Reactions of 2,3-Dimethylenebutadiene Dianion with Electrophiles. Synthesis and Conformations of 2,3-Disubstituted-1,3-butadienes

Article abstract of DOI:10.1021/jo00190a025

Reactions of 2,3-dimethylenebutadiene dianion 1 with monofunctional electrophiles were found to provide the best routes to symmetrically substituted 1,3-butadienes such as 2c - n.Evidence that at least some of these reactions go by single electron-transfer mechanisms is presented.Possible mechanisms for the formation of some unusual byproducts are discussed.Reactions of 1 with dichlorides and dibromides were found to provide the best routes to most 1,2-dimethylenecycloalkanes 3, the corresponding dimers 4 and trimers 5, and 1,1-dimethyl-3,4-dimethylene-1-metallacyclopentanes 26.From their UV spectra, the dienes 2 contain about 60percent transoid conformations, except 2,3-dimethyl-1,3-butadiene (2a) which shows about 100percent and 2,3-dineopentyl-1,3-butadiene (2h) which shows about 30percent. 1,2-Dimethylenecycloalkanes 3 display cisoid conformations in rings smaller than 9-membered and transoid conformations in rings larger than 11.When the ring size exceeds 15, the homoallylic methylene groups on both sides are also anti coplanar.With rings larger than about 30, the next methylene group on each side is also anti coplanar, and when the ring size exceeds about 45, an additional methylene on each side is anti coplanar.