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The Baeyer–Villiger oxidation of 7-phosphanorbornene 7-oxides with sterically demanding substituents on the phosphorus atom ( 4a – d ) by m -chloroperbenzoic acid afforded the title products ( 5a – d ) as a mixture of two regioisomers ( A and B ). Isomer A , the result of thermodynamic control, was stable, while isomer B , the product of kinetic control, underwent decomposition and/or epoxidation. Single crystal X-ray analysis of P -(2,4,6-triisopropylphenyl) oxaphosphabicyclooctene ( 5Ac ) was not only useful in the evaluation of its structure, but, for the first time in the literature, a low-coordinated arylmetaphosphonate ( 15c ) formed by fragmentation on X-ray irradiation could also be detected. The precursors ( 5Aa - c ) were utilized in the thermoinduced and UV light-mediated fragmentation-related phosphorylations of alcohols. Beside the well-known elimination-addition mechanism via the metaphosphonate intermediate ( 15 ), a novel addition-elimination route involving a species with a pentavalent pentacoordinated phosphorus atom ( 16 ) was also substantiated. |
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