Asymmetric Hydrogenation of Unfunctionalized Olefins

Historically, reaction of simple olefins in the presence of chiral phosphine-Rh complexes in 1968 marked the first examples of homogeneous asymmetric hydrogenation [6], However, only a few successful results have been reported for asymmetric hydrogenation of unfunctionalized olefins. Some examples with late and early transition-metal complexes are illustrated in Schemes 1.27-28 and Schemes 1.29-30, respectively. 

Asymmetric hydrogenation of unfunctionalized olefins, enamines, and imines... 

The synthesized zwitterionic iridium complexes containing various chiral P, N ligands with imidazoline or oxazoline were used as precatalysts for the asymmetric hydrogenation of unfunctionalized olefins. The cationic complexes with fluorinated borates as anions were superior catalysts in dichloromethane, whereas the iridium betaines were good catalysts in pure hydrocarbons. ... 

Figure 15.7. A selection of P,N ligands used for the asymmetric hydrogenation of unfunctionalized olefins.

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

TABLE 6-3. Chiral Titanocene-Catalyzed Asymmetric Hydrogenation of Unfunctionalized Trisubstituted Olefins... 

In this context it is worth noting that neither the titanium(IV) tartrate catalyst nor other metal catalyst-alkyl hydroperoxide reagents are effective for the asymmetric epoxidation of unfunctionalized olefins. The only system that affords high enantioselectivities with unfunctionalized olefins is the manganese(III) chiral Schiff s base complex/NaOCl combination developed by Jacobsen [42]. There is still a definite need, therefore, for the development of an efficient chiral catalyst for asymmetric epoxidation of unfunctionalized olefins with alkyl hydroperoxides or hydrogen peroxide. 

The 7 -symmetric complex ethylene bis(tetrahydroindenyl) titanium l,T-binaphth-2,2 -dithiolate has been used to catalyze the asymmetric hydrogenation of unfunctionalized trisubstituted olefins.1680 The kinetic resolution of racemic disubstituted 1-pyrrolidines via asymmetric reduction has been described.1681... 

Iridium-Phosphinooxazoline Catalysts. Asymmetric hydrogenation of simple olefins with chiral Rh or Ru catalysts generally shows low reactivity and unsatisfactory enantioselectivity (198,248,249). However, several unfunctionalized olefins can be hydrogenated in high yields and excellent enantioselectivity by using iridium catalysts with chiral phosphinooxazoline ligands (60,186,187, 189,191-194,250) (Fig. 56). To avoid catalyst deactivation, the extremely weakly coordinating anion tetrakis[3,5-bis(trifiuoromethyl)phenyl]borate has to be used (182,251). 

In the area of the asymmetric hydrogenation of C=C double bonds, the reduction of unfunctionalized olefins has been scarcely explored, in comparison with... 

The cationic Rh and Ir complexes with chiral (iminophosphoranyl)ferrocene ligands (107) and (108) were found to be very powerful catalysts for asymmetric hydrogenation of a series of unfunctionalized di- and trisubstituted olefins with almost perfect enantiomeric excesses (up to 99% ee) under mild conditions. In some cases the rhodium complexes were even better catalysts than their iridium counterparts. ... 

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