JP2012524171A - Method for producing reactive zinc by electrochemical reduction - Google Patents

Abstract

  The present invention relates to a process for producing reactive zinc by electrochemical reduction, in which iron or steel is used as the cathode material.

Description

  The present invention relates to a process for producing reactive zinc by electrochemical reduction, in which iron or steel is used as the cathode material.

  There is a great demand for a process for producing reactive zinc as a starting material for functionalized organometallic building blocks. These building blocks are used, for example, in the construction of pharmacologically relevant agents or complex pesticides. Zinc organyl, which is available from reactive zinc, can then be used in transition metal mediated coupling to C, C bonds, in which case allyl halides, aryl halides, alkenyl halides are used. And alkynyl halides can be used as coupling partners. Furthermore, zinc organyl can be added to the carbonyl compound, in which case the chiral auxiliary reagent makes these transformations even stereoselective.

  Direct synthesis of zinc organyl starting from elemental zinc is only possible in a few cases based on a passivated ZnO layer. These include so-called Reformatsky reagents, their synthesis starting from commercially available zinc powder and α-haloacetates. In addition, reactive halogen compounds, in particular alkyl iodides, can also react with unactivated zinc powder. The disadvantage of this reaction is that it is possible to obtain exclusively α-haloacetic acid esters or alkyl iodide zinc organyls and not other functionalized zinc organyls, and this process is very limited. And extremely substrate specific.

  Many zinc organyls, however, are not available starting from elemental zinc which is not activated. Various methods have been described in the state of the art for the activation of zinc and subsequent synthesis of the corresponding zinc organyl.

  P. Knochel describes various methods for obtaining zinc organil in the Handbook of Functionalized Organometallics-Applications in Synthesis, Wiley-VCH Verlag Weinheim, 2005. These include metal exchange reactions, chemical activation of zinc as well as the production of reactive zinc by chemical reduction.

  A metal exchange reaction is understood to be a reaction between a metal organyl and usually an inorganic metal salt, in which the organyl moiety is transferred from one metal to the other. Starting from Li and Mg organyl, a corresponding variety of zinc organyls can also be produced. A major disadvantage of this method is that only unfunctionalized metal organyls can often be produced, since many functional groups are not compatible with Li and Mg organyls. Functional groups such as nitriles, carboxylic acid esters, ketones or tertiary amides, on the other hand, are attacked according to the addition of Li-organyl or Mg-organyl and are therefore no longer available for further reactions. Other functional groups with moderately acidic protons such as acetylides, secondary amides or nitro compounds can be deprotonated by strong metal organyls, so that they are no longer available for further reactions. Not possible.

  The chemical activation of zinc is carried out in a classical manner using LiCl, iodine, dibromoethane or TMSCl as an auxiliary. All these reagents are used to overcome the passivated ZnO layer. The disadvantage of these reactions is that chemical auxiliaries must be added in sub-stoichiometric or stoichiometric amounts and must not be disturbed in subsequent reactions of zinc organyl. Therefore, the use of these zinc organyls is only possible with limitations.

Rieke® zinc is a reagent that contains reactive zinc and is obtained by chemical reduction of ZnCl ₂ with lithium metal in the presence of naphthalene. This material is characterized by a very high reactivity compared to chemically activated zinc. This reactivity results from the generation of zinc under oxygen-free and water-free conditions, thereby avoiding the generation of a passivating oxide layer. Disadvantageous to this reaction is that lithium must be used in stoichiometric amounts, resulting in high raw material costs and accepting more costs for safe handling of reactive alkali metals. It is not to be.

P. Knochel also describes the electrochemical activation of zinc in the Handbook of Functionalized Organometallics-Applications in Synthesis, Wiley-VCH Verlag Weinheim, 2005. International Publication (WO-A) 01/02625 describes electrochemical reduction catalyzed by transition metals. In this case, dissolution of the zinc anode takes place at the anode, whereby Zn ²⁺ is produced in solution. At the same time, the transition metal is reduced on the cathode, after which it inserts into the C-halogen bond and moves the organic group onto Zn ²⁺ . Possible transition metals are Ni, Co and Fe. Disadvantageous to this method is the presence of transition metals in later products. The generated zinc organyl is inherently contaminated with transition metals, which can then be found again in the product in subsequent stages. Especially in the synthesis of pharmacological agents, zinc organyl from the above-mentioned method is not suitable for this, since contamination with transition metals must however be avoided.

