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

Syn-[2.n]metacyclophan-1-enes (n=5, 6, 8) are synthesized by the McMurry cyclization of 1,n-bis(3-formyl-4-methoxyphenyl)alkanes in good yields. Syn-[2.n]metacyclophan-1-enes (n= 6, 8) were converted to the highly strained syn-type [2.n] metacyclophane-1-ynes by successive bromination followed by dehydrobromination with KOBut. The attempted trapping reaction of [2.5]metacyclophane-1-yne by a Diels-Alder reaction with 1,3-diphenylisobenzofuran failed due to its smaller ring size and strained structure. The triple bonds in syn-[2.n]metacyclophane-1-ynes (n= 6,8) are distorted from linearity with the bond angles, 156.7° and 161.4° as shown by X-ray crystallographic analyses. A DFT (Density Functional Theory) computational study of the synthesized compounds was conducted to determine the stability of the different configurations