ligand Factors in the Isomerization of Olefins by Palladium Complexes Wm. H. Clement and Thomas Speidel Department of Chemistry, Ithaca College, Ithaca, N . Y . 14850 The double bond isomerization of 1-hexene in methylene chloride catalyzed by a series of
1,3-dichloro-2-hex-l-ene-4 (p-Z-pyridine N-oxide)-palladium(lI) complexes has been studied. The reaction is very sensitive to the electronic nature of the trans N-oxide ligand, and olefin isomerization results only when the N-oxide ligand dissociates from the original palladium-olefin complex. When the complex contains a strongly electron-withdrawing nitro group on the N-oxide, the dissociative equilibrium is favored in the direction of free ligand and produces a very effective catalyst system. If the complex is structured with p-methylpyridine N-oxide, dissociation i s negligible and minimal isomerization occurs. The experimental results support either the metal-hydride or a pseudo-x-allyl mechanism for the isomerization
THE
palladium(I1)-catalyzed isomerization of the double bond in olefins is a well documented reaction (Cramer and Lindsey, 19661, and several papers have recently suggested mechanisms for this isomerization (Cramer, 1968). This reaction is significant as an extremely undesirable side reaction which appears when higher a-olefins are used in the Wacker and related processes (Clement and Selwitz, 1964; Stern, 1967). No detailed, systematic study of ligand effects in this reaction can be found in the literature, and nearly all reports to date essentially neglect the metal ligands as having no significant influence in the catalysis. Cramer mentioned that the over-all rate of reaction is sensitive to the metals auxiliary ligands, but gave no data to allow interpretation of just how this influence is operative. Emphasis has been placed primarily on the starting olefin and the isomerized products. Most workers depict the generally accepted olefin-palladium complex intermediate and show this intermediate with the olefin pi-bonded to the palladium; however, the remaining bonds and species associated with the metal are often ignored. This may or may not be justified. The present research was undertaken to clarify the role of the ligands in the isomerization reaction and to identify further the nature of this catalysis. The results of the isomerization of 1-hexene in methylene chloride catalyzed by a series (p-2-pyridine N-oxide)of 1,3-dichloro-2-hex-l-ene-4 palladium(I1) complexes (Z = hydrogen, nitro, methoxy, or methyl) are given. The catalysts were generated in situ from the known ethylene complexes (Clement, 1967) by simple displacement of the ethylene by 1-hexene (Pregaglia et al., 1966), and the rate of migration of the double bond to internal positions was followed with each catalyst system. This scheme utilizes systematic variations of the electron distribution in the trans ligand (the 220
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 1970
N-oxide) by way of changes in the para-substituents, and readily allows for detection and analysis of ligand influences in the reaction. Experimental
Material and Analysis. Reagent-grade dichloromethane was purchased from the J. T. Baker Co. The I-hexene was obtained from the Gulf Research and Development Co.; 99% of the olefin content was 1-hexene. The method of Clement (1967) was used to prepare the dichloroethylene-N-oxide-palladium(I1) complexes. Each complex was freshly prepared before use. The olefin product analyses were determined by gasliquid chromatography on a Perkin-Elmer Model 820, using a 7-foot column of 20% tricresyl phosphate on 60j80 Chromosorb P a t 62" C (helium carrier gas). The 1-hexene was completely separated from the cis- and trans-2- and 3-hexenes. No attempt was made to resolve the four internal isomers; rather, they were treated as a single quantity of isomerized product. Appropriate blanks and standards were run to assure quantitative data. General Isomerization Procedure. The catalyzed isomerizations of 1-hexene were carried out as follows. The appropriate ethylene-Pd complex (5 x mole) was prepared in a 100-ml round-bottomed flask equipped with a serum-stoppered side arm for sampling. A solution of 1-hexene ( 5 ml, 4.7 x lo-* mole) in dichloromethane (10 ml) was added with magnetic stirring to the complex, which dissolved rapidly as it was converted to the hexene analog. The flask was quickly fitted with a condenser surmounted by a drying tube, placed in a warm water bath, and refluxed (42°C) with stirring. Samples (2-ml) were withdrawn with a syringe periodically, and the catalyst in each was quenched with an excess of tri-
