Imines catalyst, immobilization

Several organocatalysts have been recycled efficiently (selected examples are shown in Scheme 14.2). For example, the Jacobsen group has reported results from an impressive study of the recycling of the immobilized urea derivative 6, a highly efficient organocatalyst for asymmetric hydrocyanation of imines (Scheme 14.2) [11]. It was discovered that the catalyst can be recycled and re-used very efficiently - over ten reaction cycles the product was obtained with similar yield and enantioselectivity (96-98% yield, 92-93% ee). 

A series of (1-lactams (64) have been synthesized through the use of an immobilized cinchona alkaloid catalyst. This is postulated to proceed via the cycloaddition of an imine, and a ketene formed in situ through deprotonation of an acid chloride (Scheme 4.81). Different system configurations were described in the paper however, a column filled with a 5 1 mixture of solid K2C03 and immobilized-quinine derivative 65 cooled to —45 °C was found to be the most practical. The solution of the acid chloride and imine was dripped through the column and then directed... 

In an attempt to develop a PEG-supported version of a chiral phase-transfer catalyst the Cinchona alkaloid-derived ammonium salt 15 used by Corey and Lygo in the stereoselective alkylation of amino acid precursors was immobilized on a modified PEG similar to that used in the case of 13. The behaviour of the catalyst obtained 16, however, fell short of the expectations (Danelli et al. 2003). Indeed, while this catalyst (10 mol%) showed good catalytic activity promoting the benzy-lation of the benzophenone imine derived from tert-butyl glycinate in 92% yield (solid CsOH, DCM, -78 to 23 °C, 22 h), the observed ee was only 30%. Even if this was increased to 64% by maintaining the reac-... 

Many of the above-mentioned reactions can be successfully coupled with an extraction step in the CESS procedure. In the hydrogenation of imines, the conversion of the substrate to the product is sufficient to change the solubility of the Ir catalyst in the medium [69]. The transformation and subsequent extraction can thus be carried out without changes in pressure and temperature, effectively immobilizing the catalyst for at least four subsequent cycles. The low solubility of the Gmbbs metathesis catalyst allows for a similar sequence in RCM [9],... 

A spectacular, site-isolation effect in heterogeneous asymmetric catalysis was first reported by Pugin et al. The asymmetric hydrogenation of imine 1 is important for the commercial production of fS -metolachlor, a herbicide presently produced at >10000 tons per year. In this reaction, whereas homogeneous Ir-BPPM (2) catalyst prepared with [Ir(COD)Cl]2 was deactivated after 26% conversion (turnover frequency (TOP) min = 0), the covalently immobilized Ir catalysts, Si-PPM (3)-Ir, were much more active and productive (TOP min = up to 5.1 Scheme 2.1)... 

A few years ago Cahard reported a series of studies on the use of immobilized cinchona alkaloid derivatives in asymmetric reactions with phase-transfer catalysts [17[. Two types of polymer-supported ammonium salts of cinchona alkaloids (types A and B in Scheme 8.4) were prepared from PS, and their activity was evaluated. The enantioselectivity was found to depend heavily on the alkaloid immobilized, with the type B catalysts usually giving better results than the type A catalysts. By performing the reaction in toluene at -50 °C in the presence of an excess of solid cesium hydroxide and 0.1 mol equiv of catalyst 10, benzylation of the tert-butyl glycinate-derived benzophenone imine afforded the expected (S)-product in 67% yield with 94% ee, a value very close to that observed with the nonsupported catalyst. (Scheme 8.4, Equation b) Unfortunately-and again, inexplicably-the pseudoenantiomer of 10 proved to be much less stereoselective, affording the R)-product in only 23% ee. No mention of catalyst recycling was reported [18]. 

The functionalized ligands were tested for various hydrogenation reactions (see Scheme 12.17). Ir-Josiphos bound to silica gel as well as to a water-soluble complex produced TONs in excess of 100000 and TOFs up to 20000h for the Ir-catalyzed hydrogenation of 2-methyl-6-ethyl aniline (MEA) imine to give an intermediate for (SJ-metolachlor [45a]. The polymer-bound Ir complex was much less active, and in all cases the ee-values were comparable to those for the homogeneous catalyst Selected results are summarized in Table 12.3. However, no immobilized system could compete with the homogeneous catalyst which is used to produce >10000 tonsy of enantioenriched (. S J-metolachlor and which, under optimized condi-... 

Table3. MEA imine hydrogenation with Ir-ferrocenyldiphoshine complexes Comparison of catalyst performances of (a) homogeneous catalysts, (b) immobilized on silica gel, and (c) for reductive alkylation in a two-phase system (formulas see Fig. 7)...

L. Ya. Zakharova, A. R. Ibragimova, F. G. Valeeva, A. V. Zakharov, A. R. Mustafina, L. A. Kudryavtseva, H. E. Harlampidi, A. I. Konovalov, Nanosized reactors based on polyethylene-imines from microheterogeneous systems to immobilized catalysts, Langmuir, 2007, 23,... 

Portnoy focused his attention on the immobilization of PyBox ligands [153], The polymer-supported PyBox 240 were prepared in five steps from the Wang trichloroacetamidate resin (Scheme 96). The supported-ligands 240 were allowed to react with copper (1) triflate for 24h to afford the corresponding catalysts 241. These catalysts were then used in the first heterogeneous catalyzed addition of phenylacetylene to imine. 

As an efficient catalyst for enantioselective aza-MBH reaction, bifunctional phosphine catalyst CP17 has also been immobilized on a series of den-drimers. The dendrimer-immobilized catalyst CP44 was more effective than catalyst CP17 for the aza-MBH reaction of A-sulfonyl imines with MVK, EVK or acrolein. The catalyst could be separated easily from the reaction mixture by simple filtration after the reaction and reused without obvious loss of activity (Scheme 2.233). ... 

Further examples of supported catalysts used for ATH of imines include those based on recyclable silica [84], functionalized MCM-41 [85], siliceous mesocellular foam [86] and magnetic mesoporous silica. In the latter case, the reusable immobilized catalyst exhibited high activity and enantioselectivities in the ATH of imines in FA/TEA. It was able to be separated mechanically using an external magnet to facilitate ready recycling [87],... 

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