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Theoretical Insights into the Role of a Counterion in Copper‐Catalyzed Enantioselective Cyclopropanation Reactions
Author(s) -
Fraile José M.,
García José I.,
Gil María J.,
MartínezMerino Víctor,
Mayoral José A.,
Salvatella Luis
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305161
Subject(s) - cyclopropanation , carbene , counterion , chemistry , catalysis , steric effects , enantioselective synthesis , cationic polymerization , copper , yield (engineering) , cyclopropane , oxazoline , photochemistry , organic chemistry , ion , thermodynamics , ring (chemistry) , physics
The effect of a coordinating counteranion on the mechanism of Cu I ‐catalyzed cyclopropanation has been investigated extensively for a medium‐sized reaction model by means of theoretical calculations at the B3LYP/6‐31G(d) level. The main mechanistic features are similar to those found for the cationic (without a counteranion) mechanism, the rate‐limiting step being nitrogen extrusion from a catalyst–diazoester complex to generate a copper–carbene intermediate. The cyclopropanation step takes place through a direct carbene insertion of the metal–carbene species to yield a catalyst–product complex, which can finally regenerate the starting complex. However, the presence of the counteranion has a noticeable influence on the calculated geometries of all the intermediates and transition structures. Furthermore, the existence of a preequilibrium with a dimeric form of the catalyst, together with a higher activation barrier in the insertion step, explains the lower yield of cyclopropane products observed experimentally in the presence of chloride counterion. The stereochemical predictions of a more realistic model (made by considering a chiral bis(oxazoline)–copper( i) catalyst) have been rationalized in terms of the lack of significant steric repulsions, and the model shows good agreement with the low enantioselectivities observed experimentally for these kinds of catalytic systems.