Irradiation of 4-chloro-N,N-dimethylaniline in acetonitrile in the presence of benzene and of various alkenes leads to heterolytic dehalogenation and trapping of the cation. 4-(Dimethylamino)biphenyl is formed in the first case, while with alkenes β-chloroalkylanilines, stilbenes, or allylanilines are obtained depending on the alkene structure. 4-Fluoroaniline is similarly dehalogenated.Phenyl cations have been the subject of active research from the mechanistic point of view. 1 These are usually generated under proper conditions from diazonium salts 2 and have been spectroscopically characterized both in matrix 3 and in the gas phase. 4 In the few case where the products from the reaction in solution have been characterized, both reduction and addition to O-and N-centered nucleophiles have been found to occur. 2a,5 On the contrary, C-C bond formation does not involve the cation. When biaryls are formed from diazonium salts, as in the Gomberg-Bachmann or in the Pschorr reactions, 6 the actual reacting species is the aryl radical. Recently, however, Steenken and McClelland 7 reported the formation of biphenyls by irradiation of phenyldiazonium salts in the presence of benzene derivatives and showed that the process involved the cation.We are pleased to report in the following an alternative generation of aryl cations and to show that this gives rise to synthetically useful reactions.Irradiation of both 4-chloro-and 4-fluoro-N,N-dimethylaniline (1a,b) in acetonitrile caused reductive dehalogenation to 2, but important products were also the corresponding 2,4′bis(N,N-dimethylamino)biphenyls 3 (ca. 30%, see Scheme 1). This appeared a clear indication that a phenyl cation was formed under this condition.

Photoinduced hydrogen abstraction from aliphatic cyclic hydrocarbons (C(5) to C(7), C(12), as well as adamantane) by triplet aromatic ketones in the presence of alpha,beta-unsaturated (di)nitriles offers a straightforward entry to the corresponding alkylated (di)nitriles via the alkyl radicals. Yields are moderate to good depending on the olefins structure (substitution in beta slows down the addition to mononitriles, but with alpha,alpha-dinitriles electronic activation allows efficient alkylation also of beta,beta-disubstituted substrates). A tandem alkylation-cyclization process has been obtained with (1-methylpent-4-enylidene)malononitrile.

Photoinduced electron transfer (PET) from an aliphatic donor to a sensitizer and fragmentation of the radical cation leads to alkyl radicals. Radical alkylation of electron-withdrawing substituted alkenes and alkynes has been obtained in this way, and its scope has been explored. Effective sensitizers are tetramethyl pyromellitate (TMPM), 1,4-dicyanonaphthalene (in combination with biphenyl, DCN/BP), and 1,2,4,5-tetracyanobenzene. Radical precursors are tetraalkylstannanes, 2,2-dialkyldioxolanes, and, less efficiently, carboxylic acids. Steady-state and flash photolysis experiments show that escape out of cage of radical ions is the main factor determining the yield of radical formation. This is efficient with triplet sensitizers such as TMPM, while with singlet sensitizers, the use of a “cosensitizer” is required, as in the DCN/BP system. Radical cations containing primary alkyl radicals escape and fragment more efficiently than those containing tertiary radicals. The thus-formed radicals are trapped by electron-withdrawing substituted alkenes, and the relative efficiency is determined by the rate of radical addition, in accord with the proposed mechanism. Among the alkynes tested, only dimethyl acetylenedicarboxylate reacts, and the order of radical reactivity is different. It is suggested that a different mechanism operates in this case and involves assistance by the alkyne to the radical cation fragmentation.