WILLIAM J. BAILEYAND RAYMOND A. BAYLOUNY
2126
mixture was placed around t r a p C and the system was evacuated. The catalyst was warmed cautiously with a free flame, evaporating residual ethanol. As heating was continued ( t o M . 200' as determined by separate experiments emoloying a heating bath) gases desorbed abruptly from t h e catalyst, a process accompanied by evolution of heat and light, and samples of these gases were collected in the usual manner for mass spectrographic and radioactivity assays. The data in nos. 5 and 6 of Table I were obtained on gas samples collected in this manner. Radioactivity assay of the gas sample in question followed by appropriate pressure and volume corrections indicated that the sample contained only about 36y0 of the radioactivity known (no. 5, Table 11) t o be associated with the residual catalyst. Accordingly, the catalyst remaining after the above vacuum
[CONTRIBUTION FROM THE
VOl. 81
baking was analyzed for additional radioactivity by the usual wet combustion procedure, and found to contain radioactivity t o an extent accounting for an additional 24% of t h a t known t o be present. A solid residue remaining after the above wet combustion was filtered and re-assayed in the same way, accounting for an additional 11% of the radioactivity in no. 5 , Table 11. These results suggest that the thermal desorption process for Raney nickel samples containing adsorbed hydrogen and carbon monoxide not only involves conversion of the latter to methane, as discussed above, but also conversion t o some quite refractory form of carbon, possibly a carbon-nickel alloy. This question is under further investigation. STANFORD, CALIF.
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF
MARYLAND]
Pyrolysis of Esters. XVII. Effect of Configuration on the Direction of Elimination' BY WILLIAMJ. BAILEYAND RAYMOND A. BAYLOUNY~ RECEIVED JULY
25,1958
Vapor-phase chromatography showed that the pyrolysate from methyl cis-2-acetoxycyclohexanecarboxylate contained only methyl 2-cyclohexenecarboxylateand no methyl 1-cyclohexenecarboxylate. Similarly, the pyrolysate from the methyl trans-2-acetoxycyclohexanecarboxylatecontained 97% of the conjugated ester and only 3% of the unconjugated isomer. Thus it appears that no trans elimination occurred in this cyclic system. Additional evidence that no rearrangement occurs during pyrolysis was obtained from the fact that pyrolysis of methyl 3-acetoxycyclohexanccarboxylategave a mixture of methyl 2- and 3-cyclohexenecarboxylatesbut no conjugated ester.
Since Hurd and Blunck* proposed that esters pyrolyze by a unimolecular cyclic mechanism, evidence that supports this proposal has accumulated. One of the most convincing pieces of evidence is the demonstration that y-lactones that can not sterically form an intermediate six-membered ring are stable a t 600°, while larger lactones that can form a transient six-membered ring pyrolyze normally a t 500°.4 Alexander and Mudrak6 showed also that in five- or six-membered cyclic systems cis elimination occurred more readily than did trans elimination. While both the cis- and trans-2-methylindanyl acetates gave the same product, 2-inethylindene, a temperature approximately 200" higher was required for decomposition of the trans isomer. Similarly, pyrolysis of cis-2-phenylcyclohexyl acetate gave by a normal cis elimination 93% of 3-phenylcyclohexene but by an apparent trans elimination 7% of l-phenylcyclohcxene. The mechanism of this trans elimination is still a matter of speculation. Of course, Blomquist and Goldstein6 showed that, if the ring was large enough, a so-called trans-hydrogen could be eliminated by a normal cyclic process. Bordwell, et aZ.,' recently have shown that, if the activation of the hydrogen atom is great enough, (1) Previous paper in this series,
THIS JOURNAL, 81, 651 (1959). (2) Ofiice 01 Naval Research Fellow, 1955-1957; Goodyear Tire and Rubber Co. Fellow, 1957-1958. (3) C. D. Hurd and P. H. Blunck, THIS JOURNAL, 60, 2419 (1938). (4) W. J. Bailey and C. N. Bird, Abstracts of the 132nd National Meeting of the American Chemical Society, Miami. Fla.. April, 1957. (5) E. R. Alexander and A. Mudrak, THIS JOURNAL, 75, 1810, 3194 (1950); 73, 50 (1951). (13) A. T. Blomquist and A. Goldstein, ibid., 1 7 , 1001 (1955). (7) F . G . Bordwell and R. J. Kern, ibid., 71, 1141 (1955); F. G. Dordwell and M . L. I'eterson, ibid., T I , 1145 (1955); F. G. Bordwell and J. Weinstock, ibid., 7 7 , 6700 (1955).
