SAMUEL SIEGEL AND BASILDMUCHOVSKY
3132
g. (1 .O mmole) of oc-methyl-3,4-dipropionoxycinnamicacid
Vol. 84
5 ml. of acetone and filtered to give 0.175 g. ( 7 1 % yield) of white crystals, m.p. 248-252' dec. This material was recrystallized from water to give buff-colored crystals, m.p. 258-259' dec. 9 mixture of this material with the ClaH21NO8 degradation product ( m . p . 253-258" dec.) of the antibiotic hygromycin melted a t 253-259' dec. Furthermore, the infrared spectra of the two samples were identical, and the ultraviolet spectra of the degradation product and the synthetic material were essentially the same.
in 15 ml. of thionyl chloride was allowed t o reflux on the steam-bath for 2 hr. T h e excess thionyl chbride was removed by evaporation, a n d 5 ml. of benzene was added. The benzene was removed by evaporation. T h e benzene addition and removal was twice repeated. T h e resulting acid chloride was dissolved in 10 ml. of chloroform. A solution of 0.426 g. (1.0 mmole) of penta-0-acetyl-neoinosamine-2 hydrochloride (XIV) and 0.202 g. (2.0 mmoles, 0.28 ml.) of triethylamine in20 ml. of chloroform was magnetically stirred with the acid chloride solution for 2 hr. T h e chloroform solution was washed with water, dried over magnesium sulfate, and evaporated to give a gum which was dissolved in 15 ml. of ethanol. T h e ethanol solution was concentrated a t atmospheric pressure to 5-ml. volume and diluted with 5 ml. of water. This precipitated a gum which crystallized on scratching to give 0.517 g. (765% yield) of cream-colored crystals, m.p. 157-160' after two recrystallizations from dilute alcohol. T h e material had "A$': 209 (e 20,600) and 260 m p (e 18,700); A:",: 209 (e 21,100) and 260 m p (e 19,300); AH: 252 (e 12,900)and 316 m p (e 7,130); ;:A: 2.96, 5.69, 5.98,6.10,6.56(shoulder),6.63, and 8.13 p . ilnal. Calcd. for C32H3&015: C, Ti6.71; H, 5.80; ,'uT 2.07. Found: C, 56.83; H, 5.60; X,2.16. I n a second experiment this product was obtained in 69% yield.
Anal. Calcd. for C16H2lP\TOs: C, 54.08; €1, 5.96; N, 3.94. Found: C, 53.71; H,6.01; N,4.06. Paper Chromatography.-Circular paper chromatograms were r u n in the apparatus described by K a ~ v e r a u . ~ T~ he apparatus (26-cm. diameter) was purchased from the Shandon Scientific Co., London, Eng., and modified in the manner described by Kissman and Weiss.35 The paper used was a special Whatman #l filter paper (KCT-26) which had been slotted for the Kawerau apparatus. Solvents were mixed just before use, and the paper was not equilibrated with the solvent mixture. T h e inosaniine derivatives were detected with the silver nitrate-sodium hydroxide reagent described by Trevelyan, et ul.,a6 as modified by Bnet and Reyn0lds,3~ and the chromatograms were fixed by spraying with thiosulfate . __
2-Deoxy-2-(3,4-dihydroxy-~-methylcinnamido)-neo-inosi- (34)
E. Kawerau, C l z ~ o ~ n a l o g r a p h i cJ l e l h o d i , 1, S o . 2 , 7 (1936) [published by H. Reeve Angel and Co., 52 Duane Street, S e n Y o r k 7 ,
to1 (XVI).-A solution of 0.465 g. (0.69 mmole) of 1,3,4,5,6penta-0-acetyl-2-deoxy- 2- ( C Y - methyl - 3,4-propionoxycinnamido)-neo-inositol (XV) and 0.146 g. (1.45 mmoles, 0.20 ml.) of triethylamine in 25 ml. of anhydrous methanol was allowed t o reflux for 2 hr. T h e solution was acidified with glacial acetic acid and evaporated. T h e amber residual sirup was treated u-ith 1.5 ml. of glacial acetic acid a n d allowed to stand a t room temperature for 10 min. during which time crystnls separated The mixture was diluted with
x. Y . ] . (35) H. M. Kissman and M. J. Weiss, J , A m Chum. S o r . , 80, 5559 (1058). (36) \V. E. Trevelyan, D. P. Proctor and J. S. H a r r i s m . S a t t i r e , 166, 444 (1950). (37) E. F. L. J. h n e t and T. h l . Reynolds, a b & . . 174, 9:jO [ I ! J 5 4 > . (38) S . J. Angjal. 13. J. 3IcHui.h and P 'r, Gilham. .I C'A,m . .Sw , 1482 ( I i i 5 i l ~~
[ C O X T K I BU T I O N FROM THE
DEPARTMEST O F
CHEMIISTKY, fiNIVERSITV O F - k K A N S A S , FAYETTEVILLE, -kKK.]
