United States Patent Office Patented June 27, 1961
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2,990,422 United States Patent Office Patented June 27, 1961 2 In essence, our invention is predicated upon the fact 2,990,422 that an acyclic diene, 2,6-dimethyl-2,7-octadiene, can be PROCESSFOR PREPARNGCTRONELLOL FROM AN ACYCLC. TERPENE treated with diborane to yield, a trialkenyl boron com Eugene L. Woroch, Itasca, and J. Wayne Cole, Oak Park, pound and the trialkenylboron then oxidized and hydro l, assignors to The Glidden Company, Cleveland, 5 lyzed to produce the desired alcohol. The results ob Ohio, a corporation of Ohio tained by the process of our invention were unexpected. No Drawing. Filed Dec. 9, 1957, Ser. No. 701,371 In the preferred embodiment of our invention, we 10 Claims. (C. 260-462) contact 2,6-dimethyl-2,7-octadiene with diborane, (a col Orless gas at room temperature) the contact being made This invention relates to a novel process for the prep O in a suitable solvent for the reactants and in an inert aration of citronellol. The invention is more specifically atmosphere at room or slightly above room temperature directed to a process for preparing citronellol readily to obtain tricitronellyl boron and thereafter oxidize and and inexpensively by the employment of an acyclic diene hydrolyze the compound and recover the desired cit of the terpene series, namely, 2,6-dimethyl-2,7-octadiene ronellol. The diborane can be generated in situ or in as the starting material. 5 troduced as a gas into the reaction system. in the past decade or so certain boron compounds As indicated, diborane can be employed as a gas ob have been found to be good catalysts and good reducing tained outside of the system or it can be generated in agents for reducible groups. These boron compounds, the system in situ in a suitable solvent by the treatment of which lithium or sodium borohydride (Li or NaBH4) of a boron compound with a reagent capable of gen are exemplary, have been employed for the reduction 20 erating it. The BHe gaseous compound is stable, is avail of such functional groups, as carbonyl e.g. ketones and able commercially, and can also be obtained by the fol aldehydes, esters and the like to their corresponding lowing methods: primary or secondary alcohols. Boron, itself, is known (1) The treatment of sodium borohydride with boron fo form a compound, diborane (BHs) which has here trifluoride etherate. tofore been found to react with mono-olefins, with satu 25 (2) The treatment of lithium borohydride with boron ration of the double bond and the formation of a tri trifluoride etherate. alkyl boron. (3) The treatment of sodium trimethoxyborohydride The reaction of diborane with polyene compounds, with boron trifluoride etherate, and either acyclic or cyclic, has not heretofore been report (4) The treatment of lithium hydride with boron tri ed to the best of our knowledge. All of the prior art 30 fluoride etherate, relating to the reaction of alkene hydrocarbons with di For a description of these methods of generating di borane has been limited to the treatment of mono-un borane, see J.A.C.S. 75, 187 1953. saturated olefinic materials, preponderantly those of not We have employed sodium hydride in the presence more than about six carbon atoms to obtain Saturated of boron trifluoride etherate in a solvent to yield the trialkyl boron compounds. As will be brought out here 35 desired diborane compound. We can also employ an inafter, our invention relates to a process for preparing hydrous aluminum chloride and sodium borohydride to citronellol, an unsaturated acyclic alcohol of the ter yield the diborane compound. It can thus be seen that pene series, from an acyclic diene 2,6-dimethyl-2,7-octa methods of preparing diborane are known and we do not diene. intend to limit our invention to any of the specific meth Citronellol, a Coterpene alcohol, 2,6-dimethyl-2-octen 40 ods disclosed. 8-ol, also known as 3,7-dimethyl-6-octenol, is a valuable The other reactant component of the process of our terpene alcohol per se and also as an intermediate in invention, 2,6-dimethyl-2,7-octadiene, can be obtained the manufacture of 1-menthol. It is an oily liquid with readily from turpentine. Essentially, starting with the an odor of rose for which the following physical char abundant material turpentine, there can be obtained cy. acteristics have been reported: 45 pinene which, upon hydrogenation with, for example, a Raney nickel catalyst, will yield pinane. Pinane can d.150.850-0,857, b. 220° C., n.20 14566 thereafter be pyrolyzed in the presence of a mildly acidic Citronellol is very slightly soluble in water and miscible catalyst, desirably on the surface of an absorbent such with alcohol and ether. Methods of preparing citronnel as pumice, at temperatures of from 400 to 700° C. to lol include by the action of sodium amalgam on the alde 50 yield a mixture comprising 2,6-dimethyl-2,7-octadiene hyde citronellal and from the various volatile oils, e.g. which can be recovered therefrom by fractional distilla geranium, rose, etc. These processes are, however, ex tion. We have obtained a fraction having the follow pensive and in view of the value of citronellol it would ing properties: d5 0.7583, N25 1.4363, os-7.91 be highly desirable to obtain it by a cheaper process and B. 94.5/100 mm. Any known method for obtaining from readily available raw materials. 