From lithium ketazides to isomeric silylketazine-rings – imine-enamine tautomerism

Abstract

Di( tert-butylmethyl)ketazine ( I) reacts with n-BuLi in a 1:1 molar ratio to give a monolithium salt ( II). The reaction of II with t Bu 2SiF 2 in n-hexane leads, even in a 1:1 molar ratio, to the formation of the isomeric five- and four-membered ring compounds 1 and 2. Compound 1 has an endocyclic imine and an exocyclic enamine unit. The opposite is found for 2. The acyclic monosubstitution product, t Bu 2SiFCH 2–C t Bu N–N C t BuCH 3 ( III) could not be isolated. It reacts with the lithium ketazide to give 1 or 2. I is reformed. The reaction in THF yields only the four-membered ring 2. In a comparable reaction of the lithium ketazide and (H 3C) 2SiF 2, the substitution product 3 could be isolated. A possible formation mechanism for 2 includes an intermediate silene IV. Both compounds 1 and 2 react with H 3C–OH under cleavage of the endocyclic Si–N-bond to give the addition product 5. The reaction mechanism includes a hydrogen shift from a nitrogen atom to a carbon atom via an imine-enamine tautomerism. In a 2:1 molar ratio, n-BuLi and the di( tert-butylmethyl)-ketazine ( I) form the dilithium salt, 6. Compound 6 crystallizes from THF as trimer with four imine and two enamine units. A seven-membered ring ( 7) isomeric to 1 and 2 is the result of the reaction of 6 with t Bu 2SiF 2. Compound 7 contains one imine and one enamine unit in the ring skeleton. The comparable reaction of the (CH 3) 3Si-substituted dilithium-di( tert-butylmethyl)ketazide and t Bu 2SiF 2 yields the five-membered ring compound 8 with one endocyclic imine and one exocyclic enamine unit. Quantum chemical calculations of 1, 2, 7 and the intermediate silene IV have been carried out and show a low energy difference between the cyclic silyl-ketazine isomers.