Metal-Catalysed Reactions of Hydrocarbons / 07-Hydrogenation of Alkenes and Related Processes

Figure 7.15. Formation of Z -1,2-dimethylcyclohexane from 1,2-dimethylcyclohexene: the latter is shown as the π -adsorbed state, which is converted to the half-hydrogenated state having the same geometry as the final product.

surface; stepwise addition of two hydrogen atoms from below therefore inevitably leads by the Horiuti-Polanyi mechanism to Z -dimethylcyclohexane (Figure 7.15). The same should be true for 1,2-disubstituted cyclopentenes and cyclobutenes. However, it is found that in almost every case a significant, sometimes major, amount of the E -isomer is formed; the amount depends on the kind of metal used, the hydrogen pressure, and most importantly the size and shape of the alkyl substituents.217,234 The two hydrogen atoms must in this case have been added to opposite sides of the ring, and much research has been directed to find out how this can happen; more than half a century’s effort has not however provided an unequivocal answer.235

In tackling this problem it has been found helpful to look as well at other isomers, especially 2,3-dimethylcyclohexene (G), 2- and 4-alkylmethylene- cyclohexanes (H and I) and the octalins (Table 7.6), and in particular the product of their interaction with deuterium.236 The exocyclic double bond readily moves to the stablest position within the ring, that is, to where it has most substituents. Relative stabilities are governed by the same considerations as operate with the n- butenes (see Table 7.1), and the equilibrium proportions of F,G and H are as 85:15: 0.3.237 Isomerisation from exo- to endo- positions occurs even with platinum catalysts,238,239 which (together with iridium) are least prone to give double-bond migration, but exo-isomers are much more easily hydrogenated, so much of the cycloalkane may arise from this isomer, even although its concentration may be small.

To appreciate how important alkyl reversal is in the reactions being considered, the reactions of monosubstituted C6 cycles with deuterium are informative: the results obtained239 with carbon-supported metals after 25% addition are given in Table 7.7 Remembering that deuterium numbers M of the cycloalkane greater than two mean more hydrogen exchange than cycloalkene exchange, and vice versa, the results are broadly in line with the characteristics of the three metals as exposed in the earlier sections. A certain amount of alkyl reversal must occur in all cases, but alkene desorption is only important with palladium. With Pt/C

TABLE 7.6. Structure and Nomenclature of Substituted Cycloalkenes6

O: 9,10 -octalin

The references cited against F also apply to G–J.

In most cases the substituent is a methyl group, but in the cyclohexene series isopropyl and tert-butyl groups have also been used,6,238,241,252 as have compounds where alkyl groups are in the 1,3-, 1,4- and 2,4-positions.

there must be quite variable amounts of hydrogen exchange to account for these results, and with Q the cycloalkane appears to have been considerably exchanged on one side of the ring, leading to the high values of M found with both palladium and platinum. This does not seem to happen with P and R. In the hydrogenation of Q, endocyclic species dominate and lead to the saturated product; the intervention of exocyclic species is only revealed by the use of specifically labelled reactants.240

Reverting to the 1,2-dimethylcycloalkenes, we find that on nickel film at 273 K the C4 and C5 cycles gave respectively 84 and 66% of the Z-isomer;176 the

TABLE 7.7. Reactions of Methylcyclohexenes and Methylenecyclohexanes with Deuterium over Carbon-Supported Metals: Product Analysis After 25% Addition.

M: deuterium number of methylcycloalkane.

Me :deuterium number of unchanged reactant.

Me,i :deuterium number of Q formed by isomerisation.

latter gave 75% on Ni/pumice at 298 K,176 while on platinum it gave 81%, increasing to 95% at high hydrogen pressure.237 On palladium the Z -isomer formed only 25% of the products.241 Before trying to understand how Z -addition occurs, we must look at what happens with 2,3-, 1,3- and 1,4-disubstituted cycles, as well as those having an exocyclic double bond (Table 7.6). In molecules of this type, the two sides of the ring are not the same, as one or more of the substituents has already adopted a disposition with respect to the plane of the ring, so that they will be preferentially chemisorbed at the least obstructed side. Addition of hydrogen atoms from below will force a methyl group in the 1- or 2-positions upwards, and this will happen too with the group formed from a methylene substituent (Figure 7.15). However it is not always self-evident which saturated isomer will be formed, as this depends on the conformation adopted (i.e. on nonbonding interactions within the molecule or between the molecule and the surface), either at the chemisorption or some later stage in the reaction path. Table 7.8 contains a small selection of the available results; more are to be found in references.

