Nickel-catalyzed silaboration of small-ring vinylcycloalkanes: Regio- and stereoselective (E)-allylsilane formation via C-C bond cleavage

Article abstract of DOI:10.1021/om020007k

The nickel-catalyzed silaboration of small-ring vinylcycloalkanes was analyzed. These reactions provided (ω-borylalkyl)-substituted allylsilanes in a highly regio- and stereoselective manner. The results highlight the relatively unexplored bis- and hydrometalation chemistry of vinylcycloalkanes.

Full text of DOI:10.1021/om020007k

Organometallics 2002, 21, 1537-1539
1537
Nick el-Ca ta lyzed Sila bor a tion of Sm a ll-Rin g
Vin ylcycloa lk a n es: Regio- a n d Ster eoselective
(E)-Allylsila n e F or m a tion via C-C Bon d Clea va ge
Michinori Suginome,*,† Takanori Matsuda, Takayo Yoshimoto, and
Yoshihiko Ito*,‡
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, and PRESTO, J apan Science and
Technology Corporation (J ST), Kyoto 606-8501, J apan
Received J anuary 2, 2002
Ta ble 1. Nick el-Ca ta lyzed Rea ction of VCP s 2 w ith
1a
Summary: Vinylcyclopropanes and vinylcyclobutanes
undergo regio- and stereoselective reaction with silyl-
borane in the presence of nickel catalysts, giving (ω-
borylalkyl)-substituted (E)-allylsilanes via cleavage of the
carbon-carbon σ-bond in the rings.
The versatile utility of silylborane has recently been
demonstrated in transition-metal-catalyzed additions,1
C-C coupling,2 and C-C cleavage reactions,3 which lead
to the synthesis of regio- and stereodefined organic
compounds containing both silicon and boron.4,5 In
comparison with other bimetals such as disilanes,1a,6
diborons,7 and silylstannanes,1a the high but control-
lable reactivities in the presence of a variety of transi-
tion-metal catalysts including Ni, Pd, and Pt complexes
are highly remarkable for the silylboranes. The sila-
boration products thus obtained are attractive as new
tools in organic synthesis, since many methods have
been developed for the selective functionalization not
only at the silyl but also at the boryl moieties.1c,8,9 Here,
we report new nickel-catalyzed silaboration of vinyl-
cyclopropanes (VCPs), which is accompanied by cleavage
of C-C bonds in the cyclopropane ring. Although
transition-metal-catalyzed cycloadditions of VCPs have
gained much attention in synthetic organic chemistry,10
little is known about their reactivity in transition-metal-
entry
VCP (R1, R2)
equivb PCy3/Nic product % yield of 3d
1
2
3
4
5
6
7
8
9
2a (H, Ph)
2a
2a
1.5
1.5
1.5
3.0
3.0
3.0
1.5
1.5
1.5
3.0
1.5
3.0
0
1
2
1
1
1
1
1
1
1
1
1
3a
3a
3a
3a
3b
3c
3d
3e
15e
69
<30e
89
2a
2b (H, p-FC6H4)
2c (H, Me)
2d (Ph, H)
2e (n-Bu, H)
2f (Ph, Me)
2g (Ph, Ph)
2h (H, c-Pr)
2h
81
74
82
84 (87)f
3f
3g
56
44
38
61
10
11
12
3h + 4g
3h + 4h
a
Unless otherwise noted, reactions were carried out on a 0.8
mmol scale at 90 °C for 2-8 h in toluene in the presence of a nickel
catalyst prepared from Ni(acac)2 (5 mol %) and DIBAH (5 mol %)
with or without PCy3. Molar equiv of 2 based on 1. c Molar ratio
b
d
of the added PCy3 and Ni(acac)2. Isolated yield based on 1 unless
otherwise noted. e NMR yield. f The value in parentheses indicates
g
the yield obtained in a 4.0 mmol scale reaction. 3h :4 ) 76:24
(1H NMR). 3h :4 ) 86:14 (1H NMR).
h
catalyzed bis- or hydrometalations.11 We also describe
how the same protocol was applied to vinylcyclobutanes
(VCBs). These reactions provided (ω-borylalkyl)-substi-
tuted allylsilanes in a highly regio- and stereoselective
manner.
