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

The first three periodic groups
or organoaluminium reagents on borate esters (trialkoxyboranes) or on the
boron trifluoride ether complex.
B(OMe), + 3RMgX --------+ R,B + 3Mg(OMe)X
The other boron halides BX, (X= Cl, Br, 1) cleave ethers readily; this makes them
particularly unsuitable when the Grignard method is adopted.
BCl, + EtOEt --------+ EtOBCl 2 + EtC!
Partial substitution of chlorine by alkyl or aryl is conveniently achieved using
organotin reagents.
Ph 4 Sn + 2BCI, --------+ 2PhBCI 2 + Ph 2 SnCI 2
All trialkylboranes are readily oxidized in air. The lower members up to tri-nbutylborane
are spontaneously inflammable, burning with the characteristic
green boron flame. Peroxo intermediates are involved in the oxidations.
Arylboranes are rather less sensitive to oxygen.
3. 8.1 Hydroboration
By far the most widely used method for making boron-carbon bonds is by the
addition of boranes to alkenes or alkynes. Much of the work in this area is due to
H.C. Brown. This reaction is termed 'hydroboration'. Although it is reversible the
equilibrium normally lies in favour of product rather than reactants in ether
solvents at room tempera ture, so that the addition goes to completion. On heating
above ca. 100°C organoboranes with a /3-C-H function eliminate alkene, that
is, the reverse reaction takes place. Isomerization of alkylboranes occurs by a
series ofthese addition-elimination steps,leading eventually to terminal boranes.
The alkene can then be displaced by a less volatile alkene. The direction of this
/"
B-H +
"' 1 1 B-C-C-H
/ 1 1
'\_B/
1
H3 CCH 2CH 2 -CHCH3
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