J. Chem. SOC., Faraday Trans, I , 1987,83 (8), 2359-2363 The Mechanism of Hydrogenolysis and Isomerization of Oxacycloalkanes on Metals Part 8.l-New Results on the Mechanism of Hydrogenolysis of Oxiranes on Platinum and Palladium Ferenc Notheisz, Agnes G. Zsigmond, Mihaiiy Bartdk* and Gerard V. Smith7 Department of Organic Chemistry, Jbzsef Attila University, Szeged, Hungary The transformations of methyloxirane and cis- and trans-2,3-dimethyloxir- ane have been studied on Pt/C and Pd/C catalysts at 373 K in the hydrogen pressure range 2-70 kPa in a circulation reactor. The reactions of the dimethyloxirane isomers proceed by similar mechanisms, the rate- determining step in both cases presumably being cleavage of the C-0 bond. The mechanism for methyloxirane differs from that for the dimethyloxiranes.One mechanism involves edgewise adsorption of the oxirane (probably via two non-bonding electron pairs), while the other involves flat-lying adsorption. The change in regioselectivity with increase in hydrogen pressure is caused by supression of the adsorption of oxygen via two electron pairs. Relatively few publications have appeared on kinetic studies of transformations of oxiranes. Tenma and Kwan2 investigated the vapour-phase isomerization and hydro- genolysis of oxirane on Pt, Pd and Ni catalysts. On Pt at 373 K in the hydrogen pressure interval 6-26 kPa, the rate of hydrogenolysis was found to be described by r = kpgzpZxirane (n = 0.5). They proposed the reaction mechanism depicted in scheme 1. I * 8 + H I H I \r, CH3-CH2- ?H * * Scheme 1.The transformation of ethyloxirane on Pt/C was studied by Davidova and K r a ~ s . ~ Interpretation of their results is hampered by the fact that the catalyst they used behaved as a bifunctional The reactions of the dimethyloxiranes on various metals were examined by Cornet and coworkers,6-8 who put forward the mechanism outlined in scheme 2. A study has been made of the effect of variation of the hydrogen pressure in the case of trans-2,3-dimethyloxirane on Pd film at 293 K7 The rate of isomerization did not change in a regular way with the hydrogen pressure; with regard to the rate of the hydrogenolysis, it was found that n = -0.4. Our own investigations to date have led to our drawing the following conclusions in t Present address : Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, U.S.A. 23592360 Hydrogenolysis of Oxiranes on Pt and Pd Me Me I I +"' - +H 0' bMe -Pt Pt Scheme 2.connection with the mechanism of hydrogenolysi~:~ (i) we accept the mechanism in scheme 2; (ii) the rate-determining step is different for cis- and trans-2,3- dimethyloxiranes ; (iii) the rate-determining step for methyloxirane and for cis-2,3- dimethyloxirane is the transformation of the di-adsorbed species, while for trans-2,3-dimethyloxirane it is the ring-opening process. The question arises as to the validity of the mechanisms in schemes 1 and 2. Can both be correct? Does the mechanism depend on the structure of the oxirane and/or on the reaction conditions? Is the observed negative order for hydrogen characteristic of the trans-2,3-dimethyloxirane or of the Pd catalyst? The answers to these questions necessitate a comparison of the transformations of methyloxirane and cis- and trans- 2,3-dimethyloxiranes on the same catalyst, and a study of the behaviour of Pt and Pd catalysts with the same model compound.Experimental A circulation reactor, described earlier,l0 was used for the measurements. Methyloxirane was a B.D.H. product; cis- and trans-2,3-dimethyloxiranes were prepared as reported previou~ly.