Recent advances in the chemistry of stable simple enols

Two new aspects of the chemistry of simple bulky-aryl substituted enols are described. (i) The reactions of amines such as Me₂NH, morpholine piperidine, or pyrrolidine with ditipyl ketene (tipyl = Tip = 2,4,6-tri-isopropyl-phenyl) generate the enol of the amide Tip₂C=C(OH)NR¹R² which is observed and characterized by NMR spectra. The addition reaction is reversible. Further tautomerization of the enol to the stable conformer of the amide is relatively slow and involves an additional NMR observable intermediate which is presumably the kinetically controlled unstable conformer of the amide. (ii) Di- and tri-bulky arylvinyl alcohols and their derivatives such as the ethers or acetates are chiral, having a propeller conformation. The enantiomerizations of triarylethenols usually proceed by a three ring flip, and the rotational barriers are mostly too low for resolution of stereoisomers. In the Mes₂C=C(OH)(2-X-9-anthryl) system where X = F, OMe, the α-ring lacks a C2 symmetry and enantiomerization barriers in which this ring passes through the molecular plane are higher. Resolution on an optically active column is achieved when X = F, but fast enantiomerization then takes place. This is probably due to an E⇌Z enol interconversion which was observed in other systems.