Lewis Acid Promoted Decomposition of Substituted 1,3,2λ5-Dioxaphospholanes: Kinetic and Thermodynamic Studies

Article abstract of DOI:10.1021/jo00271a039

The kinetics of Lewis acid mediated decomposition of a series of substituted 1,3,2λ⁵-dioxaphospholanes, prepared by transphosphoranylation of 1,2-diols with diethoxytriphenylphosphorane (DTPP), is reported.The rate data obtained from 31P NMR spectroscopic measurements, emphasize the influence of (i) variations in the coordination potential (i.e., cationic charge) of the Lewis acids, (ii) methyl group substitution at C-4 and C-5 in the 1,3,2λ⁵-dioxaphospholanyl substructure, and (iii) changes in solvent polarity.Also, the propensity for 1,2-hydride migratory processes attending conformationally restricted bicyclic 1,3,2λ⁵-dioxap hospholanes versus epoxide formation from the collapse of simple cyclic 1,3,2λ⁵-dioxaphospholanes were examined.The results are best explained by invoking a "site-selective" coordination by the catalyst to one of the "ethereal" oxygens within 1,3,2λ⁵-dioxaphospholanyl moiety initiating P-O bond cleavage and ultimately affording the requisite betaine intermediate(s).Methyl substitution on the 1,3,2λ⁵-dioxaphospholanyl hydrocarbon backbone decreases the rate of P-O bond cleavage, and the 1,2-hydride migratory process within conformationally rigid bicyclic 1,3,2λ⁵-dioxaphospholanes requires ca. 2.0 kcal/mol more energy than the decomposition of the monocyclic 1,3,2λ⁵-dioxaphospholanes via 3-exo-tet cyclization to the respective cyclic ethers.Mechanistic implications of various reactions are discussed.