Medium-sized cyclophanes. Part 58. Synthesis and conformational studies of [2.n]metacyclophan-1-enes and [n.1]metacyclophanes

Abstract

A series of syn- and anti-[2.n]metacyclophan-1-enes and [2.n]metacyclophane-1,2-diols are prepared in good yields by a McMurry cyclization of 1,n-bis(5-acetyl-2-methoxyphenyl)alkanes. Interestingly, in the same coupling reaction in the absence of pyridine the pinacol rearrangement of [2.n]metacyclophane-1,2-diols to afford [n.1]metacyclophanes is observed, attributable to the TiCl4 or acids generated from the McMurry reagent occurring during the cyclization reaction. In fact, protic acid- or Lewis-acid induced pinacol rearrangements of [2.n]metacyclophane-1,2-diols afford [n.1]metacyclophanes in good yield. The [2.n]metacyclophan-1-ene-to-[n.1]metacyclophane ratio of the products is strongly governed by the number of the methylene bridges. The proportion of the rearrangement product increases with increasing length of the bridge. Conformational studies of [n.1]metacyclophanes as well as of [2.n]metacyclophan-1-enes in both solution and solid state are also described.