Search

Reductive couplings of the vinylogues of cyclohepta-1,3,5-triene-1,6-dicarbaldehyde with a low-valent titanium reagent afforded the corresponding monomeric or dimeric products, i.e. singly bridged or doubly bridged annulenes in most cases. These annulenes showed diatropicity or paratropicity, depending on the number of peripheral conjugated π-electrons. The 1H n.m.r. spectra of the methanoannulenes showed a larger temperature-dependency than those of the closely related tetradehydromethanoannulenes, reflecting the lower planarity and the larger conformational mobility of the molecular skeleton in the former compounds.

The title annulenes (4a–d) have been synthesized by a Wittig reaction of cyclohepta-1,3,5-triene-1,6-dicarbaldehyde (1a) or its vinylogous dialdehydes (1b–d) with 3-methylpent-2-en-4-ynyltriphenylphosphonium bromide (2), followed by intramolecular oxidative coupling of the resulting acyclic compounds. They proved to be diatropic or paratropic, depending on the number of peripheral π-electrons. The related [1,3]cyclophanpolyenediynes (11), (14), (17), (19), (21), and (23) were prepared by a double Wittig reaction of 3-methylpent-2-en-4-ynal (8) and/or its vinylogous aldehydes (9) and (10), with 1,3-bis(triphenylphosphoniomethyl)benzene dibromide (7), followed by intramolecular oxidative coupling of the resulting acyclic compounds. Their properties are compared, on the basis of 1H n.m.r. and electronic spectral results, with those of tetradehydromethanoannulenes.