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

Reactions of P-chlorophosphaalkenes (RMe 2 Si) 2 C=PCl (1a: R = Me; 1b: R = Ph) with the disilane Me 3 SiSiCl 3 (5) furnish diphosphenes (Cl 3 Si)(RMe 2 Si) 2 C-P=P-C(SiCl 3 )(SiMe 2 R) 2 (4a: R = Me; 4b: R = Ph) by Me 3 SiCl elimination. The structure of the new compound 4b was confirmed by X-ray diffraction; it displays crystallographic inversion symmetry. Monitoring the reactions with 31 P- and 29 Si-NMR spectroscopy detected P-(trichlorosilyl)phosphaalkenes (RMe 2 Si) 2 C=PSiCl 3 (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(SiCl 3 )C(Sid 3 )(SiMe 2 R) 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 (Me 3 Si) 2 C=P-P(SiCl 3 )tBu (7), which rearranges into an unsymmetric diphosphene tBuP=PC(SiMe 3 ) 2 SiCl 3 (8). The bulkier P-chlorophosphaalkene (iPrMe 2 Si) 2 C=PCl (1c) reacts with 5, eliminates Me 3 SiCl and thereby provides the first persistent acyclic per-silylated phosphaalkene (iPrMe 2 Si) 2 C=PSiCl 3 (2c) in an incomplete reaction. 2c exhibits an exceptionally large NMR coupling 1 J( 31 P, 29 Si) = ±249 Hz. Within weeks, the mixtures of 1c and 2c undergo decomposition with loss of the P=C functions.