COMPUTATIONAL STUDIES ON OLEFIN METATHESIS REACTIONS

Abstract

Olefin metathesis is a unique carbon skeleton redistribution in which unsaturated carbon-carbon bonds are rearranged in the presence of metal carbene complexes. With the advent of efficient catalysts, this reaction has emerged as a powerful tool for the formation of C-C bonds. The number of applications of this reaction has dramatically increased in the past few years. The broad applicability of olefin metathesis has attracted attention from both academic and industrial scientists. In the last several years, computational studies on olefin metathesis reactions have proliferated. Treatment of the reactions with quantum chemical methods involves calculation of geometries and energetics of reactants, intermediates, transition states, and products. In the present work we investigate computationally some olefin metathesis reactions catalyzed by tungsten (W), molybdenum (Mo) and ruthenium (Ru) alkylidene. The mechanistic studies of tungsten catalyzed ring opening metathesis (ROM) and molybdenum catalyzed ring opening metathesis polymerization (ROMP) are carried out using the highly strained 3,3-dimethyl cyclopropene (DMCP) moiety. The stereochemistry of ring opening of asymmetric 3-methyl-3-phenylcyclopropene (MPCP) is also studied and the effect of substituents on ROM is explored. The ruthenium catalyzed ring opening-cross metathesis (ROCM) is investigated using trisubstituted cyclopentene and methyl vinyl ketone as model compounds. All calculations have been performed using the Gaussian 09W suite of programs. The thesis has been divided into six chapters: The first chapter presents a general introduction to olefin metathesis reactions and a review of the relevant literature. Emphasis is placed on the different types of metathesis in olefins and metathesis reaction mechanism. Acyclic diene metathesis (ADMET), cross metathesis (CM), ring opening metathesis (ROM), ring closing metathesis (RCM) and ring opening metathesis polymerization (ROMP) are discussed along with the mechanisms. The Schrock and Grubbs catalysts used in olefin metathesis are also described. A critical review of the available literature on computational studies of olefin metathesis reactions is presented and comparisons with relevant experiments are also made wherever possible. The second chapter outlines the computational methods used. A brief introduction to ab initio SCF and Density Functional methods and of the location and characterization of stationary points on the potential energy surface is presented. ii The third chapter deals with the computational studies of the ring opening metathesis of 3,3-dimethyl cyclopropene using tungsten model catalyst W(NH)(CH2)(OCH3)2. Two different faces of the catalyst namely COO face and CNO face have been investigated for their involvement in the cycloaddition step of ROM of the DMCP. syn and anti orientations of cyclopropene are also explored for the reaction. Optimization of the geometry of all species has been done at DFT/B3LYP level using LANL2DZ basis set. The critical geometrical parameters are also reported. Relative energies of the stationary species are presented. Frequency analyses were also performed to confirm that the structures obtained were true minima on the PES or saddle points as the case may be. IRC calculations are done from each transition state, to verify the structure moving towards the reactant and product sides. The fourth chapter incorporates computational studies on the molybdenum model catalyst Mo(NH)(CH2)(OCH3)2 mediated ring opening metathesis polymerization reaction of 3,3-dimethylcyclopropene. The COO and CNO faces of molybdenum catalyst as well as syn and anti orientations of cyclopropene are discussed for ROMP reaction. The geometries of the stationary structures are obtained at the DFT/B3LYP level using LANL2DZ basis set in each case and the nature of each stationary point was probed by frequency calculations. IRC calculations have also been performed. The fifth chapter of the thesis presents the investigation of the stereochemistry of the ring opening metathesis of asymmetric 3-methyle-3-phenylcyclopropene (MPCP). DFT/B3LYP calculations have been performed with LANL2DZ basis sets. The calculations revealed ring opening of MPCP with parallel and perpendicular conformers. Effect of substituents on ring opening metathesis of cyclopropene is also explored by substituting the phenyl group of MPCP with NH2, OH, CN and CF3 groups. The viability of reaction has been verified using energy barriers calculated for the reaction path. The sixth chapter presents the computational modeling of the whole catalytic cycle of ruthenium catalyzed tandem ring opening cross metathesis reaction to obtain enddifferentiated product. The dissociative mechanism was explored in a detailed study of the ROCM of trisubstituted cyclopentene with methyl vinyl ketone. Distal and proximal orientations of trisubstituted cyclopropene are discussed for the first catalytic cycle (ROM). In subsequent catalytic cycle (Cross metathesis) of ROCM, cis and trans orientations of methyl vinyl ketone are studied and the catalytic cycle is investigated using cis orientation to obtain end-differentiated olefin through tandem ROCM. All calculations are performed iii using B3LYP and M06-L functional. Ruthenium atom is treated with LANL2DZ basis set and 6-31G(d) basis set applied for all other atoms. Stationary points located on potential energy surface were characterized by frequency calculations as minima or transition state.