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

Bis(olefin)amines (boas) 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 5H-dibenzo[a,d]cyclohepten-5-yl chloride (tropCl) and primary amines allows the synthesis of various chiral boas. Birch reduction of phenylalanine gives (2S)-2-amino-3-cyclohexa-1,4-dien-1-yl propanoate, which is used for the synthesis of the bis(olefin)amine methyl (2S)-3-(cyclohexa-1,4-dienyl)-2-(5H-dibenzo[a,d]cyclohepten-5-yl-amino)propionate. Coupling between cyclohex-3-en-1-ylamine with tropCl gives N-(cyclohex-3'-en-1'-yl)-5H-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, 5H-dibenzo[a,d]cyclohepten-5-ylamine (trop-NH2) can react with cyclohexenyl ketones to give prochiral bis(olefin)imines, which were reduced to the corresponding bis(olefin)amines. With [Rh-2(mu-Cl)(2)(CO)(4)] or [Rh-2(mu-Cl)(2)(C2H4)(4)], a complexation of these compounds was achieved leading to chiral rhodium complexes of the type [Rh(boa)(CO)]OTf or [Rh(boa)(PR3)]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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)