United States Patent Office Patented Apr
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3,129,262 United States Patent Office Patented Apr. 14, 1964 2 The present invention provides an improved method 3,129,262 for converting organic thiocyanates to thiols, and is par REDUCTION OF ORGANIC THOCYANATES ticularly suitable for converting thiocyanophenols to mer Robert S. Lafer, Pittsburgh, Pa., assignor to Consolida captophenols. In accordance with the process of this tion Coal Company, Pittsburgh, Pa., a corporation of invention, organic thiocyanates may be converted to thiols Pennsylvania by reacting the organic thiocyanate with alkali metal No Drawing. Fied Cect. 8, 1962, Ser. No. 229,213 anhydrous liquid ammonia reagent, i.e., a solution con S2 (Caisas. (C. 260-578) sisting of alkali metal as solute dissolved in anhydrous This invention relates to an improved process for con liquid ammonia as solvent. The thiocyanate is dissolved verting organic thiocyanates to thiols. More particularly, 10 in anhydrous liquid ammonia and then reacted with alkali it relates to the preparation of mercaptophenols by the metal, preferably sodium, to form the alkali salt of the reductive cleavage of thiocyanophenols using alkali metal organic mercaptan as well as alkali cyanide. Acidifica and anhydrous liquid ammonia. tion then frees the mercaptain and hydrogen cyanide from The conversion of organic thiocyanates to mercaptains, their alkali salts. This selective cleavage technique is i.e., thiols, by chemical reduction using metal-acid com 15 generally applicable to all organic thiocyanates. Where binations or by catalytic hydrogenation is known. Chem certain reactive groups are present which would be af ical reductions carried out with, e.g., zinc and hydro fected by the sodium-ammonia reagent, such as halogen chloric acid, proceed very slowly; catalytic hydrogenation or nitro groups, these groups will also be removed or using hydrogen in the presence of molybdenum disulfide reduced. Also, certain polycyclic aromatic nuclei may as catalyst gives poor yields. Such a technique when 20 be partially hydrogenated by the use of sodium-ammonia applied to a thiocyanophenol results in substantial cleav reagent, as is known. For convenience, the alkali metal age of the entire thiocyano group from the ring, thereby used has been exemplified throughout as sodium since obtaining the phenol as one of the principal products, this would be preferred for commercial use because of with but slight yield, if any, of the desired mercapto its lower cost and availability. However, potassium and substituted phenol. Another reduction method using 25 lithium would be as effective technically, as also would lithium aluminum hydride in ether is of general utility. be the liquid sodium-potassium alloy. Thus, from a However, this method is not suitable for commercial ex technical standpoint, an alkali metal selected from the ploitation because of the high cost of the lithium alumi group consisting of lithium, sodium, potassium, and alloys num hydride, which is consumed. Thus none of the fore thereof, may be readily employed in practicing this going methods combine the desired properties of being 30 invention. of broad general application, of providing high yields of The process of this invention is particularly applicable the converted thiol, and of being relatively inexpensive. and preferred for the conversion of thiocyanophenols to Various methods have also been proposed for the prep mercaptophenols in high yield under commercially feas aration of mercaptophenols, which are phenolic com ible conditions. Inasmuch as phenols may now be readily pounds substituted in the aromatic nucleus with a mer 35 converted to thiocyanophenols by the process shown in capto group in addition to the hydroxyl group. These the copending application of E. P. Previc, S.N. 228,870, compounds and their S-alkyl derivatives, i.e., alkyl mer filed of even date herewith, the present process further captophenyl sulfides, are suitable for therapeutic use as provides a means for producing mercaptophenols from antiseptics. They are also important as direct inter phenols which is readily adaptable to commercial exploi mediates in the synthesis of various alkylmercaptophenyl 40 tation, and whereby the intermediate thiocyanophenol may phosphoric acid ester amines used as contact insecticides be reacted as a crude product in situ without further puri and as systemic agents. One such synthesis is shown in fication. Such an in situ technique is particularly useful U.S. Patent 2,978,479. inasmuch as many thiocyanophenols tend to polymerize In one method for the preparation of a mercaptophenol, on storage or even during their synthesis. The process shown in U.S. Patent 2,810,765, a phenol is sulfurized 45 of the present invention will give excellent results even by reaction with sulfur monochloride, and the polysulfide with considerably deteriorated thiocyanophenols. product formed is hydrogenated to yield the desired mer The present process is particularly advantageous over captophenol. This method is disadvantageous in that, previously employed procedures for preparing mercapto