Page 452 - Advanced Organic Chemistry Part B - Reactions & Synthesis

HOCH 2 D 425
D D C C
O SECTION 5.4
D 2
Al – D
– H Group IV Hydride
D AlOCH C CH O C H Donors
3
2
C C
H O HOCH
H 2 2 H
H D
C C
D H
The efficiency and stereospecificity of reduction is improved by using a 1:2 mixture
of LiAlH -NaOCH as the reducing agent. 174 The mechanistic basis of this effect has
4
3
not been explored in detail.
Scheme 5.7 illustrates these and other applications of the hydride donors. Entries
1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of
an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example
using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and
9 illustrate the difference in ease of reduction of alkynes with and without hydroxy
participation.

5.4. Group IV Hydride Donors

5.4.1. Reactions Involving Silicon Hydrides
Both Si−H and C−H compounds can function as hydride donors under certain
circumstances. The silicon-hydrogen bond is capable of transferring a hydride to carbo-
cations. Alcohols that can be ionized in trifluoroacetic acid are reduced to hydrocarbons
in the presence of a silane.

H OH H Ph SiH H H H
3
CF CO H
3
2
H H 92%

Ref. 175
Aromatic aldehydes and ketones are reduced to alkylaromatics under similar conditions
through reactions involving benzylic cations. 176

+ R SiH
ArCR + H + ArCR 3 ArCHR ArCHR + H O
2
CF CO H +
2
3
O OH OH
ArCHR + R 3 SiH ArCH R
+ 2
174 E. J. Corey, J. A. Katzenellenbogen, and G. H. Posner, J. Am. Chem. Soc., 89, 4245 (1967); B. B. Molloy
and K. L. Hauser, J. Chem. Soc., Chem. Commun., 1017 (1968).
175 F. A. Carey and H. S. Tremper, J. Org. Chem., 36, 758 (1971).
176
C. T. West, S. J. Donnelly, D. A. Kooistra, and M. P. Doyle, J. Org. Chem., 38, 2675 (1973); M. P. Doyle,
D. J. DeBruyn, and D. A. Kooistra, J. Am. Chem. Soc., 94, 3659 (1972); M. P. Doyle and C. T. West,
J. Org. Chem., 40, 3821 (1975).