[CONTRIBUTION FBOY THE DEPUTMWT OF CHEMISTRY OF BBYN MAWRCOLLEQE m !mz U N I W R ~ I TOF Y MICHIQ~]
ALICYCLIC-AROMATIC ISOMERIZATIONS. CATALYTIC ISOMERIZATION OF 2 ,6-DIBENZALCYCLOHEXANONE AND CARVONE E: C. HORNING Received February .& l9.@ -I,
The isomeric aromatization, or isoaromatization, of alicyclic ketones provides a relatively little used path for the synthesis of phenols. The usefulness of this route depends in part upon the availability of methods or conditions for accomplishing the isomerization. In general, a variety of conditions are known which will effect the isomerization of alicyclic isomers to aromatic structures (1). In the event that isomerization can take place with rearrangement of hydrogen atoms alone, hydrogenation catalysts have proved capable of bringing about isomerization. The experiments described here were carried out with 2,6-dibensalcyclohexanone (I) and carvone (111) with the aim of obtaining information with regard to the effectiveness of several common hydrogenation catalysts in the isomerization (on a preparative scale of 25 g.) of tbese ketones. Previous work has indicated that palladium-charcoal catalysts may act as very effective agents; this has been c o n h e d . Raney nickel and 2,6-dibenzalcyclohexone (I) were heated at 255-265' (under reduced pressure), and in a bomb with ethanol at 190"for periods of four and six hours respectively, without effecting appreciable isomerization. Platinum black (Adams' catalyst) brought about approximately 34% isomerization to 2,6-dibenglphenol when heated with 2,6-dibenzalcyclohexanone a t 255-265" for five hours. When the isomerization was carried out with palladium-charcoal, using a 10% palladium catalyst, i t ww found that heating 2,6-dibenzalcyclohexanone with the catalyst for five hours at 235-245" (reflux conditions, under reduced pressure) led to 91% conversion to 2,6dibenzylphenol (11). The palladium-charcoal catalyst was prepared by Hartung's method (2) which leads to a catalyst of uniformly high order of activity not
only for hydrogenations, but also for dehydrogenations and isomerizations involving aromatic structures. 263
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The conversion of 2,6-dibenzalcyclohexanoneto 2,6-dibenzylphenol has been reported previously (3) as a two-step process in which hydrogen was introduced into a heated mixture of 2,6-dibenzalcyclohexanone and a palladium-charcoal catalyst until hydrogenation to 2,6-dibenzylcyclohexanone was completed,
CHs
I
OH
followed by raising the temperature and dehydrogenating to 2,6-dibenzylphenol. The isomerization of carvone (111) to carvacrol (IV) by a palladium-charcoal catalyst has been observed previously. It is an undesirable side reaction which occurs when carvone is hydrogenated a t room temperature using palladium catalysts (4,5), indicating that isomerization occurs in this case even under mild conditions. The effect of variations in conditions was examined by Linstead (5) who heated carvone (3.0-cc. samples) at temperatures in the range of 20" to 230" for varying lengths of time with a palladium-charcoal catalyst (obtained through reduction with alkaline formaldehyde). Treatment for t.welve hours at 230" yielded 95% of carvacrol; for two hours at 228" yielded 81% of carvacrol. In repeating this isomerization, carvone (25.0 g.) was heated under gentle reflux (232-233") at atmospheric pressure for one hour with a palladium-charcoal catalyst (1.0 g. of Hartung's catalyst) and carvacrol was obtained in 92% yield. EXPERIMENTAL
Catalysts. The Raney nickel catalyst was that obtained in the usual fashion (6). The palladium-charcoal catalyst was prepared according to Hartung (2) by hydrogenation of an aqueous solution of palladium chloride and sodium acetate containing suspended charcoal (Norit). It contained 0.1 g. of palladium per gram of catalyst. The platinum catalyst was prepared by suspending a mixture of 0.15 g. of platinum oxide obtained by the usual procedure (7) and 1.0 g. of charcoal in 50 cc. of methanol, and reducing the oxide by hydrogenation. The catalyst was removed by filtration and employed immediately while still wet with solvent. An examination of the action of platinum black from reduction of the oxide, but without charcoal, was also carried out. Rearrangement of dibenzolcyclohexanone. A . W i t h Raney nickel. A Claisen flask with a thermometer extending into the bulb was employed as a reaction vessel. A mixture of 25.0 g. of 2,6-dibenealcyclohexanone(m.p. 116-117') and 1 g. (estimated) of Raney nickel was maintained a t 255-265'' for three hours under a pressure of about 17 mm. Attempted distillation a t this point yielded only a few drops of yellow oil. The treatment was continued for one hour longer a t 285-390", but again no appreciable rearrangement was effected. 1
All temperatures, melting points, and boiling points are uncorrected.
