WO2020245145A1

WO2020245145A1 - Process for the preparation of nootkatone by using a iron (iii) porphyrin complex catalyst

Info

Publication number: WO2020245145A1
Application number: PCT/EP2020/065274
Authority: WO
Inventors: Fridtjof Schröder
Original assignee: Givaudan Sa
Priority date: 2019-06-04
Filing date: 2020-06-03
Publication date: 2020-12-10
Also published as: GB201907915D0

Abstract

An allylic oxidation process comprising: forming a mixture containing valencene and an iron (lll)-X porphyrin complex catalyst in a sustainable solvent, introducing molecular oxygen into the mixture, and effecting allylic oxidation to produce nootkatone.

Description

PROCESS FOR THE PREPARATION OF NOOTKATONE BY USING A IRON (III) PORPHYRIN COMPLEX CATALYST

The present invention is directed to a process of preparing nootkatone 2 (4, 4a, 5, 6,7,8- hexahydro-6-isopropenyl-4,4a-dimethyl-2(3H)-naphtalenone) by allylic oxidation of vallencene 1 ((3R,4aS,5R)-4a,5-dimethyl-3-(prop-1-en-2-yl)-1 , 2, 3, 4, 4a, 5,6,7- octahydronaphthalene) using catalytic amounts of iron(lll)porphyrin complexes in the presence of molecular oxygen and in a sustainable solvent.

Unsaturated terpenes such as valencen are important substrates in the flavor and fragrance industry, giving after allylic oxidation, valuable flavor and fragrance ingredients such as nootkatone. Oxidative transformations of this substrate are especially challenging because the substrate has multiple sites which are sensitive to oxidation and can give various oxidation products arising from competing epoxidation, ene-oxidation or unselective allylic oxidation side reactions.

A large number of syntheses has been described until now to obtain nootkatone by the oxidation of valencene. One can cited, for example , the oxidation of valencene by electrochemical allylic oxidation reported by Horn and Rosen et al. in a letter to Nature, Vol. 533, 77 (5 May 2016) using co-oxidants tetrachloro-N-hydroxyphthalimide, pyridine, butyric acid and UCI0₄ in various technical grade solvents, such as acetone.

WO201 1/106166 relates to allylic oxidation catalysts. A n exemplary method comprises the step of catalyzing oxidation of an allylic compound using an allylic oxidation catalyst comprising palladium, gold, and titanium. Example 2 disclosed the oxidation of valencene to form nootkatone using the catalyst (2.5% Au + 2.5% Pd/Ti0₂) in an autoclave charged with oxygen at 30 bar.

It is also known to produce Nootkatone from Valencene by lacase catalyzed oxidaten as disclosed in US 6200786.

Nonetheless, there remains a need for an efficient allylic oxidation method to form Nootkatone in the presence of oxygen using inexpensive and nontoxic catalysts in sustainable solvents. Solvents previously used in allylic oxidation processes, such as benzene, chloroform, dichloromethane, and ether, are considered to be undesirable as pointed out by Dunn and Perry et al. Green Chemistry 10, 31 -36 (2008) for example. Other solvents, which are not considered sustainable, include carbon tetrachloride, dimethyl formamide and dimethyl acetate, among others.

In a first aspect there is provided an allylic oxidation process comprising forming a mixture containing Valencene 1 and an iron(lll)-X porphyrin complex catalyst in a sustainable solvent, introducing molecular oxygen into the mixture, and effecting allylic oxidation to produce an Nootkatone 2. X is selected from Cl, Br, I, mesylates, triflates, and carboxylates, preferably Cl, Br and I, further preferably Cl.

In one particular embodiment there is provided an allylic oxidation process as herein described, characterized in that Nootkatone 2 is produced as main product. By“main product” we mean in the context of this invention that oxidative product obtained by oxidation of Valencene 1 which is most present. This can be easily measured, e.g. by gas chromatography (GC).

