New Route to Access an Acyl‐Functionalized Phosphasilene and a Four‐Membered Si‐P‐C‐O Heterocycle

An acyl‐functionalized phosphasilene, LSi(CO t Bu)P(SiMe 3 ) (L=PhC(N t Bu) 2 ) was synthesized on a new route by the addition of t BuCOCl to the phosphinosilylene LSiP(SiMe 3 ) 2 and subsequent Me 3 SiCl elimination. DFT studies elucidated its molecular structure, the influence of the acyl group on UV/Vis transitions, and revealed the mechanism. The intermediate LSi(CO t Bu)ClP(SiMe 3 ) 2 , with a five‐coordinate silicon center, was characterized by NMR spectroscopy and X‐ray analysis. On the other hand, phosphasilene LSi(SiMe 3 )P(SiMe 3 ) reacted with t BuCOCl by a [2+2] cycloaddition of the silicon–phosphorus double bond and the carbon–oxygen double bond in addition to Me 3 SiCl elimination, thereby affording the novel, fully characterized compound LSi(SiMe 3 )[PC( t Bu)O] bearing a Si‐P‐C‐O heterocycle with a phosphorus–carbon double bond. DFT studies suggest that two mechanisms occur simultaneously.