Process for synthesizing diorganomonothiophosphinates

A new and improved method for making diorganomonothiophosphinate compounds having the formula: ##STR1## which comprises oxidizing a secondary phosphine in aqueous media to form the corresponding secondary phosphine oxide and reacting the secondary phosphine oxide thus formed with an excess of sulfur and an hydroxide compound at elevated temperature for a time sufficient until formation of the diorganomonothiophosphinate compound is substantially complete. The new and improved process of the present invention permits the preparation of diorganomonothiophosphinate. The products are stable in aqueous systems and are useful as sulfide collectors in froth flotation beneficiation of sulfide minerals.

BACKGROUND OF THE INVENTION 
The present invention relates to a new and improved method for making 
diorganomonothiophosphinate compounds. More particularly, it relates to a 
process in aqueous media wherein a diorganophosphine is oxidized to form a 
diorganophosphine oxide, which is thereafter converted in the presence of 
sulfur and base to the corresponding diorganomonothiophosphinate compound. 
Monothiophosphinate compounds are known to be useful as metal collectors 
for beneficiating certain mineral values from ores by froth flotation 
methods. The Soviet authors, P. M. Solozhenkin et al, in an article 
entitled, "Flotation Properties of Sulfur-Containing Phosphorus 
Derivatives." Dokl. Akad. Nauk Tadzh. SSR 13, No. 4, 26-30 (1970), for 
example, disclose that diethylmonothiophosphinic acid is a useful 
collector for galena, pyrite and antimonite. From the results of their 
study, they concluded that dithiophosphinate compounds performed better 
than monothiophosphinate compounds in flotations of these particular 
minerals. 
More recently, it has been discovered, as is disclosed in 
commonly-assigned, copending application Ser. No. 675,489, filed Nov. 28, 
1984, that diorganomonothiophosphinates provide exceptionally good 
metallurgical performance in selective flotation of base metal sulfide 
minerals, such as those of copper, nickel, molybdenum, cobalt and zinc, 
with selective rejection of pyrite, pyrrhotite and other gangue sulfide 
minerals, at reduced dosages over a broad range of pH values. 
Prior art methods for making diorganomonothiophosphinate compounds are 
known. In one method, a corresponding diorganothiophosphoryl chloride is 
hydrolyzed to provide the corresponding diorganomonothiophosphinate, as 
summarized by the equation: 
##STR2## 
Another method for making diorganomonothiophosphinates is disclosed in the 
above-cited article by Solozhenkin et al, which comprises reacting 
phosphorus trichloride with primary alcohol to form dialkoxy substituted 
secondary phosphine oxide, followed by reaction with a Grignard Reagent, 
sulfur, and acidification to diorganomonothiophosphinic acid as summarized 
by the following reaction sequence: 
##STR3## 
Still another method is disclosed in the article Hoffman, H. and P. 
Schellenbeck, "Dorstelling und Eigenschafter einiger Phosphorverlunding 
mit tert.-Butylgruppen". Chem. Ber. 99 1134 (1966). In accordance with 
this method, an alkyl or aryl-substituted phosphorus dichloride is reacted 
with a Grignard Reagent to form a secondary phosphorus chloride which is 
thereafter hydrolyzed to form the secondary phosphine oxide, or it is 
disclosed that a secondary phosphine may be oxidized directly with 
hydrogen peroxide to form the secondary phosphine oxide. The secondary 
phosphine oxide, in accordance with the disclosed process, may be 
sulfurized by adding sulfur to a solution of the secondary phosphine oxide 
in benzene and heating to form the corresponding 
diorganomonothiophosphinic acid solution in benzene. This method is 
summarized as follows: 
##STR4## 
These prior art methods of making diorganomonothiophosphinate compounds 
have several shortcomings. The first two methods are not very economical 
for commercial production and therefore cannot be used to provide 
commercial quantities of diorganomonothiophosphinates for use as flotation 
reagents at a commercially acceptable price. Moreover, in the latter 
method of Hoffman et al, the need to use organic solvents, such as 
benzene, because of the instability of the diorganomonothiophosphinic acid 
products produced by the process in aqueous media, generally renders the 
method both economically and environmentally unattractive. 
