Cyclopropanation copper-catalyzed mechanisms

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a, (3-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol 333 . The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a p,y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopen-tene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place 334 ). 

Various approaches to epoxide also show promise for the preparation of chiral aziridines. Identification of the Cu(I) complex as the most effective catalyst for this process has raised the possibility that aziridination might share fundamental mechanistic features with olefin cyclopropanation.115 Similar to cyclo-propanation, in which the generally accepted mechanism involves a discrete Cu-carbenoid intermediate, copper-catalyzed aziridation might proceed via a discrete Cu-nitrenoid intermediate as well. 

Scheme 1. General mechanism for the copper-catalyzed cyclopropanation of alkenes using diazoesters.

The mesityl diimine 88d was as effective a ligand in the aziridination as the 2,6-dichlorophenyl diimine 88a ( 65% ee vs 66% ee) (61). The bound face of the styrene undergoes aziridination (in contrast with Fu s selective crystallization of the wrong face of styrene in his copper-catalyzed cyclopropanation reaction, cf. Section II.A.8). Unfortunately, the potential racemization of 118 (by the mechanism... 

Abstract The application of QM/MM methods to the study of the reaction mechanisms involved in chemo-, regio-, and enantio- selective processes has been a very productive area of research in the last two decades. This review summarizes basic general ideas in both QM/MM methods and the computational study of selectivity and presents selected results on the study of three of the most representative examples of these applications rhodium-catalyzed hydrogenation, rhodium-catalyzed hydroformylation, and copper-catalyzed cyclopropanation. 

Formally, the insertion of a carbene(oid) into the 2,3-doubIe bond of the thiophene ring should result in the formation of the 2-thiabicyclo [3.1.0] hex-3-ene ring system. Copper(II)-catalyzed reaction of thiophene with diazomethane results in the formation of 24 (R = H) in modest yield (63TL1047). Analogously, the reaction of thiophene with ethyl diazoacetate yields 24 (R = COjEt) (22LA154). Although these reactions appear to be simple carbene insertion reactions, it is probable that this simple mechanism is not in operation. Rather, the cyclopropane derivatives 24 probably result from the initial formation of the ylid (e.g., 18), which subsequently rearranges. 

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