WO2015004117A1

WO2015004117A1 - Selective transfer hydrogenation of retinal

Info

Publication number: WO2015004117A1
Application number: PCT/EP2014/064566
Authority: WO
Inventors: Werner Bonrath; Jonathan Alan Medlock; Bettina WÜSTENBERG; Jan SCHÜTZ; Thomas Netscher
Original assignee: Dsm Ip Assets B. V.
Priority date: 2013-07-08
Filing date: 2014-07-08
Publication date: 2015-01-15
Also published as: EP3263554A1; EP3263554B1; EP3019459B1; MY180008A; CN105358512B; CN105358512A; EP3019459A1

Abstract

The present invention relates to a selective transfer hydrogenation of retinal to retinol.

Description

SELECTIVE TRANSFER HYDROGENATION OF RETINAL

The present invention relates to a selective transfer hydrogenation of retinal to retinol. Retinal is also called retinaldehyde or vitamin A aldehyde.

Retinol (also vitamin A^ is one form of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals.

Retinal (IUPAC:3,7-dimethyl-9-(2,6,6-trimethylcyclohexen-1 -yl)nona-2,4,6,8-tetraenal) is the compound of formula (I)

Retinol is an important precursor of vitamin A acetate (or retinyl acetate), Retinyl acetate (retinol acetate, vitamin A acetate) is a stable form of vitamin A which is the acetate ester of retinol. It has potential antineoplastic and chemopreventive activities.

Retinal as well as retinol can have various isomeric configurations. In the context of the present invention the formula (I) as well as formula (II) are to be understood in the way that they cover an tnese comigurations. Most preTerrea comiguration is aii-tj-retinai/retinoi. utner isomers can be converted into (all-E), when needed.

The goal of the present invention was to find an improved way to provide retinol (compound of formula (II)),

Surprisingly we found that by using specific kinds of catalysts and reduction agents it is possible to hydrogenate retinal selectively by a transfer hydrogenation reaction whereas the selectivity, the conversion as well as the yield of the hydrogenation are excellent. ula (I)

characterized in that the hydrogenation is carried out in the presence of

a) an amino-amide-transition-metal catalyst of formula (III) MAK-(-M)JLYj (III)

wherein

M is a transition metal, preferably chosen from the group consisting of Ir, Rh and Ru, and

X signifies H or a halogen atom, and

Y signifies a ligand, preferably an aromatic ligand, and

Z signifie

wherein

R¹ signifies d-C₆-alkyl which may be substituted with one or more fluorine atoms, C₂- Cs-alkenyl, C₂-C₈-alkynyl, C₄-C₇-cycloalkyl, aryl which may be substituted, heteroaryl, or camphor-10-yl, and

R² and R³ each independently signify hydrogen, d-d-alkyl, C₄-C₇-cycloalkyl, or aryl which may be substituted, and

R⁴ signifies hydrogen or Ci-Ce-alkyI, and

R⁵ signifies hydrogen or d-Ce-alkyl, and

R⁶ signifies hydrogen or d-Ce-alkyl, and

n signifies 0,1 ,2 or 3, and

m signifies 0,1 ,2 or 3,and

with the proviso that in formula (V) the sum of n and m is 2, 3, 4 or 5, and

b) a H₂ donor.

A preferred transfer hydrogenation (TH¹) is (TH), characterized in that the hydrogenation is carried out in the absence of water. I nis (^"in tne aosence οτ water ) means mat water is not aaaea intentionally to tne reaction solution. The water content is usually below 3 weight-% (wt-%), preferably less than 2 wt-%, based on the total weight of the reaction solution.

The selectivity, the yield and the conversion of the transfer hydrogenation according to the present invention are excellent.

The monosulphonylated diamine is present in the complex as a monoanion and is accordingly denoted in formula III as "Z-(-H)".

By the term "amino-amide transition metal catalyst" it is meant that the transition metal is complexed to Z via the amine and the amide of Z.

Transfer hydrogenation is the addition of hydrogen to a compound (molecule) from a source other than gaseous H₂.

The use of H₂ gas has some issues with the handling of the explosive gas in regard to safety and in regard to the apparatus needed.

Instead of H₂ gas a H₂ donor system is used for transfer hydrogenation. Any H₂ donor system known from the prior art can be used.

The H₂ donor (= reduction agent), which is used in the process according to the present invention, can be any commonly known H₂ donor in the field of transfer hydrogenation.

