GB2153849A - Production of pyridines - Google Patents

Abstract

Pyridines are prepared from the reaction of an alkyne and a cyanide in the presence of a cobalt complex catalyst which is electrochemically generated in situ during the reaction. In a separate embodiment, cobalt complex catalysts are electrochemically activated.

Description

SPECIFICATION Production of pyridines In one aspect, the present invention relates to a process for the production of pyridines from an alkyne and a cyanide using a cobalt complex catalyst which is electrochemically generated in the reactor during reaction. In a second aspect, this invention relates to the electrochemical activation of cobalt complex catalysts useful in catalysing such reactions.

US 3,829,429, US 4,226,061 and US 4,212,978 describe methods of preparing pyridines from alkynes, and cyanides (cyano compounds) using cobalt complex catalysts. In each of these patents, the catalyst is prepared prior to being added to the reactor. A disadvantage of previous organocobalt complex catalysts, however, is that they generally undergo degradation in air.

Thus, the catalysts of these patents must be manufactured and subsequently handled either in vacuo or under an inert gas such as nitrogen, argon or helium prior to and during use.

It is also known that organometallic compounds can be prepared by electrolysis. For example, J Grobe et al in Z. Anorg. Allg. Chemie (1981) 481,107 teaches the preparation of cobaltocene (biscyclopentadienyl-cobalt (II)) by the electrolysis of cyclopentadiene in THF using a cobalt anode. However, it has not been previously contemplated to prepare the cobalt complex catalyst in situ in the reaction vessel or to use electrolysis for the activation of cobalt complex catalysts in situ in the reaction vessel.

It has now been discovered that cyclopentadienyl cobalt catalysts or substituted cyclopentadienyl catalysts can be electrochemically prepared in situ in the reaction vessel avoiding the need to prepare the air-sensitive catalyst outside the reactor and thus avoiding any problems associated with the subsequent handling of it. Furthermore, by using a method of continual catalyst generation, problems associated with slow catalyst deactivation in the reactor may be reduced.

Moreover, it has also been found that cobalt complex catalysts useful in preparing pyridines from alkynes and cyanides can be electrolytically activated to their active catalytic species thereby reducing the necessity to employ high temperatures and/or pressures during the reaction. This is particularly important since conventional processes generally require high pressures such as up to 50 bar and/or temperatures in excess of 150"C. In addition to the added costs of commercialising high pressure processes, certain of the reactants, such as acetylene, may undergo explosive decomposition at these high pressures. Thus, the present process in a separate embodiment provides for a low pressure process mitigating these problems.

Accordingly, the present invention provides a process for the preparation of pyridines comprising contacting an alkyne and a cyanide in a reaction vessel containing (a) a solvent, (b) an electrolyte, (c) cyclopentadiene or a substituted cyclopentadiene, (d) one or more anodes containing cobalt or a alloy of cobalt, and (e) one or more cathodes, and passing an electric current between the anode and the cathode.

In a second embodiment of this invention, cobalt complex catalysts useful for the production of pyridines from the reaction of an alkyne and a cyanide can be activated into its catalytically active species by a process comprising contacting the catalyst with an electrical current.

A particularly convenient method of adding soluble cobalt to the reaction mixture is by using one or more positive electrodes (anodes) made of cobalt metal or an alloy of cobalt which are connected to the external source of electricity. Passage of electricity between the positive and negative electrodes then causes the cobalt anodes to slowly oxidise and dissolve in the reaction mixture.

The alkyne reactapt may be any compound containing an acetylenic grouping. Thus, it may be unsubstituted acetylene, a mono- or dialkyl acetylene in which the alkyl group has 1-20 carbon atoms or a mono- or diaryl acetylene in which the aryl groups can be substituted or unsubstituted. Examples of the alkyne reactant include but are not limited to acetylene, methyl acetylene (propyne), dimethyl acetylene (but-2-yne) or phenylacetylene. One or more alkynes may be used in the reaction mixture.

The cyanide which is used as a reactant is a organic cyanide compound having a cyano grouping such as a nitrile, is aprotic and is inert to the electrolysis conditions employed. Specific examples of the cyanide that may be used depending somewhat upon the electrolysis conditions include but are not limited to hydrogen cyanide, methyl cyanide (acetonitrile), alkyl thiocyanate, phenyl cyanide, cyanamide, 2-cyanopyridine, cyanogen, hydroxyethyl cyanide, 2-methoxy propionitrile, ethyl cyanoformate.

The mole ratio of the alkyne to the cyanide in the reaction mixture may vary from 0.01:1 to 2:1 with 0.1:1 to 1:1 being preferred.

The pyridines prepared in accordance with the present invention are dependent upon the specific reactants employed. From the selection of the reactants, it is clear to one skilled in the art which pyridines will be prepared. Examples of a few of the pyridines which can be prepared by the process of this invention includes pyridine, 2-picoline, 2,(2-methoxy ethyl)pyridine and collidine.

The reaction is carried out in a solvent in which the cobalt complex catalyst is soluble. The solvent can be any polar aprotic organic liquid which is stable under the reaction conditions employed. The solvent can comprise one or more of the reactants together with any of the products or byproducts of the reaction. Useful solvents include dimethyl formamide, trimethyl phosphate, acetone, triethylamine, acetonitrile, pyridine and substituted pyridines.