phenylphosphine in acetone. The resulting mixtures were cooled in a dry ice-acetone bath, then centrifuged; the supernatant liquid was analyzed using gas-liquid chromatography. Modified Procedures. With the p-methyl N-oxide complex, additional runs were made as described above, except that in one case hydrogen ( 5 x lo-' mole) was present (no reduction to P d occurred), and in the other trifluoroacetic acid (0.1 gram) was added (a blank run with this acid alone gave no isomerization). A run was made with the p-nitropyridine N-oxide complex, in which mole) of ligand was present an extra equivalent (5 x in the reaction solution. Control runs were also made using the known catalyst system palladium chloride bis(benzonitri1e) (Sparke et al., 1965), and a standard room-temperature isomerization run was made with the p-nitro N-oxide catalyst, in which hydroperoxides were removed over alumina from the 1-hexene and methylene chloride prior to use. The reagents were then thoroughly purged with nitrogen, and the reaction was carried out under nitrogen. Characterization of Catalyst. Separate runs were made in which each catalyst, after 30 minutes of reaction, was reconverted to the ethylene form and isolated from the olefin solution in the following manner: Benzene (15 ml) was added under ethylene; after several minutes the solution was cooled in ice water and 50 ml of pentane was added. A yellow-brown solid precipitated. The supernatant liquid was syringed from the solid, and following a pentane wash under ethylene, the complex was blown dry with ethylene. The infrared spectra were taken of the complexes (Nujol mulls, on a Perkin-Elmer 337 infrared spectrophotometer) and these were identical to the spectra of authentic samples of their respective ethylene N-oxide complexes. In the case of the p-nitro ligand, it was possible to isolate the catalyst directly without reconversion to its ethylene precursor by blowing the reaction solution dry with nitrogen. The infrared spectrum of this residue matched that of the compound isolated by treatment of di-p-chlorodichlorobis (1-hexene) dipalladium with two equivalents of p-nitropyridine N-oxide, and is assumed to be 1,3-dichloro-2-hex-l-ene-4-p-nitropyridine N-oxide palladium(I1). The other N-oxide ligands led to extremely unstable 1-hexene complexes that decomposed during isolation. Infrared solution spectra of the p-nitro and of the p-methyl N-oxide catalyst systems were also taken, using matched KBr cells to cancel out the olefin and solvent adsorbtions. Displacement of Ethylene by 1-Hexene. Treatment of a solution of ethylene-palladium(I1) chloride dimer (0.5 mmole in 2 1 ml of CH2C12)with 2 ml of 1-hexene (16 mmoles) in a gasometric apparatus (under ethylene) led to the evolution of 0.79 mmole of ethylene. Similar treatment of 1,3-dichloro-2-ethylene-4(p-methylpyridine N oxide)-palladium(I1) (1.0 mmole in 2 1 ml of CHqC12) with 2 ml of 1-hexene led to the evolution of 0.71 mmole of ethylene a t essentially the same rate as with the dimer.
:
Results
Large variations in the effectiveness of the various dichlorohexene-N-oxide-Pd(I1) complexes in catalyzing the isomerization of 1-hexene a t 42"C in dichloromethane were found. The catalyst prepared from p-methylpyridine
N-oxide proved to be poor, whereas the one synthesized from the p-nitro N-oxide was efficient. The p-hydrogen and p-methoxy N-oxides gave catalysts of intermediate capability. The results of the study are reported graphically in Figure 1. The only complex capable of conveniently providing isomerization a t room temperature was the one containing the 4-nitro ligand (Figure 2). The relative maximum rates of disappearance of 1-hexene were: p-NOz, 100; p-OCH3,8; p - H , 4; p-CH3,l. Cramer and Lindsey (1966) suggested that transition metal compounds become catalysts for isomerization only
/'
70
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! 10
2
3
TIME (hr) Figure 1 . Per cent isomerization of 1-hexene a t 4 2 ° C v s . time (hours) p-Z-pyridine N-oxide ligand. 0,CHI; 0, H; v,CHIO; A, NO?
