Coumarin (lacton of o-hydroxycinnamic acid, 1,2- benzopyron) is a white crystalline substance found in tonka beans and melilot ^[1]. It has a pleasant aroma similar to hay and vanilla and so it is often used in the cosmetics and food industries. However, coumarin derivatives, for example 4- hydroxycoumarin, are primarily used as a rodenticide ^[2]. Thus, alternatives to the use of coumarin are being sought.

Octahydrocoumarin is a clear liquid, δ-lacton, which is absent in nature, but it has a similar aroma and flavor to coumarin ^[1]. It is added to tobacco products to enhance their smell and taste and to improve their organoleptic properties ^[3]. Its primary advantage is non-toxic. Octahydrocoumarin can serve as a replacement fragrance for coumarin, which has hepatotoxicity and carcinogenicity in rodents and allergenic effects on the skin in humans.

The hydrogenation of aromatics in the presence of a Ru catalyst is discussed in Refs. ^{[4, 5, 6]}. The hydrogenation of coumarin and its derivatives are discussed ^{[7, 8, 9]}, but the octahydrocoumarin was not a key product. Octahydrocoumarin has been prepared in various ways, some of which were based on coumarin ^{[10, 11, 12, 13]}. In the earliest work ^[14] in this field, octahydrocoumarin was obtained from the hydrogenation of coumarin using cycloheptan or ethanol as a solvent and Ru-Ni or Cu-Cr as a catalyst. However, this method is not suitable for modern purposes because this synthesis yields very small amounts of the desired product. In a diploma thesis ^[15], the hydrogenation of coumarin was carried out in methanol in the presence of a commercial Ni catalyst. However, the selectivity was low and octahydrocoumarin could not be obtained with purity above 80%. Later, coumarin was hydrogenated in methanol using 5% Ru on carbon ^[13]. Using this approach, after the total conversion of coumarin, 59% of methyl 3-(2-hydroxycyclohexyl)propionate (“open form” of octahydrocoumarin) and only 22% of octahydrocoumarin were present in the reaction mixture.

In this paper, we prepare octahydrocoumarin using a Ru catalyst under various combinations of the following reaction conditions: hydrogen pressure, coumarin concentration, and type of solvent (n-hexane, diethylenglykol-dimethylether, tetrahydrofuran, dipropylenglykol-monomethylether, 1- methoxypropan-2-ol, n-butanol and methanol). We present the optimal reaction conditions necessary to obtain sufficiently high yields of octahydrocoumarin.

The hydrogenation of coumarin was carried out in a 25-mL stainless steel autoclave under constant hydrogen pressure. A commercial catalyst TYPE 97 PASTE (surface area 1000 m²/g, moisture content 52.3%, uniform metal location, mean particle size 25 µm) from Johnson Matthey with the composition of 5 wt% Ru on active carbon (Ru/C) was used for the hydrogenation. Into the autoclave were inserted 2.8 g of coumarin, 0.14 g of the catalyst in the powder form and 14 mL of solvent. The reaction was carried out at 130 °C and from 2-10 MPa of hydrogen pressure.

The samples were analyzed using a gas chromatograph (GC Shimadzu 17-A) equipped with a ZB-5 column (60 m, 0.25 mm diameter, 0.25 μm film). The injector temperature was 250 °C, the column temperature was ramped from 120 to 250 °C at a rate of 10 °C/min. The carrier gas flow was 1.86 mL/min. The FID temperature was 250 °C.

The components of the reaction mixture were identified using a gas chromatograph (Shimadzu GC-2010 Plus) coupled to mass spectrometry (Shimadzu GCMS-QP 2010 Ultra) equipped with a DB-1 column (50 m, 0.25 mm diameter, 0.5 μm film). The injector temperature was 250 °C, the column temperature was ramped from 250 to 330 °C at a rate of 10 °C/min. The carrier gas flow was 0.66 mL/min. The quadrupole detector temperature was 250 °C.

The selectivity S of the catalyst was calculated using equation S = B_τ/(A₀ - A_τ) × 100%, where A₀ is the substrate concentration at the start (time 0), A_τ is substrate concentration at time τ, and B_τ is the concentration of the desired product at time τ.

The complete hydrogenation mechanism of coumarin to octahydrocoumarin was not found in the literature ^{[11, 15]}. In an alcoholic solvent, the hydrogenation of coumarin leads to the opening of the heterocycle ring to give the corresponding alkylester of propionic acid. These esters were described in Ref. ^[11] as side products and can be converted back to their "closed form" during alcohol distillation under reduced pressure. The selectivity can be calculated as the sum of the two desired products (octahydrocoumarin and alkylester of 3-(2-hydroxycyclohexyl)propionic acid).

A reaction scheme of coumarin hydrogenation in methanol in the presence of Ru/C was deduced in this work. This is shown in Scheme 1. All the products were identified using a gas chromatograph coupled to a mass spectrometer. The presence of coumarin, dihydrocoumarin, and octahydrocoumarin was also verified using standards on the GC. Besides the main products (dihydrocoumarin, methyl-3-(2-hydroxyphenyl) propionate, octahydrocoumarin, and methyl-3-(2- hydroxycyclohexyl)propionate), side products were identified, which reduce the selectivity of the reaction. The formation of these side products is also explained in Scheme 1.

