Phosphabicyclo octenes

Retro-cycloaddition extrusion of the metaphosphate moiety from 2,3-oxa-phosphabicyclo[2.2.2]octene derivatives (79), in 1,2-dichloroethane at 100 °C in the presence of PrOH, has been shown to proceed via an unsymmetrical transition state in which C—P bond breakage and P=0 bond formation are more advanced than C—O breakage.39 The secondary deuterium isotope effect on H adjacent to the P—C bond is 1.060 0.008 for (79a) and 1.081 0.009 for (79b) and the oxygen kinetic isotope effect on the P—O—C bridge is 0.9901 0.0016 for (79a). 

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,... 

Synthesis of Phosphabicyclo[2.2.2]octadiene-and Phosphabicyclo[2.2.2]octene Derivatives... 

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]. 

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