Regioselective ethyl transfer reactions between diethylzinc and 1,4-dialkyl-1,4-diaza-1,3-butadienes: Synthesis of EtZn[R(Et)NCH=CHNR],

Abstract

The l/l reaction of diethylzinc with 1,4dialkyl-1,4-diaza-1,3-butadienes (R-DAB = R-N=C(R’)-C(R’)=N-R) below -50°C results in formation of the l/l complex Et*Zn(R-DAB) (R = alkyl), containing o,a-N,N’ chelate bonded R-DAB. Above -50°C these complexes are unstable and undergo selective transfer of an ethyl group from Zn to the adjacent N atom, thus producing the novel species Et%[R(Et)NCR’=CR’NR] in quantitative yield. The proposed monomeric structure consists of a three-coordinate zinc atom bound to a 1-amino-2amidoethylene mono-anionic ligand via the N atoms. Reaction of these zinc complexes with t-BuOH(D) affords an equilibrium mixture of the ene-diamine and the 1-amino-2-iminoethane tautomers, which upon reaction with diethylzinc re-forms the EtZn[R(Et)NCR’=CR’NR] compound_ The tautomer equilibrium is dependent on the nature of R and R’. Branching at Co in R shifts the equilibrium towards the 1-amino-2-iminoethane tautomer, while introduction of an Me(R’) group stabilizes the ene-diamine tautomer.