One-pot synthesis of β-diketimine ligands

Symmetric N,N'-dialkyl-2-amino-4-imino-pent-2-enes (nacnac(R)H) can be prepared in high yields in a simple one-pot reaction from acetylacetone and 2 equiv. of amine. Optimized conditions require the azeotropic removal of water, use of a minimum amount of solvent, and of 1 equiv. of acid. Either para-toluenesulfonic acid, hydrochloric acid, or a mixture of both can be employed, with the latter being most advantageous. Symmetric nacnacRH are thus obtained in higher than 95% purity and with 65% yield for R = Me and 80%-95% yields for R = n-Pr, i-Pr, i-Bu, Bn, Cy, and (+)-CH(Me)Ph.