Metal-Catalysed Reactions of Hydrocarbons / 09-Hydrogenation of Alkynes

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Figure 9.9. Hydrogenation of 2-butyne on palladium-gold wires: (A) activation energy, ln A, ln (rate of 2-butyne removal); (B) selectivities to E-2-butene and 1-butene versus composition; (C) yields of products other than Z-2-butene versus temperature for gold contents above 60%.123

of 2-butyne in that the rate of the second stage was slower.12,128 The properties of the substitutional alloy PdBx have also been noted.154

Product selectivities shown by the metals of Groups 8 to 10 plus copper for 2-butyne hydrogenation are given in Table 9.8:27,152 the few kinetic parameters known are also given. Ways of forming products other than Z -2-butene are also explained in Scheme 9.5; the occurrence of 1,2-butadiene as an intermediate is supported by its observation as a minor product on rhodium and iridium. Nickelcopper alloy powders showed an activity maximum at Ni90Cu10, and activation energies between 27 and 38 kJ mol−1: orders were close to unity in hydrogen and zero or a little negative in 2-butyne.152 The quantity of oligomers formed increased with copper content.

The reactions of 2-butyne with hydrogen and with deuterium have been studied in detail on Ru/Al2O3 and Os/Al2O3 catalysts:155 orders in hydrogen and in 2-butyne were respectively first and zero on both, and selectivities fell with increasing hydrogen pressure and decreasing temperature as expected (Figures 9.4 and 9.5). Abbreviated results for the deuterated products are given in Table 9.9: there was no exchange in the reactant, but hydrogen exchange occurred to balance

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the deuterium numbers greater than two. The mechanism has been discussed in detail; the much greater deuterium content of the 1-butene supports the notion of its being formed by reversal of a 2-butenyl species to 1,2-butadiene, followed by addition of two deuterium atoms (Scheme 9.5B).

9.5.3.Alkyl-Substituted Alkynes Having More Than Four Carbon Atoms125,156

By now it should be possible to predict what products will be observed in the hydrogenation of alkynes containing larger alkyl substituents. Table 9.10 contains relevant results, and few surprises; the very high stereospecificity shown in the reduction of 1-pentyne even on a metal such as rhodium, where the total selectivity is quite low, is especially noteworthy. Cessation of reaction after reduction of 2-pentyne can be achieved by using dimethylformamide as solvent; this acts as a selective poison, preventing adsorption of Z-2-pentene. 2-Hexyne has been reduced selectively to Z -2-hexene at a Pt/TiO2 membrane.157 Binary platinum-palladium Adams oxide has been used to catalyse hydrogenation of 1-octyne; the β-hydride phase was not formed when the platinum content exceeded 10%. 4-Undecyne unusually yielded E-4-undecene as the major product (68%); it was thought the first-formed Z -isomer isomerised before desorbing, but it is unclear why this does not occur with other molecules.

The reaction of 3-hexyne with deuterium on Pd/Al2O3 proceeded without exchange either into the alkyne or the deuterium.153 Z-3-Hexene was the major product, of which 87% was the –d2 isomer; molecules containing more than four deuterium atoms were absent, but the n-hexane was very fully exchanged. It appears that once the reaction proceeds to the n-hexyl stage, the normal αβ-exchange mechanism comes into play, and extensive exchange occurs before the radical is liberated as n-hexane. On the Northwestern series of Pt/SiO2 and Pt/Al2O3 catalysts having dispersions based on H/Pt ratios of between 7 and 109%, 3-hexyne

TABLE 9.9. Products of the Reactions of 2-Butyne with Deuterium over Alumina-Supported Ruthenium and Osmium Catalysts

Metal	Butene	%d2 -isomer	Other isomers		Ma
Rub	E -2	75	no –d0	or -d≥4	1.87
	Z -2	87	no –d0	or -d≥5	1.90
Osc	1-	36	-d1 to -d5 only		2.75
	E -2	57	-d0 to -d8		2.41
	Z -2	65			2.00
	1-	21			3.27

a M = mean deuterium number b T = 364 K

c T = 373 K

TABLE 9.10. Hydrogenation of Alkyl-substituted Alkynes in the Liquid Phase: Product Selectivities

