Process for the manufacture of thiophosphoryl chloride

The present invention relates to an improved process for the preparation of thiophosphoryl chloride which is useful as an intermediate for the synthesis of insecticidally active compounds. The improvement comprises the presence in the reaction mixture of a catalytic amount of a nitroxide free radical of the following general formula: ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 represent an alkyl group.

TECHNICAL FIELD OF THE INVENTION
 The present invention relates to an improved process for the preparation of
 thiophosphoryl chloride which is useful as an intermediate for the
 synthesis of insecticidally active compounds. The improvement comprises
 the presence of a catalytic amount of a nitroxide free radical in the
 reaction mixture.
 BACKGROUND OF THE INVENTION
 A process for the preparation of thiophosphoryl chloride is known in the
 art. British Patent 694,380 discloses the preparation of thiophosphoryl
 chloride (PSCl.sub.3) by refluxing phosphorous trichloride (PCl.sub.3) and
 excess sulfur at atmospheric pressure, in the presence of carbon
 (activated with K.sub.2 S) and about 40 mole percent of thiophosphoryl
 chloride for about three hours. German Patent 1,145,589 discloses that
 thiophosphoryl chloride is obtained by treating phosphorous trichloride
 and sulfur in liquid phase at atmospheric pressure, using aluminum or
 alloys thereof, preferably aluminum wastes, as a catalyst. Thiophosphoryl
 chloride is preferably used as the reaction medium and moderator. Further,
 U.S. Pat. No. 5,464,600 discloses that thiophosphoryl chloride is obtained
 by reacting phosphorous trichloride and sulfur in the presence of a
 catalytic amount of a tertiary amine.
 SUMMARY OF THE INVENTION
 The present invention provides an improved process for preparing
 thiophosphoryl chloride by reacting phosphorous trichloride with sulfur in
 the presence of a catalytic amount of a tertiary amine, and in the
 presence of a catalytic amount of a nitroxide free radical of the
 following general
 ##STR2##
 wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 represent an alkyl group.
 The improvement comprises the presence of a catalytic amount of the
 nitroxide free radical.
 DETAILED DESCRIPTION OF THE INVENTION
 The present invention provides an improved process for preparing
 thiophosphoryl chloride by reacting phosphorous trichloride with sulfur in
 the presence of a catalytic amount of a tertiary amine, and in the
 presence of a nitroxide free radical. The improvement comprises the
 addition of a catalytic amount of the nitroxide free radical in the
 reaction mixture. The nitroxide free radical is of the following general
 formula:
 ##STR3##
 wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 represent an alkyl group.
 In a preferred embodiment, the nitroxide free radical is
 2,2,6,6-tetramethyl-1-piperidinyloxy ("TEMPO"). TEMPO is a standard
 nitroxide free radical which may be commercially obtained from Lancaster
 [CAS # 2564-83-2]. Further, in this embodiment, the tertiary amine may be
 selected from a group of pyridines which are preferably substituted
 pyridines such as 5-ethyl-2-methylpyridine, 2-methylpyridine,
 2,4-dimethylpyridine, 2,6-dimethylpyridine, 2,4,6-trimethylpyridine,
 trialkylamines (e.g., tripropylamine and tributylamine),
 tris-[2-(2-methoxyethoxy)ethyl]amine and
 1,8-diazabicyclo[5.4.0]undec-7-ene.
 In the process of the invention, the sulfur and phosphorous trichloride can
 be employed in a ratio of sulfur to phosphorous trichloride that is in the
 range of about 1.0:1.0 to about 1.5:1.0, and preferably about 1.4:1.0. The
 nitroxide free radical may be present in a ratio of nitroxide free radical
 to phosphorous trichloride that is in the range of about 0.002:1.0 to
 about 0.004:1.0, and preferably about 0.00314:1.0. The tertiary amine can
 be used in a ratio of tertiary amine to phosphorous trichloride that is in
 the range of about 0.15:1.0 to about 0.20:1.0, and preferably about
 0.16:1.0.
 The reaction can be conducted at initial temperatures of about 110.degree.
 C. to about 140.degree. C., and preferably from about 115.degree. C. to
 about 120.degree. C.
 The reaction product containing thiophosphoryl chloride is typically
 distilled to remove the thiophosphoryl chloride. Following isolation of
 the thiophosphoryl chloride and phosphorous oxychloride by-product, the
 distillation heel which contains the tertiary amine and the nitroxide free
 radical, can be recycled for use in another reaction of phosphorous
 trichloride and sulfur.
 In accordance with the invention, the process is well suited to either a
 batch or continuous reaction. In the continuous reaction, the distillation
 heel is recycled continuously to a primary reactor stage, where the sulfur
 and phosphorous trichloride are reacted.
 The invention is further illustrated but is not intended to be limited by
 the following examples in which all parts and percentages are by weight
 unless otherwise specified.

