[4+2] Versus [2+2] Cycloadditions in the Sphere of P-Heterocycles as Useful Synthetic Tools

The Diels-Alder reaction of 1,2-dihydrophosphinine oxides with dienophiles, such as dimethyl acetylenedicarboxylate (DM AD) and maleic acid derivatives affords phosphabicyclo [2. 2.2] oct adien es or bicyclooctenes, respectively, that are precursors of low-coordinated P-fragments, methylenephosphine oxides useful in the phosphorylation of alcohols, phenols and amines. An alternative mechanism involving an intermediate with a pentavalent, pentacoordinated phosphorus atom was also substantiated and proved. Reaction of the P-2, 4, 6-trialkylphenyl dihydrophosphinine oxides with DMAD followed an unexpected protocol to give the corresponding stabilised phosphonium ylides. The intermediates are oxaphosphetes formed by the [2+2] cycloaddition of the P=O group of the P-cycle and the triple bond of DMAD. This kind of reaction, that is the consequence of the presence of the electronreleasing trialkylphenyl group, has never been observed earlier and proved to be general to take place with a variety of cyclic phosphine oxides.