Stereochemistry and the Mechanism of Hydrogenation of Cyclo-a1kenes.l IV. 4f erf-Butyl-1-methylcyclohexeneand 4-ter f-Butyl-l-methylenecyclohexane on Platinum Oxide and a Palladium Catalyst2 R 'I
SAMUEL SIEGEL AND
BASILDMUCHOVSKT~
RECEIVED OCTOBER 25, 1961 Tlie ratiu of the saturated stereoisomers obtained upon the hydrogeriatiun o f 4-te.i.t-but)-l-l-tneth~-lc~clol~exe11e (1) and 4-iert-butyl-1 -methylenecyclohexane (11) on reduced PtO? is ii function of the pressure of hydrogen. The teut-butyl group magnifies the steric effects over those previously observed. -1simple mathematical analysis of the previousll- proposed rriechanistic scheme is shown t o be consistent with the stereochemical information. T h e characteristics of the reaciion in the presence of a palladium catalyst are also accounted for with the assumption that the rate-limiting surface reaction occurs a t a later stage than that which pertains on platinum.
Previous stereochemical studies of the hydrogenation of 1,%-dimetliylcyclohexeneand several of its isomers on platinum' and palladium' catalysts had suggested that the distribution of the saturated stereoisomers, as well as the isomeric cycloalkenes formed concurrently, inay ser\Te to identify the product and/or rate-controlling surface reaction and also to define the geometry of the pertinent transition states. Arguments based upon conformational theory were employed and the treatment of mechanism was qualitative. In the present work, 4-tertbutyl - 1- methylcyclohexene and 4 - tert - butyl - 1(1) For t h e previous paper in this series, see S. Siege1 a n d G. V. Smith, J. A m . Chem. Soc., 8 8 , 6087 (1060). (2) The support b y a grant (NSF-G-9920) from t h e National Science Foundation is gratefully acknowledged. A grant from t h e Monsanto Chemical Co. provided a fellowship (B. I).) a n d additional valued assistance. ( 5 ) Taken in part from the M.S.thesis uf H. l ) . ,January l!?tiO. I I , S . Sear1 :tiid C, 1. S m i t h . .7, d w C / w m .Tor., 82, 6082 (10ti01.
methylenecyclohexane have been hydrogenated in the liquid phase (acetic acid solvent) and in contact with reduced PtO?. The bulk of the tevt-butyl group restricts the conformations of the six-inembered cycle to which it is attached t o those in which this group is equatorial or quasi equatorial5; co~iseyuently, conformational effects and aiialysis are simplified, and the resulting stereochemistry is more readily- identified with a simple, mathematical treatment of the proposed mechanistic scheme.
Experimental Preparation of 4-tert-Butyl-1-methylenecyclohexane.4-tert-Butyl-1-methylenecyclohexanewas prepared by the pyrolysis of the unsaturated acid obtained uia the Keformatsky reaction of 4-tert-butylcyclohexanoneand ethyl bromoacetate.Gs7 The crude olefin, b.p. 185-187' (730 n1rn.i (.?) S. Winstein and N.S . Holness, i b i d . , 77, 5562 (1955). :!IJ, )A'. Raker and J. B. Holdsworth, J . Chetn. Sor., 728 (19451 (7) H Cr,r,s ~ t i d0 . H. Whitham, i h i d , :38!12 ( 1 R A O I .
