Homogeneous catalysis. Catalytic production of simple enols

Article abstract of DOI:10.1021/ja00003a032

Complexes of the type [Rh(diphosphine)(solvent)₂]⁺ in dry acetone or tetrahydrofuran solutions are effective catalysts for generating synthetically useful quantities of simple enols from their corresponding allylic alcohols. A representative collection of simple enols has been produced, and the physical properties and stabilities are recorded. Although these catalysts also ketonize (tautomerize) the enols to their corresponding aldehydes and ketones, the simple enols are conveniently stable in solutions containing the milder catalysts. It is found that in the absence of catalyst simple enols are remarkably stable; the enols of propanal and methyl ethyl ketone persist for up to 2 weeks in dilute acetone solutions at 25 °C. The mechanism of catalysis has been inferred from specific isotopic labeling experiments. The conclusions are as follows: the production of enols involves a hydrogen 1,3-shift mechanism involving hydrido-π-allylic intermediates; the catalytic process is essentially irreversible; and the catalytic ketonization proceeds through a hydrido-π-oxyallylic intermediate. This process is also irreversible, and the stereoselective step is the α-hydrogen abstraction from the allylic substrate. The hydrogen abstraction is probably the turnover limiting step. Three examples of chemical reactions that are unique to enols are reported and are described as proceeding by an ene mechanism.