Insertion into C–H bonds

We said that the formation of cyclopropanes by addition of substituted carbenes to alkenes was rare—in fact, alkyl-substituted carbenes undergo very few intermolecular reactions at all because they decompose very rapidly. When primary alkyl halides are treated with base, alkenes are formed by elimination., you should expect the mechanism of this elimination to be E2 and, if you started with a deuterated compound like this, the alkene product would be labelled with two deuterium atoms at its terminus.

This is indeed what happens if the base is sodium methoxide (pKa[MeOH] about 16). If, however, it is phenylsodium (pKa[benzene] about 50), only 6% of the product is labelled in this way while 94% of the product has only one deuterium atom. Evidently a hydrogen atom has ‘migrated’ from the 2-position to the 1-position. The overall mechanism of the elimination with very strong bases like phenylsodium is thought to be: (1) formation of a carbene by α elimination and then (2) 1,2-migration of a hydrogen atom on to the carbene centre. Carbenes with β hydrogens undergo extremely rapid 1,2-migration of hydrogen to the carbene centre, giving alkenes.

The reason for the rapid migration is that the electrophilic carbene has found a nearby source of electrons—the HOMO of the C–H bond—and it has grabbed the electrons for itself, ‘inserting’ into the C–H bond, as shown in the margin. This type of reaction is better demonstrated by two examples in which the ‘insertion reaction’ is a bit more obvious: when there are no β hydrogens, the carbene inserts into C–H bonds a little further away in the same molecule or even in the solvent (cyclo hexane in the second example). In the first case, the carbene is formed by α elimination (using one of the ‘Schlosser bases’) and, in the second case, by photolysis of a diazoketone.

Because these insertion reactions create new bonds at completely unfunctionalized centres, they can be very useful in synthesis. This next carbene is created between two carbonyl groups from a diazocompound with rhodium catalysis and selectively inserts into a C–H bond five atoms away to form a substituted cyclopentanone.

In these C–H insertion reactions, the similarity with cyclopropane formation by insertion into alkenes is clear, and the mechanisms mirror one another quite closely. As with the cyclo propanation reactions, the mechanism depends on whether the carbene is a singlet or triplet. Singlet carbenes can insert in a concerted manner, with the orbitals overlapping constructively provided the carbene approaches side-on.

This mechanism implies that, if the C–H bond is at a stereogenic centre, the stereochemistry at that centre will be retained through the reaction, as in Cane’s synthesis of pentalenol actone. A nice example of this result is this ingenious synthesis of α-cuparenone using a stereospecific carbene insertion.

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مواضيع ذات صلة

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Making cyclopropanes

Carbenes react with alkenes to give cyclopropanes

The structure of carbenes depends on how they are made

Carbenes can be divided into two types

اكتشاف العناصر

العناصر

Diazomethane makes methyl esters from carboxylic acids

Intramolecular radical reactions are more efficient than intermolecular ones

Alkyl radicals from boranes and oxygen