Reactions of 1,2-diketones with vinyllithium: Addition reactions and dianionic oxy cope rearrangements of cyclic and acyclic substrates

Dianionic oxy Cope rearrangements have been shown to take place at low temp erature upon syn double addition of alkenyllithium derivatives to cyclobuta nedione compounds such as benzocyclobutenedione chromium complex 1 or squar ic acid esters. In order to obtain some insight into the more general appli cability of this type of reaction sequence beyond these special cases, a nu mber of 1,2-diketones were treated with vinyllithium. The diketones tested include benzil derivatives, aliphatic acyclic 1,2-diketones, ortho-quinones , and cyclic aliphatic 1,2-diketones. With benzil and heterobenzil derivati ves, the desired double addition/dianionic oxy Cope rearrangement was found to take place at low temperature, leading to 1,6-diketones and their intra molecular aldol adducts in up to 80% overall yield. With acyclic aliphatic 1,2-diketones as substrates, this reaction sequence was also found, albeit with somewhat lower yields and requiring higher temperatures than in the be nzil cases. A brief investigation of the intramolecular aldol adduct/1,6-he xanedione equilibrium indicated that the preferential formation of intramol ecular aldol adducts at lower temperatures and at shorter reaction times ap pears to be the result of kinetic reaction control, whereas the preference for 1,6-diketones at higher temperatures is caused by thermodynamic reactio n control, ortho-Quinones reacted with vinyllithium only by addition; no di anionic oxy Cope rearrangement was observed. This was also the case for mos t aliphatic cyclic diketones; however, in the case of 1,2-indanedione, rear rangement products were obtained in moderate yield at elevated reaction tem peratures.