Platinum-Catalyzed Silaborative Coupling of 1,3-Dienes to Aldehydes: Regio- and Stereoselective Allylation with Dienes through Allylic Platinum Intermediates

Full text of DOI:10.1021/ja980373o

4248
J. Am. Chem. Soc. 1998, 120, 4248-4249
Scheme 1. Silaboration of 2,3-Dimethyl-1,3-butadiene
Followed by Reaction with Benzaldehyde
Platinum-Catalyzed Silaborative Coupling of
1,3-Dienes to Aldehydes: Regio- and Stereoselective
Allylation with Dienes through Allylic Platinum
Intermediates
Michinori Suginome, Hiroshi Nakamura,
Takanori Matsuda, and Yoshihiko Ito*
Department of Synthetic Chemistry and Biological
Chemistry, Graduate School of Engineering
Kyoto UniVersity, Kyoto 606-01, Japan
ReceiVed February 3, 1998
Table 1. Reaction of 2,3-Dimethylbutadiene (2a), Benzaldehyde
(5a), and Silylborane 1 in the Presence of Platinum Catalyst 3a
Synthetic application of organometallic compounds containing
intermetallic σ-bonds has been developed by means of transition-
metal catalysis involving activation of the σ-bonds. The σ-bonds
between main-group elements such as Si-Si1 and B-B2 undergo
insertion reactions with carbon-carbon multiple bonds, i.e.,
bismetalation reactions, whose usefulness has been demonstrated
in stereoselective organic synthesis.3 Insertion of 1,3-dienes with
the intermetallic σ-bond promoted by transition metals is pre-
sumed to involve generation of π- or σ-allylic transition-metal
complexes accompanying regio- and stereoselective formation of
a carbon-main group metal bond.4 However, the synthetic
utilization of the allylic transition-metal intermediates generated
in situ has not thoroughly been exploited, though some bismeta-
lative dimerization reactions of 1,3-dienes has been described.5
Recently, we found that Si-B σ-bonds were effectively
activated by palladium as well as platinum catalysts for regio-
and stereoselective silaboration of C-C triple bonds.6 It should
be noted that the platinum complex also catalyzed the silaboration
of terminal C-C double bonds, in which we proposed that
insertion of alkenes into the Pt-B bond in a (silyl)(boryl)platinum
intermediate is followed by reductive coupling with the silyl group
on the platinum.7 In this paper, we disclose a silaborative
coupling of diene to aldehyde, which may involve 4-borylcrotyl-
platinum complex intermediates. The new three-component
coupling reaction features the high regio- and stereoselectivity,
PhCHO (5a)
(equiv)
temp
time
(h)
product 7a
entry
1e
2
3
4
(°C)b
yield (%)c
ratiod
1.5
1.5
1.5
1.0
3.0
120
80
50
80
80
2
4
8
4
4
74
78
61
63
85
81:19
95:5
98:2
93:7
96:4
5
a All reactions were carried out in hexane in the presence of
Pt(CH2dCH2)(PPh3)2 (3, 2 mol %) under nitrogen unless otherwise
noted. b Bath temp. c Isolated yields. d Ratios of the syn and anti isomers
1
determined by H NMR. e A reaction in octane.
providing homoallylic borane derivatives, which may be useful
for further synthetic elaboration.
On the basis of the success of the silaboration of alkynes and
alkenes, we attempted the reaction of silylborane 1 with 2,3-
dimethyl-1,3-butadiene (2a) in the presence of a catalytic amount
of Pt(CH2dCH2)(PPh3)2 (3) at 120 °C for 24 h in octane (Scheme
1). A 1,4-addition of the silicon-boron bond across the diene
took place to give 4 in good yield, but in nearly 1:1 ratio of the
E and Z isomers as judged by NMR spectroscopy of the resultant
mixture.1a,4 Addition of benzaldehyde (5a) to the reaction mixture
including 4 afforded a homoallylic alcohol 6 having a silyl group
as a 1:1 mixture of syn and anti isomers in good yield through
allylboration of the aldehyde.8
(1) (a) Okinoshima, H.; Yamamoto, K.; Kumada, M. J. Am. Chem. Soc.
1972, 94, 9263-9264. (b) For a review, see: Horn, K. A. Chem. ReV. 1995,
95, 1317-1350.
(2) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J. Am. Chem. Soc.
1993, 115, 11018-11019. Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa,
F.; Suzuki, A.; Miyaura, N. Organometallics 1996, 15, 713-720. Baker, R.
T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew. Chem., Int. Ed. Engl.
1995, 34, 1336-1337. Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem.
Commun. 1997, 689-690.
(3) For the recent application of the bis-silylation to stereoselective organic
synthesis, see: Suginome, M.; Yamamoto, Y.; Fujii, K.; Ito, Y. J. Am. Chem.
Soc. 1995, 117, 9608-9609. Suginome, M.; Matsumoto, A.; Ito, Y. J. Am.
