Arylimines, enantioselective hydrogenation

Enantioselective Hydrogenation of A -Arylimines in the Synthesis of the Chiral Herbicide, ( S)-Metolachlor... 

Spindler, F., Pugin, B., Jalett, H.-P., Buser, H.P., Pittelkow, U. and Blaser, H.U. (1996) A technically useful catalyst for the homogeneous enantioselective hydrogenation of N-arylimines a case study. Chemistry Industry Catalysis of Organic Reactions), (Dekker) 68, 153-166. 

Other 2, 2 -disubstitued biferrocene derivatives (ligands 2, Fig. 3) also tested using the dinuclear [Ir( r-Cl)(COD)]2 as the iridium source provided total conversion in arylimine hydrogenation, although with poor-to-moderate enantioselectivity (20-33% ee) [17]. The low enantioselectivies are assumed to be related to the conformational flexibility of this type of bifep ligands. 

An iridium(I) complex with the l,2-bis(tcrt-butylmethylphosphino)ethane (4) and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as the counter anion catalyzes the hydrogenation of several acyclic aromatic Ai-arylimines under atmospheric hydrogen pressure at room temperature, giving the desired chiral amines with high-to-excellent enantioselectivities (up to 99%, Fig. 6) [19]. The authors also tested (S )-BINAP (Fig. 1) and (/ )-Ph-PHOX (PHOX = 2-[2-(diphenylphosphino) phenyl]-4,5-dihydrooxazole) hgands with lower enantioselectivities [19]. Both steric and electronic properties of the ligand and the combination with the BArF anion are in the base of the efficacy of this catalytic system. On the other hand, attempted hydrogenations of Ai-(2,2,2-trifluoro-l-phenylethylidene)aniline and M-(l,2,2-trimethyl-propylidene)aniline under the same conditions resulted in... 

Later, furanoside diphosphinite ligands 12, but with C2-symmetry, provided enantioselectivities up to 70% in the asymmetric hydrogenation of several N-arylimines (Fig. 12) [26]. The electronic effect of the ligand on enantioselectivity is considerable. Results were best when electron-donating groups are present on the phenyl rings (ligand 12b). On the other hand, the use of additives was detrimental to both conversion and enantioselectivity. 

The furanoside phosphite-phosphinite ligand 13, related to the previously diphosphinite ligands 12, was also applied to the asymmetric hydrogenation of N-arylimines, increasing the enantioselectivities up to 76% (Fig. 13) [26]. Precursors based on cationic [Ir(cod)2]BF4 provided better yields and enantioselectivities than... 

Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

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