There are two main projects under current investigation:

Lead (IV) Complexes in Synthesis

Whilst organometallic compounds derived from transition metals have been extensively applied to organic synthesis, those of lead(IV) and bismuth(V) are less so. Why could these metals be so useful? In fact, they are good electrophiles, have high co-ordination numbers and oxidation potential, and there is precedent for their application in oxidation reactions and ligand coupling. However, the design of ligands for lead(IV), for example, is much more challenging than for so many other (non-oxidising) metals, since we require an electron donor which is not readily oxidised; this restricts substantially the choice of possible functional groups. We have chosen chelating carboxylates, often with pendant nitrogen donors, for investigation. Lead tetracarboxylates derived from monocarboxylic acids and dicarboxylic acids are readily accessible, by metathesis of lead tetraacetate; this method allows the preparation of mixed ligand complexes by appropriate choice of stoichiometry of the desired ligands. Our specific aim has been to identify ligands which give organic-soluble lead(IV) systems, capable of reacting both as 2-electron oxidants and in carbon-carbon bond forming reactions.

In addition to studying their synthesis and reactivity, we have recently initiated detailed structural studies of many of these compounds using crystallographic, mass spectroscopic and 207Pb nmr spectroscopic (solution and solid state) techniques; this has in turn given us a greater understanding of the reactivity of these lead(IV) compounds. More recent work has begun to focus on electrochemically mediated reaction of Pb(IV) for use in catalytic processes.

What are the applications of this work? We have been able to identify novel reactions specifically mediated by lead(IV) and it is our aim to develop these into synthetically valuable processes, including esterifications (a) and lactone ring synthesis (b).

Crystal Structures

A crucial aspect of this work has been the successful determination of x-ray structures of a variety of lead(IV) compounds.

- Lead tetraacetate.py complex

- Lead tetra(o-benzoylbenzoate)

- o-Methoxylead triacetate

- p-Methoxylead triacetate

- Di(o-methoxyphenyl)lead diacetate

- Diphenyllead diacetate

Reformatsky reagents have been known for nearly 100 years, but have not been used very widely in organic synthesis. We are working on the development of the chemistry of these useful reagents to allow their application to a wider variety of synthetic transformations. For example, we have shown that the a-arylation of esters and amides is possible under mild conditions using microwave catalysis; this represents a significant improvement over existing procedures.

 

We have also shown that Reformatsky reagents are useful nucleophiles, suitable for application in conjugate addition reactions, provided that the alkene is activated by two electron withdrawing groups. This simple transformation has been generally overlooked as an alternative to, for example, organocuprate additions.

This process is of particular value for stereocontrolled conjugate additions in lactam systems.