Azido Phosphazenes
During the past fifty years, organic azides have developed into important synthetic reagents. Organic azides have been developed for applications in photoresists, vulcanization, polymer coupling and cross-linking, and for the surface modification of polymers and metals. Aryl azides (PhN3), sulfonyl azides (RSO2N3), and azidoformates (ROC(=O)N3) have been the principal compounds analyzed due to their ability to form reactive nitrene intermediates that are capable of inserting into chemical bonds that are unreactive as C-H.
Phosphoryl azides (RO2P(=O)N3) also form nitrene compounds which undergo insertion reactions, but relatively little nitrene insertion chemistry appears to have been conducted with these materials. Most of the recent work has utilized phosphoryl azide-promoted coupling or polymerization of organic monomers to produce polypeptides, polyamides, polyureas, or polyurethanes. The lack of phosphoryl azide nitrene insertion chemistry may be due to the inherent moisture sensitivity of many phosphoryl azides or because of the difficulties in preparing phosphoryl compounds with multiple azide groups.
Cyclic phosphazene trimers (N3P3R6) offer a possible solution. The presence of three phosphorus atoms in these species allows easy access to compounds with two or three azido groups per molecule. Also, the properties of phosphazenes can be tailored by the organic cosubstituent groups that are connected to the phosphazene ring. For example, the incorporation of appropriate cosubstituent groups should provide azido compounds with acceptable stability. Despite these potential advantages, only a few phosphazene azide compounds have been prepared. Little investigation of the nitrene chemistry of phosphazene azides has been conducted, and no evidence of insertion chemistry has been described.
A variety of cyclic phosphazene trimers containing the azido group with aryloxy, alkoxy , or dialkylamino cosubstituents have been synthesized. The aryloxy and alkoxy phosphazene azides undergo complete reaction with numerous phosphorous(III) species to form phosphazene phosphinimines. The success of these reactions indicates that these materials are promising candidates for the synthesis or modification of organic polymers via phosphinimine formation. Work is currently underway to study the versatility of this route with various organic polymers and co-polymers.
Several of the phosphazene azides showed successful nitrene insertion chemistry. With aryloxy phosphazene azides, this was via a thermally initiated route. Nitrene insertion was initiated by UV light with the alkoxy phosphazene azide in an aliphatic solvent. No photolytic insertion was observed when aromatic species were involved due to their significantly stronger absorption in the UV region relative to the azido groups. The dialkylamino phosphazene azides rearranged in preference to nitrene insertion chemistry. Aryloxy phosphazene derivatives with multiple azide groups have been utilized to cross-link polyolefins using thermally initiated nitrene insertion chemistry.