In a further embodiment there is provided an allylic oxidation process as herein described, characterized in that Nootkatone is produced with a GC-purity of 50% or higher (e.g. 55, 60, 62, 64, 66, 70, 75, 80) measured in % relative peak area based on the oxidative products obtained by the allylic oxidation. The GC method used for the calculation of the GC-purity is described herein below (see non polar GC).

wherein

X is selected from Cl, Br and I; and R is selected from phenyl, pentafluorophenyl, p-methoxyphenyl, dichlorophenyl, benzenesulfonate, bromophenyl, chlorophenyl, and methylphenyl.

In certain embodiments of the subject process, the catalyst is an iron (III) porphyrin complex catalyst, having a chloride counter-ion. In preferred embodiments, the porphyrin complex is a Hemin complex (e.g. hemin chloride, CAS 16009-13-5) or complex of formula (I) wherein R is selected from phenyl, tetraphenyl, and p- methoxyphenyl. In a particularly preferred embodiment, the catalyst is chloro(tetraphenylporphyrinato)iron(lll), i.e. , X is Cl and R is phenyl.

In certain embodiments, the concentration of catalyst may be in the range of 0.01 to 10 mol %, preferably 0.5 to 2 mol %, e.g. 1 - 1 .5 mol % which includes 1 .1 mol %, based on the number of mols of Valencene.

In the subject process, the mixture may additionally contain a base coordination compound, for example, an amine, such as triethylamine or an inorganic base, such as K₂C0₃. The base coordination compound is preferably an N-heterocycle (an N- containing heterocycle), such as pyridine, imidazole or N-methylimidazole, further preferably imidazole.

The sustainable solvent used in the subject process may be selected from the group consisting of water, acetone, ethanol, 2-propanol, ethyl acetate, isopropyl acetate, methanol, methyl ethyl ketone, 1 -butanol, t-butanol and mixtures thereof, preferably an ethanol/water mixture. The suitability of aqueous ethanol as a solvent for this reaction is surprising, because ethanol as such may be oxidized, and water is not the solvent which one would typically use in organic chemistry reactions.

Additionally or alternatively, the sustainable solvent used in the subject process may be selected from the group consisting of cyclohexane, heptane, toluene, methylcyclohexane, methyl t-butyl ether, isooctane, acetonitrile, xylenes, dimethyl sulfoxide, acetic acid, ethylene glycol and mixtures thereof.

The subject process preferably includes stirring the mixture by any suitable means known in the art. In a preferred embodiment, the reaction may proceed from -10°C (freezing point of the water / ethanol reaction mixture) to 78°C (boiling point of the water/ethanol reaction mixture). A preferred range is about 35 to about 55 °C.

In the subject process, the step of introducing molecular oxygen into the mixture comprises bubbling oxygen gas into the mixture. In certain embodiments, introducing molecular oxygen into the mixture comprises bubbling air into the mixture. Alternatively, molecular oxygen may be introduced into the mixture by reacting in an oxygen atmosphere with strong stirring. Molecular oxygen may also be introduced into the mixture by processing the reaction in a flow reactor.

Although not required, the subject process may include exposing the mixture to electromagnetic radiation, preferably UV and visible light radiation. The wavelength range of the light used to expose the mixture may be in the range of about 200 nm to about 800 nm. Alternatively, the process may be carried out in the dark, or under ambient light conditions.

In the subject process, produced Nootkatone, obtained in a mixture may be further purified by means known to the person skilled in the art. For example the crude mixture may be purified by bulb-to-bulb distillation, e.g, at about 150 - 230°C at 0.04 - 0.1 mbar, optionally followed by flash chromatography.

Valencene is a natural chemical compound that has been isolated from a variety of plant sources. For example, it is obtained inexpensively from Valencia oranges. Mixtures comprising valencene used for the process described herein, preferably contain at least 15 weight % (which includes 20 wt%, 30 wt%, 40 wt%, 50 wt %, 60 wt %, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt %, 90 wt% or more) valencene.

The disclosure is now further described with reference to the following non-limiting examples. These examples are for the purpose of illustration only and it is understood that variations and modifications can be made by one skilled in the art.

