David J. Cole-Hamilton

Highly Versatile Catalytic Hydrogenation of Carboxylic and Carbonic Acid Derivatives using a Ru-Triphos Complex: Molecular Control over Selectivity and Substrate Scope

The complex [Ru(Triphos)(TMM)] (Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane, TMM = trimethylene methane) provides an efficient catalytic system for the hydrogenation of a broad range of challenging functionalities encompassing carboxylic esters, amides, carboxylic acids, carbonates, and urea derivatives. The key control factor for this unique substrate scope results from selective activation to generate either the neutral species [Ru(Triphos)(Solvent)H2] or the cationic intermediate [Ru(Triphos)(Solvent)(H)(H2)](+) in the presence of an acid additive. Multinuclear NMR spectroscopic studies demonstrated together with DFT investigations that the neutral species generally provides lower energy pathways for the multistep reduction cascades comprising hydrogen transfer to C═O groups and C-O bond cleavage. Carboxylic esters, lactones, anhydrides, secondary amides, and carboxylic acids were hydrogenated in good to excellent yields under these conditions. The formation of the catalytically inactive complexes [Ru(Triphos)(CO)H2] and [Ru(Triphos)(μ-H)]2 was identified as major deactivation pathways. The former complex results from substrate-dependent decarbonylation and constitutes a major limitation for the substrate scope under the neutral conditions. The deactivation via the carbonyl complex can be suppressed by addition of catalytic amounts of acids comprising non-coordinating anions such as HNTf2 (bis(trifluoromethane)sulfonimide). Although the corresponding cationic cycle shows higher overall barriers of activation, it provides a powerful hydrogenation pathway at elevated temperatures, enabling the selective reduction of primary amides, carbonates, and ureas in high yields. Thus, the complex [Ru(Triphos)(TMM)] provides a unique platform for the rational selection of reaction conditions for the selective hydrogenation of challenging functional groups and opens novel synthetic pathways for the utilization of renewable carbon sources.

Polymer precursors from catalytic reactions of natural oils

Dimethyl 1,19-nonadecanedioate is produced from the methoxycarbonylation of commercial olive, rapeseed or sunflower oils in the presence of a catalyst derived from [Pd2(dba)3], bis(ditertiarybutylphosphinomethyl)benzene (BDTBPMB) and methanesulphonic acid (MSA). The diester is then hydrogenated to 1,19-nonadecanediol using Ru/1,1,1-tris-(diphenylphosphinemethyl)ethane (triphos). 1,19-Nonadecadienoic acid is hydrogenated to short chain oligoesters, which can themselves be hydrogenated to 1,19-nonadecanol by hydrogenation in the presence of water.

Catalytic applications of rhodium complexes containing trialkylphosphines

Abstract The uses of trialkyl complexes of rhodium as homogeneous catalysts are reviewed. Although they have been much less studied than their triarylphosphine counterparts, rhodium trialkylphosphine complexes do have certain properties which make them suitable for a wide range of catalytic reactions, for some of which, they are the only systems available, or are the catalysts of choice. The major difference between complexes containing trialkylphosphines and those with triarylphosphines is that the electron donating trialkylphosphines lead to a much higher electron density on the metal and hence make oxidative addition reactions, even of difficult substrates, more facile. Examples of where this is particularly beneficial include the photocatalysed activation of alkanes, either to produce alkenes and H 2 or, in the presence of CO, to produce homologous aldehydes; the activation of amines in the hydroamination of alkenes; their ability to dehydrogenate alcohols, directly, via transfer of hydrogen to acceptors such as alkenes, or in hydrocarbonylation reactions where the alcohol is the hydrogen source; the water-gas shift reaction; their ability to catalyse the carbonylation of allylchlorides and CH 2 I 2 . Another example of the effect of the high electron density on the metal is the production of alcohols, rather than aldehydes as the primary products from alkenes under hydroformylation reaction conditions, which is believed to proceed via an hydroxycarbene complex formed by protonation of an acyl intermediate. The hydrogenation of CO catalysed by trialklyphosphine complexes of rhodium may also be possible because the high electron density on the rhodium makes a hydrido ligand sufficiently hydridic for it to be transferred to CO coordinated to another rhodium centre to form the crucial formyl intermediate. In addition to these rather unusual reactions, trialkylphosphine complexes of rhodium also show activity for a variety of other more classical reactions, such as hydrogenation (especially of aldehydes and ketones), hydrosilylation, hydroboration and condensation reactions of alkenes and alkynes. For most of these reactions, apart from the hydrogenation of aldehydes, these catalysts tend to be inferior to their triarlyphosphine counterparts.

Carbon Dioxide Induced Phase Switching for Homogeneous‐Catalyst Recycling

SwitchPhos: Rhodium complexes formed from PPh(3) ligands functionalized with weakly basic amidine groups are highly active catalysts for the hydroformylation of alkenes. On bubbling with CO(2) in the presence of water, the yellow rhodium complexes move into the water phase, whereas bubbling with N(2) at 60 degrees C causes them to switch back into the organic phase. The catalysts can be used for reactions in water or an organic phase.

Continuous flow homogeneous alkene metathesis with built-in catalyst separation

Continuous flow homogeneous alkene metathesis using a supported ionic liquid phase (SILP) catalyst with CO2 as a transport vector allows the self-metathesis of methyl oleate with only a slight loss of activity for at least 10 h; cross-metathesis of dimethyl maleate with methyl oleate ceases after 3 h, but the catalyst remains active for methyl oleate metathesis. The reasons for this unusual behaviour are explored and a practical system for the cross-metathesis of methyl oleate with dimethyl maleate, under batch conditions, is described.

