Novel generation of parent, alkyl, dialkyl and alicyclic nitrenium ions in photolyses of pyridinium, quinolinium, bipyridinium and phenanthrolinium salts and aromatic N-substitution by nitrenium ions

Abstract

The photolyses of 1-aminopyridinium, 1-aminoquinolinium, 1-amino-2,2′-bipyridinium, 1,1 ′-diamino-2,2′-bipyridinium and 1-amino-1,10-phenanthrolinium salts 1 and 10–13 in arene–trifluoroacetic acid (TFA) mixtures gave aromatic N-substitution products such as anilines and toluidines, and radical products such as benzyltoluene and (dimethylbenzyl)mesitylene. The effect of heavy atoms and O₂ indicates that the N-substitution and radical products are characteristic of a singlet and a triplet species, respectively. The intermediacy of a nitrenium ion species was suggested by a Hammett plot with ρ– 1.9 and by the exclusion of other mechanisms. Selectivity factors (S_f), log[2 ×(%para)/(%meta)], for the formation of toluidines are influenced by both thesubstituenton the pyridine ring of the substrate 1 and the nature of the counter-ion. This demonstrates that the parent nitrenium ion is not free but interacts intimately with both the heterocyclic nitrogen atom and the counter-ion. The interaction with the heterocyclic nitrogen stabilises selectively the singlet state rather than the triplet to favour aromatic amination. Similarly the photolyses of 1-(amino, methylamino, dimethylamino and piperidino)-2-methyl-4,6-diphenylpyridinium tetrafluoroborates 1m–p in benzene–TFA gave the corresponding anilines. Photolyses of substrates 1m and 1n in toluene–TFA yielded the toluidine derivatives. However, photolyses of substrates 1o and 1p in TFA–toluene or –mesitylene did not produce the toluidine derivatives, but instead gave benzyltoluene and (dimethylbenzyl)mesitylene. These allowed us to propose that the singlet reaction giving the aromatic amines is prevented by a fast triplet reaction with toluene or mesitylene according to Le Chatelier's principle when the nitrenium ion has a very small S–T energy gap; the singlet state of the dialkyl or alicyclic nitrenium ion (having a smaller energy gap than the parent or alkyl nitrenium ion) is selectively stabilised by interaction with the heterocyclic nitrogen, resulting in a very low S–T energy gap. The proposal is well supported by the fact that a very low S–T energy gap of a five-membered intermediate gives an unusually low yield of toluidines and a relatively high yield of benzyltoluene in the photolysis of compound 13 in toluene–TFA.