General:

Nonpolar GC: 100 °C / 2 min, 15 °C / min to 240 °C, 240 °C / 5 min. Thermo Focus GC. Nonpolar column: Agilent Technologies J&W Scientific DB-5 (nonpolar, 5% phenylmethyl-polysiloxane). Column dimensions: 30 m length, 0.32 mm I D, 0.25 mpi film thickness. Injector: Split. Injector temperature: 240 °C. Detector: FID. Detector temperature: 270 °C. Injection volume: 1 pi. Carrier gas: Helium. Split ratio: 1 2.3. Pressure: 70 kPa. Integrator: Hewlett Packard. The purity of Valencene was 65% as determined by this GC-method.

Non-polar GCMS: 50 °C / 2 min, 20 °C / min 200 °C, 35 °C / min 270 °C. GC/MS Agilent 5975C MSD with HP 7890A Series GC system. Non-polar column: BPX5 from SGE, 5 % phenyl 95% dimethylpolysiloxane 0.22 mm x 0.25 mm x 12m. Carrier Gas: Helium. Injector temperature: 230 °C. Split 1 :50. Flow: 1 .0 ml/min. Transfer line: 250 °C. MS-quadrupol: 106 °C. MS-source: 230 °C.

Example 1 . Preparation of Nootkatone from Valencene by allylic 0₇-oxidation under Fed Ill-catalysis

Chloro(tetraphenylporphyrinato)iron(ll l) (34 mg, 0.05 mmol) and imidazole (6.7 mg, 0.1 mmol) are added to Valencene 1 (65% purity, 1 g, 3.2 mmol) in ethanol / water 1 : 1 (20 ml) under stirring. The greenish mixture is stirred at 45 °C and with oxygen bubbling under the surface of the reaction mass for 46 h . The brown mixture is evaporated under reduced pressure and the residue extracted with tert-butyl methyl ether against water. The combined organic layers are dried over MgS0₄, filtered and evaporated. The residual brown oil (1 .03 g) contains Valencene 1 (8%), Valencen epoxide ((4R,4aS,6R)- 4,4a-dimethyl-6-(prop-1 -en-2-yl)octahydro-3H-naphtho[1 ,8a-b]oxirene; 9%) and

Nootkatone 2 (76%) according to GCMS. Bulb-to-bulb distillation at 25 - 230 °C / 0.03 mbar giving 0.75 g Nootkatone 2 with 76% purity (81 % corrected yield) and 0.19 g of a residue. The analytical data of Nootkatone were identical with the ones from the literature (e.g. as described by S. Serra et al. Eur. J . Org. Chem. 6472 -6478, 2015).

The same reaction was repeated with the same amount of Valencene 1 (65% purity, 1 g, 3.2 mmol) giving under identical conditions and after the same work-up a residual brown oil which was purified by flash chromatography using silicagel and heptane / tert- butyl methyl ether 4: 1 as eluent, giving after evaporation of the solvent 0.33 g (48% corr. yield) of Nootkatone with 100 % purity. The analytical data of Nootkatone 2 were identical with the ones from the literature.

Example 2. Preparation of Nootkatone from Valencene at 10 g scale

Chloro(tetraphenylporphyrinato)iron(lll) (0.7 g, 1 mmol) and imidazole (67 mg, 1 mmol) are added to Valencene 1 (65%, purity, 10 g, 32 mmol) in ethanol / water 1 :1 (50 ml) under stirring. The greenish mixture is stirred at 45 °C and with oxygen bubbling under the surface of the reaction mass for 45 h. The brown mixture is evaporated under reduced pressure and the residue extracted with tert- butyl methyl ether against water The combined organic layers are dried over MgS0₄, filtered and evaporated. The residual brown oil (8 g) contains Valencene 1 (1 1 %), Valencene epoxide ((4R,4aS,6R)- 4,4a-dimethyl-6-(prop-1 -en-2-yl)octahydro-3H-naphtho[1 ,8a-b]oxirene; 2%) and

Nootkatone 2 (69%) according to GCMS. Bulb-to-bulb distillillation at 25 - 230 °C / 0.05 mbar gives 6.8 g Nootkatone 2 with 68% purity as light yellow oil (67% corrected yield) and 1 g of a residue. The analytical data of Nootkatone 2 were identical with the ones from the literature.

