Nickel-catalyzed silaborative dimerization of alkynes

Article abstract of DOI:10.1021/om9807250

Double insertion of alkynes into the Si-B bond of (dimethylphenylsilyl)pinacolborane proceeded in the presence of nickel(0) catalysts to give cis,cis-1-silyl-4-boryl-1,3-butadiene derivatives in a regio- and stereo-selective manner. Involvement of (silyl)(2-boryl-1-alkylvinyl)nickel(II) intermediates in the silaborative dimerization reaction was suggested by the regiochemical preference as well as on the basis of reactions of the corresponding germylborane with alkynes in the presence of Ni, Pd, and Pt catalysts.

Full text of DOI:10.1021/om9807250

Organometallics 1998, 17, 5233-5235
5233
Nick el-Ca ta lyzed Sila bor a tive Dim er iza tion of Alk yn es
Michinori Suginome, Takanori Matsuda, and Yoshihiko Ito*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Kyoto 606-8501, J apan
Received August 27, 1998
Summary: Double insertion of alkynes into the Si-B
bond of (dimethylphenylsilyl)pinacolborane proceeded in
the presence of nickel(0) catalysts to give cis,cis-1-silyl-
4-boryl-1,3-butadiene derivatives in a regio- and stereo-
selective manner. Involvement of (silyl)(2-boryl-1-alkyl-
vinyl)nickel(II) intermediates in the silaborative dimeri-
zation reaction was suggested by the regiochemical
preference as well as on the basis of reactions of the
corresponding germylborane with alkynes in the presence
of Ni, Pd, and Pt catalysts.
aldehydes.9 The appropriate choice of the metal as well
as the ligands was crucially important for attaining not
only the high yields but also the high regio- and
stereoselectivities. Herein, we describe new dimeriza-
tion reactions of alkynes with formation of a B-C bond
as well as a Si-C bond through the activation of the
silicon-boron bond by nickel(0) catalysts.10
Reaction of silylborane 1 with 2.5 molar equiv of
1-hexyne was carried out at 80 °C in the presence of a
catalyst prepared from 5 mol % of Ni(acac)2 with
diisobutylaluminum hydride (DIBAH) as a reductant
(Ni/Al ) 1:2) (eq 1).11,12 The reaction afforded silabo-
Catalytic activation of σ-bonds between metallic ele-
ments such as boron, silicon, germanium, and tin has
gained much attention, leading to development of new
synthetic methods for organometallic compounds con-
taining boron and group 14 elements.1-4 It has been
reported that palladium and platinum complex catalysts
are effective for the activation of the interelement bonds,
enabling the addition reactions across carbon-carbon
multiple bonds as well as σ-bond metathesis reactions.1
In contrast, nickel catalysts have hardly achieved the
successful σ-bond activation except for the Si-Si bonds
with neighboring participating groups5 and those in the
highly strained small rings.6 Recently, we reported that
regioselective addition of a silicon-boron bond of si-
lylborane across carbon-carbon triple bonds was cata-
lyzed by palladium complex catalysts in a highly regio-
and stereoselective manner.7 Furthermore, platinum
complex catalysts were effective for the activation of the
silicon-boron bond, promoting regioselective silabora-
tion of alkenes8 and silaborative coupling of dienes to
rative dimerization products 2a and 3a in 50% yield as
a 3:1 mixture along with a minor amount of silaboration
product 4a (Table 1; entry 1). NOE experiments
revealed that the Z,Z-isomer 2a was formed as a major
isomer with head-to-head dimerization of alkyne, while
the Z,Z-isomer 3a was formed as a minor isomer with
head-to-tail dimerization. No products derived from
tail-to-tail or tail-to-head13 dimerization nor those with
trans CdC bonds were formed in the reaction. More-
over, the minor silaboration product 4a was obtained
as a single isomer, which was identical to that obtained
in the palladium-catalyzed silaboration of 1-hexyne.7
(1) For Si-Si bonds, see: (a) Sharma, H. K.; Pannell, K. H. Chem.
Rev. 1995, 95, 1351-1374. (b) Horn, K. A. Chem. Rev. 1995, 95, 1317-
1350. (c) Suginome, M.; Ito, Y. J . Chem. Soc., Dalton Trans. 1998,
1925-1934.
