Recent Developments of Free-Radical Substitutions of Heteroaromatic Bases

k R I + A r . * R. + A r I . k>109 M-~s ' (18) A l t h o x h ba th r e a c t i o n s gave good r e s u l t s i n many cases (Table 8), some l i m i t a t i o n s a re encountered wi th both r a d i c a l sources . Aroyl peroxides do r e a c t wi th t a l k p l iod ides , hu t do not produce t a l k y l rad i c a l s due t o i n compet i t ive i o n i c r e a c t i o n s . Moreover t h e thermal o r induced decomposition of a r o y l peroxides a t f i r s t produce aroyloxy r a d i c a l s , A r C O O , which r e a c t wi th s e v e r a l s u b s t r a t e s ( a c t i v a t e d aromatics, o l e f i n s , a lcohols , e t h e r s , amines) i n s t e a d o f gene ra t ing a r y l r a d i c a l s by decarboxyla t ion. HETEROCYCLES, Vol 28, No 1, 1989 The main limitation encountered with diazonium salts is due to the high addition rates of the nucleophilic radicals to the diazonium group, leading to the "free-radical diazo-coupling reaction" 27 (eq. 19). This competition can be in part restricted by keeping low the stationary concentration of the diazonium salt during the reaction. TABLE 8 Alkylation of heteroaromatic bases by diazonium salt, benzoyl peroxide and alkyl iodides 11 Heteroaromatic base Radical source Alkyl iodide Orientation Yield % a )