Graham Ronald Eastham

Phosphine Ligands in the Palladium‐Catalysed Methoxycarbonylation of Ethene: Insights into the Catalytic Cycle through an HP NMR Spectroscopic Study

Novel cis-1,2-bis(di-tert-butyl-phosphinomethyl) carbocyclic ligands 6-9 have been prepared and the corresponding palladium complexes [Pd(O(3)SCH(3))(L-L)][O(3)SCH(3)] (L-L=diphosphine) 32-35 synthesised and characterised by NMR spectroscopy and X-ray diffraction. These diphosphine ligands give very active catalysts for the palladium-catalysed methoxycarbonylation of ethene. The activity varies with the size of the carbocyclic backbone, ligands 7 and 9, containing four- and six-membered ring backbones giving more active systems. The acid used as co-catalyst has a strong influence on the activity, with excess trifluoroacetic acid affording the highest conversion, whereas excess methyl sulfonic acid inhibits the catalytic system. An in operando NMR spectroscopic mechanistic study has established the catalytic cycle and resting state of the catalyst under operating reaction conditions. Although the catalysis follows the hydride pathway, the resting state is shown to be the hydride precursor complex [Pd(O(3)SCH(3))(L-L)][O(3)SCH(3)], which demonstrates that an isolable/detectable hydride complex is not a prerequisite for this mechanism.

Highly selective formation of unsaturated esters or cascade reactions to α,ω-diesters by the methoxycarbonylation of alkynes catalysed by palladium complexes of 1,2-bis(ditertbutylphosphinomethyl)benzene

The methoxycarbonylation of phenylethyne catalysed by Pd/1,2-bis-(ditertiarybutylphosphinomethyl)benzene gives the unusual linear product, methyl cinnamate with high activity (initial turnover frequency, TOFo > 1700 mol product·(mol catalyst·h)−1) and regioselectivity (>90%). Terminal aliphatic alkynes give α,β–unsaturated esters after short reaction times or α,ω-diesters, including dimethyl 1,6-hexanedioate (dimethyl adipate), from 1-butyne after longer times. The diesters are formed by a cascade methoxycarbonylation–isomerisation–methoxycarbonylation sequence. Methoxycarbonylation of internal alkynes (e.g. 4-octyne) leads to the formation of the mono-carbonylated product as a result of the low propensity of the tri-substituted double bond of the product towards isomerisation. Hydroxycarbonylation of phenylethyne gives predominantly E-3-phenylpropanoic acid with smaller amounts of branched and disubstituted products as well as 3-phenylpropanoic acid. Evidence is presented that the reactions occur via a hydride mechanism.

Highly active and selective catalysts for the production of methyl propanoate via the methoxycarbonylation of ethene

A highly active and selective catalyst for the production of methyl propanoate via the methoxycarbonylation of ethene is described, based on the new zero valent palladium complex L2Pd(dba) [where L2 = 1,2-bis(di-tert-butylphosphinomethyl)benzene and dba = trans,trans-dibenzylideneacetone].

Highly selective formation of linear esters from terminal and internal alkenes catalysed by palladium complexes of bis-(di-tert-butylphosphinomethyl)benzene

The methoxycarbonylation of terminal or internal alkenes catalysed by palladium complexes of bis-(di-tert-butylphosphinomethyl)benzene under mild conditions leads to linear esters in 99% selectivity via a hydride mechanism.

N-Functionalised heterocyclic carbene complexes of silver

N-Functionalised imidazol-2-ylidene carbene complexes of AgI have been prepared by interaction of the corresponding imidazolium salt with Ag2O in dichloromethane or 1,2-dichloroethane in the presence of molecular sieves or with Ag2CO3 in 1,2-dichloroethane. In an analogous way disilver(I) complexes of o-phenylenedimethylenebis(imidazol-2-ylidene) have been obtained. Their structures, as determined by X-ray crystallography, indicate that the carbene ligand acts as monodentate from the carbene end or as a bridge between two different metal atoms. In one case the silver is coordinated by two carbene ligands. In the majority of the structurally characterised complexes the metal centres adopt linear geometries with M–C(carbene) bond lengths in the range of 207–210 pm characteristic of a single bond.

Synthesis and reactivity of palladium hydrido-solvento complexes, including a key intermediate in the catalytic methoxycarbonylation of ethene to methyl propanoate

The sequence of reaction steps and the role of each reactant, required for the transformation of the Pd(0) precursor [Pd(dtbpx)(dba)] [dtbpx = 1,2-(CH2PBut2)2C6H4; dba = trans,trans-(PhCHCH)2CO], 1, into [Pd(dtbpx)H(MeOH)]+, 2a, the active Pd(II)-hydride catalyst for the methoxycarbonylation of ethene to methylpropanoate, have been delineated using a combination of spectroscopic and crystallographic methods. The preparation and characterisation of a variety of related complexes are described including some unusual examples involving bidentate sulfonate complexes and mono-cationic and neutral palladium hydride complexes. X-Ray crystal structures have been determined for [Pd(dtbpx)(η2-O2)], 3, [Pd(dtbpx)(η2-BQ)] (BQ = benzoquinone), 4, [Pd(dcpx)(dbaH)]+ [dcpx = 1,2-(CH2PCy2)2C6H4], 7, and [Pd(dtbpx)(η2-MeSO3)]+, 9b.

