Sterically Hindered Diazabutadienes (DABs): Competing Reaction Pathways with MeLi

Treatment of N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene with MeLi results in carbon−carbon bond formation and selective reduction of one of the imine arms to give the N-lithiated salt of the imine/amine, N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3,3-trimethyl-1-butene. This compound crystallizes from Et2O as a solvent adduct, exists as a monomer both in solution and the solid state, and features a three-coordinate lithium nucleus that is chelated to oxygen, amido, and imino donors. In contrast, treatment of N,N′-bis(2,4,6-trimethylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene with MeLi gives the N-lithiated imine anion N-(2,4,6-trimethylphenyl)-1-aza-2-methyl-3-N′-(2,4,6-trimethylphenyl)-1,3-butadiene, as the ether adduct, by proton abstraction. With a more hindered base, N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene undergoes proton abstraction with lithium diisopropylamide to give the N-lithiated imine anion of the ene/amine, N-(2,6-diisopropylphenyl)-1-aza-2-methyl-3-N′-(2,6-diisopropylphenyl)-1,3-butadiene, again isolated as the ether adduct. A further equiv of MeLi·LiBr added to N-(2,4,6-trimethylphenyl)-1-aza-2-methyl-3-N′-(2,4,6-trimethylphenyl)-1,3-butadiene gives the doubly N-lithiated salt of the diene/diamide, 2,3-N,N′-bis(2,4,6-trimethylphenyl)-1,3-butadiene, in which both nitrogen centers have been reduced, and which adopts a cisoid stereochemistry with respect to the butadiene unit in the solid state.