Page 451 - Advanced Organic Chemistry Part B - Reactions & Synthesis

424 The occurrence of a radical intermediate is also indicated in the reduction of 2-octyl
iodide by LiAlD since, in contrast to the chloride or bromide, extensive racemization
4
CHAPTER 5 accompanies reduction.
Reduction of The presence of transition metal ions has a catalytic effect on reduction of halides
Carbon-Carbon Multiple 166
Bonds, Carbonyl and tosylates by LiAlH . Various “copper hydride” reducing agents are effective
4
Groups, and Other for removal of halide and tosylate groups. 167 The primary synthetic value of these
Functional Groups
reductions is for the removal of a hydroxy function after conversion to a halide or
tosylate.
Epoxides are converted to alcohols by LiAlH in a reaction that occurs by nucleo-
4
philic attack, and hydride addition at the less hindered carbon of the epoxide is usually
observed.

H
PhC CH 2 +LiAlH 4 PhCHCH 3
O OH

Cyclohexene epoxides are preferentially reduced by an axial approach by the nucle-
ophile. 168
H
LiAlH
) C 4 (CH ) C
(CH 3 3 3 3
O OH
O OH
LiAlH 4
(CH ) C (CH ) C
3 3
3 3
H
Lithium triethylborohydride is a superior reagent for the reduction of epoxides that are
relatively unreactive or prone to rearrangement. 169
Alkynes are reduced to E-alkenes by LiAlH . 170 This stereochemistry is comple-
4
mentary to that of partial hydrogenation, which gives Z-isomers. Alkyne reduction
by LiAlH is greatly accelerated by a nearby hydroxy group. Typically, propargylic
4
alcohols react in ether or tetrahydrofuran over a period of several hours, 171 whereas
forcing conditions are required for isolated triple bonds. 172 This is presumably the
result of coordination of the hydroxy group at aluminum and formation of a cyclic
intermediate. The involvement of intramolecular Al–H addition has been demonstrated
by use of LiAlD as the reductant. When reduction by LiAlD is followed by quenching
4
4
2
with normal water, propargylic alcohol gives Z-3- H-prop-2-enol. Quenching with
2
2
D O gives 2- H-3- H-prop-2-enol. 173
2
166 E. C. Ashby and J. J. Lin, J. Org. Chem., 43, 1263 (1978).
167 S. Masamune, G. S. Bates, and P. E. Georghiou, J. Am. Chem. Soc., 96, 3686 (1974); E. C. Ashby,
J. J. Lin, and A. B. Goel, J. Org. Chem., 43, 183 (1978).
168
B. Rickborn and J. Quartucci, J. Org. Chem., 29, 3185 (1964); B. Rickborn and W. E. Lamke, II, J.
Org. Chem., 32, 537 (1967); D. K. Murphy, R. L. Alumbaugh, and B. Rickborn, J. Am. Chem. Soc.,
91, 2649 (1969).
169 H. C. Brown, S. C. Kim, and S. Krishnamurthy, J. Org. Chem., 45, 1 (1980); H. C. Brown, S.
Narasimhan, and V. Somayaji, J. Org. Chem., 48, 3091 (1983).
170
E. F. Magoon and L. H. Slaugh, Tetrahedron, 23, 4509 (1967).
171
N. A. Porter, C. B. Ziegler, Jr., F. F. Khouri, and D. H. Roberts, J. Org. Chem., 50, 2252 (1985).
172 H. C. Huang, J. K. Rehmann, and G. R. Gray, J. Org. Chem., 47, 4018 (1982).
173
J. E. Baldwin and K. A. Black, J. Org. Chem., 48, 2778 (1983).