David E. Webster

Homogeneous catalysis of olefin isomerisation. Part V. Pent-1-ene isomerisation catalysed by solutions of RuHCl(PPh3)3 and of RuHCl(CO)(PPh3)3; variation of the isomeric composition of pent-2-ene and its attribution to steric factors

Isomerisation of pent-1-ene to pent-2-ene in the range 35 to 80 °C is catalysed by solutions of RuHCl(PPh3)3 and of RuHCl(CO)(PPh3)3 in benzene. Preferential formation of cis-olefin occurs at higher catalyst concentrations, but a change to preferential trans-olefin formation is observed as catalyst concentration is reduced. This change in selectivity, which is not accompanied by a change in mechanism, is attributed to a progressive decongestion of the catalytic site brought about by a gradual increase in the degree of dissociation of the catalyst by loss of triphenylphosphine ligands.

Homogeneous catalysis of olefin isomerisation. Part IV. The isomerisation of pent-1-ene catalysed by solutions of IrH(CO)(PPh3)3, IrHCl2(PEt2Ph)3, IrCl(CO)(PPh3)2 IrCl(O2)(CO)(PPh3)2, and of PtH(SnCl3)(PPh3)2

The isomerisation of pent-1-ene to pent-2-ene at 80 °C is catalysed by solutions of (i) IrH(CO)(PPh3)3, (ii) IrHCl2(PEt2Ph)3, (iii) IrCl(CO)(PPh3)2 or IrCl(O2)(CO)(PPh3)2 each in the presence of an oxygen source, and (iv) PtH(SnCl3)(PPh3)2 in benzene. Systems (i), (ii), and (iv) catalyse the preferential formation of cis-pent-2-ene (initial cis:trans ratios typically 2·0 to 3·5) whereas (iii) provides preferential formation of trans-pent-2-ene (initial cis:trans ratio ca. 0·6).The isomerisation of [1, 2-2H2]pent-1-ene catalysed by (i), (iii), and (iv) is reported. Intermolecular hydrogen transfer typical of isomerisation via the formation of pentyl complexes was observed in each case. Stepwise movement of the double bond occurred in systems (i) and (iii), whereas multiple movement was observed in (iv). The relative probabilities of the three elementary reactions (viz : formation of pent-1-yl complex, formation of a pent-2-yl complex, and isomerisation) are calculated for each system.Gross variations of the cis:trans ratio are interpreted in terms of the presence or absence of steric interaction between bulky ligands and the pent-2-yl group.

The activation of saturated hydrocarbons by transition-metal complexes in solution. Part IV. Oxidation of benzene and of alkanes by hexachloroplatinate(IV)

Benzene is oxidised to chiorobenzene by H2PtCl6 in aqueous trifluoroacetic acid at 120 °C. The initial formation of a water-soluble complex of platinum(IV) and benzene is accelerated by an increase in the H2PtCl6 concentration and by addition of K2[PtCl4], and is reduced by the addition of chloride ion. Subsequent reactions give chlorobenzene; some of it complexes with platinum. An analogous study has been made of the oxidation of alkanes, particularly hexane, using the same system. Evidence for the formation of chlorohexanes from hexane and of platinum complexes with hydrocarbon ligands has been obtained. 2- and 3-chlorohexanes are further oxidised and, from determinations of the amount of PtIV reduced during the reaction, it appears likely that polychlorinated carboxylic acids are formed. The mechanisms of aromatic and alkane oxidation in this system are initially very similar. The reaction of cyclohexane with H2PtCl6 is more complex. In addition to chlorination, dehydrogenation occurs to give benzene and chlorobenzene.

Homogeneous catalysis of olefin lsomerisation. Part VI. Pent-1-ene isomerisation catalysed by solutions of dodecacarbonyltri-iron(0) and of bis(benzonitrile)dichloropalladium(II) in benzene

Isomerisation of pent-1-ene to cis- and trans-pent-2-ene is catalysed at 50 °C and above by solutions in benzene of Fe3(CO)12 and of PdCl2(C6H5CN)2. In both cases, preferential formation of trans-pent-2-ene occurs. Isomerisation of [1,2-2H2]pent-1-ene reveals that each reaction proceeds by the intramolecular transfer of hydrogen and deuterium atoms. The reaction mechanisms involve π-allylic intermediates; for the palladium-catalysed reaction bis-π-allyl complexes may also participate.

