Reactions of 1,4-Dihalocubanes with Organolithiums. The Case for 1,4-Cubadiyl

摘要

Recently Eaton and Maggini reported that reaction of 1,2-diiodocubane with tert-butyllithium produces 1,2-dehydrocubane (cubene). We report now our work on the reactions of 1,4-dihalocubanes with organolithiums undertaken in search of 1,4-dehydrocubane. Addition of 1,4-diiodocubane in THF to an excess of tert-butyllithium in THF/pentane at -78 deg C followed by carbomethoxylation gives primarily 1,4-dicarbomethoxycubane (from 1,4-dilithiocubane, 2) and 5 in a ratio of ca. 2:1. The latter comes via elimination of HI from 4-iodolithiocubane (1) giving 4-lithiocubene (3), followed by addition of tert-butyllithium to this strained olefin (yields 4). No substantial amounts of bicubyls are found. On the other hand, dropwise addition of tert-butyl-lithium to a solution of 1,4-diiodocubane in THF at -78 deg C followed by carbomethoxylation gives a diverse mixture of products including 4-tert-butylcarbomethoxycubane (6), 4,4'-dicarbomethoxybicubyl (7), and 4-carbomethoxybicubyl (8); 6 and the bicubyls 7 and 8 are thought to arise, respectively, from reaction of tert-butyllithium, (iodocubyl)lithium (1), or cubyllithium with 1,4-dehydrocubane. When 1,4-dibromocubane or 4-bromoiodocubane is used in place of the 1,4-diiodide in either mode of addition, the principal products are derived via lithiocubene 3. The rate of formation of 1,4-dehydrocubane by loss of LiBr from 4-bromolithiocubane is apparently slow by comparison to loss of LiI from 1, as is the rate of bromine-lithium exchange. Evidently, tert-butyllithium-induced dehydrobromination (yields 3) can compete effectively. Reaction of 4-chloroiodocubane with tert-butyllithium in THF at -78 deg C gives 4-chlorolithiocubane.