Metal-Catalysed Reactions of Hydrocarbons / 11-Hydrogenation of Small Alicyclic Rings

Scheme 11.1. Mechanisms involved in the reactions of cyclopropane with hydrogen and with deuterium.

positive cyclopropane order depending upon the ranges of surface coverage in which the measurements were made: the change in hydrogen order from positive to negative (at low temperatures) as its pressure is increased suggests that the slow step requires both a hydrogen atom and a vacant site on which cyclopropane can chemisorb, either as structure B or structure C (Figure 11.2). Where the orders are respectively unity and zero (Table 11.2), the slow step may be the collision of a gaseous cyclopropane molecule with a fully covered layer of hydrogen atoms. Once the propyl radical stage is reached, the way is open for the mechanisms envisaged for alkane exchange (Chapter 6) to operate. There are however one or two difficulties. (1) It is possible that the rate equation based on the above mechanism is not universally applicable. (2) There is no analogue to the stepwise exchange of alkanes, because the initial propyl radical must have at least one deuterium atom, and even although multiple exchange is usually the favoured process there are often subsidiary maxima at propane-d4 and -d2. The latter is perhaps easier to account for, but the former is less easily explained. The idea mentioned above (Section 11.2.2) that there was a ‘pool’ of atoms containing about half of each kind with which the C3 species equilibrated was severely criticised, with some justification because four or sometimes five disposable parameters were needed to model the whole propane distribution. The concept was however resurrected by

R.L. Burwell Jr. to interpret the way in which unsaturated hydrocarbons interacted with deuterium (Section 8.3.5).

We have already noted that the occurrence of hydrogenolysis of cyclopropane is easier than that of propane, because the initial chemisorption needs a smaller energy input. The fact that methane and ethane are the usual products shows that a second C––C bond breaking is followed by the speedy desorption of the C2 fragment, and the somewhat special and exigent conditions needed for total conversion to methane reflects the known greater stability of the C––C bond in ethane (or its predecessor). There have been several detailed discussions of likely intermediate strucutres; those shown in Scheme 11.1 are as reasonable as any.

11.3. HYDROGENATION OF ALKYLCYCLOPROPANES47

11.3.1. Mono-alkylcyclopropanes

The substitution of one of the six hydrogen atoms of cyclopropane for an alkyl group creates differentiated C––C bonds in the ring, so that for example the hydrogenation of methylcyclopropane can give either n- or isobutane; small amounts of the corresponding alkenes are also sometimes found. The first work on this reaction showed1,2,42 that, with a Pt/pumice catalyst, n-butane constituted 95% of the products at room temperature, a difference in activation energy of only some 7 kJ mol−1 being sufficient to decrease this figure to about 70% at 523 K. It reacted somewhat faster than cyclopropane, and although the kinetics were of similar form the differences in the orders of reaction under equivalent conditions suggested that it was more strongly adsorbed, and reacted faster because it covered more of the surface. This should provide some clue as to the mode of chemisorption of cyclopropane ring. The steric effect of the methyl group on the process of chemisorption was clearly negligible, and its role is more likely to have been as a partial electron donor to the ring, with a consequent stronger engagement to the surface through delocalised electrons. Other explanations are no doubt possible, as the substituent might affect the strength of C––H bonds on the opposite carbon atoms or of the C––C bond between them, thus favouring adsorption as the analogue of structures B or C (Figure 11.2). The apparent absence of structural information on methylcyclopropane does not help to resolve this question. The presence of the methyl group, or particularly of a longer chain alkyl group, also provides possible further modes of adsorption.

The easier breaking of the bond opposite the substituent has been frequently confirmed,19,27,47 although as we shall see there are some exceptions. A steric effect due to a methyl group may just be sufficient to make the C2––C3 bond more likely to break, but a weakening of this bond relative to the C1––C2 bonds in the

adsorbed state is also possible. This question also has not received much attention in the literature,42 although the reactions of more heavily substituted molecules provide some further insights (Section 11.3.2).

The regiospecificty of the ring opening is not simply a characteristic of the methylcyclopropane molecule; the nature of the catalytic metal is important as well. The percentages of isobutane found on metal films are:27 for platinum, 97% (in harmony with that cited above); for palladium, 85%; for nickel, 75%; and for tungsten about 50%. It may be that different intermediate species are formed (e.g. π -propenyl on Pd and Ni). In the reactions with deuterium, the extents of exchange were not the same in the two isomers. On nickel there was more stepwise and less multiple exchange in the n-butane than in the isobutane; with platinum, it was the opposite, while on palladium the two processes overlapped. All ten hydrogen atoms were exchangeable. There was however no exchange in the reactant except on tungsten film, where only the five ring hydrogen atoms were substituted.

