DYNAMICS OF ANILINIUM RADICAL ALPHA-HETEROLYTIC FRAGMENTATION PROCESSES - ELECTROFUGAL GROUP, SUBSTITUENT, AND MEDIUM EFFECTS ON DESILYLATION, DECARBOXYLATION, AND RETRO-ALDOL CLEAVAGE PATHWAYS

A single electron transfer (SET) photosensitization technique in conju nction with time-resolved, laser spectroscopy has been employed to gen erate and kinetically analyze decay processes of anilinium radicals de rived by one-electron oxidation of alpha-anilinocarboxylates, beta-ani linoalcohols, and alpha-anilinosilanes. In this manner, the races of u nimolecular decarboxylation of aniliniumcarboxylate radicals were dete rmined to be in the range 10(6)-10(7) s(-1) and dependent upon solvent polarity, the nature of the metal cation, and substituents on the ani line ring, nitrogen, and alpha-carbon. In addition, kinetic analysis o f base-induced retro-aldol fragmentations of cation radicals arising b y SET oxidation of beta-anilinoalcohols has shown that they occur with bimolecular rate constants which vary from 10(4) to 10(5) M-1 s(1). T hese values are close to those for alpha-deprotonation reactions of re lated N,N-dialkylanilinium radicals. The retro-aldol fragmentation rat es, like those for alpha-decarboxylation, also vary in a patterned way with changes in arene ring, nitrogen, and alpha- and beta-carbon subs tituents. An investigation of the dynamics of methanol-promoted reacti ons of alpha-(trimethylsilyl)methyl-substituted anilinium. radicals, h as demonstrated that a change in the nitrogen substituent from alkyl t o acyl causes an ca, 10-fold increase in the desilylation rate. Parall el photochemical studies have been conducted to gain chemical evidence to support assignment of the anilinium radical decay pathways in the LFP experiments and to demonstrate the preparative consequences of the kinetic results. First, clean formation of products derived by coupli ng of the (N-methylanilino)methyl radical in photochemical reactions o f 1,4-dicyanobenzene with either tetra-n-butylammonium N-methyl-N-phen ylglycinate or beta-(N-methyl-N-phenyl)aminoethanol shows that the res pective decarboxylation and retro-aldol cleavage processes occur with exceptionally high efficiencies. Second, in accord with the high races observed for aminium radical decarboxylation and base-induced retro-a ldol fragmentation, tethered cyclohexenone-alpha-aminocarboxylates and -beta-aminoethanols undergo high-yielding SET-promoted photocyclizati on reactions under both direct and SET-sensitized conditions. Last, re sults which depict how the rates of aminium radical alpha-fragmentatio n correlate with quantum efficiencies of SET-promoted reactions of ter tiary amines and amides have come from a study of photocyclization rea ctions of N-(aminoethyl)and (amidoethyl)phthalimides. The quantum yiel ds for these SET-promoted processes are observed to vary with the elec trofugal group and nitrogen substituent in the manner predicted on the basis of the LFP-determined fragmentation rates.