[CONTRIBUTION FROM
THE
FRICICCHEMICAL LABORATORY, PRINCETON UNIVBIRBITY]
CATALYTIC CIS-TRANS-ISOMERIZATION AND RESTRICTED ROTATION OF BIPHENYL DERIVATIVES W. I. GILBERT*, JOHN TURKEVICH,
AND
EVERETT 5. WALLIS
Received July 88, 1938; revised October 88, 1958
It is well known that halogen atoms, halogen halides, oxides of nitrogen oxygen, and colloidal sulfur catalyze cis-truns-isomerizations.l More recently Kuhn2has shown that such transformations can be brought about by certain metals. In his experiments on the isomerization of the dimethyl ester of maleic acid, and of cis-stilbene into the trans modifications he has observed that the change is catalyzed by alkali metals and catalytic platinum and palladium, but not by bivalent metals such as zinc, cadmium, mercury, magnesium, calcium, strontium, and barium. His results with gallium, indium, and thallium are inconclusive. Since all the above substances with the exception of the halogen acids have the electron configuration necessary for paramagnetism, Kuhn attempts to correlate his observations by stating that substances which possess a permanent magnetic moment, that is, are paramagnetic, bring about this transformation. Kuhn’s theory finds further support in the bromine photosensitized transformation in which the bromine atoms are the active agents. He explains the action of hydrogen iodide and hydrogen bromide as being due to the presence of a small but sufficient concentration of bromine and iodine atoms.* To explain the action of hydrogen chloride Kuhn invokes the strong polarizing action of the protons to produce the distortion of the ethylene bond necessary for isomerization. The first part of this paper deals with an attempt to extend Kuhn’s theory to other substances such as chlorides and oxides and to determine whether in these cases also there is a correlation between catalytic activity and magnetic behavior. I n Table I in the experimental part of this paper is given the magnetic characteristics of the substances tested and the experimental results obtained. From these observations it can be readily * Research Assistant on Special Funds from The Chemical Foundation. 1 For references see GRIGNARD AND BAUD,“Trait6 de Chimie Organique,” I, 1088 (1935), Paris. 3 R. KUHN,see FREUDENBERG, “Stereochemie,” p. 917 (1933), Leipzig. See also KHARASCH and co-workers, J . Am. Chem. Soc., 66,2468 (1933); ibid., 69, 1405 (1937). J. ORG. CHEM.,2, 288, 298 (1937). Further references are given in these papers. 611 J
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W. I. GILBERT, J. TURKEVICH, AND E. 8. WALLIS
seen that there is no direct correlation between the magnetic characteristics of the compound tested and their catalytic activity. Thus, the diamagnetic zinc chloride produces isomerization in five hours while the strongly ferromagnetic magnetite requires fifteen hours to produce a similar change. Furthermore, the paramagnetic ferric oxide and nickel chloride produce no change in the times indicated. The diamagnetic aluminum chloride must be left out of consideration because of the undoubted presence of small traces of hydrogen chloride always associated with this substance, which would catalyze the transformation. The difficulty of formulating an explanation of the results obtained in this study is one common to any problem in catalysis. At the start one one must assume a definite interaction between the catalyst and the ethylene bond. An examination of the structure of this double bond indicates that it has two types of electrons, the u electrons, and the 7r electrons, which are available for bond formation. These latter electrons are also the very electrons which stabilize the special configuration of the molecule (in this case maleic ester). Therefore, in any process of isomerization that does not involve a complete rupture of the ethylene compound into radicals it is these electrons that are involved. During the isomerization process, therefore, the coupling between these two electrons must be affected either by a distortion of their orbits (polarizing action) or by actual bond formation with the catalyst itself. Thus, the catalyst has definite conditions placed upon it. Kuhn claims that this polarizing action is exerted by the odd electrons present in the catalyst. The presence of an odd electron produces an unsaturation on the catalyst surface which results in the deformation of the orbits of the ethylene 7r electrons and thus produces the isomerization. The presence of the odd electron, however, is not necessary. It is to be remembered that the catalyst may have unsaturation in the Werner sense. By that, we mean that the catalyst molecule may have an incomplete octet, as for example, has aluminum chloride, or it may have an octet capable of being expanded into one of higher electron number in which the central atom exhibits its maximum covalency. Such a configuration can also accommodate the two electrons of the ethylene bond, and, thus, form a complex, and make possible the equilibrium. Ethylene bond
+ Catalyst
$ Complex,
which of necessity will ultimately produce the spacial form which is consistent with the thermodynamic requirements of the system cis @ trans.
