Alkene metathesis

In this example, and in many before it, the formation of the carbene is initiated by a metal— often copper, rhodium, or silver. The carbene intermediates in these reactions are formed as reactive complexes with those metals, but in other cases the complexes are extremely stable. For example, decomposition of phenyldiazomethane in the presence of a ruthenium (II) com plex gives a carbene complex stable enough to be isolated and stored for months. This com plex, and a family of related Ru complexes, are among the most important of carbene-derived reagents because of a remarkable reaction known as alkene (or olefin) metathesis.

The reaction is most easily understood when a simple diene reacts with a very small amount (in this case 2 mol%) of the catalyst. A cyclization reaction occurs and the product is also an alkene. It contains no atoms from the catalyst: indeed, it has lost two carbon atoms, which are given off as ethylene.

What happens is a metathesis—an exchange of groups between the two arms of the molecule. But how? The mechanism is not difficult, but is unlike any other you have met before, except, perhaps, the Wittig reaction, which also forms alkenes. First, the carbene complex adds to one of the alkenes in what can be drawn as a [2 + 2] cycloaddition to give a four-membered ring with the metal atom in the ring (a ‘metallacyclobutane’).

Now the same reaction happens in reverse, either unproductively to give back the starting materials or, by cleavage of the other two bonds, a new carbene complex and styrene.

This new complex has the same reactivity as the catalyst we started with, so it will quickly find another alkene to undergo [2 + 2] cycloaddition with. There is now one in the same molecule, so a fast intramolecular reaction joins up the five-membered ring and produces a second metallacyclobutane. As before, there are two alternative ways for this metallacyclo butane to break down, and the productive one gives a third carbene complex and the cyclic product.

This new carbene complex is then ready to attack another molecule of starting material and the cycle is repeated, with the minor difference that ethylene (ethene) is now lost instead of styrene in the first step.

Not a lot can go wrong in this sequence, which is one reason why the yield is so high. Most of the steps are reversible, and the overall reaction is driven by the only irreversible step—the loss of ethylene as a gas. Even if the carbene complex adds the wrong way round to the alkene, nothing is lost because the only thing the resulting metallacycle can do is revert back to start ing materials.

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مواضيع ذات صلة

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct

Cross metathesis

Ene-yne metathesis

Metathesis catalysts

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Insertion into C–H bondsInsertion into C–H bonds

Making cyclopropanes