Alkene metathesis
Abstract
The alkene metathesis is a transition-metal-catalyzed reaction in which two alkenes undergo exchange of their terminal fragments through a sequence of [2+2] cycloaddition and cycloreversion steps. Since its discovery, considerable efforts have been devoted to the development of well-defined ligands, driven by the need to improve activity, selectivity and stereochemical control over the E/Z geometry of the newly formed product.
Keywords
Alkene, C-C bond formation
Category
Alkene, Organometallic catalysis
Mechanism
Initiation: The catalytic cycle is initiated by the coordination of a first monosubstituted olefin (R1-CH=CH2) to the metal center (0), following dissociation of a labile ligand (in this example with the Grubbs 1st generation, a phosphine) (1). The resulting complex undergoes a [2+2] cycloaddition with metal carbene to form the first metallacyclobutane intermediate (2). This four-membered ring then undergoes [2+2] cycloreversion, releasing ethylene as the first small-molecule by-product and generating a new alkylidene [Ru]=CH-R1 (3). This latter is the catalytic active species.
First cycle possibility, the cross-metathesis: The active alkylidene then coordinates the second terminal monosubstituted olefin R2-CH=CH2 and undergoes a new [2+2] cycloaddition affording a new metallacyclobutane bearing R1 and R2 on terminal carbons (A). After another [2+2] cycloreversion, the new alkene is obtained (B). The coordination and cycloaddition of another olefin R1-CH=CH2 (C) finally release an ethylene molecule as by-product and regenerate the active species (D).
Second cycle possibility, the homo-metathesis: The active alkylidene coordinates a second equivalent of R1-CH=CH2 and undergoes a new [2+2] cycloaddition forming a second metallacyclobutane. [2+2] cycloreversion of this species release another ethylene by-product and products the homo-metathesis product R1-CH=CH-R1.
About the stereoselectivity, both substituents R1 and R2 can adopt either an anti or syn conformation: The anti intermediate is strongly favored with classical Grubbs catalysts, due to steric repulsion between R1 and R2. After [2+2] cycloreversion step, the E configuration is obtained as the major product. The syn intermediate leads to Z configuration upon cycloreversion and can be promoted by Hoveyda-Grubbs 2nd generation where the steric bulk of the ligands enforces a syn approach of the incoming olefin.
