Communications
DOI: 10.1002/anie.200805406
À
C H Functionalization
Stereoselective Synthesis of cis-b-Methyl- and Phenyl-Substituted
Alkenylboronates by Platinum-Catalyzed Dehydrogenative
Borylation**
Toshimichi Ohmura, Yuta Takasaki, Hideki Furukawa, and Michinori Suginome*
Increasing attention is being paid to the development of
efficient methods for the synthesis of alkenylboron com-
pounds, which serve as versatile building blocks in organic
synthesis.[1] For example, a variety of alkenylboronates that
are difficult to obtain by classical hydroboration have been
synthesized with high regio- and stereoselectivity by the
[2]
À
transition-metal-catalyzed addition of B R bonds (R = H,
S,[3] B,[4] Si,[5] Sn,[6] CN,[7] alkynyl[8]) to alkynes. Transition-
A toluene solution of 1a (R1,R2 = Et), prepared easily
from the corresponding homoallylic alcohols with
ClPh2SiB(pin),[14] was stirred at 1108C in the presence of
[Pt(dba)2] (5 mol%) and PPh3 (11 mol%; entry 1, Table 1).
The starting 1a was completely consumed after 21 h with no
intramolecular silaboration products in the reaction mixture.
We found that alkenylboronate 2a was formed by dehydro-
genative borylation in 84% yield with high stereoselectivity
(> 98:2). The configuration of the double bond in the product
was determined to be E by NOE. Compounds 1b–d derived
from homoallylic alcohols were subjected to the same
reaction conditions (entries 2–4, Table 1). Dehydroborylation
took place efficiently for both tertiary-alcohol-derived 1b and
secondary-alcohol-derived 1c (entries 2 and 3, Table 1). The
silyl group was readily removed by treatment with 1n HCl aq.
to give the corresponding alcohol 2c’ (entry 3, Table 1).
Stereoselective formation of the b,b-disubstituted alkenyl-
boronate was also achieved in the reaction of (À)-isopulegol
derivative 1d, affording 2d in 87% yield (entry 4, Table 1).
The reaction of the allylic substrate 3a[14] was then
examined (entry 5, Table 1). After optimizing the reaction
conditions,[15] we found that a platinum complex bearing the
electron-deficient phosphine ligand P(4-CF3C6H4)3 catalyzed
the reaction of 3a effectively even at 808C, giving 4a in high
yield. The reaction produced small amounts of side products,
one of which might be the stereoisomer. After hydrolysis of
À
metal-mediated C C bond formation with 1-alkynylboro-
nates[9] and vinylboronates[10] is an alternative route to
stereodefined alkenylboronates. However, stereoselective
synthesis of alkenylboranes bearing a b-methyl group cis to
the boryl group is still limited,[11] although such alkenylbor-
anes are attractive for construction of the terpenoid unit
widely found in natural products (Scheme 1).
Scheme 1. A potential synthetic intermediate of terpenoids.
In our recent study on platinum-catalyzed intramolecular
silaboration of alkenes,[12] we have found that dehydrogen-
ative borylation was the major side reaction under certain
conditions. Because dehydrogenative borylation of alkenes is
an attractive route to alkenylboranes,[13] our interest was then
focused on the efficiency and selectivity of this process. In this
paper, we describe a new platinum-catalyzed transformation
of geminally disubstituted alkenes, which results in the
À
stereoselective functionalization of alkenyl C H bonds
[Eq. (1); dba = dibenzylideneacetone, B(pin) = 4,4,5,5-tetra-
À
methyl-1,3,2-dioxaborolan-2-yl].
the Si O bond in 4a, the corresponding alcohol was obtained
as the isomerically pure form in 57% yield. Various allylic
substrates 3b–e were then reacted under the same reaction
conditions (entries 6–9, Table 1). Silylboronates 3b–d derived
from secondary and tertiary alcohols bearing sterically bulky
substituents reacted well under these conditions, giving the
corresponding alkenylboronates 4b–d in 73–86% yield
(entries 6–8, Table 1). Substrate 3e, which has a phenyl
group on the double bond, also reacted under the same
conditions to give 4e in 59% yield (entry 9, Table 1). In
contrast, reaction of the sterically less hindered 2-methyl-1-
hepten-3-ol derivative (R1 = nBu, R2 = H, R3 = Me, n = O)
suffered from a slow reaction rate and provided the desired
product in low yield (< 10%). In addition, no reaction took
[*] Dr. T. Ohmura, Y. Takasaki, H. Furukawa, Prof. Dr. M. Suginome
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2722
E-mail: suginome@sbchem.kyoto-u.ac.jp
[**] This work was supported in part by a Grant-in-Aid for Young
Scientists (B) (No. 20750076) from the Ministry of Education,
Culture, Sports, Science and Technology (Japan).
Supporting information for this article is available on the WWW
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 2372 –2375