions, 2nd ed.; De Meijere, A., Diederich, F., Eds.; Wiley-VCH:
Weinheim, 2004; p 163. (b) Hiyama, T. In Metal-Catalyzed Cross-Coupling
Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998;
p 421
.
(3) (a) Blumenkopf, T. A.; Bratz, M.; Castaneda, A.; Look, G. C.;
Overman, L. E.; Rodriguez, D.; Thompson, A. S. J. Am. Chem. Soc. 1990,
112, 4386. (b) Blumenkopf, T. A.; Look, G. C.; Overman, L. E. J. Am.
Chem. Soc. 1990, 112, 4399
.
(6) Miura, K.; Itaya, R.; Horiike, M.; Izumi, H.; Hosomi, A. Synlett
2005, 3148.
(4) (a) Ikenaga, K.; Kikukawa, K.; Matsuda, T. J. Chem. Soc., Perkin
Trans. 1 1986, 1959. (b) Hatanaka, Y.; Hiyama, T. Synlett 1991, 845
(5) For the Pt(II)-catalyzed reaction of aldehydes with allylsilanes, see:
Fu¨rstner, A.; Voigtla¨nder, D. Synthesis 2000, 959.
.
(7) PtCl4, PtBr2, PtI4, and PtCl2L2 (L2 ) 2,2′-bipyridine, COD, (PPh3)2,
(PhCN)2) were much less effective than PtI2. Pt(0)Ln (Ln ) (PPh3)4, (dba)2,
(CH2dCHSiMe2)2O) showed no catalytic activity.
10.1021/ol902060r CCC: $40.75
Published on Web 10/05/2009
 2009 American Chemical Society

by PtCl2. These iodides effectively promoted the Pt-catalyzed
vinylation. In particular, the use of LiI completed the reaction
within 2 h.8
Table 1. Addition of Vinylsilanes to Aldehydesa
Scheme 1
product,
yieldb
(%)
aldehyde
(R)
vinylsilane
(R1, R2)
time
(h)
entry
1
2
3
4
5
6
7
8
1b (1-naphthyl)
1c (4-O2NC6H4)
1d (4-MeO2CC6H4) 2a
2a (n-C10H21, Me)
2a
24 3ba, 84
24 3ca, 84
48 3da, 77
24 3ea, 89
1e (4-FC6H4)
1f (4-ClC6H4)
1g (2-ClC6H4)
1h (4-BrC6H4)
1i (4-IC6H4)
1j (4-AcOC6H4)
1k (4-MeC6H4)
1l (4-MeOC6H4)
1m (Ph(CH2)2)
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
(E)-2a
5
3fa, 90
48 3ga, 86
48 3ha, 91
36 3ia, 71
24 3ja, 69
We next carried out the reaction of 2a with various
aldehydes 1 under the optimized conditions. 1-Naphthalde-
hyde (1b) and substituted benzaldehydes 1c-k were smoothly
converted into allyl TMS ethers 3ba-ka (entries 1-10 in
Table 1). C-I and C-Br bonds, which are reactive toward
oxidative addition to Pt(0) species,9 are compatible with the
present vinylation. The reaction of 4-methoxybenzaldehyde
(1l) gave 3la in low yield due to the decomposition of 3la
(entry 11).
9
10
11
12c
13c
14c
15
16
17
18
19
20
21
22
23
5
2
4
4
4
3ka, 58
3la, 18
3ma, 64
3na, 71
3oa, 52
1n (c-C6H11
)
1o (t-Bu)
1a (Ph)
1a
1f
1a
1a
1a
1a
1a
24 3aa, 60
3ab, 78
2b (Me, Me)
2b
2c (c-C6H11, Me)
2d (Ph, Me)
2e (4-F3CC6H4, Me) 24 3ae, 6
2f (H, Me)
2
24 3fb, 91
24 3ac, 5
10 3ad, 69
The addition of 2a to aliphatic aldehydes 1m-o resulted
in low or no yield of 3ma-oa under catalysis by PtCl2-LiI.
Self-aldol condensation of 1m and silylation of 1n to the
corresponding TMS enolate were observed under these
conditions. As the result of screening of other metal iodides,
MnI2 was found to serve as an effective cocatalyst of the
vinylation of aliphatic aldehydes (entries 12-14).
Other vinylsilanes were applied to the Pt-catalyzed addition
to 1a. The E-isomer of 2a was not as reactive as 2a (entry
15). Vinylsilane 2b as well as 2a showed high reactivity
(entries 16-17). As shown in the case of 2c, the presence
of a bulky alkyl group at the ꢀ-position largely reduced the
reactivity to 1a (entry 18). Phenyl-substituted vinylsilane 2d
was reactive enough; however, vinylsilane 2e, bearing a
4-trifluoromethylphenyl group, reacted sluggishly (entries 19
and 20). Thus, the steric and electronic natures of the
ꢀ-substituents strongly affected the reactivity of 2. Use of
the parent vinylsilane 2f led to a complex mixture of products
(entry 21). The replacement of the methyl group on silicon
by a tert-butyl or phenyl group decelerated the vinylation
of 1a (entries 22 and 23).
