Synthetic approaches to five-membered heterocycles containing both ring and side-chain phosphorus atoms

Abstract

A brief review of the importance and variety of symmetrical and unsymmetrical bisphosphines is given and the possible theoretical and synthetic value of bisphosphines in which one of the phosphorus atoms is included in a phosphole ring is outlined. This is followed by a brief account of the electronic structure of phospholes in the context of the possible aromaticity of such systems and the present conflicting position is discussed. The catalytic activity of phosphole complexes is also briefly reviewed in the context of possible extension to catalytically active systems based upon phosphino-substituted phospholes. Synthetic approaches to bisphosphines in general, and the [superscript 31] P and [superscript 13] C NMR spectral properties of these and some of their metal complexes are discussed in some detail. In the new work reported in this thesis, a number of synthetically useful and structurally interesting five-membered heterocyclic bisphosphines containing both ring and side-chain P atoms have been synthesized. Their [superscript 31] P, [superscript '] H and [superscript 13] C spectral properties, particularly unusual [superscript 31] P shifts and long-range P - C couplings, have been thoroughly investigated and their structures have been unambiguously established. It has been found that many of the reactions used in these various syntheses are stereospecific or stereoselective. A new and possibly general route to 4-oxo-2-phospholene oxides or sulfides from the corresponding 4-methy1ene compounds is described. This highly unusual reaction involves free-radical catalysed oxidative cleavage of the 4-methylene group by molecular oxygen and the reaction was found to proceed very smoothly when AIBN is used as the radical initiator for the oxidation. A possible mechanism for this oxidation is proposed.