A study of the reactions of phosphetanium and vinylphosphonium salts.
thesisposted on 19.11.2015, 08:47 by John Richard. Shutt
Successful and unsuccessful attempts were made to synthesise vinylphosphonium salts containing an active methylene group attached to the phosphorus atom. An investigation was carried out into the use of benzyldiphenylvinylphosphonium bromide in a Wittig reaction without the use of base, by the addition of a nucleophile to the vinyl group followed by proton transfer to give the benzylidene ylid. The hydrolyses of vinyl- and beta-substituted ethyl phosphonium salts were investigated; also the reactions of alkylidenediphenyl-vinylphosphoranes were studied in an attempt to prepare four membered heterocycles. In both cases, interesting and surprising results were obtained. A study was made of the stereochemistry of substitution reactions at the phosphorus atom of phosphetanes. Alkaline hydrolyses of benzyl salts proceed with loss of the benzyl group. One geometrical isomer of 1-benzyl-2,2 ,3-trimethyl-l-phenylphosphetanium bromide and the corresponding pentamethyl salt hydrolysed with retention of configuration at the phosphorus atom, whereas the other geometrical isomer of these salts hydrolysed with partial inversion of configuration. In comparison, 1-benzyl-2 ,2,3,3-tetramethyl-l-phenylphosphetanium iodide, possessing no geometrical isomerism, was shown to hydrolyse with complete retention of configuration by optically active techniques. A discussion of the possible mechanisms is presented, ylid inversion being favoured. Other substitution reactions in phosphetanes brought about either ring expansion or ring opening reactions. All the reactions of phosphetanes were governed by the preference of the four membered ring to occupy an apical-equatorial position in the trigonal-bipyramidal intermediate. Further proof of this preference was obtained by the enhanced stability observed in the pentacoordinate phosphoranes prepared from phosphetanes as compared with the adducts of their acyclic analogues.