2
-, 0
2g (n-C10H21, t-Bu)
2h (n-C10H21, Ph)
24 3ag, 29
10 3ah, 60
1a
a All reactions were carried out with an aldehyde (0.50 mmol), a
vinylsilane (1.00 mmol), PtCl2 (0.05 mmol), and LiI (0.10 mmol) or MnI2
(0.05 mmol) in 1,2-dichloroethane (1.5 mL) at 70 °C. b Isolated yield. c MnI2
(10 mol %) was used instead of LiI.
Scheme 2
of 1a with 2a-D revealed 1,2-shift of the deuterium atom
(Scheme 3). Crossover experiments using a 1:1 mixture of
2i and 2j were conducted under different sets of reaction
conditions (Table 2). Under the standard conditions, the
reaction of 1a with 2i and 2j gave non-crossover products
3ai and 3aj mainly (3ai/3aa ) 94:6, 3aj/3ak ) 93:7, entry
1). Use of a 2-fold amount of the solvent hardly affected
the ratios of non-crossover to crossover products (entry 2).
In contrast, increased concentrations promoted the formation
of crossover products 3aa and 3ak (entries 3 and 5). These
results suggest that the catalytic cycle should involve the
formation of a transient intermediate from a vinylsilane
molecule by elimination of the silyl group. In subsequent
reaction of the intermediate, incorporation of the original silyl
The PtCl2-catalyzed reaction of 2a with benzaldehyde
dimethyl acetal also proceeded at the ꢀ-position to give allyl
methyl ether 4 but in low yield (Scheme 2).
To gain mechanistic insight, we performed some experi-
ments using D-labeled vinylsilanes 2a-D and 2i. The reaction
(8) PtI2-2LiCl showed a catalytic activity similar to that of PtCl2-2LiI.
The activity of PtI2-2LiI was lower than that of PtI2. Use of LiCl as
cocatalyst did not enhance the catalytic activity of PtCl2.
(9) (a) Hartley, F. R. In ComprehensiVe Organometallic Chemistry I;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Elsevier: Oxford, 1982;
Vol. 6, p 471. (b) Anderson G. K. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds; Elsevier:
Oxford, 1995; Vol. 9, p 431.
Org. Lett., Vol. 11, No. 21, 2009
5067

complex (Si-Pt transmetalation),11 (4) addition of 6 to the
activated aldehyde proceeds rapidly at the position ꢀ to
platinum to form carbene complex 7,12 and (5) 7 undergoes
1,2-H-shift and elimination of PtX2 to give allyl silyl ether
3.13 LiI likely serves to enhance the π-Lewis acidity of
Li[PtX3] by introduction of the iodide ion, which is a soft
anion. In addition, the LiI-induced formation of anionic
complexes 5 and 6 may accelerate the transmetalation and
ꢀ-addition steps.
Scheme 3
group would be faster than that of the silyl group from other
vinylsilane molecules, although the latter crossover path is
facilitated by an increase in concentration. As shown in
entries 3 and 4, the product ratio did not depend on the
reaction time. This fact indicates that the interconversion
between non-crossover and crossover products does not occur
under the reaction conditions.
Scheme 4
Table 2. Crossover Experiments Using Vinylsilanes 2i and 2ja
The low reactivity of 2c and 2g is probably because the
cyclohexyl and TBS groups suppress the coordination step
by steric repulsion. The sluggish addition of 2e is attributable
to deceleration of the same step by the electron-withdrawing
ability of the CF3 group.
volume
of
solvent
(mL)
yield of
3ai + 3aa (%),
3ai:3aab
yield of
3aj + 3ak (%),
3aj:3akb
time
(h)
entry
To ascertain the proposed mechanism, the reaction of 2a
with MeOH was performed under catalysis by PtCl2-LiI
(Scheme 5). We predicted methyl ether 8 would be formed
by protonation of vinylplatinum 6a to platinum carbene
complex 9 and its insertion into the O-H bond.14,15 Indeed,
8 was obtained as a minor product, although the desilylation
1
2
3
1.5
3.0
0.75
0.75
0.40
4
4
4
2
4
48, 94:6
21, 92:8
43, 87:13
14, 86:14
13, 79:21
48, 93:7
21, 93:7
45, 86:14
14, 86:14
17, 81:19
4
5c
a All reactions were carried out with 1a (0.50 mmol), 2i (0.50 mmol),
2j (0.50 mmol), PtCl2 (0.05 mmol), and LiI (0.10 mmol) in 1,2-
dichloroethane (0.4-3 mL) at 70 °C. b The yield and ratio were determined
by GC-MS analysis of the mixture of products. c The reaction stopped
before complete consumption of 1a.
