Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing organic compound
Reexamination Certificate
2000-11-02
2002-07-16
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing organic compound
C205S422000, C205S424000, C205S425000, C205S456000
Reexamination Certificate
active
06419814
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to iodonium salts, and more particularly, to high yield methods for the electrochemical synthesis of both known and novel symmetrical and unsymmetrical diaryliodonium salts, and others, such as polyiodonium salts, cyclic iodonium salts, and so on, which methods provide for greater product selectivity and improved economics.
BACKGROUND OF THE INVENTION
Diaryliodonium salts have a variety of use applications. A few representative examples include photoinitiators (U.S. Pat. Nos. 4,136,102 and 3,981,897) , fungicides (U.S. Pat. Nos. 3,944,498 and 3,763,187), bactericides (U.S. Pat. Nos. 3,885,036 and 3,712,920), many active chemicals, including pharmaceutical intermediates, and so on.
Diaryliodonium salts were first produced by the chemical route, but often relied on costly reagents. In some instances, hazardous by-products were synthesized which were unstable and potentially explosive. By contrast, electrochemical methods for synthesizing diaryliodonium salts have been found more attractive, in general. They avoid the use of highly toxic reagents, can be performed under more hazard-free conditions without forming potentially explosive by-products, and can operate at zero effluent, since chemical change is initiated by electron transfer at electrode surfaces. In addition, a redox reagent is not required in the chemistry, so process costs can be more economic.
One electrochemical process disclosed in U.S. Pat. No. 4,759,833 by Lentz et al provides for the simultaneous preparation of a diaryliodonium salt and an alkoxide salt. However, the process is conducted in a membrane divided electrolytic cell equipped with a platinum anode. While the Lentz et al process was an advance in the art, capital cost requirements for electrolytic cells so equipped can be high.
U.S. Pat. No. 5,277,767 by Cushman et al provides a further important advance in the electrochemical synthesis of diaryliodonium salts by conducting the reaction in a single compartment electrolytic cell equipped with more economical carbon anodes, like carbon felts, vitreous or glassy carbon, graphite carbon or carbon cloth. The process of Cushman et al is also based on the discovery that. anodic oxidation of an aryl iodide in the presence of an aromatic. hydrocarbon can be performed in an undivided electrolytic cell without the iodonium salts generated at the anode being reduced at the cathode. More specifically, the Cushman et al process demonstrated the electrochemical synthesis of diaryliodonium salts, such as ditolyliodonium salts wherein an unsubstituted or a substituted iodoaryl compound like iodotoluene, and an unsubstituted or substituted aromatic hydrocarbon, such as toluene are electrolyzed in a reaction medium. The medium consisted of a solvent for the iodoaryl compound, e.g., alcohols and organic acids, with acetic acid being most preferred. The reaction medium also included a supporting electrolyte, such as a strong acid, like sulfuric acid and a minor amount of a drying agent. The latter ranging from 1 to 10 percent by-weight of the reaction mixture. The drying agent could include an anhydride corresponding to the organic acid. That is, when acetic acid was used as the solvent, the preferred drying agent was acetic anhydride.
The electrochemical synthesis according to the methods of Cushman et al can be demonstrated by the following reaction:
Cushman et al either failed to appreciate or concern themselves with the fact that substantial portions of the costly 4-iodotoluene reactant in this electrochemical coupling reaction were also converted to iodobenzyl carboxylate ester by-products (not shown), instead of the more valuable diaryliodonium salt.
Weinberg, et al,
Proceedings Electrochem. Soc. Spring Meeting, Abstract
976, May, 1996 subsequently reported that 4-iodotoluene and toluene could be converted electrochemically to ditolyliodonium salts in high yields in a coupling reaction at the carbon anode in an undivided electrochemical cell in a reaction medium consisting of a mixture of acetic acid, 2 percent acetic anhydride and 5 percent sulfuric acid. In spite of this report, no attempt was made by Weinberg, et al to examine the generality of this anodic coupling procedure, the electrochemical synthesis of unsymmetrical diaryliodonium salts, and the discovery of important iodobenzyl ester by-products.
However, when the anodic oxidation of 4-iodotoluene in the presence of toluene was repeated by the immediate inventors using a carbon felt anode in the same reaction mixture of acetic acid, 2 percent acetic anhydride and 5 percent sulfuric acid it was found that the same iodonium salt was prepared, namely 4,4′-dimethyldiphenyliodonium cation. But, they also isolated in this reaction for the first time, 4-iodobenzyl acetate. Importantly, these inventors also discovered for the first time the yields of the less valuable iodobenzyl acetate by-product were practically as high as those of the more important 4,4′-dimethyl-diphenyliodonium salt.
Consequently, this finding meant that the more preferred process for the electrochemical synthesis of aryliodonium salts, as practiced heretofore, had generally low product selectivity, and unfavorable economics because a very significant percentage of the costly aryliodide reactants used in the process were being expended in the generation of the less valuable by-products, e.g., iodobenzyl acetate.
Accordingly, there is a need for an improved, more economic process for the electrochemical synthesis of both symmetrical and unsymmetrical diaryliodonium salts, etc., which enables greater control over selectivity over the end-product produced, and which provides significantly higher yields than achieved heretofore.
SUMMARY OF THE INVENTION
Accordingly, it is one principal object of the invention to provide for the electrochemical synthesis of organoiodonium salts, and more particularly, to methods for the electrosynthesis of known and certain novel symmetrical and unsymmetrical diaryliodonium salts at yields not heretofore readily achievable.
It is yet a further object of the invention to provide an novel and inventive method for electrolyzing reaction mixtures comprising aryliodides and/or aromatic substrates in novel reaction mediums which impart a high degree of product selectivity in electrochemical synthesis reactions.
For purposes of this invention, the term “selectivity” or “selectivities” as appearing in this specification and appended claims is defined as the percentage of an aryliodide reactant consumed that is converted to the end product desired.
The foregoing objects can be demonstrated by the electrochemical synthesis of diaryliodonium salts of the structure:
Y-Aryl-I
+
-Aryl′-Z,
which comprises the steps of:
(i) introducing an electrolysis reaction mixture into an electrochemical cell equipped with a cathode and an electrically conductive carbon anode. The electrolysis reaction mixture comprises an aryliodide, designated by Y-Aryl-I; an aromatic substrate, designated H-Aryl′-Z and a reaction medium comprising a strong acid electrolyte; a lower carboxylic acid and an acid anhydride in an amount >10 percent by-weight, i.e., at least 11. The Aryl moiety of the aryliodide comprises at least one aromatic carbocyclic ring or aromatic heterocyclic ring structure. Y of the aryliodide reactant is at least one coupling reaction promoting substituent bonded to the Aryl moiety. The Aryl′ moiety of the H-Aryl′-Z aromatic substrate comprises at least one aromatic carbocyclic ring or aromatic heterocyclic ring structure each with at least one nuclear hydrogen. Z is at least one coupling reaction promoting substituent bonded to the Aryl′, and
(ii) imposing a voltage across the anode and cathode of the electrochemical cell to electrolyze the electrolysis reaction mixture to provide improved selectivities of the diaryliodonium salts.
The present inventors discovered, in general, that selectivities of the more valuable diaryliodonium salts can be substantially increased accordin
Pletcher Derek
Weinberg Norman L.
Cornell Development LLC
Ellis Howard M.
Wong Edna
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