Geraniol, hydrogenation

The bis(phosphane oxide) resolution route to (S)-MeOBIPHEP (S-29), the ligand for the geraniol hydrogenation, was successfully developed to a larger scale process (Fig. 11). In this process, phosphinoylation of 3-bromoanisole was realized... 

The nature of the catalyst deactivation in the geraniol hydrogenation and the exact role of the carrier surface require further research... 

Another important use of a-pinene is the hydrogenation to i j -pinane (21). One use of the i j -pinane is based on oxidation to cis- and /n j -pinane hydroperoxide and their subsequent catalytic reduction to cis- and /n j -pinanol (22 and 23) in about an 80 20 ratio (53,54). Pyrolysis of the i j -pinanol is an important route to linalool overall the yield of linalool (3) from a-pinene is about 30%. Linalool can be readily isomerized to nerol and geraniol using an ortho vanadate catalyst (55). Because the isomerization is an equiUbrium process, use of borate esters in the process improves the yield of nerol and geraniol to as high as 90% (56). 

Uses ndReactions. Nerol (47) and geraniol (48) can be converted to citroneUol (27) by hydrogenation over a copper chromite catalyst (121). In the absence of hydrogen and under reduced pressure, citroneUal is produced (122). If a nickel catalyst is used, a mixture of nerol, geraniol, and citroneUol is obtained and such a mixture is also useful in perfumery. Hydrogenation of both double bonds gives dimethyl octanol, another useful product. 

CitroneUol (27) is manufactured on a commercial scale by the hydrogenation of nerol (47) and geraniol (48) made either from a- or P-pinene. 

Geranyl acetate (a diene) takes up 2 moles of hydrogen unselectively in 48 hours to give the saturated acetate, 3,7-dimethyloctyl acetate, bp 109-110712 mm, 1.4261. (Geraniol itself has an allylic hydroxyl and appears to suffer decarbonylation under these reaction conditions.)... 

Following this initial SN1 reaction, loss of the pro-R hydrogen gives geranyl diphosphate, itself an allylic diphosphate that dissociates a second time. Reaction of the geranyl carbocation with water in a second S>jl reaction, followed by loss of a proton, then yields geraniol. 

Selective hydrogenation of citral Rh/SiO, Geraniol, nerol Fragrances... 

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). 

Geraniol can be converted into citronellol and menthol over Cu/A1203 under catalytic hydrogenation conditions owing to chemoselective hydrogenation and a three-functional process taking place on the catalyst surface. 

Here we report that geraniol 1, under catalytic hydrogenation conditions in the presence of a Cu/A1203 catalyst, gives two valuable products, namely citronellol 2 and menthol 3. 

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%. 

Both the rhodium and ruthenium catalysts have been used to successively hydrogenate the terpene geraniol (3) to citronellol (4) and 3,7-dimethyl-octanol (J08) ... 

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]. 

Enantioselective hydrogenation of unsaturated alcohols such as allylic and homoallylic alcohols was not very efficient until the discovery of the BINAP-Ru catalyst. With Ru(BINAP)(OAc)2 as the catalyst, geraniol and nerol are successfully hydrogenated to give (S)- or (R)-citronellol in near-quantitative yield and with 96-99% ee [3 c]. A substratexatalyst ratio (SCR) of up to 48 500 can be applied, and the other double bond at the C6 and C7 positions of the substrate is not reduced. A high hydrogen pressure is required to obtain high enantioselec-... 

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. 

Ru(II)-BINAP complexes (1) can effect enantioselective hydrogenation of pro-chiral ally lie and homoallylic alcohols, without hydrogenation of other double bonds in the same substrate.1 The alcohols geraniol (2) and nerol (3) can be reduced to either (R)- or (S)-citronellol (4) by choice of either (R)- or (S)-l. Thus the stereochemical outcome depends on the geometry of the double bond and the chirality... 

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