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Heating the same quantities of compound and catalyst in a hydrogenation bomb with a little ethanol at 190" for six hours was also without pronounced success, although on distillation there was obtained about 2 g. of oil boiling at 205-210°/1 mm. B . With palladium-charcoal. A mixture of 25.0 g. of 2,6-dibenzalcyclohexanone and 1.0 g. of 10% palladium-charcoal catalyst was heated in a Claisen flask under reduced pressure (approx. 15 mm.) at a temperature of 235-245" (thermometer in the mixture) for five hours. Under the conditions employed the mixture was boiling; reflux condensation was aided by a stream of air directed against the flask neck. At the end of five hours the phenol was removed by distillation. There was collected 22.8 g. (91%) of 2,6-dibenzylphenol, distilling as a light yellow, viscous oil at 247-249"/15 mm. On long standing the phenol solidified and then melted at about 30" [reported, b.p. 235-238"/10 mm., m.p. ca. 30" (8, 9); b.p. 210°/3 mm., m.p. 30" (3)l. The acetate of 2,6-dibenzylphenol was prepared by the action of acetic anhydride in pyridine on the phenol. Recrystallization from aqueous acetic acid resulted in colorless needles, m.p. 76-77", in agreement with previous observations (3,s). C. With platinum. The platinum-charcoal mixture was employed in the same fashion as the palladium-charcoal catalyst. A mixture of the catalyst, obtained as described, and 25.0 g. of 2,6-dibenzalcyclohexanonewas heated for five hours at a temperature of 255-260" under reflux conditions (about 17 mm. pressure). The temperature was then raised (to 265"/17 mm.) until distillation ensued. There was collected 8.4 g. of 2,6-dibenzylphenol. This represents a conversion of about 34%. The remainder was an intractable gum. The effect of platinum black alone was also examined. The oxide, 0.15 g., was hydrogenated in 30 cc. of methanol. The platinum was removed by filtration, and added to 25.0 g. of the melted ketone in a Claisen flask. A few chips of dry ice allowed the transfer of the catalyst without sparking. The mixture was maintained at 260-265" under 17 mm. pressure for two hours. On attempted distillation (to 265"/17 mm.) there was obtained 7-8 cc. of oil. The residue was then heated for five hours longer at a temperature of about 270". Distillation was again attempted, but only about 1cc. more was obtained. The total yield was 8.6 g.; the residue was a tar. Rearrangement of carvone: With palladium-charcoal. A mixture of 25.0 g. of redistilled carvone [b.p. 224-226", n t 1.4983; the index reported by Linstead (5) for a sample purified through the semicarbazone was ng 1.49951 and 1.0 g. of 10% palladium-charcoal catalyst was heated in a Claisen flask for one hour under atmospheric pressure. A gentle reflux was maintained during this period; the internal temperature rose rapidly t o 232" and remained a t 232-233". The product was distilled from the flask; there was collected 23.1 g. (92%) of carvacrol, b.p. 232-234') n: 1.5213. The index of refraction found by Linstead (5) was n t 1.5223. The identity of the product was confirmed by the preparation of &nitrosocarvacrol, m.p. 153-154") in agreement with the literature (10). SUMMARY
The isomerization of 2,6-dibenzalcyclohexanone to 2,6-dibeazylphenol can be effected readily and in nearly quantitative yield by the use of a palladiumcharcoal catalyst. Platinum black was a less effective isomerization catalyst, and Raney nickel was without appreciable action. The use of a palladiumcharcoal catalyst also led to a nearly quantitative isomerization of carvone to carvacrol. ANN ARBOR,MICH. REFERENCES (1) HOFWING, Chem. Rev., 33, 89 (1943). (2) HARTUNG, J . Am. Chem. SOC.,SO, 3370 (1928). See also J . Am. Chem. Soc., 66, 888 (1944), footnote 7.
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Du FEU, MCQUILLIN, AND ROBINSON, J . Chem. SOC., 53 (1937). WALLACH, Ann., 381, 64 (1911); 403, 87 (1914); 414, 349 (1918). AND THOMAS, J . Chem. Soc., 1139 (1940). LINSTEAD,MICHAELIS, COVERTAND ADKINS,J’. A m . Chem. Soc., 64, 4116 (1932). ADAMS,VOORHEES, ANI) SHRINER, OTg. Syntheses, Coll. Vol. I, John Wiley and Sons, Inc., New York, 1941, p. 463. (8) WEISSAND EBERT,Momtsh., 66, 399 (1935). (9) SHORTAND STEWART, J . Chem. Soc., 553 (1929). Ber., 32, 1516 (1899). (10) KLACIES, (3) (4) (5) (6) (7)