(2) For B-B bonds, see: (a) Ishiyama, T.; Matsuda, N.; Miyaura,
N.; Suzuki, A. J . Am. Chem. Soc. 1993, 115, 11018-11019. (b)
Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.;
Miyaura, N. Organometallics 1996, 15, 713-720. (c) Baker, R. T.;
Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew. Chem., Int. Ed. Engl.
1995, 34, 1336-1337. (d) Ishiyama, T.; Kitano, T.; Miyaura, N.
Tetrahedron Lett. 1998, 39, 2357-2360, and references therein.
(3) For Si-Sn bonds, see: (a) Mitchell, T. N.; Killing, H.; Dicke, R.;
Wickenkamp, R. J . Chem. Soc., Chem. Commun. 1985, 354-355. (b)
Chenard, B. L.; Zyl, C. M. V. J . Org. Chem. 1986, 51, 3561-3566. (c)
Tsuji, Y.; Obora, Y. J . Am. Chem. Soc. 1991, 113, 9368-9369. (d)
Murakami, M.; Amii, H.; Takizawa, N.; Ito, Y. Organometallics 1993,
12, 4223-4227, and references therein.
(9) Suginome, M.; Nakamura, H.; Matsuda, T.; Ito, Y. J . Am. Chem.
Soc. 1998, 120, 4248-4249.
(10) Nickel-catalyzed hydrosilative dimerization of terminal alkynes
was reported, see: (a) Lappert, M. F.; Takahashi, S. J . Chem. Soc.,
Chem. Commun. 1972, 1272. (b) Lappert, M. F.; Nile, T. A.; Takahashi,
S. J . Organomet. Chem. 1973, 72, 425-439. For an application to
diynes, see: (c) Tamao, K.; Kobayashi, K.; Ito, Y. J . Am. Chem. Soc.
1989, 111, 6478-6480.
(11) Typical procedure for the Ni-catalyzed silaborative dimerization
of alkynes: To a mixture of Ni(acac)2 (7.7 mg, 0.030 mmol) and alkyne
(2.5-6.0 equiv based on 1 employed) in toluene (0.15 mL) was added
DIBAH (11 µL, 0.060 mmol) at 0 °C.; the mixture was stirred for 30
min at 0 °C. To the mixture was then added 1 (158 mg, 0.60 mmol) at
room temperature under nitrogen; the mixture was stirred at 80 °C
for 48 h. The mixture was diluted with hexane and stirred under air
for 6 h at room temperature. Filtration of the mixture and evaporation
of the volatile material followed by column chromatography on silica
gel or preparative GPC gave the products.
(12) Use of n-BuLi or MeMgBr as a reductant gave results identical
to that with DIBAH.
(13) The tail-to-head silaborative dimerization would give the diene
product with a boryl group at the substituted alkyne carbon.
(4) For Sn-B bonds, see: Onozawa, S.-y.; Hatanaka, Y.; Sakakura,
T.; Shimada, S.; Tanaka, M. Organometallics 1996, 15, 5450-5452.
(5) For hydrodisilanes, see: Okinoshima, H.; Yamamoto, K.; Ku-
mada, M. J . Am. Chem. Soc. 1972, 94, 9263-9264. For vinyldisilanes,
see: Ishikawa, M.; Nishimura, Y.; Sakamoto, H.; Ono, T.; Ohshita, J .
Organometallics 1992, 11, 483-484.
(6) Ishikawa, M.; Naka, A. Synlett 1995, 794-802.
(7) Suginome, M.; Nakamura, H.; Ito, Y. Chem. Commun. 1996,
2777-2778.
(8) Suginome, M.; Nakamura, H.; Ito, Y. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2516-2518.
10.1021/om9807250 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 11/04/1998

5234 Organometallics, Vol. 17, No. 24, 1998
Communications
Sch em e 1. P ossible Mech a n ism for
Nick el-Ca ta lyzed Sila bor a tive Dim er iza tion of
Alk yn es
Ta ble 1. Rea ction s of Silylbor a n e 1 w ith Ter m in a l
Alk yn es in th e P r esen ce of Nick el Ca ta lystsa
alkyne R
(equiv)
selectivityb,c yield/%d regiob
entry
additive
(2 + 3)/4
of 2 + 3
2/3
1
2
Bu (2.5)
Bu (4.0)
Bu (6.0)
Bu (4.0)
Bu (4.0)
c-Pen (4.0)
none
none
none
none
PBu3
none
94/6
92/8
96/4
93/7
96/4
87/13
50
64
78
52
68
39
76/24
74/26
75/25
75/25
75/25
59/41
3
4e
5
6
a
Ni(acac)2 (5 mol %) and DIBAH (10 mol %) were used unless
otherwise noted. Determined by 1H NMR of the reaction mix-
b
tures. c Isolated yields. In all cases, the compounds 4 were
d
obtained as single isomers. e Ni(acac)2 (2 mol %) and DIBAH (4
mol %) were used.
