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 (tropNH2) can react with cyclohexenyl ketones to give prochiral bis(olefin)imines, which were reduced to the corresponding bis(olefin)amines. With [Rh2(μ‐Cl)2(CO)4] or [Rh2(μ‐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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) A number of chiral [bis(olefin)amino]rhodium(I) complexes containing a tropylidenyl moiety (blue) were synthesized and tested in enantioselective transfer hydrogenations by using ethanol as “green” hydrogen donor. The best results were obtained with a ligand containing a cyclohex‐3′‐en‐1′‐yl group (red) (up to 44 % ee at a substrate/catalyst ratio S/C = 10000).