Incomplete Electrocyclization of a Sterically Hindered 2,3‐Diphosphabutadiene

: 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. 31 P 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 31 P 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.