Patent Application
20050167283
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1. Field of Invention
The present invention concerns electrochemical preparation of heterocyclic and mixed heterocyclic/carbocyclic diaryliodonium salts in an undivided electrolytic compartment or cell.
2. Description of Prior Art
The electrochemical formation of diaryliodonium salts is known for benzene plus iodobenzene (see Wendt: H. Hoffelner, H. W. Lorch, H. Wendt, Journal of Electroanalytical Chemistry, 66 (1975), pp 183-194) and toluene plus iodobenzene (see Miller: Larry L. Miller, A. K. Hoffman, JACS 89 (1967), pp 593-597). Both references require expensive platinum electrodes and the use of divided electrochemical cells, an expensive alternative, and do not anticipate the undivided cell process or the use of heterocyclic reactants.
In U.S. Pat. No. 5,277,767 Cushman teaches making diaryl iodonium salts with carbocyclic rings of 6 to 11 carbon atoms, but not heterocyclic rings, in an undivided cell and with carbon electrodes and acetic acid solvent and sulfuric acid electrolyte. U.S. Pat. No. 5,277,767 does not teach the reacting of heterocyclic ring reactants.
In U.S. Pat. No. 6,419,814 Pletcher teaches making diaryl iodonium salts containing heterocyclic rings. Pletcher teaches that ring substituents should be electron donating groups at least as potent as hydrogen. Pletcher teaches the use of alkyl, alkoxy, aryl and aralkyl substituents. Pletcher does not anticipate the use of substituents on the rings that are more electron withdrawing than hydrogen.
In U.S. patent application Ser. No. 10/753,588 Wojcik teaches making heterocyclic and mixed heterocyclic/carbocylic iodonium salts in two steps. This discovery teaches one step can be used.
This present invention is directed to the electrolytic process for the preparation of heterocyclic or mixed heterocyclic/carbocyclic diaryliodonium salts comprising charging an electrolytic cell fitted with an anode and cathode in a single compartment with a reaction mixture comprising an iodoaryl compound, an aryl compound, a stable electrolyte, and a solvent, and applying an electric potential to the cathode and anode under conditions to promote formation of the desired heterocyclic or mixed heterocyclic/carbocyclic diaryliodonium salt product in essentially a single step.
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While this invention may have many different forms, the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.
The iodoaryl compound employed as a starting material in the process of the present invention is a heterocyclic compound containing at least 4 carbon atoms and an oxygen, nitrogen, or sulfur atom or carbocyclic aromatic compounds containing 6 to 11 carbon atoms. The iodoaryl compound can be substituted with electron donating or withdrawing groups such as halides, alkyl groups having 1 to 12 carbon atoms, vinyl groups, carboxylic acids or esters, ethers, and the like. Preferred iodoaryl compounds include iodotoluene.
The aryl compound employed as a starting material in the process of the present invention is a heterocyclic compound containing at least 4 carbon atoms and an oxygen, nitrogen, or sulfur atom or carbocyclic aromatic compounds containing 6 to 11 carbon atoms. The aryl compound of the invention is distinguished from the iodoaryl compound of the invention in that the latter is substituted with iodine and the former may or may not. It is possible that the aryl compound can be substituted with either electron donating or withdrawing groups such as halides, alkyl groups having 1 to 12 carbon atoms, vinyl groups, carboxylic acids or esters, ethers, and the like. Preferred aryl compounds include chlorothiophenes. The iodoaryl and aryl specie must be able to withstand the environment of the electrochemically active anode without decomposing.
In general, the optional substituents on the aryl and iodoaryl compounds can be any group or groups that do not have substantial adverse effects on the preparation of the desired heterocyclic or mixed heterocyclic/carbocyclic diaryliodonium compound.
The method of the invention is conducted using a solvent, the iodoaryl compound, the aryl compound, and electrolyte. The solvent can be selected from the group consisting of polar solvents, and preferably acyclic polar solvents. Examples of solvents suitable for use with the present invention are halogenated hydrocarbons such as dichloromethane and chloroform, acetonitrile, organic acids, and the like. The most preferred solvent is acetic acid.
The electrolyte for use in the process of the present invention is one which will conduct an electric current and not have substantial adverse effects on preparation of the desired diaryliodonium compound. Also, the electrolyte can function partially or totally as the reacting solvent. Examples of suitable electrolytes include strong acids such as p-toluene-sulfonic acid and, preferably, sulfuric acid. Other useful electrolytes include organic salts.
The electrolyte and/or solvent must be capable of contributing a negative ion as the counter ion of the heterocyclic diaryliodonium salt. Typical salts include, for example, sulfates, halides, acetates, phosphates, and the like. It may be desirable to perform an ion exchange of the counter ion.
The process of the invention is carried out in an undivided or single compartment electrolytic cell equipped with a cathode and anode. Use of an undivided cell is more economical than the use of a divided cell.
The nature of the anode for use in the process of the invention is important to achieve increased current efficiency. The anode is comprised of, or preferably consists essentially of, carbon. The form of the carbon anode is not particularly critical. Thus, the anode can be carbon felt, graphitic carbon, or carbon cloth. Graphitic carbon is preferred.
The nature of the cathode for use in the process of the invention is not critical, other than avoiding excessive overvoltage. Thus, the cathode can be comprised of zinc, platinum, nickel, cadmium, tin, copper, stainless steel, vanadium, carbon, and the like. Preferred is carbon.
The reaction mixture for the process of the present invention preferably contains a minor amount, for example about 1% to about 10% based on the total weight of the reaction mixture of a drying agent. The preferred drying agent is acetic anhydride.
To perform the process of the invention, the single compartment is charged with the reactants, solvent, drying agent, and electrolyte in any order. An electric potential preferably about 1 to 25 volts, more preferably 2 to 8 volts, depending on cell configuration and electrode spacing, is then applied to the anode and cathode. The electric potential is normally applied to the anode and cathode for a period of time of about 2 to 10 hours. The reaction can be conducted under varied conditions. For example, temperatures of about 15 C to about 60 C and pressures of 1 atm to 5 atm are typical. In general, solution electrical conductivity increases as temperature is raised from room temperature to the boiling point of one of the reactants or solvents. The electric potential is generally applied to the anode and cathode as a constant electric potential, but may be applied as constant current and may be applied as direct current, pulsing or constant, or alternating current.
The molar ratio of the iodoaryl compound: aryl compound is preferably about 40:1 to about 1:40. The amount of electrolyte can vary widely, 0.05% to about 5% of the reaction mixture.
The invention is further illustrated by the following non-limiting example:
In a cell was placed 1.91 grams p-iodotoluene, 2.12 grams 2-chlorothiophene, 2.0 grams sulfuric acid, 43 grams glacial acetic acid, and 1 gram acetic anhydride. The contents of the cell were blanketed with a dry nitrogen layer. Carbon electrodes were used for cathode and anode. The mixture was electrolyzed at a constant current of 50 mA until 1,600 Coulombs of charge had passed. The cell contents were removed and liquids reduced under reduced pressure. The 4-methylphenyl, 2-(2-chlorothienyl) iodonium cation was recovered as the chloride.
