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 esters with AlHCl(2) and are characterized by (31)P, (13)C, and (1)H NMR and by HMRS. They can be kept several weeks in the refrigerator after purification. They lead then to the corresponding phosphaalkenes 3a-g by HCl elimination. For the volatile alpha-chlorophosphines 2a-e HCl elimination occurs in the gas phase on solid potassium carbonate under VGSR conditions (vacuum gas-solid reactions); the corresponding phosphaalkenes 3a-e are characterized by real time HRMS analysis of the gaseous flow (VGSR/HRMS coupling) and by solid-phase IR spectroscopy after condensation of the gaseous flow on a KBr window cooled to 77 K. The decomposition of phosphaalkenes at this temperature is monitored 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 (31)P 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 (31)P NMR data of the simple phosphaalkenes 3a-g are for the first time reported. They are consistent with the proposed structure. The stereochemistry of the (Z)- and (E)-isomers is established according to the "cis-rule". Phosphaalkenes 3a-g are also characterized by chemical trapping in solution with various dienes, dipoles, or thiols. All of these experiments confirm the transient character of these species. The synthetic potential of this route is evaluated.
Gaumont
Annie-Claude
AC
Groupe de Physicochimie Structurale, Equipe Associée au CNRS 704, and CRMPO, Université de Rennes I, 35042 Rennes, France, and Ecole Polytechnique, Equipe Associée au CNRS, 91128 Palaiseau, France.
Pellerin
Bruno
B
Cabioch
Jean-Luc
JL
Morise
Xavier
X
Lesvier
Mickaël
M
Savignac
Philippe
P
Guenot
Pierre
P
Denis
Jean-Marc
JM