THEORETICAL STUDIES OF HYDROBORATIONAND ALLIED REACTIONS

Abstract

Hydroboration of olefins is one of the most important reactions in synthetic organic chemistry, because the organoboranes formed are very useful intermediates and are subject to a large number of organic transformations under mild conditions. In the present work we investigate the computationally hydroboration of strained cyclopropane ring, which is known to exhibit in several instances olefin like behavior. Hydroboration of substituted cyclopropanes are also studied. The analogous hydroalumination of cyclopropane using A1H3 is also studied. Addition of MgH2 and LiH to cyclopropane are also investigated. All calculations have been performed using the Gaussian 98W suite of programs. The thesis is divided into six chapters, whose contents are outlined below. V The First chapter presents a general introduction and an overview of hydroboration and allied reactions. Emphasis is placed on the transition structures along the reaction path. The related hydroalurnination reaction and addition of hydrides of magnesium and lithium are also reviewed. A critical review of the available literature on computational studies on hydroboration and related reactions are presented, comparisons with relevant experiments are also made wherever possible. The Second chapter outlines the computational methods used. A brief introduction to the techniques of geometry prediction, using ab initio SCF and Density Functional methods, and of characterization of stationary points on the potential energy surface are given. The Third chapter deals with the computational studies of the hydroboration of cyclopropane with borane. Optimization of the key species, complex, transition structure and the addition product formed is the reaction done at Hartree-Fock, MP2 and DFT/133LYP level, using 631G**, 6-31 l++G**, AUG-cc-pVTZ and Dunnings basis, cc-pVDZ. Two types of transition structures have been reported: a three-centered transition structure in which borane approaches along the plane of the cyclopropane ring and the second in which borane approaches perpendicular to the plane of the ring. The critical geometrical parameters are also reported. The bond making and bond breaking phenomena are shown using HOMO's of the complex and transition structure. Relative energies of the stationary species are presented. (i) 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, showing clearly the transition state moving towards the reactant and product sides. Single point energies computed at B3LYP optimized geometries at different ab initio level have also been reported. The Fourth chapter of the thesis incorporates computational studies on the hydroboration of substituted cyclopropane, the substituents used are —F, -Cl, -CN, - NC, -CH3 and —(CH3)2. The geometries of the stationary structures are performed at the DFT/B3LYP level using 631G** basis set in each cases and the nature of each stationary point was probed by frequency calculations. Single point calculation at different ab initio level have also been performed. These studies point to different possible path'ays for the addition of BH3 to cyclopropanes. The Fifth chapter of the thesis reports investigation of the hydroalumination of cyclopropanes. Ab initio and DFT/B3LYP calculations have been performed with 631G**, 6311++G** and Dunning's cc-pVDZ and AUG-cc-pVTZ basis sets. These calculations revealed four-centered transition states with B located in the plane of along and perpendicular to the cyclopropane ring. The viability of reaction has been confirmed by using energy barriers calculated for the reaction path. The Sixth chapter reports the theoretical study of the addition of hydrides of lithium and magnesium i.e. and MgH2 and LiH to cyclopropane. First the addition of MgH2 to cycloproapne have been discussed. All calculation are performed using RHF, MP2 and DFT/B3LYP level of theory using 631G** and 6311++G** basis set. Both the possible approaches along the plane of the ring and perpendicular to the ring are investigated. A study of LiH addition follows. Same levels of calculations as used for addition of MgH2 to cyclopropane are used. Single point calculations at the higher level of theory are also reported.