# Patent

## Title
Catalytic hydrogenation

## Abstract

Catalytic hydrogenation of a double bond, notably a C═C or C═N double bond, is carried out using a homogenous catalyst which is a complex of rhodium or other transition metal containing at least one ligand which is a nitrogenous organic base. Preferably the complex is phosphorus-free and the ligand is a bicyclic base having formula




where R1 and R4 are hydrocarbon chains. R1 preferably is a saturated chain of two carbon atoms and R4 preferably is a saturated chain of three to five carbon atoms.

## Background
FIELD OF THE INVENTION
This invention relates to the catalytic hydrogenation of organic compounds.
BACKGROUND OF THE INVENTION
The hydrogenation of a double bond between two carbon atoms or between a carbon atom and a hetero atom is a chemical reaction which can be carried out in the presence of a catalyst. Transition metal complexes have been used as homogeneous catalysts for this reaction, but in many cases these complexes have contained phosphine ligands which tend to be expensive, unpleasant to handle and also toxic.
There have been few examples of non-phosphine ligands in catalysts for hydrogenation. Those that have been described include the following.
Frediani et al. “Quinoline transfer hydrogenation by a rhodium bipyridine catalyst”, Inorganica Chimica Acta, 359, 2650-2657 (2006) have described the use of the rhodium complex cis-[Rh(bipy)2Cl2]Cl 2H2O containing the bipyridyl ligand as a catalyst for hydrogenation.
Brunner and Agrifoglio “Hydrogenation of prochiral olefins with rhodium complexes of optically active amidines”, Monatshefte für Chemie, 111, 275-287 (1980) have disclosed olefin hydrogenation using a rhodium compound in the presence of an amidine compound. They used amidines having a general formula:




in which R1 was 1-methylbenzyl and R2 was benzyl, phenyl, isopropyl or 1-methylbenzyl.

SUMMARY OF THE INVENTION
The present invention provides a process for hydrogenating a compound at a double bond between carbon and another atom—by contacting the compound with hydrogen in the presence of a catalytic complex, characterized in that the complex is a transition metal complex containing a ligand which is a heterocyclic organic base of the formula




wherein R1 comprises a substituted or unsubstituted hydrocarbon chain of at least two carbon atoms and R2 and R3 are substituted or unsubstituted hydrocarbon groups or R2 and R3 together are a substituted or unsubstituted hydrocarbon chain of at least two carbon atoms. It is preferred that R2 and R3 provide saturated aliphatic hydrocarbon chains but they may be substituted and the substituent groups may be aliphatic or aromatic.

When the ligand is a bicyclic aliphatic compound in which R2 and R3 together form a single hydrocarbon chain, the ligand can be represented by the formula




where R1 is as defined above and R4 represents the hydrocarbon chain of at least two carbon atoms provided by R2 and R3 together.

R1 may be a chain of 2 up to 5 or possibly up to 8 carbon atoms especially 2 or 3 carbon atoms. R4 may be an aliphatic hydrocarbon chain of 2 up to 6 or possibly up to 10 carbon atoms, especially of 3 to 5 carbon atoms. R1 and R4 may both be saturated aliphatic groups, optionally bearing side-chain groups which may be aliphatic, notably C1 to C4 alkyl, or may be aromatic.

The catalyst may contain at least one other ligand. A second ligand may be a cycloaliphatic diene, notably 1,5-cyclo-octadiene. Preferably the catalyst does not contain phosphorus.
A catalytic complex used in this invention may contain one or two metal atoms, generally with at least two ligands per metal atom. The transition metal in the complex is preferably from groups VB, VIIB, VIIB, or VIIIB of the periodic table. It may be a transition metal with atomic number in the ranges from 41 to 45 and 73 to 77. The metal may be rhodium, which has atomic number 45. It is particularly envisaged that the molecule may contain one rhodium atom with one ligand of the formula




discussed above and one other ligand which may be a cycloaliphatic diene, notably 1,5-cyclo-octadiene.

