Reactions of the Disilane Me3SiSiCl3 with P-Chlorophosphaalkenes: Transient and Persistent Per-Silylated Phosphaalkenes

Reactions of P-chlorophosphaalkenes (RMe2Si)(2)C=PCl (1a: R = Me; 1b: R = Ph) with the disilane Me3SiSiCl3 (5) furnish diphosphenes (Cl3Si)(RMe2Si)(2)C-P=P-C(SiCl3)(SiMe2R)(2) (4a: R = Me; 4b: R = Ph) by Me3SiCl elimination. The structure of the new compound 4b was confirmed by X-ray diffraction; it displays crystallographic inversion symmetry. Monitoring the reactions with P-31- and Si-29-NMR spectroscopy detected P(trichlorosilyl)phosphaalkenes (RMe2Si)(2)C=PSiCl3 (2a, R = Me; 2b, R = Ph) as the primary intermediates from reductive P-silylation of 4a, 4b, and P-[(trichlorosilyl)phosphanyl]phosphaalkenes (Rme(2)Si)(2)C=P-P(SiCl3)C(SiCl3)(SiMe2R)(2) (3a: R = Me; 3b: R = Ph) as unsymmetric dimerisation products that rearrange to provide 4a, 4b in step III of the reaction sequence. This step (the P -> C 1,3-trichlorosilyl shift reaction) was mimicked by the synthesis of (Me3Si)(2)C=P-P(SiCl3)tBu (7), which rearranges into an unsymmetric diphosphene tBuP=PC(SiMe3)(2)SiCl3 (8). The bulkier P-chlorophosphaalkene (iPrMe(2)Si)(2)C=PCl (1c) reacts with 5, eliminates Me3SiCl and thereby provides the first persistent acyclic per-silylated phosphaalkene (iPrMe(2)Si)(2)C=PSiCl3 (2c) in an incomplete reaction. 2c exhibits an exceptionally large NMR coupling (1)J(P-31,Si-29) = +/- 249 Hz. Within weeks, the mixtures of 1c and 2c undergo decomposition with loss of the P=C functions.