THEORETICAL STUDIES ON SOME ORGANC REARRANGEMENT REACTIONS

Abstract

Rearrangement reactions play an important role in organic and Biochemistry and are very interesting instance of unimolecular reactions. These reactions play a significant role in biological system. The beauty of rearrangement lies in their simplicity. They do not require catalyst and sometime can be initiated by light or slight pH variation. Sigmatropic reactions are an important class of organic rearrangement reactions which forms a sub class of pericyclic reaction. The mechanism of some of these reactions has been subject of the most heated controversies. Woodward and Hoffmann defined a pericyclic reactions as a concerted reaction in which all bonds are made or broken around a circle. Although the Wodward-Hoffmann rules declare what may and may not happen, the rule served not to settle mechanistic question but to raise the stake on what were already lively controversies. This thesis deals with computational studies carried out on Cope, Claisen and Walk rearrangements. These reactions in some form were investigated from mechanistic point of view at several levels of calculation, with several basis sets. All calculations have been performed using the Gaussian 98 suite of programs. The thesis is divided into the following chapters. The First chapter presents a general introduction and an overview of organic rearrangement reactions. A discussion of Pericyclic reactions with emphasis on sigmatropic reactions is presented. A general introduction to Cope, Claisen and Walk rearrangements are given and a few interesting uses of these reactions are cited. Relevant literature is reviewed in the following chapters dealing with each reaction. (i) 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 is given. The third chapter deals with our studies on the cope rearrangement. Cope rearrangement of 1,5-hexadiene was investigated extensively . The reaction pathway through boat and chair configuration were studied at RHF, MP2 and B3LYP levels using 6-31G(d), 6-31G(d,p) and D95V(d,p) basis sets. As expected the chair pathway is energetically favorable and the barriers along the chair pathway are calculated at various higher levels like MP3, CCD, CCSD, CCSD(T) and CASSCF levels. Following a remark by Davidson in their DFT studies on the C2h cut through the PES for this reaction we reinvestigated the C2h cut at different levels i.e. RHF, MP2 and DFT methods using both non-hybrid and hybrid Density Functionals. To allow for the involvement of a diradical intermediate while obviating the need for a spin multiplicity change, a singlet diradical state is studied at the various levels mentioned. The effect of substituting one hydrogen by fluorine and the effect of substitution of two fluorines at 1,6 and 2,5 position were investigated. The barriers obtained were analysed using the Marcus equation following the treatment of Murdoch et al. The reaction in barbaralane was also investigated as it provides the mechanism for fluxionality in the molecule, and is a model for the more complex situation obtaining in Bullvalene.

The fourth chapter deals with Claisen rearrangement. Boat and chair configurations were studied at RHF, MP2 and B3LYP levels as in the case of cope rearrangement. Solvent effect on the Claisen rearrangement was studied using the SCRF model. Effect of water on the barrier was studied by explicitly incorporating one and two water molecules in the reaction. The fifth chapter reports the investigation on walk rearrangements. [4.1.0] bicycloheptadiene was studied at RHF and B3LYP levels to explore the relative preface for walk rearrangement and the ring opening reaction. The fluoro substituted [4.1.0] bicycloheptadiene was also studied and different possibilities of the walk rearrangement was investigated. Walk rearrangement in [2.1.0] pentene was also studied. Both the concerted mechanism and a pathway via a biradical intermediate were studied, and the biradical pathway appears to be the more favourable The thesis concludes with a few suggestions for further work.