GAS-PHASE REACTIONS .82. THE THERMAL ELIMINATION OF WATER FROM ORGANOPHOSPHORUS OXYGEN COMPOUNDS

P = O double bonds possess high bond dissociation enthalpies and, therefore, are considered to be chemically rather inert; their energetically favored formation is one of the essential principles in phosphorus chemistry. Surprisingly, however, dimethylphosphane oxide (H3C)2HP = O selectively splits off water to 2-phospha-propene H3C-P = CH2 in its PE spectroscopically analyzed 800 K gasphase thermolysis. Only at higher temperature, fragmentation to a mixture containing H2O, CH4 and HC = P is observed and, in addition, the disproportionation products (H3C)2PH and (H3C)2(HO)P = O are formed. In attempts to isolate H3C-P = CH2, Which can also be generated by thermal HCl elimination from (H3C)2PCl, besides a polymeric (H3CPCH2)n residue small amounts of a dimer (n = 2) can be trapped, for which its PE spectrum suggests a trans-dimethyl-1,3-diphosphetane structure. Furthermore, diethylphosphane oxide (H5C2)2HP = O and trimethylphosphane oxide (H3C)3P = O as well as methylphosphonous acid (H3C)(HO)HP = O, the PE spectrum of which is also assigned based on MNDO calculations, on heating partly eliminate H2O, but predominantly the disproportionation products R2PH + R2(HO)P = O and RPH2 + R(HO)2P = O form. For the microscopic pathway of the unexpected thermal dehydration under nearly unimolecular conditions, an approximate energy hypersurface for {(H3C)2HP = O --> H3C-P = CH2 + H2O] suggests the "chemically activated" tautomer (H3C)2P-OH as intermediate.