A Theoretical Investigation Of Novel Reactions For Selected Heterocyclic Compounds

Submission note: A thesis is submitted in total fulfilment of the requirement for the degree of Doctor of Philosophy to the Department of Chemistry and Physics, School of Molecular Science, College of Science, Health and Engineering, La Trobe University, Victoria, Australia.

In this thesis, computational chemistry methods have been employed to investigate the mechanism of a variety of novel chemical reactions involving heterocyclic compounds, focused on two streams. The first section is a computational study of ring expansion reactions (RER) with Nheterocyclic carbenes (NHC) and analogues containing alternative heteroatoms. A predictive theoretical study has been carried out to investigate the insertion pathway of N-heterocyclic carbene (NHC) rings containing one N atom and one P, N, C, O, or S heteroatom. Ring expansion was determined to follow a pathway common to NHCs, leading to a final expanded six-membered ring. Consideration of kinetics (barrier heights) enabled us to predict which heterocyclic carbenes might undergo expansion reactions. Another part of this study was carried out in collaboration with experimental work, for which the theoretical investigation served to rationalize experimental observations. A mechanism was determined for both observed products, for which the major product from hydride migration is calculated to be both thermodynamically and kinetically favoured over the minor product that arises from phenyl migration. The potential for P-heterocyclic carbenes (PHCs) to act as analogues of NHCs in ring insertion reactions recently observed with element hydrides has been explored. Similar reactivity is not expected for PHCs, although initial adduct formation and hydrogen transfer from the element hydride is thermodynamically favoured. The rate-determining step of ringexpansion is calculated to be too high to be experimentally achievable. The second part of this thesis is focused on boron heterocycles. A mechanism of ring insertion for 9-borafluorene by 1-adamantyl phosphaalkyne has been investigated. Results indicate that the insertion reaction gives a seven-membered ring via a single transition state. X A separate study of Diels-Alder reactivity of boroles with different dienes; 2,3- dimethyl-1,3-butadiene and 1,3-cyclohexadiene. Reaction with the butadiene indicates that the borole is acting as the dienophile, followed by rearrangements to furnish bicyclic species. For 1,3-cyclohexadiene, a [4 plus 2] process is observed in which the cyclodiene serves as the dienophile, with the borole acting as the diene. The calculation of the mechanism rationalises the experimental observations.