(11) For activation of the sp2-C-Si bond in vinylsilanes by Pt(II)
complexes, see ref 9 and the following references: (a) Mansuy, D.; Pusset,
J.; Chottard, J. C. J. Organomet. Chem. 1976, 110, 139. (b) Poist, J. E.;
Kraihanzel, C. S. Chem. Commun. 1968, 607.
(12) Some vinyl-transition metals are known to react with carbon
electrophiles at the ꢀ-position to give carbene complexes. Kusama, H.;
Yamabe, H.; Onizawa, Y.; Hoshino, T.; Iwasawa, N. Angew. Chem., Int.
Ed. 2005, 44, 468, and references cited therein.
Pt(0) complexes showed no catalytic activity in the
reaction of 1a with 2a.6 Judging from this result and the
high compatibility of C-X bonds, the participation of Pt(0)
species in the catalytic cycle is not likely. A possible
mechanism for the PtCl2-LiI-catalyzed vinylation of alde-
hydes is as follows (Scheme 4): (1) Li[PtX3] (X ) Cl, I), a
monomeric, coordinatively unsaturated Pt(II) species, is
formed from PtCl2 and LiI, (2) coordination of a vinylsilane
to Li[PtX3] affords alkene complex 5,10 (3) 5 reacts with an
aldehyde 1 to give vinylplatinum 6 and a halosilane-aldehyde
(13) The transformation of Pt-carbene complexes into alkenes has been
proposed as a key step of the Pt(II)-catalyzed cycloisomerization of 1,6-
enynes. (a) Fu¨rstner, A.; Szillat, H.; Stelzer, F. J. Am. Chem. Soc. 2000,
122, 6785. (b) Oi, S.; Tsukamoto, I.; Miyano, S.; Inoue, Y. Organometallics
2001, 20, 3704. (c) Echavarren, A. M.; Me´ndez, M.; Mun˜oz, M. P.; Nevado,
C.; Mart´ın-Matute, B.; Nieto-Oberhuber, C.; Ca´rdenas, D. J. Pure Appl.
Chem. 2004, 76, 453.
(14) For protonation of vinylplatinum complexes to carbene complexes,
see: Bell, R. A.; Chisholm, M. H.; Couch, D. A.; Rankel, L. A. Inorg.
Chem. 1977, 16, 677
.
(15) The Pt(II)-catalyzed reaction of diazo compounds with alcohols is
known to give ethers by insertion into the O-H bond, which proceeds
probably via Pt-carbene complexes. (a) Bertani, R.; Biasiolo, M.; Darini,
K.; Michelin, R. A.; Mozzon, M.; Visentin, F.; Zanotto, L. J. Organomet.
Chem. 2002, 642, 32. (b) Schils, R.; Simal, F.; Demonceau, A.; Noels, A. F.;
Eremenko, I. L.; Sidorov, A. A.; Nefedov, S. E. Tetrahedron Lett. 1998,
(10) K[PtCl3CH2dCHSiMe3], a vinylsilane-Pt(II) complex, can be
prepared from CH2dCHSiMe3 and Zeise’s salt (K[PtCl3CH2dCH2]).
Haschke, E. M.; Fitch, J. W. J. Organomet. Chem. 1973, 57, C93.
39, 7849
.
5068
Org. Lett., Vol. 11, No. 21, 2009

to 1-dodecene occurred mainly. As expected from the
postulated mechanism, use of MeOD gave R,ꢀ-dideuterated
ether 8-D in a similar yield.16 These results support the
validity of the presence of intermediates 6 and 7 in the Pt-
catalyzed vinylation.
position ꢀ to silicon. Besides the novelty in regiochemistry,
the reasonable applicability and easy access to vinylsilanes
make it synthetically useful. It is also noteworthy that the
Pt(II) catalysis efficiently activated the sp2-C-Si bond of
TMS-protected vinyl donors without any stoichiometric
activators.17 The applicability of the present vinylation and
its detailed mechanism are now under further investigation.
Scheme 5
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Area “Synergistic
Effects for Creation of Functional Molecules” from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. We thank previous reviewers for suggesting the
reaction mechanism and mechanistic experiments.
Supporting Information Available: Experimental details
and characterization data (1H NMR, 13C NMR, IR, elemental
analysis). This material is available free of charge via the
Internet at http://pubs.acs.org.
OL902060R
(17) The Hiyama cross-coupling reactions of vinylsilanes generally
requires a heteroatom-substituted silyl group (or its precursor) and a
stoichiometric activator such as a fluoride ion. See ref 2. Narasaka and
co-workers have recently reported an example of the catalytic C-C bond-
forming reaction of the TMS-protected vinyl donor without any stoichio-
metric activators. Yamane, M.; Uera, K.; Narasaka, K. Bull. Chem. Soc.
Jpn. 2005, 78, 477.
In conclusion, we have developed a novel type of
nucleophilic addition of vinylsilanes, which occurs at the
(16) Approximately one of two methylene hydrogens was displaced by
deuterium in both the R- and ꢀ-positions.
Org. Lett., Vol. 11, No. 21, 2009
5069