EXAMPLES
 Example 1
 To a 500 ml 4-necked round bottomed flask (fitted with a mechanical
 stirrer, thermometer, Friedrich condenser cooled to (-10.degree. C.),
 nitrogen blanket adaptor and Erlenmeyer flask containing 5% sodium
 hydroxide through which gases are bubbled) was charged about 1.4 moles
 (44.9 grams) of sulfur and 1.0 mole (140.1 grams) of phosphorous
 trichloride. About 0.16 mole (30 grams) of tributylamine (TBA) and 0.00314
 mole (0.5 gram) of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) were then
 charged to the reaction mixture. The reaction was monitored by a gas
 chromatograph. The reaction temperature was gradually raised to about
 120.degree. C., cooked, and then the reaction mixture was subjected to
 atmospheric distillation. The time at reflux (i.e., the reaction time) was
 about 2 hours and 8 minutes. The time for distillation was about 40
 minutes. The thiophosphoryl chloride yield in the distilled product was
 about 78.5%. The heel residue comprising thiophosphoryl chloride,
 tributylamine, and TEMPO, was saved for the next cycle.
 In this second reaction, about 1.4 moles (44.9 grams) of sulfur and 1.42
 moles (199.0 grams) of phosphorous trichloride were charged to the reactor
 vessel. Since the distillation heel contained tributylamine and TEMPO, no
 additional tributlyamine and TEMPO were needed in the reaction mixture.
 The reaction mixture was heated to 120.degree. C., cooked, and then the
 reaction mixture was subjected to atmospheric distillation. The time at
 reflux (i.e., the reaction time) was about 40 minutes. The time for
 distillation was about 1 hour and 3 minutes. The cumulative yield of
 thiophosphoryl chloride in the distilled product was about 82.5% based on
 the phosphorous trichloride charged. The heel residue comprising
 thiophosphoryl chloride, tributylamine, and TEMPO, was saved for the next
 cycle.
 In this third reaction, about 1.4 moles (44.9 grams) of sulphur and 1.42
 moles (199.0 grams) of phosphorous trichloride were charged to the reactor
 vessel. Since the distillation heel contained tributylamine and TEMPO, no
 additional tributlyamine and TEMPO were needed in the reaction mixture.
 The reaction mixture was heated to 120.degree. C., cooked, and then the
 reaction mixture was subjected to atmospheric distillation. The time at
 reflux (i.e., the reaction time) was about 30 minutes. The time for
 distillation was about 43 minutes. The cumulative yield of thiophosphoryl
 chloride in the distilled product was about 84.9% based on the phosphorous
 trichloride charged. The heel residue comprising thiophosphoryl chloride,
 tributylamine, and TEMPO, was saved for the next cycle.
 In this fourth reaction, about 1.4 moles (44.9 grams) of sulphur and 1.42
 moles (199.0 grams) of phosphorous trichloride were charged to the reactor
 vessel. Since the distillation heel contained tributylamine and TEMPO, no
 additional tributlyamine and TEMPO were needed in the reaction mixture.
 The reaction mixture was heated to 120.degree. C., cooked, and then the
 reaction mixture was subjected to atmospheric distillation. The time at
 reflux (i.e., the reaction time) was 22 minutes. The time for distillation
 was about 55 minutes. The cumulative yield of thiophosphoryl chloride in
 the distilled product was about 87.9% based on the phosphorous trichloride
 charged. The heel residue comprising thiophosphoryl chloride,
 tributylamine, and TEMPO, was saved for a subsequent cycle.
 Table I shows the results of the reactions.
 TABLE I
 PCL.sub.3 /Sulfur/TBA/TEMPO Reaction Time Cumulative
 Reaction (Moles) (Hr/min) Yield (%)
 Heel 1.00 1.40 0.16 0.00314 2 hr 8 min 78.5
 Recycle 1 1.42 1.40 -- -- 40 min 82.5
 Recycle 2 1.42 1.40 -- -- 30 min 84.9
 Recycle 3 1.42 1.40 -- -- 22 min 87.9
 Example 2
 The test conducted in Example 1 was repeated, with the exception that the
 reaction was carried out in the absence of TEMPO. The thiophosphoryl
 chloride yield in the distilled product was about 72.8%, after a reaction
 time of 3 hours and 44 minutes.
 As described above in Example 1, the distillation heel remaining in the
 flask following the distillation, was used in three subsequent cycles.
 Since the distillation heel contained tributylamine, no additional
 tributlyamine was needed in the reaction mixture. The cumulative yield of
 distilled product over 3 batches, amounted to 91.6% based on the
 phosphorous trichloride charged. Table II shows the results of the
 reactions.
 TABLE II
 PCL.sub.3 /Sulfur/TBA Reaction Time Cumulative
 Reaction (Moles) (Hr/min) Yield (%)
 Heel 1.00 1.40 0.16 3 hr 44 min 72.8
 Recycle 1 1.42 1.40 -- 34 min 83.5
 Recycle 2 1.42 1.40 -- 20 min 89.7
 Recycle 3 1.42 1.40 -- 24 min 91.6
 Although the invention has been described in detail in the foregoing for
 the purpose of illustration, it is to be understood that such detail is
 solely for that purpose and that variations can be made therein by those
 skilled in the art without departing from the spirit and scope of the
 invention except as it may be limited by the claims.