= 0
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-
-
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I
2
3
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TIME (hr) Figure 2. Per cent isomerization of 1-hexene a t room temperature vs. time (hours) Catalyst.
A,p-NOrpyridine N-oxide ligand; 8,PdC12. PPhCN
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
221
when they are converted to hydrides. I n support of this argument they have shown that LinPdCl, is activated as a catalyst by acid or hydrogen (hydride-generating reactions). I n the present work, the presence of hydride was not detected in the infrared studies. However, supporting evidence, but not proof, of a hydride mechanism was indirectly obtained when the complex prepared from the p-methyl N-oxide (the poorest catalyst) was used in concert with either hydrogen (no reduction to P d metal) or trifluoroacetic acid. I n the presence of these additives the rate of isomerization increased (Figure 3). This is in agreement with Cramer's hydride theory. The hydride mechanism also calls for inhibition by base. However, when the p-nitro ligand catalyst was used in the presence of free p-nitro N-oxide (a base which would not destroy the catalyst if ligand exchange occurred), negligible change in the rate of isomerization resulted. Infrared spectra of the reaction solutions and of the recovered catalysts gave no evidence of metal-hydride formation during the reaction. No infrared absorption was found in the 1700- to 2300-cm-' region. This does not preclude hydride formation; however, it does indicate that if hydride formed, it was present only in small quantities or had only transient existence. Significant information was obtained from the infrared spectra taken on the reaction solutions of the two extreme catalyst systems. I n the case of the active p-nitro catalyst, the prominent N - 0 band of the complexed N-oxide a t about 1200 cm-' was absent from the spectrum of the reaction solution. Instead, only the free ligand N - 0 band was observed, split a t approximately 1290 cm-' in the hexene-CH&l* solution. With the inactive p-methyl catalyst, however, only bonded ligand N - 0 was found a t about 1200 cm-'. The intense band of the free ligand, about 1240 cm-', was missing entirely. Consideration was given to the possibility that *-allyl complexes formed, which would deactivate the catalysts. Since the p-methyl complex was the weakest catalyst, it would be most suspect of having been deactivated through *-allyl formation. However, essentially all of this catalyst was recoverable as the ethylene derivative after 30 minutes of reaction time. A *-allyl, being very stable, could not have been converted back to the ethylene N-oxide complex by simple passage of ethylene through the solution. Also, a separate attempt to convert dichloro-
hex-1-ene-p-methylpyridine N-oxide-Pd(I1) to a 7-allyl via the procedure of Ketley and Broatz (1968) was made. No *-allyl formed. Therefore, it is safe to say that the p-methyl complex, as well as the others studied, did not react to produce deactivated 7-allyls. The removal of peroxides and the expulsion of air from the reaction system had little if any effect on the isomerization results. Only a 2% difference, which is within experimental error (26% us. 24% isomerization after 2 hours, room temperature, p-nitro catalyst), was detected between runs carried out with and without these experimental variations. It was determined also that 1-hexene exchanges as readily with the ethylene of 1,3dichloro-2-ethylene-4-(p-methylpyridineN-oxidei-palladium(I1) as with ethylene-palladium(I1) chloride dimer. At times, a slight decomposition of the catalyst was evidenced-a minute deposition of Pd metal on the wall of the flask-after approximately one hour of reaction. However, the isomerization pattern showed no change after decomposition was detected, and the solutions remained a clear orange color. N o significant alteration of the rate curves was found a t about one hour (Figure 1). I n any case, the differences between the catalysts were well established before decomposition occurred. At room temperature, no decomposition of the catalyst resulted during the 6 hours the reactions were followed. Discussion