In principle a transfer hydrogenation can be carried out by using

(option a) at least one solvent which also serves as hydrogen donor or

(option b) at least one hydrogen donor and at least one inert solvent.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH²), which is (TH) or (TH¹), wherein the hydrogenation is carried out by using at least one solvent which also serves as hydrogen donor.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH³), which is (TH) or (TH¹), wherein the hydrogenation is carried out by using at least one hydrogen donor and at least one inert solvent. AS tne nyarogen aonor ana at tne same time (option a) tne solvent mere can oe usea especially alcohols, preferably secondary alcohols, e.g. isopropanol.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁴), which is (TH), (TH¹) or (TH²), wherein the hydrogenation is carried out by using isopropanol as hydrogen donor and solvent.

As mixtures of hydrogen donors with an inert solvent (option b) there are conveniently used mixtures of alcohols with inert solvents, preferably a mixture of an alcohol with a lower aliphatic halogenated hydrocarbon, e.g. methylene chloride or 1 ,2-dichloroethane. Other suitable hydrogen donor/solvent mixtures are mixtures of formic acid with an inert solvent or a salt of formic acid with an inert solvent.

Suitable inert solvents are i.e. hydrocarbons, esters, alcohol and ethers.

Suitable salts of formic acids are any commonly known salts. Preferred salts are those of the following formula (VI)

QOOCH (VI) ,

wherein

Q is an alkali cation or NH₄ ⁺, NH₃(R⁷)⁺, NH₂(R⁷)₂ ⁺, NH(R⁷)₃ ⁺ or N(R⁷)₄ ⁺, wherein all R⁷ are independently from each other -CH₃ or -CH₂CH₃.

Preferably Q is Na⁺, K⁺, NH₄ ⁺, more preferably Q is K⁺.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁵), which is (TH), (TH¹) or (TH³), wherein the hydrogenation is carried out by using formic acid (or a salt of formic acid QOOCH) and triethylamine and wherein

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁵), which is (TH⁵), wherein Q is Na⁺, K⁺, NH₄ ⁺.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁵ ), which is (TH⁵), wherein Q is K⁺. vvnen mixtures οτ nyarogen aonors ana inert solvents are usea, in principle an mixture ratios can apply.

When a mixture of QOOCH and (CH₃CH₂)₃N (=QOOCH/(CH₃CH₂)₃N) is used then ratio is from 1 :1 to 10:1 , preferably 1 :1 to 6:1 (by molar ratio).

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁶), which is (TH), (TH¹), (TH³), (TH⁵), (TH⁵) or (TH^5"), wherein a mixture of QOOCH and (CH₃CH₂)₃N wherein Q is an alkali cation or NH₄ ⁺, NH₃(R⁷)⁺, NH₂(R⁷)₂ ⁺, NH(R⁷)₃ ⁺ or N(R⁷)₄ ⁺, wherein all R⁷ are independently from each other -CH₃ or -CH₂CH₃, is used in a ratio from 1 :1 to 10:1 , preferably 1 :1 to 6:1.

In the scope of the present invention the term "substituted with one or more fluorine atoms", otherwise expressed as "mono- or multiply-fluorinated", means having at least one fluorine substituent to as many fluorines as the alkyl group so modified is capable of accepting; however one to five fluorines is preferred.

In the scope of the present invention the term "alkyl" embraces straight-chain or branched alkyl groups with 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms. Methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl and tert. -butyl, are examples such alkyl groups. Trifluromethyl, 2,2,2-trifluoroethyl and pentafluoroethyl are examples of mono- or multiply-fluorinated alkyl groups.

The term "alkenyl" embraces straight-chain or branched alkenyl groups with 2 to 8 carbon atoms, preferably 2 to 6 carbon atoms, e.g. allyl, 2-butenyl and 3-butenyl.

The term "alkynyl" signifies a straight-chain or branched alkynyl group with one triple bond and 2 to 8 carbon atoms, preferably 2 to 6 carbon atoms, e.g. propynyl and butynyl.

The term "cycloalkyl" signifies a 4- to 7-membered alicyclic group, namely cyclobutyl, cyclo- pentyl, cyclohexyl or cycloheptyl, of which cyclopentyl and cyclohexyl are preferred.

