Phosphonium anhydrides, reactivity

In 1975, Hendrickson and Schwartzman reported a different mode of phosphorus-based hydroxyl activation using bis(phosphonium) electrophiles (R3P—O—PR3)2+. These highly reactive electrophiles are generated from the reagent combination of phosphine oxide and trifluoromethanesulfonic (triflic) anhydride [96]. Mukaiyama... 

In the first step (ij) the ylides 3 (R1 = H) can be alkylated with primary alkylhalogenide or with ethyl bromoacetate they can also be acylated with acetic anhydride in a reverse addition operation, (In spite of trans-ylidation process, the acylated phosphonium salt can be easily purified by liquid -liquid extraction with bases). Semi-stabilized ylides3(R1= 0) are not reactive enough to be alkylated or acylated in this way. 

Quaternary Arsonium and Stibonium Compounds. More recently, experimental work in this laboratory has revealed that quaternairy arsonium compounds (18) are extremely effective as latent accelerators for epoxy-anhydride resius. Surprisingly, it has also been found that quaternary stibonium compounds are very much less effective as accelerators than quaternary arsonium, phosphonium or ammonium compounds. In fact, the order of gelation reactivity of these compounds with epoxy-anhydride resins would appear to be as follows ... 

Quaternary phosphonium compounds have been found to be extremely effective latent accelerators for anhydride-cured epoxy resins employed in electrical insulation applications. Used at very low concentrations (0.01 to 0.25% level) in these resins, they have been found to give very fast gel times at elevated temperatures combined with very good storage stability properties at room temperature. Results show that the quaternary phosphonium compounds having non-halide anions give the best compromise of gel reactivity, storage stability and electrical properties. Comparison with other types of latent accelerators reveal that the quaternary phosphonium compounds are among the most effective additives yet discovered for epoxy-anhydride resin systems. 

Nucleophilic attack of the phenoxide anion opens the phosphonium ring due to enhanced electrophilic reactivity of the mixed anhydride and acid structures. Salicylyl phenylphosphonite, however, in combination with 2-methyl-2-oxazoline behaves as a Me monomer. ... 

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