ionic elimination reactions that normally proceed by way of a trans mechanism can be made to proceed through a cis mechanism. More recently, Bordwell and Landis* have shown that, if the trans hydrogen is sufficiently activated, the Chugaev elimination proceeds in a trans manner. It, therefore, seemed possible that, if the activation of the hydrogen atom was great enough, a trans elimination might take place during vaporphase ester pyrolysis. I n fact, a study of models showed that a transient six-membered ring was possible in a trans arrangement of the ester group and the hydrogen atom if the two groups were both equatorial and impossible if the two groups were both axial. Earlier work had shown that a carboxy group has a strong directional influence on the elimination so that pyrolysis of a p-acetoxybutyric ester produced almost exclusively the conjugated o1efin.g It seemed possible, therefore, that pyrolysis of methyl cis-2-acetoxycyclohexanecarboxylate (I) might give rise to some trans elimination. Mousseron and Canet'O pyrolyzed both ethyl cis- and trans-2-acetoxycyclohexanecarboxylates (as well as the corresponding xanthates) and reported, as the only product, the conjugated olefin, ethyl 2-cyclohexenecarboxylate. Although the product suggests a trans elimination, the authors pointed out the possibility that the elimination might have proceeded by a normal cis elimination to produce the unconjugated olefin IV which then isomerized to the more stable conjugated olefin 111. (8) F. G. Bordwell and P. S. Landis, sbid., 80, 2450 (1958). (9) W. J . Bailey and C. King, J . Org. Chcm., 41, 858 (1956). (10) M. Mousseron and M. Cane:, Compr. read., 448, 525 (1051); Bull. IOC. chim. France, 190 (1952).
Alay
,-,, l!).-)!)
ELIMINATION IN THE PYROLYSIS OF HSTEI~S
Recent work in this Lnboratoryll has shown that undcr carefully controlled conditions no rearrangement occurs during pyrolysis even with isomers of aromatic compounds. It seemed likely, therefore, that any rearrangement of the initial product could be prevented in the pyrolysis of the esters I and TI. It sccmed imperative to demonstrate
that the unconjugated ester IV would not isomerize to the conjugated ester I11 under the conditions of pyrolysis. I n many cases described in the literature evidence that an olefin did not isomerize when it was dropped through the pyrolysis furnace was accepted as evidence that isomerization did not occur during ester pyrolysis. However, there is always a possibility that the experimental conditions are not the same in these two cases. I t seemed a much surer procedure to synthesize the olefin independently by the pyrolysis of an isomeric ester under conditions at least as vigorous as those employed in the pyrolysis in question. Thus in order to establish whether methyl 3-cyclohexenecarboxylate (VI), if formed from the pyrolysis of I, would isomerize, methyl 3-acetoxycyclohexanecarboxylate (V) was prepared and 'pyrolyzed. Hydrogenation of methyl m-hydroxybenzoate produced an S5% yield of a mixture of methyl cis- and trans-3-hydroxycyclohexanecarboxylate which was acetylated with acetic anhydride to give an 80% yield of corresponding 3-acetoxy compound V. Pyrolysis of V a t 450" through a Vycor tube packed with Pyrex helices liberated 92% of the theoretical amount of acetic acid and produced a mixture of unsaturated esters. Vapor-phase chromatographic analysis indicated the pyrolysate consisted of a mixture of methyl 2- and 3-cyclohexenecarboxylates (IV and VI) with no methyl 1-cyclohexenecarboxylate (111). Infrared analysis indicated further that the mixture consisted of 58y0 of the 2-isomer IV and 4270 of the 3-isomer VI. For the two analyses the three possible unsaturated esters, 111, IV and VI, were synthesized separately. A Diels-Alder reaction of butadiene and methyl acrylate produced the methyl 3cyclohexenecarboxylate (VI). Bromination of cy( 1 1 ) W. J. Bailey and J. Rosenberg, THISJ O U R N A L , 77, 73 (1955); W. J. Bailey, J. Rosenberg nnd L. J. Young, i b i d . . 77, 1163 (1985); W. J. Bailey, R . L. Hudson nod C.-W. Liao, ibid., 80, 4358 (1958).
