PHOTOCHEMICAL FRAGMENTATION OF THE 2-PHOSPHABICYCLO[2.2.2]OCTA-5,7-DIENE RING-SYSTEM AS A VERSATILE METHOD FOR GENERATING 3-COORDINATE METHYLENE PHOSPHINE OXIDES AND SULFIDES

1,6-Dihydrophosphinine 1-oxides with 4-chloro-3(or 5)-methyl substitue nts give cycloadducts with dimethyl acetylenedicarboxylate having the 2-phosphabicyclo[2.2.2]octa-5,7-diene ring system that can be fragment ed easily on irradiation with ultraviolet light (254 nm). The phosphor us-containing bridge is released as a derivative of the 3-coordinate m ethylene phosphine oxide species, R-P(O)=CH2. This species is too reac tive to observe but is readily trapped by an addition reaction with al cohols to give phosphinates (R = alkyl or aryl) or phosphonates (R = O -alkyl). These experiments constitute the first demonstration of the e xistence of RO-P(O)=CH2. Phosphoryl oxygen in the cycloadducts has bee n replaced by sulfur, and these sulfides fragment smoothly on irradiat ion. The 3-coordinate species Ph-P(S)=CH2 has been generated by other workers, but the species RO-P(S)=CH2 was generated here for the first time. The power of the bicyclic fragmentation route for the generation of a variety of 3-coordinate P-methylene derivatives is made evident by this study.