Scheme 1. Coumarin hydrogenation. Reaction conditions: coumarin concentration 20 wt% in methanol, 0.5 wt% catalyst (based on coumarin), 130 °C, 10 MPa.

The reactions of the main components in the reaction mixture shown in Scheme 1, particularly the hydrogenation-dehydrogenation and esterification-hydrolysis, are indicated with bold arrows. The reactions of the side reaction components are indicated by a thin arrow. The equilibrium was shifted in the arrow direction. The reactions for opening and closing the lactone ring (dihydrocoumarin, octahydrocoumarin) were in equilibrium, indicated by the equilibrium arrow, and the equilibrium was shifted towards a given product by the methanol concentration in the reaction mixture. To shift the equilibrium towards methyl-3-(2-hydroxyphenyl) propionate or methyl-3-(2-hydroxycyclohexyl)propionate in the reaction mixture needed an excess of methanol. To shift the equilibrium in the opposite direction, methanol was distilled off from the reaction mixture. The change of the concentrations of the coumarin hydrogenation components with time is shown in Fig. 1. The time course of the coumarin hydrogenation minority products is shown in Fig. 2.

Fig. 1. Typical course of the coumarin hydrogenation. Reaction conditions see Scheme 1.

Fig. 2. Time course of the coumarin hydrogenation minority products. Reaction conditions see Scheme 1.

The hydrogenation of coumarin was carried out at 205 °C in the presence of a Ni catalyst ^[15]. This reaction proceeds in the presence of a Ru catalyst at milder conditions with good results ^[11]. The monitoring of the influence of pressure (2, 5, and 10 MPa) was performed at 130 °C and 5 wt% of catalyst TYPE97 PASTE in methanol.

At 2 MPa of hydrogen pressure, octahydrocoumarin was not found and the total conversion of coumarin was not achieved in 3 h. A higher selectivity (81.3%) was observed at 10 MPa than at 5 MPa (63.9%) at the total conversion of coumarin. It can be concluded that the reaction can be performed at 130 °C and 10 MPa.

The hydrogenation of coumarin was performed in various solvents. In some cases, solvents such as methanol ^{[13, 15]}, ethanol orcycloheptane ^[14] were used. In this work, solvents with different polarity were tested (Fig. 3): n-hexane, diethylenglykol-dimethylether (DIGLYM), tetrahydrofuran (THF), dipropylenglykol-monomethylether (DOWANOL DME), 1- metoxy-2-propanol (DOWANOL PM), n-butanol (n-BuOH), and MeOH.

Fig. 3. Selectivity to OHC (octahydrocoumarin + methyl-3-(2- hydroxycyclohexyl)propionate) in the presence of various solvents. Reaction conditions: coumarin 2.8 g, catalyst 0.14 g, solvent 14 mL, 130 °C, 10 MPa, 100% conversion of coumarin.

The selectivity dependence on the solvent polarity was not unambiguous. Except for n-hexane, DIGLYM and DOWANOL DME, the total conversion of coumarin was achieved within 2 h in all cases with the use of a solvent. The reaction mixtures in the presence of other solvents contained large quantities of side products 3-phenylpropionic or 3-cyclohexylpropionic acid. n-Hexane was not chosen as a suitable solvent because in the reaction mixture, undesirable hexahydrochroman, which is separated with difficulty, was present. In contrast, in the reaction mixture with methanol, unreacted methyl-3-(2- hydroxyphenyl)propionate was present, which can be converted to the desired octahydrocoumarin (after the hydrogenation by the distillation of methanol). The highest selectivity (93.4%) to octahydrocoumarin and its "open form" was observed in the presence of methanol, which was chosen as the optimal solvent.

The monitoring of the influence of the amount of coumarin (20, 40, 60, 80 and 100 wt%) was performed in methanol with Ru/C at 130 °C and 10 MPa (3 h). The comparison of the reaction product components in the mixture at different mass fractions of coumarin is shown in Fig. 4 at 100% conversion of coumarin.

Fig. 4. Content of reaction components in the mixture at different mass fractions of coumarin. Reaction conditions: 130 °C, 10 MPa, catalyst 0.5 wt%, 100% coumarin conversion, solvent methanol. DHC = dihydrocoumarin + methyl-3-(2-hydroxyphenyl)propionate, OHC = octahydrocoumarin + methyl-3-(2-hydroxycyclohexyl)propionate, ACID = 3- phenylpropionic acid + 3-cyclohexylpropionic acid.

The use of 20, 40, 80, and 100 wt% of coumarin had a negative effect on the selectivity at the total conversion of coumarin. The content of acids as undesired side products was more than the desired products. In the case of 80 and 100 wt% of coumarin, the total conversion of DHC (dihydrocoumarin + methyl-3-(2-hydroxyphenyl)propionate) was not achieved at the total conversion of coumarin. The monitoring of the reaction at the higher concentration of coumarin in the reaction mixture (80 and 100 wt% of coumarin) was significantly more complicated. Due to the viscosity of the reaction mixture, it was not possible to simply get samples from the autoclave using a sampling needle. The highest content of OHC (89.6%) in the reaction mixture was achieved at 60 wt% of coumarin.