~ The preparation and characterization of the 18.5% Pt/C catalyst (D = 0.22) has also been published.1° The 5% Pd/C catalyst (D = 0.13) was purchased from the Engelhard Co. The catalyst dispersion was measured by hydrogen chemisorption.Before use, the catalyst ( 5 x g) was pretreated as reported earlier.lO Results and Discussion Fig. 1 shows how the transformation of cis-2,3-dimethyloxirane on Pt/C varied as a function of the partial pressure of the oxirane. Fig. 1 shows that at oxirane pressures > 1.3 kPa the initial rate is independent of the partial pressure of the oxirane. The transformations of cis- and trans-2,3- dimethyloxiranes have also been studied as functions of hydrogen pressure [see fig. 6 and 7 in ref. (lo)]. The rate of isomerization exhibited a maximum as a function of hydrogen pressure, this maximum being particularly sharp for the cis isomer. The rate of hydrogenolysis rose continuously as the hydrogen pressure was increased. The shapes of the curves indicate that the ketone is formed basically via a dissociative mechanism, and the alcohol via an associative mechanism.Table 1 lists the kinetic parameters calculated from the experimental data. These suggest that the equation derived by Tenma and Kwan holds for both isomers on a Pt catalyst. The data also reveal that the optimum hydrogen coverage for ketone formation is the same for the two isomers. The identical hydrogen dependences make it improbable that the rate-determining step differs for the two isomers. The appreciable differenceF. Notheisz, k. G. Zsigmond, M . Bartdk and G. V. Smith 236 1 1 i plkPa Fig. 1. Effect of partial pressure of cis-2,3-dimethyloxirane on the initial rate of appearance of butan-2-01 over Pt/C catalyst (pH2 = 2 kPa, temperature = 373 K).Table 1. Rate constants for the transformation of 2,3- dimethyloxiranes at 373 K over Pt/C catalyst compounds k / 1 O-* mol m-2 s-l n kPa cis trans cis trans isomerization 5.2 0.58 0.5-2 0.93 0.56 0.5-2 h ydrogenol ysis 3.8 0.55 2-15 0.95 0.58 2-15 Table 2. Transformation of the mixture of 2,3-dimethyloxiranes at 373 K over Pt/C catalyst in a circulation reactor ~~ ~ t/min [trans]" [cis]" butan-2-one" butan-2-01" 0 8.03 9.54 0 0 2 8.01 8.02 0.48 0.34 16 8.03 7.14 0.96 0.8 1 a Units: mol. Table 3. Initial rates and selectivities of transformation of methyloxirane on a Pd/C catalyst at 373 K initial selectivities initial rates of disappearance p,JkPa of methyloxirane/ low6 mol min-l m-2 hydrocarbons acetone propan-2-01 2.1 6.4 9.8 12.8 63.8 5.6 11.3 12.7 14.3 16.6 5.3 49.3 45.4 2.6 41.1 56.2 2.4 38.1 59.5 2.0 34.4 63.6 1.8 30.5 67.72362 Hydrogenolysis of Oxiranes on Pt and Pd Table 4.Initial rates and selectivities of transformation of methyloxirane on Pt/C catalyst at 373 K initial rates of disappearance initial selectivities of methyloxirane/ pH2/kPa loh5 mol mh-l m-2 hydrocarbons acetone propan-2-01 propan- 1-01 0.65 0.87 2.15 1.71 6.7 2.35 13.3 3.52 20.0 5.1 1 36.0 7.78 48.7 8.84 5 25 65 5 5 23 60 12 4 22 59 15 4 22 58 16 4 22 57 17 3 21 53 23 3 21 52 24 Table 5. Initial selectivities of the transformation of oxiranes over Pt/C catalyst at 373 K meth yloxirane Q,a(2,3-dimethyloxirane) PH1 SOU Qob cis trans 0.65 0.048 0.75 - 2.15 0.12 3.45 - 6.7 0.152 13.3 0.174 20 0.168 36 0.253 48.7 0.246 65 - 0.288 0.273 0.339 0.361 0.305 0.253 0.226 0.223 0.280 0.90 0.80 0.65 0.62 0.42 0.40 0.32 0.20 - - - - - - 0.10 0.10 - - So = (propan-1 -01 + propanal)/(propan- 1-01 + propanal + acetone + propan-2-01).Qo = ke- tone/(ketone + secondary alcohol). observed in the rates for the cis and trans isomers indicates that the identical rate- determining step is the ring-opening stage. Table 2 presents data on the transformation of a mixture of the cis and trans isomers in a circulation reactor. In the presence of the cis isomer, the trans isomer underwent no conversion. The failure of the trans isomer to react is probably due to its weaker adsorption. Initial rates of the formation of propan-2-01 increased with increasing partial pressure of hydrogen on both catalysts. The kinetic orders with respect to hydrogen are 0.4 on Pd/C and 0.5 on Pt/C.The two mechanistic conceptions mentioned in the Introduction were examined. If it is assumed that the formation of the carbonyl compound is not an equilibrium reaction, the ratio of carbonyl compound and