Alkyne		Metal	Form	Stot	1	2	3	Reference
1-C5		Pd	/C	97	>99	—	—	172
		Rh	/C	63	>99	—	—	172
		Pt	/C	93	>99	—	—	172
		Ir	/C	55	75	25Z	—	172
2-C5		Pd	/C	100	98	2E	—	172
		Rh	/C	81	96	4E	—	172
4-C8		Pd	/Zeolite Aa	99	>90	—	—	172
4-C	d	Pd	/various	—	—	68E	—	27
	11	Pt	/C	90	92	8E	—	153
		Ir	/C	60	91	9E	—	162
		Ni	Raney	100	100	—	—	173
3-C6		Pd	/Al2 O3	94	92	5E	3E 2 + Z	173
1-C7 b		FeCu	/SiO2	100	—	—	—	173
1-C	c	Pd	/sepiolite	89	100	—	—	173
	8		/Zeolite Aa
3-C9		Pd		97	97	—	—	27

1: selectivity to expected alkene by Z -addition without isomerisation

2,3: superscript letters identify minor product, e.g. for 1-pentyne on Ir, 25% Z -2-pentene.

a Treated with Ph2 (EtO)2 Si. b Solvent C2 H5 OH

c Solvent CH3 OH d Various solvents.

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was hydrogenated with Stot of about 87% and SZ 3 of about 80%, both independent of dispersion.158,159 A curious feature of this reaction was the self-activation of the catalyst, more marked with the smallest particles; this was thought due to ‘reconstruction’ of the particles to give structures more conducive to the reaction: unfortunately this idea was not checked by examination of the catalyst after use.

Considerable insights into the interaction of unsaturated molecules has been obtained by using molecules containing bulky substituents such as the tBu group, which might be expected to inhibit or indeed prevent chemisorption; nevertheless di-tert-butylethyne (and the corresponding E -di-tert-butylethene) are easily hydrogenated.142

On the same series of Pt/SiO2 and Pt/Al2O3 catalysts just mentioned, the TOF for di-tert-butylethyne hydrogenation relative to that of cyclopentene (which was almost structure-insensitive) fell by a factor of three as dispersion increased.158,159 This was quite unexpectedly contrary to the predicted trend, which expected that TOF would be larger on small particles, because of the prevalence of surface atoms of low co-ordination number on which the molecule might the more comfortably sit. It was speculated that the potential gain in energy minimisation through chemisorption was sufficiently large to withdraw surface atoms from their normal places, so that the alkynes could interact with them more effectively. If this were to be a general phenomenon, we should have another way of explaining the origin of structure-insensitivity. There were also other interesting effects of dispersion and support. On catalysts of low dispersion, isomersation of the alkenes and their hydrogenation to alkane proceeded freely before all the alkyne had reacted, presumably because the alkyne molecules could not pack tightly enough to prevent these reactions occurring: but on catalysts of high dispersion this hardly happened until all the alkyne had reacted, values of Stot being about 96% on Pt/Al2O3, because the stronger adsorption of the alkyne prevented re-adsorption of the alkenes. Values of Stot and of SZ were however consistently greater with Pt/Al2O3 than with Pt/SiO2, for which there was no ready explanation.

‘Borides’ of palladium154 and of several base metals,160 prepared by reduction of metal salts with sodium borohydride, have been successfully used for the selective hydrogenation of alkynes.

Terminal alkynes are more reactive than internal alkynes, presumably because they are more strongly adsorbed; thus 1-octyne has been reduced to 1-octene in the presence of 4-octyne, which remained unaffected.160

9.5.4. Aryl-Substituted Alkynes112,160

Phenyland diphenylethyne have been the subject of a number of studies, no doubt because of the simplicity of the products formed, but the aromatic ring is much less reactive than the triple bond, and in most circumstances remains untouched while the triple bond is reduced.161,162 Only occasionally has a

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phenylalkylethyne been examined (e.g. 1-phenyl-1-pentyne, or phenylpropylethyne163); even then on Pd/montmorillonite with THF as solvent the expected Z-2-pentene was formed with 96% total selectivity and 96% stereoselectivity.163 Otherwise the complications due to possible double-bond migration and Z-E isomerisation are absent, and this makes life much simpler. The reactions may be easily followed in the liquid phase, so that solvent effects can be looked at. It is even possible for reaction to be obtained by physically mixing the solid diphenylethyne with Pd/C, so that on exposure to hydrogen the reactant is attacked by spillover hydrogen.164 Selective reduction of phenylethyne has also been achieved using the amorphous alloy Pd81Si19 in supercritical carbon dioxide.165

Particle size effects have been sought. On Pd/pumice the reaction of phenylethyne was structure-insensitive in the range of dispersion between 14 and 62%,166 but on Pd/C and Pd/SiO2-Al2O3 in n-hexane both rate and selectivity decreased with increasing dispersion.167 Bimetallic systems have also been examined. With PdCu/pumice, pre-reduction at 298 K gives rates that were independent of copper content up to 8%, and selectivities that increased slightly;168 it was thought that under these conditions the copper remained in the +2 state, because pre-reduction at 623 K led to rates that decreased with copper content. The same reaction on PdAu/Al2O3-La2O3-Nb2O5 gave high selectivities that were independent of gold content,169 as was found with 2-butyne hydrogenation at the palladium-rich end of the same system. Addition of tin to Pt/Nylon depressed the rate, but did not improve selectivity.161

