Process for preparation of trimethylsulfoxonium salts

A method for the production of trimethylsulfoxonium salts is described in which trimethylsulfonium salts are oxidized with ruthenium tetroxide in the presence of an inert solvent to form the corresponding trimethylsulfoxonium salts.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the preparation of 
trimethylsulfoxonium salts. Trimethylsulfoxonium salts have a number of 
uses within the chemical industry, and are particularly useful when 
utilized as the trimethylsulfoxonium salt of N-phosphonomethylglycine 
which is an effective herbicide. The salts are not easy to produce. One 
method disclosed in the prior art, U.S. Pat. No. 3,534,105, involves 
reacting a sulfonium salt with aqueous hydrogen peroxide in the presence 
of an inert solvent at a temperature of from 20.degree. to 100.degree. C. 
Efforts to duplicate the process disclosed in that patent have not been 
successful. Another method of manufacture involves the reaction of 
dimethyl sulfoxide with methyl iodide, as described by R. Kuhn and H. 
Trischmann, Ann., 611, 11721 (1058). The disadvantage of this method is 
that it involves the use of expensive methyl iodide and the reaction is 
very slow. Still another method involves the reaction of dimethyl 
sulfoxide with methyl bromide, this reaction being described in U.S. Pat. 
No. 4,141,920. The disadvantage of this reaction is that it requires 
greater than atmospheric pressure, the reaction is slow, and explosions 
during the reaction have been reported. Consequently alternative methods 
of producing trimethylsulfoxonium salts are continually being sought. The 
present invention relates to one such alternative method of producing the 
salts. 
DESCRIPTION OF THE INVENTION 
It has now been discovered that trimethylsulfoxonium salts can be produced 
in good yields by the oxidation of trimethylsulfonium salts with ruthenium 
tetroxide (also known as ruthenium (VIII) oxide) in the presence of water 
or other inert solvent. The trimethylsulfonium salts used as starting 
compounds in the process of the instant invention have the general formula 
EQU (CH.sub.3).sub.3 S.sup.+ X.sup.-. 
wherein X represents an anion selected from the group consisting of 
chloride, bromide, methosulfate, alkyl or aryl sulfonate wherein the alkyl 
group is from 1 to 10 carbon atoms and the aryl group is from 6 to 12 
carbon atoms. 
Ruthenium tetroxide is used in the process of the invention to oxidize the 
trimethylsulfonium salts. No other oxidant has been found to be suitable. 
The tetraoxide can be produced by any method and used alone in the process 
in stoichiometric amounts. Any source of a soluble, low valence ruthenium 
species can be used, however, the most convenient method to prepare it is 
to prepare it in situ in catalytic amounts from ruthenium dioxide hydrate 
or ruthenium [III] chloride with a suitable co-oxidant being present in 
stoichiometric amounts. The co-oxidant must be capable of oxidizing Ru 
from the III or IV to VIII oxidation states. Suitable co-oxidants are 
sodium periodate, sodium hypochlorite, and tertiary-butylhypochlorite, for 
example. In the process of the invention, the reaction of ruthenium 
tetroxide and the trimethylsulfonium salts regenerates ruthenium dioxide, 
which is then reoxidized to ruthenium tetroxide by the co-oxidant to 
continue the catalytic cycle. 
The preferred trimethylsulfonium salt for use as starting material in the 
process of the invention is trimethylsulfonium chloride and when this 
preferred salt is used in conjunction with ruthenium tetroxide and 
co-oxidant sodium hypochlorite in water as solvent, the reaction can be 
represented as follows: 
##STR1## 
Another method of preparing the ruthenium tetroxide is to generate it from 
the reaction of ruthenium dioxide hydrate with sodium hypochlorite or any 
of the co-oxidants listed above. 
While the amount of ruthenium tetroxide used must be sufficient to catalyze 
the reaction the exact amount is not critical. 
The preferred inert solvent for use in the process of the invention is 
water, however other solvents such as organic solvents inert under the 
reaction conditions such as carbon tetrachloride or chloroform can also be 
used. The solvent is normally used in excess, although the exact amount is 
not critical. When organic solvents such as carbon tetrachloride are used, 
they are normally used in conjunction with water, forming an aqueous phase 
and an organic phase. 
The instant process yields end product trimethylsulfoxonium salts in 
substantial quantities at a higher efficiency level than can be achieved 
by the methods of the prior art.

This invention will be more fully understood by reference to the following 
examples. 
EXAMPLE I 
Preparation of Trimethylsulfoxonium Chloride 
A solution of 50% trimethylsulfonium chloride in water was prepared, and to 
5 grams (g) of this solution was added 0.05 g of ruthenium dioxide hydrate 
and 10 milliliters (ml) of water. Thereafter, sodium metaperiodate (4.77 
g) was added to the reaction mixture in portions over a period of several 
hours. A thick slurry of sodium iodate formed, and this mixture was 
filtered. The filtrate was analyzed by nuclear magnetic resonance (nmr) 
spectroscopy and was found to contain trimethylsulfoxonium chloride with 
only a trace (&lt;2%) of unreacted trimethylsulfonium chloride. No other 
products were detected. 
EXAMPLE II 
Preparation of Trimethylsulfoxonium Chloride 
A mixture of 5 g of trimethylsulfonium chloride, 3 g of water, and 3 mg of 
ruthenium dioxide hydrate was stirred under a chlorine atmosphere. A 
solution of sodium hypochlorite (11.5%) in water was added over 5.5 hours. 
The pH of the mixture remained below 7 throughout the reaction. The 
aqueous product mixture was extracted three times with chloroform to 
remove ruthenium tetroxide. Analysis of the product solution by nmr showed 
78% conversion to trimethylsulfoxonium chloride. 
The process of the invention is preferably carried out at a pH of between 
about 2 to about 7. Normally speaking, the incipient pH of the reaction 
mixture will be on the acidic side. When concentrated sodium hypochlorite 
is used as a co-oxidant, however, the reaction mixture becomes alkaline 
and in that instance the pH should be adjusted downward. The pH of the 
solution can be adjusted with an acid such as HCl to achieve the correct 
balance. The pH can also be adjusted by the use of a chlorine atmosphere 
over the reaction solution. The other co-oxidant reagents which can be 
used are more or less neutral and do not require the addition of acid. 
Even use of a low concentration hypochlorite solution (i.e., 5.25%) does 
not require an adjustment of the pH. 
The reaction is preferably carried out at ambient temperature. The 
temperature is not critical, however. 
In the practice of the process of this invention, it is convenient to 
generate sodium hypochlorite in situ by the addition of sodium hydroxide 
solution to the reaction mixture of the sulfonium salt, ruthenium 
catalyst, and water, with agitation under a chlorine atmosphere. Thus, pH 
is maintained below 7 as is required by the reaction, and the volume of 
water added to the reaction may be reduced. 
Excess co-oxidant may be used to drive the reaction to completion. 
It will be appreciated by those skilled in the art that variations in the 
amounts of constituents used, as well as temperature, and other reaction 
conditions, etc., can be made without departing from the spirit and scope 
of the invention as described herein.