When dealing with heterogeneous systems it is also to be noted that the efficiency of a catalyst depends not only on its electronic structure
cis-trans-ISOMERIZATION OF BIPHENYL DEMVATIVES
613
but also on the number of catalyst molecules available to the substrate. It is well known that this number can be increased by spreading the catalyst on an inert porous support. On the other hand the number may be increased during the process of isomerization itself. For, if the binding between the substrate and the catalyst is strong enough to overcome the binding between the catalyst molecules themselves, the latter will dissolve in the substrate and thus increase the number of effective catalyst inolecules. In other words under such conditions we shall have a very efficient catalyst. PART I1
Quantum mechanical considerations suggest an analogy between the restricted rotation of the ethylene bond and the restricted rotation which gives rise to optical activity in certain diphenyl compounds. Pauling and Sherman in their discussion4 of the resonance energy of biphenyl postulate a contribution of the following type
to the ordinary structural formula for diphenyl. Theoretical considerations of Lennard-Jones and Turkevich6 suggest that in biphenyl the carbon-carbon distance between the two rings is not 1.54A, the value found in aliphatic single carbon-carbon linkages but 1.44$ involving a shortening of the single bond and an approach to a carbon-carbon double bond. Finally, X-ray investigations of the structure of biphenyl and related compounds give as the distance of the axial carbon-carbon bond, a value of 1.4& again a value intermediate between that observed for the carboncarbon single bond and the corresponding double bond.6 Quantum mechanical theories of valence account for this shortening by resonance. Thus, on this basis one would expect to find in the carbon-carbon bond connecting the two rings in biphenyl some of the characteristics of the ethylene double bond, as for example, a shortening of the carbon-carbon distance, a diminished freedom of rotation, and a tendency to assume a planar configuration of the rings. Such double bond characteristics will have two possible effects on the optical stability of the substituted biphenyls. (1) The presence of asymmetry in biphenyl compounds is generally conceded to be due to a spatial repulsion of the ortho substituents, which does not permit the benzene rings to assume a planar configuration. The fact that the benzene rings do not lie in one plane insures the existence 4
6
PAULINO AND SHERMAN, J . Chem. Phys., 1, 633 (1933). LENNARD-JONES AND TURKEVICH, Proc. Roy. SOC.,A168,297 (1936). PICKETT, Proc. Roy. SOC.,A231,213 (1933); DHAR,Ind. J . Physics, 7 , 43 (1932).
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W. I. GILBERT, J. TURKEVICH, AND E. 8. WALLIS
of optical activity. The presence of a double-bond character in the axial carbon-carbon bond tends to counteract this steric repulsion of the ortho substituents in the rings and make the rings co-planar. Such an effect would tend to produce optical instability. (2) On the other hand, it should be remembered that isolation of the optically active form of a potentially asymmetric compound depends also on the ease of racemization. For instance the molecule RIR2R3N is potentially asymmetric since it has a pyramidal structure. Yet the ease of racemization
N
Ri Ra Rs
Ri Ra Ra
N
/I\
\I/
is so great that it cannot be isolated in an optically active form. The double bond character of the axial carbon-carbon bond in the biphenyls would decrease the ease of racemization. I n the first place it would shorten the carbon-carbon distance and thereby increase'the steric repulsion of the ortho substituents. In the second place the process of racemization involves rotation around the carbon-carbon bond. The double bond character of this bond would decrease the ease of rotation and consequently the ease of racemization. This effect would produce optical stability. In view of these considerations, attempts were made to determine whether those experimental conditions which produce the cis-trans isomerization of the ethylene double bond, and which temporarily destroy the double bond character, would racemize an optically active biphenyl derivative. For this study optically active 3 ,5-dinitro-6-a-naphthylbenzoic acid was chosen. The partially resolved material had a specific rotation [CY]:' = +8.39". From the results of the experiments described in the experimental part of this paper it must be concluded that the existence of the double bond in the carbon-carbon atom linkage between the two phenyl groups in biphenyl derivatives cannot be detected by use of those chemical agents which bring about the cis-trans isomerization. Two explanations may be given for this fact. (1) In reality there may be no contribution of the type
to the ordinary structural formula for biphenyl as postulated by Pauling and Sherman' in their discussion of resonance energy. (2) This contribution may be present, but, due to the size of the substituents on the phenyl groups, steric factors may come into play and prevent the catalyst from
cis-trans-ISOMERIZATION OF BIPHENYL
DERIVATIVES
616
affecting the coupling between these two P electrons, either by distorting their orbits or by actual bond formation with the catalyst, and thus prevent the formation of the necessary complex which of necessity on decomposition would give an equal number of d, and E forms. This point of view finds substantiation in certain experimental results of Adkins and his coworkers.’ In their work on the catalytic hydrogenation of substituted diphenyls these investigators have shown that one may easily hydrogenate all compounds of this type except those which possess the structure necessary for molecular asymmetry. These latter compounds resist hydrogenation even under the most drastic conditions. TABLE I *‘CORRELATION” OF MAGNETIC CHARACTERISTICS WITH CATALYTIC ACTIVITY BUBBTANCE
MAGNETIC IUBCEPTIBILITP
TIYE NECESSARY TO FORM CRYSTAL8
+0.51 -0.60
10 min. 1 hour 3 hours 5 hours
(X X 10s e.8.u.)
