Charles D. Murray

Acylarylnitrosamines. Part VIII. 15N-Labelling experiments and their relevance to the mechanisms of formation of benzyne from benzenediazonium acetate and of the benzenediazonium ion from hydroxyazo-compounds

Experiments using 15N-labelled N-nitrosoacetanilide and related compounds have shown that the complex reaction leading to benzyne and/or phenyl radicals does not involve the reversible extrusion of nitrogen. The contrasting observation that nitrogen exchange did occur in the formation of benzenediazonium ions by the in situ nitrosation of 15N-labelled acetanilide is attributed to the generation of unlabelled benzenediazonium ions by the reaction of phenyl radicals with the nitrosating agent. The reactions of nitrosating reagents such as p-chlorobenzoyl nitrite with 1-phenylazo-2-naphthol and related hydroxyazo-compounds to give benzenediazonium ions have been investigated, and it is suggested that nitrosation of the hydrazone from of the hydroxyazo-compound followed by rearrangement and cleavage provides the primary source of the diazonium ions but that these are again regenerated in the system from phenyl radical sand the nitrosating reagent. In accord with this, azobenzene and 4-phenylazo-1-methoxynaphthalene, which cannot exist as hydrazones do not so react, in contrast to phenylazotetrachlorocyclopentadiene, which does exist in the hydrazone form. Reactions of p-chlorobenzoyl nitrite or phentyl nitrite with 1-phenylazo-2-naphthol in benzene at 50° lead to biphenyl, via phenyl radicals, in 30 and 41% yield (82% based on azo-compound consumed).

Acylarylnitrosamines. Part IX. Use of benzyne promoters in the conversion of N-nitrosoacetanilide into benzyne

N-Nitrosoacetanilide (NNA), and hence benzenediazonium acetate, decomposes in solution to give benzyne and/or phenyl radicals. Tetraphenylcyclopentadienone diverts the decomposition to give benzyne mainly, while furan, despite being an arynophile, diverts the reaction towards phenyl radicals. The addition of 1,1-diphenylethylene, styrene, 2,3-diphenylindenone, trans-stilbene, triphenylethylene, 1,3,4-triphenylbut-2-ene-1,4-dione, or 3,4diphenylbut-3-en-2-one to the latter reaction promotes the formation of benzyne relative to phenyl radicals, even in the presence of furan. The relevance of these results to the tetracyclone effect is discussed in terms of removal of chain-initiating phenyl radicals by reaction with the unsaturated compounds thus allowing the competing eliminative formation of benzyne from benzenediazonium acetate to become dominant.

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

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.

Spin trapping by tetraphenylcyclopentadienone and 2,3-diarylindenones. New long-lived radicals and their relevance to benzyne promotion from benzenediazonium acetate

Benzenediazonium acetate, formed from N-nitrosoacetanilide in situ, in benzene in the presence of tetraphenylcyclopentadienone or 2,3-diarylindenones give e.s.r. signals corresponding to new long-lived radicals (2) and (3) formed by trapping of phenyl radicals by the dienones, thus providing evidence relating to the mechanism of benzyne promotion in these reactions.

Acylarylnitrosamines. Part VII. [2H] Labelling experiments and the mechanism of formation of benzyne from N-nitrosoacetanilide and benzenediazonium acetate

Reaction of N-nitroso[2′,4′,6′-2H3]acetanilide, formed in situ from [2′,4′,6′-2H3]acetanilide (99%[2H3]) and p-chlorobenzoyl nitrite, with tetraphenylcyclopentadienone and acetic acid in benzene gave biphenyl, from which no deuterium had been lost, and 1,2,3,4-tetraphenylnaphthalene which had lost one g atom of deuterium. Similar results were obtained from reaction of [2,4,6-2H3]benzenediazonium fluoroborate (99%[2H3]) with potassium acetate in benzene. The corresponding reaction using anthracene as a benzyne trap gave similar result. These results exclude the operation of a ‘pre-equilibrium’ type of E1cB mechanism in the formation of benzyne from the benzenediazonium acetate ion pair, but do not allow a distinction to be made between E1cB (irreversible) or concerted E2 mechanisms, particularly in view of the low (1·5–1·8) isotope effects observed with [2-2H]benzenediazonium salts.

Use of benzyne promoters in the conversion of benzenediazonium acetate into benzyne

The observations that 1,1-diphenylethylene and a series of related alkenes act as promoters for the formation of benzyne from benzenediazonium acetate even in the presence of furan, which otherwise promotes the competing radical reaction, leads also to an explanation of the hitherto puzzling promoting effect of tetraphenylcyclopentadienone.

The role of [2H]-labelling in the determination of the mechanism of formation of benzyne from benzenediazonium acetate

Contrary to a recent report,1 we find that [2H]–[1H] exchange does not occur during the formation of [2H2]- and [2H]-benzynes from [2,4,6-2H3]- and [2-2H]-benzenediazonium acetates in the presence of acetic acid.