The regiochemical outcome revealed that the boryl
groups were exclusively introduced to the terminal
alkynyl carbon of 1-hexyne for all the products.
The yields of the dimerization products were improved
by use of a large excess of the alkyne (entries 2 and 3);
78% yield was attained by use of 6 molar equiv of
1-hexyne. The reaction also proceeded with a catalyst
loading as low as 2 mol % of Ni complex but in slightly
lower yield (entry 4). Although additives such as PPh3
and P(OEt)3 resulted in significant retardation of the
reaction, P(n-Bu)3 on the nickel catalyst resulted in
slight improvement of the yield for the dimerization
products (entry 5). The dimerization of a terminal
alkyne with the bulkier cyclopentyl group (R ) cyclo-
Pen) required higher temperature (110 °C) to give
dienes 2b and 3b in 39% yield with lower regioselec-
tivity (entry 6).
tive dimerization of alkynes is illustrated in Scheme 1.
The (silyl)(boryl)Ni(II) intermediate A, which may be
formed by oxidative addition of the Si-B bond onto the
Ni(0) complex, undergoes cis-insertion of a terminal
alkyne into the B-Ni bond with highly regioselective
Ni-C bond formation at the internal alkynyl carbon to
give (silyl)(2-boryl-1-alkylvinyl)nickel(II) intermediate
B. The following insertion of alkyne into the Si-Ni
bond of B takes place with the moderate regiochemical
preference for the Ni-C bond formation at the internal
alkynyl carbon to give C and C′, whose reductive
elimination furnishes silaborative dimerization products
2 and 3, respectively. Another possibility for the second
insertion, in which the alkyne inserts into the Ni-C
bond of B to form (silyl)(dienyl)Ni(II) species, may be
excluded, since the resultant (silyl)(dienyl)Ni(II) species
should be susceptible to further insertion of alkynes into
the Ni-C bond to give silaborative trimerization prod-
ucts, which were not indeed detected in the reaction
mixture.17 The insertion of alkyne into the Si-Ni bond
of B may be much faster than the insertion into the
B-Ni bond of A, since the single-insertion product 4 was
obtained only in low yield. Moreover, attempted silabo-
rative cross-dimerization of 1-hexyne and diphenyl-
acetylene afforded 5d in moderate yield together with
the homodimerization products 2a and 3a in 22% total
yield without formation of any other cross-dimerization
products (eq 4). Thus, diphenylacetylene, which is a
stronger ligand for nickel(0) than 1-hexyne but is
reluctant to insert into the Ni-B bond of A, may be
selectively inserted into the Ni-Si bond of B.
Internal alkynes also afforded the silaborative dimer-
ization products in higher yields. Thus, 3-hexyne and
5-decyne (2.5 equiv) furnished (Z,Z)-dimerization prod-
ucts 5a and 5b in 90% and 88% yields, respectively, in
the presence of the nickel catalyst (eq 2).14 Moreover,
1,8-bis(tert-butyldimethylsilyloxy)-4-octyne afforded the
corresponding diene 5c in good yield. It is remarked
that diphenylacetylene, one of the most reactive internal
alkynes in the palladium-catalyzed silaboration,7 was
inert under the identical conditions.
The silaborative dimerization was applicable to in-
tramolecular cyclization of diyne 6, leading to dimeth-
ylenecyclohexane derivative 7 in 55% yield (eq 3).15,16
(15) In the absence of PBu3, 7 was obtained in 33% yield under
otherwise identical conditions.
(16) Similar silaborative cyclization was reported in the reaction of
6 with a silylborane having an N,N′-dimethylethylenediamine ligand
on the boron atom in the presence of a palladium catalyst. See:
Onozawa, S.-y.; Hatanaka, Y.; Tanaka, M. Chem. Commun. 1997,
1229-1230.