It is noteworthy that in an article published in J. Org. Chem 27, 2198 
(1962), it is disclosed that a secondary phosphine sulfide cannot be 
oxidized to form a diorganomonothiophosphinate, i.e., 
##STR5## 
Accordingly, to overcome the disadvantages of the prior art methods, it is 
an object of the present invention to provide a new and improved method 
for making diorganomonothiophosphinate compounds in aqueous media in a 
commercially economical manner from readily available starting materials. 
SUMMARY OF THE INVENTION 
In accordance with this and other objects, the present invention provides a 
new and improved process for preparing diorganomonothiophosphinate 
compounds having the formula: 
##STR6## 
wherein R.sup.1 and R.sup.2 are each, independently, selected from 
saturated and unsaturated hydrocarbyl radicals, alkyl polyether radicals, 
and aromatic radicals; and such radicals optionally and independently 
substituted with polar groups selected from halogen, nitrile and nitro 
groups; or 
wherein R.sup.1 and R.sup.2 together form a heterocyclic ring having the 
formula: 
##STR7## 
wherein R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are each, 
independently, selected from hydrogen and C.sub.1 to C.sub.12 alkyl, and X 
is selected from hydrogen, alkali or alkaline earth metals and NH.sub.4 ; 
said process comprising: 
(a) providing a reactive mixture of at least one diorganophosphine compound 
of the formula: 
##STR8## 
in an aqueous medium; (b) gradually or incrementally adding an amount of 
an oxidizing agent sufficient to oxidize substantially all of the 
diorganophosphine in (a) to the corresponding diorganophosphine oxide, 
##STR9## 
(c) heating the reactive mixture of step (b) to an elevated temperature 
and adding an excess amount of sulfur and an hydroxide compound selected 
from the group consisting of water, alkali metal or alkaline earth metal 
hydroxides and ammonium hydroxide; and 
(d) thereafter, permitting the reaction to proceed until formation of the 
diorganomonothiophosphinate compound is substantially complete. 
In accordance with the present invention, a diorganophosphine is first 
oxidized to the corresponding diorganophosphine oxide by the addition of 
an oxidizing agent. The oxidation reaction is exothermic and for this 
reason, in order to control the temperature, the oxidizing agent is added 
slowly or in increments, such that the temperature of the reactive mixture 
is controlled at from about 40.degree. to 60.degree. C., preferably from 
about 50.degree. to 55.degree. C. The oxidizing agent is added in an 
amount sufficient to oxidize substantially all of the secondary phosphine 
to the corresponding secondary phosphine oxide, and generally an equimolar 
amount of oxidizing agent is used. Suitable oxidizing agents include, for 
example, air, oxygen, hydrogen peroxide, and hydrogen peroxide-liberating 
solids which give off H.sub.2 O.sub.2 when introduced in an aqueous 
mixture, such as, alkali metal perborates, alkali metal carbonate 
peroxyhydrates and histidine perhydrate. Hydrogen peroxide is especially 
preferred because it is rapid, inexpensive and readily available. 
After the secondary phosphine oxide has been obtained, it is reacted with 
an excess of sulfur and base to yield the diorganomonothiophosphinate. 
Generally, and without limitation, the sulfurization reaction is conducted 
at elevated temperatures on the order of from about 60.degree. to 
90.degree. C., preferably from about 65.degree. to about 75.degree. C., 
for a time sufficient to provide the liquid diorganomonothiophosphinate 
product. Generally, the reaction mixture is heated for a period of from 
about 1 to about 7 hours, or until formation of the 
diorganomonothiophosphinate compound is substantially complete. 
Remaining excess elemental sulfur can be readily removed from the reaction 
mixture by filtration. Depending on the concentration of the starting 
materials, the product obtained will vary from viscous oil to aqueous 
solution. In any form, the diorganomonothiophosphinate products are 
indicated by .sup.31 P NMR spectra characterized by a --71 ppm shift with 
respect to phosphoric acid (85%), used as a reference. In concentrated 
form, the product is a viscous oil which will not recrystallize, so that a 
melting point is not readily determined. The product may be diluted to any 
desired concentration for use as a metal collector flotation reagent in 
the form of an aqueous solution, or it can be added in oil form. 
