Iron complexes, with cyclo-octa-1,5-diene

Corey-Winter elimination dibenzobarralene (442). In a similar way, spirohex-4-ene was obtained in 80% yield from (443). Daub and his co-workers have synthesized several uncommon olefins, among these (442), from parent cyclic thionocarbonates, using iron pentacarbonyl as the efficient elimination promoter. Semmelhack and Stauffer have reported that bis-(cyclo-octa-l,5-diene)nickel (444) converts thionocarbonates of vic-diols into the corresponding olefins in excellent yields and with a high degree of stereospecificity. In connection with a discussion of the mechanistic pathways of this reaction, the authors presented evidence for the intermediacy of the carbene complex (445). 

Reaction of 2,4,6-cyclo-octatrieneone (281) with 3-penten-2-oneiron tricarbonyl gave the iron tricarbonyl complex of its bicyclic valence tautomer. Oxidative cleavage at low temperature with ceric ammonium nitrate on alumina then gave bicyclo[4,2,0]octa-2,4-diene-7-one. At 0°C, (282) reverted to (281) with k = 5.7 X IO s SAG = 20 kcalmol . Combination ofthis data with an earlier estimate of the rate for the reverse reaction led to an estimate of the = 0.0052 at 25 °C. Direct measurement of the equilibrium concentrations of (281) and (282) by FT- H n.m.r. gave a value K 0.(X)7, in good agreement. 

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