However, before attempting a short survey of the factors determining these observations, results of a very detailed study of the reactions of 1,9- (N) and 9,10-octalin (O) must be presented6,211,215 (Table 7.10). The product decalin can exist in either the Z - or the E -configuration (Figure 7.16), depending on whether the two hydrogen atoms are on the same or different sides of the common C––C bond. Thus 9,10-octalin ought to give Z -decalin, and 1,9-octalin either the Z - or the E - form, depending on which of the non-equivalent sides of the molecule faces the surface (Figure 7.17). The double bond is stablest in the 9,10 position, where it is tetra-substituted, but isomerisation (and exchange) of the

TABLE 7.9. Stereochemistry of the Reactions of Octalins with Deuterium on Various Carbon-Supported Metals at 298 K

Me is the deuterium number of the reactant; Me,i , that of the isomerised reactant; ME,Z is the mean deuterium number of the E- and Z-decalin isomers (the individual values are generally very similar).

a Conversions between 64 and 97%. b Conversions between 37 and 77%.

notwithstanding that isomerisation in that direction is unfavourable, and was not actually observed.

A thorough explanation of all these results cannot be given in a short paragraph (or perhaps at all); the cited papers contain much more detailed discussion of mechanisms than is possible here. A cardinal question has been the point at which

Figure 7.16. Formation of Z -1,2-dimethylcyclohexane from (A) 2, 3-dimethylcyclohexane and (B) 2- methylmethylencyclohexane, after the manner of Figure 7.14.

Figure 7.17. Formation of Z - and E - decalin from two different orientations of 1,9 -octalin shown in the π -adsorbed forms.

the stereochemistry of the product is determined. Assuming the cycloalkene to be adsorbed as the π state,217 in which the geometry of the free molecule is only slightly distorted (Section 4.4.2), where there is little or no isomerisation (e.g. with platinum, and where the double-bond is already in a fairly stable position) product structure should be fixed at this point (Figures 7.15 and 7.16). Where there is more likelihood of isomerisation (e.g. with palladium, and when the doublebond is exocyclic), then extensive cycloalkene-cycloalkyl interconversion may lead to product structure being decided by interactions within the cycloalkyl species. The nature of the product is fixed when the last hydrogen atom is added, but isomerisation is not important if the reactant is hydrogenated much faster than its isomer: this is the case with 1,9- octalin and with molecules containing exocyclic double bonds.

The use of molecules forming enantiomeric pairs (e.g.(+)- and (−)- 4-methylcyclohexene and (+)- and (−)-p-menthene (1-methyl-4-isopropyl- cyclohexene)) is helpful because isomerisation creates a distinguishable product that has the same strength of adsorption and reactivity as the reactant, unlike the dialkylcyclohexenes. Isomerisation of (+)-α-pinene can however occur through movement of the double bond first to the exocyclic position, giving β-pinene, so attention has focussed on the corresponding molecule lacking the side chain, i.e. apopinene (Figure 7.18). Isomerisation of (+)-apopinene occurs readily, even on platinum catalysts:242 this has been attributed to the rigidity of the carbon skeleton, which allows movement of the double bond without alteration of the geometry, unlike cyclohexene, where conformational change giving flattening of the ring is needed. While this is easy with palladium, via a π3-species, it is more difficult with platinum, where such species are not favoured. In the presence of deuterium, the double-bond migration appears to involve a 1,3-sigmatropic shift of a hydrogen

Figure 7.18. Structures of (+)- and (−)- apopinene.

or deuterium atom, although a π3-intermediate would serve just as well. Use of palladium230 and platinum243 catalysts of various dispersions showed that the reactions of (+)-apopinene are mildly structure-sensitive on both, with isomerisation and addition peaking at 60% dispersion. The effect of inhibitors has also been examined; both thiophene and carbon tetrachloride showed minima in plots of ki/kh vs. inhibitor concentration.

