Expedient Synthesis of Homochiral 1‐Aryl‐Substituted 4,5‐Dihydro‐1H‐imidazoles and Their Modification to N‐Heterocyclic Carbene Precursors

Abstract

AbstractThe amidoamines (S)‐Ar1CONHCHRCH2NHAr2 [Ar1 = o‐C6H4SO3H, R = Bn, iBu, iPr; Ar2 = 2,6‐iPr2C6H4, 2,6‐Et2C6H4, 2,4,6‐Me3C6H2] cyclise to (S) 1‐aryl‐substituted 4,5‐dihydro‐1H‐imidazolinium species with HC(OEt)3 in moderate‐to‐excellent yields on heating to 150–175 °C (nine examples, four isolated yields of 48 to >97 %). They are attained as their o‐C6H4(SO3–)(CO2Et) salts. The latter are readily deprotonated to afford analytically pure (S) 1‐aryl‐substituted 4,5‐dihydro‐1H‐imidazoles (imidazolines). The purification of the intermediate sulfonate salts is not always necessary, and analytically pure imidazolines are isolated by simple kugelrohr distillation (nine examples, 45–95 %) after basification. Imidazoline alkylation provides a library of (S)‐N‐alkylimidazolinium salts (23 examples, 74–97 %). As the initially required amidoamines are available in simple one‐pot reactions, the overall approach constitutes a rather efficient approach to this useful family of chiral N‐heterocyclic carbene (NHC) ligand precursors (effectively three steps from commercial N‐Boc‐α‐amino alcohols; BOC = tert‐butyloxycarbonyl).