Graham R. Eastham

The utilisation of the PNP iridium pincer complex [Ir(PNP)(COE)][BF ₄ ] [PNP = 2,6-bis{(di- tert -butylphosphino)methyl}pyridine; COE = cyclooctene] in the sp ³ C–H activation of methyl propanoate and other related esters was explored.

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.

The coordination chemistry and solution behaviour of Rh(I) and Ru(II) complexes derived from mixed anhydride ligands of carboxylic acids and phosphorus acids were explored. Similar to the free ligand systems, mixed anhydride complexes rearranged in solution via a number of pathways, with the pathway of choice dependent on the mixed anhydride employed, the auxiliary ligands present as well as the nature of the metal centre. Plausible mechanisms for some of the routes of rearrangement and by-product formation are proposed. Where stability allowed, new complexes were fully characterised, including solid state structures for four of the unrearranged mixed anhydride complexes and two of the interesting rearrangement products.

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, TOF_o > 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.

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^tBu₃ were found to be more effective than the less donating dppe or PPh₃. The addition of D₂O led to deuteration of the aromatic ring mainly in the position ortho to the hydroxyl group. Phenol can also be generated by SO₃extrusion from 4-hydroxybenzenesulfonic acid catalysed by highly electron rich palladium complexes.

Catalysts prepared in situ from 1,2-(Bu^t ₂PCH₂)₂C₆H₄ (DTBPMB) and [RhCl(CO)₂]₂ catalyse the carbonylation of methanol in the presence of MeI at rates (4.12 and 8.90 × 10⁻⁴ mol dm⁻³ s⁻¹ at 150 and 180 °C respectively) faster than those in the absence of added phosphine, although they decompose to [RhI₂(CO)₂]⁻ over time. HPIR studies suggest that MeI adds to [Rh(DTBPMB)X(CO)] (X = Cl or I), which have been isolated and fully characterised, to give [Rh(DTBPMB)MeI₂(CO)]. [Rh(DTBPMB)MeI₂(CO)] reacts with CO to give [Rh(DTBPMB)(C(O)Me)I₂(CO)], but that methanol promotes decomposition to [RhI₂(CO)₂]⁻ and [1,2-(MeBu^t ₂PCH₂)₂C₆H₄)][I₃]₂, which has been isolated and characterised. HPNMR studies show that [Rh(DTBPMB)Cl(CO)] reacts with CO to give [Rh(DTBPMB)Cl(CO)₂], which is square pyramidal with an apical Cl.

Amides can be hydrogenated to amines using a catalyst prepared in situ from [Ru(acac)₃] and 1,1,1-tris(diphenylphosphinomethyl)ethane; water is required to stabilize the catalyst and primary amines can only be formed (selectivity up to 85%) if ammonia is also present.

Density functional calculations suggest that intermolecular attack of methanol may be important in the methanolysis of simple Pd–acyl systems and that the energetics of this process are strongly dependent on the metal coordination environment.

A catalyst system based on palladium-1,2-bis-(di-tert-butylphosphinomethyl)benzene (BDTBPMB) shows good activity for the methoxycarbonylation of strongly activated aryl chlorides, like 4-chloromethylbenzoate or 4-chlorocyanobenzene. Surprisingly, the use of less activated aryl chlorides, like 4-chloroacetophenone, leads to the formation of dimethyl terephthalate amongst other products arising from organic reactions of methoxide ion and/or CO. Less nucleophilic alcohols such as 2,2,2-trifluoroethanol promote the formation of carbonylation products even from 4-chloroacetophenone and chlorobenzene. Labelling studies involving CD₃OH, CD₃OD or ¹³CO give information on the origin of many of the products.

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.