Classification of Fischer type metal carbenes

Abstract

Fischer type metal carbene complexes are used as catalysts in various organic synthesis reactions, e.g. metathesis, cyclopropanation and benzannulation. Nevertheless, the mechanisms of the above-mentioned reactions are not properly understood, especially with regard to frontier orbital interactions. Therefore, the focus of this study is on the classification of various Fischer type metal carbene complexes based on their catalytic activity for various reactions. A modified molecular modelling method used to classify these complexes was developed in previous studies in the Catalysis and Synthesis Group at the North-West University. The Fischer type carbene complexes with heteroaromatic groups investigated in this study were synthesised by a research group at the University of Pretoria. The heteroaromatic groups of interest in this study are furan, bithiophene, N-methyl-thieno[3,2-b]pyrrole, 2-(2’-thienyl)furan and N-methyl-2-(2’-thienyl)- pyrrole. Twenty five Fischer carbene complexes were optimized using Materials Studio 6.0 DMol3 density functional theory (DFT) module with the GGA/PW91 functional and the DNP basis set. Electronic and steric parameters were calculated for each Fischer carbene complex using Gaussian09, Chemissian and Solid-G. The data obtained from these calculations were analysed using principal component analysis within Statistica version 12 in order to establish trends. Therefore, computational techniques such as NBO analysis, NPA charges, shielding, frontier orbital analysis and multivariate analysis were used to classify these Fischer type carbene complexes according to their chemical properties. Results obtained from NPA charge analysis of the TM-C bonds indicate that the carbene carbons attached to chromium have a higher positive charge than those attached to tungsten. This explains the preference for chromium based Fischer type carbene complexes for benzannulation reactions. Furthermore based on electrophilicity indices we conclude from this study that complexes A3 (furyl-substituted), B9 (N-methyl-thieno[3,2-b]pyrrolyl-substituted) and C15 (bithienyl substituted) (Chapter 4) are suitable candidates for nucleophilic attack reactions; while complexes C12 (bithienyl-substituted), C13 (bithienyl-substituted) and D23 (2-(2’-thienyl)furylsubstituted) (Chapter 4) are suitable for benzannulation and metathesis reactions. Complexes A4 (furyl-substituted) and B6 (N-methyl-thieno[3,2-b]pyrrolyl-substituted) are suitable for both nucleophilic attack, metathesis and benzannulation reactions.