Page 485 - Advanced Organic Chemistry Part B - Reactions & Synthesis

458 Similarly, carbanionic character to a leaving group can lead to -fragmentation.

CHAPTER 5 Y X + + + Y –
–
2e – X
Reduction of
Carbon-Carbon Multiple
Bonds, Carbonyl
Groups, and Other A classical example of the -elimination reaction is the reductive debromination of
Functional Groups vicinal dibromides. Zinc metal is the traditional reducing agent. 291 A multitude of other
reducing agents have been found to give this and similar reductive eliminations. Some
examples are given in Table 5.7. Some of the reagents exhibit anti stereospecificity,
whereas others do not. A stringent test for anti stereospecificity is the extent of
Z-alkene formed from a syn precursor.

X X
R′ R′
R R
R R′
Y Y

Anti stereospecificity is associated with a concerted reductive elimination, whereas
single-electron transfer fragmentation leads to loss of stereospecificity and formation
of the more stable E-stereoisomer.

X –
X – . . e – R′
R′ e R′ R′
R R R R
Y Y Y

As vicinal dibromides are usually made by bromination of alkenes, their utility
for synthesis is limited, except for temporary masking of a double bond. Much more
frequently it is desirable to convert a diol to an alkene, and several useful procedures
have been developed. The reductive deoxygenation of diols via thiono carbonates was

Table 5.7. Reagents for Reductive Dehalo-
genation
Reagent Anti stereoselectivity
a Yes
Zn, cat TiCl 4
b
Zn, H 2 NSNH 2 ?
SnCl 2 , DiBAlH c ?
Sm, CH 3 OH d No
Fe, graphite e Yes
f
C 2 H 5 MgBr, cat Ni dppe Cl 2 No
a. F. Sato, T. Akiyama, K. Ida, and M. Sato, Synthesis, 1025
(1982).
b. R. N. Majumdar and H. J. Harwood, Synth. Commun., 11,
901 (1981).
c. T. Oriyama and T. Mukaiyama, Chem. Lett., 2069 (1984).
d. R. Yanada, N. Negoro, K. Yanada, and T. Fujita, Tetra-
hedron Lett., 37, 9313 (1996).
e. D. Savoia, E. Tagliavini, C. Trombini, and A. Umani-
Ronchi, J. Org. Chem., 47, 876 (1982).
f. C. Malanga, L. A. Aronica, and L. Lardicci, Tetrahedron
Lett., 36, 9189 (1995).
291
J. C. Sauer, Org. Synth., IV, 268 (1965).