D.Sc. / Please refer to full text to view abstract

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D.Sc. / Please refer to full text to view abstract.

This research has been directed at the study of organosulfur frontier orbital interactions with transition metal d-orbitals. Two novel thioether complexes tricarbonyl(1,4,7-trithiacyclononane)molybdenum(O) and tricarbonyl(2,5,8-trithianonane)molybdenum(O), have been prepared and structurally characterized by single crystal x-ray crystallography. The facial configuration of the carbonyl ligands provides a unique point of reference for describing the two polythioether ligands in terms of the free ligand's frontier orbitals. The relative carbonyl stretching frequencies of the two metal complexes indicate that 1,4,7-trithiacyclononane is a poorer donor than 2,5,8-trithianonane. This result is explained in terms of mechanical constraints placed on the mesocyclic polythioether which are absent in its acyclic analogue. The coordinatively unsaturated species (n⁵-C₅H₅)MO(NO)(SC₆H₅)₂ has been characterized by x-ray crystallography and its electronic structure has been modeled using Fenske-Hall molecular orbital calculations. The monomeric nature and chemical inertness of (n⁵-C₅H₅)MO(NO)(SC₆H₅)₂ are attributed to dπ-pπ bonding between the thiolate ligands and an empty molybdenum dπ orbital. The dπ-pπ interaction simultaneously strengthens the metal thiolate bond and makes the complex less susceptible to nucleophilic attack by raising the energy of the LUMO. The rotational orientations of the thiolate ligands observed in the solid state support this electronic model. For (n⁵-C₅H₅)Fe(CO)₂SR, the dπ-pπI antibonding interaction between the thiolate ligand and the metal has been modeled using Fenske-Hall molecular orbital calculations and experimentally investigated by photoelectron spectroscopy. The calculations predict that the HOMO is metal-sulfurn-antibonding and largely sulfur 3p in character. The observed HOMO ionization energies of (n⁵-C₅H₅)Fe(CO)₂SC₆H₄-p-Z; Z = OMe, H, Cl, CF₃, N0₂; correlate with several chemical properties including the rate of electrophilic attack on the sulfur by alkyl halides to give the thioether complex [(n⁵-C₅H₅)Fe(CO)₂(SR₂)]X and by electron-deficient alkynes to give the heterometallacycle (n⁵-C₅H₅)(CO)FeS(R)-C=C=C=0. The latter reaction is compared to the similar reaction of alkenes and alkynes with (n⁵-C₅H₅)Fe(CO)₂PR₂ to give (n⁵-C₅H₅)(CO)FeP(R)₂-C=C-C=0. X-ray crystal structures of one of the sulfur-containing and one of the phosphorus-containing heterometallacycles have been obtained.

Section I of this thesis is concerned with the rearrangement of allylic sulphinate esters to sulphones. Isomerisation of cyclohex-2-enyl toluene-p-sulphinate (1) and several related substituted sulphinates was investigated particularly in ionising solvents such as formamide. The substituted systems, 1-[²H]-cyclohex-2-enyl toluene-p- sulphinate and 3-methylcyclohex-2-enyl toluene-p-sulphinate (2), gave sulphones with the label scrambled between C-1 and C-3. Rearrangement of the conformationally biased esters, cis- and trans-5-t-butylcyclohex-2-enyl toluene-p-sulphinate, was not stereospecific. Sulphinate (1), both diastereoisomers of which reacted at the same rate, rearranged more slowly than did sulphinate (2). Rearrangement of sulphinate (1) in the presence of sodium benzenesulphinate lead to formation of some of the phenyl sulphone. A dissociative mechanism involving ion-pairs is proposed for the rearrangement of the above allylic sulphinates in ionising media. The [1,3] sulphonyl group migration of some allylic sulphones is discussed in Section II. 3-Methyl-3-(p-tolylsulphonyl)- 1-butene rearranged on heating in acetic acidwater (3:2 v/v) containing toluene-p-sulphinic acid or its sodium salt, and in this case slow rearrangement did occur in the absence of such additives. Substituted 3-(p-tolyl-sulphonyl)cyclohexenes rearranged in hot aqueous acetic acid containing sodium toluene-p-sulphonate, but were stable in the absence of added sulphinate. Under these conditions, epimerisation of cis- and trans-5-t-butyl-3-(p-tolylsulphonyl)- cyclohexene occurred at the same rate as deuterium was scrambled between C-1 and C-3 starting with 5-t-butyl-3-[²H]- 3-(p-tolylsulphonyl)cyclohexene. Benzenesulphinate was incorporated in a crossover experiment with 3-(p-tolyl-sulphonyl)cyclohexene. A chain addition - elimination mechanism involving either radicals or radical anions (an S_RN1 process) is suggested for the [1,3] rearrangement of the allylic sulphones under the above conditions. Attempts to develop a stereospecific 5-membered ring annulation procedure involving allylic sulphones are described in Section III. As a model system, 1-(p-tolylsulphonyl)-1-vinylcyclopropane, prepared from 3-(p-tolylsulphonyl)propene, rearranged on flow pyrolysis to 1-(p-tolylsulphonyl)- cyclopentene.

M.Sc. / Please refer to full text to view abstract

Yu Pui-yee. / Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaves [114]-117.

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