hide
Free keywords:
alkyne metathesis catalysts;
cyclo-oligomerization;
molybdenum alkylidyne;
organic synthesis
Abstract:
This chapter illustrates the scope and limitations of alkyne metathesis, which has become a powerful tool of contemporary organic synthesis. It begins with a discussion on classical catalyst systems and state-of-the-art catalysts. The generally accepted mechanism of alkyne metathesis, originally proposed by Katz and McGinnis, follows the logic of the Chauvin cycle of olefin metathesis. It consists of a sequence of - a priori - reversible [2 + 2] cycloaddition and cycloreversion steps. To render the reaction preparatively meaningful, the resulting equilibrium needs to be constantly perturbed by removing one of the products from the mixture. Limitations are encountered when working with very bulky substrates or with alkyne derivatives that display protic functionality able to exchange with the silanolate ligands that guarantee the outstanding performance of the operative molybdenum alkylidyne. An interesting reaction format is the cyclo-oligomerization of substrates containing two or more acetylene moieties in an appropriate spatial orientation.
