Page 476 - Advanced Organic Chemistry Part B - Reactions & Synthesis

OH CH 2 CO 2 CH 3 449
O
SmI 2
(CH ) CH CHCO CH 3 SECTION 5.6
2 2
2
87% Dissolving-Metal
H
Reductions
Ref. 261
SmI has also been used to form cyclooctanols by cyclization of 7,8-enones. 262 These
2
alkene addition reactions presumably proceed by addition of the ketyl radical to the
double bond, followed by a second electron transfer.

O – O –
. Sm
R R
O O – (CH ) (CH )
2 n
2 n
RC(CH ) CH CH 2 RC(CH ) CH CH 2 or
2 n
.
2 n
CH 2 . CH Sm
2
O – O –
R R
(CH ) (CH )
2 n –1
2 n –1
The initial products of such additions under aprotic conditions are organosamarium
reagents and further (tandem) transformations are possible, including addition to
ketones, anhydrides, or carbon dioxide.
HO CH 3
O
1) SmI 2
CH 2
CH C(CH ) CH CH 2 OH
3
2 3
2) cyclohexanone
80%
Ref. 263
Another reagent that has found use in pinacolic coupling is prepared from VCl 3
and zinc dust. 264 This reagent is selective for aldehydes that can form chelated interme-
diates, such as -formylamides, -amidoaldehydes, -phosphinoylaldehydes, 265 and
-ketoaldehydes. 266 The vanadium reagent can be used for both homodimerization and
heterodimerization. In the latter case, the reactive aldehyde is added to an excess of
the second aldehyde. Under these conditions, the ketyl intermediate formed from the
chelated aldehyde reacts with the second aldehyde.
R′ R′ OH
.
R CH R CH R′CH O R CH O V 2+ R CH O – R R′
.
X O X O – X O – – X O – – X OH
–
V 2+ V 3+ V 3+ V 3+
The VCl -Zn reagent has also been used in cyclization reactions, as in Entries 4 and
3
5 in Scheme 5.13.
261 E. J. Enholm and A. Trivellas, Tetrahedron Lett., 30, 1063 (1989).
262
G. A. Molander and J. A. McKie, J. Org. Chem., 59, 3186 (1994).
263
G. A. Molander and J. A. McKie, J. Org. Chem., 57, 3132 (1992).
264 J. H. Freudenberg, A. W. Konradi, and S. F. Pedersen, J. Am. Chem. Soc., 111, 8014 (1989).
265 J. Park and S. F. Pedersen, J. Org. Chem., 55, 5924 (1990).
266
A. S. Raw and S. F. Pedersen, J. Org. Chem., 56, 830 (1991).