The term "unsubstituted or substituted aryl" or equally "aryl which may be substituted" or "optionally mono- or multiply-substituted aryl" preferably embraces a phenyl or naphthyl group, which can be unsubstituted, mono-substituted or multiply-substituted. As substitutents come into consideration e.g. phenyl, halogen and straight-chain and branched alkyl and alkoxy groups witn in eacn case i to b caroon atoms, wnereoy tne mumpiy-suDstitutea pnenyi or naphthyl groups can have the same or different substituents. Of the alkyl and alkoxy groups the methyl and, respectively, methoxy group is preferred. Examples of optionally substituted aryl groups are phenyl, chloro-, bromo- and fluorophenyl, tolyl, anisyl, as well as naphthyl. The place of the substitution can be at any position of the aromatic ring.

The term "heteroaryl" embraces 5- or 6-membered heterocyclic groups featuring O, S or N as a ring member, i.e. heteroatom, such as, for example, furyl, thienyl, benzofuryl, dibenzofuryl, xanthenyl, pyrrolyl and pyridinyl. The heterocyclic groups featuring O as the heteroatom are especially preferred.

A preferred embodiment of the present invention relates to a transfer hydrogenation of retinal, wherein the catalyst of formula (III)

M is chosen from the group consisting of Ir, Rh and Ru, and

X signifies H or a halogen atom, and

Y signifies an aromatic ligand, preferably a 6π electrons aromatic system-ligand, and

Z signifie

wherein

R¹ signifies d-C₂-alkyl which may be substituted with one or more fluorine atoms, C₂- C₆-alkenyl, C₂-C₆-alkynyl, C₄-C₇-cycloalkyl, aryl which may be substituted, heteroaryl, or camphor-10-yl, and

R² and R³ each independently signify hydrogen, Ci-C₂-alkyl, C₄-C₇-cycloalkyl, or aryl which may be substituted, and n signiTies nyarogen or Ui-u₂-aiKyi, ana

R⁵ signifies hydrogen or C₁-C₂-alkyl, and

R⁶ signifies hydrogen or C₁-C₂-alkyl, and

n signifies 0,1 ,2 or 3, and

m signifies 0,1 ,2 or 3,and

with the proviso that in formula (V) the sum of n and m is 3 or 4.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁷), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵), (TH^5") or (TH⁶), wherein the catalyst is a compound of formula (III) MX[Z-(-H)][Y], wherein

M is chosen from the group consisting of Ir, Rh and Ru, and

X signifies H or a halogen atom, and

Y signifies an aromatic ligand, 6π electrons aromatic system-ligand, and

Z signifie

wherein

R² and R³ each independently signify hydrogen, Ci-C₂-alkyl, C₄-C₇-cycloalkyl,or aryl which may be substituted, and

R⁴ signifies hydrogen or d-C₂-alkyl, and

R⁵ signifies hydrogen or d-C₂-alkyl, and

R⁶ signifies hydrogen or Ci-C₂-alkyl, and

n signifies 0,1 ,2 or 3, and m signiTies υ,ι ; or 6, ana

with the proviso that in formula (V) the sum of n and m is 3 or 4.

In the catalyst of formula (III) preferably Y is chosen from the group consisting of pentame- thylcyclopentadienyl, benzene, p-cymene, toluene, anisole, xylene, 1 ,3,5-trimethylbenzene, p-dicyclohexylbenzene, naphthalene and tetralin.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁸), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵), (TH^5"), (TH⁶) or (TH⁷), wherein the lig- and Y in the catalyst of formula (III) is chosen from the group consisting of pentamethylcyclo- pentadienyl, benzene, p-cymene, toluene, anisole, xylene, 1 ,3,5-trimethylbenzene, p- dicyclohexylbenzene, naphthalene and tetralin.

A more preferred embodiment of the present invention relates to a transfer hydrogenation of retinal, wherein the catalyst of formula (III)

M is chosen from the group consisting of Ir, Rh and Ru, preferably Ru, and

X signifies H or CI, and

Z signifies a group of formula (IV)

wherein

R² and R³ each independently signify hydrogen, C₁-C₂-alkyl, C₄-C₇-cycloalkyl, or aryl which may be substituted, and

R⁴ signifies hydrogen or d-C₂-alkyl, and

R⁵ signifies hydrogen or d-C₂-alkyl, and t" signiTies nyarogen or Ui-u₂-aiKyi, ana

n signifies 0,1 ,2 or 3.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH⁹), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵), (TH^5"), (TH⁶), (TH⁷) or (TH⁸), wherein wherein the catalyst of formula (III)