Further evidence of the stereochemical constraints that determine the course of reactions of cycloalkenes with hydrogen or deuterium is obtained by studying the racemisation of (R)-(−)-10-methyl- 1,9-octalin.244 The paper describing this work, and the review of G.V. Smith’s contributions,242 admirably summarise the current status of the subject.

A number of explanations have been offered for the formation of the stabler E -1,2-dialkylcyclohexanes from the corresponding cycloalkene: these can be summarised as follows. (1) The molecule transiently isomerises to the less stable 2,3-isomer or to the exo-isomer, from which the E -cycloalkane is more easily formed, because adsorption with the more hindered face towards the metal is not impossible, especially if the σ -form is adopted. In support of this possibility, small amounts of the 2,3-isomer have been seen in the products of the reaction of 1,2-dimethylcyclopentene.217 It also accounts for the larger amounts of the stabler but unexpected E -products given by palladium (Tables 7.8 and 7.9), and the larger extent of exchange in the reactant with this metal.239 E -products are also the more extensively exchanged on nickel catalysts, but only when formed from the 1,2-dialkylcycloalkene.176 (2) The product is formed in an Eley-Rideal step245 by a hydrogen molecule from the gas phase attacking the top side of an adsorbed cycloalkene. This possibility is hard to evaluate, and there is no direct evidence for it. (3) A hydrogen atom may move within an adsorbed cycloalkene to a top-side position by a sigmatropic shift; this has been suggested as the way in which (+)-apopinene isomerises to the (−)-form. (4) Vinylic dissociative adsorption of a cycloalkene would give a planar intermediate to which addition of a hydrogen atom from either side would be equally easy:234,246 this type of adsorption has been implicated in the exchange of cycloalkenes on nickel176 and platinum.247 (5)

Carbonaceous residues could act as bridges for top-side addition to adsorbed cycloalkenes, or in solution adsorbed solvent molecules could perform the same role. (6) Finally – a possibility not previously considered, but one which follows logically from (5) – a hydrogen or deuterium atom from an alkyl radical may add top-side to a chemisorbed alkene as one step in the alkyl-alkene-alkyl chain shown in process 7.K, or by alkyl disproportionation. This is most likely to happen when species are packed closely together, i.e. when they are small or at low hydrogen pressure.

Robert Augustine16,189,239,246,248 and others237,247 have made extensive studies of the effect of varying hydrogen pressure on product stereochemistry chiefly using palladium catalysts. For example, the yield of Z -isomer from 1,2-dimethylcyclohexene increases with hydrogen pressure from 80 to 95%, while that of the 2,3-isomer falls from 80 to 40%. It was believed that the same product composition at low pressure was due to the 2,3-isomer changing to the stabler 1,2-isomer the E -product resulting from the former in equilibrium with the latter. As hydrogen pressure is increased, isomerisation in the sense 1,2- to 2,3- is less favoured than addition so the amount of Z -isomer increases; but the same factor when applied to the 2,3-isomer explains the increase in the E -yield. Hydrogenation of the methylmethylenecyclohexanes favours the more unstable isomer (2-methyl-, Z ; 3-methyl-, E ; 4-methyl-, Z ) and their yields increase with hydrogen pressure.246,248

A clear example of the repulsive forces that exist between alkyl substituents in adsorbed cycloalkenes is provided by the observation248 that 1,2- dimethylcyclopentene gives much more of the Z -product than does the corresponding cyclohexene: the lack of flexibility in the C5 ring means that adjacent Z -substituents experience a strong repulsion, while in the more flexible C6 ring this is lessened. This effect, felt in the cycloalkyl intermediates, is sufficient to persuade the C5 reactant to give the stabler isomer, perhaps via isomerisation to the 2,3-isomer.

The processes considered in this section – hydrogenation, disproportionation, and isomerisation – are frequently encountered in the chemistry of the terpenes and steroids, but even with the simpler cycloalkenes many problems remain to be solved. It is unfortunate that no LEED or vibrational spectroscopy seems to have been performed on substituted cycloalkenes, so that stereochemical preferences cannot be related to adsorbed structure as defined for example by the π σ factor (Section 4.42).

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