9.5.5. Multiply-Unsaturated Molecules

Further interesting information concerning the relative adsorption strengths and reactivities of various types of carbon-carbon unsaturation is obtained by the use of molecules containing two or more such reactive groups. Much of this treasury lies buried in the literature of organic chemistry, and does not readily come to the notice of the catalytic chemist. It is often concerned just with yields, not with kinetics and mechanism; the Lindlar catalyst is most often used.112,160 The two functions may be conjugated (e.g. CC––CC) or non-conjugated; they may be the same (e.g. CC––CC), because the types will diverge upon reaction. When multiple bonds are conjugated, the intervening single bond is shortened due to electron delocalisation, as with 1,3-butadiene (Section 8.12); in 1,3-butadiyne it is only 136 pm long, but in 1,4-hexadiyne it is 138 pm because part of the charge moves to the terminal single bonds. Addition of deuterium to 1,3-pentadiyne on Pd/α-Al2O3 gave Z-1,3-pentadiene-d4 as the major product, but the reaction did not proceed in well-defined stages, and other possible products appeared simultaneously.170

1-Ethynylcyclohexene has been reduced selectively to 1-ethenylcyclohexene by the Lindlar catalyst; addition of alkyl groups to give for example the arrangement CC––CC––C––C was without effect, but the grouping CC––CC––CC

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led to endless complications.112 Hydrogenation of 2-methyl-1-buten-3-yne (valylene) to isoprene proceeded with 88% selectivity on Pd/α-Al2O3 reduced at 773 K, better rates being obtained with larger particles; addition of small amounts of antimony, tin or lead from their n-butyl compounds further improved selectivity.129 Compounds containing either non-conjugated or conjugated triple bonds are usually reduced with Lindlar catalyst successively to the enyne and then to the diene.112,160

9.6. CONCLUSION

It may be helpful to try to draw together some of the characteristics that are shared by alkyne hydrogenations in their many manifestations, and to attempt some generalisations: these by definition will be statements that are more or less true most of the time.

Quite evidently alkynes are strongly chemisorbed on all relevant metals, due to the simultaneous engagement of both sets of π orbitals simultaneously with those of the metal surface. The accurate representation of this bonding, and that in intermediates, as in Scheme 9.1, is hard to depict, and at a certain modest level of mechanistic understanding is largely irrelevant. Orders of reaction in hydrogen, being frequently about first, signify that the involvement of two hydrogen atoms or species containing them, e.g. alkenyl + atom, in the slowest step; there is little evidence for the reversible chemisorption of the molecule during reaction, even (and this is somewhat surprising) when disubstituted alkynes are used. Although in many cases there appears to be no competition between the reactants for the surface, nevertheless the alkyne blocks the dissociation of the hydrogen. Sometimes however the order in alkyne is somewhat negative, which implies that free sites may be needed for the hydrogen to access the reactive layer. Such negative orders are slightly temperature-dependent, which means that derived activation energies are not quite true, although their values are such as to suggest this.

The strength of the alkyne chemisorption is usually blamed for the inverse particle size effect, namely, for small particles being less active per unit area than large ones because the alkyne is less reactive. This supposition does not seem to have been validated by thermochemical or kinetic information, but is logical in that low coordination atoms abundantly present on the surface of small particles should be able to form chemisorption complexes analogous to those known in organometallic chemistry. The strange behaviour of di-tert-butylethyne, noted above, supports this contention, and there are even indications when the reactant is in the liquid phase that metal can be eluted from the catalyst, presumably as an alkyne complex. This particular mode of adsorption does however allow highly selective formation of alkene, so a compromise has to be struck in terms of optimum particle size between good selectivity and high activity.

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The outstanding quality of palladium in both these respects is well known but not well understood. Foremost among the complexities that have to be addressed are (i) the tendency to form hydride phases, and (ii) the frequent formation, especially with ethyne, of a carbonaceous overlayer, derived perhaps from oligomers, some of which escape into the fluid phase. Both these factors are invoked to account for palladium’s remarkable properties, but both are responsive to reaction conditions and especially to particle size; support effects may also operate. So many factors have to be kept in play when discussing mechanisms that one may safely conclude that none so far suggested is wholly satisfactory. The rich literature on alkyne hydrogenation deserves careful attention, and should be a fine source of inspiration for further research.

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