The catalyst maybe Rh(DBU)(COD)Cl or Rh(DBN)(COD)Cl where DBU denotes 1,8-diazabicyclo[5.4.0]undec-7-ene which has the formula




DBN denotes 1,5-diazabicyclo[4.3.0]non-5-ene which has the formula





and COD denotes 1,5 cyclo-octadiene which has the formula





DBN and DBU are strong uncharged nitrogenous bases but are bulky, sterically hindered molecules and consequently are weak nucleophiles.
The hydrogenation reaction will generally be carried out using hydrogen gas with the substrate molecule and the catalyst in solution in an organic solvent. The reaction temperature may possibly range from 0 to 80° C. although temperatures outside this range are also possible.



BRIEF DESCRIPTION OF THE DRAWING
The sole drawing FIGURE illustrates the structure of the complex Rh(DBU)(COD)Cl as determined by X ray crystallography.



DETAILED DESCRIPTION
In these examples, all operations with air-sensitive complexes were performed under an inert atmosphere of dry N2 using standard Schlenk techniques or in a glove box (MBRAUN Labmaster SP). All solvents (including deuterated solvents) were dried and deoxygenated using sodium/benzophenone, and then distilled under nitrogen. IR spectra were obtained using a Nicolet 800 FT-IR spectrometer and NMR spectra were recorded on a Bruker AVANCE 400 spectrometer (1H at 400.1 MHz and 13C at 100.6 MHz) and spectra are reported relative to tetramethylsilane.
Preparative Example
Preparation and Characterisation of Rh(DBU)(COD)Cl
38.1644 mg [Rh(COD)Cl]2 (0.0774 mmol) was dissolved in ca. 5 ml toluene, and 0.3719 mmol DBU (56.6181 mg) was then added. The reaction mixture was stirred at 70° C. for 16 hours. The reaction mixture was dried in vacuum, and then washed with hexane. The remaining solid was dried again, and then dissolved in benzene. By carefully layering the benzene solution with hexane, yellow crystals were obtained which were used for single crystal X-ray diffraction study. The yield of Rh(DBU)(COD)Cl was 50.8612 mg (0.1275 mmol, 82.36%).
The results for 13C {H} NMR spectroscopy were (100.6 MHz, benzene-d6, 298K):













δ 82.91 ppm (d, 1C, J (103Rh, 13C) = 12 Hz),
Four carbon atoms


δ 81.83 ppm (d, 1C, J (103Rh, 13C) = 12 Hz),
of COD, coordinated to


δ 73.42 ppm (d, 1C, J (103Rh, 13C) = 14 Hz),
rhodium


δ 73.22 ppm (d, 1C, J (103Rh, 13C) = 15 Hz),


δ 32.24 ppm (s, 1C), δ 31.68 ppm (s, 1C),
Four carbon atoms of


δ 31.04 ppm (s, 1C), δ 30.71 ppm (s, 1C),
COD


δ 163.36 ppm (s, 1C), δ 52.84 ppm (s, 1C),
Nine carbon atoms of


δ 47.21 ppm (s, 1C), δ 41.17 ppm (s, 1C),
DBU


δ 39.33 ppm (s, 1C), δ 29.37 ppm (s, 1C),


δ 27.70 ppm (s, 1C), δ 25.35 ppm (s, 1C),


δ 22.34 ppm (s, 1C).









In this 13C {H} NMR spectrum the carbon of C═N of bound DBU shows a singlet at δ 163.36 ppm; the carbon nuclei of bound C═C trans to N show two doublets at δ 82.91 ppm and δ 81.83 ppm, respectively, with J(Rh, C) of 12 Hz each, while the carbon nuclei of bound C═C trans to Cl show two doublets at δ 73.42 ppm (J(Rh, C)=14 Hz) and δ 73.22 ppm (J(Rh, C)=15 Hz), respectively.
The corresponding results for 1H NMR spectroscopy were (400.1 MHz, benzene-d6, 298K):













δ 7.27 ppm (s)
solvent, benzene-d6


δ 5.10 ppm (bs, 1H,) δ 5.02 ppm (bs, 1H,),
Four protons at double


δ 3.70 ppm (bs, 1H,) δ 3.64 ppm (bs, 1H,),
bonds of COD


δ 3.78 ppm (m, 1H) δ 3.09 ppm (m, 1H)
DBU protons


δ 3.56 ppm (m, 1H) δ 3.24 ppm (m, 1H)


