Studies relating to the coupling of carbon dioxide and propene at nickel (0).

Abstract

The work described in this thesis is concerned with the regioselective coupling of carbon dioxide and propene in the presence of nickel catalysts to give methacrylic acid, the catalytic cycle for which is given m Figure 1.0. An important aim of the work is to obtain information which can lead to a better understanding of the chemistry involved and to a viable process for the industrial production of methacrylic acid. Carbon dioxide is a remarkable source of carbon both as gas or bound in carbonate rocks. The total amount of carbon existing in the atmosphere as carbon dioxide is estimated to be 720 x 10(^9) tons(^1). The chemical utilisation of carbon dioxide as a resource for the production of useful substances has been rather limited, however. The large quantities of carbon dioxide available at relatively inexpensive cost is one driving force for the development of the use of carbon dioxide in industry. Another pressure to find ways to use carbon dioxide is related to its perceived role in the greenhouse effect. The important result of this work is the demonstration that the complexes of the type [L(_2)Ni] (I) and [L(_2)Ni](_9) (II) can be synthesised from methacrylic acid (I) and crotonic acid (II) respectively via an extension of the chemistry developed by Yamammoto et al and reviewed in chapter four(i). The same complexes can also be synthesised from propene and carbon dioxide via an oxidative coupling reaction at phosphine liganded nickel(O). The synthesis of the intermediates (type I and II) from the corresponding carboxylic acid is a two step process. The oxidative addition of methacrylic/crotonic acids to nickel(0) only occurs in the presence of one molar equivalence of tricyclohexylphosphine, a highly electron donating sterically demanding phosphine. The second reaction to form complexes of die type [L(_2)Ni] (I) and [L(_2)Ni] (II) is a ligand displacement reaction. The following compounds have been synthesised. Type (I) [L(_2)Ni] where = DPPE(IB), DCPE(1C), DCPP(1D), DCPy(lG), Bipy(lH), Phen(lJ), TMEDA(II). Type (II) [L(_2)NiCH(CH(_3))CH(_2)COO] where L(_2) = DPPE(IE), DCPE(IF).The complex [(DCPE)Ni] has been used for further reaction shidies, chosen because it has been shown that it can be synthesised from methacrylic acid as well as from propene/carbon dioxide mixtures. Ways were sought to destabilise the metallacycle to liberate methacrylic acid, the use of maleic anhydride and heat/pressure are reported. The final section of this work involved the oxidative coupling of propene and carbon dioxide at liganded nickel(O). Successful coupling in high yield was obtained using die ligands DCPE and DCPy only to yield complexes of the type [L(_2)Ni] and [L(_2)Ni]. A small degree of coupling was indicated using DCPP but this was only a minor component of the nickel containing species isolated at the end of the reaction. The amine ligands tried all appeared to form carboxylates and/or carbonates as indicated by die IR spectra of the solids isolated.