alcohol at a given temperature in the mechanism of scheme 1 must be independent of the hydrogen pressure, whereas in scheme 2 this ratio must decrease as the hydrogen pressure is raised. In an attempt to distinguish between the two mechanisms, an investigation was made of the initial selectivities' So and Q, (see tables 1 and 2) of the transformations of methyloxirane and cis- andF. Notheisz, AI. G. Zsigmond, M . Bartbk and G. V . Smith 2363 trans-2,3-dimethyloxiranes on Pt/C (table 5). So denotes the regioselectivity of C-0 bond breaking, while Q, stands for the selectivity of ketone formation.Table 5 illustrates the variation in the selectivities Q, as functions of the hydrogen pressure. For methyloxirane, Q, does not change. For the dimethyloxiranes, however, ketone is formed with high selectivity at low hydrogen pressure, while mainly secondary alcohols are formed at high hydrogen pressure. It was earlier founds? ' 1 l1 that the ketone does not undergo hydrogenation as long as unchanged epoxide is present in the system; the difference in behaviour therefore indicates that two different mechanisms are involved. The mechanism in scheme 1 is followed in the case of methyloxirane, and that in scheme 2 for the dimethyloxiranes. We concluded previously1° that edgewise adsorption occurs for methyloxjrane, and flat-laying adsorption for the dimethyloxir- anes.Thus the mechanism of scheme 1 is associated with edgewise adsorption, and that of scheme 2 with flat-lying adsorption. The regioselectivity So also varied markedly with hydrogen pressure. On both Pt/C and Pd/C, the probability of cleavage of the C-0 bond adjacent to the substituent increased as the hydrogen pressure was raised. This observation can be explained if we accept that cleavage in the sterically less hindered direction is connected with the adsorption of oxygen via two electron pairs. As the hydrogen pressure is raised, the probability of adsorption of the oxygen via two electron pairs will progressively diminish, whereas in the adsorption via one electron pair both C-0 bonds may undergo cleavage.Our new findings may be summarized as follows: (i) the alcohol is basically formed through an associative mechanism; (ii) in the transformations of cis- and trans- 2,3-dimethyloxiranes, the mechanisms and the rate-determining steps are the same; (iii) the rate-determining step is probably cleavage of the C-0 bond; (iv) the mechanisms of transformation of the oxiranes on Pt and Pd catalysts are the same; (v) the mechanisms of transformation of methyloxirane and dimethyloxirane differ : one mechanism (scheme 1)) is connected with the edgewise adsorption of the oxirane, and the other (scheme2) with the flat-lying adsorption of the oxirane; (vi) the change in the regioselectivity as the hydrogen pressure is increased is caused by the fact that the two-electron-pair adsorption of the oxygen is suppressed. We acknowledge support for this research provided by the Hungarian Ministry of Culture (grant P-167/1984), and wish to thank a referee for his thorough review of the original manuscript. References 1 Part 7, M. Bartok, F. Notheisz, A. G. Zsigmond and G. V. Smith, J. Mol. Catal., to be published. 2 S. Tenma and T. Kwan, Shokubai, 1958, 15, 11. 3 H. Davidova and M. Kraus, J. Catal., 1980, 61, 1. 4 M. Bartok, F. Notheisz and J. T. Kiss, J , Catal., 1981, 68, 249. 5 M. Kraus and H. Davidova, J. Catal., 1981,68, 252. 6 D. Comet, Y. Gault and F. G. Gault, Proc. 3rd International Congress of Catalysis (North-Holland, 7 G. Senkchal and D. Comet, Bull. SOC. Chim. Fr., 1971, 773. 8 G. Senkhal, J. C. Duchet and D. Comet, Bull. SOC. Chim. Fr., 1971, 783. 9 M. Bartok, F. Notheisz and A. G. Zsigmond, J . Catal., 1980, 63, 364. Amsterdam, 1965), p. 1184. 10 M. Bartok, F. Notheisz, A. G. Zsigmond and G. V. Smith, J. Catal., 1986, 100, 39. 11 F. Notheisz, A. Molnar, A. G. Zsigmond and M. Bartok, J. Catal., 1986, 98, 131. Paper 611375; Received 9th July, 1986