δ 2.48 ppm (m, 8H)
Four protons of DBU and



four protons of COD


δ 2.07 ppm (m, 1H)
DBU proton


δ 1.76 ppm (m, 4H)
Four protons of COD


δ 1.64 ppm (m, 1H) δ 1.60 ppm (m, 1H)
DBU protons


δ 1.28 ppm (bs, 1H) δ 1.40 ppm (bs, 1H)


δ 1.30 ppm (bs, 1H) δ 1.07 ppm (bs, 2H)









In this 1H NMR spectrum the HC═CH trans to N shows two broad singlets at δ 5.10 ppm and δ 5.02 ppm compared with the HC═CH trans to Cl showing two broad singlets at δ 3.70 and δ 3.64 ppm.
The observed elemental analysis (expressed in weight percent) was: C, 51.30; H, 7.05, N, 6.98, Cl: 8.34, which is in good agreement with the calculated elemental percentages: C, 51.20; H, 7.08; N, 7.02; Cl, 8.89.
The drawing shows the X-ray crystal structure of the complex Rh(DBU)(COD)Cl with hydrogen atoms omitted. The DBU is coordinated with a Rh center via an imino-nitrogen located trans to a C═C in COD. The complex adopts a slight distorted square planar geometry around the Rh center, which is similar to the complex Rh(PPh3)3Cl (Ph=phenyl) that is known as Wilkinson's catalyst. The bond lengths of Rh(1)-C(6) and Rh(1)-C(5) trans to the nitrogen atom are longer than those of Rh(1)-C(2) and Rh(1)-C(1), which are trans to the chlorine atom.
Rh(DBU)(COD)Cl crystals were still stable after being exposed to air for 4 days, indicating low sensitivity to oxygen and moisture.
Examples of Catalytic Hydrogenation of C═C and C═N Double Bonds
The catalytic activity of Rh(DBU)(COD)Cl to the hydrogenation of various C═C, C═N and C═O bonds was evaluated. Table 1 shows the compounds hydrogenated using the catalyst Rh(DBU)(COD)Cl, the hydrogenation products, the conversion (or yield) of the reaction and the turnover number (TON) of the catalytic cycle. Turnover number is the average number of substrate molecules hydrogenated per catalyst molecule The details of the reactions are described in the following examples.
Example 1
Hydrogenation of N-Benzylidenebenzylamine to Dibenzylamine
Working in a glove box, a mixture of Rh(DBU)(COD)Cl (6.30 mg, 0.0158 mmol), N-benzylidenebenzylamine (403.19 mg, 2.065 mmol) and approximately 2 ml dimethyl sulphoxide (DMSO) was prepared in a 20 ml glass vessel with a gas admission valve. The reaction mixture was pressured with 1.5 bar hydrogen gas and stirred for 16 hours at a temperature of 50° C., after which time the solution turned from its original colour of bright yellow to a dark brown. Integration of peaks in the 1H NMR spectrum of the reaction mixture revealed a conversion rate of 13.4% and a TON of 18.
Example 2
Hydrogenation of N-Benzylideneaniline to N-Benzylaniline
A mixture of Rh(DBU)(COD)Cl (4.46 mg, 0.0112 mmol), N-benzylideneaniline (246.4 mg, 1.360 mmol) and approximately 2 ml DMSO was prepared in a 20 ml glass vessel with a gas admission valve, located in a glove box. The reaction mixture was pressured with 1.5 bar hydrogen gas and stirred for 16 hours at a temperature of 50° C., after which time the solution turned from its original colour of bright yellow to a dark brown. Integration of peaks in the 1H NMR spectrum of the reaction mixture showed a conversion rate of 91.6% and a TON of 111.
Example 3
Hydrogenation of Cyclohexene to Cyclohexane
A mixture of Rh(DBU)(COD)Cl (5.14 mg, 0.0129 mmol), cyclohexene (1.14065 g, 13.887 mmol) and sufficient toluene to dissolve the catalyst was prepared in a 20 ml glass vessel with a gas admission valve, located in a glove box. The reaction mixture was pressured with 1.5 bar hydrogen gas and stirred for 16 hours at a temperature of 50° C. Integration of peaks in the 1H NMR spectrum of a small sample of the reaction mixture dissolved in deuterated chloroform (CDCl3) showed a conversion rate of 20.6% and a TON of 222.
Example 4
Hydrogenation of 2,3-Dimethyl-2-Butene to 2,3-Dimethyl-2-Butane
A mixture of Rh(DBU)(COD)Cl (5.97 mg, 0.0150 mmol), 2,3-dimethyl-2-butene (0.93469 g, 11.106 mmol) and sufficient toluene to dissolve the catalyst was prepared in a 20 ml glass vessel with a gas admission valve, located in a glove box. The reaction mixture was pressured with 1.5 bar hydrogen gas and stirred for 16 hours at a temperature of 50° C. Integration of peaks in the 1H NMR spectrum of a small sample of the reaction mixture dissolved in CDCl3 showed indicated a conversion rate of 7.8% and a TON of 58.
Example 5
Hydrogenation of 1,4-Cyclohexadiene to Cyclohexene and Cyclohexane
A mixture of Rh(DBU)(COD)Cl (4.79 mg, 0.0120 mmol), 1,4-cyclohexadiene (0.92612 g, 11.558 mmol) and sufficient toluene to dissolve the catalyst was prepared in a 20 ml glass vessel with a gas admission valve, located in a glove box. The reaction mixture was pressured with 1.5 bar hydrogen gas and stirred for 16 hours at a temperature of 50° C. Integration of peaks in the 1H NMR spectrum of a small sample of the reaction mixture dissolved in CDCl3 showed a conversion rate of 16.1% for cyclohexene and 5.1% for cyclohexane, which corresponded to a TON of 155 for cyclohexene and a TON of 49 for cyclohexane, respectively.
Example 6
Hydrogenation of Oleic Acid to Stearic Acid
A mixture of Rh(DBU)(COD)Cl (4.02 mg, 0.0101 mmol), oleic acid (1.04317 g, 3.693 mmol) and approximately 0.3 ml of deuterated benzene (C6D6) was prepared in a 20 ml glass vessel with a gas admission valve, located in a glove box. The reaction mixture was pressured with 1.5 bar hydrogen gas and stirred for 14 hours at a temperature of 50° C. All of the liquid had disappeared during the course of the reaction and only a white solid was observed. A small sample of the white solid was dissolved in benzene-d6. Integration of peaks in the 1H NMR spectrum of this solution indicated a conversion rate of 89.0% and a TON of 329.







