PREPARATION OF TERTIARY ALKYL ISOCYANATES

Abstract: A process for the production of tertiary alkyl isocya­ nates, such as tetramethylxylylene diisocyanates, is dis­ closed in which the corresponding tertiary alkyl halide is reacted with an alkali metal cyanate utilizing a zinc-pyridine complex as a catalyst which has been prereacted with the cyanate before reaction with the halide.

Description: PREPARATION OF TERTIARY ALKYL ISOCYANATES Thi.s. invention relates to tertiary alkyl isocyanates and in particular provides a method for producing monoand poly-isocyanates b.y reaction of the corresponding tertiary alkyl halides with alkali metal cyanates. This invention has general applicability to the preparation of alkyl isocyanates in wh.ich the isocyanato group is attached to a tertiary carbon atom and in particular isocyanates in which the tertiary alkyl carbon atom is a substi.tuent on an aromatic nucleus. The invention has particular applicability to the preparation of diisocyanates, such as tetramethylxylylene di.isocyanates, and is illustrated in that context below, but is also applicable to the preparation of a variety of isocyanates such as tertiary butyl isocyanate, dipentene diisocyanate. and tri-(isocyanato isopropyl) benzenes. This invention specifically relates to production of tertiary alkyl isocyanate.s by the. reaction of the corresponding tertiary alkyl halide with an alkali metal cyanate, such as. is. described in United States Patent Nos. 4,13Q,577 and 4,224,238. These patents describe the preparation of a w.ide variety of tertiary alkyl isocyanates by such reaction in an aprotic organic solvent and in the pres.ence of a catalyst selected from the mineral acid salts and carboxylic acid salts of zinc, iron, antimony, tin and cobalt. The catalyst can also desirably include promoters, such as tertiary amines. I.t has been re.cogni.ze.d that the. proportion of the catalyst promoter., in the case of pyridine, should he on the order of 2 moles. per mole. of catalyst metal salt, sucb as: zinc chloride. This is presumably because the catalytic agent is a complex involving one. mole of zinc s-alt and 2 moles of the promoter, pyridine. In accordance with. the. present invention i.t has been found that when using catalysts with promoters, such. as a zinc salt with pyridine, the order of addition of the reactants is important. Greatly improved yields are. obtained through reaction of the initial zinc-pyridine complex with the alkali metal cyanate prior to contact with the tertiary alkyl halide. Thus in accordance with the present invention the complex between the zinc salt and pyridine is first prepared, generally in a 1:2 mole ratio of the.se components. Thereafter, the complex is reacted with the alkali metal cyanate at least in a quantity of 2 equivalents of cyanate for each atom of zinc present,. thus forming a complex of zinc, pyridine and cyanate anion containing 2 moles of pyridine and 2 equivalents of cyanate anion per atom of zinc. Thereafter, the zinc-pyridine-cyanate complex is reacted wi.th the tertiary alkyl halide to form the tertiary alkyl isocyanate. It has also been found as described in co-pending application Serial No. filed 1982, that the presence of water is desireable in promoting the catalyzed reaction between alkali metal cyanate and a tertiary alkyl halide. Desirably the amount of water should be on the. order of 0.Q2% to 0.2% based on the weight of the. entire reaction mixture including solvents. Thus. in accordance with this invention, as indicated above., the alkyl halide utilized can be any alkyl halide in which thehalogen is bonded to a tertiary carbon atom. The reaction is generally selective., and the. same. or similar functi.onal groups, such. as halides, attached to secondary or primary carbon atoms are nonreactive. under