Methyl carbenoid asymmetric cyclopropanation

Carbenoids derived from the aryldiazoacetates are excellent donor/acceptor systems for the asymmetric cyclopropanation reaction [22]. Methyl phenyldiazoacetate 3 cyclopropanation of monosubstituted alkenes catalyzed by Rh2(S-DOSP)4 is highly diaster-eo- and enantioselective (Tab. 14.5) [22]. Higher enantioselectivities can be obtained when these reactions are performed at -78°C, as the catalyst maintains high solubility and activity at this temperature. The phenyldiazoacetate system has been evaluated using many popular rhodium(II) and copper catalysts the rhodium(ll) prolinates have proven to be superior catalysts for this class of carbenoids [37, 38]. 

I 74 Rhodium(ll)-Stabilized Carbenoids Containing Both Donor and Acceptor Substituents Tab. 14.5 Catalytic asymmetric cyclopropanation using methyl phenyidiazoacetate (3). 

The cyclopropanation utilizing donor/acceptor rhodium carbenoids can be extended to a range of monosubstituted alkenes, occurring with very high asymmetric induction (Tab. 14.4) [40]. Reactions with electron-rich alkenes, where low enantioselectivity was observed at room temperature, could be drastically improved using the more hydrocarbon-soluble Rh2(S-DOSP)4 catalyst at -78°C. The highest enantioselectivity is obtained when a small ester group such as a methyl ester is used [40], a trend which is the opposite to that seen with the unsubstituted diazoacetate system [16]. 

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