TABLE 1







Hydrogenation of compounds with C═C and C═N bonds












Example
Substrate
product
Conversion
Substrate/Cat.
TON















1








13.4%
130.8
 18


 


2








91.6%
121.6
111


 


3








20.6%
1077.6
222


 


4








 7.8%
742.6
 58


 


5








16.1%/5.1%
963.3
155/49


 


6
C8H17CH═CHC7H14COOH
C8H17CH2—CHC7H14COOH
89.0%
369.3
329

## Claims

1. A process for hydrogenating a compound at a double bond between carbon and another atom by contacting the compound with hydrogen in the presence of a catalytic complex, characterized in that the complex is a rhodium complex containing a ligand which is a heterocyclic organic base of the formula



where R1 comprises a substituted or unsubstituted saturated hydrocarbon chain of 2 to 8 carbon atoms and R4 is a substituted or unsubstituted saturated hydrocarbon chain of 2 to 10 carbon atoms,
the complex further containing a second organic ligand which is a cycloaliphatic diene.

2. The process of claim 1 wherein R1 is a substituted or unsubstituted saturated hydrocarbon chain of 2 to 5 carbon atoms and R4 is a substituted or unsubstituted saturated hydrocarbon chain of 2 to 6 carbon atoms.

3. The process of claim 1 wherein R1 is a substituted or unsubstituted saturated hydrocarbon chain of 2 or 3 carbon atoms and R4 is a substituted or unsubstituted saturated hydrocarbon chain of 3 to 5 carbon atoms.

4. The process of claim 1 wherein the cycloaliphatic diene is a 1,5-cyclo-octadiene.

5. The process of claim 1 wherein hydrogenation takes place at a carbon-carbon double bond.

6. The process of claim 1 wherein hydrogenation takes place at a carbon-nitrogen double bond.

7. A process according to claim 4 wherein R1 is a substituted or unsubstituted saturated hydrocarbon chain of 3 carbon atoms and R4 is a substituted or unsubstituted saturated hydrocarbon chain of 3 to 5 carbon atoms.