Hydrogenation citronellal

In this case, the aldehydic function hydrogenation is very selective even at very high conversion. A very small amount of citronellal is detected in the early time of reaction and disappears as the reaction proceeds. No product from citronellal hydrogenation has been detected owing to the accuracy of the analysis. 

According to Skita, the reaction proceeds in a different manner if the reduction be effected with palladium chloride and hydrogen. In this case the citral in alcoholic solution is mixed with an aqueous solution of palladium chloride and the whole thickened with gum-arabic. Hydrogen gas is then forced into this solution under pressure. The products of the reduction include citronellal and citronellol and a di-molecular aldehyde, C Hj O, which probably has the following constitution —... 

Natural products citronellal (35) and linalool (36) have the same skeleton as (34). Hydrogenation of linalool gave alcohol (37). Dehydration and hydrogenation would be the obvious way to make (34) from (37) but an alternative was used here. The halide (38) was reduced with sodium in liquid ammonia. 

A striking difference in selectivity was observed. According to the non-acidic character of the support Cu/Si02 showed to be an excellent hydrogenation catalyst. The conjugated double bond was selectively reduced giving citronellal with fairly good selectivity and then citronellol. 

On the contrary, the Lewis acid sites present on the snrface of sepiolite make the Cn/sepiolite catalyst extremely active in promoting the ene reaction of citronellal. Thns, citronellal never accnmnlates in the reaction mixtnre bnt it is com erted into isopulegol as soon as it forms. Hydrogenation of isopnlegol is very slow nnder these reaction conditions, bnt this simple catalyst is able to produce menthol in a one-pot-one-step reaction under very mild experimental conditions. Notably dehydration products, which give account of 40% of the reaction mixture obtained over Ni-H-MCM-41 [4], are kept under 20% over both Cu catalysts. 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

The reactor system works nicely and two model systems were studied in detail catalytic hydrogenation of citral to citronellal and citronellol on Ni (application in perfumery industty) and ring opening of decalin on supported Ir and Pt catalysts (application in oil refining to get better diesel oil). Both systems represent very complex parallel-consecutive reaction schemes. Various temperatures, catalyst particle sizes and flow rates were thoroughly screened. 

To evaluate the performance of the reactor system, the catalytic hydrogenation of citral to citronellal and citronellol in ethanol was nsed as a sample reaction. The reaction scheme is displayed below. 

Citronellol is formed through selective hydrogenation of the C=C bond activated by the presence of the OH group, whereas menthol 3 is the product of a three-functional process involving isomerization of the allylic alcohol 1 to citronellal 4, ene reaction to isopulegol 5 and final hydrogenation (Scheme 2). 

Hydrogen peroxide Ketones, Nitric acid Ozone Citronellic acid See other cyclic peroxides... 

Catalytic hydrogenation of citronellal provides (+)-citronellol with high optical purity (Equation (3)). 

Another example from Liu s team in this field concerns the selective hydrogenation of citronellal to citronellol by using a Ru/PVP colloid obtained by NaBH4 reduction method [112]. This colloid contains relatively small particles with a narrow size distribution (1.3 to 1.8 nm by TEM), whereas the metallic state of Ru was confirmed by XPS investigation. This colloid exhibited a selectivity to citronellol of 95.2% with a yield of 84.2% (total conversion 88.4%), which represented a good result for a monometallic catalyst. 

Isomerization of allylic amines is another example of the application of the BINAP complex. Rh BINAP complex catalyzes the isomerization of N,N-diethylnerylamine 40 generated from myrcene 39 with 76-96% optical yield. Compound (R)-citronellal (R)-42. prepared through hydrolysis of (R)-41, is then cyclized by zinc bromide treatment.49 Catalytic hydrogenation then completes the synthesis of (—)-menthol. This enantioselective catalysis allows the annual production of about 1500 tons of menthol and other terpenic substances by Takasago International Corporation.50... 

S )-citronellal 42 can also be prepared similarly from 40. Asymmetric hydrogenation of (R)-43 provides 44, which can be used to make the side chain of vitamins E and K (Scheme 6-22). 

Chapter 2 to 6 have introduced a variety of reactions such as asymmetric C-C bond formations (Chapters 2, 3, and 5), asymmetric oxidation reactions (Chapter 4), and asymmetric reduction reactions (Chapter 6). Such asymmetric reactions have been applied in several industrial processes, such as the asymmetric synthesis of l-DOPA, a drug for the treatment of Parkinson s disease, via Rh(DIPAMP)-catalyzed hydrogenation (Monsanto) the asymmetric synthesis of the cyclopropane component of cilastatin using a copper complex-catalyzed asymmetric cyclopropanation reaction (Sumitomo) and the industrial synthesis of menthol and citronellal through asymmetric isomerization of enamines and asymmetric hydrogenation reactions (Takasago). Now, the side chain of taxol can also be synthesized by several asymmetric approaches. 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

When the hydrogenation of citral is performed with supported nanoparticles of rhodium metal, for example Rh/Si02 under classical conditions [liquid phase, rhodium dispersion 80% (particles in the range of 1-2nm), citral/Rhs = 200, P(ti2) = 80bar, T = 340 K], the catalytic activity is very high but most of the above products are obtained and the reaction is totally non-selective, even if the major product was citronellal. 