M is chosen from the group consisting of Ir, Rh and Ru, preferably Ru, and

X signifies H or CI, and

Y signifies an aromatic ligand, preferably a 6π electrons aromatic system -ligand, and

Z signifies a group of formula (IV)

wherein

R¹ signifies C₁-C₂-alkyl which may be substituted with one or more fluorine atoms, C₂- Ce-alkenyl, C₂-C₆-alkynyl, C₄-C₇-cycloalkyl, aryl which may be substituted, heteroaryl, or camphor-10-yl, and

R² and R³ each independently signify hydrogen, d-C₂-alkyl, C₄-C₇-cycloalkyl, or aryl which may be substituted, and

R⁴ signifies hydrogen or Ci-C₂-alkyl, and

R⁵ signifies hydrogen or Ci-C₂-alkyl, and

R⁶ signifies hydrogen or d-C₂-alkyl, and

n signifies 0,1 ,2 or 3.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH¹⁰), which is (TH⁹), wherein the ligand Y in the catalyst of formula (III) is chosen from the group consisting of pentamethylcyclopentadienyl, benzene, p-cymene, toluene, anisole, xylene, 1 ,3,5-trimethylbenzene, p-dicyclohexylbenzene, naphthalene and tetralin. i-urtnermore tne present invention aiso relates to preterrea transter nyarogenations wnerein the catalyst is of formula (III')

wherein

M is Ru, Rh, Ir, preferably Ru, and

Y is an aromatic ligand chosen from the group consisting of

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH^{1 1}), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵ ), (TH^5"), (TH⁶), (TH⁷), (TH⁸), (TH⁹) or (TH¹⁰), wherein

the catalyst is of formula (III')

wherein

M is Ru, Rh, Ir, preferably Ru, and γ is an aromatic ngana cnosen trom tne group consisting ot

Preferred transfer hydrogenations according to the present invention are such wherein the following catalysts are used:

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH¹²), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵ ), (TH^5"), (TH⁶), (TH⁷), (TH⁸), (TH⁹), (TH¹⁰) or (TH^{1 1}), wherein the catalyst is chosen from the group consisting of

More preferred transfer hydrogenations according to the present invention are such wherein the following catalysts are used:

and

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH¹³), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵ ), (TH^5"), (TH⁶), (TH⁷), (TH⁸), (TH⁹), (TH¹⁰), (TH^{1 1} ) or (TH¹²), wherein the catalyst is chosen from the group consisting of

wherein Y is an aromatic ligand chosen from the group consisting of

Preferably the catalyst is prepared in situ starting from a metal precursor complex and various ligands:

0.5 Mol [M(Y)X₂]₂ and 1 Mol Z, which is as stated above represented by formula (IV) or formula (V)

are mixed to form the catalyst.

All the substituents (in formula (IV) and in formula (V)) have the same meanings as defined above as well as the same preferences.

The reaction temperature of the process of production of the catalyst (compound of formula (III)) is usually 0 - 100 °C. The reactants are usually mixed for a period of time and then all the reactants for the selective transfer hydrogenation of the present invention are added and the necessary reaction conditions of the hydrogenation are applied. I ne selective transTer nyarogenation reaction is usually carriea out at a temperature οτ u - 1 00 °C, preferably 25 - 90 °C.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH¹⁴), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵ ), (TH^5"), (TH⁶), (TH⁷), (TH⁸), (TH⁹), (TH¹⁰), (TH^{1 1} ), (TH¹²) or (TH¹³), wherein the transfer hydrogenation reaction is carried out at a temperature of 0 - 100 °C, preferably 25 - 90 °C.

The selective transfer hydrogenation reaction is usually carried out at a normal (ambient) pressure. It could also been carried out at elevated and preferably at reduced pressure. Usually the transfer hydrogenation according to the present invention is carried out at a pressure of 50 - 1 000 mbar.

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH¹⁵), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵ ), (TH^5"), (TH⁶), (TH⁷), (TH⁸), (TH⁹), (TH¹⁰), (TH^{1 1}), (TH¹²), (TH¹³) or (TH¹⁴), wherein the transfer hydrogenation reaction is carried out at a pressure of 50 - 1000 mbar.

