Geraniol, asymmetric hydrogenation

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). 

Complexes containing one binap ligand per ruthenium (Fig. 3.5) turned out to be remarkably effective for a wide range of chemical processes of industrial importance. During the 1980s, such complexes were shown to be very effective, not only for the asymmetric hydrogenation of dehydroamino adds [42] - which previously was rhodium s domain - but also of allylic alcohols [77], unsaturated acids [78], cyclic enamides [79], and functionalized ketones [80, 81] - domains where rhodium complexes were not as effective. Table 3.2 (entries 3-5) lists impressive TOF values and excellent ee-values for the products of such reactions. The catalysts were rapidly put to use in industry to prepare, for example, the perfume additive citronellol from geraniol (Table 3.2, entry 5) and alkaloids from cyclic enamides. These developments have been reviewed by Noyori and Takaya [82, 83]. 

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. ... 

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... 

Asymmetric hydrogenation of allylic and homoallylic unsaturated alcohols was not very efficient until the discovery of the BINAP-Ru catalyst. With Ru(BrNAP)(OAc)2 as catalyst, geraniol 70 and nerol 72 are successfully hydrogenated to give ([S)- or (i )-citronellol (71 and 73, respectively) in high overall yield with good enantioselectivity of 98 and 99% ee.59... 

Optically active citronellol can be prepared in high optical purity by asymmetric hydrogenation of geraniol using a rhodium [(5)-BESfAP]2" catalyst (BINAP = 2,2 -bis (diphenylphosphino)-1,1 -binaphthyl) [ 165-168]. 

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. 

The 1-pro-7 -hydrogen is lost on oxidizing geraniol with a cell-free extract from Cannabis sativa (Vol. 7, p. 9, ref. 96), asymmetric microbial reduction of ( )-citronellal to (-)-citronelloI is reported, and callus cultures of Nicotiana tabacum selectively hydroxylate linalool, dihydrolinalool, and the derived acetates at the -methyl group [e.g. to give (59)]. ... 

Allylic alcohols also provide a suitably activated substrate for hydrogenation. Ryoji Noyori s asymmetric reduction of the prochiral geraniol (E-double bond geometry) and nerol (Z-double bond geometry) to enantiomerically pure citronel-lol in the presence of Ru(OAc)2 BINAP is a well-known example. The nonallylic olefin is not reduced appreciably, which indicates the importance that the allylic alcohol functionality plays in this reduction. (Homoallylic alcohols are also reduced by this system, but when the olefin in question is three or more bonds distant from the alcohol moiety, the compound is inert.) Either enantiomer of cit-ronellol is accessible regardless of which substrate is used depending on the chirality of the Ru-BINAP catalyst used (8). This type of relationship implies that the reaction s mechanism possesses high facial selectivity. 

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