Hydrogenation of acetophenone to 1-phenylethanol

The asymmetric epoxidation of acyclic )S,)3-disubstituted o, )3-enones in acetonitrile, by peracetic acid and catalysed by an iron complex in which Fe(OTf)2 was coordinated by two 2-[l-(l-naphthyl)-2-naphthyl]-l,10-phenanthroline ligands (35) (R = m-xylyl), to the corresponding Q ,j8-epoxyketones with yield up to 88% and up to 92% ee was achieved. The epoxy ketone was further converted to functionalized )8-keto-aldehydes with an all-carbon quaternary centre." The transfer hydrogenation of acetophenone to 1-phenylethanol in isopropanol in the absence of added base was catalysed by a five-coordinated Fe(II) complex (36) and certain analogues. ... 

A number of donor-functionalized NHC-based iridium complexes have been tested in the transfer hydrogenation of ketones using isopropanol as hydrogen source. Good to excellent conversions in the transfer hydrogenation of acetophenone to 1-phenylethanol were obtained using 46 [68b] and 47 [69], while amine-NHC Ir(III) complexes 48 described by Morris and coworkers afforded lower activities in the same reaction [68a]. 

Moreover, the combination of ILs and supercritical carbon dioxide (SCCO2) can be used in the preparation of IL-supported Pd NPs and in the acetophenone hydrogenation reaction [31]. The catalyst system consists of in sitw-formed Pd NPs stabihzed by [BMIm][PFg] and [Hexyl4N]Br. SCCO2 can behave as the extractant to remove the impurities produced in the NP preparation step and promote the separation of the hydrogenation products in the catalytic cycle. Figure 2.16. Such IL-supported Pd NPs have a narrow size distribution of 3.5 0.6nm and exhibit excellent catalytic performance in the hydrogenation of acetophenone to 1-phenylethanol with >90% conversion and selectivity. No noticeable deactivation was observed after six cycles. 

The conversion of 2-phenylbutene to 2-phenylbutane and acetophenone to 1-phenylethanol over HZSM-5 modified by Pt and a chiral ligand has been reported. An asymmetric hydrogenation of N-acyldehydrophenylalanine derivatives with enantioselectivities 95% over zeolite-suported chiral rhodium complexes has been reported. 

Polysiloxane-bound ruthenium-based hydrogen-transfer catalysts were reported by liese et al. [45] [Eq. (20)]. The ruthenium-bound system was used for the asymmetric reduction of acetophenone in a continuously operated membrane reactor for the reduction of acetophenone to (S)-phenylethanol. With a constant dosage of base and an initial catalyst concentration of 0.5%, a stable operation was possible for more than 150 h (residence time 1 h). The maximum space-time yield was 580 g d" for the membrane reactor for more than 200 residence times, with a maximum ee of 92% (er = 24). 

The mechanism of the catalysis (Scheme 20.8) is quite unlike that of the rhodium-DuPhos catalysis of prochiral olefins described above, since the ketone substrate does not bind to the metal (ruthenium) atom. When a substrate binds the metal, as in the rho-dium-DuPhos systems, there are opportnnities for unwanted pathways that terminate the catalysis. On the other hand, a conseqnence of the metal being protected by its ligands in the Noyori-Ikariya catalysis in principle rednces the likelihood of catalyst deactivation and increases the expectation for achieving very high catalyst utilization (substrate/catalyst ratios). Thus, in the asymmetric hydrogenation of acetophenone to (i )-l-phenylethanol, Noyori et al. reported an astounding molar snbstrate/catalyst ratio of 2,400,000 1. ... 

Acetophenone can react with formaldehyde to yield light-resistant resins which are used as additives in nitrocellulose paints. It is also used as a photoinitiator, and in the pharmaceuticals, perfumery, and pesticide industries (344). It can be hydrogenated to 1-phenylethanol which is used for the production of aromatic ester fragrances (345). Technical-grade acetophenone is available at 2.29/kg perfume-grade acetophenone was 6.50/kg in October 1994. 

Acetophenone is separated for hydrogenation to 1-phenylethanol, which is sent to the dehydrator to produce styrene. Hydrogenation is done over a fixed-bed copper-containing catalyst at 115—120°C and pressure of 8100 kPa (80 atm), a 3 1 hydrogen-to-acetophenone ratio, and using a solvent such as ethylbenzene, to give 95% conversion of the acetophenone and 95% selectivity to 1-phenylethanol (186,187). 