The substrate to catalyst ratio (s/c ratio) in the selective transfer hydrogenation is usually from 20:1 to 10000:1 , preferably 50:1 to 1 000:1 . This ratio is related to mol. Mol of substrate (which is compound of formula (I)) to mol of catalyst (compound of formula (III)).

Therefore a further embodiment of the present invention is a transfer hydrogenation (TH¹⁶), which is (TH), (TH¹), (TH²), (TH³), (TH⁴), (TH⁵), (TH⁵ ), (TH^5"), (TH⁶), (TH⁷), (TH⁸), (TH⁹), (TH¹⁰), (TH^{1 1}), (TH¹²), (TH¹³), (TH¹⁴) or (TH¹⁵), wherein molar substrate to catalyst ratio (s/c ratio) is from 20:1 to 10000:1 , preferably from 50:1 to 1000:1 .

The transfer hydrogenation according to the present invention allows obtaining retinol in excellent yield.

The retinol which is obtained by the transfer hydrogenation according to the present invention can be used as such or in further application as well as a starting material for further synthesis.

The invention is illustrated by the following Examples. All temperatures are given in °C and all parts and percentages are related to the weight. Examples

Example 1 : Production of the catalysts

In a 25 mL 2-necked round bottomed flask equipped with thermometer and a magnetic stirrer, 0.020 mmol of metal complex (M(Y)X₂]₂) and 0.044 mmol of a ligand (Z) were dissolved in 9 mL solvent at room temperature. The mixture was stirred for 30 min.

The solvent was ethyl acetate, isopropanol or methanol.

All of the following catalysts [(C1 ) - (C8)] were produced according to the process of Example 1

Catalyst 1 (C1 )

Catalyst 2 (C2)

with Y= \^

uataivst ci (UcS)

Catalyst 4 (C4)

Catalyst 5 (C5)

Catalyst 6 (C6)

with Y= uataivst / (U/ )

Catalyst 8 (C8)

Example 2 (Procedure A): Selective transfer hydrogenation of Retinal

299 mg of retinal and 24.2 mg (5 mol%) of C1 were dissolved in 4 ml of anhydrous dichloro- methane. To the red solution 169 μΙ of formic acid/triethylamine complex (5:2) were added drop wise and the reaction was monitored by TLC. After stirring at 23 °C for 2.5 hours the reaction mixture was diluted with 15 ml of dichloromethane and washed with 15 ml of water. The layers were separated and the organic phase was again washed with water (2 x 15 ml). The aqueous phases were re-extracted with 15 ml of dichloromethane. The combined organic extracts were dried over sodium sulphate, filtered and evaporated to dryness. The crude product (325 mg) was obtained as dark brown oil and was purified by column chromatography [Si0₂ 60 (10 g), cyclohexane/ethyl acetate 8:2]. The purified retinol was obtained in 83% yield and 75.2% purity at 97% conversion. For further results see Table 1 a. txampie aa ana 3D (^roceaure ay. Preparation of the active catalyst solution

In a 5 ml glass vial under argon atmosphere a stock solution of the catalyst was prepared as follows: 41 .4 mg of catalyst C1 were weighed into a glass vial with magnetic stir bar. The vial was capped with a septum and flushed with argon. Then, 900 μΙ of dichloromethane and a solution of 41 .1 mg of sodium formate in 465 μΙ of deionized water were added. The biphasic mixture was warmed to 35 °C in a water-bath and stirred for 30 min. After phase separation, samples of the organic phase were used as active catalyst solution for several reactions on the same day.

Reaction

0.73 g of retinal were weighed into a 20 ml glass vial equipped with magnetic stir bar. The vial was sealed with a septum and flushed with argon via cannula. 4.29 ml of anhydrous iso- propanol were added via syringe and the mixture was warmed to 38 °C in a water-bath until all retinal had dissolved. After that, 263 μΙ (1 .0 mol%) of the freshly prepared catalyst solution were added via microliter-syringe at 38 °C and the reaction was monitored by TLC and HPLC. After 30 min the reaction was complete. For results see Table 1 b.

