Photochemical Additions of Alkenes to Phthalimides. Mechanistic Investigations on the Stereochemistry of Alkene Additions and the Effect of Aryl Substituents on the Regiochemistry of Alkene Additions

Article abstract of DOI:10.1021/jo00166a009

The mechanism of the photochemical addition of alkenes to phthalimides was investigated by determining the stereochemistry of the addition and the effect of aryl substituents on the regiochemistry.Intra- and intermolecular examples were examined.The stereochemistry of the addition of cis- and trans-2-butene to N-methylphthalimide to give 2,3,4-trimethyl-2-benzazepine-1,5-dione was studied.It was found that both alkenes added stereospecifically, each giving the corresponding cis- or trans-2,3,4-trimethyl-2-benzazepine-1,5-dione with >95percent stereospecificity.The mechanistic implication of this result is either that the photochemical addition is a concerted (2 + 2) addition or that any intermediate biradical closes faster than rotation around the C-C bond which would result in loss of stereochemistry.A second approach to this problem employed the directive effects of aryl substituents.The proposed biradical intermediate is similar in structure to the phthalimide radical anion.The directive effects of aryl substituents have been experimentally determined in this system and are consistent with theoretical predictions.Theoretical predictions for aryl-substituent directive effects in the alternative concerted (2 + 2) process are opposite to those for the biradical case, which predicts that donors will direct meta and acceptors para.Irradiation of 4-methoxy- and 4-carbomethoxy-N-methylphthalimide in the presence of 1-hexene afforded the benzazepinedione addition products that resulted from addition to the para and meta C(O)-N bonds, respectively.These results are entirely consistent with a concerted process.