Synthesis and Structures of [2.n]Metacyclophan‐1‐enes and their Conversion to Highly Strained [2.n]Metacyclophane‐1‐ynes

Abstract

The syntheses of syn‐[2.n]metacyclophan‐1‐enes (n = 5, 6, 8) in good yields using the McMurry cyclization of 1,n‐bis(3‐formyl‐4‐methoxyphenyl)alkanes are reported. Conversion of syn‐[2.6]‐ and [2.8]metacyclophan‐1‐enes to the corresponding highly strained syn‐type [2.6]‐ and [2.8]metacyclophane‐1‐ynes was achieved by successive bromination and dehydrobromination reactions. An attempted trapping reaction of the putative corresponding [2.5]metacyclophane‐1‐yne by Diels–Alder reaction with 1,3‐diphenylisobenzofuran failed due to its smaller ring size and strained structure. X‐ray crystallographic analyses show that the triple bonds in syn‐[2.6]‐ and [2.8]metacyclophane‐1‐ynes are distorted from linearity with bond angles of 156.7° and 161.4°, respectively. A DFT (Density Functional Theory) computational study was conducted to determine the stabilities of different conformations of the target compounds.