the. condi.tions of the reaction. Poly tertiary alkyl halides can be reacte.d to produce. the correaponding polyisocyanates. The alkali me.tal cyanate can he any alkali. metal cyanate, but preferably is sodium cyanate. The proportions of halide to cyanate are generally stoichiometric or with. a slight excess of cyanate and range between rati.os of 1:0.8 to about 1:4, preferably in the range of 1:1-1:2, of equivalents of halide to moles of cyanate. The preparation of the catalyst with. promoter, pre-reaction of alkali. metal cyanate and final reaction with alkyl halide are all preferably carried out in an aprotic organic solvent for the organic components which is non-reactive with. hydrogen halides and forms neither salts nor adducts with hydrogen halides. The. solvents which can be used are those known in the art for this reaction. Particularly suitable are halogenated aliphatic and aromatic hydrocarbons, methylene dichloride being preferred. Generally the reaction of cyanate with alkyl halide is carried out between O c. and lOO0c for a sufficient period of time to convert the tertiary alkyl halide substantially to the corresponding tertiary alkyl isocyanate. In some circumstances higher temperatures may be used. Tertiary halides are usually unstable over 50 C, however and dehydrochiorination can occur at higher temperatures.. The solvent can also limit the. temperature. Methylene dichlori.de, for example, boils at 4QOC. Normally the reactions are carried out at atmospheric pressure. The. preparation of the. catalyst with promoter and pre-reaction with. alkali metal cyanate take place readi.ly at ambient, room temperatures, but higher and lower temperatures may be used. Generally catalysts effective. to promote the reacti.on of a tertiary alkyl halide with an alkali metal cyanate. in accordance w.i.th this invention are zinc salts of mineral acids and various carboxylic acids. Zinc chloride has been found particularly effective. As is known in the art, only small amounts of catalyst are required, and preferably the amount is jus.t that sufficient to obtain the de.sired reaction. Usually 0.5 to about 5Q mole percent of catalyst, and preferably about 1 to 1Q mole percent of catalyst, based on the starting tertiary alkyl halide, is utilized. As noted, in accordance with this invention the catalyst is employed in conjunction with a promoter such. as pyridine. The amount of pyridine i.s based on the. amount of catalyst employed and preferably is 2, moles per mole of zinc salt. Preferably zinc chloride is employed as catalyst, and the zinc chloride and pyridine are first complexed and then reacted with the alkali metal cyanate. While two equivalents of cyanate are required to form the zinc-pyridine-cyanate complex, usually the cyanate requirements for reaction with the alkyl halide wi.ll dictate a substantial excess of cyanate over that required by the stoichiometry of the zinc-pyridine-cyanate complex. In carrying out the reaction of a tertiary alkyl halide with an alkali metal cyanate in a solvent to produce the corresponding tertiary alkyl isocyanate utilizing as a catalyst a material, such. as zinc chloride, promoted with a material, such as pyridine, the reaction i.s slow and the yields are poor. To illustrate a reaction mixture of 198g of -tetramethylxylylene dichloride, 226g of sodium cyanate (.90% pure, Q.5% H2Ql 5.36g of zinc chloride. and 6.lg of pyridine in 575g of methylene dichloride was stirred at 20-25 C. for many hours. After 3 hours an additional 5.4g of zinc chloride was added. The reaction process. was followed by gas-liquid chromatography with the results indicated in Table 1. TABLE 1 Time, Hrs. Organic Product Distributi.on, Area % p-DIPER p-TMI p-TMDXDI 2 43.5 41.2 13.6 3 39.7 43.2 16.7 4 36.5 45.5 16.9 20 25.2 43.5 24.2 p-DIPES-para-diisopropenbenzene-the