Pincer ligands as powerful tools for catalysis in organic synthesis

Abstract

Present trends in homogeneous catalysis are moving toward the development of multi-step, one-pot processes where salt waste is reduced to a minimum and in the lowest possible volumes of solvent. Energy efficiency is also maximized to produce the most cost-effective end product(s). However, these one pot cascade, tandem, or domino catalyzed reaction sequences rely on catalyst, substrate and solvent compatibility, on catalyst stability and on the possibility to protect these catalysts from mutual deactivation. This can be facilitated, for example, by binding these to suitable (in)soluble supports or by using compatible, stable catalysts that can be used to mediate a variety of different reactions. Pincer organometallics are powerful tools that can be used to achieve many of these objectives. The synthesis and properties of a variety of “molecularly enlarged” pincer organometallics displaying both surprising stability and versatile catalytic properties are discussed. A fascinating, recent development is the site-selective inhibition of a lipase, i.e., Cutinase, with a single pincer-metal catalytic site (see X-ray). Similarly, partial inhibition of lipases immobilized on beads with a Ru-based racemization catalyst yields a bio-organometallic hybrid catalyst, which in combination with still uninhibited lipase on these beads, can operate as a heterogeneous bifunctional catalytic material for the kinetic resolution of racemic alcohols to afford enantiopure product(s). --------------------------------------------------------------------------------