21 27
clohexanecarboxylic acid, followed by lreatirient of the product with methanol] gave the methyl 1-bromocyclohexanecarboxylate. Suhsequcn t treatment of the bronio ester with sym-collidine gave the desired methyl 1-cyclohexenecarboxylate (111). The methyl 2-cyclohexenecarboxyl~tc(IV) was prepared by the pyrolysis of methyl cis-2acetoxycyclohexanecarboxylate (I). These three unsaturated esters were used as standards for the vapor-phase chromatographs and infrared nnalyses. Since it was established that 110 rearrangement of the unsaturated IV or VI to the conjugated isomer I11 occurred during pyrolysis at 450°, the syntheses and pyrolyses of the methyl cis- and frans-2acetoxycyclohexanecarboxylates were undertaken. A Diels-Alder reaction between 1-acctoxy-1,3butadiene and methyl acrylate gave an SO% yield of a mixture of adducts from which the solid methyl cis-2-acetoxy-3-cyclohexenecarboxylatccould be crystallized in an over-all yield of 20Cj0. Catalytic reduction of this unsaturated ester gave tlie desired methyl cis-2-acetoxycyclohexanec~~l1oxylate (I) in a 62% yield. Pyrolysis of the cir-acetate I a t 435" liberated COYo of the theoretical amount of acetic acid. Vapor-phase chroniatoqraphy of the pyrolysate showed only a single peak corresponding to the methyl 3-cyclohcxenec~rbouplate (IV), with no evidence of the presence of the conjugated isomer 111. Catalytic reduction of methyl salicylate gave a 54y0 yield of a mixture of the methyl cis- and fruns2-hydroxycyclohexanecarboxylatc, which ~ K L S hydrolyzed and isomerized to give a 3s';: yield of the acid. solid trans-2-hydroxycycloliexanecarl~oxy~ic Acetylation, followed by esterification with diazomethane, gave the desired methyl tran\--?-acetoxycyclohexanecarboxylate (11). Pvrolpsis of this trans ester at 435' liberated S4(z of the theoretical amount of acetic acid. 11 vapor-phase chromatograph indicated that the unsaturated portion of the pyrolysate consisted of 97% .of conjugated isomer I11 and only 3yoof the unconpgated isomer IV. It must be concluded that under ideal conditions no rearrangement takes place duriny such a pyrolysis, even though a tendency to form thc conjugated isomer exists. I t also is obvious that the activation supplied by a carbomethoxy group is not sufficient to promote a trans elimination in spite of the fact that models show that the traus hydrogen can get very close to the carbonyl group. It appears that under the conditions (liquid-phase pyrolysis) employed by Mousseron and Canet l o the cis ester underwcnt a cis elimination to produce the unconjugated ester IV, which subsequently rearranged to the conjugated isomer 111. Very likely the trans eliminations observed by Alexandcr and Mudrak6 proceeded through a mechanism much different from that usually assigned to ester pyrolysis. The possibility remains that the presence of a group that would activate the trans hydrogen more than a carbornethoxy group would promote a trans elimination during the pyrolysis of a carboxylic ester.
