Synthesis of the First Chiral Chelating Nitrogen Ligand of the Alkyl-BIAN Family

The number of applications of bis-imines as ligands in homogeneous catalysis has much increased in recent years. Derivatives of acenaphthenequinone (R-BIAN) are especially useful for their chemical stability and rigidity. Until recently only Ar-BIAN ligands in which the aryl group contained electrondonating or moderately electronwithdrawing groups were known. We have recently expanded the available range of derivatives to ligands where the aryl group bears strongly electronwithdrawing substituents,1 two different aryl groups are present,2 and even to Alkyl-BIAN compounds.3,4 The latest had never been isolated before because an isomerization reaction occurred leading to their decomposition. We identified the cause of the decomposition in the ring strain of the five-membered ring of the acenaphthene moiety, which is partly released upon isomerization of the C=N double bond. The problem could be solved by employing ring-strained amines, best cyclopropylamine, for which this isomerization is thermodinamically unfavorable.3,4 Here we report the first synthesis of a chiral cyclopropylamine derived from enantiomerically pure -pinene. The amine was obtained in good yields in four diastereomers by the procedure reported in the scheme. These can be separated by column chromato-graphy. Two isomers are most abundant. One of them was employed in the synthesis of the corresponding BIAN derivative and the structure of the ZnCl2 complex of the obtained ligand is shown below. A strong differentiation between adjacent quadrants is evident from the front view.