Phosphabicyclo octene 2-oxides

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. 

Keywords 1,2-Dihydrophosphinine oxide, 1,2,3,4,5,6-Hexahydrophosphinine oxide, P-Oxophosphoranes, 2-Phosphabicyclo[2.2.2]octene derivative, 1,2,3,6-Tetrahydrophosphinine oxide,... 

An overview is given on the synthesis, reactivity, and properties of six-membered P-heterocycles, such as 1,2-dihydro-, 1,2,3,6-tetrahydro- and 1,2,3,4,5,6-hexahydrophosphinine oxides, as well as bridged P-heterocycles, such as phosphabicyclo[2.2.2]octene derivatives and finally heterocyclic P -oxophosphoranes. 

The 1,2-dihydrophosphinine oxides are versatile intermediates [4,5] and are suitable to take part in Diels-Alder reaction with different dienophiles, like N-phenyl maleimide and dimethyl acetylenedicarboxylate (DMAD) [47]. These cycloadditions were also performed on MW irradiation in the absence of any solvent. Starting from the 1-phenyl-dihydrophosphinine oxide (23), the reactions were 25 times faster as compared to the thermal variations, and the valuable cycloadducts 2-phosphabicyclo[2.2.2]octene 24 and 2-phosphabicyclo[2.2.2]octadiene 25 were obtained in excellent yields with high purity (Scheme 8) [48]. 

---