Methylenephosphines

This section will provide details of recent efforts to polymerize phosphaalkenes. It will begin with an introduction to the factors that must be considered when attempting to polymerize P=C bonds. In addition, a historical context will be provided since, perhaps ironically, it was so-called polymerization reactions that plagued early efforts to prepare compounds possessing heavier element multiple bonds. Finally, it will close with the first successful polymerization of a P=C bond to give poly(methylenephosphine)s. 

This review has shown that the analogy between P=C and C=C bonds can indeed be extended to polymer chemistry. Two of the most common uses for C=C bonds in polymer science have successfully been applied to P=C bonds. In particular, the addition polymerization of phosphaalkenes affords functional poly(methylenephosphine)s the first examples of macromolecules with alternating phosphorus and carbon atoms. The chemical functionality of the phosphine center may lead to applications in areas such as polymer-supported catalysis. In addition, the first n-conjugated phosphorus analogs of poly(p-phenylenevinylene) have been prepared. Comparison of the electronic properties of the polymers with molecular model compounds is consistent with some degree of n-conjugation in the polymer backbone. 

Synthesis of bis(trimethylsilyl)methylenephosphinous chloride (Structure 18, Scheme 4)... 

Phosphabenzene, the phosphorus analogue of pyridine, is a species that, as with the methylenephosphines, formally contains a doubly coordinated phosphorus singly bound to one carbon and doubly bound to another. The first isolation of a phosphabenzene derivative was reported in 1966 using the reaction as shown below (equation 10). The parent phosphabenzene molecule was first isolated by the reaction (equation 11) of phosphorus tribromide with a staimane derivative, albeit in low yield.Further spectroscopic investigation of this parent molecule indicated it to be planar with indications of multiple bond character between phosphoms and carbon and aromatic delocalization. ... 

The Diels-Alder reaction of 1,2-dihydrophosphinine oxides with dienophiles, such as acetylenic derivatives and maleic acid derivatives affords 2-phosphabicyclo [2.2.2]octadiene and 2-phosphabicyclo[2.2.2]octene 2-oxides that may be regarded as the precursors of low-coordinate, methylenephosphine oxides that are useful in the phosphorylation of 0- and N-nucleophiles. It was observed that the photo-chemically induced fragmentation-related phosphorylation may follow a novel addition-elimination mechanism instead of the classical elimination-addition protocol. 

The living anionic polymerization of a novel phosphaaUcene (11) has recently been achieved to yield a well-defined poly(methylenephosphine) (Scheme 5.14) [168-171]. The addition polymerization of the C=P bond in 11 proceeds exclusively in a regioselective fashion to form a red-colored diphenylmethyl carbanion via a nucleophilic attack on the phosphorus atom. The resultant poly(methylenephosphine)... 

Also, methylenephosphine oxides undergo [4 +2] cycloaddition reactions with phenylbenzoylketene to give the cycloadducts 47. ... 

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