Homogeneous catalysis of olefin isomerisation. Part II. (a) Isomerisation of pent-1-ene catalysed by hydridochlorotris(triphenylphosphine)-ruthenium(II) in benzene; and (b) a novel procedure for evaluating velocity constants for a network of three first-order reversible reactions

The isomerisation of pent-1-ene to cis-pent-2-ene (60%) and trans-pent-2-ene (40%) is catalysed at 50° by solutions of RuHCl(PPh3)3 in benzene. Reaction is of the first order with respect both to complex and to pent-1-ene when the concentrations of each are low. Departures from first-order behaviour occur because the catalytically active species are formed by dissociation of the complex, and because of changes in the nature of the solvent at the higher pentene concentrations. Interference of the reaction by-products is slight. The six velocity constants for the interconversion of the three pentene isomers have been determined by a new procedure involving the use of Laplace–Carson transforms.Isomerisation of deuterium-labelled pent-1-ene has revealed (i) that equilibrium is established between uncoordinated and co-ordinated pent-1-ene, (ii) that redistribution of deuterium in pent-1-ene accompanies its isomerisation, (iii) that normally isomerisation involves the movement of the double bond to the adjacent position only, (iv) that cis-and trans-pent-2-ene are each formed by a mechanism involving a pentyl intermediate, (v) that a mechanism involving a π-allylic intermediate also contributes to the formation of the trans-isomer, and (vi) that some processes require the formation of transient species having two hydrogen atoms as ligands of ruthenium and in which two phosphine ligands have been lost by dissociation.

Homogeneous catalysis of olefin isomerisation. Part III. The isomerisation of pent-1-ene catalysed by solutions of tetrakis(triethyl phosphite)-nickel(o) and of hydridochlorocarbonyltris(triphenylphosphine)osmium(II)

The isomerisation of pent-1-ene to cis- and trans-pent-2-ene is catalysed at 35 °C by solutions of Ni{P(OEt)3}4 in benzene containing trifluoroacetic acid, and at 80 °C by solutions of OsHCl(CO)(PPh3)3 in pure benzene. The initial rate of the nickel-catalysed reaction is not diminished by the addition of triethyl phosphite, and NiH{P(OEt)3}4+ is suggested as the catalyst. The initial rate of the osmium-catalysed reaction is retarded by the addition of triphenylphosphine, and catalyst generation is suggested to occur by dissociation of triphenylphosphine from the original complex.The initial cis:trans-ratios in the pent-2-ene were 2·3(Ni) and 6·0(Os).Isomerisation of [1,2-2H2]pent-1-ene has shown that, for each reaction, isotopic redistribution in the reactant is rapid, and involves only the intermolecular exchange of hydrogen and deuterium bonded to the olefinic carbon atoms. Initially, pent-2-ene contained more deuterium than the starting material; no deuterium entered the ethyl group.The suggested mechanism involves pentyl and 1-methylbutyl intermediates. Dissociation or displacement of pent-2-ene from the catalyst occurs before the formation of 1-ethylpropyl complexes. Steric hindrance about the carbon-metal bond in 1-methylbutyl complexes is suggested in order to reconcile the conclusion (i) that the formation of 1-methylbutyl complexes from pent-1-ene is a favoured process, and (ii) that the rate of conversion of 1-methylbutyl complexes into pent-2-ene is relatively slow.

Homogeneous catalysis of olefin isomerisation. Part I. Reactions of trans-[2H2]ethylene catalysed by complexes of cobalt, ruthenium, rhodium, and palladium

Reactions of trans-[2H2]ethylene with solutions of CoHN2(PPh3)3, RuHCl(PPh3)3, RuCl2(PPh3)3, and of RhH(CO)-(PPh3)3 in benzene at 50° have been investigated. Redistribution of hydrogen isotopes in the ethylene occurred, leading eventually to a random distribution, the reaction proceeding via ethyl intermediates. Solutions of the hydrido-complexes were of comparable activity, and were more active than those of RuCl2(PPh3)3. Hydrogen exchange was observed between deuterium atoms of the olefin and protium atoms in the ortho-positions of the triphenylphosphine ligands of RuHCl(PPh3)3.A solution of PdCl2(PhCN)2 in benzene at 50° catalysed the conversion of trans-into cis-[2H2]ethylene without simultaneous isotope redistribution. Routes by which this can occur are discussed.