Principal Investigator Brian Yates Project g29

Department of Chemistry Machine VP

University of Tasmania


Theoretical Studies on the Mechanisms of the Stevens rearrangement and Related Reactions

We have used computational chemistry to calculate the structures and energies of organic molecules, reactive intermediates and transition structures, with a view to investigating the mechanisms of chemical reactions, including specifically the Stevens and Sommelet-Hauser rearrangements.

The Stevens rearrangement of an alkylammonium ylide to an amine was discovered in 1928 and has found recent application in organic synthesis in the preparation of new heterocyclic compounds. This reaction and the Sommelet-Hauser rearrangement are used in various ring-expansion procedures. With the aid of the VP2200 we have carried out a thorough investigation of the mechanisms of these reactions, including the effect of electron withdrawing and electron donating substituents at various positions on prototype chemical systems. The use of the VP2200 has made it possible to include solvation effects and to study systems of a realistic experimental size. All of this information has provided us with a broader understanding of the mechanisms and enabled us to better interpret the experimental studies.

This project was supported by a large ARC grant.


What are the basic questions addressed?

For the Stevens and Sommelet-Hauser rearrangement, which is lower in energy?

How does this answer change as a function of different steric and electronic substituent effects?

How does this answer change when the size of the molecule is increased?

How does this answer change as a function of solvation?

What are the results to date and future of the work?

We have investigated the Stevens and Sommelet-Hauser rearrangements for a prototype ylide, N-methyl-3-propenyl ammonium methylide, using ab initio and semi-empirical molecular orbital methods.

The results of this project show that the activation energies for the two chemical processes are remarkably close, separated by only 2 kJ mol-1 at ROMP/6-311+G(d,p). Increasing the size of the basis set leads to a relative stabilisation of the Sommelet-Hauser transition geometry, while higher levels of electron correlation (such as CCSD(T)) favour the Stevens rearrangement. Incorporation of solvent effects via the SCRF polarisable continuum model leads to a lowering of the energy barrier of the concerted rearrangement, but has little effect on the dissociative pathway.

The activation energies of both pathways have been calculated for ylides bearing substituents on the ammonium nitrogen and the double bond. Substituents at nitrogen lead to an ylide which is sterically unstable, and hence a preference for the dissociative rearrangement. Electron-withdrawing substituents on the double bond show a preference for the rearrangement, while mildly electron-donating alkyl substituents have very little effect on activation energies.

Further calculations are planned to more closely relate the systems studied to those available experimentally, and to investigate competing mechanisms in organometallic reactions.

What computational techniques are used and why is a supercomputer required?

We have used quantum chemistry techniques (mainly conventional ab initio SCF and MP2 methods together with density functional methods) with the Gaussian 94 program. These calculations make good use of the vector facility. We require a supercomputer to enable us to study large molecules with high levels of theory. Such big calculations are beyond our local workstation resources.

Publications

Theoretical Evaluation of Alternative Pathways in the Stevens Rearrangement, G. L. Heard, B.F.Yates Aust. J. Chem., 48, 1413-1423 (1995)

A hybrid supermolecule-polarisable continuum approach to solvation: application to the mechanism of the Stevens rearrangement, G. L. Heard, B. F. Yates, J. Comp. Chem., in press.

Competing Mechanisms in the Carbonylation of Palladium (II) Complexes Containing Bidentate Ligands: Theoretical Insights K E Frankcombe, K. J. Cavell, R. B. Knott, B. F. Yates, J. Chem. Soc., Chem. Commun., in press.

Competing Rearrangements of Ammonium Ylides: A Quantum Theoretical Study, G. L. Heard, B.F. Yates, J. Org. Chem., in press.

Ligand substitution: an assessment of the accuracy of ab initio calculations K.E. Frankcombe, K.J. Cavell, R.B. Knott, B. F. Yates, J. Phys. Chem, submitted.