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Activation of cyclopropanes by transition metals

(Redirected from Cyclopropane carbon-carbon bond activation)

In organometallic chemistry, the activation of cyclopropanes by transition metals is a research theme with implications for organic synthesis and homogeneous catalysis.[1] Being highly strained, cyclopropanes are prone to oxidative addition to transition metal complexes. The resulting metallacycles are susceptible to a variety of reactions. These reactions are rare examples of C-C bond activation. The rarity of C-C activation processes has been attributed to Steric effects that protect C-C bonds. Furthermore, the directionality of C-C bonds as compared to C-H bonds makes orbital interaction with transition metals less favorable.[2] Thermodynamically, C-C bond activation is more favored than C-H bond activation as the strength of a typical C-C bond is around 90 kcal per mole while the strength of a typical unactivated C-H bond is around 104 kcal per mole.

Structure of the platinacyclobutane PtC3H6(bipy) derived from activation of cyclopropane.

Two main approaches achieve C-C bond activation using a transition metal. 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). Because of the large ring strain energy of cyclopropanes (29.0 kcal per mole), they are often used as substrates for C-C activation through oxidative addition of a transition metal into one of the three C-C bonds leading to a metallacyclobutane intermediate.

Substituents on the cyclopropane affect the course of its activation.[3]

Reaction scope

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Cyclopropane

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The first example of cyclopropane being activated by a metal complex was reported in 1955, involving the reaction of cyclopropane and hexachloroplatinic acid. This reaction produces the polymeric platinacyclobutane complex Pt(C3H6)Cl2.[4][5] The bis(pyridine) adduct of this complex was characterized by X-ray crystallography.[6]

The electrophile Cp*Ir(PMe3)(Me)OTf reacts with cyclopropane to give the allyl complex:[7]

Cp*Ir(PMe3)(Me)OTf + C3H6 → [Cp*Ir(PMe3)(η3-C3H5)]OTf + CH4
 
Oxidative addition into cyclopropane C-C bond gives a metallacyclobutane.

Fused and spiro-cyclopropanes

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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. In the presence of carbon monoxide, migratory insertion of CO into one of the carbon-rhodium bonds gives a rhodacyclopentanone intermediate. Beta-carbon elimination to form an alkene from the other carbon-rhodium bond leads to a rhodacyclohexanone intermediate with an exocyclic double bond. Reductive elimination of the two carbon-rhodium bonds followed by isomerization of the exocyclic double bond leads to the desired beta-substituted cyclopentenone product. This reaction was applied to the total synthesis of (±)-β-cuparenone.

 

Using the same rhodium(I) catalyst and C-C bond activation strategy one can access compounds with fused rings.[9] Once again the reaction involves oxidative addition to give a rhodacyclobutane eventually affording a rhodacycloheptene 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.

 

Cyclopropyl halides

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Nickel(0) complexes oxidatively cleave halocyclopropanes to give allyl)Ni(II) halides.[10]

Cyclopropylketones

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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 metallacyclobutane intermediate, 1,2-migratory insertion into an alkyne followed by reductive elimination yields a substituted cyclopentene product. Examples of intramolecular reactions with a tethered alkyne[11] and intermolecular reactions with a nontethered alkyne[12] both exist with use of a nickel or rhodium catalyst. 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.

 
 

Cyclopropylimines

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Oxidative addition into cyclopropylimines gives a metalloenamine intermediate similar to oxidative addition to cyclopropylketones giving alkylmetalloenolates. These intermediates can also reaction with alpha-beta unsaturated ketones to give disubstituted cyclopentane products following reductive elimination.[16]

With rhodium, the intermediate metalloenamine reacts with tethered alkynes.[17] and alkenes[18] to give cyclized products such as pyrroles and cyclohexenones, respectively.

 

Alylidenecyclopropanes

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Alkylidenecyclopropanes more readily undergo C-C bond oxidative addition than cyclopropanes.

Following oxidative addition, 1,2-insertion mechanisms are common and reductive elimination yields the desired product. The 1,2-insertion step usually occurs with an alkyne,[19] alkene,[20] or allene[21] and the final product is often a 5 or 7 membered ring. Six-membered rings may be formed after dimerization of the metallocyclobutane intermediate with another alkylidenecyclopropane substrate and subsequent reductive elimination.[22] Common transition metals utilized with alkylidenecyclopropanes are nickel, rhodium, and palladium. It has been shown that the metallacyclobutane intermediate following oxidative addition to the distal C-C bond can isomerize.[23]

 
 
 

Vinylcyclopropanes

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Oxidative addition of vinylcyclopropanes primarily occurs at the proximal position, giving pi-allyl intermediates. Through subsequent insertion reactions (e.g. with alkynes,[24] alkenes,[25] and carbon monoxide[26]), rings of various sizes and fused ring systems[27] can be formed.

 

Cyclopropenes

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Oxidative addition into cyclopropenes normally occurs at the less hindered position to yield the metallacyclobutane. This reaction can result in formation of cyclopentadienones,[28] cyclohexenones,[29] and phenols.[29]

 

References

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