Scheme 2
temperature and the use of 1.3 equiv. of iodine under similar
It is well known that the reaction of boron compounds with
organometallic compounds, where the metal is significantly
more electropositive than boron, results in transmetallation to
give organoboranes.4 As an example of transmetallation of
alkenyl groups, alkenyldialkylboranes can be obtained by
treatment of alkenylalanes with B-methoxydialkylboranes.6 On
the other hand, when the electronegativity of the metal is close
to that of boron, the direction of transmetallation can be
reversed under appropriate conditions. Organomercury com-
pounds are typical of those having such reversibility. For
example, treatment of alkenylboranes with mercury(II) acetate
followed by sodium chloride yields alkenylmercuric chloride
with retention of configuration.7 The present reaction is, to the
best of our knowledge, the first case of the transfer of an alk-
1-enyl group from boron to aluminium.
In conclusion, we have successfully developed a novel
pathway to (E)-alk-1-enyldiisobutylalanes 2 via the reaction of
(E)-alk-1-enyldicyclohexylboranes 1 with DIBAL-H in the
presence of hex-1-ene. This transfer of an alk-1-enyl group is
achieved with complete retention of configuration at the double
bond under very mild conditions. The preparation of 2 from
conjugated alk-1-ynes can be performed efficiently, making this
a superior method to the hydroalumination reaction.1 We are
currently investigating the scope and limitations of this
reaction.
conditions to the literature2 gave the best result in which the
desired product (E)-1-iododec-1-ene (3a) was obtained in 78%
yield based on the starting amount of dec-1-yne. The 1H NMR
spectrum of 3a revealed that only one pair of signals appeared
at d 5.97 (dt, J 14.3, 1.5 Hz) and 6.51 (dt, J 14.3, 7.0 Hz) in the
alkenyl region, hence the geometry of the product was assigned
to be E-configuration. Although it is well known that treatment
of 1a with iodine produces not 3a but rather (Z)-1-cyclohex-
yldec-1-ene via a sequence of addition–migration–elimination
reactions,5 this product was not observed in the reaction
mixture. Bromination of 2a derived from 1a was also explored
and, in consequence, N-bromosuccinimide (NBS) proved to be
more selective and efficient than bromine which brought about
the cleavage not only of alkenyl carbon–aluminium bond but of
the isobutyl carbon–aluminium bond.2 Thus, consecutive
reaction of 1a with 1.2 equiv. of DIBAL-H and 1.1 equiv. of
NBS under the same conditions as described above afforded
(E)-1-bromodec-1-ene (4a) in 82% yield based on the starting
amount of dec-1-yne. The geometry of 4a was assigned to be E-
configuration by its 1H NMR spectrum where siganls at d 6.01
(d, J 13.4 Hz) and 6.15 (dt, J 13.4 and 6.4 Hz) were
observed.
Table 1 shows the results of iodination and bromination of 2
prepared via transfer reaction of 1 derived from alkynes with
substituents, including bulky alkyl, phenyl, alkenyl and chloro-
alkyl groups. Halogenations proceeded smoothly to provide the
corresponding (E)-1-haloalk-1-enes in good yields. The process
tolerates such functional groups as alkenyl and chloroalkyl. It is
noteworthy that transformation of conjugated alk-1-ynes such
as phenylethyne and cyclohexenylethyne into (E)-1-haloalk-
1-enes through alkenylalane intermediates has been realised
with satisfactory results.
Notes and references
1 For example, see: (a) G. Zweifel, in Comprehensive Organic Chemistry,
ed. D. Barton and W. D. Ollis, Pergamon, London, 1979, vol. 3, pp.
1013–1041; (b) J. J. Eisch, in Comprehensive Organometallic Chemistry
II, ed. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, New
York, 1995, vol. 11, pp. 277–311.
Table 1 Iodination and bromination of (E)-alk-1-enyldiisobutylalanes 2
derived from (E)-alk-1-enyldicyclohexylboranes 1a
2 G. Zweifel and C. C. Whitney, J. Am. Chem. Soc., 1967, 89,
2753–2754.
3 M. Hoshi, K. Shirakawa and A. Arase, Chem. Commun., 1998,
1225–1226.
Yield of productsb (%)
4 For example, see: (a) A. Pelter and K. Smith, in Comprehensive Organic
Chemistry, ed. D. Barton and W. D. Ollis, Pergamon, London, 1979, vol.
3, pp. 791–940; (b) A. Pelter, K. Smith and H. C. Brown, in Borane
Reagents, Academic, London, 1988; (c) D. S. Matteson, in Stereodirected
Synthesis with Organoboranes, Springer, Berlin, 1995.
5 G. Zweifel, H. Arzoumanian and C. C. Whitney, J. Am. Chem. Soc., 1967,
89, 3652–3653.
6 E. Negishi and L. D. Boardman, Tetrahedron Lett., 1982, 23,
3327–3330.
7 (a) R. C. Larock and H. C. Brown, J. Organomet. Chem., 1972, 36, 1–12;
(b) R. C. Larock, S. K. Gupta and H. C. Brown, J. Am. Chem. Soc., 1972,
94, 4371–4373.
R
3
4
n-C8H17
t-C4H9
Ph
Cyclohexenyl
Cl(CH2)3
a
b
c
d
e
78
81
80
83
72
82
82
87
a Both iodination and bromination were carried out at 250 °C to room
temperature using 1.3 equiv. of iodine or 1.1 equiv. of NBS. b GLC yields
based on alk-1-yne employed.
CHEM. COMMUN., 2002, 2146–2147
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