The new and improved aqueous process of the present invention permits 
preparation of diorganomonothiophosphinate products in aqueous media and 
in a single reactor. The products of the process, because sulfurization is 
conducted in the presence of base, are stable in aqueous media for a 
substantial period. The starting materials are either commercially 
available or readily prepared from available materials. 
Other objects and advantages of the present invention will become apparent 
from the following detailed description and illustrative working examples. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the new and improved process of the present invention, 
diorganomonothiophosphinate compounds are prepared from diorgano, i.e., 
secondary phosphines by first oxidizing the secondary phosphine in aqueous 
media in the presence of an oxidizing agent to provide a diorganophosphine 
oxide; and thereafter sulfurizing in the presence of a base at elevated 
temperature to provide a diorganomonothiophosphinate salt. 
The secondary phosphines for use as starting materials in accordance with 
the process of the present invention, are generally represented by the 
formula: 
##STR10## 
wherein R.sup.1 and R.sup.2 are the same as defined above. Several of the 
secondary phosphines, such as the dialkyl phosphines, are commercially 
available from a number of commercial suppliers. The secondary phosphines 
may also be made in accordance with well known methods, by reacting 
phosphine (PH.sub.3) with a mono olefin in the presence of: strong bases 
as disclosed by M. M. Rauhut et al in J. Am. Chem. Soc., 81, 1103 (1959); 
or by free radical initiation, e.g. 
EQU PH.sub.3 +RCH=CH.sub.2 +initiator H.sub.2 PCH.sub.2 CH.sub.2 R+HP(CH.sub.2 
CH.sub.2 R).sub.2 +P(CH.sub.2 CH.sub.2 R).sub.3 
as disclosed in U.S. Pat. No. 2,803,597 and M. M. Rauhut et al, J. Org. 
Chem. 26, 5138 (1961), each of the above citations being specifically 
incorporated herein by reference. The secondary phosphines are easily 
separated from the primary and tertiary products by distillation. The 
addition reaction for making dialkyl, bis alkyl-, aryl alkyl- and bis- or 
di- aryl-substituted phosphines are presently very well known to those 
skilled in this art, and further details are amply provided in the 
above-cited references and elsewhere in the chemical literature. 
For the particular embodiments, wherein R.sup.1 and R.sup.2 together form a 
heterocyclic ring, the secondary phosphine starting material will comprise 
a phosphine compound having the formula: 
##STR11## 
These heterocyclic secondary phosphines may be prepared by reacting 
phosphine (PH.sub.3) with a corresponding alkyl or aryl aldehyde under 
acid catalyzed conditions. 
More particularly, and with reference to certain preferred compounds for 
illustrative purposes, 1,3,5-triisopropyl-4,6-dioxa-2-phosphacyclohexane 
(TIP) compounds. e.g. 
##STR12## 
are prepared by reacting isobutyraldehyde and phosphine at a 3:1 molar 
ratio, respectively, using molar quantities of 100% phosphoric acid, as 
catalyst. The reaction is generally complete within 1 to 2 hours at 
50.degree. C. TIP product may be isolated by distillation. A 63% yield of 
TIP with negligible amounts of side products are obtained. 
In accordance with the present process, the aqueous reaction mixture of the 
diorgano, or secondary, phosphine is carefully oxidized by addition of an 
oxidizing agent in an amount sufficient to oxidize substantially all of 
the diorganophosphine to form the corresponding diorganophosphine oxide. 
By careful oxidation is meant, that the oxidation reaction is performed by 
gradual or incremental addition of the oxidizing agent at a rate which 
provides a controlled temperature of from about 40.degree. C. to 
60.degree. C., and preferably from about 50.degree. to about 55.degree. C. 
The amount of oxidizing agent added should be sufficient to oxidize 
substantially all of the secondary phosphine and generally an equimolar 
amount of oxidizing agent is used. The time of addition will vary 
depending on the starting amounts of secondary phosphine used. Generally, 
oxidation under controlled temperature conditions will be complete with 
gradual or incremental addition of the oxidizing agent over a period of 
from about 1 to about 3 hours. 
