Selective mixed coupling of carboxylic acids (I). - Electrolysis, thermolysis and photolysis of unsymmetrical diacyl peroxides with acyclic and cyclic alkyl groups

Abstract

14 Unsymmetrical diacyl peroxides (R 1CO 2-O 2CR 2 with R 1: undecyl; R 2: e.g. methyl, propyl, pentyl, nonyl, 2-methylpropyl, 2-propyl, 2-pentyl, cyclopropyl, cyclobutyl, cyclohexyl) are prepared in 85 to 92 % yield. Square pulse electrolysis of dodecanoyl octanoyl peroxide ( li ) affords the unsymmetrical coupling product octadecane( 4 )ins poor yield and selectivity. Thermolysis or photolysis in solution produces preferentially 4 , but also considerable amounts of disproportionation products. At −78°C the neat peroxides are photolysed selectively to the mixed dimers. With straight chain and β-branched alkyl groups high yields are obtained (63 – 76 %), with cycloalkyl groups medium yields (42 – 56 %), and with α-branched diacyl peroxides moderate yields (20 – 33 %). A comparison of the mixed Kolbe-electrolysis with the low temperature photolysis of the neat peroxide demonstrates the superiority of the latter method in small scale conversion with regard to yield and selectivity.