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United States Patent (19) (11) 4,055,601 Ehmann 45) Oct United States Patent (19) (11) 4,055,601 Ehmann 45) Oct. 25, 1977 (54) PROCESS FOR THE OXDATION OF PRIMARY ALLYLCALCOHOLS OTHER PUBLICATIONS Djerassi, "Organic Reactions', vol. VI, chapt. 5, pp. 75 Inventor: William J. Ehmann, Orange Park, 207-234. Fla. Adkins, et al., "J. Amer. Chem. Soc.' vol. 71, pp. (73) Assignee: SCM Corporation, New York, N.Y. 3622-3629. Batty et al., "Chem. Society Journal' (1938), pp. 21 Appl. No.: 582,114 175-179. Filed: May 30, 1975 22 Primary Examiner-Bernard Helfin Related U.S. Application Data Attorney, Agent, or Firm-Richard H. Thomas 63) Continuation-in-part of Ser. No. 437,188, Jan. 28, 1974, 57 ABSTRACT abandoned. Improved conversions of 3-substituted and 3,3-disub (51) Int. Cl’.............................................. CO7C 45/16 stituted allyl alcohols to the corresponding aldehydes (52) U.S. C. ............................ 260/593 R, 260/603 C, are obtained in an Oppenauer oxidation process, under 260/347.8; 260/599; 260/600 R; 260/598 Oppenauer oxidation conditions, by carrying out the (58) Field of Search ............ 260/603 HF, 599, 603 C, oxidation employing furfural as the hydrogen acceptor. 260/593 R, 600 R, 598 The invention is particularly applicable to the oxidation of geraniol and nerol to citral, which can be converted (56) References Cited directly to pseudoionone without purification. U.S. PATENT DOCUMENTS 2,801,266 7/1957 Schinz ........................... 260/603 HF 13 Claims, No Drawings 4,055,601 1. tion produces water as a by-product which hydrolyzes PROCESS FOR THE OXDATION OF PRIMARY and consumes the aluminum catalyst. This requires ALLYLCALCOHOLS nearly stoichiometric quantities (as compared to cata lytic quantities) of the aluminum catalyst (notice page This application is a continuation-in-part of prior 224 of Djerassi, supra). In addition, the hydrolyzed application Ser. No. 437,188, filed Jan. 28, 1974, now catalyst is in the form of a gel-like precipitate which is abandoned and assigned to assignee of the present appli difficult to dispose of and which also presents mechani cation. cal problems in carrying out the oxidation reaction. Still BACKGROUND OF THE INVENTION further, large amounts of solvent are required to dis O solve the correspondingly large amount of catalyst The present invention relates to the Oppenauer oxida employed for the oxidation reaction. A second disad tion of primary allylic alcohols to their corresponding vantage is that if the reaction is carried to high conver aldehydes. The invention is particularly applicable to sion, the yield tends to fall off. the Oppenauer oxidation of 3-substituted and 3,3-disub Substituting hydrogen acceptors such as cyclohexa stituted allyl alcohols, and will be described primarily 15 none, which are less likely to undergo an aldol conden with respect to the Oppenauer oxidation of geraniol sation, for the acetone may improve the aldehyde yield. (trans) and nerol (cis)(3,7-dimethyl-2,6-octadien-1-ol) to Still, relatively high reaction temperatures are required citral (3,7-dimethyl-2,6-octadienal), although it will be when using ketones as hydrogen acceptors to carry out apparent to those skilled in the art that the present in the oxidation to high conversion in a reasonable time. vention has other applications. 20 High temperatures would be a disadvantage with heat Citral which is a mixture of citral-a (geranial) and sensitive aldehydes such as citral, as these are capable of citral-b (neral) is a highly useful intermediate in the self-condensation at high temperatures. In addition, syntheses of several desirable products. The syntheses other ketonic hydrogen acceptors present problems of of ionones and methyl ionones begin with citral. It may availability or low equilibrium constants, the latter ne be converted via a condensation with acetone, directly 25 cessitating a large excess of hydrogen acceptor which to pseudoionone, which in turn can be converted into causes difficulty in subsequent isolation of products. a-ionone and £8-ionone. The latter is a key intermediate Djerassion page 230 points out: "Until recently alde in the production of Vitamin A. Pseudoionone is useful hydes have been used only infrequently as hydrogen in the production of Vitamin E. Citral has many other acceptors.' Aldehydes are traditionally difficult prod uses, for instance, in synthetic flavors. 