published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The aim of the work carried out in this thesis is to develop new efficient catalytic systems for the synthesis of enantioenriched secondary alcohols. The access to this class of important synthetic intermediates is possible either by nucleophilic addition of organometallic reagents to aldehydes or by reduction of prochiral ketones catalysed with chiral complexes. We have thus developed two new catalytic systems for the asymmetric hydrosilylation of ketones using polymethylhydrosiloxane (PMHS) as an inexpensive reducing agent. A first approach is based on the synthesis of new N, S ligands and zinc complexes bearing a ferrocenyl backbone with planar chirality. Those compounds were prepared and evaluated first in the addition of diethyl zinc to aldehydes, reaching enantiomeric excesses up to 74%. The precatalysts were also evaluated in the reduction of prochiral ketones with PMHS. In that case, a good reactivity was observed and enantioselectivities up to 61% could be reached. It is assumed that a zinc hydride complex formed by thermal ß-elimination of the corresponding ethyl complex is the active species. This catalytic system also allows the hydrosilylation of dialkyl ketones, a challenging goal in asymmetric catalysis. In a second part of this work, we have developed a new catalytic system for hydrosilylation of ketones based on the hypothetical formation of a chiral cobalt hydride catalyst. This intermediate is supposed to be formed by reaction of the corresponding neutral cobalt (II) complex with tetrabutyl ammonium triphenyldifluorosilicate (TBAT) as activator and a silane. Following a preliminary screening, salen type ligands were found to be the most efficient when activated with TBAT in presence of PMHS. Modifications on this structure by replacing the phenol group of the ligand by various sulfonamide groups led, after optimisation of the reaction parameters, to a highly efficient catalytic system with enantioselectivities reaching 84 % for the reduction of tetralone. This work should thus open new perspective for the design of highly enantioselective and low cost catalytic systems for the hydrosilylation of prochiral ketones.

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Hydrometalation of α,β-unsaturated carbonyl compounds provides access to reactive metal enolates, which can then be trapped by a suitable electrophile. The coppercatalysed reductive aldol reaction involves hydrometalation of an α,β-unsaturated carbonyl compound, followed by an inter- or intramolecular aldol reaction. While there have been numerous examples of copper-catalysed reductive aldol reactions reported in the literature, the corresponding reductive Michael reaction has been relatively understudied. Herein, the copper-catalysed reductive Michael cyclisation of substrates containing two α,β-unsaturated carbonyl moieties is described. A range of structurally and electronically diverse substrates were prepared by various different methods. Both α,β-unsaturated ketones and esters underwent cyclisation, in the presence of a copper catalyst, a bisphosphine ligand, and a stoichiometric reductant, to afford 5- and 6- membered carbocyclic and heterocyclic products, with good-to-excellent levels of diastereo- and enantiocontrol. Furthermore, the diastereochemical outcome of these reactions is dependent on the specific reaction conditions used.

A general straightforward synthetic approach for the enantioselective construction of medium and large ring cyclopropyl ketones via diastereoselective Simmons-Smith cyclopropanation is discussed. Medium and large 2-cycloalkenone 1,4Di-O-benzyl-L-threitol ketals undergo efficient and diastereoselective cyclopropanation when treated with an excess of the Simmons-Smith reagent. The cyclopropyl ketals are obtained in 80-94% with >23:1-8:1 diastereoselectivity. The first enantioselective synthesis of (-)-modhephene, a natural product possessing the carbocyclic (3.3.3) propellane skeleton, by the use of diastereoselective cyclopropanation methodology is described.

