Cyclopropanations trifluoromethanesulfonate

Even an oligopeptide has been attached to (Table 4.3, compound 130) [111]. This was achieved by a coupling reaction of the carboxylic group in the side chain of the cyclopropane ring as well. First, the tert-butylcarboxylate 129 was synthesized by the reaction of the corresponding diazomethylbenzoate with Cgg. After hydrolysis with trifluoromethanesulfonic acid, the acyl chloride was generated by treatment with oxalyl chloride. Finally, in a one-step procedure the fullerene peptide 130 was obtained by the reaction with the N-deprotected pentapeptide H-(L-Ala-Aib)2-L-Ala-OMe. 

The cyclopropane is inert to tributyltin chloride, but reacts readily with more highly Lewis acidic tributyltin trifluoromethanesulfonate (triflate) [11]. The 3-stannyI ester 13 showed no sign of internal chelation Eq. (16). 

Enantioselective Cyclopropanation of Alkenes. Cationic Cu complexes of methylenebis(oxazolines) such as (1), which have been developed by Evans and co-workers, are remarkably efficient catalysts for the cyclopropanation of terminal alkenes with diazoacetates. The reaction of styrene with ethyl diazoacetate in the presence of 1 mol % of catalyst, generated in situ from Copper(I) Trifluoromethanesulfonate and ligand (1), affords the (rans -2-phenylcyclopropanecarboxylate in good yield and with 99% ee (eq 3). As with other catalysts, only moderate transicis selectivity is observed. Higher transicis selectivities can be obtained with more bulky esters such as 2,6-di-r-butyl-4-methylphenyl or dicyclohexylmethyl diazoacetate (94 6 and 95 5, respectively). The efficiency of this catalyst system is illustrated by the cyclopropanation of isobutene, which has been carried out on a 0.3 molar scale using 0.1 mol % of catalyst derived firom the (R,R)-enantiomer of ligand (1) (eq 4). The remarkable selectivity of >99% ee exceeds that of Aratani s catalyst which is used in this reaction on an industrial scale. 

Until the end of the 1970 s, interest in such reactions concentrated on catalysis by copper salts (review Burke and Grieco, 1979), obviously influenced by the long, broad, and successful experience with copper +- and copper-ions in aromatic diazo chemistry (Sandmeyer, Pschorr and Meerwein reactions, see Zollinger, 1994, Chapts. 8 and 10). A landmark was the discovery of Salomon and Kochi (1973), who found that cyclopropanations with diazomethane in the presence of copper(i) trifluoromethanesulfonate (triflate OTf) resulted in reduction of Cu + to Cu +, and that the rate of dediazoniation is inversely proportional to the alkene concentration. These results strongly indicate that formation of an alkene-Cu+ complex (8-47 2) precedes the complex formation with the diazoalkane. 

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