Kinetics and mechanism of alkali-induced decomposition of 2-alkoxyethylcobaloximes

Abstract

Kinetics of the base-induced decomposition of five 2-alkoxyethyl(aquo)cobaloximes, ROCH 2CH 2- Co(D 2H 2)OH 2 (R = C 6H 5, CF 3CH 2, CH 3, CH 3CH 2, (CH 3) 2CH), have been studied manometrically in aqueous base, ionic strength 1.0 M (KC1) at 25.0± 0.1 °C under an argon atmosphere. For the complexes with good leaving group alkoxide substituents (R = C 6H 5 and CF 3CH 2) the reactions are first- order in cobaloxime and first-order in hydroxide ion and produce stoichiometric amounts of ethylene and leaving group alcohol (ROH). NMR observation of decomposing solutions and workup of cobalt chelate products show that the reaction is initiated by hydroxide ion attack on an equatorial quaternary carbon leading to formation of an altered cobal- oxime product in which one of the Schiff's base linkages has become hydrated. For the remainer of the complexes the yield of ethylene is less than stoichiometric and pH-dependent, and the ethylene evolving reaction is second-order in hydroxide ion activity. The yield-limiting side reaction is shown to be base-catalyzed formation of a base-stable but photolabile alkoxyethylcobaloxime analog in which a Schiff's base linkage of the chelate has become hydrated, β-Elimination to form alkyl vinyl ethers was not observed for any of the alkoxyethylcobal- oximes. The second-order dependence of ethylene formation on hydroxide ion activity for R = CH 3, CH 2CH 3, and CH(CH 3) 2 is discussed at some length, but is not well understood at present.