Methathesis Reactions

Methathesis Reactions-8
Grubbs’ ruthenium metathesis catalysts are widely used catalysts in organic synthesis because of their stability, selectivity, and functional group tolerance.

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Ring-closing metathesis is a variant of the olefin metathesis reaction in which alkylidene moieties are exchanged to form a ring.

The most common catalysts for this reaction are complexes of molybdenum or ruthenium.

In other words, it occurs through the concerted interaction of one molecule with another.

In petroleum refining, heating alkenes over metal oxide surfaces results in the formation of longer-chain alkenes.

In contrast to an orbital picture for an alkene, an orbital picture for an alkylidene pi bond suggests orbital symmetry that can easily interact with the LUMO on an alkene.

That's because a metal-carbon \(pi\) bond likely involves a d orbital on the metal, and the d orbital has lobes alternating in phase like a \(pi\) antibonding orbital.Olefin metathesis involves the exchange of two alkylidene groups to generate two new olefins from one or more starting alkenes.Cleavage of the carbon-carbon double bond is accompanied by the formation of two new carbon-carbon double bonds.This mechanism is called the Chauvin mechanism, after its first proponent, Yves Chauvin of the French Petroleum Institute.Chauvin's proposal of this mechanism shortly after the discovery of metal alkylidenes by Dick Schrock at Du Pont earned him a Nobel Prize in 2005.The [2 2] addition and retro-[2 2] reactions occur in equilibrium with each other.Each time the metallacyclobutane forms, it can form two different pairs of double bonds through the retro reaction.Chauvin and Schrock shared the prize with Bob Grubbs, who made it possible for the reaction to be adapted easily to the synthesis of complex molecules such as pharmaceuticals.Why does olefin metathesis lead to the formation of internal alkenes?The reaction, it turns out, involves a [2 2] cycloaddition of an alkene to a metal alkylidene (to the metal-carbon double bond).This reaction results in a four-membered ring, called a metallacyclobutane.

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