Highly Active Carbene Potassium Complexes for the Ring-Opening Polymerization of ε‑Caprolactone

Herein
we report the synthesis of two complexes of potassium employing strongly
nucleophilic carbenes, such as cyclic “(alkyl)­(amino)­carbene
(cAAC) and abnormal N-heterocyclic carbene (aNHC). Both complexes are dimeric in the solid state and
the two potassium centers are bridged by trimethylsilylamide. In these
complexes, the carbene- - -K interaction is predominantly
electrostatic in character, which has been probed thoroughly by NBO
and AIM analyses. Indeed, the delocalization energy of the cAAC lone pair calculated from the second-order perturbation
theory was only 5.21 kcal mol–1, supporting a very
weak interaction. The solution-state behavior of these molecules,
as inferred from NOESY spectra, hints that the weak carbene- - -K
interaction is retained in nonpolar solvents, and the bond is not
dissociated at least on the NMR time scale. We took advantage of such
a weak interaction to develop highly effective ring-opening polymerization
catalysts for ε-caprolactone and rac-lactide.
The efficacy of these catalysts is prominent from a very high substrate/metal-initiator
ratio as well as very low dispersity index of the obtained polymer
chains, reflecting significant control over polymerization.