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.

中文翻译：

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立体受阻的1,4-二磷酸丁二烯的不完全电环化。

丁二烯是最简单的中性无环闭壳π共轭体系，通常足够稳定，可以避免电环化为环丁烯。相反，大多数含有较重元素的丁二烯同源物很容易转化为相应的四元环丁烯系统。在这里，我们证明，空间受限的2,3-双（亚膦基）-1,4-二硅丙烷中的gauche 1,4-二磷酸丁二烯（P = CC = P）骨架会由于“不完全电环化”而受到显着扰动，其中P = CC = P通过[2 + 2]电环化部分形成相应的1,2-二氢二磷（3,4-二磷环丁烯）。在溶液中获得的31 P NMR数据表明，闭环的共存大大降低了P = CC = P部分的开环特性。但是，31 P CP-MAS频谱为2，3-双（亚膦酰基）-1,4-二硅烷基烷表明，P = CC = P结构在固态中占主导地位。单晶X射线分析表明，降低温度促进了结晶状态下少量不完整的1,2-二氢二磷酸体系的生成。此外，在加热单晶时1，2-二氢二磷酸单元消失，并且这种独特的固态电环化反应是可逆的。