Studies toward hydrocarbon activation mediated by high valent transition metal Lewis acids.

摘要

During the last 15 years, there has been many reports on C-H and C-C activation of hydrocarbons by transition metals. Such activation proceeds by breaking the C-H or C-C bond to create a carbon-metal bond. That new bond can then undergo a subsequent chemical transformation leading to a functionalized compound. Such a process would be extremely useful in the petroleum industry where feedstocks, which consist of long saturated chains, are extremely difficult to transform into functionalized molecules because of their chemical inertness. Currently, this process is achieved by means of solid superacids. Our goal is to synthesize a highly electrophilic titanium complex which would be able to activate alkanes.; The reaction of 2-phenylpropene with transition metals such as TiCl{dollar}sb4{dollar}, was studied so that a reference can be established for comparison with our future catalyst. It appeared that the reaction leading to cyclic oligomers, at room temperature, is proton mediated. In this case, the transition metal Lewis acid has the role of a co-initiator. By contrast, at low temperature where a linear polymer is obtained, the reaction could be initiated by the only temperature where a linear polymer is obtained, the reaction could be initiated by the only presence of a transition metal. That polymerization mechanism probably involves a precoordination of the titanium center to the aromatic ring of 2-phenylpropene.; The synthesis of a catalyst involved the reaction of TiCl{dollar}sb4{dollar} and TiCl{dollar}sb2{dollar}(OiPr){dollar}sb2{dollar} with 4-alkylamino-3-penten-2-one and 3-mercapto-1,3-diphenylprop-2-en-1-one ligands. Those ligands bound to the titanium center through the oxygen atom and gave dicarbonyl adducts of TiCl{dollar}sb4{dollar} and TiCl{dollar}sb3{dollar}OiPr. A TiCl{dollar}sb3{dollar}OiPr moiety has been isolated when TiCl{dollar}sb2{dollar}(OiPr){dollar}sb2{dollar} is reacted with any of the above ligands, and it suggests an equilibrium between TiCl{dollar}sb2{dollar}(OiPr){dollar}sb2{dollar} and TiCl{dollar}sb3{dollar}OiPr plus TiCl(OiPr){dollar}sb3{dollar}. X-ray structures have been performed on two of those compounds, one of them showed a hydrogen chloride bond which is quite unusual. None of the adducts obtained can dimerize 2-phenylpropene at room temperature. This suggests that, in presence of water, they do not release the HCl required to initiate the reaction. They also failed to polymerize 2-phenylpropene and that is probably due to their steric hindrance which prevents any precoordination to the olefin.