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

Scheme 11.4. Unsaturated cyclopropanes and some of their reactions

isomerising propensity of palladium also enables an alkene in which the ring has been broken to desorb, and where there are unsaturated functions at C1 and C2, it is the intervening C––C bond that breaks preferentially (see also Scheme 11.4C).

Cyclopropene is a highly strained system, but loss of a hydride ion gives the cyclopropenyl cation, which is formally an aromatic compound; the tri(cyclopropyl) cyclopropenyl cation is very stable and can even exist in aqueous solution. Notwithstanding the very great strain in the parent hydrocarbon, the hydrogenation of methyl-substituted cyclopropenes has been reported.11

Phenyl substituents have the same effect as alkenic unsaturation in promoting ring fission adjacent to the point of substitution, but a study of the hydrogenation with deuterium of a number of phenyland phenyl-methyl-substituted hydrocarbons at temperatures between 369 and 443K adds some further refinement to this simple picture.66 In most cases the products contained only two deuterium atoms, added across a C––C bond adjacent to the phenyl group, but where the carbon atom bearing the phenyl group also carried a hydrogen atom this also was liable to be exchanged after ring opening: there was no exchange in any of the reactants. With the 1,2-phenylmethylcyclopropanes, the bond broken was that opposite the methyl group. Placing a methylene group between the phenyl group and the ring again interfered with the conjugative effect and lowered the reactivity.

Consideration of the likely steric interference between the substituents and the surface led to the view that reaction involved arrival of the hydrocarbon at

the surface with one of the unsubstituted ring carbons pointing towards it (‘corner orientation’),66 followed by attack by a deuterium atom. It is clear that in some cases ‘flat’ chemisorption (as in structure A, Figure 11.2) would be at best weak and that αγ -diadsorption by opening of the bond that predominantly broke would be almost impossible. The proposed mechanism therefore is the only remaining alternative; catalysis is, like politics, the art of the possible. It is however risky to believe that conclusions based on heavily substituted rings inform on the mechanism of reaction of the unsubstituted ring, and that product analysis alone can allow a definitive model to be reached. In the absence of kinetic information (reaction orders, activation energy etc.), no inkling of the effect of substituents on chemisorption strength can be obtained. In the interpretation offered, the electronic effects of the substituents worked only on the free molecules, and affected only the disposition of charge within them, and hence their reactivity. In particular no thought was given to the possibility that the aromatic ring might engage with the surface holding it in place while the small ring suffered attack. This concept has neatly explained the effects of alkenic substituents.2

11.5.HYDROGENATION OF ALKYLCYCLOBUTANES AND RELATED MOLECULES

Very little is known about the hydrogenation of cyclobutane itself. On Pt/Al2O3 the rate was first order in the hydrocarbon, but the order in hydrogen was expressed indirectly in terms of assumed loss of hydrogen atoms in the adsorbed state.67 There have however been a number of publications concerning its alkyl derivatives. As we have seen (Section 11.1 and Table 11.1), cyclobutane resembles much more an open-chain alkane than does cyclopropane, so the motivation for studying its alkyl derivatives is to see how the position of ring opening is affected by substituents, and what intermediate species may be formed. Once again however it will be hard to separate steric and electronic effects in deciding what products are formed.

The two modes of ring-opening of methylcyclobutane give n-pentane and 2-methylbutane; on films of nickel, palladium and platinum, the latter predominated, its selectivity S2MB decreasing with increasing temperature as befits the easier path: it also rose with increasing hydrogen pressure on platinum at 323 K, but fell at 403 K (Figure 11.8).60 Orders of reaction in hydrogen on platinum were negative (−1 to −2), at 323–403 K; nickel gave some hydrogenolysis, platinum a little and palladium none, and activation energies were respectively 33.5, 69 and 82 kJ mol−1. On Pt/Al2O3 model catalysts and on EUROPT-1 (6% Pt/SiO2), orders were positive in both reactants at low hydrogen pressure,68 but increasing this caused rates to pass through maxima, the position of which increased with temperature but was not affected by dispersion. The reactants were clearly competitively

Figure 11.8. Hydrogenation of methylcyclobutane: dependence of 2-methylbutane selectivity on temperature for films of nickel ( ), palladium ( ), and platinum ( ): hydrogen pressures 0.017 atm (filled points), 0.057 atm (open points), 0,17 atm (half-filled points).60

adsorbed, and the fact that reaction occurred as low as 373 K suggests that the mode of the ring’s chemisorption was by C––C bond breaking and not by C––H dissociation. TOFs increased with hydrogen pressure, and values of S2MB passed through minima; they decreased with rising temperature (as in Figure 11.8), values being between 50 and 70%. Model Rh/TiO2 catalyst began to lose activity due to the SMSI effect when reduced above 373K, although Rh/A12O3naturally did not: values of S2MB were 90% independent of temperature.69