The methylcyclopropane-hydrogen reaction has been used in extensive studies to evaluate particle size effects. Series of Pt/SiO2, Pt/A12O3, Pd/SiO2 and Rh/SiO2catalysts have been prepared in various ways, and their dispersions (D) exhaustively characterised by selective gas chemisorption and X-ray line profile analysis; values of TOF and of isobutane selectivity (Si ) for this reaction have been reported.19,20,30,47−49 Standard pretreatment sequences were applied to the precursor (oxidation, helium flushing, reduction (variable temperature) and further helium flushing (optional)). The ranges of dispersion used and changes in TOF and Si observed as dispersion was increased are recorded in Table 11.4; the standard reduction temperature (Tred) was 623 K and final helium flushing was used. The most startling observation was the extent to which TOF in particular varied with Tred. With Pt/SiO2,19,20,30,47 good activities were found after hydrogen treatment at ambient temperature, although reductions may not have been complete, but minimal rates were found after reduction at 473 ± 50 K, and rates some

TABLE 11.4. Effect of Degree of Dispersion (D) on the TOF and isoButane Selectivity (Si ) in the Hydrogenation of Methylcyclopropane at 273 K

a Change in TOF on increasing the dispersion

b Where two figures are given, they refer respectively to minimum and maximum dispersion c Difference in activation energies for C2––C3 and C1––C2 bond-breaking processes

d The effect is small at D < 40%

e TOF is maximal at D = 65%: the increase given is for 15–65% dispersion, and the factor depends somewhat on Tred. See text for other details

since composition and not ratio is measured, values of ratios may be subject to large experimental errors.

11.3.2. Poly-alkylcyclopropanes

The hydrogenation of Z- and E-1,2-dimethyl-, 1,1-dimethyl- and 1,1,2, 2-tetramethylcyclopropanes has been thoroughly investigated on silica-supported nickel,55 copper,56,57−59 rhodium,51,56 palladium52,56 and platinum54,56 catalysts by Hungarian scientists at the University of Szeged. The results in the main have been reported graphically as TOFs for individual products at a single temperature as a function of hydrogen pressure. Emphasis was placed on TOF rather than on product selectivity, which in most cases can only be estimated roughly. No attempt has been made to model the results kinetically, and rate dependence on the hydrocarbon pressure was not investigated: this might have thrown light on their strengths of adsorption, which can now only be guessed through the rate dependence on hydrogen pressure. It is usually preferable to report total rates and selectivities.

Great variation in the reaction parameters was found, depending on the reactant and catalyst used. Selectivities to the preferred product (i.e. 2-methylbutane; neopentane; 2,3,3-trimethylbutane) in most cases exceeded 90% and often were even higher: in general they were independent of hydrogen pressure, or showed only slight dependence. Rates fell in the sequence

Pt >> Rh > Pd > Cu ≥ Ni

Molecules containing a quaternary carbon atom were especially reactive over Cu/SiO2,59 which also at low hydrogen pressures afforded quite large amounts of isomerised alkenes having the preferred carbon skeleton. 1,2-Dimethylcyclopro- pane reacted about 2000 times faster over Pt/SiO2 than over Pd/SiO2.

The dependence of rate on hydrogen pressure adopted a variety of forms; some representative examples are shown in Figure 11.6, and some further information relating to Z- and E-1,2-dimethylcyclopropane is contained in Table 11.5. It is difficult to discern any systematic trends. Rates for alternative processes usually showed the same dependence on hydrogen pressure, but there were several exceptions. There was no regular dependence of reactivity on reactant structure for each metal. In some cases there was no maximum rate as hydrogen pressure was raised (Figure 11.6 (a)); with Pt/SiO2 at 318 K, 1,1-dimethylcyclopropane showed a maximum rate followed by a gentle decline, characteristic of competitive chemisorption54 (Figure 11.6 (c)), whereas the same reactant on Pd/SiO2 at 373 K showed an extremely sharp maximum52 (Figure 11.6 (a)). The strangest behaviour was shown by the tetramethylcyclopropane on Rh/SiO512 (Figure 11.6 (d)): other reactants performed as in Figures 11.6 (a) and (c). Further examples

TABLE 11.5. Parameters of the Hydrogen Pressure Dependence for the Hydrogenation of Z- and E-1,2-Dimethylcyclopropane on Silica-Supported Metals51,52,54,55,57

Notes:

1.The figures in columns 3, 4 and 5 relate to the process for the formation of 2-methylbutane, or the point at which this is maximal.

2.Temperatures were: for Ni, 473 K; for Cu, 498 K; the others, 318 K.

3.Forms of the rate vs. pressure curves are generally similar for both processes, and for fresh and run-in catalysts.

4.Similar results for ethyl-, 1,1-dimethyl- and 1,1,2,2-tetramethylcyclopropane will be found in the cited papers.

a As in Figure 11.6(b)

b As in Figure 11.6(c)

c Rate attains maximum value and stays constant. d As in Figure 11.6(d)

e At PH = 65 kPa.

f S2MB varies with PH : this value is for low PH

of irrational behaviour will appear in Section 11.5. This situation calls to mind Ovid’s definition of chaos, which he called rudis indigestaque moles - a rough and indigestible mass. More recently it has been described as A state of order that we do not yet understand.

