Transfer of alk-1-enyl group from boron to boron: preparation of
B-[(E)-alk-1-enyl]-9-borabicyclo[3.3.1]nonane
Masayuki Hoshi,*† Kazuya Shirakawa and Akira Arase
Department of Applied and Environmental Chemistry, Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan
Treatment of (E)-alk-1-enyldicyclohexylborane
1
with
unreacted B-MeO-9-BBN and the other at d 3.69 arising from
those of dicyclohexylmethoxyborane. These results suggest that
the (E)-hex-1-enyl group transferred from the boron atom of 1a
to the boron atom of B-MeO-9-BBN with complete retention of
the stereochemistry. The reaction at 0 °C for 2 h improved the
ratio of 1a:2a to 8:92. The reaction with 1.2 equiv. of B-MeO-
9-BBN under otherwise identical conditions resulted in a slight
increase for 2a; the 1a:2a ratio was 6:94 (entry 1, Table 1).
However, no further improvement in the ratio was achieved by
increasing the amount of B-MeO-9-BBN to 1.5 equiv.
B-methoxy-9-borabicyclo[3.3.1]nonane (B-MeO-9-BBN) at
0 °C results in transfer of alk-1-enyl group from boron to
boron to give B-[(E)-alk-1-enyl]-9-BBN 2 with retention of
configuration.
Alkenylboranes have been widely used as one of the most
important intermediates in organic synthesis, and generally they
are prepared by hydroboration of alkynes with borane deriva-
tives.1 In some cases, however, hydroboration is not necessarily
the most reliable route. For example, a stoichiometric hydro-
boration of alk-1-yne with 9-BBN is not efficient because it
causes dihydroboration to give a significant amount of 1,1-di-
boryl adduct.2,3 In order to suppress the dihydroboration, the
reaction requires a 100% excess of alk-1-yne and must be
carried out at 0 °C for 18 h.2
We recently reported that hydroboration of alk-1-ynes with
1,3,2-benzodioxaborole (catecholborane) and hydroboration of
1-haloalk-1-ynes with 9-BBN, both of which are sluggish at
room temperature in THF,4 are accelerated by addition of a
catalytic amount (5 mol%) of dicyclohexylborane in THF to
afford B-[(E)-alk-1-enyl]catecholborane and B-[(Z)-1-haloalk-
1-enyl]-9-BBN in high yields, respectively.5 Dicyclohexyl-
borane probably plays a critical role in a catalytic cycle. In the
former reaction, it is presumed that dicyclohexylborane would
hydroborate the alk-1-yne and the resulting (E)-alk-1-enyl-
dicyclohexylborane would undergo exchange of the alk-1-enyl
group for a hydrogen atom of catecholborane to give B-[(E)-alk-
1-enyl]catecholborane with retention of configuration and
regeneration dicyclohexylborane. The latter dicyclohexyl-
borane-promoted hydroboration appears to include a similar
catalytic cycle. These exchange reactions may belong to the
category of not hydroboration but transfer reaction. It seems
probable that such transfer reactions provide a method for the
preparation of some alkyl- or alkenyl-boranes whose formation
is very difficult or inefficient via hydroboration. We report here
an efficient and stereoselective preparation of B-[(E)-alk-
1-enyl]-9-BBN 2 via treatment of (E)-alk-1-enyldicyclohex-
ylborane 1 with a slightly excess of B-MeO-9-BBN at 0 °C [eqn.
(1)].
The reaction of various compounds 1 with 1.2 equiv. of
B-MeO-9-BBN was carried out at 0 °C for 2 h. These results
including NMR data for 1 are summarised in Table 1. The
reaction of (E)-3,3-dimethylbut-1-enyldicyclohexylborane 1b
proceeded smoothly and stereoselectively to give B-[(E)-
3,3-dimethylbut-1-enyl]-9-BBN 2b; the 1b:2b ratio was 5:95
(entry 2). In the reaction of (E)-2-phenylethenyldicyclohexyl-
borane 1c the reaction mixture was analysed by 11B NMR
spectroscopy, since the alkenyl protons of B-[(E)-2-phenyl-
ethenyl]-9-BBN 2c were indistinguishable from those of 1c.
From the 11B NMR spectrum of the reaction mixture in THF–
hexanes‡ 1c was found to be converted to 2c with a high ratio
(1c:2c = 1:99) (entry 3). 3-Substituted B-[(E)-alk-1-enyl]-
9-BBN, having a functionality at a position very close to the
alkenyl moiety, may be a potential intermediate because of its
poly-functional properties. The present transfer reaction is
applicable to such functionalised (E)-prop-1-enyldicyclo-
hexylboranes without any difficulties. Thus, (E)-3-chloroprop-
1-enyldicyclohexylborane 1d and (E)-3-methoxyprop-1-enyl-
dicyclohexylborane 1e were converted to the corresponding
compounds 2 stereoselectively (entries 4 and 5). 3-Substituted
(E)-prop-1-enyldicyclohexylboranes 1f–h having an oxygen
protected with Ac, THP and TMS groups were also converted to
B-[(E)-3-acetoxyprop-1-enyl]-9-BBN 2f, B-[(E)-3-(tetrahydro-
2H-pyran-2-yloxy)prop-1-enyl]-9-BBN 2g and B-[(E)-3-(tri-
methylsiloxy)prop-1-enyl]-9-BBN 2h, respectively (entries
6–8).
Previously Brown and Gupta reported that boron–carbon
bond formation via redistribution between trialkylborane and
borate required temperatures above 100 °C.7 It should be noted
that the present reaction proceeds smoothly at 0 °C despite the
redistribution reaction involving boron–oxygen bond cleavage,
and thus appears to be applicable to transfer of alkenyl groups
containing a thermally unstable functionality.
One of the characteristic reactions of B-alkenyl-9-BBN is the
1,4-addition reaction with but-3-en-2-one.8 In situ addition of 2
to but-3-en-2-one under conditions identical to those described
in the literature gave the corresponding 4-(alk-1-enyl)butan-
2-one, while the yields were a little lower than those reported.
Thus, the present reaction is expected to be synthetically useful,
although there may still be room for improvement.
)2B
H
H
R
B
H
H
R
C C
+
(1)
)2BOMe
+
BOMe
C C
2
1
The reaction of (E)-hex-1-enyldicyclohexylborane 1a in THF
with an equimolar amount of B-MeO-9-BBN in hexanes was
carried out at 0 °C for 1 h, and the reaction mixture, after
removal of solvents, was analysed by 1H NMR spectroscopy. In
the alkenyl region, the two double triplets at d 6.20 and 6.73
arising from 1a decreased considerably while two double
triplets appeared at d 6.23 and 6.83 (J 17.3 Hz, trans alkenyl
protons), indicating that B-[(E)-hex-1-enyl]-9-BBN 2a2,6 had
been formed in a stereoselective manner, the ratio of 1a:2a
being 20:80. In the same spectrum we also observed two
singlets, one at d 3.76 arising from the methyl protons of
In conclusion, B-[(E)-alk-1-enyl]-9-BBN 2 can be produced
efficiently via the reaction of (E)-alk-1-enyldicyclohexylborane
1 with B-MeO-9-BBN. This transfer of an alk-1-enyl group is
performed with complete retention of configuration under very
mild conditions. We note that this study provides, to the best of
our knowledge, the first example of the transfer of an alk-1-enyl
group from boron to boron in stoichiometric amounts. The
Chem. Commun., 1998
1225
Hoshi, Masayuki
