Cyclopentanone carbene formation

From the reactions presented in this section one can conclude that cyclic acetal formation via addition to a carbene intermediate is a general reaction for type I cleavage of cyclobutanones, tricyclic compounds, and certain bridged bicyclics as minor products. No acetal has been isolated from photolyses of cyclopentanones or cyclohexanones except for the special case of an a-sila ketone previously discussed. 

Intramolecular carbene C-H insertion frequently leads to the formation of five-membered rings [967,990,1021,1113-1128], In particular l-diazo-2-alkanones tend to yield cyclopentanones exclusively when treated with rhodium(ll) carboxylates. The use of enantiomerically pure catalysts for diazodecomposition enables the preparation of non-racemic cyclopentane derivatives [1005,1052,1074,1092,1129]. Intramolecular 1,5-C-H insertion can efficiently compete with 1,2-C-H insertion... 

Concerning the possible rearrangement of the lithiooxirane into the alkoxy carbene 155, calculations have also shown that the activation energies of the 1,2-H shifts (to cyclopentanone enolate or cyclopentenol) are extremely high (at least 23 kcalmol" ) from 155, whereas they are much lower (between —0.4 kcalmol" and 8.8 kcalmol" ) from carbene 154. This is explained by a strong intramolecular stabilization of the carbene by the alcoholate moiety, as depicted in Scheme 66. This stabilization could signify that the formation of a carbene from the carbenoid is a disfavored process, and that the carbenoid itself is involved in the rearrangement reaction. 

In alkenyl- and alkynylcarbene complexes the addition of nucleophiles to the carbene carbon competes with the addition to the 3-carbon of the conjugated C-C multiple bond. [17] The regioselectivity of the addition of amines to alkynylcarbene complexes is temperature dependent 1,2-addition is favoured by lower temperatures. [17c] Enolates turned out to be efficient C-nucleophiles for Michael addition reactions to unsaturated metal carbenes. The product distribution may depend on steric factors as shown in Scheme 7 for the addition of different enolates to alkenylcarbene complex 10. The less bulky acetone enolate 11 adds to the carbene carbon protonation of the primary addition product results in demetalation and in the formation of a mixture of isomeric enones 12. In contrast, the more bulky cyclopentanone enolate 13 adds to the less shielded vinylic position. 

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