Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
1999-04-30
2001-03-27
Wong, Edna (Department: 1741)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C568S038000, C568S054000, C568S058000, C205S422000, C205S431000, C205S444000
Reexamination Certificate
active
06207838
ABSTRACT:
TECHNICAL FIELD
This invention relates to electrochemical generation of organothiating agents, which may then be reacted with appropriate substrates to effect the synthesis of organothioaromatic compounds.
BACKGROUND
Electrophilic thiation is typically accomplished by the use of a sulfenyl chloride, disulfide, or thiosulfonate, with a Lewis or Bronsted acid. Silica gel has been used to catalyze the reactions of sulfenyl chlorides with arenes, while zeolites have been used for catalysis of hydrocarbyl disulfide reactions with aromatic alcohols. A method has also been reported for thiation using electrochemically generated sulfenium ion in dichloromethane, with low yields; see Do et al.,
Tetrahedron Lett.,
1998, 4657. Electrochemical preparation of organothiating agents is desirable because organothiation can be achieved in the absence of a Lewis acid catalyst; high yields of organothiated products are also desirable.
THE INVENTION
This invention provides for the electrochemical generation of organothiating species from organic disulfides (R
2
S
2
) in liquid sulfur dioxide, and further provides for the reaction of the organothiating species thus generated with appropriate substrates to effect the synthesis of organothioaromatic compounds. Because the organothiating agent is generated electrochemically, no catalyst is required; the absence of a catalyst minimizes the formation of side products. Moreover, the process of the invention provides organothiated products in high yield. Another advantageous feature of this invention is that it enables the organothiation of strongly to weakly activated aromatic compounds in high yield. In fact, when the substrate is a phenol, a high yield of the para-organothiated product is obtained, even though no catalyst, much less a para-directing catalyst, is used in the process. In short, this invention makes possible non-catalytic regioselective organothiation in a wide variety of aromatic compounds.
Thus, in accordance with one of the embodiments of this invention, there is provided a process for generating an organothiating agent in liquid sulfur dioxide via electrolysis of an organic disulfide, such as an alkyl disulfide, in which the organic groups are free of nonaromatic unsaturation.
Another embodiment of the invention is a process which comprises generating an organothiating agent in liquid sulfur dioxide via electrolysis of an organic disulfide in which the organic groups are free of nonaromatic unsaturation, and contacting all or a portion of the resultant solution from the electrolysis with an organothiatable substrate. In this way, a wide variety of useful organothiated aromatic compounds can be produced with high efficiency and in high yield.
These and other embodiments and features of this invention will be apparent from the ensuing description and appended claims.
The organic disulfides utilized in the practice of this invention can be represented by the formula R—S—S—R where R is the organic group, each of which typically contains no more than about 24 carbon atoms. Although there are two organic groups in the molecule, when the organic groups are the same, such compounds are often named, for example, as methyl disulfide instead of dimethyl disulfide. The organic groups of the organic disulfides utilized in the practice of this invention can be hydrocarbyl groups (i.e., the organic groups consist of carbon and hydrogen), or they can be functionally substituted hydrocarbyl groups wherein the substituent(s) on the hydrocarbyl group are innocuous in the sense that they do not materially interfere with the formation of the organothiating agent. Tertiary hydrocarbyl disulfides are not desired as organic disulfides in this invention because they do not generate thiating agents; see Elothmani et al.,
J. Chem. Soc., Chem. Comm.,
1993, 715. Because alkenes are known to react with thiating agents, nonaromatic unsaturation is also undesirable in the organic disulfide. Preferably, the organic disulfide used is a hydrocarbyl disulfide.
The hydrocarbyl groups of the disulfides used in the practice of this invention can be primary or secondary aliphatic, cycloaliphatic, or aromatic hydrocarbyl groups. Of such hydrocarbyl disulfides, preferred are the primary aliphatic disulfides. The aliphatic hydrocarbyl groups can be linear or branched. Preferably, the hydrocarbyl groups will each contain up to about 18 carbon atoms, and more preferably, up to about 8 carbon atoms.
The organic disulfides in which the organic groups contain one or more innocuous functional substituents are compounds in which the substituents are, for example, halogen atoms, alkoxy groups, aryloxy groups, nitro groups, esterified carboxyl groups, nitrile groups, heterocyclic groups in which the heteroatom(s) is/are oxygen, and the like.
Examples of suitable organic disulfides include methyl disulfide, ethyl disulfide, 2-hydroxyethyl disulfide, propyl disulfide, 3-carboxypropyl disulfide, isopropyl disulfide, n-butyl disulfide, sec-butyl disulfide, 2,2,4,4-tetramethylcyclobutyl disulfide, heptafluorocyclobutyl disulfide, pentyl disulfide, cyclopentyl disulfide, cyclohexyl disulfide, cyclooctyl disulfide, 2-methylphenyl disulfide, 4-methylphenyl disulfide, 3-nitrophenyl disulfide, 1-naphthyl disulfide, 2-naphthyl disulfide, and the like. Preferred hydrocarbyl disulfides are the primary aliphatic disulfides, RCH
2
—S—S—CH
2
R, where R is a hydrocarbyl group, preferably having up to about 17 carbon atoms. Particularly preferred hydrocarbyl disulfides are methyl disulfide and ethyl disulfide, especially methyl disulfide.
The electrolysis of the organic disulfide can be carried out in a two-compartment electrochemical cell by passing current through the electrochemical cell in the presence of the organic disulfide and a supporting electrolyte. The supporting electrolyte may be any salt that is soluble in liquid sulfur dioxide, not redox active in the potential range used, and unreactive toward the electrolysis products. Particularly preferred supporting electrolytes are salts of the tetra(n-butyl)ammonium cation, especially the tetrafluoroborate salt, and more particularly the hexafluorophosphate salt.
Because the electrolysis is carried out in liquid sulfur dioxide, the electrochemical cell is usually maintained at conditions such that sulfur dioxide is a liquid. At atmospheric pressure, this condition is satisfied when the temperature is in the range of from about −73° C. to about −10° C. A preferable temperature at atmospheric pressure is in the range of from about −60° C. to about −10° C.; more preferred temperatures are in the range from about 45° C. to about −10° C. It is preferred to conduct the electrolysis at atmospheric pressure rather than under either increased or reduced pressure. However, in the event it is desired pursuant to this invention to operate at temperatures above −10° C a suitable pressure is applied to the electrolysis system so as to keep the sulfur dioxide in a liquid state of aggregation. The desirability of performing this pressurized embodiment of this invention will at least to some extent be governed by the relative costs of pressurized equipment and operation as compared to refrigeration equipment and operation.
The electrolysis may be carried out under constant current or constant potential with the potential maintained in a range from about 0.5V less than the oxidation potential of the organic disulfide to about 1.0 V more than the oxidation potential of the organic disulfide. Preferably, the potential is in a range of from about 0.3 V less than the oxidation potential of the organic disulfide to about 0.5V more than the oxidation potential of the organic disulfide. Actual potential values will vary, depending on the redox potential of the chosen organic disulfide and choice of reference electrode. The current passed during the electrolysis corresponds to the amount of electricity in the range of from about 90000 Coulombs to about 205000 Coulombs per mole of organic disulfide. A preferred range is from about 1
Glass Richard S.
Jouikov Viatcheslav V.
Spielman, Jr. E. E.
The University of Arizona
Wong Edna
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