Suitable oxidizing agents for use in the present process, as has been 
mentioned above, include oxygen, air, hydrogen peroxide, solids which 
liberate hydrogen peroxide such as alkali metal perborates, alkali metal 
carbonate peroxyhydrates and histidine perhydrate, as well as other 
peroxides and other oxidizing agents which will suggest themselves to 
those skilled in this art. The selection of a particular oxidizing agent 
is not critical, so long as it is effective to oxidize the secondary 
phosphine to the secondary phosphine oxide. Hydrogen peroxide is the 
preferred oxidizing agent for use herein, because it is inexpensive, 
readily available, and the temperature and rate of the oxidation reaction 
are easily controlled with its use. 
After substantially all of the secondary phosphine has been converted to 
the corresponding secondary phosphine oxide, the aqueous reaction mixture 
is heated to an elevated temperature of between about 60.degree. to about 
90.degree. C., and preferably from about 65.degree. C. to about 75.degree. 
C. Thereafter, an excess of sulfur and excess of an hydroxide compound are 
added to convert the secondary phosphine oxide to the corresponding 
diorganomonothiophosphinate compound. The aqueous sulfurization reaction 
in the presence of base is conducted at temperatures of between about 
60.degree. C. to 90.degree. C., and allowed to proceed substantially to 
completion. Generally, the reaction is complete within a period of from 
about 1 to about 5 hours at temperatures of 60.degree. C. to 90.degree. C. 
The process of the present invention provides quantitative yields of 
diorganomonothiophosphinate compounds. Any excess sulfur present in the 
aqueous reaction product mixture may be removed by filtration. Depending 
on the concentration of starting reactants, the products will be a viscous 
oil in more concentrated forms or an aqueous solution. The 
diorganomonothiophosphinate products are characterized by a .sup.31 P NMR 
spectral shift at about --71 ppm with respect to phosphoric acid (85%), 
used as a reference. 
The new and improved process of the present invention provides a simple 
two-step/one reactor method for making diorganomonothiophosphinate 
compounds in aqueous media which are stable in aqueous systems. The 
process of this invention provides a commercially-suitable method for 
making diorganomonothiophosphinates without the use of environmentally 
harmful organic solvents or expensive organometallic compounds. 
Other objects and advantages of the present invention will become apparent 
from the following working examples which are provided by way of 
illustration only, to enable those skilled in this art to better 
understand and practice the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
PREATION 1 
Synthesis of Heterocyclic Secondary Phosphine 
The following reaction was performed in a phosphine autoclave reactor, and 
more particularly, in a one-gallon stirred autoclave reactor equipped with 
internal and external heating/cooling coils, gas inlets and temperature 
and pressure gauges. 
The autoclave was charged with 780 g (10.8 moles) of isobutyraldehyde and 
333 g of 100% phosphoric acid. The autoclave lines were purged 3 times 
with nitrogen at 400 psig. A total of 116 g of phosphine (PH.sub.3) was 
transferred to the autoclave and the mixture was stirred at 2000 rpm. Some 
cold water was passed through the internal cooling coil to promote the 
rate of phosphine take-up. The autoclave was heated to 50.degree. C. using 
steam through the external heating system and the following 
temperature/pressure profile was observed: 
______________________________________ 
Time Temp. Presence 
(min.) (.degree.C.) 
(Psig) Remarks 
______________________________________ 
0 25 315 Heat introduced 
15 70 276 Some exotherm noted, 
External heat shut-off 
30 47 109 External heat on 
45 51 86 
60 53 78 
75 52 67 
90 50 59 Cooling Introduced 
95 25 50 Phosphine Vented 
______________________________________ 
The autoclave was discharged under nitrogen into a 3-necked stainless steel 
flask. Total weight of the autoclave contents was 1170 grams. 
The reaction product was transferred from the flask to a 2-liter separatory 
funnel under nitrogen and the brown spent acid lower layer weighing 405 g 
was separated. The top layer weighed 755 g. Analysis by .sup.31 P NMR 
showed that the product contained high concentrations of 
2,4,6-triisopropyl-1,3-dioxa-5-phospha cyclohexane (TIP) with very few 
side products. 
