Taniaphos catalysts

The Taniaphos catalyst (6) has been found to give S 2 products in excellent yields (90-98%) with a high enantioselectivity (90-98% ee) in the allylic substitution of (7e) several different leaving groups by Grignard reagents in the presence of CuBr.SMe2 in r-BuOMe at -78 °C.9. 

Taniaphos (139), developed by Knochel (with cooperation with Degussa), can be electronically fine-tuned by variation of the phosphine groups and the groups on the chiral sidechain (R1).177,178 Taniaphos catalysts have been used to reduce enamide esters to a-amino esters,178-181 P-acylami-doacrylates esters to P-amino esters,175 1,3-diketones,179 180 P-keto esters,179,180 enol acetate,179,180 enamides to amide,179 and itaconate esters179,180 with enantioselectivities >90%. 

Interestingly, TaniaPhos catalysts used in the asymmetric hydrogenations can be synthesized from the (7 )-Ugi amine 15 in two steps [14], The (/ )-Ugi amine can be synthesized via a Mannich reaction between ferrocene 12, dimethyl amine (13), and acetaldehyde (14) (Scheme 15.6) [15]. 

Pre-Catalyst Systems Using TaniaPhos/MandyPhos Ligands... 

TaniaPhos active catalyst discussion As shown by Salzer (2) such complexes with half sandwich stracture result in the catalyst cycle into a hydride species where the pentadienyl moiety can be hydrogenolyticaUy liberated (2, 6). This was verified in the case of BINAP complexes (2, diss. Podewils, Geyser). In accordance to this fact and other mechanistic aspects from Noyori s work (3, 5) it is likely that the pre-catalyst species undergoes the same reaction pathway and that the reactive part of the pre-catalyst, the pentadienyl moiety, will be liberated under hydrogenolytic conditions as shown below in Scheme 23.9 ... 

The recent introduction of the TaniaPhos ligands (34) provides another excellent catalyst system for the reduction of dehydroamino acid derivatives and enol... 

A broad screening of ligands and ionic liquids was carried out by Feng et al. [104]. For rhodium-catalyzed hydrogenation of enamides the best catalysts were found to be the rhodium-ferrocenyl-diphosphine complexes with taniaphos, josiphos, walphos and mandyphos as ligands (Fig. 41.9). 

The second approach starts from achiral allylic substrates and uses a chiral metal catalyst. Among all ligands which have been tested binaphtol-derived phosphoramidites like 174 have led to excellent regio- and enantioselectivities (equation 103) °. Highly stereoselective substitutions are also obtained using ligand 175 (Taniaphos, equation 104... 

A catalytic enantio- and diastereo-selective aldol reaction of ketones with ketene (g) silyl acetals, H2C=C(OTMS)-OMe, gives fair to good yields and ee.129 With further substitution of the vinyl function, the reaction is diastereoselective, up to 97%. A highly developed catalyst/promoter protocol is employed a copper fluoride complex is combined with a Taniaphos auxiliary (a chiral ferrocenyldiphosphine), plus (EtO)3SiF. Evidence for the formation of species (EtO)4 SiF (n > 2) as active... 

The screening of the different reaction media proved that the application of an ionic liquid/water mixture - a so-called wet ionic liquid - led to improved catalyst recycling compared to the reaction in ionic liquid without co-solvent. This comparative study revealed good results for the hydrogenation of various enamides in wet ionic liquids with respect to conversion, enantioselectivity and catalyst separation. The results obtained with the Rh-taniaphos system in different solvent systems are summarized in Table 5.3-4. 

The ferrocene-based ligand Taniaphos (137) has been found particularly effective in copper-catalyzed AAA using Grignards and allylic bromides. " Here, CuBr SMe2 is the precursor of choice (1-5 mol%), used in DCM at -78 "C. Cinnamyl systems typically react in high yields and excellent ee s. Catalyst loading of only 1% is sufficient. Reactions in MTBE (f-BuOMe) as solvent were not as selective. 

The use of ort/jo-substituted cinnamyl substrates in Cu-catalysed AAA is a particularly difficult transformation, especially when the reaction is performed with MeMgBr, as an erosion on the regio- and enantioselectivity is generally observed. Feringa and co-workers demonstrated that, by employing their previously developed copper-taniaphos (L6) catalyst, a variety of ort/io-substituted cinnamyl... 

Scheme 15.5 Asymmetric hydrogenation of 11 with Ir/TaniaPhos and asymmetric transfer hydrogenation with Noyori catalyst toward 10.

An efficient diastereo- and enantio-selective reductive coupling of alkenylazaarenes with ketones has been developed, which provides aromatic heterocycles bearing tertiary-alcohol-containing side chains, with copper(II) acetate and Taniaphos SL-TOOl-1 as the catalytic system and phenylsilane is the hydride source." Mixed phosphorus-sulfiir ligands derived from carbohydrates are good catalyst precursors... 

---