Mechanistic and structural investigations of decarboxylation reactions

Abstract

The utilization of carbon dioxide as a C1 source for the production of organic substrates presents a great challenge to researchers. One such process would be the carboxylation of hydrocarbons to afford carboxylic acids. Relevant to this objective research efforts in our group have focussed on measuring the energetics of the closely related process, decarboxylation. In particular our studies have included defining the mechanistic aspects of the catalytic decarboxylation of cyanoacetic acid utilizing W(CO)5O2CCH2CN- as a catalyst. Since this species reacts to yield the nitrile complex W(CO)5NCCH2COOH in the presence of excess cyanoacetic acid, it was necessary to investigate the displacement of the nitrile ligands in W(CO)5NCCH2COOH, as well as in the W(CO)5NC(CH2)10COOH and W(CO)5NCCH3 complexes. The solution and solid-state structure of the W(CO)5NCCH2COOH complex was determined. From these investigations we found that the displacement of cyanoacetic acid by CO proceeds via a dissociative interchange process in THF and by a dissociative process in CH2CI2 . This is evidenced by the activation parameters found for the displacement reaction, where ΔH = 19.4 kcal/mol and ΔS = -14.7 e.u. in THF and ΔH = 29.2 kcal/mol and ΔS = 11.8 in CH2CI2 . Also examined was the development of other catalysts for the decarboxylation of cyanoacetic acid. In this effort we found that W(CO)5O2CCH2CN-, W(CO)4PPh2CH2COO-, W(CO)4PPh2CH2CH2S-, and W(CO)4PPh2CH2CH2NMe2 are active catalysts for the decarboxylation of cyanoacetic acid. Molecular modeling of the proposed intermediates W(CO)4(O2CCH2CN)(NCCH2COOH)- and W(CO)4(PPh2CH2COO)(NCCH2COOH)- indicated that hydrogen transfer could readily occur between the oxygen of the bound cyanoacetic acid ligand and the basic end of the chelated ligand. The active intermediate W(CO)4(PPh2CH2CH2NMe2)(NCCH2COOH) has been observed in the infrared after reaction of cyanoacetic acid with the W(CO)4PPh2CH2CH2NMe2 complex...