2128
WELIAM J. BAILEYAND RAYMOND A. BAYLOUNY
Experimental 1 Methyl 3-Hydroxycyclohexanecarboxylate.--A mixture of 139 g. (0.915 mole) of methyl m-hydroxybenzoate, m.p. 71-73" (reported13 m.p. 68-70'), 110 nil. of absolute alcohol and 5 g. of Raney nickel catalyst was hydrogenated at 180" and 240 atmospheres. After 80 minutes, the pressure drop was 100 atmospheres (measured a t room temperature). The catalyst was removed by filtration and the filtrate was distilled through a 6-inch, helix-packed column t o give 128 g. (85%) of a liquid mixture of methyl cis- and trans-3-hydroxycyclohexanecarboxylate, b.p. 134-146" (19 mm.), n 2 5 ~1.4667 [reported b.p. 140-150" (14 tnm.)14; trans isomer1&b.p. 129-130" (12 mm.), ~ Z 1.4G60j. D Methyl 3-Acetoxycyclohexanecarboxylate (V).-To 200 g. (2.0 moles) of refluxing acetic anhydride, a mixture of 127 g. (0.80 mole) of methyl 3-hydroxycyclohesanecarboxylate and 50 g. of acetic acid was added dropwise. When the addition was completed, heating was continued for an additional 30 minutes. Distillation of the mixture through an l&inch, helix-packed column gave 130 g. (80%) of methyl 3-acetoxycyclohexanecarboxylate (I-), b.p. 86-87' (1.0 m m . ) , ~ * S D 1.4508. Anal. Calcd. for CloHItOI: C, 59.98; H , 8.05. Found: C, 60.28; H, 7.82. Pyrolysis of Methyl 3-Acetoxycyc!ohexanecarboxylate (V). -In two portions a total of 51 g. (0.25 mole) of methyl 3acetoxycyclohe:tanecarboxylate was pyrolyzed at a rate of 30 drops per niinute in a vertical Vycor tube packed with 1/8-inch Pyrex helices and externally heated a t 450' with a Hoskins 303 A electric furnace, as previously described.lG The pyrolysate, which was faintly yellow, was dissolved in ether and extracted several times with distilled water t o remove the liberated acetic acid. (Titration of an aliquot portion of the aqueous washings with standard base showed that of the theoretical amount of acetic acid was libcrated. ) The ethereal solution was dried over anhydrous magnesium sulfate and distilled through a 12-inch, helix-packed column to give in nine fractions 24.6 g. (70.3%) of a mixture of rnethyl 2- and 3-cyclohexenecarboxylate (IV and VI), b.p. 47-48' ( 3 . 5 nixn.), n% 1.4582-1.4587. Anal. Calcd. for CBHI?OZ:C , 68.57; H, 8.57. Found: C, 68.35; H , 8.6%. This mixture w a s analyzed in the Pcrkin-Elmer model 154 vapor fractometer on the "A" column a t a temperature of 150" with the gas pressure a t 20 Ih. per square inch and a recording range of 4. Under these conditions the retention time for the liquid was about 24 min., but the curve was not symmetrical. It had a definite shoulder on one side, indicating that there were two compounds in the mixture. Methyl 3-Cyclohexenecarboxylate (VI).--.& mixture of 27 g. (0.5 mole) of butadiene and 43 g. (0.5 mole) of methyl acrylate was heated at 130" for 1.5 hr. in a 300-ml. steel hydrogenation vessel. After the apparatus had cooled, the contents were distilled at reduced pressure through a 20inch Podbielniak column t o give 21 g. (307,) of methyl 3cyclohexenecarboxylate (VI), h.p. 80-82" (23 mm.), n z 6 1.4575 [reported b.p. 116" (100 xnm.),17 n% 1.460217; ~ Vapor-phase chrornab.p. 72" (I5 mm.),IS n 1 61.4635)18]. tography showed a single peak with a retention time of 24 min. under the conditions described above. Methyl cis-2-Acetoxy-3-cyclohexenecarboxylate.-A mixture of 74 g. (0.85 mole) of methyl acrylate, 95 g. (0.85 and 3.4 g. of N-phenyl-2mole) of l-acetoxy-1,3-b~tadiene~~ (12) T h e author; are grateful t o Miss Kathryn Gerdeman and Miss Jane Swan frrr the microanalyses and t o hliss Kathryn Gerdeman for the infrared spectra. T h e infrared spectra were determined on t h e pure liquids with a Beckman I R - 4 spectrometer. All melting points are corrected. (13) J. B. Colien and 11. W. Diidley, J. Chem. SOL.,1742 (1910). (I 1) A . Rinliorn and I;. Coblits, A n n . , 2 9 1 , 300 (1896). (1:) XI. I
