Page 365 - Advanced Organic Chemistry Part B - Reactions & Synthesis

PhSeOSO Ar ArSO 2 O H PhSe H 337
2
CH (CH ) C CH +
3
2 7
SePh CH (CH ) OSO Ar SECTION 4.5
2
CH (CH ) 3 2 7
2 7
3
Addition at Double
PhSeOSO Ar ArSO O CH 3 85% 55:45 Bonds via Organoborane
2
2
Intermediates
PhC CCH 3
Ph SePh 75%
Some of the most synthetically useful addition reactions of alkynes are with
organometallic reagents, and these reactions, which can lead to carbon-carbon bond
formation, are discussed in Chapter 8.
4.5. Addition at Double Bonds via Organoborane Intermediates

4.5.1. Hydroboration

Borane, BH , having only six valence electrons on boron, is an avid electron pair
3
acceptor. Pure borane exists as a dimer in which two hydrogens bridge the borons.

H H H
B B
H H H

In aprotic solvents that can act as electron pair donors such as ethers, tertiary amines,
and sulfides, borane forms Lewis acid-base adducts.

+ – + – + –
R O BH 3 R N BH 3 R S BH 3
3
2
2
Borane dissolved in THF or dimethyl sulfide undergoes addition reactions rapidly with
most alkenes. This reaction, which is known as hydroboration, has been extensively
studied and a variety of useful synthetic processes have been developed, largely through
the work of H. C. Brown and his associates.
Hydroboration is highly regioselective and stereospecific. The boron becomes
bonded primarily to the less-substituted carbon atom of the alkene. A combination of
steric and electronic effects works to favor this orientation. Borane is an electrophilic
reagent. The reaction with substituted styrenes exhibits a weakly negative value
+
(−0 5). 156 Compared with bromination =−4 3 , 157 this is a small substituent
effect, but it does favor addition of the electrophilic boron at the less-substituted end of
the double bond. In contrast to the case of addition of protic acids to alkenes, it is the
boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is
reinforced by steric factors. Hydroboration is usually done under conditions in which
the borane eventually reacts with three alkene molecules to give a trialkylborane. The

156 L. C. Vishwakarma and A. Fry, J. Org. Chem., 45, 5306 (1980).
157
J. A. Pincock and K. Yates, Can. J. Chem., 48, 2944 (1970).