† E-mail: suginome@sbchem.kyoto-u.ac.jp.
‡ Present address: Faculty of Pharmaceutical Science, Kyoto Phar-
maceutical University, Yamashina, Kyoto 607-8412, J apan.
(1) (a) Review: Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221.
(b) Suginome, M.; Matsuda, T.; Yoshimoto, T.; Ito, Y. Org. Lett. 1999,
1, 1567. (c) Suginome, M.; Ohmori, Y.; Ito, Y. J . Am. Chem. Soc. 2001,
123, 4601.
Initially, VCP 2a (R1 ) H, R2 ) Ph) was reacted with
silylborane 1 in toluene at 90 °C in the presence of nickel
catalysts generated from Ni(acac)2 with diisobutyl-
aluminum hydride (DIBAH) (eq 1; Table 1, entries 1-4).
The reactions afforded silaboration product 3a in vary-
ing yields, in which the proximal carbon-carbon bond
in the cyclopropane ring was concomitantly cleaved.
Whereas 3a was formed only in low yield in the absence
of phosphine ligand (entry 1), addition of PCy3 (tri-
cyclohexylphosphine, 1 equiv to Ni) improved the yield
to 69% (entry 2).12 Although further increase in the P/Ni
ratio lowered the yield significantly (entry 3), use of
(2) (a) Suginome, M.; Nakamura, H.; Matsuda, T.; Ito, Y. J . Am.
Chem. Soc. 1998, 120, 4248. (b) Suginome, M.; Matsuda, T.; Ito, Y.
Organometallics 1998, 17, 5233.
(3) (a) Suginome, M.; Matsuda, T.; Ito, Y. J . Am. Chem. Soc. 2000,
122, 11015. (b) Pohlmann, T.; de Meijere, A. Org. Lett. 2000, 2, 3877.
(4) For noncatalyzed reactions of silylboranes, see: (a) Buynak, J .
D.; Geng, B. Organometallics 1995, 14, 3112. (b) Suginome, M.;
Fukuda, T.; Nakamura, H.; Ito, Y. Organometallics 2000, 19, 719. (c)
Hata, T.; Kitagawa, H.; Masai, H.; Kurahashi, T.; Shimizu, M.;
Hiyama, T. Angew. Chem., Int. Ed. 2001, 40, 790.
(5) For the convenient synthesis of (dimethylphenylsilyl)pinacolbo-
rane, see: Suginome, M.; Matsuda, T.; Ito, Y. Organometallics 2000,
19, 4647.
(6) Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351.
(7) (a) Marder, T. B.; Norman, N. C. Top. Catal. 1998, 5, 63. (b)
Ishiyama, T.; Miyaura, N. J . Organomet. Chem. 2000, 611, 392.
(8) Colvin, E. W. Silicon in Organic Synthesis; Krieger: Malabar,
1985.
(11) For Rh-catalyzed hydrosilylation of VCPs, see: Bessmertnykh,
A. G.; Blinov, K. A.; Grishin, Y. K.; Donskaya, N. A.; Beletskaya, I. P.
Tetrahedron Lett. 1995, 36, 7901.
(12) PBu3 was a little less effective than PCy3, whereas reactions
using PPh3, PCyPh2, P(t-Bu)3, or PMe2Ph resulted in lower yields
(trace-10%) of 3a than that in the absence of the phosphine ligands.
(9) Matteson, D. S. Stereodirected Synthesis with Organoboranes;
Springer: Berlin, 1995.
(10) Wender, P. A.; Barzilay, C. M.; Dyckman, A. J . J . Am. Chem.
Soc. 2001, 123, 179, and references therein.