Selective catalytic hydrogenation with chromium-promoted Raney nickel is reported (e.g. citral and citronellal to citronellol) NaHCr2(CO)io and KHFe(CO)4 reduction of a/3-unsaturated ketones (e.g. citral to citronellal) has been described (cf. Vol. 7, p. 7). The full paper on selective carbonyl reductions on alumina (Vol. 7, p. 7) has been published." Dehydrogenation of monoterpenoid alcohols over liquid-metal catalysts gives aldehydes and ketones in useful yields. ... 

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)]. ... 

R = vinyl or ethynyl) predominates (c/. Vol. 1, p. 36 Vol. 2, p. 35). Base treatment of a 2-alkoxypyridinium tosylate of nerol gives expected e.g. limonene 82%) cyclic hydrocarbons whereas the corresponding geraniol salt yields similar amounts of cyclic and acyclic hydrocarbons. SnCU-catalysed cyclization of the N-benzylimine derived from R-(+)-citronellal yields the expected menthylamines after catalytic hydrogenation. ... 

In a similar approach, Shishido et al. (241) used oxime 215 [derived from the monoterpene (+)-citronellal (214)] for the synthesis of (—)-mintlactone (218) and (+)-isomintlactone (219), sweet compounds isolated from some Mentha species (Scheme 6.89). Bicyclic isoxazoline 216 was obtained in good yield from the cycloaddition. As expected, the product possessing tra i-l,4-substimtion prevailed. Reductive hydrolysis of the major isomer of 216 using hydrogen-Raney Ni-trimethyl borate provided p-hydroxyketone 217. This compound was dehydrated to give an enone and this was followed by carbonyl reduction-lactonization to complete the synthesis of both lactones 218 and 219 (241). 

Hydrolysis of the enamine 14 furnishes citronellal (15) in high optical purity (ca. 99% ee) which gives 17 via ene cyclization with zinc bromide as catalyst. The diastereoselectivity of this step is the result of simple diastereoselection in a trans-decalin-like transition state 16. Catalytic hydrogenation converts the olefin 17 into (—)-menthol (18). Despite its elegance this novel route has not been able to replace the older resolution-based procedure described earlier in this section. 

The hydrogenation of Claisen rearrangement products (R)-(E)- and (S)-(Z)-3,7-dimethyl-4-octenal [obtained highly selectively from (7 )-( )-6-methyl-4-vinyloxy-2-heptene, see p421] to give (S)- and (7 )-3,7-dimethyloctanal, respectively. Authentic samples of these aldehydes were obtained from (5)- and (7 )-citronellal 14°. 

Citronellol undergoes the typical reactions of primary alcohols. Compared with geraniol, which contains one more double bond, citronellol is relatively stable. Citronellol is converted into citronellal by dehydrogenation or oxidation hydrogenation yields 3,7-dimethyloctan-l-ol. Citronellyl esters are easily prepared by esterification with acid anhydrides. 

Synthesis of (+)- and ( )-Citronellol from the Citronellal Fraction of Essential Oils. (+)-Citronellal is obtained by distillation of Java citronella oil and is hydrogenated to (+)-citronellol in the presence of a catalyst (e.g., Raney nickel). Similarly, (zb)-citronellol is prepared from the ( )-citronellal fraction of Eucalyptus citriodora oil. 

Pure citronellal is a colorless liquid with a refreshing odor, reminiscent of balm mint. Upon catalytic hydrogenation, citronellal yields dihydrocitronellal, citro-nellol, or dihydrocitronellol, depending on the reaction conditions. Protection of the aldehyde group, followed by addition of water to the double bond in the presence of mineral acids or ion-exchange resins results in formation of 3,7-dimethyl-7-hydroxyoctan-l-al (hydroxydihydrocitronellal). Acid-catalyzed cycli-zation to isopulegol is an important step in the synthesis of (-)-menthol. 

Synthesis from Citral. Selective hydrogenation of citral to citronellal can be accomplished in the presence of a palladium catalyst in an alkaline alcoholic reaction medium [65]. 

Isopulegol can be isolated from this mixture and hydrogenated to (-)-menthol. The remaining isopulegol stereoisomers can be partly reconverted into (+)-citronellal by pyrolytic cleavage and reused in the cyclization procedure [79]. 

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