Chem. Soc. 1996, 118, 3061-3062. Suginome, M.; Matsumoto, A.; Ito, Y. J.
Org. Chem. 1996, 61, 4884-4885. Suginome, M.; Iwanami, T.; Matsumoto,
A.; Ito, Y. Tetrahedron: Asymmetry 1997, 8, 859-862. Suginome, M.; Ito,
Y. J. Synth. Org. Chem. Jpn. 1997, 55, 1040-1051.
Since the lack of stereoselectivity was presumed to be due to
isomerization of the primarily formed E or Z alkenes 4 under the
reaction conditions, we examined the silaboration reaction in the
presence of 5a, which would trap 4 before the isomerization, if
any. The reaction at 120 °C proceeded smoothly (2 h) until
silylborane 1 was completely consumed and provided an aldehyde
adduct without the formation of the primary silaboration product
4. However, to our surprise, the product was not 6 but 7a, i.e.,
silyl-protected homoallylic alcohol having boryl group at the
terminal carbon of the diene (eq 1; Table 1, entry 1).
(4) Transition-metal-catalyzed 1,4-bismetalation reactions of dienes have
been reported. For Si-Si, see: (a) Matsumoto, H.; Shono, K.; Wada, A.;
Matsubara, I.; Watanabe, H.; Nagai, Y. J. Organomet. Chem. 1980, 199, 185-
193. (b) Tsuji, Y.; Lago, R. M.; Tomohiro, S.; Tsuneishi, H. Organometallics
1992, 11, 2353-2355. For Si-Sn, see: (c) Tsuji, Y.; Obora, Y. J. Am. Chem.
Soc. 1991, 113, 9368-9369. For B-B, see: (d) Ishiyama, T.; Yamamoto,
M.; Miyaura, N. Chem. Commun. 1996, 2073-2074.
(5) For Si-Si, see: (a) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. Chem.
Lett. 1975, 887-890. (b) Tamao, K.; Okazaki, S.; Kumada, M. J. Organomet.
Chem. 1978, 146, 87-93. (c) Carlson, C. W.; West, R. Organometallics 1983,
2, 1801-1807. (d) Sakurai, H.; Eriyama, Y.; Kamiyama, Y.; Nakadaira, Y.
J. Organomet. Chem. 1984, 264, 229-237. (e) Finckh, W.; Tang, B.-Z.; Lough,
A.; Manners, I. Organometallics 1992, 11, 2904-2911. (f) Obora, Y.; Tsuji,
Y.; Kawamura, T. Organometallics 1993, 12, 2853-2856. For Sn-Sn, see:
(g) Tsuji, Y.; Kakehi, T. J. Chem. Soc., Chem. Commun. 1992, 1000-1001.
For B-B, see: ref 4d.
(6) Suginome, M.; Nakamura, H.; Ito, Y. Chem. Commun. 1996, 2777-
2778.
(7) Suginome, M.; Nakamura, H.; Ito, Y. Angew. Chem., Int. Ed. Engl.
1997, 36, 2516-2518.
The unexpected B-C and C-C bond formation along with
Si-O bond formation took place even at 80 °C to give 7a with
high (95:5) stereoselectivity in 78% yield (Table 1, entry 2). The
reaction at lower temperature (50 °C) improved the diastereose-
lectivity up to 98:2, but resulted in decrease in yield (Table 1,
entry 3). Molar amount of the aldehyde employed was also
important to attain high yield and stereoselectivity (Table 1, entries
4 and 5). Use of 3 molar amounts of 5a gave the highest yield
(8) Hoffmann, R. W.; Zeiss, H.-J. Angew. Chem., Int. Ed. Engl. 1979, 18,
306-307. Hoffmann, R. W.; Zeiss, H.-J. J. Org. Chem. 1981, 46, 1309-
1314.
S0002-7863(98)00373-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/22/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 17, 1998 4249
Table 2. Reaction of Dienes (2), Aldehydes (5), and Silylborane
1 in the Presence of Platinum Catalyst 3a
Scheme 2. Possible Mechanism for the Silaborative Coupling
of Dienes to Aldehydes
dienes
aldehydes
temp products 7
entry
(R1, R2)
(R′)
(°C)b yield (%) ratio
1
2
3
4
2b (H,Ph)
2b
2b
2b
2b
2b
5a (Ph)
120
120
120
120
120
120
50
b (79)
c (77)
d (83)
e (80)
f (71)
g (60)
h (63)
i (60)
99:1
99:1
99:1
99:1
93:7
96:4
95:5
99:1
5b (p-MeOPh)
5c (o-MeOPh)
5d (p-NCPh)
5e (n-Hex)
5f (c-Hex)
5
6
7c,d
8c
2c (H,H)
2d (-(CH2)4-) 5a (Ph)
5a (Ph)
80
a All reactions were carried out in octane using 1.5 equiv of aldehyde
unless otherwise noted. b Bath temp. c 3.0 equiv of aldehydes. d A
reaction in hexane under atmospheric pressure of 1,3-butadiene.
