Cyclopropanation and C-H Insertion

Equation 14.16 shows how carbene insertion into CH bonds is also possible where a CH bond is favorably located. In this case, the less thermodynamically preferred cis geometry of the Me and R groups is 

Kinetic resolution (KR) in the reduction of racemic mixtures of R and S starting materials requires a chiral catalyst that reacts very much faster with one substrate enantiomer. Suppose we have a catalyst that reduces only the R reactant in Eq. 14.18, to give the R product. If the kisom is zero, we will ideally end up with a 50% yield of the S starting 

Asymmetric hydrogenation (Section 9.3) of C=C, C=0, and C=N bonds is widely employed with numerous catalysts. The example shown in Eq. 14.20 uses a Noyori catalyst that is believed to operate by an outer-sphere mechanism of Section 9.3 with transfer of H from the metal to carbonyl carbon and H+ from the amino hgand to the carbonyl oxygen, the carbonyl substrate not being directly coordinated to the metal. 

When the substrate C=C bond is tri- or tetra-substituted, the [(cod)-IrLL JX series is most useful (Section 9.3) LL is typically a homochiral P,N mixed donor chelate, and X is the noncoordinating BAi anion. In Eq. 14.21, hydrogenation does not proceed as usual because an intermediate vinylrhodium complex is completely trapped to form a new C-C bond. Since the postcoupling intermediate is Rh(III) and 

The Pauson-Khand reaction forms cyclopentenones from three groups, a C=C, a C=C, and a CO molecule (Eq. 14.28). Originally, stoichiometric and based on Co, the Rh-catalyzed version is now widely adopted. Equation 14.29 shows the formation of the carbon skeleton of guanacastepene A, a novel antibiotic candidate.  

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The carbenoid fragment reacts as an electron-deficient carbon centre. Substituents both at rhodium and at the carbene centre can make it more electron-deficient. If the carbenoid is given the choice between a cyclopropanation and C-H insertion reaction, the preference for C-H insertion increases with the electron deficiency [19], Figure 17.10. 

For the rhodium-catalyzed cyclopropanation of olefin substrates with activated allylic C—H groups, the selectivity between cyclopropanation and C—H insertion is sometimes unsatisfactory. Very recently, Davies and Thompson (128) reported a selective silver-catalyzed cyclopropanation of olefins to give predominately cyclopropane products. Aryldiazoacetates were used as carbene precursors with the aryl groups helping to stabilize the carbene intermediate and facilitate the reaction. Notably, phenallyldiazoacetate with different olefins also gave cyclopropane products (Fig. 26). 

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