10.1021/om020007k CCC: $22.00 © 2002 American Chemical Society
Publication on Web 03/21/2002

1538 Organometallics, Vol. 21, No. 8, 2002
Communications
Sch em e 1
more equivalents of VCP 2a gave 3a in sufficiently high
yield (entry 4). It is noteworthy that the silaboration of
VCP proceeded with highly selective Si-C and B-C
bond formations at the terminal vinyl carbon and the
2-position of the cyclopropane ring of 2a , respectively,
with cleavage of the 1,2-C-C bond in the cyclopropane
ring. Furthermore, the formal 1,5-addition of silylborane
provided the allylsilane 3a , in which the geometry of
the CdC bond was 100% trans. Under the optimized
reaction conditions, VCP 2b and 2c bearing a substitu-
ent R2 at the internal vinyl carbons afforded the
corresponding products 3b and 3c selectively in good
yields (entries 5 and 6). VCPs 2d and 2e having a
substituent (R1) at the terminal vinyl carbons provided
respective products 3d and 3e in high yields even with
use of smaller amounts of VCPs (entries 7 and 8).13
Although the presence of more substituents on the vinyl
group slightly lowered the product yields, the regio- and
stereoselectivity were very high (entries 9 and 10).
Reaction of 1,1-dicyclopropylethene (2h ) afforded a
double addition product 4 as a minor product along with
the major formation of 3h (eq 2; entry 11). The ratio of
3h to 4 could be improved to 86:14 by use of a larger
amount of 2h (entry 12).
the P/Ni ratio of 1:1 (ca. 35% yield each based on 2a ).
The change in the product distribution implies that the
two reactions proceeded through different reaction
pathways. A possible mechanism for the silaboration of
VCPs may involve formation of (silyl)(boryl)nickel(II)
intermediate A via oxidative addition of 1 onto a Ni(0)
complex (Scheme 1). Coordination of VCPs onto the
nickel (to form B) may be followed by migratory B-C
bond formation with cleavage of the C-C bond, giving
allylnickel species C. Subsequent reductive elimination
of the Si-C bond proceeds facilely only at the allylic
carbon cis to the silyl group without possible syn-anti
as well as cis-trans isomerization of C, leading to the
highly regioselective formation of 3. The exclusive
formation of the trans-alkenes may be rationalized by
the preference for s-trans conformation of VCPs in the
migratory B-C bond formation step.10 The undesired
VCP-cyclopentene isomerization may arise from direct
interaction of VCPs with the Ni(0) species. The proposed
catalytic cycle seems to be most probable in comparison
with the other mechanistic possibilities from the fol-
lowing considerations: (1) Most stoichiometric reactions
of VCPs with transition-metal complexes are driven by
the formation of π-allyl complexes;17 (2) the observed
insensitivity of the reaction rate and yield to the
substitution pattern at the CdC bonds of VCPs suggests
that the insertion of the CdC is not involved in the cycle;
and (3) our previous study on the Ni-catalyzed sila-
boration reaction strongly suggests that exclusive mi-
gration of the boryl group rather than the silyl group of
the (silyl)(boryl)nickel(II) to the coordinated organic
substrate is involved in the cycle.2b,18
We found that the nickel-catalyzed silaboration of
VCP 2a was accompanied by ring-enlargement isomer-
ization to 1-phenylcyclopentene.14 Although nickel-
catalyzed VCP-cyclopentene isomerization of VCPs
having additional CdC or cyclopropyl groups was also
demonstrated by using a Ni(cod)2-PBu3 (1:1) catalyst,
no reaction took place for 2a .15 In sharp contrast to that
report, the isomerization of 2a was found to proceed in
high yield (>95%) with the Ni(acac)2-DIBAH-PCy3 (1:
1:1) system, even in the absence of the silylborane (eq
3).16
To elucidate the mechanistic relationship between the
two competing Ni-catalyzed reactions of VCPs, i.e.,
silaboration and isomerization, we carried out the
silaboration of 2a (1.5 equiv) with varying PCy3/Ni
ratios (1:1-4:1; Ni: 5 mol %) at 90 °C for 1 h. Interest-
ingly, the formation of 5 became dominant (ca. 90% yield
based on 2a ; 3a :5 ) 1:9) with the P/Ni ratio of 4:1,
whereas ca. 1:1 formation of 3a and 5 was observed for
Application of the present silaboration reaction to
vinylcyclobutanes was quite successful (eq 4; Table 2).
Although higher temperature and concentration as well
as larger catalyst loading were required, the reaction
of VCBs (1.5-3.0 equiv) with 1 afforded 7a -c with
100% trans CdC double bonds in good yields via
cleavage of the C-C bond in the cyclobutane ring.19,20
The characteristic regioselectivity for the introduction
(13) VCPs 2d and 2e were used as mixtures of E and Z isomers.