Chart 1
as well as high stereoselectivity (Table 1, entry 5), while use of
1 molar equivalent of 5a resulted in an unsatisfactory yield with
slight decrease in stereoselectivity (Table 1, entry 4).
The silaborative diene-aldehyde coupling reaction was ap-
plicable to unsymmetrical 2-phenyl-1,3-butadiene (2b), though
higher temperature was required for the completion of the
reactions. Thus, in the reaction with 5a, the B-C and C-C bond
formation occurred at the less substituted CdC bond with another
CdC bond having the phenyl group left intact (Table 2, entry 1).
Benzaldehyde derivatives having methoxy group at either para
(5b) and ortho (5c) position as well as that having a p-cyano
group (5d) similarly gave the corresponding coupling products 7
with the high regio- and stereoselectivities in good yields (Table
2, entries 2-4). Diene 2b also reacted with aliphatic aldehyde
5e in 71% yield with a diastereoselectivity of 93:7 (Table 2, entry
5). Higher selectivity was attained in the reaction of branched
aliphatic aldehyde 5f (Table 2, entry 6). It is noted that a reaction
of 1 and 5a under butadiene (2c) atmosphere (1 atm) at 50 °C
furnished the corresponding coupling product 7h in 63% yield
with 95:5 stereoselectivity (Table 2, entry 7). The silaborative
coupling to 5a also took place with diene 2d, in which two CdC
bonds are fixed in a s-cis fashion, giving 7i with almost complete
diastereoselectivity (Table 2, entry 8). Treatment of 7i with basic
hydrogen peroxide followed by acetonidation afforded 8 as a
single diastereomer (eq 2).
(silyl)(diene)platinum(II) intermediate A. Insertion of the coor-
dinated diene to the B-Pt bond takes place with regioselective
B-C bond formation at the terminal carbon of the less-substituted
CdC bond to give (4-borylcrotyl)(organosilyl)platinum(II) com-
plex B having a cis geometry. The cis-crotylplatinum complex
B can react with an aldehyde with C-C bond formation at the
position γ to the platinum atom to give (alkoxy)(silyl)platinum-
(II) C,10 which undergoes a reductive elimination to give the
coupling product 7 with Si-O bond formation. The finding that
the silaborative diene-aldehyde coupling reaction proceeds at
lower temperature than the silaboration of dienes in the absence
of aldehydes suggests that the reductive elimination of a Si-C
bond from B is sufficiently slow to allow the further reaction of
B with aldehydes. The observed high stereoselectivity may be
attributed to the cyclic transition state, in which platinum serves
as intramolecular Lewis acid (Chart 1). Presumably, in the
absence of the aldehydes, the cis-crotylplatinum intermediate B
may be gradually isomerized to the trans isomer, resulting in the
formation of the E alkene via the subsequent reductive elimination.
In summary, a new catalytic C-C bond forming reaction
involving the regio- and stereoselective formation of allylic
platinum intermediates from 1,3-dienes through the activation of
the Si-B bond has been reported. The highly stereoselective
reaction of the platinum intermediate with aldehydes may lead
to further exploitation of new synthetically useful reactions.
It is remarked again that the s-cis diene 2d reacted with 1 in
the absence of aldehydes to give 1,4-silaboration product 9 in
high yield (eq 3).
Acknowledgment. This study was partially supported by a Grant-
in-Aid for Scientific Research on Priority Areas, “The Chemistry of Inter-
element Linkage” (09239226) from Ministry of Education, Science,
Sports, and Culture, Japan. H.N. acknowledges fellowship support of the
Japan Society for the Promotion of Science for Japanese Young Scientists.
Supporting Information Available: Detailed experimental proce-
dures, characterization of the new compounds, and stereochemical
assignment of 7 (7 pages, print/PDF). See any current masthead page
for ordering information and Web access instructions.
JA980373O
The finding that the platinum-catalyzed reaction did not take
place with 1 and 5a in the absence of diene 2a suggests that some
reactive platinum intermediate may be formed from silylborane
1 and dienes in the presence of the platinum catalyst. We propose
the following mechanism for the platinum-catalyzed silaborative
diene-aldehyde coupling (Scheme 2).9 Oxidative addition of the
Si-B bond onto the platinum(0) complex and subsequent
coordination of a diene with s-cis conformation may form (boryl)-
(9) The proposed mechanism, involving the generation of nucleophilic
allylic transition-metal intermediate from diene, may be related to intramo-
lecular, hydrosilylative diene-aldehyde coupling reaction catalyzed by a Ni
catalyst. See: Sato, Y.; Takimoto, M.; Hayashi, K.; Katsuhara, T.; Takagi,
K.; Mori, M. J. Am. Chem. Soc. 1994, 116, 9771-9772.
(10) Nakamura, H.; Iwama, H.; Yamamoto, Y. J. Am. Chem. Soc. 1996,
118, 6641-6647.