_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Organoboron compounds
isomerization is opposite to that predicted from relative stabilities of alkenes, for
which the interna! isomers are thermodynamically favoured. It is also opposite to
that normally obtained using transition metal catalysts.
Although the reaction between diborane and an alkene proceeds only very
slowly in the gas phase, in the presence of weak Lewis bases such as diethyl ether
or dimethyl sulphide it occurs rapidly at room temperature. In early work it was
customary to generate diborane in situ with the alkene substrate from Et 2 0BF 3
and NaBH 4 dissolved in an ether such as diglyme. Diborane is not evolved until
the second reaction is complete. Nowadays a solution of diborane in THF (approx.
3NaBH 4 + 4F 3 B·0Et,----> 2B 2 H 6 + 3NaBF 4 + 4Et 2 0
7NaBH 4 + 4F,B·OEt 2 ----> NaB 2 H, + 3NaBF 4 + 4Et 2 0
1 moi dm- 3 ), which is obtained by passing the gas, generated as described, into
the solvent, is often used. It is stable for severa! months at ooc under a dry inert
atmosphere and is commercially available. Another useful borane source is the
complex Me 2 S · BH 3 which is supplied as a liquid m.p. - 40°C and which is stable
at room temperature. Dimethyl sulphide (b.p. 38 C) is volatile and is thus readily
removed from products after reaction.
The addition of B-H takes place from the less hindered side of the double bond
stereospecifically cis. The reaction is also generally quite regiospecific, boron
entering at the less substituted position. Steric effects are important in determining
the site of attack and the use of bulky boranes therefore leads to greater
selectivity. The most generally accepted mechanism involves a four centre
transition state.
Rer' H H
___. BR2 i
o
The cis addition of 'R 2 BH' to a substituted alkene can sometimes be
complemented by the photochemical cis addition of a trialkylborane to the
unsubstituted olefin, which affords the other geometrica! isomer. Redistribution
of organic groups about boron occurs very readily. Even though trialkylboranes
exist as monomers, alkyl groups readily transfer between boron atoms. Unsymmetrical
trialkylboranes are seldom sta bie above about 1 oooc.
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