Vinylferrocene as paradigm for the two-step reaction mode with ketenes. The case of tert-butylcyanoketene (Moore’s ketene)

Abstract

The reaction of tert-butylcyanoketene (TBCK, Moore’s ketene; 1) with ethenylferrocene (2) yields four products: the cyclobutanone 2-trans-tert-butyl-2-cyano-3-ferrocenylcyclobutanone (3), two δ-lactones as 2:1 ketene:alkene cis and trans adducts 3-cis-tert-butyl-3-cyano-6-(1-cyano-2,2-dimethylpropylidene)-4-ferrocenyltetrahydro-2-pyrone (4) and 3-trans-tert-butyl-3-cyano-6-(1-cyano-2,2-dimethylpropylidene)-4-ferrocenyl-tetrahydro-2-pyrone (5), and an α,β-unsaturated ketone 3-cyano-1E-ferrocenyl-5,5-dimethylhex-1-en-3-one (6). The configuration of the bulky groups (t-Bu and ferrocenyl) in cyclobutanone 3 is trans. The two δ lactones are stereoisomers as a result of the cis and trans configuration of t-Bu and ferrocenyl groups. The cyclobutanone 3 isomerizes by heating or catalytically into the α,β-unsaturated ketone 6. Assigned structures for the compounds were made based on IR, 1H, 13C NMR, and for 5 also by single-crystal X-ray diffraction.