Example 3. Preparation of Nootkatone from Valencene under Hemin catalysis

Hemin (CAS 16009-13-5; 9.6 mg, 0.015 mmol) and imidazole (6.7 mg, 0.1 mmol) are added to Valencene 1 (65% purity, 1 g, 3.2 mmol) in ethanol / water 1 :1 (20 ml) under stirring. The brownish mixture is stirred at 45 °C and with oxygen bubbling under the surface of the reaction mass at 1 -2 bubbles of oxygen per second as measured by a bubble counter. Conversion is measured by GC. After 22 h (45% conversion, 13% Nootkatone) a second portion of Hemin (9.6 mg, 0.015 mmol) and ethanol (10 ml) is added (to compensate ethanol loss by evaporation) and after 77 h a third portion of Hemin (9.6 mg, 0.015 mmol) and ethanol (10 ml). After 96 h (85% conversion, 56% Nootkatone, rpa) the oxygen stream is stopped, replaced by a nitrogen stream bubbled under the surface of the reaction mass, sodium hydroxyde (0.1 g, 2.5 mmol) is added and the orange-brown mixture heated to 100 °C for 7 h (to destroy remaining peroxides). At room temperature the mixture is partially evaporated under reduced pressure (to remove remaining ethanol). After addition of sodium chloride (to facilitate phase separation) the residue is extracted with tert-butyl methyl ether (2 x 30 ml) against water (30 ml) and saturated NaCI (30 ml). The combined organic layers are dried over MgS04, filtered and evaporated. The residual brown oil (0.9 g) contains Valencene 1 (19%), Valencene epoxide ((4R,4aS,6R)-4,4a-dimethyl-6-(prop-1 -en-2- yl)octahydro-3H-naphtho[1 ,8a-b]oxirene ; 5%) and Nootkatone 2 (64%) according to GCMS. Purification by flash chromatography using silicagel and heptane / tert-butyl methyl gradient 98:2 to 80:20 as eluent, gives after evaporation of the solvent 0.24 g (34% corr. yield) of Nootkatone 2 with 98 % purity. The analytical data of Nootkatone 2 are identical with the ones from the literature.

Claims

1 . An allylic oxidation process comprising:

forming a mixture containing valencene ((3R,4aS,5R)-4a,5-dimethyl-3-(prop-1 -en-2-yl)- 1 ,2,3,4,4a,5,6,7-octahydronaphthalene) and an iron (lll)-X porphyrin complex catalyst wherein X is selected from the group consisting of Cl, Br, I, mesylates, triflates, and carboxylates, in a sustainable solvent,

introducing molecular oxygen into the mixture, and

effecting allylic oxidation to produce nootkatone (4,4a,5,6,7,8-hexahydro-6-isopropenyl- 4,4a-dimethyl-2(3H)-naphtalenone).

2. The process according to claim 1 , wherein the mixture additionally contains a base coordination compound.

3. The process according to any one of claims 1 to 2, including exposing the mixture to electromagnetic radiation.

4. The process according to any one of claims 1 to 3, wherein the sustainable solvent is selected from the group consisting of water, acetone, ethanol, 2-propanol, ethyl acetate, isopropyl acetate, methanol, methyl ethyl ketone, 1 -butanol, t-butanol and mixtures thereof.

5. The process according to any one of claims 1 to 4, wherein the sustainable solvent is selected from the group consisting of cyclohexane, heptane, toluene,

methylcyclohexane, methyl t-butyl ether, isooctane, acetonitrile, xylenes, dimethyl sulfoxide, acetic acid, ethylene glycol and mixtures thereof.

6. The process according to any one of claims 1 to 5, wherein the catalyst is an iron (III) porphyrin complex catalyst, having a chloride counter-ion.

7. The process according to any one of claims 1 to 6, wherein the porphyrin complex is a tetraphenylporphyrin complex.

8. The process according to any one of claims 1 to 7, wherein the catalyst is selected from chloro(tetraphenylporphyrinato)iron(l ll) and hemin chloride.