Page 1140 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1116 Epoxides can also be converted to allylic alcohols using electrophilic reagents.
The treatment of epoxides with trialkyl silyl iodides and an organic base gives the
CHAPTER 12 silyl ether of the corresponding allylic alcohols. 154
Oxidations
CH 3
) SiC(CH )
(CH 3 2 3 3
O OSiCC(CH )
3 3
I
CH
N 3
N 70–80%
Similar ring openings have been achieved using trimethylsilyl triflate and 2,6-di-t-
butylpyridine. 155
Each of these procedures for epoxidation and ring opening is the equivalent of
an allylic oxidation of a double bond with migration of the double bond.
OH
R CHCH CHR′ R C CH CHR′
2
2
In Section 12.3, other means of effecting this transformation are described.

12.3. Allylic Oxidation

12.3.1. Transition Metal Oxidants

Carbon-carbon double bonds, apart from being susceptible to addition of oxygen
or cleavage, can also react at allylic positions. Synthetic utility requires that there be
good selectivity between the possible reactions. Among the transition metal oxidants,
the CrO -pyridine reagent in methylene chloride 156 and a related complex in which
3
3,5-dimethylpyrazole replaces pyridine 157 are the most satisfactory for allylic oxidation.
CH
CH 3 CH 3 CH 3
3
CrO –3,5-dimethyl-
3
pyrazole
O
Ref. 158
Several pieces of mechanistic evidence implicate allylic radicals or cations as
14
intermediates in these oxidations. Thus C in cyclohexene is distributed in the product
cyclohexenone indicating that a symmetrical allylic intermediate is involved at some
stage. 159
O
∗ . ∗ ∗ ∗ + ∗
∗ ∗ . ∗ ∗ ∗
O

154 M. R. Detty, J. Org. Chem., 45, 924 (1980); M. R. Detty and M. D. Seiler, J. Org. Chem., 46,
1283 (1981).
155
S. F. Martin and W. Li, J. Org. Chem., 56, 642 (1991).
156
W. G. Dauben, M. Lorber, and D. S. Fullerton, J. Org. Chem., 34, 3587 (1969).
157 W. G. Salmond, M. A. Barta, and J. L. Havens, J. Org. Chem., 43, 2057 (1978); R. H. Schlessinger,
J. L. Wood, A. J. Poos, R. A. Nugent, and W. H. Parson, J. Org. Chem., 48, 1146 (1983).
158 A. B. Smith, III, and J. P. Konopelski, J. Org. Chem., 49, 4094 (1984).
159
K. B. Wiberg and S. D. Nielsen, J. Org. Chem., 29, 3353 (1964).