Entropy-controlled diastereoselectivity in the photocyclization of rigid derivatives of o-allylaniline

Two rigid derivatives of o-allylaniline, namely 8-allyl-2-phenyl-1,2,3,4-tetrahydroquinoline (1b) and 7-(trans-2-cinnamyl)-2-methylindoline (1c), have been chosen as suitable systems to study the potential stereoselectivity of the photocyclization process. Photolysis of 1b leads to a mixture of diastereomeric lilolidines 4 (translcis), while 1c produces a mixture of 4 (translcis) and the tetrahydropyrrolo[3,2,1-hi]indole derivatives 5 (trans/cis). To disclose whether the diastereoselectivity could be entropy dependent, photolysis of 1b and 1c has been performed at several temperatures. In both cases, linear relationships have been observed when ln(k(t)/k(c)) (the relative reaction rate constants calculated from the diastereomeric excess) is plotted against the reciprocal temperatures. However, significant entropy-controlled diastereoselectivity has only been found for the photocyclization of 1c to 4. The fluorescence spectra of 1b,c show formation of intramolecular charge-transfer exciplexes, which is in agreement with the proposed excited-state electron-transfer mechanism for photocyclization.