reactant tetramethylxylylene dichloride (p-TMXDC) is observed on GLC as the diene as a result of thermal dehydrochlorination during measurement. Part of the observed -DIPEB may be real, as a result of dehydrochlorination in the reaction mixture. p-TMI= the corresponding monoisocyanate which. may be present in the reacti.on mixture both as p-chloroisopropyl-a,a-dimethylbenzyl isocyanate and as p-isopropenyl a, dimethyl benzylisocyanate. p-TMXDI=para-tetramethylxylylene diisocyanate. It is believed the poor yields obtained when the catalyst and reactants are mixed together occurs because the solid zinc chloride reactant does not dissolve with sufficient rapidity to interact with the pyridine "promoter". Consequently the pyridine reacts with the alkyl halide. to form the corresponding olefin and pyridine hydrochloride. In small s.cale experiments. utilizing the same ingredients. with. equipment provided with. magnetic stirrers. effective. closer to the b.ottom of the reaction vessel the he.avy zinc chloride crystals were b.etter dispersed and reacti.on was much. improved. It has nevertheless been found that the. formation of the zinc-pyridine-cyanate complex prior to addition of the alkyl halide provides superior results as shown in the. following series of small scale runs set forth in Table II. In the experiment shown in Table II the following components were utilized: 20.g p-TMXDC (0.087 mole) 22.4g NaOCN (90% pure;0.31 mole) 0.54 ZnCl2 (0.0040 mole) 0.61g pyridine (0.0077 mole) 58 ml CH2C12 Table II further sets forth the procedure and order of addition utilized. The symbology is the same as in Table I. TABLE II Experiment Procedure (at 25 C) Time, Hrs. Reaction Progress (Area %, by GLC Analysis) No. p-DIPEB p-DIPEB p-TMXDI TMI A (a) Added ZnCl2 to CH2Cl2 and stirred 15 min. 2.5 7.8 23.3 64.1 (b) Added TMXDC, stirred 10 min. 4 4.8 20.7 69.1 (c) Added Pyiridine, stirred 10 min. (d) Added NaOCN, stirred B (a) and (b) as in Ex. No. A 2 No Apparent Reaction (c) no pyridine added (d) as in Ex. No. A C Repeat Ex. No. A 2.5 9.7 25.4 61.8 D (a) Added TMXDCand NaOCN to CH2Cl2 (b) Added ZnCl2, stirred 2.5 10.5 24.0 62.7 10 min. 4.5 9.0 24.1 62.2 (c) Added Pyridine dropwise with stirring, continued stirring E (a) Added ZnCl2 and Pyridine to CH2Cl2, stirred 15 min. 2.5 2.1 15.8 76.0 (b) Added NaOCN, stirred 10 min. 4.5 0.7 9.5 82.9 (c) Added TMXDC, stirred In Experiments A and C, above, the zinc chloride pyridine complex was allowed to interact directly with the- p-TMXDC before introduction of the cyanate salt and dehydrochlorination occurred. It is believed the: zinc was partially deactivated by crystallization of the difficultly soluble salt, pyridinium tetrachlorozincate. In the absence of pyridine, however, as shown in Experiment B, no reaction takes place whatsoever. ExperimentD shows that addition of the sodium cyanate at the same time as the halide does not alleviate the problem e.ven though contact of these precedes the addition of pyridine.. It has been found in accordance with. the present invention with the addition of the halides being deferred until after the zinc chloride has been solubilized by its reaction with pyridine substantially greater yie.lds are. obtained. It has been found that yields are further improved hy allowing time for the initially solubilized complex of zinc salt and pyridine to react with. the cyanate anion and form the zinc-pyridine complex having two equivalents of cyanate anion and two moles. of pyridine for each atom of zinc. Conversion of the zinc-pyridine complex in solvent mediums such as methylene dichloride with added alkyl metal cyanate, such as sodium cyanate, takes place in a short time, i.e., in 10 minutes as is shown in Experiment E. The benefits of this procedure can be illustrated in an experiment in which 