Complexes of the type 1,3-dichloro-2-hex-l-ene-4-p-Zpyridine N-oxide catalyze the isomerization of the double bond in 1-hexene, but a t greatly different rates. These catalysts differ only in the substituent a t the para position of the pyridine N-oxide ligand, and therefore variances in their ac:ivities may be attributed to this structural change. More specifically, the electronic nature of the trans-N-oxide ligand, as determined by the character of the p-substituent on the ligand, greatly influences the catalysis. Reference to Figure 1 shows that the electron1968), on the withdrawing nitro group, '6 (Nelson et d., ligand produces a catalyst which is very active, whereas substitution of an electron-releasing methyl group, u - , at the para position of the N-oxide yields a very ineffective catalytic complex. A Hammett sigma-rho type relationship is, however, not obtained. The p-methoxy substituent is far out of line and does not provide the poorest catalyst
z 0 ca Figure 3. Per cent isomerization of 1-hexene at vs. time (hours)
42"C
Cocatalyst with p-CH1-pyridine N-oxide ligand. o, None 0 ,Tritluoroacetic acid
V , Hydrogen
ca
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TIME ( h r ) 222
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 2, 1970
system, as would be expected in a Hammett relationship; in fact, it yields a catalyst more active than the unsubstituted ligand. The infrared studies show that the active nitro catalyst exhibits ligand (the N-oxide) dissociation in the reaction solution, while the inactive methyl compound remains undissociated. Since all the catalysts are recoverable with the N-oxide ligand bonded to the metal via a precipitation procedure from the reaction medium, this dissociation must be reversible. It therefore appears that a ligand is lost from the palladium, probably in an equilibrium exchange with solvent in order to “activate” the complex for catalysis. The induction period which was observed in the room-temperature isomerizations using the nitro ligand (Figure 2) is in agreement with this suggestion. Significantly, since the olefin is already bonded to the palladium, this vacant coordination site is not needed, as has been suggested (Webster and Wells, 1968), to provide an empty ligand site where the olefin can enter into the catalyst complex. The most generally accepted scheme for the Pd(I1) chloride-catalyzed isomerization of olefins is
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T-
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In fact, this type of reaction is rapid and predominates when the initial catalyst complex contains aniline as the organic ligand. Many implications arise from this research. Certainly, ligand electronic factors do affect isomerization, a t least to the extent that they influence the bonding between the metal and ligand. Electron-withdrawing groups, which weaken the ligand-to-metal bond, cause dissociation of the complex, which leads to the necessary intermediates for catalysis. If the complex, however, retains its four original ligands, including an olefin, no isomerization will occur. Therefore, it may prove possible that in palladium(I1)-catalyzed reactions which convert olefins to other useful chemicals, the double bond isomerization can be prevented by careful ligand control. This phase of the investigation is being pursued. Acknowledgment
JI
(I)
The explanation for the results obtained with the p-methoxy ligand is that this olefin complex disproportionates partially and furnishes some free, catalytically active dimer-i.e.,
We are grateful to the National Science Foundation and the Gulf Oil Corp. for partial support of this research. literature Cited
R
(11)
Although the entire path from I to I1 has not been clearly established, and in fact the present experimental results may be interpreted to support either a hydride or a *-allyl type mechanism (Cramer, 1968), it is now apparent that a ligand must dissociate from I, or catalysis will not occur. s o Iv e n t
(I)
$
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py/ C1
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2
(111)
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Clement, W. H., J . Organometal. Chem. 10, p. 19 (1967). Clement, W. H., Selwitz, C. M., J. Org. Chem. 29, 241 (1964). Cramer, R., Accounts Chem. Res. 1, 186 (1968). Cramer, R., Lindsey, R . V., J . Am. Chem. Sac. 88, 3534 (1966). Ketley, A. D., Broatz, J., Chem. Comm 1968, 169. Nelson, J. H., Nathan, L. C., Ragsdale, R . O., J . Am. Chem. SOC.90,5754 (1968). Pregaglia, C. F., Donati, M., Conti, F., Chem. Ind. (London) 1966, 1923. Sparke, M. B., Turner, L., Wanham, A. J. M., J . Catalysis 4, 332 (1965). Stern, E. W., Catalysis Rev. 1, 73 (1967). Webster, D. E., Wells, P. B., Discussions Faraday SOC 46,37 (1968). RECEIVED for review August 29, 1969 ACCEPTED January 10, 1970
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