A clearer picture emerges from limited work on metal films,60 where the Z-isomer of 1, 2-dimethylcyclopropane gave much more of the disfavoured product (n-pentane) than did the E-isomer, especially on platinum (see Table 11.6). The disposition of the methyl groups in the E-isomer clearly militates against flat chemisorption of the ring and the fission of the C1––C2 bond, whereas in the Z-isomer this is possible if less likely than C2––C3 bond fission, perhaps because

TABLE 11.6. 2-Methylbutane Selectivity in the

Hydrogenation of 1,2-Dimethylcyclopropane

Isomers on Metal Films85

Figure 11.7. Representations of the adsorbed state of (a) Z - and (b) E -1-dimethylcyclopropane; (c) 1,1-dimethylcyclopropane; (d) 1,1,2,2-tetramethylcyclopropane.

of an electronic factor. Steric inhibition due to methyl groups in the chemisorption of the other molecules is represented in Figure 11.7.

The reactions of 1,1-dimethyl- and 1,1-diethylcyclopropanes with hydrogen and with deuterium has also been examined on films of all the metals of Groups 8 to 10 (except Ru and Os);61 the results have a pleasing simplicity. Ring fission was exclusively at the C2––C3 bond and with deuterium the 1, 3-dideuteroalkanes were the chief products, except on platinum and to some extent on iridium, where -d3 and -d4 alkanes were also found. The presence of the quaternary carbon atom forbids the easy αβ exchange, so αα-diadsorbed species must have been responsible for further exchange. Hydrogenolysis on the base metals gave methane and isobutane from the dimethyl-compound.62,63 Further studies61 with hydrogen-deuterium mixtures led to the conclusion that hydrogenation occurred on platinum by simultaneous addition of two atoms to 2,3-diadsorbed species. A study of the reaction of 1,1- dimethylcyclopropane with deuterium on Cu/SiO2 has also been reported.64

11.3.3. The Cyclopropane Ring in More Complex Hydrocarbons1

The cyclopropane ring can exist within larger molecules. For example the whole family cyclopropylmethanes (C3H5)xCH4−x (x = 1–4) has been synthesised, and detailed studies of dicyclopropylmethane (x = 2) have been carried out with various platinum and nickel catalysts.65 The routes whereby it is converted into C7 products on platinum is shown in Scheme 11.2: steps in which bond scission adjacent to a substituent occurs are a consequence of dissociative chemisorption on the side-chain. Clearly the product formed by two-fold breaking of the bonds opposite the substituent predominates, although others are significant. This molecule provides an easy way of assessing mass-transport within porous catalysts through

Scheme 11.2. Reactions of dicyclopropylmethane with hydrogen on platinum and nickel catalysts; the numbers relate to the reactions on platinum.

measuring the relative amounts of the intermediate and final products formed. On nickel catalysts there was some demethanation and less of the intermediates, 2-methylhexane forming some 70% of the C7 products.

Modes of breaking the cyclopropane ring within more complex hydrocarbons give some further insights into the flexibility with which it can chemisorb.1,10 Thus spiropentane (bicyclo[2.2.0]pentane) is hydrogenated to 1,1-dimethyl- cyclopropane, although neither ring can easily lie flat on the surface. With bicyclo[2.1.0]pentane, however, the bond between the points of substitution breaks first because more strain is thus released, and the product is cyclopentane. Similarly addition of methylene to 9,10-octalin forms a cyclopropyl ring between the pre-existing rings, and this is hydrogenated to give E - and Z -9-methyldecalins. In the reaction of 3,3-dimethylnortricyclene to 7,7-dimethylnorbornane, it is hard to see how the reactant can chemisorb so that the relevant bond is broken: considerations of strain-release must again predominate. These processes are illustrated in Scheme 11.3.

11.4.HYDROGENATION OF CYCLOPROPANES HAVING OTHER UNSATURATED GROUPS

Methylenecyclopropane in the free state may also be regarded as trimethylenemethane9 (Scheme 11.4A), but on chemisorption the former structure is the more appropriate because with this compound hydrogenation gives simultaneously methylcyclopropane and butanes in which the n-isomer predominates.1,2,66 Breaking at the C––C bond opposite the point of substitution also occurs with ethenylcyclopropanes, and prior chemisorption at the CC bond facilitates the splitting of the nearer C––C bond. However the insertion of a methylene group

Scheme 11.3. Some further reactions of compounds containing the cyclopropane ring.

between the double bond and the ring insulates the latter, and the normal mode is resumed.

A detailed study of the hydrogenation of methylenecyclopropane (MCPr) on Pt/pumice was reported many years ago.2 The hydrocarbon was more strongly chemisorbed than hydrogen (and even than isobutene, with which it was compared), the rate passing through a maximum at low MCPr pressure. The reactants were competitively adsorbed, but in the reverse order from that seen with cyclopropane and hydrogen. The activation energy was 73 kJ mol−1. At 290 K the butanes were the sole products (n-butane selectivity 70%), but at 373 K the major products were the butenes, all of which were found. Diadsorbed intermediates having had the ring broken were displaced as butenes by MCPr and they only began to be hydrogenated after about 60% of the MCPr had reacted.

Hydrogenation of 3-carene on platinum gave Z -carane, but on palladium migration of the double bond into conjugation with the ring allowed splitting of the strained ring C––C bond to give a cycloheptane10 (Scheme 11.4B). The strong