HYDROGENATION OF CYCLO-ALKENES WITH PLATINUM OXIDE
Aug. 20, 1962
3133
TABLE I HYDROGEKATION OF ON
4-k??'t-BUTYL-l-MEl"YLCYCLOHEXENE
PLATINIC OXIDE L,G
cas Isumrr
85 3,5
36' 3ti :3 9 40 :3 9 47 47 a
'
,l4the ratio changes in Rearranging 7 yields the predicted way when the pressure of hydrogen is eEa (k-3' f k4C6")lhtC/kac6" (8) increased. The chosen model for adsorption accounts for the closer approach to unity of the ratio which is substituted into 6 to obtain, after rearof stereoisomers obtained a t high presswe from 4- rangement of terms tert-butyl-1-methylcyclohexenethan from 4-tertkzck3C[E10~ ORc = (9) butyl-1-methylenecyclohexanefor in the former the + k-scOH k3C0~2 n-orbital of the double bond is equally approachable from either side of the molecule, whereas in and the latter, the cycle, which is frozen in the chair conformation by the tert-butyl group, provides greater hindrance from the one direction which yields the t r a m isomer than the other which yields An equation, identical in form, is obtained for the cis. The same limiting ratio of isorr,ers is obtained a t high pressure in the hydrogenation of 2 - the rate of formation of the trans isomer. The assumption that the rate-limiting surface methyl-1-methylenecyclohexane and, presumably, reaction is the formation of the "half-hydrogenfor the same reason. ated" state (reaction 3) provides the condition that k-3c/k4c k&, c.g., the pressure of hydrogen is sufficiently low, then the equation reduces to 13
Aug. 20, 1962
HYDROGENATION OF CYCLO-ALKENES WITH
where K c and K t are the equilibrium constants for the adsorption of the alkene to form, respectively, the cis or trans diadsorbed alkane. Under these conditions, the ratio of the isomeric alkanes is determined by the ratio of the rate constants for the formation of the respective “half-hydrogenated states” from the alkene. At high pressures of hydrogen, i t is probable that k16H > k-? arid accordingly the cis to t r a m ratio, would approach kZc/k2t, the ratio of the rate constants for the two modes of adsorption of the alkene. The manner in which OH will change with the external pressure of hydrogen will depend upon both the rate of the surface reactions involving hydrogen and the rate of transport of the hydrogen to the surface. If the former are relatively slow, OH will vary approximately as the square root of the pressure of hydrogen a t low relative pressures of hydrogen. When palladium catalysts are employed, the isomerization of the alkene is extensive, and the more stable saturated isomer (1,4-trans)is obtained in the larger amount. These results are also in harmony with those previously recorded, and we choose to interpret them in the same way:’ namely, that on palladium, the rate-controlling surface reaction is the reduction of the “half-hydrogenated state,” the previous steps being rapid and reversible by comparison. This condition also follows from the preceding analysis in that if reaction 4 is the rate-limiting surface reaction, k-3c/kdc > 1, and if in addition k3C5 k--?c,then the first term in the denominator will be dominant (note OH 6 1). Equation 10 reduces to
where K2,S represents the equilibrium constant relating the alkene to the half-hydrogenated state. Likewise d [ t r ~ n s ] / d= t k,tKz,,t[E]8~z
(15)
and the ratio
PL.crImk1
OXIDE
3135
terms of the properties of a postulated “stereochemically symmetrical intermediate.]’ This is a hypothetical surface complex which by further reaction with hydrogen can yield both cis and trans saturated disubstituted cyclo-alkanes.16 Structurally, i t corresponds to a dissociatively adsorbed alkene. The principal basis for advancing this proposal is the observation that upon hydrogenation over PtOl, 36y0 of the cis product is obtained from 4-methyl-1-isopropylidenecyclohexane(111) while 1-isopropyl-4-methylcyclohexe1ie(IV) yields 58%, and it is presumed that the latter is the most likely and indeed only observed isomerization product. In our view, an important isomerization product should be trans-I-isopropenyl-4-methylcpclohexane (V) which should yield almost exclusively the trans isomer, and thus account for the difference in stereochemical reduction of 111 and IV. This would require that the preferred configuration for the adsorption of 111 would lead, by further addition of hydrogen, to the trans isomer in contrast to the result predicted and found with 2-, 3- and 4alkyl-1-methylenecycloalkanes. The failure to observe V when reduced platinum oxide is the catalyst is to be expected on the basis of the considerable selectivity this catalyst displays for the reduction of alkenes which differ in the degree of substitution a t the double bond. I t is observed in the reduction of 111 and IV on a palladium catalyst where the olefins are equilibrated before half the initial charge has been reduced.Iti Apparently the most likely product of the reduction of the “stereochemically symmetrical intermediate” is an alkene. Hamilton and Btirwell17 have shown that the hydrogenation of 2-butyne over a palladium catalyst yields only &-’)-butene so long as the alkyne is in excess. H n-butane t/-
H
H G