Luk To. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 115-123). / Abstracts in English and Chinese. / Contents --- p.i / Acknowledgement --- p.ii / Abstract --- p.iii / Abbreviation --- p.vi / Chapter 1. --- Introduction --- p.1 / Chapter 1-1 --- Background --- p.1 / Chapter 1-2 --- Sharpless Epoxidation --- p.2 / Chapter 1-3 --- Mn-Salen Complexes for Epoxidation --- p.4 / Chapter 1-4 --- Oxaziridinium Salts --- p.6 / Chapter 1-5 --- Dioxiranes --- p.9 / Chapter 1-6 --- Asymmetric Epoxidation by Dioxirane --- p.12 / Chapter 2. --- Results and Discussion --- p.31 / Chapter 2-1 --- Epoxidation of trαns-disubstituted and trisubstituted alkenes catalyzed by L-αrαbino-4-uloses --- p.31 / Chapter 2-2 --- Epoxidation of cis-alkenes catalyzed by L-arabino-4-uloses --- p.47 / Chapter 2-3 --- Epoxidation of alkenes catalyzed by new L-arabino-4-uloses --- p.55 / Chapter 3. --- Conclusion --- p.77 / Chapter 4. --- Experimental Section --- p.80 / Chapter 5. --- References --- p.115 / Chapter 6. --- Appendix --- p.124

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The photoelectron spectra of a series of ketone compounds have been investigated in the gas phase. Core ionization energies were measured to probe the effects of molecular size and connectivity number on core ionization energies. It was found that core ionization energies roughly decrease with increasing connectivity number as expected. However, subdividing into several categories based on the molecular type gives improved correlations. These results show both the effect of molecular size and that of bringing the polarizable group closer to the core ionized center. In some cases, compounds with the same connectivity number have different core ionization energies and, in others, compounds with different connectivity number have the same core ionization energies. These discrepancies indicate that the simple method for calculating connectivity numbers must be modified to reflect different molecular types. Other discrepancies indicate that this method does not correctly predict the effects of remote hydrogens or the effects of aromatic substituents, where conjugation may influence the relaxation energy. / Graduation date: 1995

El treball experimental que ha permès redactar la present Tesi Doctoral ha estat dividit en dues parts. A la primera part es presenten els resultats referents a la síntesi estereocontrolada de noves heteroarilglicines (quinolil i pirazolilglicines) a partir de cetones acetilèniques, substrats quirals que permeten accedir a l'esquelet de diferents heterocicles (quinolines i pirazoles), la posterior obtenció dels corresponents quinolil i pirazolil--aminoalcohols i les diferents metodologies d'oxidació per tal d'accedir a les corresponents quinolil i priazolilglicines objectiu. A la segona part d'aquesta memòria s'ha estudiat, en dissolució, l'habilitat del grup alquilsulfona com a grup sortint eficaç en reaccions d'ipso-substitució nucleofilica. El desenvolupament d'aquesta reacció ha servit de punt de partida per a la creació de llibreries d'heterocicles amb alta diversitat molecular i potencial interès biològic sobre fase sòlida. / These work have been divided in two sections. In the first section, a new and efficient methodology towards the stereocontrolled synthesis of novel -acetylenic ketones containing a masked -amino acid functionality in enantiomerically pure form has been developed. This new chiral building block is the starting material for the synthesis of enantiomerically pure quinolyl and pyrazolylglycines, important structures with potential biological activity and very useful for the synthesis of peptidomimetics. The methods begins from the Garner's aldehyde and takes place through an alkyne derivative via a carbonyl-alkyne homologation reaction. Condensation of the lithium acetylide at low temperatures with different aldehydes yielded the corresponding propargyllic alcohol derivatives in good yields, which under mild oxidative conditions gave the corresponding acetylenic ketones algmost quantitatively.In the second part of this work, the ability of alkylsulfonyl groups as efficient leaving groups in heteroaromatic mucleophilic ipso-substitution reactions has been studied. The rational behind this study has been the possibility of transferring the developed methodology to the solid support and thus to synthesize in a parallel fashion a small library of molecularly diverse heterocycles with potential biological interest. An in solution optimised methodology has been successfully transferred to the solid support and a small library of molecularly diverse benzothiazoles (-excedent heterocylces) was prepared in a parallel fashion in good overall yields and purities of the final products.