Selective formation of α,ω-ester amides from the aminocarbonylation of castor oil derived methyl 10-undecenoate and other unsaturated substrates†

The reaction of long chain alkenes with CO and aniline in the presence of palladium complexes of 1,2-bis-(ditertbutylphosphinomethyl)benzene produces amides with high linear selectivity, with much higher rates and catalyst stability when 2-naphthol and sodium or potassium iodide are added. Unsaturated esters including methyl 10-undecenoate from castor oil give α,ω-ester amides, whilst reactions in THF give N-phenylpyrrolidine.

Efficient and chemoselective ethene hydromethoxycarbonylation catalysts based on Pd-complexes of heterodiphosphines o-C6H4(CH2PtBu2)(CH2PR2)

The synthesis and properties of a series of unsymmetrical diphosphines o-C6H4(CH2PtBu2)(CH2PR2) are reported where R = C6H5 (La); p-CH3C6H4 (Lb); o-CH3C6H4 (Lc); o-CH3CH2C6H4 (Ld); o-CH3OC6H4 (Le); 2,4,6-(CH3)3C6H4 (Lf); CH(CH3)2 (Lg); or PR2 = PCg (Lh) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions is catalysed by the Pd complexes of each of the ligands La–ha–h to give methyl propanoate in >99% selectivity with catalytic activities comparable to those obtained with o-C6H4(CH2PtBu2)2 (L1) or o-C6H4(CH2PCg)2 (L55). The catalysts derived from Lc, Ld and Lh are more active than the catalyst derived from La or L1; these ligands have in common, a PR2 donor that is more bulky than the PPh2 present in La. Treatment of [PtCl(CH3)(cod)] with La–ha–h gave [PtCl(CH3)(L)] as mixtures of 2 isomers 1a–h and 2a–h. The major isomer in each case was 1a–h with the CH3 ligand trans to the PtBu2 group; the diastereoselectivity of this reaction for products 1a–h ranges from 88% to over 99%. The crystal structures of 1b, 1c and 1e have been determined. Fluxionality associated with chelate ring inversion has been detected for 1c by variable temperature 31P NMR spectroscopy. The 31P NMR data are compared for the complexes [PtCl(CH3)(L)], [Pt(CH3)2(L)] and [PtCl2(L)] where L = Lh, L1 or L55. The crystal structure of [Pt(CH3)2(Lh)] (4h) has been determined and shows that PCg is sterically less demanding than PtBu2 in this complex. Treatment of 4h with HCl gave a 1:1 mixture of 1h and 2h that equilibrated over 5 h to a 70:1 mixture. Treatment of an equilibrium mixture of 1h and 2h with isotopically labelled 13CO gas gave a single acyl complex [PtCl(13COCH3)(Lh)] (5h) with retention of configuration at Pt, i.e. the 13COCH3 is trans to the PtBu2 group. Mechanisms for the CO insertion are discussed which are consistent with the observed stereochemistry. The palladium complexes [PdCl(CH3)(Lh)] (7/8) also reacted with labelled 13CO to give a single acyl species [PdCl(13COCH3)(Lh)]. Treatment of [PdCl(13COCH3)(Lh)] with MeOH rapidly gave CH313COOMe.

Preparation of phenolic compounds by decarboxylation of hydroxybenzoic acids or desulfonation of hydroxybenzenesulfonic acid, catalysed by electron rich palladium complexes

Phenolic compounds can be prepared by catalytic decarboxylation of 4-hydroxybenzoic acid or desulfonation of 4-hydorxybenzene sulfonic acid. Palladium complexes are shown to be highly active in the decarboxylation reaction, but complexes of platinum or ruthenium also show some activity in this reaction. Highly electron donating diphosphines such as BDTBPMB or monophosphines such as P(t)Bu(3) were found to be more effective than the less donating dppe or PPh(3). The addition of D(2)O led to deuteration of the aromatic ring mainly in the position ortho to the hydroxyl group. Phenol can also be generated by SO(3) extrusion from 4-hydroxybenzenesulfonic acid catalysed by highly electron rich palladium complexes.

The Putative Mevalonate Diphosphate Decarboxylase from Picrophilus torridus Is in Reality a Mevalonate-3-Kinase with High Potential for Bioproduction of Isobutene

ABSTRACT Mevalonate diphosphate decarboxylase (MVD) is an ATP-dependent enzyme that catalyzes the phosphorylation/decarboxylation of (R)-mevalonate-5-diphosphate to isopentenyl pyrophosphate in the mevalonate (MVA) pathway. MVD is a key enzyme in engineered metabolic pathways for bioproduction of isobutene, since it catalyzes the conversion of 3-hydroxyisovalerate (3-HIV) to isobutene, an important platform chemical. The putative homologue from Picrophilus torridus has been identified as a highly efficient variant in a number of patents, but its detailed characterization has not been reported. In this study, we have successfully purified and characterized the putative MVD from P. torridus. We discovered that it is not a decarboxylase per se but an ATP-dependent enzyme, mevalonate-3-kinase (M3K), which catalyzes the phosphorylation of MVA to mevalonate-3-phosphate. The enzymes potential in isobutene formation is due to the conversion of 3-HIV to an unstable 3-phosphate intermediate that undergoes consequent spontaneous decarboxylation to form isobutene. Isobutene production rates were as high as 507 pmol min−1 g cells−1 using Escherichia coli cells expressing the enzyme and 2,880 pmol min−1 mg protein−1 with the purified histidine-tagged enzyme, significantly higher than reported previously. M3K is a key enzyme of the novel MVA pathway discovered very recently in Thermoplasma acidophilum. We suggest that P. torridus metabolizes MVA by the same pathway.