Abstract
The cyclopropanation reaction using chiral-at-metal Fischer carbene complexes is examined. Racemic molybdenum and chromium carbene complexes fac- and mer-1 and mer-2, respectively, undergo cycloaddition to acrylonitrile, methyl acrylate and 1-hexene to give cylopropanes 3 and 4 with trans/cis ratio up to 4.7:1. The cyclopropanation of acrylonitrile with the enantiopure carbene complex (+)-mer-1 resulted in a modest asymmetric induction of 2:1 for the trans- isomer 3a.
Highlights
The asymmetric transfer of carbene moieties onto the prochiral face of an olefin is one of the fundamental reactions in organic chemistry
In 1966 Noyori and Nozaki described the use of a copper complex for asymmetric cyclopropanation that constituted the first asymmetric catalytic reaction in a homogeneous phase.[1]
While transition metal catalyzed asymmetric cyclopropanations have become a powerful tool for the construction of chiral three-membered ring systems,[4,5] the related process starting from Fischer carbene complexes has met with far more difficulties
Summary
The asymmetric transfer of carbene moieties onto the prochiral face of an olefin is one of the fundamental reactions in organic chemistry. While transition metal catalyzed asymmetric cyclopropanations have become a powerful tool for the construction of chiral three-membered ring systems,[4,5] the related process starting from Fischer carbene complexes has met with far more difficulties. The reason for the mentioned difficulties in the use of Fischer carbene complexes for direct cyclopropanation[9] can be found in the relatively high stability, due to heteroatom substitution, which results in diminished reactivity under standard reaction conditions. Modified Fischer carbene complexes 1-2 with a chiral bisphosphite ligand P^P
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