Enantiomerically Pure Isophorone Diamine [3-(Aminomethyl)-3,5,5-trimethylcyclohexylamine]:  A Chiral 1,4-Diamine Building Block Made Available on Large Scale

Isophorone diamine [IPDA, 3-(aminomethyl)-3,5,5-trimethylcyclohexylamine] is a chiral non-C₂-symmetric 1,4-diamine which is produced industrially on large scale as the mixture of all four stereoisomers (cis/trans ca. 3:1). Starting from this industrial bulk product, the preparation of the bis-tosyl, bis-Fmoc, bis-Boc and bis-Z derivatives of cis-IPDA, the preparation of the pure cis enantiomers by HPLC on chiral stationary phase, and the assignment of absolute configurations to the isolated enantiomers are described. We furthermore report an efficient method for the optical resolution of IPDA by salt formation with dibenzoyl tartaric acid. The latter method conveniently affords enantiomerically pure cis-IPDA in 100 g quantities. A number of salen ligands have been prepared from this enantiomerically pure 1,4-diamine and fully characterized. The nickel complex of one of the salen ligands was prepared and analyzed by X-ray crystallography. The crystal structure of the Ni₄L₄ complex illustrates the pronounced preference of cis-IPDA for adopting the chair conformation in which both the amino- and the aminomethyl substituents occupy equatorial positions. As a consequence, the two salicylidene imine moieties of one ligand molecule do not converge on one metal ion, but act as bridging ligands between two nickel ions.