The Diverse Carbenic and Cationic Chemistry of 3-Diazo-2,5-diphenylpyrrole

Article abstract of DOI:10.1021/jo00175a012

3-Diazo-2,5-diphenylpyrrole (1) thermolyzes and photolyzes to 2,5-diphenyl-3H-pyrrolylidene (3), which inserts into methylene hydrogen of cyclohexane and methyne hydrogen of cumene.Hydrogen abstraction to give 2,5-diphenylpyrrole (7) occurs competitively in these systems.Carbene 3 reacts with cyclohexene, allylbenzene, and 2,3-dimethyl-2-butene to give 3-(allylically substituted)-2,5-diphenylpyrroles (15, 20, 21, and 29) as the only products of olefin incorporation along with 7.The initial position of the double bond in the olefin may be altered in the overall insertion process, and cyclopropanes are not isolable.The apparent behavior of 3 with saturated and olefinic hydrocarbons is as singlet 8s and triplet 9t.Reactions of 3 with anisole (31a) and with toluene (31b), benzenes substituted by electron-donor groups, result in selective ortho and/or para substitution to give 2,5-diphenyl-3-(substituted-phenyl)pyrroles (38a, 35, and 38b) and in hydrogen abstraction to 7.Insertion into the methyl groups and hydrogen abstraction also occur in reactions of 3 with 31b, yielding 3-benzyl-2,5-diphenylpyrrole (39) and 1,2-diphenylethane (40).Benzene (42a), however, reacts thermally or photolytically with 1 to form 1,3-diphenyl-2H-cyclooctapyrrole (46a), a member of a new heterocyclic system.Ring expansions to 4-, 5-, and 6-cyano-1,3-diphenyl-2H-cyclooctapyrroles (46b, 46b', and 46b'') and 4-, 5-, and 6-nitro-1,3-diphenyl-2H-cyclooctapyrroles (46c, 46c', and 46c'') are the principal reactions of 3 with benzonitrile (42b) and nitrobenzene (42c). 3-(m-Nitrophenyl)-2,5-diphenylpyrrole (47b) is also formed from 1 and 42c at 170 deg C.Thermolysis and photolysis of 1 to effect substitution and ringexpansion of benzenes may involve electrophilic attack of 8s to form spiropyrrolonorcaradienes (32).Directed heterolytic ring opening of 32 and (1,5 sigmatropic) rearrangements of hydrogen will rationalize the selective ortho and/or para substitution processes.Cyclooctapyrroles may arise from (electrocyclic) isomerization of 32 to spirocycloheptatrienes 44, (1,5 sigmatropic) rearrangement involving ring expansion to 45, and then hydrogen migration.Triplet photosensitization of 1 in 42a and 42b leads to 2,3,5-triphenylpyrrole (47a) and 3-(o-cyanophenyl)-2,5-diphenylpyrrole (47c), products of aromatic substitution rather than ring expansion.Such photolytic processes may involve generation and then addition of 9t to 42a and 42b, spin inversion of the triplet to singlet diradical intermediates, and successive hydrogen migrations.Aniline (59a), N-methylaniline (59b), and N,N-dimethylaniline are nucleophiles in that they are pyrrylated on nitrogen by 1 at 180 deg C.Primary and secondary alcohols and 1 undergo oxidation/reduction to carbonyls and 7; conversion to 3-alkoxy-2,5-diphenylpyrroles is minor except in the presence of ...