(17) For the hydrosilative dimerization, Lappert and Takahashi
proposed a mechanism involving a first insertion of alkyne into the
Ni-H bond of a (hydride)(silyl)Ni(II) intermediate followed by a second
insertion into the newly formed Ni-C bond. See ref 10.
A possible mechanism for the Ni-catalyzed silabora-
(14) Single-insertion products corresponding to 4 were also formed
in 5-8% yields.

Communications
Organometallics, Vol. 17, No. 24, 1998 5235
Ta ble 2. Rea ction s of Ger m ylbor a n e 8 w ith
1-Hexyn e in th e P r esen ce of Nick el, P a lla d iu m ,
a n d P la tin u m Com p lexes
9 yield/% 10 yield/%
entry
catalyst
(ratio)d
(ratio)e
9/10f
1
2
3
Ni(acac)2/DIBAHa
Pd(acac)2/t-OcNCb
Pt(CH2dCH2)(PPh3)2
74 (74/26) trace
96/4
39 (96/4) 46 (>99/1) 47/53
c
Related reactions of germylborane 8 with 1-hexyne,
which were found to be catalyzed by not only nickel
complex but also palladium and platinum complexes (eq
5, Table 2), may be relevant to the reaction mechanism.
0
87 (91/9) <1/99
a
Ni(acac)2 (0.05 eq), DIBAH (0.10 eq), 1-hexyne (4.0 eq), and 8
b
were reacted at 80 °C. Pd(acac)2 (0.02 eq), 1,1,3,3-tetramethyl-
butyl isocyanide (0.08 eq), 1-octyne (2.5 eq), and 8 were reacted
at 110 °C. c Pt(CH2dCH2)(PPh3)2 (0.02 eq), 1-octyne (1.5 eq), and
d
8 were reacted at 80 °C. Regioisomeric ratios of head-to-head and
head-to-tail dimers. e Regioisomeric ratios of 1-boryl and 2-boryl
derivatives. f Determined by 1H NMR of the crude reaction
mixtures.
the corresponding germyl-palladium bond of B′ ([M] )
Pd), which is, in turn, less susceptible to undergoing the
alkyne insertion than the corresponding germyl-nickel
bond of B′ ([M] ) Ni).
In summary, silaborative dimerization of 1-alkyne
was achieved by nickel complex catalyst. We proposed
a mechanism involving the (silyl)(boryl)nickel(II) com-
plex intermediate, which is formed via oxidative addi-
tion of the silicon-boron bond and undergoes stepwise
insertion of 1-alkyne into the boron-nickel bond and
then the silicon-nickel bond.
The nickel catalyst also promoted the germaborative
dimerization of 1-hexyne, producing 9 selectively as a
3:1 mixture of regioisomers corresponding to 2 and 3 in
moderate yield (entry 1). On the other hand, the
palladium-isonitrile complex catalyst, which afforded
only a single-insertion product 4 in the reaction of
1-hexyne with silylborane 1, gave a 1:1 mixture of 9 and
10 in high total yield in the reaction with germylborane
8 (entry 2). However, the platinum-triphenylphos-
phine complex catalyzed the germaboration of 1-hexyne,
producing 10 exclusively in high yield (entry 3).
The product selectivities in the germaboration reac-
tions of 1-hexyne might be rationalized by a possible
involvement of (germyl)(2-boryl-1-alkylvinyl)[M] com-
plexes B′ ([M] ) Ni, Pd, Pt), which undergo either
reductive elimination giving 10 or further insertion of
1-hexyne into the germyl-[M] bond followed by reduc-
tive elimination from the resultant bis(alkenyl)[M]
intermediate C′′ leading to 9 (eq 6). The results
summarized in Table 2 may suggest that the germyl-
platinum bond of B′ ([M] ) Pt) is relatively less
susceptible to the insertion of 1-hexyne leading to 9 than
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research on Priority Areas,
“The Chemistry of Inter-element Linkage” (09239226)
from the Ministry of Education, Science, Sports, and
Culture, J apan.
Su p p or tin g In for m a tion Ava ila ble: Text giving detailed
experimental procedures and characterization data for the new
compounds (5 pages). Ordering information is given on any
current masthead page.
OM9807250