Citronellal catalytic

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. 

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

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... 

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. ... 

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. ... 

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. 

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. 

Addition of dihydrosilane to a, /j-unsaturated carbonyl compounds such as citral (49), followed by hydrolysis, affords saturated citronellal (50) directly. The reaction is used for the selective reduction of conjugated double bonds[45,46]. In addition to Pd catalyst, the use of a catalytic amount of... 

All ( )-menthol is made by synthetic methods. One method involves the cydization of (+)-citronellal (68). Using a mild acid catalyst, (+)-citronellal [2385-77-5] undergoes an ene-reaction to produce a mixture of isopulegols (142). Catalytic hydrogenation of the isopulegol mixture gives a mixture of menthol and its isomers. The (zt)-menthol is obtained after efficient fractional distillation and the remaining isomers can be equilibrated, usually with sodium mentholate or aluminum isopropoxide. An equilibrium mixture is obtained, comprised of 62 wt % (d=)-menthol, 23 wt % (+)-neomenthol, 12 wt % ( )-isomenthol, and 3 wt % (+)-neoisomenthol. The equilibrium mixture can be distilled to recover additional ( )-menthol. 

Synthetic methods for the production of citronellal include the catalytic dehydrogenation of citronellol (110), the telomerization of isoprene (151), and the lithium-catalyzed reaction of myrcene with secondary alkylamines (128). 

The selectivity for citronellal increases up to a value of 81% (at 100% canv.) for a Sn/Rh ratio of 0.12. Above these values, the selectivity far citronellal decreases and the selectivity for geraniol and nerol increases up to 96% (at 100% conv.) for a Sn/Rh ratio of 0.92. Significant variations of activities are simultaneously observed suggesting selective metallic surface poisoning fallowed by enhanced catalytic activity due to a new catalytic material which contains a tin din-butyl fragment. 

This process represents a Knoevencigel condensation,3 a reaction in which a compound with an acidic methylene group, such as dimethyl malonate (8). condenses with a carbonyl compound like citronellal (9) to give an alkene. Reaction occurs in a weakly basic or neutral medium. Catalytic amounts of piperidinium acetate suffice to deprotonate malonic ester 8. The resulting anion 15 adds to the aldehyde citronellal (9) to give alkoxide 16. 

Olefinic double-bond isomerization is probably one of the most commonly observed and well-studied reactions that uses transition metals as catalysts [1]. However, prior to our first achievement of asymmetric isomerization of allylamine by optically active Co(I) complex catalysts [2], there were only a few examples of catalytic asymmetric isomerization, and these were characterized by very low asymmetric induction (<4% ee) [3], In 1978 we reported that an enantioselective hydrogen migration of a prochiral allylamine such as AVV-diethylgerany-lamine, (1) or N V-diethylnerylamine (2) gave optically active citronellal ( )-enamine 3 with about 32% ee utilizing Co(I)-DIOP [DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane] complexes as the catalyst (eq 3.1). 

This is probably the single most important material used by the fragrance industry. Several million pounds are used annually, mainly in soaps and detergents. The principal method of manufacture shown in Figure 10 is by hydration of citronellal via the bisulfite addition product (2). The aldehyde moiety must be protected before hydration. A second manufacturing process starts with citronellol which is hydrated under acid conditions. The primary alcohol end of the molecule is then dehydrogenated catalytically or by oxidation to the aldehyde. 

The first catalysts used were Co(I)-DIOP (DIOP, see Figure 11b) based systems. Up to 32% ee was achieved with 39% yield in the isomerization of (A, AO-diethylgeranylamine to (i )-citronellal. Subsequently, some rhodium (I)-DIOP or BINAP (BINAP = 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) systems proved to be very active. In particular, cationic rhodium(I)-BINAP complexes show very high selectivities and catalytic activities for this isomerization. BINAP is an atropoisomeric diphosphine (Figure 20) which was first synthesized by Noyori and Takaya and since then its metal complexes have been extensively used as catalysts in a variety of asymmetric syntheses. 

For the synthesis of (+)-citronellol (6), the mixed pinenes were catalytically hydrogenated to give (-)-cis-pinane (7) which was pyrolysed to (-)-citronellene. Application of the Ziegler reaction with aluminium hydride proceeded selectively at the more reactive disubstituted double bond and following atmospheric oxidation and aqueous work-up, (+)-citronellol was isolated identical with that derived by the reduction of natural citronellal by the Ponndorf-Meerwein-Verley method (ref.9) as shown. 

Additional examples of the bismuth chloride-catalyzed C-C bond forming reactions are summarized in Scheme 5.28. Active methylene compounds can react smoothly with aldehydes and a,j8-unsaturated carbonyl compounds in the presence of bismuth chloride without using organic solvents [92CL1945, 97TL1449]. A catalytic amount of bismuth chloride (2 mol%) can efficiently induce the intramolecular ene cyclization of (-l-)-citronellal as well as the intermolecular ene reaction between (—)-)8-pinene and chloral... 

The other major producer of synthetic L-menthol is the Japanese company Takasago. They produce about 1000 tonnes per annum using elegant chemistry developed by Noyori (Scheme 4.22). Pyrolysis of / -pinene gives myrcene, to which diethylamine can be added in the presence of a catalytic amount of strong base. This produces N,N-diethylgeranylamine. Isomerization of this with the rhodium 2,2 -(diphenylphosphino)-1,1-binaphthyl (BINAP) complex produces the enamine of citronellal. The elegance of this route stems from the fact... 

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