Homoallylic coupling in 1,4-dihydronaphthalenes

Abstract

The Birch reduction of 5-acenaphthoic acid using lithium in ammonia affords a high yield of cis-2a,5-dihydro-5-acenaphthoic acid as the sole isolated product whereas reduction with sodium gives a mixture of the cis- and trans-isomers. The ¹H n.m.r. spectrum of the cis-dihydro-acid exhibits the largest homoallylic coupling constant yet seen in a 1,4-dihydronaphthalene ring system (ca. 8 Hz), a result which is consistent with considerable puckering expected for the dihydrobenzene ring in this strained system. This result is compared with that observed in 4-methyl-1,4-dihydronaphthoic acid.