N. Kurono, T. Inoue and M. Tokuda describe another method for producing reactive zinc by electrochemical reduction, so-called electrolytically prepared zinc (EGZn), in Tetrahedron 2005, 61, 11125-11131. In this method, Zn is dissolved at the anode in order to generate Zn ²⁺ ions in the solution. In the following, Zn ²⁺ is reduced directly by a redox mediator such as naphthalene or also on the cathode, and elemental zinc is formed in the solution. This is highly reactive due to insertion into the C-halogen bond, since it does not contain a passivating oxide layer. Disadvantages of this method are the use of Pt electrodes that are not usable on an industrial scale and are too expensive and low temperatures in the range of 0-10 ° C. that would result in increased costs. is there.

  The object of the present invention is therefore to provide a process in which reactive zinc can be produced as inexpensively as possible without the use of chemical reducing agents or reagents and which can also be used on an industrial scale. That is.

This task involves the following steps:
a) preparing an electrolysis cell comprising a cathode and a zinc anode;
b) The electrolysis cell contains a conductivity salt selected from the group of quaternary ammonium salts, organometallic salts and inorganic metal salts, and N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl Filling an electrolyte selected from the group of pyrrolidone and other tertiary amides;
c) applying a current to the cell until a 2-20% suspension of reactive zinc is formed in the electrolyte, wherein an iron or steel cathode is used as the cathode and electrochemical reduction Is solved by a process for the production of reactive zinc, which is carried out at a temperature in the range of 20-60 ° C.

  The method according to the invention is advantageous when N, N-dimethylformamide is used as the electrolyte.

  The process according to the invention is advantageous when tetrabutylammonium tetrafluoroborate is used as the conductivity salt.

  In the method according to the invention, the electrolyte further comprises naphthalene, N, N-dimethyl-1-naphthalene and another 1-substituted naphthalene and phenanthrene, anthracene, 4,4'-dipyridyl, and 4,4'-di It is advantageous if it contains a redox mediator selected from the group of t-butylbiphenyl.

  The process according to the invention is advantageous when the temperature at which the electrochemical reduction is carried out is in the range from 35 to 45 ° C.

The method according to the invention is advantageous when it is adjusted to the current density of 1~4A / dm ^2.

  The method according to the invention is advantageous when undivided electrolysis cells are used.

  The method according to the invention is advantageous when an iron or steel tube is used as the cathode and the zinc anode is arranged concentrically inside the cathode.

  The process according to the invention is advantageous when carried out batchwise.

  The process according to the invention is advantageous when carried out continuously.

  In the process according to the invention, activated zinc is produced in the electrolysis cell by electrochemical reduction of zinc ions provided by dissolution of the zinc anode. For this purpose, each electrolysis cell known to the person skilled in the art, for example a split or non-split flow cell, a capillary gap cell or a plate gap cell, is suitable. Non-divided flow cells are preferred.

  The electrolysis cell is provided with a zinc anode and an iron or steel cathode in the process according to the invention. Each shape known to those skilled in the art of iron cathode or steel cathode as the cathode is, for example, rod-shaped, as a thin plate, as a thin iron plate or steel plate formed into a tube, as a thin iron plate or steel plate formed into a conical shape, Is suitable.

  The zinc anode itself may also have each shape known to the person skilled in the art, for example as a rod, as a thin plate, as a taper / cone or as a bulk electrode. Particular preference is given to zinc anodes in the form of rods, cylinders or cones.