where an alkyl phenol is sulfurized, the alkyl group, e.g., phenols, particularly p-mercaptophenols in that it is t-alkyl, is migratory, assuming a more stable configura 50 adaptable to semi-continuous operation and to commer tion under acid catalysis. Thereby mixtures of mercapto cial development at relatively low cost. In practicing phenols are obtained comprising the expected product and the process of this invention in its preferred aspects, no an isomer in which the t-alkyl phenol has isomerized undesired by-products are obtained. Furthermore, only prior to its sulfurization. Thus, a sample of 4-mercapto a single isomer is obtained in each instance, no isomeriza 6-t-butyl-o-cresol prepared from 6-t-butyl-o-cresol via 55 tion occurring resulting in mixtures of products, such as sulfurization-hydrogenation will contain appreciable quan occurs in the sulfurization process shown in U.S. 2,810, tities of 6-mercapto-4-t-butyl-o-cresol. It is well known 765. As mentioned, in the sulfurization process, the t that 6-t-butyl-o-cresol is readily isomerized to 4-t-butyl-o- alkyl group is mobile and migrates to form a more stable cresol. Further, the sulfurization route is variable, and configuration. low yields of mercaptophenols are obtained because of 60 By use of the present process, a mercaptophenol may thiobisphenol formation. In another method, where be prepared from a phenol in high yield and purity with mercaptophenols are prepared from aminophenols via out isolation of the intermediate thiocyanophenol. Thus the Leuckart synthesis, the process is too elaborate and as taught in the copending application of E. P. Previc, costly for commercial use and requires as starting ma S.N. 228,870, filed of even date, a phenol is reacted with terials the more expensive aminophenols. 65 thiocyanogen (either free thiocyanogen or prepared as In a method shown in U.S. Patent 2,923,743, the phenol nascent thiocyanogen by reaction of an inorganic thio is reacted with a dialkyl disulfide in the presence of an cyanate with a halogen) whereby the corresponding thio acidic condensation agent to yield the S-alkyl derivative cyanophenol and thiocyanic acid are formed. The so of a mercaptophenol. Syntheses using the Bayer process, formed thiocyanic acid is neutralized by reacting it with e.g., reaction of dialkyl disulfides with an acid catalyst, 70 ammonia to form ammonium thiocyanate. Then in com also yield alkyl mercapto derivatives of phenols rather bination with the present process, the crude thiocyano than mercaptophenols. phenol is reacted with the alkali metal-liquid ammonia 3,129,262 - 3 4. solution whereby the thiocyanophenol is reductively ried out on a commercial scale in stainless steel equip cleaved to form the corresponding mercaptophenol. This ment. When stainless steel equipment is used, no evidence combination process is uniquely adapted for the conver of sodamide formation is noted. This undesirable side sion of a phenol to a corresponding mercaptophenol with reaction can occur in non-stainless steel apparatus be out isolation and elaborate purification of interemdiate cause of the known catalytic effect of iron, cobalt, and products. It is particularly of utility for the preparation nickel ions. of p-mercaptophenol inasmuch as the thiocyanation of The thiocyanate is dissolved in Sufficient liquid am phenols having an unsubstituted para position proceeds monia, and sodium is added in small pieces. Alterna more rapidly than the thiocyanation of p-alkyl phenols. tively, the sodium may be metered as a liquid by first The complex procedures heretofore known for the re O dissolving it in liquid ammonia and adding this solution ductive cleavage of organic thiocyanates or for converting to the reaction vessel; or the sodium may be added as phenols to mercaptophenols are not feasible for commer the molten metal; or the low-melting liquid sodium-potas cial development because of either their low yield or sium alloy may be used. Liquid ammonia is the solvent their prohibitive cost. The instant process brings mer of the system, and is ordinarily used at or below its boil captophenols into the realm of low cost commercial ing point of -33°C. In order to avoid undue losses of chemicals. Such chemicals are of particular interest be product caused by heat generation during the reaction, cause of their serving as direct intermediates for the preferable operation is at temperatures about 5 degrees formation of many important thiophosphoric acid esters elow -33", to approximately -40° C. Removal of which are useful as insecticides, acaricides, mitricides, the heat of reaction through the walls of the reactor is ovicides, etc., for example as shown in U.S. Patent 2,978,- 20 preferred to cooling by reflux to avoid frothing of the 479. The mercaptophenol compound 4-methylmercapto reaction mixture. Im-cresol is a particularly important intermediate for these While the reductive cleavage may be conducted at syntheses. temperatures up to 100° C. using pressurized equipment, Somewhat surprisingly, the sodium-ammonia reagent there is ordinarily no advantage obtained in operating effects a selective cleavage of the thiocyanophenol.