Some aspects of organo-phosphorus chemistry.
thesisposted on 19.11.2015, 09:19 by Hassan Abu-el-Seoud. Aly
Chapter 1 represents a review on the synthesis and structure of 1-substituted 2,2,3,4,4-pentamethylphosphetans. In Chapter 2 the stereochemistry and mechanism of base-catalysed phosphetanium salt hydrolysis are envisaged and shown to depend upon the reaction conditions, in particular the pH of the medium. The results are interpreted in terms of the intermediacy of TBP structural phosphoranes, and the most significant role of the BPR as a viable mechanism for intramolecular isomerization of such intermediates, in determining the products distribution, is discussed. The possibility of a pre-decomposition equilibrium between diastereomeric salts via pseudorotation is considered and shown to occur, specifically, in very dilute alkaline media. A change in the reaction rate-limiting step from a slow isomerization process to a phosphorane decomposition is postulated to account for variation in product composition on varying the conditions. The alkaline hydrolysis of r-1-substituted acetylenic 2,2,3-trans-4,4-pentamethylphosphetanium bromides proceeds, in contrast to the generally reported retention as the stereochemically preferred pathway for phosphetans, with complete inversion of configuration. Base-catalysed ring expansions of a number of salts are shown to proceed, stereospecifically, to give only one observable product oxide. This is attributed to a very short-lived intermediate phosphorane. In the end a qualitative scale of the relative apicophilicities of the studied groups, is observed. In Chapter 3, an attempt to synthesise the 1:2 adducts of 1-substituted acetylenic-2,2,3,4,4-pentamethylphosphetan with HFA resulted in formation of other products for which a probable mechanism and structures, based on spectroscopic and elemental analysis data are discussed and suggested for each product. A variety of phosphoranes containing phosphetan rings was synthesised. A kinetic study of the rate of reaction of 2-ethoxy-1,3,2-dioxaphospholan with benzil showed the reaction to follow a bimolecular step-wise mechanism. Relative apicophilicities of a number of groups attached to the phosphorus atom in phosphetan adducts were determined by monitoring the high energy isomerization pathways, accessible to these adducts, in which the five-membered rings span diequatorial positions, using 1H n.m.r, spectroscopy techniques. The values so determined are rationalised in terms of substituent electronegativities and the extent of a probable ?-electron interaction with the phosphorus d-orbitals.