In this instance, the probable intermediate G cannot be converted to a saturated hydrocarbon withThe significance of this expression is that the out the intervention of the desorbed alkene. And, ratio of saturated stereoisomers is a function of the consequently, structures such as these cannot be distribution of half-hydrogenated states (IT%,$, significant intermediates in the hydrogenation of K Z , ~and ~ )the rates a t which these react with hydro- alkenes. This result can be deduced from the principle gen ( k d C , k 2 ) . It appears likely that the most stable “half-hydrogenated” states whether “cis” or “trans” which has been employed successfully to rationalize will be bonded to the surface via equally substituted the stereochemistry of displacement,l8 ndditionlg carbon atoms, primary where possible, but other- and elimination*Oreactions in homogeneous systems. wise secondary, and the rate constants for the Fundamentally, a reaction proceeds most rapidly further reduction of these several intermediates v i a the transition state which conserves the overlap (kqC,k4t) should depend primarily on the immediate of the molecular orbitals involved in the concerted environment of the surface-bonded carbon atom. bond breaking and forming processes. Thus, nuClearly the reaction of these cycloalkenes on the cleophilic displacements on a carbon atom occur palladium catalystsfits a mechanism in which reaction with inversion of configuration. And both the addi4 is rate controlling, for not only does the cis/trans tion of electrophiles to a double bond and the elimratio approach the expected equilibrium distribu- ination of a,&substituents to form a double bond21 tion of the saturated isomers, but equilibrium (16) R. L. Burwell, Jr., B . K. Shim and H. C. Rowlinson, ibid., 74, among the isomeric alkenes is also realized. 5142 (1957). Sauvage, Baker and Hussey15 suggest that data (17) W. M. Hamilton and R. L. Burwell, Jr., Paper No. 44, Record such as described here and for the reduction of cyclo- of International Congress on Catalysis, Paris, June, 1960. (18) H. V. Hartel and M. Polanyi, 2. p h y s i k . Chem., B l l , 97 (1930). alkenes on a palladium catalyst are best explained in (15) S. Sauvage, R . H. Baker and A. S. Hussey, J . A m . Chem. SOC., 83, 3874 (1961).
(19) E. Bergmann, M. Polanyi and A. Szabo, ibid., B20, 161 (1933). (20) H. Eyring and M. Polanyi, ibid., B l 8 , 279 (1931). (21) D. H. R. Barton, J . Chem. SOL.,2174 (1949).
s. SIEGEL, G. IT.SMITH, B. DMUCHUVSKY, D. DUBBELL AND iv. HALPERN
3136
occur via transition states whose probable geometry conforms to the above principle. The surface reactions in which carbon to hydrogen bonds are formed appear to be analogous to the electrophilic displacement reactions on carbon atoms which take place with retention of configuration.?”-?4 In this transition state, the molecular orbital joining the carbon atom to the surface is divided so as to overlap both a surface orbital and the hydrogen atom being removed from the surface Of the possible one-step transformation products of the “stereochemically symmetrical intermediate” which would not yield an alkene directly but would be a reasonable step along the path toward saturation, the following appears to us to he most likely*6 R
I
K
R
* !4
(22) S . Winstein and T. Traylor, J . .lm. Cheni. .So,-,, 78, 2697 il$l56).
VOl. 84
Because the a-orbital in H is directed parallel to the surface, i t is not properly oriented to rehybridize in a manner which will both overlap with a hydrogen atom leaving the surface and the second indicated surface orbital without a prioy destruction of the r-bond. This reaction path would, therefore, be expected to have a higher energy barrier than one yielding the alkene directly. I n conclusion, the data and arguments presented here are consistent with the Horiuti-Polanyi mechanism for the hydrogenation of unsaturated hydrocarboils on an active surface.*& Our study shows how the stereochemistry of the reaction may be employed to define the rate-limiting surface reaction as well as to describe the geometry of the pertinent transition states. (23) H. B. Charman, E. 11. Hughes and C. K . Ingold, J . Chem. Soc., 2530 (1959). (24) M. M. Kreevoy and R. L. Hansen, J . A m . Chem. Soc., 83, 626 (1961). (25) R . L. Burwell, J r . , C h e m Revs., 57, 895 (1957). (26) F o r a detailed review of theliterature, see T. I. Taylor, “Catalysis,” Vol. V , edited by P. H . E m m e t t , Reinhold Puhlishing Corp., New York, ?;. Y . , 19.57. Chap. 5 .
[CONTRIBUTION F R O M THE DEPAKT.MEST O F CHEMISTKY, vNIVEKSITY O F ARKASSAS, FAYETTEVILLE, A!LRK.]
The Stereochemistry of the Hydrogenation of the Isomeric Xylenes and p-fertButyltoluene over a Platinum Catalystl~~’~ R Y S.SIEGEL,G. V. SMITH, B. DMUCHOVSKY, D. DUBRELL AND W. HALPERN RECEIVEDOCTOBER 2 5 , 1961 The ratio of cis and trans disuhstituted cyclohexanes which is obtained from the hydrogenation of the isomeric xylenes and p-ifvt-butyltoluene, dissolved in acetic acid and in contact with reduced platinum oxide, is a function of the structure of the substrate and the pressure of hydrogen. The proportion of the cis isomer increases with a n increase in the pressure of hydrogen for pressures over ti\-
