Page 1156 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1132 O O
Cr(VI)
PhCH CPh PhCCPh + PhCH + PhCO H + PhCH CHPh
CHAPTER 12 2 2
Oxidations O 7 O CPh CPh 8
O O

Both the diketone and the cleavage products were shown to arise from an
-hydroxyketone intermediate (benzoin) 9.

O
H 2 CrO 4 PhCH CPh + Cr(IV) products
PhCH 2 CPh PhCH CPh Ph CH CH Ph
H 2 O O CrO 3 H OH O
OH 9
The coupling product is considered to involve a radical intermediate formed by one-
electron oxidation, probably effected by Cr(IV). Similarly, the oxidation of cyclohex-
anone involves 2-hydroxycylohexanone and 1,2-cyclohexanedione as intermediates. 208

O O O
OH O CO 2 H
Cr(VI) H
CO 2

Owing to the efficient oxidation of alcohols to ketones, alcohols can be used as the
starting materials in oxidative cleavages. The conditions required are more vigorous
than for the alcohol to ketone transformation (see Section 12.1.1).
Aldehydes can be oxidized to carboxylic acids by both Mn(VII) and Cr(VI).
Fairly detailed mechanistic studies have been carried out for Cr(VI). A chromate ester
of the aldehyde hydrate is believed to be formed, and this species decomposes in
the rate-determining step by a mechanism similar to the one that operates in alcohol
oxidations. 209
OH
RCH O + H Cr (VI) O 4 RC O CrO H RCO H + [Cr (IV) O H] – + H +
3
3
2
2
H
Effective conditions for oxidation of aldehydes to carboxylic acids with KMnO 4
involve use of t-butanol and an aqueous NaH PO buffer as the reaction medium. 210
4
2
Buffered sodium chlorite is also a convenient oxidant. 211 Both KMnO and NaClO 2
4
can be used in the form of solid-supported materials, using silica and ion exchange
resins, respectively, 212 which permits facile workup of the product. Silver oxide is one
of the older reagents used for carrying out the aldehyde to carboxylic acid oxidation.
208 J. Rocek and A. Riehl, J. Org. Chem., 32, 3569 (1967).
209 K. B. Wiberg, Oxidation in Organic Chemistry, Part A, Academic Press, New York, 1965, pp. 172–178.
210
A. Abiko, J. C. Roberts, T. Takemasa, and S. Masamune, Tetrahedron Lett., 27, 4537 (1986).
211 E. Dalcanale and F. Montanari, J. Org. Chem., 51, 567 (1986); J. P. Bayle, F. Perez, and J. Cortieu,
Bull. Soc. Chim. Fr., 565 (1996); E. J. Corey and G. A. Reichard, Tetrahedron Lett., 34, 6973 (1993);
P. M. Wovkulich, K. Shankaran, J. Kiegiel, and M. R. Uskokovic, J. Org. Chem., 58, 832 (1993);
B. R. Babu and K. K. Balasubramaniam, Org. Prep. Proc. Int., 26, 123 (1994).
212
T. Takemoto, K. Yasuda, and S. V. Ley, Synlett, 1555 (2001).