55 2,6-dimethyl-2,7-octadiene can be employed, however. It In accordance with the foregoing an object of our in should be pointed out that we prefer to start with a diene vention is the preparation of citroneliol from a readily produced from an optically active pinane from 3-pinene available raw material. obtained from American turpentine using a nickel cata A further object of our invention is the provision of a lyst to obtain an optically active citronellol. The cit process for the conversion of 2,6-dimethyl-2,7-octadiene 60 ronellol can be dehydrogenated to optically active cit to citronellol. ronellal, a precursor of optically active 1-menthol using A still further object of our invention is the provision the well known process involving cyclization of citronel of novel intermediates suitable for conversion to citronel lal to isopulegol then hydrogenating the latter to men. lol. thoi. From what has been stated above it will be rec Other objects will be apparent from a description of our 65 ognized that we provide a process for producing an ex invention which follows. pensive and valuable p-menthane alcohol possessing or In accordance with the foregoing, we have found that ganoleptic properties by a relatively simple procedure We can convert 2,6-dimethyl-2,7-octadiene to citronel by employing cheap readily available raw materials. lol by a process which comprises contacting said 2,6- We have prepared and isolated tricitronellyl boron, an dimethyl-2,7-octadiene with diborane, oxidizing the re 70 intermediate (and new compound) in the process of our Sulting trialkenyl boron and hydrolyzing in an aqueous invention as will be illustrated in the specific examples. medium. The oxidation of the tricitronellyl boron can be ac 3 2,990,422 4. complished with known oxidizing agents such as hydrogen drying, the ethereal solution was concentrated and the peroxide, oxygen, nitric acid, alkaline potassium perman residue steam distilled, giving 41.3 gms. of volatile ma ganate, etc. We presently prefer to employ hydrogen terial. The infrared pattern of this material showed that peroxide in an aqueous alkaline medium, for example, it was principally citronellol. an alkali metal hydroxide solution. By this procedure, Example 2 the oxidation and hydrolysis of the trialkenyl boron is satisfactorily accomplished. In regard to hydrolysis, the Sixty-nine gms. (0.5 mole) of 2,6-dimethyl-2,7-octa intermediate trialkenyl borate can also be hydrolyzed by diene was added to a stirred solution of 9.5 gms. (0.25 treatment with water or dilute acids. - tmole) of sodium borohydride and 11.2 gms. (0.084 mole) In that embodiment of our invention wherein we gen 0 of anhydrous aluminum chloride in 250 cc. of dry di erate diborane in situ we employ a solvent for the re glyme under a nitrogen atmosphere. The reaction was actants as a means for facilitating the preparation. Thus, carried out as described in Example 1. where diborane is generated from sodium hydride and The trialkenylborane was separated by extraction and boron trifluoride etherate or from anhydrous aluminum concentrated. After oxidation and hydrolysis with alka chloride and sodium borohydride, we employ diethylene 5 line hydrogen peroxide the total product (61.5 gms.) was glycol dimethyl ether (diglyme) as a solvent. Other sol separated by distillation giving the following results by vents that are applicable are dioxane, tetrahydrofuran, infrared pattern: etc. The preparation of the boron trifluoride etherate is 6.5% unknown fraction possessing strong hydroxyl and described by Schlesinger et al. in J.A.C.S. 75, 195 (1953) carbonyl absorption. and essentially consists of passing BF3 gas into anhydrous 20 42% citronello. ethyl ether until the liquid is saturated. The etherate -30% unknown fraction. is then obtained by distillation and recovery of the frac 16% viscous residue. tion desired. - . - . Although we can carry out the reaction of diborane Example 3 and 2,6-dimethyl-2,7-octadiene at temperatures between 25 The reaction of Example 2 was repeated but this time about 0° C. and 160° C., we prefer to employ room employing 69 gms. of 2,6-dimethyl-2,7-octadiene, 6.27 temperature or slightly above room temperature.