Page 1171 - Advanced Organic Chemistry Part B - Reactions & Synthesis

A related method for conversion of carboxylic acids to bromides with decar- 1147
boxylation is the Hunsdiecker reaction. 276 The usual method for carrying out this
transformation involves heating the carboxylic acid with mercuric oxide and bromine. SECTION 12.6
Selective Oxidative
HgO Cleavages at Functional
CO H Br Groups
2
Br 2 41– 46% Ref. 277
The overall transformation can also be accomplished by reaction of thallium(I)
carboxylate with bromine. 278 Phenyliodonium diacetate and bromine also lead to
brominative decarboxylation. 279

CCH )
PhI(O 2 3 2
Br , hν Br
2
CO H Br 56%
2
H
CO 2
1,2-Dicarboxylic acids undergo bis-decarboxylation on reaction with lead tetraac-
etate to give alkenes. This reaction has been of occasional use for the synthesis of
strained alkenes.

O
O
O
Pb(OAc) 4
pyridine, O
O 80°C
O
O
O
O 39% Ref. 280
The reaction can occur by a concerted fragmentation process initiated by a two-electron
oxidation.
O R R O R R
IV
II
H O C C C C O Pb (OAc) 3 2 CO + C C Pb (OAc) + CH CO H
2
2
2
3
R R
R R R
A concerted mechanism is also possible for -hydroxycarboxylic acids, and these
compounds readily undergo oxidative decarboxylation to ketones. 281
O H
R C R C O + CO + Pb (OAc) + CH CO H
II
2
3
2
2
2
2
IV
C O Pb (OAc) 3
O
276
C. V. Wilson, Org. React., 9, 332 (1957); R. A. Sheldon and J. Kochi, Org. React., 19, 326 (1972).
277 J. S. Meek and D. T. Osuga, Org. Synth., V, 126 (1973).
278
A. McKillop, D. Bromley, and E. C. Taylor, J. Org. Chem., 34, 1172 (1969).
279
P. Camps, A. E. Lukach, X. Pujol, and S. Vazquez, Tetrahedron, 56, 2703 (2000).
280 E. Grovenstein, Jr., D. V. Rao, and J. W. Taylor, J. Am. Chem. Soc., 83, 1705 (1961).
281
R. Criegee and E. Büchner, Chem. Ber., 73, 563 (1940).