Introduction. Catalysis enables: it opens up new areas of ‘chemical space’, increases efficiency, and provides control over both the substrate utilised and the product formed. Linking together several catalysed reactions has the potential to provide complex molecules selectively and sustainably for numerous applications. This approach is used by nature, evolution having provided exquisitely active and accurate enzyme catalysts for the control of biosynthetic pathways [see for example Scott’s in vitro biosynthesis of vitamin B(12)].
The design and synthesis of small molecule catalysts has transformed synthetic chemistry, as recognised by the award of several Nobel prizes to some of the leading protagonists in this field. The objective of such studies is usually to achieve high product selectivity (especially enantioselectivity), or to discovery new catalysed reaction pathways (and sometimes both). Successful as these approaches have been, the efficiency of most catalysts remains very low requiring high loadings and long reaction times. In addition most catalysts are incompatible with one another for multi-reaction applications, and most catalysed reactions involve thermodynamically favourable addition across multiple bonds (C=C, C=O, C≡C etc). The incorporation of external sources of energy to drive ‘uphill’ reactions is in its infancy.
To address some of these challenges, especially within the field of asymmetric catalysis, extensive use is made of metallocene ligands and metallocene based metallacycle complexes. These usually contain several elements of chirality, including planar chirality. Variation of the metal(s), the stereochemistry, and the ligating or catalytically active groups employed enables the identification of an efficient catalyst for a given reaction. Underpinning this work is stereoselective and stereospecific organometallic synthesis, and the catalysts developed are applied to organic synthesis. These studies developing and evolving catalytic efficiency are also anticipated to give some insight into how pre-enzymatic catalysts developed the specificity, selectivity and activity that may well have been required for the emergence of life.