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

The syntheses ofsyn-[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 ofsyn-[2.6]- and [2.8]metacyclophan-1-enes to the corresponding highly strainedsyn-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 insyn-[2.6]- and [2.8]metacyclophane-1-ynes are distorted from linearity with bond angles of 156.7 degrees and 161.4 degrees, respectively. A DFT (Density Functional Theory) computational study was conducted to determine the stabilities of different conformations of the target compounds.