Some reactions of cyclic phosphonium salts.
thesisposted on 19.11.2015, 08:45 by Wilfred. Hawes
A review of the Wittig olefin synthesis and some of its synthetic applications is given. The synthesis of substituted l, 6-diphenylhexa-l, 3, 5-trieness from phospholenium salts is described. Some interesting allylic rearrangements found while investigating the optimum conditions for olefin formation from allylic phosphine oxides are described. Factors influencing the formation of phosphetanes are discussed and the syntheses of highly substituted phosphetane derivatives are described. Following a discussion of the mechanism of the alkaline hydrolysis of phosphonium salts, the alkaline hydrolysis of phospholenium and phosphetanium salts is described. Ring expansion of the methyl and iodomethyl salts of 2, 2, 3, 4, 4-pentainethyl-l-phenylphosphetane occurs during hydrolysis; the benzyl salt gives 2, 2, 3, 4, 4-pentamethyl-l-phenylphosphetane oxide with retention of configuration about phosphorus. This is the first example of the hydrolysis of a phosphonium salt proceeding with retention rather than inversion of configuration. Two other Walden cycles in which substitution at phosphorus proceeds with retention of configuration are described and a mechanism is proposed involving "pseudo-rotation" of a trigonal bipyramidal intermediate. "Pseudo-rotation" is discussed in the elucidation of the mechanism of the alkaline hydrolysis of phosphate esters. A neo-pentyl effect observed in the hydrolysis of esters with two t-butyl groups attached to phosphorus is described. The stability of pentacovalent phosphorus compounds is found to be increased by a) increased electronegativity of the substituents, and b) the presence of rings constrained to have a preferred angle of 90° at phosphorus.