Cross Metathesis Reactions

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Molybdenum catalyst 1 was developed before the Grubbs-type catalysts and is highly active, but sensitivity of this catalyst to air and water limits its applicability.

Ruthenium catalysts 2 and 3 are less active and cannot be recycled, but exhibit better functional-group tolerance than the rather indiscriminate catalyst 1.

However, amides and other protected amines can be used in RCM reactions with Grubbs-type catalysts (Eq. As in the synthesis of carbocycles, some level of conformational bias in the substrate is important in the synthesis of medium and large heterocycles.

For example, bicyclic carbamates containing an eight-membered ring are efficiently synthesized by ring-closing metathesis (Eq. Cyclic boronates are formed in cross-metathesis reactions of allylic alcohols and allylboron reagents.

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.Heterocycles containing a carbon-carbon double bond can also be prepared via RCM.Although nitrogen- and oxygen-containing rings are the most common products, heterocycles containing phosphorus, silicon, boron, sulfur, and other elements have also been prepared.Molybdenum catalyst 1 and second-generation ruthenium complexes 4 and 5 are most effective for the synthesis of substituted cyclic alkenes.Terminal alkenes are usually the preferred substrates because of their relatively high reactivity.This reaction was first observed in 1931, investigated by Du Pont and other manufacturers in the 1950's, Partly due to its relevance to petrochemical industry, olefin metathesis reactions have been investigated extensively. Although initial examples of ring-closing metathesis used poorly defined metal catalysts, subsequent development of Schrock-type molybdenum catalysts such as 1 and Grubbs-type ruthenium catalysts such as 2 - 6 greatly expanded the scope and utility of RCM (Eq. In general, molybdenum catalysts display high activity but are unstable toward air or water; ruthenium catalysts are less active but exhibit good selectivity and functional-group compatibility.Four general classes of reactions have emerged: cross metathesis, an intermolecular reaction of two alkenes; ring-opening metathesis polymerization (ROMP), in which a cyclic alkene opens to form a polyolefin; ring-opening metathesis (ROM), the opening of a cyclic alkene to form a diene; and ring-closing metathesis (RCM), in which reaction of a diene affords a cyclic alkene and a small olefinic byproduct. RCM has been employed extensively in organic synthesis to establish both saturated and unsaturated rings; the reaction can be used to form carbocycles or heterocycles.In RCM reactions, reactants are typically designed so that the desired cyclic alkene is accompanied by a small gaseous olefin such as ethylene or propene, the loss of which drives the reaction forward.Highly dilute conditions discourage intermolecular metathesis and thereby also promote RCM.Intramolecular cycloaddition yields new metallacyclobutane 9, which undergoes cycloreversion to expel the metal carbene catalyst and generate the product cyclic alkene.Because this mechanism relies on both [2 2] cycloadditions and retro-cycloadditions, in general each step is reversible, resulting in an equilibrium mixture of olefinic products.

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