A 171 g portion of this top material was distilled, first at atmospheric 
pressure to remove unreacted isobutyraldehyde (33 g) and second at 
60.degree.-61.degree. C./0.4 mm to collect 118 g of product (63% yield). 
A .sup.31 P NMR spectrum of the distilled product showed three peaks at +70 
ppm, +72 ppm and +112 ppm, against 85% phosphoric acid reference. 
Subsequent analysis by capillary GC-mass spectroscopy showed that the 
three components indicated by NMR were for the +112 ppm peak a noncyclic 
intermediate, and for the +72 ppm peak a tertiary phosphine impurity, and 
the +70 ppm peak corresponded to TIP. The spectral data were consistent 
with the TIP structure: 
##STR13## 
EXAMPLE 1 
Preparation of Ammonium Diisobutyl Monothiophosphinate 
In a stainless steel reactor were charged 570 grams of a diisobutyl 
phosphine, containing 95.5% of diisobutyl phosphine and 2.4% of 
triisobutyl phosphine impurity. 506 grams of a 24.29% H.sub.2 O.sub.2 
solution in water (1 mole) were added to the reactor over a period of 1 
hour and 45 minutes, while controlling the temperature at 
52.degree.-53.degree. C. 
Thereafter, the temperature of the reaction mixture was raised to between 
about 68.degree.-70.degree. C. A mixture prepared by adding 308 grams of a 
28% solution of aqueous ammonia to 125 grams of sulfur, was added to the 
reaction vessel and the temperature was maintained at 
68.degree.-70.degree. C. for a period of 21/2 hours. The reaction mixture 
was thereafter filtered to remove the excess sulfur. A quantitative yield 
of a viscous oil was obtained. .sup.31 P NMR spectra of the oil product 
showed a --71 ppm shift with respect to 85% phosphoric acid used as a 
reference. 
EXAMPLE 2 
Preparation of Heterocyclic Secondary Phosphine Oxide 
78 g of 2,4,6-triisopropyl-1,3-dioxa-5-phospha cyclohexane (TIP) prepared 
in Preparation 1 were dissolved in 250 mls of isopropyl alcohol and an air 
stream was passed through the solution at a rate which kept the 
temperature at between 40.degree.-45.degree. C., over a period of about 
21/2 hours. The solution was stoppered overnite. The following morning an 
airflow was again introduced to the solution and the flow was increased. 
The temperature was observed to go up to 40.degree. C. and gradually went 
down to 20.degree. C. over 2 hours at the same airflow. 
The solution was stripped of isopropyl alcohol in a Rotovac to yield an 
oil. Some crystal formation occurred upon standing. The oily liquid seeded 
with crystals was dissolved in benzene and extracted two times with a 
dilute NaHCO.sub.3 solution. 
The organic layers were dried with Na.sub.2 SO.sub.4, filtered and stripped 
on the Rotovac to yield 59.0 g of an oil. 
The oil was seeded with crystals from earlier aliquots and let stand 
overnite. The following morning, the sample was solid. The solid was 
pressed on a clay plate. IR spectra was consistent with the structure for 
TIPO, 
##STR14## 
If the TIPO is diluted with water and reacted at a temperature of between 
68.degree.--70.degree. C. with excess ammonium hydroxide and sulfur in 
accordance with the method of Example 1, then the corresponding ammonium 
1,3,5-triisopropyl-4,6-dioxa-2-phospha cyclohexane monothiophosphinate 
will be obtained. 
Although the present invention has been described with reference to certain 
preferred embodiments, modifications or changes may be made therein by 
those skilled in this art. Instead of diisobutyl phosphine, other 
secondary phosphines such as diisoamyl, di-sec-butyl, or diisopropyl 
phosphine may be employed as the starting material. Instead of ammonium 
hydroxide, sodium, lithium or potassium hydroxide may be employed in the 
sulfurization reaction. All such obvious modifications may be made herein 
by those skilled in this art without departing from the scope and spirit 
of the present invention as defined by the appended claims.