A mechanistic study of alkene hydrogenation

• Main Author: Hamid, S. A.
• Published: University of Cambridge 2001
• Subjects: 547.05
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.603611

The work described in this thesis explores the influence of the nature of transition metals, the electronic properties of ligands and the effect of counter-ions in the regioselectivity of transition metal-hydride addition (hydrometalation) to carbon-carbon double bonds. The method used to investigate the regioselectivity of the hydrometalation was based on the established methodology [Mode a (M<SUP>δ+</SUP> - H<SUP>δ-</SUP>) and Mode b (M<SUP>δ-</SUP> - H<SUP>δ+</SUP>)]. The <I>cis-</I>alkenes were subjected to hydrogenation using deuterium gas. Following the hydrometalation step, the rotation of one end of the substrate would give a <I>trans-</I>conformation, which would transform to the <I>trans</I>-isomer <I>via </I>β-hydride elimination. The location of the deuterium on the double bond would indicate the regioselectivity of the hydrometalation. Chapter 2 investigates the intrinsic nature of transition metal-hydrides, namely Pd/C, Pt/C and Rh/C. The behaviour of each M-H addition to the double bond of alkenes is compared, in an attempt to reveal the periodic trend in the transition metals. Information on the nature of these catalysts would be useful for modification of the catalysts in order to improve selectivity. Chapter 4 demonstrates the effect of the electronic properties of ligands in controlling the regioselectivity of hydrometallation. Wilkinson's catalyst is taken as a model system. A range of ligands bearing different electron-withdrawing and electron-donating substituents are synthesized and are then used to make Wilkinson's type catalysts. These catalysts are used in the hydrogenation of <I>cis</I>-β-methoxystyrene using deuterium gas. The implication of the findings for chiral induction is discussed. Chapter 5 describes an extension of the methodology to another class of catalyst, Ir-phosphinooxazolines. These complexes have been shown to be efficient catalysts for the asymmetric hydrogenation of alkenes. The effect of the counter-ions of the complexes on the regioselectivity of the hydrometalation is investigated.