Dirhodium intermolecular cyclopropenation

Chiral rhodium(II) carboxamides are exceptional catalysts for highly enantio-selective intermolecular cyclopropenation reactions (50). With ethyl diazoacetate and a series of alkynes, use of dirhodium(II) tetrakis[methyl 2-pyrrolidone-5-(R)-carboxylate], Rh2(5R-MEPY)4, in catalytic amounts ( 1.0 mol %) results in the formation of ethyl eyelopropene-3-earboxylates (eq 4) with enantiomeric excesses... 

Dirhodium(ll) tetrakis[methyl 2-pyrrolidone-5(R)-oarboxylate], Rh2(5R-MEPV)4, and its enantiomer, Rh2(5S-MEPY)4, which is prepared by the same procedure, are highly enantioselective catalysts for intramolecular cyclopropanation of allylic diazoacetates (65->94% ee) and homoallylic diazoacetates (71-90% ee),7 8 intermolecular carbon-hydrogen insertion reactions of 2-alkoxyethyl diazoacetates (57-91% ee)9 and N-alkyl-N-(tert-butyl)diazoacetamides (58-73% ee),10 Intermolecular cyclopropenation ot alkynes with ethyl diazoacetate (54-69% ee) or menthyl diazoacetates (77-98% diastereomeric excess, de),11 and intermolecular cyclopropanation of alkenes with menthyl diazoacetate (60-91% de for the cis isomer, 47-65% de for the trans isomer).12 Their use in <1.0 mol % in dichloromethane solvent effects complete reaction of the diazo ester and provides the carbenoid product in 43-88% yield. The same general method used for the preparation of Rh2(5R-MEPY)4 was employed for the synthesis of their isopropyl7 and neopentyl9 ester analogs. 

Dirhodium(II) catalysts that possess chiral 2-pyrrolidone-5-carboxylate ester ligands (mepy) are the most effective among those of dirhodium or copper for highly diastereoselective and enantioselective intermolecular cyclopropenation reactions between l-alkynes and diazoesters (eq. (9)). Product yields are moderate, and enantiomeric excesses range from 40 to 98 %. Interestingly, the (R) or (5) catalyst produces the cyclopropene-l-carboxylate respectively with the (/ ) or (5) configuration [26]. 

The addition to a carbon-carbon triple bond results in the formation of cyclo-propene products, and with diazoacetates the catalyst of choice for intermolecular addition is the dirhodium(II) carboxamidate 13 (e.g., Eq. 26). The reactions are general, except for phenylacetylene whose cyclopropene product undergoes [2 + 2]-cycloaddition, and selectivities are high. However, high selectivities have not been reported for reactions with allenes. 

Dirhodium(II) tetrakis(carboxamides), constructed with chiral 2-pyrroli-done-5-carboxylate esters so that the two nitrogen donor atoms on each rhodium are in a cis arrangement, represent a new class of chiral catalysts with broad applicability to enantioselective metal carbene transformations. Enantiomeric excesses greater than 90% have been achieved in intramolecular cyclopropanation reactions of allyl diazoacetates. In intermolecular cyclopropanation reactions with monosubsti-tuted olefins, the cis-disubstituted cyclopropane is formed with a higher enantiomeric excess than the trans isomer, and for cyclopropenation of 1-alkynes extraordinary selectivity has been achieved. Carbon-hydro-gen insertion reactions of diazoacetate esters that result in substituted y-butyrolactones occur in high yield and with enantiomeric excess as high as 90% with the use of these catalysts. Their design affords stabilization of the intermediate metal carbene and orientation of the carbene substituents for selectivity enhancement. 

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