The trimethylaluminum-mediated Michael addition of ethyl phenyl-H-phosphinate to 1,2-dihydrophosphinine oxides (1A.) yielded 3-(EtOPhP(O))-1,2,3,6-tetrahydrophosphinine oxides (4) in a selective manner, as a mixture of only two diastereomers. In the above type of reactions (e.g., in that of 1Aa and Ph2P(O)H), Me3Al could not be substituted by microwave irradiation due to low efficiency. Catalytic hydrogenation of the Michael adducts (4) led to 3-(EtOPhP(O)- 1,2,3,4,5,6-hexahydrophosphinine oxides 5, in the case of P-phenyl substituent (5a), as a mixture of only two diastereomers, while in the instance of the P-ethoxy derivative (5b), as a mixture of four isomers. Stereostructure of the products (5) was substantiated on the basis of analogies and stereospecific NMR couplings. The predominant conformations of compounds 4a, 4b, 5a, and 5b-1 were determined by HF/6-31G* calculations. Reduction of P(1)-Ph heterocycles 4a and 5a by phenylsilane resulted in monodeoxygenation to afford P-ligands 6 and 8, respectively, that were protected as the corresponding phosphine boranes (7 and 9, respectively).
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