Activation of cyclopropanes by transition metals

In organometallic chemistry, the activation of cyclopropanes by transition metals is a research theme with implications for organic synthesis and homogeneous catalysis.

One strategy is to increase the ring strain and the other is to stabilize the resulting cleaved C-C bond complex (e.g. through aromatization or chelation).

[6] The electrophile Cp*Ir(PMe3)(Me)OTf reacts with cyclopropane to give the allyl complex:[7] Rhodium-catalyzed C-C bondactivation of strained spiropentanes leads to a cyclopentenones.

[8] In terms of mechanism, the reaction proceeds by apparent oxidative addition of the 4-5 carbon-carbon bond, leading to a rhodacyclobutane intermediate.

Insertion of carbon monoxide into one of the carbon-rhodium bonds form a rhodacyclooctenone intermediate that can reductively eliminate to yield a 6,7-fused ring system.

The authors propose that the regioselectivity of the initial oxidative addition is controlled by coordination of the endocyclic double bond to the rhodium catalyst.

[10] With cyclopropylketones, transition metal can coordinate to the ketone to direct oxidative addition into the proximal C-C bond.

The resulting metallacyclobutane intermediate can be in equilibrium with the six-membered alkyl metal enolate depending on presence of a Lewis acid (e.g. dimethylaluminum chloride[11]).

With the six-membered alkyl metal enolate intermediate, dimerization[13][14] or reaction with an added alpha-beta unsaturated ketone[15] yields a 1,3-substituted cyclopentane product.