+86.
-0.47 -44. Ferromagnetic
-44.7 -0.58 -0.19 -0.23 -0.7
+m.
9 hours 15 hours None in 5 hours None in 5 hours None in 5 hours None in 5 hours None in 5 hours None in 5 hours
I t should be noted that our experiments on the interaction of sodium and the dimethyl ester of maleic acid were also carried out in the absence of oxygen. In all cases an appreciable amount of a red, water-soluble sodium derivative was formed, This could be easily isolated by taking up the reaction mixture in anhydrous ether. This fact, which was apparently not noted by Kuhn in his experiments, is significant, and i t suggests possibilities which should be further investigated. EXPERIMENTAL
Experiments on d i m t h y 1 maleate-Five-tenths of a gram of dimethyl maleate together with 0.1 g. catalyst was placed in a small glass test-tube, stoppered, and heated a t 100’ for varying amounts of time. The tube was cooled rapidly and the appearance of solid material was used as a criterion of isomerization since dimethyl fumarate is insoluble in dimethyl maleate. The above table gives the magnetic characteristics of the substances tested and the experimental results obtained. Experiments with d-S,6-dinitro-6-~~-naphthylbenzoic acid.-The 3,5 dinitro-6-CY naphthylbenzoic acid used in these experiments was prepared and partially resolved _____ 7
WALDEMAND, ZARTMAN,AND ADKINS,J. Am. Chem. SOC., 66, 4234 (1933).
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W. I. GILBERT, J. TURKEVICH, AND E. 8. WALLIS
according to the directions of Wallis and Moyeru. The specific rotation of the partially resolved material was [a]: = +8.38'. The ethyl ester of this acid was also made in accordance with directions described in the article referred to above. The ester so prepared was recrystallized from aqueous alcohol. It melted a t 94-96'; [a]: = +7.08. Effect of Platinum Black.-Two hundred and one milligrams of the acid was dissolved in 25 cc. of absolute alcohol containing 8 mg. of freshly reduced platinum black. The suspension was shaken for twenty-four hours. The optical activity was then measured and found to be a',"= 0.08". The result for a blank solution containing no platinum black waa a',"= 0.08". No racemization took place under the above conditions. Action of Metallic Sodium on the Ester.-A dry benzene solution of the ester containing 0.08 g. in 20 cc. was treated with 4 g. of metallic sodium wire. The mixture was thoroughly freed from air by repeated freezing and evacuation and then sealed off in an evacuated tube. The sodium wire turned a brownish-copper color. After shaking for fifteen hours the rotation was a',"= $0.06 while that of the blank was a: = +0.07. The same experiment was repeated a t 100' for forty-eight hours. The sodium in this case was molten and finely divided. There was no evidence of complete racemization under these conditions. Action of Ferric Chloride on the Ester.-Sixty-three and six-tenths milligrams of the ester in 20 cc. of absolute alcohol was treated with 1cc. of ferric chloride solution in absolute alcohol (0.1 g. of the salt). A blank containing no iron chloride was similarly prepared. Both solutions were allowed to stand for one day a t room = $0.10 for temperature. The optical activity was measured and found to be a'," = +0.09 for the blank. Thus, no racemization the ferric chloride solution and a'," occurred. Action of Light on the Optically Active A c i d . 4 n e hundred and twelve milligrams of the optically active acid was dissolved in 25 cc. of a chloroform-carbon tetrachloride solution, placed in a quartz tube, and exposed to bright summer sunlight for eight hours. No racemization occurred. Action of Bromine Atoms on Optically Active Acid.-The same solution was treated with 0.1 cc. of a concentrated bromine tetrachloride solution and exposed to sunlight and the strong electric light of a 150-watt lamp for twenty hours. No racemization was observed. A parallel experiment using the dimethyl ester of maleic acid showed that three hours time was sufficient to convert the maleic acid ester into the fumaric acid form. Other catalysts were studied but in no case was racemization observed under conditions which bring about the cia-trans isomerization.
We wish to take this opportunity to express our thanks to the Chemical Foundation for a grant-in-aid for this work. SUMMARY
The catalytic influence of sodium, and of certain metallic oxides and halides on the cis-trans isomerization of dimethyl maleate has been studied. Results have been obtained which show that there is no correlation between catalytic activity and magnetic susceptibility. 8
WALLISAND MOYER,ibid., 66,2598 (1932).
C~~-~T~TL~-ISOMERIZATION OF BIPHENYL DERIVATIVES
617
A study has also been made of possible catalytic effects of these substances on the racemization of d-3,5-dinitro-6-a-naphthylbenzoicacid in an attempt to ascertain by chemical means whether there be present in an optically active diphenyl derivative a contribution to the resonance energy of the double bond type,
as suggested by certain quantum mechanical considerations. A theoretical discussion is given of the results so obtained.
