Hydrogenation of geraniol

Asymmetric hydrogenation of geraniol and nerol in methanol at room temperature and an initial hydrogen pressure of 90-100 atm gives citronellol in 96-99% ee and in quantitative yields. The allylic and non allylic double bonds in the substrate can be clearly differentiated to obtain the product contaminated with less than 0.5% dihydrocitronellol (Mookherjee, 1997). 

The hydrogenation of geraniol over Cu/A1203 in hydrocarbon solvents gives mixtures of citronellol 2 and menthol 3. 

This is a quite remarkable result, as the chemoselective hydrogenation of geraniol over a heterogeneous catalyst has rarely been reported. It can be carried out over platinum containing zeolite (9), over Pt/Al203 modified with carboxylic acids (10), over Ni/diatomaceous earth and alkali hydroxides or carbonates (11) or NiRaney and alkali or alkaline earth metal hydroxides (12), yields never exceeding 85%. 

Takaya and co-workers46 found that BINAP-based Ru(II) dicarboxylate complexes 31 can serve as efficient catalyst precursors for enantioselective hydrogenation of geraniol (2E)-32 and nerol (2Z)-32. (R)- or (iS )-citroncllal 33 is obtained in nearly quantitative yield with 96-99% ee. The nonallylic double bonds in geraniol and nerol were intact. Neither double bond migration nor (fi)-/(Z)-isomerization occurred during the catalytic process. Furthermore, the S/C ratio was extremely high, and the catalyst could easily be recovered (Scheme 6-18). This process can be applied to the asymmetric synthesis of a key intermediate for vitamin E. 

A characteristic example for the application of homogeneous catalysts in enantiose-lective and regioselective hydrogenation of dienic compounds is the hydrogenation of geraniol and nerol to citronellol with Ru-BINAP catalyst (equation 8)26,27. The high enantiomeric excesses (96-98%), the nearly quantitative yields (>95%) and the very low catalyst/substrate ratio (1 50000) are attractive attributes of this process. 

Crude BINAP-Ru complex with consistent spectral characteristics can be used for hydrogenation of geraniol (98.7% pure commercial geraniol containing 1.3% of nerol, distilled from 4 A molecular sieves, 4.7 M substrate in 95% aqueous methanol, 2.8 mM Ru(OCOCH3)2[(R)-BINAP], 100 atm of H2, 20°C, 8 hr), to give (S)-citronellol in 96% ee, 97% isolated yield. 

The procedure for the synthesis of the title compound is a representative example of asymmetric hydrogenation in the presence of BINAP-Ru(ll) diacetate.5 The method is based on the synthesis of BINAP-Ru(ll) dicarboxylate complexes and enantioselective hydrogenation of geraniol.7 The present method provides the first practical means for asymmetric synthesis of (S)- and (R)-citronellol. (S)-(-)-Citronellol of optical purity up to 92% can be obtained in a limited quantity from rose oil. A microbiological reduction of geraniol was reported to give enantiomerically pure (R)-(+)-citronellol. ... 

Similar effects can be observed in the hydrogenation of molecules with two competing C=C double bonds (54). As an example, the hydrogenation of geraniol to citronellol or 3,7-dimethyl-2-octen-l-ol has been performed with supported Pt°. With Pt particles in a non-shape selective environment such as MCM-41, the chemoselectivity for citronellol amounts to 85% at 80% conversion. However, in a shape selective host such as zeolite Pt-HBeta, 100% chemoselectivity is obtained at the same conversion level ... 

Similarly, catalytic hydrogenation of geraniol (1.60) in the presence of Ru-(i )-BINAP complex yields (S)-(—)-citronellol (1.61) in high yield . ... 

A characteristic example for the application of homogeneous catalysts in enantioselective and regioselective hydrogenation of dienic compounds is the hydrogenation of geraniol and nerol to citronellol with Ru-BINAP catalyst (equation The high... 

Fig. 5 Ru-BINAP enantioselective hydrogenation of geraniol/nerol by Takaya and Noyori et al. [58, 59],...

Tab. 1 Enantioselective hydrogenation of geraniol (13) with chiral Ru(tfa)2-complexes.

In the study of Ru-BINAP catalyzed asymmetric hydrogenation of geraniol 10, a striking isomerization to y-geraniol 11 was observed, Eq. (5) [24]. The reaction is noteworthy in that it promotes the olefin migration from an inner to an exo position. Originally the presence of 11 was supposed to be an intermediate... 

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