Cyclobutane-Cyclopentane Interconversion of Coordinated Adducts of Cycloheptatriene and Tetracyanoethylene Via the [2,2]-Sigmahaptotropic Rearrangement

Abstract

The reaction of (3,7,7-trimethyl-cycloheptatriene)Fe(CO)₃ with tetracyanoethylene gave an equilibrium mixture of the 2+2 and 3+2 isomeric adducts, in a 2:3 ratio. The rate constants for the interconversion process in CDC1₃ were evaluated by the spin saturation transfer technique, and in CD₃OD by variable temperature line shape analysis. The rate constant for the 3+ 2 2+ 2 rearrangement in CDC1₃ k₃₂ 0.19 ±0.03 s^-1 with ΔG 18.37 + 0.10 kcal/mol at 24°C. In CD₃OD k₃₂ 1.3 + 0.6 s^-1 with ΔG 17.13 + 0.27 kcal/mol at 22°C. Other activation parameters were ΔH= 15 kcal/mol, AS- 10 e.u. (in CDC1₃) and ΔH- 20 kcal/mol, ΔH= +12 e.u. (in CD₃OD), respectively. Addition of strong dienophiles, such as (carbomethoxy)maleic anhdride, to the eqilibrium mixture did not interfere with the reaction. This, together with the kinetic data and the small observed solvent effect k(CD₃OD)/k(CDCl₃) = 6.8, support a mechanism in which isomerization proceeds via a single step pericyclic [2,2]-sigmahaptotropic rearrangement.