Topochemical [2+2] Dimerization of Kinetically Stabilized 1‐Phosphaallenes in the Solid State

The thermolysis of two bulky 1‐phosphaallenes in the solid state afforded diphosphanylidenecyclobutane or 2,4‐dimethylene‐1,3‐diphosphacyclobutane. The regioselectivity of the [2+2] dimerization depends largely on the crystal structure of the 1‐phosphaallenes, and the reaction path seems to be affected by the presence of bulky 2,4,6‐tri‐ tert ‐butylphenyl (Mes*) group. This topochemical reaction was also controlled by the substituents at the 3‐position of the 1‐phosphaallene skeleton. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)