Another work on the Hquid phase hydrogenation of acetophenone is that of Casagrande et al. The reaction was studied over a series of silica-supported bimetallic catalysts with various Ru/Cr atomic ratios, which were prepared by reduction at room temperature with aqueous sodium tetrahydroborate. The nanostructured catalysts are very active in the low-pressure hydrogenation of acetophenone, although the selectivity towards 1-phenylethanol did not surpass 22% at 90% conversion. The addition of chromium salts to the starting solution gave rise to... 

Table 6.13 Racemic hydrogenation of acetophenone" initial reaction rate rf, selectivity to products at 100% conversion. Results for the chemical reduction with NaBH4 are included for comparison. PE 1-phenylethanol, CHMK cyclohexyl methyl ketone and CHE 1-cyclohexylethanol. (Reproduced from Reference [34])...

In contrast to hydrogenation over noble metals hydrogenation of acetophenone over different nickel catalysts and over copper chromite results in the formation of 1-phenylethanol without hydrogenolysis [43,45,49,50]. 

Aromatic ketones are often hydrogenated with the aim to prepare the corresponding aromatic alcohols - Acetophenone was chosen here as a reasonably complex molecule to effect a modelization of this family of catalytic hydrogenations. Previous papers have proved that Raney nickel is an effective catalyst in this type of reaction (refs. 1-4).Acetophenone is principally hydrogenated to 1-phenylethanol which is a very valuaole product of the perfumery industry. 

In the course of irradiation of acetophenone in the presence of 1-phenylethanol, the actual quantum yields for pinacol formation do not exceed 50%, but rise to 71% when PhCH(OD)Me is used for photoreduction of acetophenone in acetonitrile683,684. A conclusion has been reached from this inverse DIE that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants. A transfer of an O-bonded hydrogen to a triplet ketone is taking place (equation 318) besides the abstraction of hydrogen from... 

To specify the position and the nature of the transferred hydride, the reaction was performed with 2-propanol-dj as solvent/donor, sodium 2-propylate as base and Fe3(CO)12/PPh3/TerPy as catalyst under optimized conditions. In the transfer hydrogenation of acetophenone a mixture of two deuterated 1-phenylethanols was obtained (Scheme 4.7, 9a and 9b). The ratio between 9a and 9b (85 15) indicated a specific migration of the hydride, albeit some scrambling was detected. However, the incorporation is in agreement with the monohydride mechanism, implying the formation of metal monohydride species in the catalytic cycle. 

Nishimura and Kasai studied the hydrogenation of acetophenone in f-butyl alcohol using carefully prepared ruthenium and rhodium blacks.176 The selectivities for the formation of cyclohexyl methyl ketone and 1-phenylethanol as simultaneous products have been determined by application of the equation in Scheme 11.7. The values of K and/as well as the composition of the final products obtained are summarized in Table 11.13. Three ruthenium blacks—Ru (A), Ru (N), and Ru (B)—were prepared from the ruthenium hydroxide precipitated at pH 5,7, and 7.8, respectively, by adding lithium hydroxide solution to an aqueous solution of ruthenium chloride. It is seen that the selectivity for the saturated ketone (see figures in parentheses) was considerably higher over Ru (B) (43%) than over Ru (A) (25%) and Ru (N) (20%). The selectivity over Ru (N) increased markedly to 65% at 100°C and 5.9-7.8 MPa H2. Over rhodium... 

A further selectivity issue is the hydrogenolysis of the alcohol, which is promoted by the use of acidic media or supports. Attempts can be made to avoid this if the reaction is stopped after the addition of one equivalent of hydrogen. In some instances this is the desired reaction and although a few drops of concentrated hydrochloric, or perchloric acid may be added, the use of acetic acid will be less corrosive in the stainless steel autoclaves typically employed. An example is depicted in Scheme 1 for the hydrogenation of acetophenone I to either ethylbenzene II or 1-phenylethanol III. 

Other sources of hydrogen can be used instead of elemental hydrogen gas. In transfer hydrogenations a secondary alcohol has been employed as hydrogen donor. Noyori et al. have also made advances in this field. They use a catalyst system from [RuCbC -mesitylene)]2, iV-(p-toluenesulfo-nyl)-l,2-diphenylethylenediamine, and KOH in isopropanol. At room temperature acetophenone can be reduced in 15 h with this complex prepared in situ (substrate catalyst = 200 1) to 1-phenylethanol in 95% yield with an optical purity of 97 %. [15, 16] The... 

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