0.22 mole of sodium cyanate and Q. 10 mole of p-TMXDC were added simultaneously to a solution of O.QQ67 mole.Zn(pyrl2C12 dissolved in 35 ml of methylene dichioride. The reaction was followed at 250 b.y GLC. Conversi.on to p-TMXDI was only 28% in 2 hours; When the sodium cyanate was added first and stirred with the zinc chloride-pyridine complex solution for at leas 1Q minutes before the dichloride was added a 4.8%. conversion to -TMXD° was obtained in 2 hours. The. following Examples further illustrate the preparation of tertiary alkyl i.socyanates in accordance with. this invention. EXAMPLE 1 a,-Dimethyl.-4.-.I.s.op.ropenylb.e.nzyl Isocyanate (p-TMI 1067 g (6.7 mole.) of p-diis.opropenyl benzene was dissolved in three liters of methylene dichloride and treated at 0 C. with 6.7 moles dry, gaseous hydrogen chloride. The resulting crude solution contained &alpha;.&alpha;-dimethyl-4-isopropenylbenzyl chloride as a predominant organic product. A catalyst solution was prepared by stirring a mixture of 64.4g zinc chloride, 75g of pyridine and 1580. ml of methylene dichloride (0.002% H20) until an essentially clear solution was obtained (30 minutes). An additional 1800 ml of methylene chloride and 1065g (14.7 moles) of 90% sodium cyanate (0.5% H2 0) were then added, and the suspension was stirred for 15 minutes. The. chloride reagent solution prepared above was then gradually added over 10 minutes to the catalyst solution, and the reaction mixture was held at 20-25 with. gentle cooling. After a 2 hour reaction period no further change was. found to be occurring as determined hy gas chromatography, and the reaction products, so determined were present in molar ratios of approximately 17:74:3 of diisopropenyl benzene: p-TMI; p-TMXDI. The solvent was then distilled from the reaction mixture., and the. residue was extracted with 3 litera of hexane. This extract was concentrated to recover 1448g of crude p-TMI containing 4.Q6 mole C60.6t yield) of E-TMI. The crude p-TMI was. then purified by flash distillation through. a wiped film evaporator to remove polymeric by-products and residual zinc catalyst, and thereafter was fractionally distilled through a packed column; B..P. 1210C at 5 mm Hg. EXAMPLE 2 &alpha;,&alpha;-Dimethyl-3-Isopropenylbenzene Isocyanate m--TMI This compound was prepared in the same manner as described with reference to p- TMI in Example 1, above, starting with 1,3-diisopropenylbenzene. Comparable yields were obtained; B.P. 118 C. at 5 mm Hg. EXAMPLE 3 1,3,5-Tris(1-Isocyanato-1-Methylethyl) Benzene A catalyst solution was prepared by stirring a mixture of zinc chloride (1.0 moles), pyridine (2.0 moles and methylene dichloride until the solid dissolved. 1.42 moles of 90g sodium cyanate containing Q.5% H2O were added to 100 ml. of the-solution so prepared which contained the equivalent of 24 m.moles of Zn(pyrl2C12. The mixture was stirred for an hour,and thereafter 0.33 moles of 1,3,5-tris(l- chloro-1-methylethyl)benzene di.ssolved in 530 methylene chloride was. added. After stirring overnight at room tempe.rature gas: chromatographi:c analysis indicated 81% conversi.on to triisocyanate. had been achieved. The solvent was evaporated, and the product was recrystallized from hexane giving a 59% yi.eld; M.P. 65.5-66.50c. EXAMPLE 4 2,6-Bis(1-Isocyanato-1-Methylethyl)Naphthalene 2,6-Bis(1-chloro-1-methylethyl)naphthalene (0.178 mole1 was prepared by passing a stream of dry gasous hydrogen chloride through a methylene dichloride suspension of 2, 6-di.isopropenylnaphthalene (37.2g, 0.18 mole) which had previously been cooled to 40C. After conversion of the dii.sopropenylnaphthalene to the dichloride the solvent was removed in vacuo at 300C. leaving an off-white solid. The solid was redissolved in 250 ml. of methylene dichloride, cooled to 40C. and slowly added