Examples 4a-e (Procedure C):

719 mg of retinal (2.5 mmol) were weighed into a round bottomed flask. The apparatus was evacuated to p = <0.1 mbar and then inflated with argon. This procedure was done three times. 9.6 ml of 2-propanol (125 mmol = 50 eq.) were added with a syringe. With magnetic stirring at r.t. the suspension was evacuated and inflated three times as described above (p = <0.1 mbar, foams heavily at the beginning!). Then 3.03 mg of ruthenium catalyst C1 (0.0063 mmol = 0.25 mol% = s/c 400) were weighed into a GC vial. The catalyst was dissolved in ~1 ml of dichloromethane and transferred with a syringe into the reaction mixture. Evacuated (p = -0.4 mbar, foams heavily at the beginning!) and inflated with argon three times as described above. 5 μΙ_ of potassium hydroxide solution (2.5148 g KOH to 50 ml in deionized water, 0.0039 mmol = 0.625 eq. rel. to Ru-catalyst), were also transferred with a 10 μΙ_ GC syringe via the rubber seal into the reaction mixture. Evacuated and inflated with argon three times as described above.

At an argon pressure of -0.3 bar, the reaction mixture was stirred for 18 hours in a preheated oil bath (Tj = 38 ^<€). For the work-up the reaction mixture was cooled to r.t., transferred into a separating funnel with -150 ml of diethyl ether and extracted three times with ~bu mi oT aeionizea water, ι ne organic layer was ariea over soaium suipnate, Tiiterea ana distilled on a rotary evaporator at Tj = 35 °C, p = 10 mbar. Most of the remaining 2-propanol was distilled off by switching to high vacuum, Tj = 35 p = <0.1 mbar). 0.76 g of clear and sticky oil, amber in colour, 93.1 wt% of retinol by qNMR were obtained. Conversion 99.3%, yield 98.8% relative to retinal, selectivity 100%.

For further results see Table 2.

Table 1a: with Cat C1 and Formic acid/ triethylamine (ratiol :1 ) as red agent and dichloro- methane as solvent

Table 1 b: with Cat C1 and i-propanol/ sodium formate as red agent and i-propanol as solvent

Table 2: with Cat C1 and i-propanol/KOH as red. agent and dichloromethane as solvent

Exp. s/c t Conv. Yield

[h] [%] [%]

4a 100 2.75 98.2 94.7

4b 200 2.75 99.3 99.1

4c 300 24 99.2 97.3

4d 400 18 99.3 98.8

4e 400 24 97.3 95.5 Examples 5a - 5c I ranster hydrogenation ot retinal - in situ preparation ot the catalyst

In the following examples the catalyst is formed in situ. The following ligands have been used to form the catalyst in situ. The numbering of the ligands corresponds to the Examples they are used in.

6 mg of dichloro(p-cymene)ruthenium(ll) dimer (CAS No. 52462-29-0) and 7 mg of the ligand (in table 3 the numbering of the Example corresponds to the numbering of the ligand) were dissolved in 3 ml of ethyl acetate.

After stirring for 5 minutes, a solution of 280 mg of retinal in 2 ml of ethyl acetate was added. The mixture was stirred for an additional 5 minutes and afterwards 0.45 ml of a 5:2 mix- ture of formic acid:triethylamine (5was added and the reaction mixture was stirred at room temperature. After 30-120 minutes, HPLC analysis indicated complete consumption of retinal (<99% conversion). For the results (in regard to the conversion and the yield) see the table 3 below. Table 3:

Exp. Time (min) Conversion [%] Product [%]

5a 120 >99 89.4

5b 45 >99 96.7

5c 45 >99 90.1

Claims

The present invention relates to a transfer hydrogenation of a compound of formula

characterized in that the hydrogenation is carried out in the presence of

a) an amino-amide-transition-metal catalyst of formula (III)

MX[Z-(-H)][Y] (III)

wherein

X signifies H or a halogen atom, and

Y signifies a ligand, preferably an aromatic ligand, and

Z signifies a group of formula (IV) or (V)

wherein

R¹ signifies Ci-C₆-alkyl which may be substituted with one or more fluorine atoms, C₂-C₈-alkenyl, C₂-C₈-alkynyl, C₄-C₇-cycloalkyl, aryl which may be substituted, het- eroaryl, or camphor-10-yl, and

R² and R³ each independently signify hydrogen, Ci-C₆-alkyl, C₄-C₇-cycloalkyl, or aryl which may be substituted, and

R⁴ signifies hydrogen or d-CValkyl, and

R⁵ signifies hydrogen or d-CValkyl, and

R⁶ signifies hydrogen or d-Ce-alkyl, and

n signifies 0,1 ,2 or 3, and

m signifies 0,1 ,2 or 3,and

with the proviso that in formula (V) the sum of n and m is 2, 3, 4 or 5, and

b) a H₂ donor.

