Hydrogenation of N-arylimines

Fig. 2 f-Binaphane ligands 1. Summary of the best results obtained in the Ir-catalyzed hydrogenation of N-arylimines using la... 

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... 

Fig.3. Schematic catalytic cycle postulated for the Ir-diphosphine catalyzed hydrogenation of N-arylimines. Not shown are halide ligands...

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. 

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. 

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]. 

The transition metal catalyzed synthesis of arylamines by the reaction of aryl halides or tri-flates with primary or secondary amines has become a valuable synthetic tool for many applications. This process forms monoalkyl or dialkyl anilines, mixed diarylamines or mixed triarylamines, as well as N-arylimines, carbamates, hydrazones, amides, and tosylamides. The mechanism of the process involves several new organometallic reactions. For example, the C-N bond is formed by reductive elimination of amine, and the metal amido complexes that undergo reductive elimination are formed in the catalytic cycle in some cases by N-H activation. Side products are formed by / -hydrogen elimination from amides, examples of which have recently been observed directly. An overview that covers the development of synthetic methods to form arylamines by this palladium-catalyzed chemistry is presented. In addition to the synthetic information, a description of the pertinent mechanistic data on the overall catalytic cycle, on each elementary reaction that comprises the catalytic cycle, and on competing side reactions is presented. The review covers manuscripts that appeared in press before June 1, 2001. This chapter is based on a review covering the literature up to September 1, 1999. However, roughly one-hundred papers on this topic have appeared since that time, requiring an updated review. 

The nature of the substituent directly attached to the N-atom influences the properties (basicity, reduction potential, etc.) of the C=N function more than the nature of the substituents at the carbon atom. For example, it was found that the Ti-ebthi catalyst (Fig. 1) can hydrogenate only hf-alkylimines but not N-arylimines [6]. Oximes and other C=N-X compounds show even a more pronounced variation in their reactivity. The following sections give a short summary of the results obtained for different classes of C=N groups. Only catalysts with synthetically useful selectivities or otherwise of interest were included in Tables 2, 3, 4, 5, and 6 (s/c substrate/catalyst ratio, tof turnover frequency at high conversion). 

Acyclic aromatic A-arylimines, Ar1-C(Me)=N-Ar2, have been reduced to the corresponding amine with up to 99% ee, using 1 atm of hydrogen and an iridium(I) catalyst bearing a chiral diphosphinoethane chelating ligand.52... 

The spectra of the s-triazole 4-sulfonylimines differ clearly from those of the pyridine Af-sulfonylimines.131 These latter compounds possess the following four principal fragmentation processes N-S bond cleavage, rearrangement reactions to ionized V-arylimines and azacarbazoles, the loss of hydrogen atom from the molecular ion, and the cleavage a to the S02 group. 

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