Hydroformylation in perfluorinated solvents; improved selectivity, catalyst retention and product separation

Abstract The hydroformylation of linear terminal alkenes using rhodium based catalysts under fluorous biphasic conditions in the presence and absence of toluene is reported. Using fluorinated ponytails to modify triarylphosphites and triarylphosphines, good selectivities and reactivities can be obtained, along with good retention of the catalyst and ligand within the fluorous phase. Using P(O–4-C 6 H 4 C 6 F 13 ) 3 (P/Rh=3:1) as the ligand in toluene/perfluoro-1,3-dimethylcyclohexane, good results are obtained at 60°C, but decomposition of the catalyst and/or ligand occurs on increasing the temperature. More impressive results are obtained by omitting the toluene, with higher rates, better l/b ratios, and better retention of the catalyst and the phosphite within the perfluorocarbon solvent. Competing isomerisation restricts linear aldehyde selectivities to 6 H 4 C 6 F 13 ) 3 is used as the ligand in the absence of toluene, even more impressive results can be obtained, with linear aldehyde selectivities up to 80.9%, high rates, and the retention of up to 99.95% of the rhodium and up to 96.7% of the phosphine within the fluorous phase. These results are compared with those of commercial systems for propene hydroformylation and with those previously reported in the literature for hydroformylation under fluorous biphasic conditions. Phase behaviour studies show that 1-octene is completely miscible with the fluorous solvent under the conditions used for the hydroformylation experiments, but that the product nonanal, phase separates.

The production of low molecular weight oxygenates from carbon monoxide and ethene

Abstract Transition metal catalysed reactions of CO and ethene can lead to a variety of products ranging from small molecules to perfectly alternating long chain polyketones. In this review, we discuss the formation of small molecules with chain lengths up to 12 C atoms. Palladium based complexes of monodentate tertiary phosphines tend to give methyl propanoate under most conditions, but the selectivity can be varied by altering the electron donating power of the ligand or the nature of added acid co-catalysts. In addition to methyl propanoate, the major products can be co-oligomers, 3-pentanone or propanal. Using rhodium catalysts, the same products can be obtained, but the different selectivities depend upon the electron donating power of the ligand and the potential for chelate binding. In some cases, the extra H atoms required for the formation of 3-pentanone or oligoketones can be abstracted from the solvent, whereas in others they come from hydrogen formed by the water–gas shift reaction. The different reaction selectivities are discussed in terms of the reaction mechanisms operating.

Continuous flow homogeneous hydroformylation of alkenes using supercritical fluids

The hydroformylation of long chain alkenes, carried out as a continuous flow process by using a catalyst made in situ from [Rh(CO)2(2,4-pentanedioate)] and [1-propyl-3-methylimidazolium][Ph2P(3-C6H4SO3)] dissolved in 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonamide), is reviewed. The substrates are introduced into and the products are removed from the reactor dissolved in continuously flowing supercritical carbon dioxide. Optimisation studies allow long term operation with rates up to 500 catalyst turnovers h−1 and rhodium leaching into the product as low as 0.012 ppm. Using a different ligand based on the xantphos skeleton, linear ∶ branched ratios in the product aldehydes can be as high as 40 ∶ 1, but the rate is somewhat lower (272 h−1) and the rhodium retention is slightly less efficient. By removing the ionic liquid and dissolving the catalyst in the mixture of products and substrates that develops during the reaction, it is possible to carry out the reaction at much lower pressures (125 bar). 1-alkyl-3-methylimidazolium salts of [Ph2P(3-C6H4SO3)] are used as ligands and the success of the reaction depends crucially on the alkyl chain employed in the imidazolium cation. Too long a chain causes large amounts of rhodium leaching, whilst too small a chain causes the solubility of the ligand to be too low so that the reaction is very poor and unliganded rhodium is rapidly extracted from the system. The best results are obtained using a pentyl chain, which gives a rate of 162 h−1 and rhodium leaching of 0.1–0.5 ppm. Comparisons of these systems with current commercial systems are made and consideration is given to the design of a plant suitable for operating these reactions in a totally emissionless fashion.

Rapid thermal hydrogen production from alcohols catalysed by [Rh(2,2′-bipyridyl)2]Cl

Turnover numbers of up to 100 h–1 are observed for hydrogen production from alcohols catalysed by [Rh(2,2′-bipyridyl)2]Cl.

Phosphine-containing carbosilane dendrimers based on polyhedral silsesquioxane cores as ligands for hydroformylation reaction of oct-1-ene

Radical additions of diethyl- and diphenylphosphine have been used to prepare 1st and 2nd generation dendrimers based on polyhedral oligosilsesquioxane cores by a divergent synthetic method. The 1st generation dendrimer is built on either 16 and 24 vinyl or allyl arms formed by successive hydrosilation and vinylation or allylation of vinyl-functionalised polyhedral silsesquioxanes. Successive hydrosilation/allylation followed by hydrosilation/vinylation and addition of phosphine produce the 2nd generation dendrimer. The dendrimers have been used as ligands for the hydroformylation of oct-1-ene catalysed by [Rh(acac)(CO) 2 ]. Using the alkylphosphine-containing dendrimers as ligands, alcohols (nonan-1-ol and 2-methyloctanol) are obtained, whilst the diphenylphosphine counterparts lead to the formation of aldehydes (nonan-1-al and 2-methyloctanal). Linear to branched ratios of 3/1 are obtained for the diethylphosphine compounds while ratios of 12 to 14/1 are given by the diphenylphosphine dendritic molecules.