Transfer hydrogenation according to claim 1 , wherein the hydrogenation is carried out in the absence of water.

Transfer hydrogenation according to claim 1 or claim 2, wherein the hydrogenation is carried out by using at least one solvent which also serves as hydrogen donor.

Transfer hydrogenation according to claim 1 or claim 2, wherein the hydrogenation is carried out by using at least one hydrogen donor and at least one inert solvent.

Transfer hydrogenation according to claims 1 , 2, or 3, wherein the hydrogenation is carried out by using isopropanol as hydrogen donor and solvent.

Transfer hydrogenation according to claims 1 , 2, or 4, wherein the hydrogenation is carried out by using formic acid (or a salt of formic acid QOOCH), wherein Q is an alkali cation or NH₄ ⁺, NH₃(R⁷)⁺, NH₂(R⁷)₂ ⁺, NH(R⁷)₃ ⁺ or N(R⁷)₄ ⁺, wherein all R⁷ are independently from each other -CH₃ or -CH₂CH₃.

7. I ranster hydrogenation according to claim b\ wherein the molar ratio ot tormic acid (or a salt of formic acid) and triethylamine is from 1 :1 to 10:1 , preferably 1 :1 to 6:1 .

8. Transfer hydrogenation according to anyone of the preceding claims, wherein the catalyst is a compound of formula (III)

MX[Z-(-H)][Y] (III), wherein

M is chosen from the group consisting of Ir, Rh and Ru, and

X signifies H or a halogen atom, and

Y signifies an aromatic ligand, preferably a 6π electrons aromatic system- ligand, and

Z signifies a group of formula (IV) or (V)

wherein

R¹ signifies C₁-C₂-alkyl which may be substituted with one or more fluorine atoms, C₂-C₆-alkenyl, C₂-C₆-alkynyl, C₄-C₇-cycloalkyl, aryl which may be substituted, het- eroaryl, or camphor-10-yl, and

R² and R³ each independently signify hydrogen, Ci-C₂-alkyl, C₄-C₇-cycloalkyl, or aryl which may be substituted, and

R⁴ signifies hydrogen or d-C₂-alkyl, and

R⁵ signifies hydrogen or d-C₂-alkyl, and

R⁶ signifies hydrogen or Ci-C₂-alkyl, and

n signifies 0,1 ,2 or 3, and

m signifies 0,1 ,2 or 3, and

with the proviso that in formula (V) the sum of n and m is 3 or 4.

9. Transfer hydrogenation according to anyone of the preceding claims, wherein the ligand Y in the catalyst of formula (III) is chosen from the group consisting of pen- tamethylcyclopentadienyl, benzene, p-cymene, toluene, anisole, xylene, 1 ,3,5- trimethylbenzene, p-dicyclohexylbenzene, naphthalene and tetralin.

10. Transfer hydrogenation according to anyone of the preceding claims, wherein

the catalyst of formula (III)

M is chosen from the group consisting of Ir, Rh and Ru, preferably Ru, and X signifies H or CI, and

Z signifies a group of formula (IV)

wherein

R⁴ signifies hydrogen or d-C₂-alkyl, and

R⁵ signifies hydrogen or d-C₂-alkyl, and

R⁶ signifies hydrogen or Ci-C₂-alkyl, and

n signifies 0,1 ,2 or 3.

11. Transfer hydrogenation according to anyone of the preceding claims, wherein the catalyst is of formula (III')

wherein M is Ku, Kh, ir, preferably Ku, and

Y is an aromatic ligand chosen from the group consisting of

13. Transfer hydrogenation according to anyone of the preceding claims 1 -1 1 , wherein the catalyst is chosen from the group consisting of

wherein Y is an aromatic ligand chosen from the group consisting of

14. Transfer hydrogenation according to anyone of the preceding claims, wherein transfer hydrogenation reaction is carried out at a temperature of 0 - 100 °C, preferably 25 - 90 °C.

15. Transfer hydrogenation according to anyone of the preceding claims, wherein the transfer hydrogenation reaction is carried out at a pressure of 50 - 1000 mbar.

16. Transfer hydrogenation according to anyone of the preceding claims, wherein the molar substrate to catalyst ratio (s/c ratio) is from 20:1 to 10000:1 , preferably from 50:1 to 1000:1 .