DIVISION OF ORGANOMETALLIC CHEMISTRY: PROTOTROPIC SHIFTS IN WEAKLY ACIDIC HYDROCARBONS*

Abstract

The production of carbanions from slightly acidic hydrocarbons has been the subject of long intensive study.l For example, Pines2 has developed supported alkali metal catalysts for simple hydrocarbon anion reactions at moderate temperatures, and Shatenstein3 has explored deuterio-amide liquid ammonia systems. The former systems suffered from irreproducibility and the incursion of radial anion-type reactions while the latter systems a r e relatively inactive. Recently, it has been shown that dipolar, aprotic solvents activate bases so that the resulting systems a r e active for the shift of a proton in a simple olefin.4,5The discovery of a rapid hydrocarbon prototropic shift led to extensive experimental work on a host of similar reactions involving weak hydrocarbon acids, including aromatic oxidation, elimination,? disproportionation,8 and Michaels a d d i t i~n .~, lo The olefinic prototropic shift offered the opportunity f o r an in-depth study of the influence of solvent, base, and substrate structure variations in a model carbanion precursor system. It was anticipated that information developed on this model could be broadly applied to understanding and predicting modes of behavior in carbanion reactions.If the base-catalyzed isomerization of an olefin involves carbanion formation, then a reasonable mechanism involves the formation of a carbanion (step l), allylic resonance of the intermediate (step 2), and finally, reformation of the isomerized olefin hydrocarbon (step 3). In this sequence, formation ofcontact ion pairs and solvent-separated ion patrs i s neglected for pictorial purposes, but their mode of formation and reaction is of obvious importance and will be discussed. Many questions arise concerning the prototropic shift:Is the reaction strictly a 1,3-intramolecular shift o r does the anion abstract *This paper, illustrated with slides,