E/Z ratios: 76/24 for 2d ; 7/93 for 2e.
(14) Hudlicky, T.; Reed, J . W. In Comprehensive Organic Synthesis,
Trost, B. M., Fleming, I., Paquette, L. A., Eds.; Pergamon: Oxford,
1991; Vol. 5, p 899.
(15) Murakami, M.; Nishida, S. Chem. Lett. 1979, 927.
(16) Besides 2a (>95%), VCPs 2b (>95%), c (<5%), f (40%), g (50%),
and h (90%) underwent the nickel-catalyzed isomerization under the
same reaction conditions. The values in parentheses indicate the
conversion of the VCPs after 8 h at 90 °C. As for reactions of VCPs 2d
and 2e, products were not identifiable due to the low conversion. In
these reactions, cis isomers were more rapidly consumed than the
corresponding trans isomers.
(17) Khusnutdinov, R. I.; Dzhemilev, U. M. J . Organomet. Chem.
1994, 471, 1.
(18) We have proposed that not only the Ni-catalyzed silaborations,
but also Pd- and Pt-catalyzed silaborations generally proceed via
insertion into the boron-metal bond.1-3 Recently, Ozawa et al. reported
that stoichiometric reaction of cis-(PhMe2Si)[(pin)B]Pt(PR3)2 with
phenylacetylene proceeded via exclusive insertion of the triple bond
into the B-Pt bond, giving cis-(PhMe2Si)[(pin)BHCdCPh]Pt(PR3)2 in
high yield, which subsequently underwent reductive elimination
cleanly on heating. Sagawa, T. ; Asano, Y.; Ozawa, F. The 48th
Symposium on Organometallic Chemistry, J apan, Yokohama, Sep-
tember 18-19, 2001; Abstract PA234.

Communications
Organometallics, Vol. 21, No. 8, 2002 1539
Ta ble 2. Nick el-Ca ta lyzed Rea ction of VCBs 6 w ith
1a
isomerization, selective double-bond isomerization oc-
curred to give methylenecyclobutane derivative 8 with-
out any detectable VCB-cyclohexene isomerization. In
the absence of 1, almost quantitative formation of 8 was
observed under the same reaction conditions.
In summary, we report that new silaboration reac-
tions of VCPs and VCBs were efficiently catalyzed by
nickel complexes. The silaboration was accompanied by
C-C bond cleavage of the cyclopropane and cyclobutane
ring, resulting in highly regio- and stereoselective
formation of (ω-borylalkyl)-substituted (E)-allylsilanes.
These findings may open up new possibilities of rela-
tively unexplored bis- and hydrometalation chemistry
of vinylcycloalkanes.
entry
VCP (R1, R2)
equivb
product (% yield)c
1
2
3
6a (H, Ph)
6b (Ph, Me)
6c (Ph, Ph)
3.0
3.0
1.5
7a (65)
7b (69)
7c (77)
a
Reactions were carried out on a 0.4 mmol scale at 110 °C in
the presence of a nickel catalyst prepared from Ni(acac)2 (20 mol
%) and DIBAH (20 mol %). b Molar equiv of 6 based on 1. c Isolated
yield based on 1.
of the silyl and the boryl groups to the allylic and
ω-positions, respectively, indicates that the silaboration
of VCBs proceeds via essentially the same mechanism
as that proposed for VCP silaboration. It is interesting
to note that the VCB silaboration was accompanied by
a side reaction that was quite different from that for
VCP silaboration (eq 5). Unlike the VCP-cyclopentene
Ack n ow led gm en t. T.M. thanks the J apan Society
for Promotion of Science for fellowship support.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectral data for the new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(19) For palladium-mediated reactions of vinylcyclobutanes, see:
Larock, R. C.; Varaprath, S. J . Org. Chem. 1984, 49, 3435. Larock, R.
C.; Yum, E. K. Tetrahedron 1996, 52, 2743.
(20) For rhodium-catalyzed intramolecular [6 + 2] cycloadditions
of 2-vinylcyclobutan-1-ones to alkenes, see: Wender, P. A.; Correa, A.
G.; Sato, Y.; Sun, R. J . Am. Chem. Soc. 2000, 122, 7815.
OM020007K