In order to pass an electrical current through the reaction mixture, it is necessary to have dissolved in the solvent an electrolyte which forms ions on dissolution. The ions thus formed are able to carry electrical current between the two electrodes. Any electrolyte can be used with the provisos that (i) it is soluble in the reaction mixture and (ii) it remains inert under the reaction conditions. An example of such an electrolyte is tetraethylammonium tetrafluoroborate which upon dissolution undergoes the following ionisation Et4N + BF4-eEt4N + + BF4 The electrolyte may itself be generated in the reactor by electrochemical reaction, for example quarternisation of a tertiary amine or phosphine. Typically, the amount of electrolyte added is such as to correspond to between 0.1 and 5% by weight of the solvent.

When preparing the cyclopentadienyl cobalt complex catalyst, it is necessary to have in the reactor cyclopentadiene or a substituted derivative of cyclopentadiene. Suitable substituents on the cyclopentadiene include C, to C10 alkyls, such as pentamethylcycopentadiene or ethyltetramethylcyclopentadiene; aryls such as indene and fluorene; and silanes such as trimethylsilane. In the case of cyclopentadiene, it is preferable to use compounds which have been freshly prepared from commercial dicyclopentadiene by thermal cracking. Methods of cracking dicyclopentadiene are well known to those who are skilled in the art.

The cyclopentadiene or substituted derivative thereof may be present in large concentrations in the reaction solvent, and hence form a component of the solvent, or it may be mixed with one or both of the feedstocks, at a low concentration, and fed continuously to the reactor (if the process is operated in a continuous mode). If the cyclopentadiene or substituted derivative thereof is fed continuously to the reactor, admixed with one or more of the reactants, it should be added to the feed in such a way that the concentration of this component in the reactant or reactants is in the range 0.01 to 15% by weight, preferably, 0.1 to 10% by weight.

The solvent in which the reaction is carried out should also contain two or more electrodes which are connected to an external supply of electricity. The electrodes can either be wholly or partially immersed in the solvent and any reactor design incorporating the electrodes can be employed. Conveniently, the electrodes are made of any electrically conducting inert material such as carbon, steel, platinized titanium, platinum and the like. In the electrolyte preparation of the cobalt complex catalyst, the positive electrode(s) (anodes) should be made of or contain cobalt metal or an alloy of cobalt.

The source of electricity should be such as to apply a voltage across the cell to generate a current density in the range of 1 to 500 mA cm-2 at the electrode surface.

It is thought that the electrochemical system used in preparing the cobalt complex catalyst causes the oxidation of cobalt from the cobalt anode which subsequently dissolves in the solvent and reacts with cyclopentadienyl anion generated at the cathode to further generate a cyclopentadienyl cobalt complex, such as cobaltocene. Further current will activate the complex into the species which is catalytically active. In this sense, the present invention is both the preparation of the cobalt complex and its activation into its catalytically active species.

Thus, in a separate embodiment herein, a cobalt complex catalyst can be prepared outside of the reaction vessel, placed into the reaction vessel by any suitable means and then electrolytically activated into its catalytically active species thereby allowing for the reaction to occur at lower pressures and/or temperatures than conventionally employed. The electrochemical activation of this invention of cobalt complexes is not limited to those containing cyclopentadienyl groups. For example, contemplated equivalent catalysts which can be activated by this process include borabenzene cobalt complex catalysts and allylic cobalt complex catalysts.

While not intending to be bound by theory, it is believed that the activation occurs due to the reduction of the cobalt species resulting in a ligand in the complex becoming labile which exposes coordination sites on the metal necessary for catalysis. For example, in the cyclopentadienyl cobalt catalyst, cobatocene is electrolytically reduced and one or more of the cyclopentadienyl reactants becomes labile which produces the catalyst moiety which is active.

The reaction may be carried out over a wide temperature range, preferably at elevated temperature in the range 0 to 170"C. In accordance with the inventive process, the electrochemical activation provides for the use of temperatures of less than 20"C.

The pressure may be in the range from 1 to 20 bar. Preferably, the pressure is from 1 to 3 bar when the catalysts is electrolytically activated. The pressure may be generated by one or more of the reactants, if they are gases at the temperature of reaction, or by a gas which is chemically inert to the contents of the reactor, such as nitrogen, helium or argon.

The process as described herein may be operated either batchwise or in a continuous fashion.

The invention will now be illustrated by the following Examples. However, these Examples should not be construed as limiting the scope of this invention hich incudes equivalent embodiments, modifications and variations which fall within the scope of the attached claims.

Examples Example 1-Generation of catalytic activity using cobalt anode and cyclopentadiene Acetonitrile (150 cm3), tetrabutylammonium tetrafluoroborate (0.1 m) and freshly cracked cyclopentadiene (1 cm3) were electrolysed at room temperature (20"C) in cell fitted with a steel working electrode, (1 /4" x 4 cm) cobalt auxilliary electrode (5 mm X 4 cm) and At/0.1 M AgNO3 in CH3CN reference electrode. Toluene (1 cm3) was added as an internal standard and a flow of acetylene was maintained (5 ml min-'). A constant potential of - 2.7V was maintained between the working electrode and the reference electrode. Samples were taken at intervals during the electrolysis. Traces of 2-picoline and benzene appeared after 1 24 min of electrolysis.