Extensive studies of the hydrogenation of methylcyclobutane have also been reported by the Hungarian scientists at Szeged.70−72 Forms of rate dependence for each of the products as a function of hydrogen pressure on fresh and run-in (‘working’) catalysts comprising silica-supported nickel, rhodium, palladium and platinum at two temperatures have been given, and these display a bewildering variety (see Figure 11.9). In most cases the two rates run closely parallel (see however Figure 11.9(f ) for an exception), but the forms do not in general match any of those found in the reactions of the alkylcyclopropanes (Figure 11.6). Only results for Pd/SiO2 at 523 K show71 sensible behaviour characteristic of competitive adsorption; in most other cases there are clear signs of inhibition at low hydrogen pressure, giving apparent orders in hydrogen greater than unity. This may be due to the formation of hydrogen-depleted species, of which authors were well aware, although in a number of cases the form of rate-pressure curve was similar of both the fresh and used catalysts (Figure 11.8). The very peculiar behaviour of Pt/SiO2at 573 K disappeared on raising temperature by 50 K; it defies rational explanation. Unfortunately not enough detail on experimental procedure has been provided, as it is unclear whether the hydrogen pressure was varied randomly, and whether checks were performed on reproducibility of rates. Values of S2MB were independent of hydrogen pressure (for Pt/SiO2,0.55 ± 0.05 at 573 K; 0.58 ± 0.05 at 623 K; for Pd/SiO2, 0.54–0.61 at 523 K; 0.45–0.60 at 673 K), and were similar

Figure 11.9. Hydrogenation of alkylcyclobutanes: dependence of rate of 2-methylbutane formation on hydrogen pressure for silica-supported metals.

Methylcyclobutane: (a) on Pt at 573 K; (b) on Pt (fresh) at 623 K; (c) on Pd at 523 K; (d) at 673 K; (e) on Pt (run-in) at 623 K, showing rates of both routes.71,72

n-Propylcyclopropane: (f) on Ni at 523 K.70

Note: except for Pt at 623 K, curves for fresh and run-in catalysts are similar, although rates for the latter are of course much slower.

for fresh and run-in catalysts. Measurements were also undertaken in the pulse reaction mode,73 which minimises carbon deposition: selectivities, independent of temperature, were close to those just mentioned for Pt/SiO2 and Pd/SiO2 but some hydrogenolysis and ring enlargement was noticed. Rh/SiO2 showed much hydrogenolysis above 473 K and Ni/SiO2 above 523K; values of S2MB were respectively 0.6 to 0.75 and 0.70 ± 0.05.

The number of deuterium atoms in the products of the reaction of methylcyclobutane with deuterium gives some indication of the structure of adsorbed intermediates. Thus simple alkyl reversal with αβ-diadsorbed species would give either -d3 or -d7 products depending on whether or not ring inversion via the methyl group occurred or not, whereas formation of an exocyclic π -alkenylic species would give alkanes having up to nine exchanged atoms.74 On palladium film there was stepwise exchange to -d5 and -d6 at 298 K, and at 418 K products contained

mainly five to nine deuterium atoms. On platinum at 273 K there was again stepwise exchange, but at 415 K most products had two to six deuterium atoms. This distinction between the two metals in terms of their ability to form π -alkenylic forms parallels their behaviour in alkane exchange generally (see Chapter 6).

Over platinum black at 573 to 663 K, ethylcyclobutane showed selectivities to 3-methylpentane that increased from 30 to 60% with rising hydrogen pressure at 663 K, and decreased from 52 to 30% with rising temperature at low hydrogen pressure.75,76 Extension of the side-chain permits other processes to occur, so that ring enlargement to methylcyclopentane and aromatisation to benzene was seen at low hydrogen pressures. Adsorption through the alkyl group also becomes easier, and breaking the ring close to the substituent more likely.

Results analogous to those for methylcyclobutane are available for the n- propyl compound.77–81 Kinetic studies showed the same variety of rate as hydrogen pressure curves, but in general the two molecules exhibited some similarity.80 On Pd/SiO2 it appeared that the n-propyl compound was the one strongly chemisorbed, as the rate maximum occurred at a higher hydrogen pressure. Selectivities to the ‘preferred’ product (3-methylhexane) were broadly in the same ranges, but were notably lower for Ni/SiO2 (0.30–0.55 at 523 K). Once again Pd/SiO2 showed no hydrogenolysis or ring-enlargement, and Rh/SiO2 was very active for the former (83% at 673 K). Under the same conditions the regiospecific effect of a single alkyl group appears to become smaller the longer the alkyl chain.

A constant and recurrent theme in the study of metal-catalysed hydrocarbon transformations has been the complicating role of carbonaceous deposits, which are recognised as forms of the reactants that are dehydrogenated and form multiple bonds to the surface to an extent depending critically on temperature and hydrogen pressure. Analysis of the behaviour of alkanes and alicyclics9,82,83 has led to the conclusion that such species are inimical to the reactions of the latter, but are that the former may still be reactive in their presence and may even require their participation. This of course is not a new idea, but it still needs quantitative evaluation.

The phenomenon of bistability was shown in the hydrogenation of n- propylcyclobutane, but curiously only with run-in Rh/SiO2 catalyst.84

The 1, 2-dimethylcyclobutanes contain three non-equivalent C––C bonds, the breaking of which gives either n-hexane or 3-methylpentane or 2,3-dimthylbutane; Table 11.7 records the proportions of each formed over films of several metals at the lower of the temperatures used.85 Not surprisingly, with the E -isomer the breaking of the C1––C2 bond is disfavoured compared to the Z -isomer except over palladium where formation of the planar π -alkenyl species may be possible. Fission of the C3––C4 bond is markedly easier in the E -isomer, which cannot lie flat on the surface before bond-breaking occurs; this inhibits C2––C3 breaking, this being especially marked with nickel and rhodium.

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