Page 480 - Advanced Organic Chemistry Part B - Reactions & Synthesis

reaction, so they are evidently not intermediates. The Clemmensen reaction works 453
best for aryl ketones and is less reliable with unconjugated ketones. The mechanism
is not known in detail but may involve formation of carbon-zinc bonds at the metal SECTION 5.7
surface. 273 The reaction is commonly carried out in hot concentrated hydrochloric acid Reductive Deoxygenation
of Carbonyl Groups
with ethanol as a cosolvent. These conditions preclude the presence of acid-sensitive
or hydrolyzable functional groups. A modification in which the reaction is run in ether
saturated with dry hydrogen chloride gave good results in the reduction of steroidal
ketones. 274

Zn, HCl
ether
O

The Wolff-Kishner reaction 275 is the reduction of carbonyl groups to methylene
groups by base-catalyzed decomposition of the hydrazone of the carbonyl compound.
It is thought that alkyldiimides are formed and then collapse with loss of nitrogen. 276

– N
–
R C N NH 2 + – OH R C N NH R C N N H 2 R CH 2
2
2
2
2
H
The reduction of tosylhydrazones by LiAlH or NaBH also converts carbonyl groups
4
4
to methylene. 277 It is believed that a diimide is involved, as in the Wolff-Kishner
reaction.
NaBH H H
R C NNHSO Ar 4 R CHN N SO 2 Ar R CHN NH R CH 2
2
2
2
2
2
Excellent yields can also be obtained using NaBH CN as the reducing agent. 278
3
The NaBH CN can be added to a mixture of the carbonyl compound and
3
p-toluenesulfonylhydrazide. Hydrazone formation is faster than reduction of the
carbonyl group by NaBH CN and the tosylhydrazone is reduced as it is formed.
3
Another reagent that can reduce tosylhydrazones to give methylene groups is
CuBH PPh . 279
4 3 2
Reduction of tosylhydrazones of -unsaturated ketones by NaBH CN gives
3
alkenes with the double bond located between the former carbonyl carbon and the
-carbon. 280 This reaction is believed to proceed by an initial conjugate reduction,
followed by decomposition of the resulting vinylhydrazine to a vinyldiimide.
Ar N NH
NNHSO Ar NHNHSO 2 – N
2
NaBH 3 CN 2 RCH CH CHR′
RCH CHCR′ RCH CH CR′ RCH CH CR′ 2
2
2
273 M. L. Di Vona and V. Rosnatti, J. Org. Chem., 56, 4269 (1991).
274 M. Toda, M. Hayashi, Y. Hirata, and S. Yamamura, Bull. Chem. Soc. Jpn., 45, 264 (1972).
275
D. Todd, Org. React., 4, 378 (1948); Huang-Minlon, J. Am. Chem. Soc., 68, 2487 (1946).
276 T. Tsuji and E. M. Kosower, J. Am. Chem. Soc., 93, 1992 (1971). Alkyldiimides are also converted to
hydrocarbons by a free radical mechanism; A. G. Myers, M. Movassaghi and B. Zheng, Tetrahedron
Lett., 38, 6569 (1997).
277
L. Caglioti, Tetrahedron, 22, 487 (1966).
278
R. O. Hutchins, C. A. Milewski, and B. E. Maryanoff, J. Am. Chem. Soc., 95, 3662 (1973).
279 B. Milenkov and M. Hesse, Helv. Chim. Acta, 69, 1323 (1986).
280
R. O. Hutchins, M. Kacher, and L. Rua, J. Org. Chem., 40, 923 (1975).