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Ring-Expanding Olefin Metathesis: A Route to Highly Active Unsymmetrical Macrocyclic Oligomeric Co-Salen Catalysts for the Hydrolytic Kinetic Resolution of Epoxides
journal contribution
posted on 2007-02-07, 00:00 authored by Xiaolai Zheng, Christopher W. Jones, Marcus WeckIn the presence of the third generation Grubbs catalyst, the ring-expanding olefin metathesis of
a monocyclooct-4-en-1-yl functionalized salen ligand and the corresponding Co(II)(salen) complex at low
monomer concentrations results in the exclusive formation of macrocyclic oligomeric structures with the
salen moieties being attached in an unsymmetrical, flexible, pendent manner. The TOF-MALDI mass
spectrometry reveals that the resulting macrocyclic oligomers consist predominantly of dimeric to tetrameric
species, with detectable traces of higher homologues up to a decamer. Upon activation under aerobic and
acidic conditions, these Co(salen) macrocycles exhibit extremely high reactivities and selectivities in the
hydrolytic kinetic resolution (HKR) of a variety of racemic terminal epoxides under neat conditions with
very low catalyst loadings. The excellent catalytic properties can be explained in terms of the new catalyst's
appealing structural features, namely, the flexible oligomer backbone, the unsymmetrical pendent
immobilization motif of the catalytic sites, and the high local concentration of Co(salen) species resulting
from the macrocyclic framework. This ring-expanding olefin metathesis is suggested to be a simple way to
prepare tethered metal complexes that are endowed with key features(i) a high local concentration of
metal complexes and (ii) a flexible, single point of attachment to the supportthat facilitate rapid and efficient
catalysis when a bimetallic transition state is required.