Computed structures and reaction enthalpies of the rearrangements of certain cyclic hexaalkynes to ortho-arene cyclynes

Abstract

Density-functional theory (BLYP/6-31G*) and semiempirical theory was used to study compounds related to a strained cyclic hexaalkyne that was postulated by Staab et al. as a trimeric intermediate in the copper-mediated oxidative coupling of 2,2′-diethynylbiphenyl. Among the products of this reaction is an ortho-arene cyclyne. It was found that the hexaalkyne→ ortho-arene cyclyne rearrangement is quite exothermic (ca. 130 kcal/mol). Of the semiempirical methods tested, the AM1 results were closer than PM3 or MNDO to BLYP/6-31G*. For the intermediate, two conformations were found: the expected D 3-symmetric conformation and an unexpected C 2-symmetric conformation. Structures and energies of hexamethyl- and dodecamethyl-substituted analogues of the intermediate and product were also calculated at the AM1 level, in order to test whether steric effects might stabilize the intermediate, relative to the product. It was found that even very sterically crowded products were much more stable than the relevant intermediates. This suggested that the large exothermicity of the hexaalkyne→ ortho-arene cyclyne rearrangement might be used to drive formation of interesting strained products. An example used to illustrate this is the rearrangement of a binaphthyl analogue of Staab's biphenyl intermediate. BLYP and AM1 calculations suggest that the rearrangement of this intermediate would be exothermic by ca. 105–110 kcal/mol, despite strain induced by formation of three [5]helicene moieties in the product.