Olefinmetathese

Olefinmetathese, or olefin metathesis, is a class of reactions in which alkene fragments exchange partners through metal-carbene catalysts to form new carbon–carbon double bonds. The process is catalyzed primarily by ruthenium, molybdenum, or tungsten complexes and is widely used in organic synthesis and polymer chemistry. The main variants are ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), cross metathesis (CM), and acyclic diene metathesis (ADMET).

The reaction proceeds via a catalytic cycle that involves [2+2] cycloadditions between a metal carbene and an olefin to form a metallacyclobutane intermediate, which then undergoes cycloreversion to give a new alkene and regenerate the metal carbene. This exchange of alkene partners allows the redistribution of alkene fragments under relatively mild conditions.

Catalysts and examples: Early catalysts were molybdenum- or tungsten-based alkylidenes (Schrock catalysts), which are highly active but air sensitive and limited in functional-group tolerance. More robust and user-friendly ruthenium catalysts, beginning with Grubbs I, were followed by second-generation Grubbs catalysts bearing N-heterocyclic carbenes, and Hoveyda–Grubbs variants with chelating ligands. These catalysts are tolerant of air and many functional groups, enabling metathesis in complex molecules.

Variants and applications: ROMP uses strained cyclic olefins to form polymers with defined backbones. RCM allows the formation of medium to large rings. CM enables exchange of fragments between alkene partners, valuable for late-stage modifications and natural-product synthesis. ADMET extends metathesis to reactions of dienes to access diverse polymer architectures.

History: Olefin metathesis emerged from work by Chauvin proposing the mechanism in the 1970s, with practical catalysts developed in the 1990s. In 2005, Chauvin, Grubbs, and Schrock were awarded the Nobel Prize in Chemistry for the development of metathesis catalysts.