Chiral [Bis(olefin)amine]rhodium(I) Complexes – Transfer Hydrogenation in Ethanol

B is( o lefin) a mines ( boa s) are a new class of ligands for the synthesis of transition metal complexes, which can be used in various homogeneous catalytic reactions. A simple straightforward coupling reaction between 5 H ‐dibenzo[ a , d ]cyclohepten‐5‐yl chloride (tropCl) and primary amines allows the synthesis of various chiral boa s. Birch reduction of phenylalanine gives (2 S )‐2‐amino‐3‐cyclohexa‐1,4‐dien‐1‐yl propanoate, which is used for the synthesis of the bis(olefin)amine methyl (2 S )‐3‐(cyclohexa‐1,4‐dienyl)‐2‐(5 H ‐dibenzo[ a , d ]cyclohepten‐5‐yl‐amino)propionate. Coupling between cyclohex‐3‐en‐1‐ylamine with tropCl gives N ‐(cyclohex‐3′‐en‐1′‐yl)‐5 H ‐dibenzo[ a , d ]cyclohepten‐5‐ylamine, which was separated into its enantiomers. Bicyclic cyclohexenylamine derivatives like bicyclo[2.2.2]oct‐5‐en‐2‐ylamine and 2‐(methoxycarbonyl)bicyclo[2.2.1]hept‐5‐en‐3‐ylamine were likewise coupled with tropCl to give chiral bis(olefin)amines. Alternatively, 5 H ‐dibenzo[ a , d ]cyclohepten‐5‐ylamine (tropNH 2 ) can react with cyclohexenyl ketones to give prochiral bis(olefin)imines, which were reduced to the corresponding bis(olefin)amines. With [Rh 2 (μ‐Cl) 2 (CO) 4 ] or [Rh 2 (μ‐Cl) 2 (C 2 H 4 ) 4 ], a complexation of these compounds was achieved leading to chiral rhodium complexes of the type [Rh( boa )(CO)]OTf or [Rh( boa )(PR 3 )]OTf. The complexes have a strongly distorted saw‐horse‐type structure (determined by X‐ray diffraction studies) and were tested in transfer hydrogenations with ethanol/2‐propanol as hydrogen donor. Only complexes with tightly bound olefinic binding sites and a pronounced saw‐horse‐type structure give significant activities. Furthermore, a phosphane ligand in trans position to the coordinated amine function has a positive impact of the catalysts performance. None of the investigated catalysts gave an impressive enantiomeric excess ( ee < 60 %) in the transfer hydrogenation of acetophenone.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)