Page 466 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Reduction of acetylenes can be done with sodium in ammonia, 220 lithium in low 439
molecular weight amines, 221 or sodium in HMPA containing t-butanol as a proton
source, 222 all of which lead to the E-alkene. The reaction is assumed to involve SECTION 5.6
successive electron transfer and protonation steps. Dissolving-Metal
Reductions
R R R R H
e – S H e – S H C C
RC CR C C C C C C
R H R H R H R

5.6.2. Reductive Removal of Functional Groups

The reductive removal of halogen can be accomplished with lithium or sodium.
Tetrahydrofuran containing t-butanol is a useful reaction medium. Good results have
also been achieved with polyhalogenated compounds by using sodium in ethanol.
Cl
Cl O CCH 3
2
Cl O CCH 3
2
Cl Na, C H OH
2 5
Cl 70%
Cl
Ref. 223
An important synthetic application of this reaction is in dehalogenation of dichloro- and
dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition
reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce
deuterium at a specific site. The mechanism of the reaction involves electron transfer
to form a radical anion, which then fragments with loss of a halide ion. The resulting
radical is reduced to a carbanion by a second electron transfer and subsequently
protonated.

e – – –X – e – – S H
R X R X R R R H

Phosphate groups can also be removed by dissolving-metal reduction. Reductive
removal of vinyl phosphate groups is one method for conversion of a carbonyl
compound to an alkene. 224 (See Section 5.7.2 for other methods.) The required vinyl
phosphate esters are obtained by phosphorylation of the enolate with diethyl phospho-

rochloridate or N,N,N ,N -tetramethyldiamidophosphorochloridate. 225
O
LiNR 2 OPO(X) 2
RCH CR′ RCH CR′ Li, RNH 2 RCH CHR′
2
(X) POCl t-BuOH
2
X = OEt or NMe 2
220
K. N. Campbell and T. L. Eby, J. Am. Chem. Soc., 63, 216, 2683 (1941); A. L. Henne and K. W. Greenlee,
J. Am. Chem. Soc., 65, 2020 (1943).
221 R. A. Benkeser, G. Schroll, and D. M. Sauve, J. Am. Chem. Soc., 77, 3378 (1955).
222
H. O. House and E. F. Kinloch, J. Org. Chem., 39, 747 (1974).
223 B. V. Lap and M. N. Paddon-Row, J. Org. Chem., 44, 4979 (1979).
224 R. E. Ireland and G. Pfister, Tetrahedron Lett., 2145 (1969).
225
R. E. Ireland, D. C. Muchmore, and U. Hengartner, J. Am. Chem. Soc., 94, 5098 (1972).