Behind the Science: Main Group Metals for Homogeneous Catalysis

Dr. Anne Deveson, Chemistry – A European Journal, talked to Professor Robert Mulvey, University of Strathclyde, Glasgow, UK, about his article on alkali-metal hydride surrogates for catalytic dehydrogenative coupling and hydroboration applications.

The controlled formation of boron–nitrogen bonds by dehydrocoupling of amine boranes is a reaction that attracts widespread attention in the synthesis of novel polymers and ceramics, and in the arena of hydrogen storage materials. Robert Mulvey’s and Stuart Robertson’s latest paper at Chemistry – A European Journal reports on the use of alkali metal dihydropyridine compounds that can function as a precatalyst to convert Me₂NH⋅BH₃ to [NMe₂BH₂]₂. The lithium compound was also successful in promoting hydroboration reactions between pinacolborane and a selection of aldehydes and ketones. The mechanisms of these rare examples of Group 1 metal-catalysed processes are discussed.

What was the inspiration behind this study?

We had specific and general reasons for following this line of research. In earlier work, we synthesized and isolated lithium dihydropyridine complexes that exhibited excellent solubility in hydrocarbon and arene solvents and noticed that on gentle heating they released lithium hydride. Lithium hydride and its Group One congeners are insoluble in such solvents so it was obvious to us that the dihydropyridine systems could make novel soluble surrogates for their insoluble salt parent compounds.

One of the most exciting recent developments in inorganic chemistry has been the emergence of main group catalysts for homogeneous catalysis. We, therefore, wanted to trial our Group 1 surrogate hydrides in useful catalytic applications. This proved successful.

Why was your attention focused on Group 1 surrogate hydrides?

Our primary focus was on establishing the validity of the lithium-mediated catalysis since in general Group One complexes are rarely thought of as being feasible catalysts/pre-catalysts. In addition, the compounds of the type studied here are relevant to the synthesis of novel boron-nitrogen polymers and ceramics, while the study of dehydrogenative coupling reactions could have implications within the area of hydrogen storage materials.

What is the broader impact of this paper for the scientific community?

It may take years of fundamental research but the challenge is: Can main group metals start to make significant inroads into the dominance of precious transition metals in homogeneous catalysis? Results like this are just the beginning of a long journey, but a worthwhile journey given the general greater earth abundance and thus sustainability of main group metals versus their precious transition metal counterparts.

How do you plan to follow up on this discovery?

There is a myriad of possible follow-up reactions in the catalytic arena. For example, can these alkali metal hydride surrogates be applied for hydrometallation of more challenging unsaturated substrates such as esters or alkenes? Can we synthesize bimetallic ate modifications of these surrogates to enhance their nucleophilicity and increase the rate of catalysis? Can catalytic reactions of this type be extended to asymmetric synthesis?

Which part of the work proved the most challenging?

When you have highly skilled and enthusiastic researchers at your disposal the inert-atmosphere experimental work, though challenging, is accomplished straightforwardly. The problematic part of this study was trying to develop appropriate control reactions to give insights into all the possible mechanistic pathways that might be taking place in the catalytic cycles.

The article they talked about