30: ucts to make, being unstable and subject to side reac tions. Use of an aldehyde as a reactant or hydrogen DESCRIPTION OF THE PRIOR ART acceptor is subject to the same problems, being equally The Oppenauer oxidation of secondary alcohols to unstable and subject to side reactions. In the Oppenauer ketones is a useful and well-known textbook reaction. oxidation process, it is likely to undergo both aldol and The oxidation is carried out generally in the presence of 35 Tischenko condensation reactions, with both itself and an aluminum catalyst such as aluminum tert-butoxide or with the Oppenauer oxidation product. aluminum isopropoxide employing a large excess of A number of studies have been conducted by Adkins acetone, as a hydrogen acceptor. The general applica and others (for instance, Adkins et al., J. An. Chem. tion of this reaction is, however, for secondary alcohols. Soc, Vol. 71, pages 3622-3629) to determine the appar It is reported in "Organic Reactions”, Vol. VI, chapter 40 ent oxidation potentials of various compounds (primar 5, on "The Oppenauer Oxidation', (pages 222-223) by ily ketones). Although it can be concluded that a high Carl Djerassi John Wiley and Sons Inc., 1951; “Until oxidation potential is desirable, a relatively low one (as very recently the Oppenauer reaction, except in isolated pointed out by Djerassion page 228) can be offset by instances, has not been used as a preparative method for using a large excess of hydrogen acceptor, and other the oxidation of primary alcohols to aldehydes because 45 factors such as rate of reaction and potential for side the aldehydes condensed with the hydrogen acceptor.” reactions may be more controlling. For instance, ace As indicated by Djerassi, experimental modifications in tone has a relatively low oxidation potential but is inex the usual Oppenauer procedure are necessary. These pensive and can be used in large excess. Cyclohexanone include the use of expensive or difficult to come by on the other hand has a higher oxidation potential, but hydrogen acceptors, use of stoichiometric amounts of 50 in comparative tests conducted with this compound, the catalyst and careful distillation of the product as it is oxidation of geraniol resulted in only 15% conversion formed. The methods are expensive, difficult to carry to citral. out on a large scale and are employed only when no other method is available. The Djerassi text in "Organic Reactions' is incorpo 55 SUMMARY OF THE INVENTION rated by reference herein. The present invention resides in the discovery that Previous observations of the oxidation of geraniol and furfural unexpectedly constitutes a superior oxidizing nerol with acetone as a hydrogen acceptor show that agent or hydrogen acceptor for the conversion of 3-sub the aldehydes produced undergo a subsequent aldolcon stituted and 3,3-disubstituted allyl alcohols to their cor densation reaction with the acetone and little aldehyde 60 responding aldehydes, and particularly for the conver (citral) is actually recoverable. Although the end prod sion of geraniol and nerol to citral. The reaction of the uct of the aldol condensation of citral is pseudoionone, present invention is carried out under mild Oppenauer two major problems have kept this reaction from being oxidation conditions in the presence of an aluminum employed in the production of psuedoionone commer catalyst, and can be represented by the following equa cially. One problem is that the aldol condensation reac tion: 4,055,601 3 4. (1) N / W Aluminumcatalyst G OH -- O CHO furfural nerol N / \ FO -- O CHOH citral-a itral- furfuryl (geranial) ES - alcohol Under the mild reaction conditions, it was discovered that furfural does not undergo a Tischenko reaction as is process in which acetone and a base are added directly common with many aldehydes. Moreover, furfural, 20 to the reaction mixture of equation (1). having no alpha-protons, does not undergo an aldol Preferably, the reaction of equation (1) is carried out condensation with the aldehyde product, for instance in the presence of furfural with a molar ratio of furfural citral, as does acetone. Hence, the amount of aluminum to primary allylic alcohol of about 1:10 to about 10:1. A catalyst required for the reaction is greatly reduced to preferred range for nerol/geraniol is about 1:2 to about catalytic quantities, eliminating the attendant mechani 25 3:1 which results in high yields of the desired products cal and pollution problems, and reducing catalyst cost. without excessive amounts of unreacted starting materi Surprisingly the reaction of furfural with substituted als. The specific ratio selected, however, depends on allyl alcohols such as nerol and geraniol has a high rate the end products desired. In the production of pseudoi of reaction which permits it to be carried out under onone, furfural also undergoes aldol condensation, so very mild conditions. This is important for such heat 30 that it is desirable to employ a molar ratio of furfural to sensitive compounds as citral. Specifically, at the mild nerol/geraniol less than 1:1 in the order of about 3:4 to conditions of the reaction of the present invention, cit avoid the necessity of separating unreacted furfural ral, itself, undergoes no aldol self-condensation. Also, at prior to the conversion of citral to pseudoionone. On the mild conditions of the present invention, no appre the other hand, if it is desired to recover citral, a rela ciable side reactions occur of the type resulting in such 35 tively high molar ratio of furfural to nerol/geraniol in compounds as isocitrals and dimers.
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