Cone vs an Alternate Structure in the Synthesis of Organometallic Calixarene Inclusion Complexes

Reaction of 1,3-di-O-methylcalix[4]arene ligands with 2 equiv of R2Zn results in the mixture
of the two bimetallic products:  an organometallic alkylzinc inclusion complex with the ligand
in the conic conformation and an isomeric complex, where the calixarene scaffold affords
the flattened cone structure. This reaction has been systematically studied with regard to
a variety of factors, such as temperature, concentration, and steric bulk. All were found to
influence the ratio between the products, with the low temperature, high R2Zn concentration,
and low steric hindrance favoring the formation of inclusion complexes with the calixarene
ligand in the conic conformation. An excess of the dialkylzinc reagent also facilitates the
exchange with the alkylzinc coordinated to the calixarene ligand. However, only the external
alkylzinc fragment is capable of participating in this Schlenk-type equilibrium process.