Doubly Diastereoconvergent Preparation and Microsolvation-Controlled Properties of (Z)- and (E)-1'-Lithio-1'-(2,6-dimethylphenyl)propenes.

Abstract

A doubly diastereoconvergent reaction can ad libitum generate either one or the other of two diastereomeric products with complete consumption of the diastereomeric precorsors or their mixtures. Thus, the preparation of configurationally pure (Z)-1'-lithio-1'-(2,6-dimethylphenyl)propene [(Z)-1] from any Z,E mixture of the corresponding bromoalkenes with n-butyllithium succeeded by means of a user-friendly (E)-1 → (Z)-1 configurational interconversion. The subsequent treatment of (Z)-1 with a minimum amount of THF afforded exclusively (E)-1 as the other diastereomeric product and was mediated by a beneficial (Z)-1 → (E)-1 interconversion. This behavior provided microsolvation-controlled choices of highly diastereoselective derivatizations of 1. Low-temperature 13C NMR spectra established that (Z)-1 was dissolved as a trisolvated monomer in THF but as a disolvated dimer in monodentate, ethereal, non-THF solvents, whereas (E)-1 was always monomeric. Backed by such knowledge, kinetic experiments indicated that the electrophiles 1-bromobutane or ClSiMe3 in Et2O reacted at 32 °C with the tiny (NMR-invisible) population of monomeric (Z)-1 that was formed in a mobile equilibrium from the inactive, predominantly dimeric (Z)-1. The equilibration of monomeric (Z)-1 and (E)-1 in THF as the solvent was fast (seconds on the 1H NMR time scale), whereas the corresponding stereoinversion of both solvated and unsolvated (E)-1 → (Z)-1 in non-THF solvents occurred on the laboratory time scale (minutes at ambient temperatures). Dicyclopropyl ketone added rapidly to the monomers (Z)-1&3THF and (E)-1&3THF with a rate ratio of at least 14:1 in THF at -78 °C. Di-tert-butyl ketone added less rapidly to the less shielded (Z)-1 [but never to (E)-1]; this singly diastereoconvergent process was much more slowly reversible in THF.