Synthesis of new zinc and cobalt complexes : application to the enantioselective hydrosilylation of ketones
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.
School:Université catholique de Louvain
Source Type:Master's Thesis
Keywords:asymmetric ketones hydrosilylation cobalt zinc complexes catalysis
Date of Publication:11/03/2006