Aft 340 min the picoline concentration had reached 0.2% with a selectivity (moles picoline/(moles picoline + moles benzene) x 100) of 83%. The charge passed was 205 Coulomb, giving a catalyst efficiency of 1.24 turnovers per cobalt (moles product/moles cobalt) and 1.24 turnovers per Faraday. The cobalt auxilliary electrode (anode) had lost 0.0607 g in weight corresponding to a 96.9% current efficiency for CoeCo'l at the electrode.

In a similar experiment using indene and - 3.OV constant potential, 2-picoline appeared after 104 min of electrolysis and reached a concentration of 0.2% after 329 min when 663 Coulomb had passed.The selectivity was 90% with 1 turnover per cobalt and 0.5 turnovers per Faraday.

Anode current efficiency was 97.2%.

Example 2-Generation of catalytic activity using cobaltocene in DMF Dimethyl formamide (150 cm3), acetonitrile (50 cm3), tetrabutylammonium tetrafluoroborate (13.2 g), toluene internal standard (5 cm3, 4.2749 g) and 1.79 g cobaltocene were put in a cell under nitrogen and saturated with acetylene (flow ca. 10 ml min-1 after saturation). The electrodes consisted of a stainless steel gauze (5 cm x 5 cm) cathode and a platinised titanium gauze anode (5 cm X 5 cm). Electrolysis was carried out at room temperature using 110 mA constant current, which gave a cell voltage of 3V. After 269 min the total charge passed was 1625 Coulomb. Analysis showed the presence of 5.9% 2-picoline and 0.57% benzene. The temperature rose to ca. 60"C during the reaction.Thus a 0.047M cobaltocene solution resulted in 1 3.4 turnovers per cobalt and 8.2 turnovers per Faraday. 1.97 Faraday per cobalt were passed.

Example 3-Generation of catalytic activity using cobaltocene in (MeO)3PO solvent Trimentyl phosphate (150 cm3), acetonitrile (29 cm3), tetrabutylammonium tetrafluoroborate (0.2M), toluene internal standard (1 cm3) and cobaltocene (0.52 g) were put in a cell under nitrogen and saturated with acetylene. The acetylene flow was then adjusted to 10 cm3 min-1.

The electrode consisted of a mild steel working electrode (4 cm x 1 /4" dia) and a carbon auxilliary electrode (4 cm X 0.9 cm dia) with a reference electrode of Ag/0. 1 m AgNO3 in acetonitrile. A p.d of - 3.0V between the reference and working electrodes was maintained resulting in a current of 20-30 mA. The cell voltage was 6V. After 340 min, 520 Coulomb had passed. Analysis showed 5.7% 2-picoline concentration and 0.28% benzene. The temperature rose to ca. 60"C during the reaction. The a 0.018M cobaltocene solution gave 37.8 turnovers 2-picoline per cobalt and 23.9 per Faraday at 94 mile % selectivity. 1.58 Faradays per cobalt were used.

Claims (10)

1. A process for the preparation of pyridines comprising contacting an alkyne and a cyanide in a reaction vessel, containing a) a solvent, b) an electrolyte, c) cyclopentradiene or a substituted cyclpentadiene, d) one or more anodes containing cobalt or an alloy of cobalt, and e) one or more cathodes and passing an electrical current between the anodes and the cathodes.

2. The process of any preceding claims wherein the electical current generates a current density from 1 to 500 mAcm-2 at the electrode surface.

3. The process of Claim 2 wherein the mole ratio of alkyne to cyanide is from 0.01:1 to

4. The process of any preceding claims wherein the alkyne is selected from acetylene, a mono- or dialkyl acetylene and a mono- or diaryl acetylene and the cyanide is a compound which has a cyano grouping, is aprotic and is inert to the electrolyses conditions.

5. The process of any preceding claims wherein the alkyne is acetylene and the cyanide is acetonitrile.

6. A process for the activation of a cobalt complex catalyst, useful in the production of pyridines from the reaction of an alkyne and a cyanide, into its catalytically active species, the activation comprising contacting the catalyst with an electrical current.

7. The process of Claim 6 wherein the cobalt complex catalyst is present in solution in an electrochemical cell containing at least two electrodes, an electrolyte, an alkyne and a cyanide.

8. The process of Claim 7 wherein the catalyst is prepared in the electrochemical cell prior to the activation of the catalyst, the preparation comprising contacting an alkyne, a cyanide and cyclopentadiene or substituted cyclopentadiene in the presence of an electrical current and wherein at least one electrode is a anode containing cobalt or an alloy of cobalt.

9. The process of Claim 8 wherein the cobalt complex catalyst is selected from a cyclopentadienyl cobalt complex or a substituted cyclopentadienyl cobalt complex.

10. The process of Claim 9 wherein the pressure employed is from 1 to 3 bar.