DEHYDROCHLORINATION OF ALPHA-CHLOROPHOSPHINES, A SIMPLE AND GENERAL-ROUTE TO PHOSPHAALKENES

Primary and secondary alpha-chlorophosphines 2a-g are prepared in ca. 70% yield by chemoselective reduction of the corresponding phosphonic and phosphinic eaters with AlHCl2 and are characterized by P-31, C-13, and H-1 NMR and by HMRS. They can be kept several weeks in the refrig erator after purification. They lead then to the corresponding phospha alkenes 3a-g by HCl elimination. For the volatile alpha-chlorophosphin es 2a-e HCl elimination occurs in the gas phase on solid potassium car bonate under VGSR conditions (vacuum- gas-solid reactions); the corres ponding phosphaalkenes 3a-e are characterized by real time HRMS analys is of the gaseous flow (VGSR/HRMS coupling) and by solid-phase IR spec troscopy after condensation of the gaseous flow on a KBr window cooled to 77 K. The decomposition of phosphaalkenes at this temperature is m onitored by IR spectroscopy. The alpha-chlorophosphines 2a-g undergo a HCl elimination in the liquid phase in the presence of a Lewis base; the formation of the transient phosphaalkenes is monitored by P-31 FT- NMR. The temperature of HCl elimination is dependent both upon the P-H acidity of the phosphine precursors and the nature of the base. The P -31 NMR data of the simple phosphaalkenes 3a-g are for the first time reported. They are consistent with the proposed structure. The stereoc hemistry of the (Z)- and (E)-isomers is established according to the ' 'cis-rule''. Phosphaalkenes 3a-g are also characterized by chemical tr apping in solution with various dienes, dipoles, or thiols. All of the se experiments confirm the transient character of these species. The s ynthetic potential of this route is evaluated.