to a cooled (40C.) methylene dichloride. suspension of sodium cyanate/zinc chloride catalyst solution. The catalyst solution had previously been prepared by vigorously stirring a suspension of zinc chloride (2.0g,0.014 mole), pyridine (2.46g, 0.03 moles anhydrous, 90% sodium cyanate (34g 0.47 mole) and 0.25 ml of water in 300 ml methylene dichloride for 2 hours at ambient temperature. The reaction mixture was allowed to stand at 40C. for two hours and then was allowed to warm to ambient temperature and stirred an addition 18 hours. Magnesium sulfate and Hyflo (a propri.etary filtration aid) were then added. After 5 minutes the solution was filtered to remove. solids. The solvent was then removed in vacuo leaving 57g of an oily solid. The oily solid was nilxed with 600 inl. of hexane, heated at 70 0C. for one hour, filtered through. Hyflo and then cooled to ambient temperature. The solid which crystallized was collected and air dried, affording 25g (first crop) of product. The mother liquor was then concentrated and cooled affording a second crop of 12g. The final product, 37g (70% yield), was found to have a chloride level of 370 ppm and an isocyante content equivalent to 6.66 meq/g. (melting range 85.5-87 C.) EXAMPLE 5 &alpha;,&alpha;-Dimethylbenzylisocyanate &alpha;,&alpha; ;-Dimethylbenzylchloride was prepared by passing a stream of dry hydrogen chloride gas through a solution of methylene. didiloride (750 ml.) and a-methylstyrene (500g. 4.2 moles) at 40C. The reaction and work-up were employed for the paration of 2,6-bis(1-isocyanato- 1-methylethyl)naphthalene described in Example 4. The resulting monochloride was dissolved in 750 ml. methylene dichloride and was added to a cooled C40C) catalyst solution previously prepared from anhydrous, 90% sodium cyanate (389g,5.39 moles), pyridine (50 ml., 0.633 mole), zinc chloride (39.15g,0.28 mole) and I ml. H2O in 1075 ml methylene di.chloride. After the reaction mixture had been stirred at ambient temperature for 3 days magnesium sulfate and Hyflo were added and the solution filtered to remove solids. Solvent was then removed in vacuo leaving an oil which. was. found to be 66% dimethyl benzylisocyanate and 33% a methylstyrene. The isocyanate was further purfied by fractional distillation affording 275 g (45%) of &alpha;,&alpha;- dimethylhenzylisocyanate.

Claims: THE CLAIM 1. A process for production of a tertiary alkyl isocyanate from the corresponding tertiary alkyl halide which. comprises: a. preparing a complex of a zinc salt of an acid selected from mineral acids and carboxylic acids and pyridine in a 1:2 mole ratio, b. there.after reacting said zinc-pyridine complex with. an alkali. metal cyanate to form a zinc-pyridinecyanate. complex containing 2 equivalents of cyanate anion per atom of zinc, and c. reacting said zinc-pyridine-cyanate complex with said tertiary alkyl halide in an aprotic solvent at a temperature of from 0 0C. to 1000C to form said tertiary alkyl isocyanate. 2. A process according to claim 1 in which said zinc salt is zinc chloride. 3. A process according to claim 2 in which said solvent is methylene. di.chloride 4. A process according to claim 2 in which said halide. is a di.(.2-chloroisopropyl)benzene dichloride. 5. A process according to claim 2 in which said halides is an a,a-dimethyl-isopropenyl benzyl chloride. 6. A process according to claim 2 in which said halide. is. a tris (2-chloroisopropyl)benzene. 7. A process according to claim 2 in which said halide is a di(2-chlorisopropyl)naphthalene. 8. A process according to clam 2 in which said halide. is ara-dimeth.ylbenzyl chloride

Inventors: FELDMAN, ALLAN MURRY, FORGIONE, PETER SALVATORE, SCHAFFER, FREDERIC CHARLES
Applicants: AMERICAN CYANAMID COMPANY
Application year: 1983
Main classification: C07C118
IPC codes: C07C265, C07C263