DOI: 10.1002/anie.201106077
Olefin Isomerization
(E)- and (Z)-b-Borylallylsilanes by Alkyne Silaboration Followed by
Regio- and Stereoselective Double-Bond Migration**
Toshimichi Ohmura,* Kazuyuki Oshima, and Michinori Suginome*
Allylic boranes and silanes bearing additional boryl or silyl
groups on their double bonds serve as useful building blocks
in organic synthesis.[1–3] These reagents allow the nucleophilic
introduction of modifiable allylic groups possessing boryl or
silyl groups, which can be converted into a variety of
functional groups through oxidation, cross-coupling, halogen-
ation, etc. It is hence highly desirable to develop efficient
methods for their synthesis. b-Borylallylsilanes are one such
class of useful allylation reagents in organic synthesis.[4,5]
These compounds are utilized not only in simple Lewis acid
promoted allylation,[4] but also in cyclizative allylations[4] and
three-component reactions to form a decalin-type skeleton by
a Prins-type cyclization.[5] These particular b-borylallylsilanes
are best synthesized by the catalytic silaboration of terminal
bismetallation is synthetically much more accessible than
allene bismetallation,[1] therefore a synthetic pathway involv-
=
ing alkyne bismetallation followed by controlled C C bond
migration would be attractive (Scheme 1, right). We have
recently found that (Z)-1-boryl-2-silyl-1-alkenes, which were
prepared by palladium-catalyzed silaboration of terminal
alkynes, underwent selective Z-to-E isomerization rather
[12]
=
than C C bond migration. However, our recent study on
catalysts for the regiocomplementary silaboration of terminal
alkynes[13] prompted us to examine the possibility of the
conversion of the 2-boryl-1-silyl-1-alkenes into b-borylallylsi-
=
lanes by catalytic C C bond migration. Herein, we report on
the successful synthesis of b-borylallylsilanes by controlled
=
C C bond migration. It should be noted that the isomer-
=
allenes, in which the internal C C bond undergoes an
ization led ultimately to the thermodynamically favored
stereoisomers through selective formation of the kinetically
favored isomerization products, which were also isolable
when a modified catalyst system was used.
addition reaction in
a
highly regioselective manner
(Scheme 1, left).[6–8] This system was extended to asymmetric
The double-bond migration of b-borylalkenylsilane (E)-
1a was examined in toluene at 508C in the presence of a
palladium catalyst (1.0 mol%; Table 1).[14] No reaction took
place in the presence of [Pd(dba)2] with or without PPh3
Table 1: Palladium-catalyzed double-bond migration of (E)-1a.[a]
Scheme 1. Synthetic strategies for b-Borylallylsilane.
silaboration giving b-borylallylsilanes with high diastereo-
and enantioselectivities and these compounds were success-
fully utilized for asymmetric cyclization reactions.[9] Terminal-
selective silaboration of terminal allenes using a platinum
catalyst has also been developed.[7b,8] However, in these
syntheses, the use of allenes as the starting material is not
ideal because of their low availability and high cost. It would
be preferable to synthesize them using more readily available
starting materials.
Entry
Pd cat
Additive
–
Yield [%][b]
E/Z[c]
1
2
3
4
5
[Pd(dba)2]
[Pd(dba)2]
[Pd{P(tBu)3}2]
[Pd{P(tBu)3}2]
[Pd{P(tBu)3}2]
no reaction
no reaction
95
94
94
–
–
[d]
PPh3
We were interested in utilizing the transition-metal-
–
83:17
95:5
9:91
[e]
=
catalyzed C C bond migration in the synthesis of boryl- and
P(tBu)3
o-bromotoluene[d]
the silyl-substituted allylic boranes and silanes.[10,11] Alkyne
[a] A palladium complex (1.0 mol%), additive (0-2.0 mol%), and 1a
(0.20 mmol) in toluene (0.2 mL) was stirred at 508C for 24 h. [b] Yield of
the isolated product. [c] Determined by GC analysis of the crude mixture.
[d] 2.0 mol%. [e] 1.0 mol%. dba=dibenzylideneacetone, pin=pinaco-
lato.
[*] Dr. T. Ohmura, K. Oshima, Prof. Dr. M. Suginome
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Kyoto 615-8510 (Japan)
E-mail: ohmura@sbchem.kyoto-u.ac.jp
(Table 1, entries 1 and 2), whereas [Pd{P(tBu)3}2][15] catalyzed
the double-bond migration efficiently to give b-borylallylsi-
lane 2a in quantitative yield as a mixture of stereoisomers
(E/Z=83:17; Table 1, entry 3). To improve the stereoselectiv-
ity, we tested several additives with [Pd{P(tBu)3}2]. We found
that the stereoselective formation of the E isomer was
[**] This work was supported by Grant-in-Aid for Scientific Research on
Priority Areas (No. 20037031, “Chemistry of Concerto Catalysis”)
from the Ministry of Education, Culture, Sports, Science and
Technology (Japan). K.O. acknowledges JSPS for fellowship support.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2011, 50, 12501 –12504
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12501