Acylarylnitrosamines. Part X. An electron spin resonance spectroscopic investigation of the reactions of aryl radicals with tetraphenylcyclopentadienone and 2,3-diarylindenones : detection of 1-oxo-2,2,3-triphenylindenyl and related radicals and their relevance to dienonepromoted formation of benzyne from benzenediazonium acetate

Abstract

Decomposition of benzenediazonium acetate, formed in situ in benzene from N-nitrosoacetanilide (NNA), in the presence of tetraphenylcyclopentadienone (tetracyclone) gives e.s.r. signals corresponding to new long lived radicals (4) formed by addition of a phenyl radical to position 2 of tetracyclone. Similar reactions of NNA with 2,3-diphenylindenone gave long lived signals corresponding to the 1-oxo-2,2,3-triphenylindenyl radical (5), an assignment supported by computer simulation. The use of dibenzoyl peroxide or pentyl nitrite–aniline as alternative sources of phenyl radicals also led to the formation of radical (5). The assignments of structures (4) and (5) were supported by analysis of the related signals obtained from reactions of NNA with 2-phenyl-3-(p-t-butylphenyl)- and 3-phenyl-2-(p-t-butylphenyl)-indenone, and by reactions of p-t-butyl-NNA with 2,3-diphenylindenone and with 2-phenyl-3-(p-t-butylphenyl)indenone. Related studies with 2-methyl-3,4,5-triphenyl-, 2,5-diethyl-3,4-diphenyl-, and 3,4-diphenyl-2,5-di-p-tolylcyclo-pentadienones were also carried out. These results with those described in the preceding paper lend support to the suggestion that benzyne promotion from NNA by tetracyclone and 2,3-diphenylindenone is explicable on the basis of inhibition of the competing NNA-derived radical chain phenylation of the solvent.