The CAS-Croucher Funding Scheme for Joint Laboratories is the outcome of an agreement between The Chinese Academy of Sciences (CAS) and the Croucher Foundati...
Molecular catalysts for carbon bond-forming reactions
The development of effective catalysts for clean processes with energy or reagent economy is of fundamental importance both in basic research and industrial applications.
Professor Zuowei Xie
In view of the importance of ligand designs in developing new catalysts or their precursors for optimising activity and selectivity, Professor Xie and his colleagues have designed a series of new ligands and their associated metal complexes. Some of these demonstrate very high catalytic activities and selectivity in Carbon-Carbon and Carbon-Nitrogen bond forming reactions.
We wanted to make the carbon-carbon bonds more efficient or more environmentally friendly, without any waste formation and so that the reaction can be done at room temperature or in very convenient reaction conditions, Xie explains. Catalysts make these reactions faster, and more efficient.
The catalyst is a metal plus a ligand. Usually the metal is fixed because the number of metals is limited but the number of ligands is unlimited, to a certain extent. So the ligands will make the reactions faster, more efficient, Xie says. Additionally, there is more selectivity so theres more room for manipulation.
For the catalytic formation of Carbon-Carbon bonds, Xie and his team have made some important inroads. He describes these in some detail:
- A series of tetraphenylene derivatives and their metal complexes were prepared which catalysed asymmetric hydrogenation of aminoalkenes with an excellent enantioselectivity and quantitative conversion,
- an efficient catalytic asymmetric Nazarov reaction was developed using tris(oxazoline)Cu(II) as a catalyst. This lead to the formation of multicyclic compounds with up to 98% ee,
- an Ir-catalysed asymmetric hydrogenation of a-substituted a,b-unsaturated ketones, a, ?-unsaturated esters, allyl alcohols, and imines were established to produce the corresponding saturated compounds of very high yields with excellent ee values,
- a Pd-catalysed asymmetric allylic alkylation reaction was developed to convert allylated products into the corresponding alcohol, ester, and ketone with the retention of stereochemistry, and
- a ring-opening reaction catalysed by chiral palladium complex was discovered to produce optically active 1-hydroxy-2-aryl-1,2-dihydronaphthalenes.
For the catalytic formation of Carbon-Nitrogen bonds, the main results are:
- a new type of N-nucleophiles in Pd-catalysed asymmetric allylic amination was developed using chiral phosphine SIOCPhox as ligand, affording the corresponding branched allyl amines in high regio- and enantioselectivities,
- a half sandwich titanacarborane monoamide was found to be a very efficient and robust catalyst for the cascade formation of Carbon-Nitrogen bonds both inter- and intra-molecularly, leading to the formation of substituted isoindoles, isoquinolines and imidazoles,
- titanacarborane monoamide was able to efficiently catalyse the hydroamination reaction of di-/tri-amines with carbodiimides, leading to one-step synthesis of mono-/bi-cyclic guanidines from commercially available starting materials, and
- a cationic [N-O-S]zirconium complex was discovered to be an excellent catalyst for the intramolecular hydroamination of aminoalkenes with a large substrate scope from terminal alkenes to internal alkenes, and primary amines to secondary amines, which tolerated various functional groups and performed sequential hydroamination of primary aminodienes.
Xie reports that the team have achieved all the objectives of this joint project. During the process six PhD students have been trained and the research resulted in the publication of 30 scientific articles in high-impact international refereed journals.