  For the implementation of the method according to the invention, each arrangement known to those skilled in the art of the anode with respect to the cathode is possible, for example, a face-to-face arrangement, a parallel plane arrangement or a concentric arrangement in which the anode is located concentrically inside the cathode. . The zinc anode is preferably arranged concentrically inside the cathode.

  The electrolysis cell is filled with an electrolytic solution. The electrolyte is then selected from the group of N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidone and other tertiary amides. N, N-dimethylformamide and N, N-dimethylacetamide are preferred. The use of N, N-dimethylformamide is particularly preferred.

  The electrolyte also contains a conductivity salt selected from the group of quaternary ammonium salts, organometallic salts and inorganic metal salts for the process according to the invention. Tetrabutylammonium tetrafluoroborate, sodium methylsulfonate and zinc chloride are preferred. Very particular preference is given to tetrabutylammonium tetrafluoroborate.

  As another additive, it is advantageous if the electrolyte contains a redox mediator. This is preferably naphthalene, N, N-dimethyl-1-naphthalene and another 1-substituted naphthalene and phenanthrene, anthracene, 4,4'-dipyridyl, and 4,4'-di-t-butylbiphenyl. Selected from the group. Naphthalene is particularly preferred.

  For the process according to the invention, the electrolyte is heated to a temperature in the range of 20-60 ° C., preferably in the range of 30-50 ° C., very particularly preferably in the range of 35-45 ° C. Temperature control is performed through a heat exchanger incorporated in the electrolyte circuit.

In the case of the method according to the invention, a current density in the range of 1-4 A / dm ² is applied on the cathode and anode. Preferably, the current density is in the range of 1.5~3A / dm ^2. Particularly preferably, it is in the range of 1.5 to 2.5 A / dm ² .

  The electrolysis is terminated when the solid content of reactive zinc in the electrolytic solution is in the range of 2 to 20% by mass, particularly preferably in the range of 2 to 10% by mass.

  The process according to the invention can be carried out batchwise or continuously. In the case of the continuous operation method, an electrolytic solution having a reactive zinc content in the range of 2 to 20% by mass, preferably in the range of 2 to 10% by mass is discharged from the cell. At the same time, the same amount of fresh electrolyte is post-filled. This is done until the zinc anode has to be replaced based on almost complete decomposition.

  In the process according to the invention, it is advantageous if the electrolyte is pumped during electrolysis when using a tube cathode surrounding the anode. A pump circulation output of 100 to 600 L / h, particularly preferably 300 to 600 L / h is preferred.

a) Reactive zinc in beaker cells: current efficiency (9120-155)
In a beaker cell equipped with a zinc anode and an iron cathode (electrode dimensions 70 × 20 × 3 mm, immersion area 45 × 20 mm, spacing 9 mm), 0.65 g Bu ₄ NBF _{4 in} 61.00 g DMF and naphthalene 1. Charge 25g. After heating the electrolyte to 40 ° C., electrolysis is started with a current strength of 0.2 A (corresponding to a current density of ² A / dm ² ). In the process of electrolysis, the electrolyte becomes dark and the voltage drops from 9.0V to 5.5V. After 12 hours of operation time, the electrolysis is over. A finely dispersed dark suspension of zinc is obtained.
Elemental analysis of the electrolyte gives a zinc (0) value of 2.7%, which corresponds to a current efficiency of about 60%.

b) Reactive zinc in beaker cell: Reactive (9120-172)
0.64 g Bu ₄ NBF ₄ and 1.30 g naphthalene in 61.00 g DMF in a beaker cell with a zinc anode and an iron cathode (electrode dimensions 70 × 20 × 3 mm, immersion area 45 × 20 mm, spacing: 9 mm, respectively) Is charged. After heating the electrolyte to 40 ° C., electrolysis is started with a current strength of 0.2 A (corresponding to a current density of ² A / dm ² ). In the process of electrolysis, the electrolyte becomes darker and the voltage rises from 8.0V to 8.7V. After 3.4 hours of operation time, the electrolysis is complete. A finely dispersed dark suspension of zinc is obtained.

