Phosphabicyclo octadienes

Diels-Alder reactions form a well-studied group of MW-assisted reactions (Keglevich et al., 2007). The cycloaddition of 1,2-dihydrophosphinine oxides and dimethyl acetylenedicarboxylate (DMAD) or A-phenylmaleimide (NPMI) results in the formation of 2-phosphabicyclo octadiene and bicyclooctene, respectively. Under traditional heating in boiling toluene, the reaction was rather slow. On MW irradiation at 110°C in the absence of any solvent, the cycloaddition became 30 times faster... 

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. 

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

As thermal examinations suggested, the phosphabicyclo[2.2.2]octadienes were used in thermo-induced fragmentation-related phosphorylations. It was possible to phosphinylate non-volatile phenol and naphthol derivatives by heating their sample with precursor 80 at 240 °C in the absence of any solvent. The esters (PhP(0)(OAr) Me) were obtained in 51-73% yield after chromatography [54] (Scheme 34). 

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]. 

G. Keglevich, R. Kovacs, L. Drahos, Diels-Alder cycloadditions of 1,2-dihydrophosphinine oxides and fi agmentation-related phosphorylations with 2-phosphabicyclo[2.2.2]octadiene oxides under green chemical conditions - the role of microwave and ionic liquids. Phosphorus Sulfur Sfflcon 186 (2011) 2172-2179. 

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