Incomplete Electrocyclization of a Sterically Hindered 1,4‐Diphosphabutadiene

Abstract

Butadiene is the simplest neutral acyclic closed‐shell π‐conjugated system and is typically sufficiently stable enough to avoid electrocyclization to cyclobutene. In contrast, most congeners of butadiene containing heavier elements are easily converted into the corresponding 4‐membered cyclobutene system. Herein, we demonstrate that the gauche 1,4‐diphosphabutadiene (P=C−C=P) skeleton in a sterically encumbered 2,3‐bis(phosphanylidene)‐1,4‐disilinane can be remarkably perturbed due to “incomplete electrocyclization” where P=C−C=P partially form the corresponding 1,2‐dihydrodiphosphete (3,4‐diphosphacyclobutene) by [2+2] electrocyclization. 31P NMR data obtained in solution indicated that the coexistence of a closed ring substantially reduces the open‐ring characteristics of the P=C−C=P moiety. However, the 31P CP‐MAS spectrum of 2,3‐bis(phosphanylidene)‐1,4‐disilinane showed that the P=C−C=P structure is predominant in the solid‐state. Single‐crystal X‐ray analysis revealed that decreasing the temperature promoted the generation of small amounts of incomplete 1,2‐dihydrodiphosphete system in the crystalline state. Furthermore, the 1,2‐dihydrodiphosphete units disappeared upon warming the single crystal, and this unique solid‐state electrocyclization reaction was reversible.