Ring Fragmentation Approaches to Low-Coordinate Phosphorus Species

Abstract

The photochemical fragmentation of phosphine oxides with the 2-phosphabicyclo[2.2.2]octene framework constitutes a useful method for the generation of methylene(oxo)phosphoranes, R-P(O)=CH2. The previously uncharacterized and highly reactive P-phenyl and P -methyl derivatives were prepared and trapped with alcohols as phosphinates. The fragmentation also occurred when alkoxy substituents were present on phosphorus, but significant amounts of dialkyl phosphites were formed as by-products. Some new observations on the photochemical fragmentation of the 7-phosphanorbornene system have also been made. Contrary to literature reports, this system is resistant to fragmentation and does not extrude the 2-coordinate phosphoryl species. Fragmentation with loss of the phosphorus bridge does occur readily when an alcohol is also present in the medium. The data support a mechanism in which an initial photo-promoted addition reaction of the 7-phosphanorbornene and the alcohol occurs to form a 5-coordinate species, which undergoes the fragmentation. Thermal fragmentation of the 7-phosphanorbornenes is slow, but does appear to release the 2-coordinate phosphoryl species.We have demonstrated in several recent studies (1-4) that certain unsaturated bicyclic frameworks containing phosphorus functions can be fragmented in solution to release a simple phosphorus-containing moiety. The nature of the bond breaking process leaves phosphorus in these fragments in a condition of low coordination (number of attached atoms) for the particular oxidation state concerned, and the method has proved of practical value for the synthesis of several low coordination species. As a general rule, low-coordination phosphorus species are of very high reactivity and some types cannot be directly observed or isolated even when stabilization through kinetic or thermodynamic effects is attempted. In such cases, their intermediacy in chemical processes is usually demonstrated by trapping reactions. We continue to explore applications of the ring fragmentation approach in the low-coordination chemistry of phosphorus, and in this paper we will describe the first extension of our methods to the generation of two types of the 3-coordinate methylene(oxo)-

Rearrangement of a W(CO)5-Complexed 7-Phosphanorbornadiene Analyzed by ab Initio MO Calculations

“…Phosphinidenes gain rapidly in popularity as versatile reagents in the syntheses of low-coordination organophosphorus compounds. , Through transition metal complexation, the otherwise triplet species are stabilized to singlets . Nucleophilic phosphinidenes, tailored after the Schrock complexes, use Ta and Zr in, for example, the phospha-Wittig reaction .…”