  In order to investigate the reactivity of the electrochemically produced zinc, 1.00 g of 2-bromopyridine is added into the electrolysis effluent and the reaction mixture is heated to 80 ° C. for 0.5 h. After cooling, 2 mL of the reaction mixture is mixed with 4 mL of water and extracted with 4 mL of MTBE (methyl-t-butyl ether). A gas chromatographic investigation of the organic phase shows 86% conversion of 2-bromopyridine to pyridine (Example a), according to 60% current efficiency, which corresponds to 70% reactivity. To do).

c) Reactive zinc in tube cell: Current efficiency at full pump output (9120-169)
Steel tube (φ = 5.0 cm, l = 55 cm, active electrode area 864 cm ² ) as cathode and zinc rod (φ = 3.7 cm, l = 55 cm, active) arranged concentrically with cathode as anode In an electrolysis cell having an electrode area of 639 cm ² ), an electrolytic solution consisting of 25 g of Bu ₄ NBF ₄ and 50 g of naphthalene in 2425 g of DMF is pumped at 40 ° C. with a pump power of 600 L / h. Over 8.4 h, 12.8 A is applied to the electrolysis cell, with the voltage rising from 6.0 V to 6.8 V. A finely dispersed dark suspension of zinc is obtained.
Elemental analysis of the electrolyte gives a zinc (0) value of 3.4%, which corresponds to a current efficiency of 68%.

d) Reactive zinc in tube cell: Current efficiency at half pump output (9120-190)
A steel tube (φ = 5.0 cm, l = 55 cm, active electrode area 864 cm ² ) as a cathode and a zinc rod (φ = 3.7 cm, l = 55 cm) disposed inside and concentrically with the cathode as an anode, In an electrolysis cell having an active electrode area of 639 cm ² ), an electrolytic solution consisting of 25 g of Bu ₄ NBF ₄ and 50 g of naphthalene in 2425 g of DMF is pumped at 40 ° C. with a pump power of 300 L / h. Over a period of 8.4 h, 12.8 A is applied to the electrolysis cell, where the voltage first rises from 6.5 V to 9.9 V and then further decreases to 1.2 V. A finely dispersed dark suspension of zinc is obtained.
Elemental analysis of the electrolyte gives a zinc (0) value of 4.1%, which corresponds to a current efficiency of 82%.

Claims (10)

A method for producing reactive zinc comprising the following steps:
a) preparing an electrolysis cell comprising a cathode and a zinc anode;
b) The electrolysis cell contains a conductivity salt selected from the group of quaternary ammonium salts, organometallic salts and inorganic metal salts, and N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl Filling an electrolyte selected from the group of pyrrolidone and other tertiary amides;
c) applying a current to the cell until a 2-20% suspension of reactive zinc is formed in the electrolyte, wherein an iron or steel cathode is used as the cathode and electrochemical reduction At a temperature in the range of 20-60 ° C.,
A method for producing reactive zinc.   The method according to claim 1, wherein N, N-dimethylformamide is used as the electrolytic solution.   The method according to claim 1, wherein tetrabutylammonium tetrafluoroborate is used as the conductivity salt.   The electrolyte further includes naphthalene, N, N-dimethyl-1-naphthalene and another 1-substituted naphthalene and phenanthrene, anthracene, 4,4'-dipyridyl, and 4,4'-di-t-butylbiphenyl. The method according to any one of claims 1 to 3, comprising a redox mediator selected from the group consisting of:   The process according to any one of claims 1 to 4, wherein the temperature at which the electrochemical reduction is carried out is in the range of 35 to 45 ° C. 6. The method according to claim 1, wherein the current density is adjusted to 1 to 4 A / dm < ² >.   7. A method according to any one of claims 1 to 6, wherein an undivided electrolysis cell is used.   The method according to claim 1, wherein an iron pipe or a steel pipe is used as the cathode, and the zinc anode is arranged concentrically inside the cathode.   9. The process as claimed in claim 1, wherein the process is carried out batchwise.   The method according to claim 1, wherein the method is carried out continuously.