Structures and stereoinversions of β-shielded α-Li (or K or Cs)-acrylonitriles: A mechanism with neighborly assistance

Abstract

Neighborly intermolecular assistance is common within the clusters of aggregating organolithium compounds but rarely as continual and kinetically helpful as for the β-shielded α-lithioacrylonitrile 2Li of the type N≡C–C(Li) = C(tert-alkyl)2. Intermolecular NLi coordination with the α-cyano substituent of 2Li was discovered not only in single crystals of a cyclohexamer or of a (Li–C–C≡N)2 eight-membered ring; it provided also in several solvents for a single kind of a THF-solvated cluster (presumably a cyclodimer or cyclooligomer). These NLi connections are thought to be the “glue” in this cluster and are proposed to substantially accelerate the molecular cis/trans stereoinversion. Due to such a neighborly assistance, the rate constants of this stereoinversion are surprisingly insensitive to usually very effective factors (cations, solvents, and electron-pair donor ligands at Li) if the NLi connections are not weakened or suppressed. The corresponding α-potassio and α-cesio congeners resemble 2Li with regard to both the ground states in solution and the fast stereoinversion processes with extrapolated rate constants roughly on the microsecond time scale at 25 °C.