Removal of Transition Metal Contamination

This invention relates to the removal of metal contamination in the form of complexes and decomposition products thereof from the products of synthetic organic processes and, in particular, to the removal of transition metal catalyst species or residues from homogeneous product mixtures.

In synthetic organic chemistry, especially for the synthesis of biologically-active compounds for pharmaceutical use, transition metals have widespread potential as catalysts for hydrogenation, oxidation and other reactions, including reactions such as olefin metathesis reactions. For example, cross metathesis methodologies have been shown to be highly effective routes in the synthesis of insect pheromones, flavour and fragrance compounds and valuable synthetic intermediates such as novel α, β-unsaturated carbonyl systems. Such reactions are recognised as offering a novel and highly efficient way in which olefmic bonds are created and functionalised. However, especially where the reaction products are ultimately intended for ingestion as pharmaceutical compounds, it is important to reduce to a minimum the quantity of any transition metal residues, including spent catalyst and catalyst by-products, present in the reaction product; by way of example, palladium has been used extensively on a research scale in various catalytic reactions, especially as a hydrogenation catalyst, and ruthenium is used in olefin metathesis reactions but such reactions are potentially compromised for use on an industrial scale due to the difficulty of removal of toxic catalytic residues from catalytic reaction mixtures. FDA recommended levels are now routinely less than 10 ppm at least. Methods which have been reported for removal of catalytic residues include the use of scavenger resins; the immobilisation of organometallic catalysts on solid supports including organic polymers and inorganic materials such as alumina, zeolites or silica; micro-encapsulation on, for example, a cross-linked polyurea matrix; the use of chelating reagents and exposure to lead(IV) acetate and chromatography procedures, for example using mixtures of dimethyl sulphoxide and triphenylphosphine. Again, however, although such procedures can be successful on a laboratory scale, they are not amenable for use on an industrial scale for various reasons such as the use of large quantities of toxic chelating reagents or merely high expense.

Catalytically-active transition metals typically include metals of the platinum group, which are expensive. There is therefore an economic incentive for providing for their residues to be removed from reaction solutions for recovery and re-use purposes.

We have now found that transition metal residues can be removed from catalytic reaction mixtures, remaining therein in very low levels, by a work-up process which comprises reacting the product solution with an oxidising agent and filtration of the reaction solution, with, in some examples, autocatalytic removal of the oxidising agent.

In one aspect, the present invention provides a process for removal of transition metal catalyst species from solutions of products obtained from a synthetic organic process, the method comprising:

treating a solution of the contaminated product, dissolved in a water-immiscible solvent, with an oxidising agent in which the transition metal catalyst species is rapidly oxidised, resulting in transition metal oxide products;

washing the treated solution with an aqueous water-soluble reducing agent, in order to remove excess oxidant, and

treating the resulting suspension to remove the transition metal oxides and allow for their recovery.

Oxide products of transition metals in the context of the present invention include water- soluble ionic species or hydroxides that precipitate from solution and are thus capable of being removed for example by filtration or other process, for example centrifugation.

In the method of the invention, the water-immiscible solvent comprises such solvents as petrols, ethers such as diethyl ether or t-butyl methyl ether, benzene, toluene, ethyl acetate and/or chlorinated hydrocarbons such as dichloromethane or chloroform and the water- soluble reducing agent may comprise sodium sulfite or a similar reducing agent such as sodium thiosulphate. In some cases, the formed transition metal oxide can catalyse the decomposition of the oxidising agent spontaneously.

Preferably, the transition metal comprises one or more metals selected from Groups 8, 9, 10 and 11 of the Periodic Table, especially including Ru, Pd, Pt, Rh, Ir, Ni and Cu. It has been found that oxidic products of these metals act as redox catalysts for reduction of the oxidising agent and are susceptible of removal from the reaction solution by filtration through, for example, a plug or pad of silica gel.

Oxidising agents for use in the process of the invention preferably comprise peroxy compounds such as hydrogen peroxide and percarbonate, perborate and peroxysuphate compounds, for example as their sodium salts. The oxidising agent is preferably added as an aqueous solution to the organic reaction medium and stirred or otherwise agitated to effect reaction contact with the catalyst residue. Hydrogen peroxide is generally preferred for economic reasons and because its by-products are water and oxygen, posing no environmental or toxicity problems in disposal. Nitrogen or other inert gas may be added to the reaction vessel or other elements of the apparatus to minimise any risk of explosion. However, where solid oxidising agents are used, these may be added direct to the organic reaction solution. The use of phase transfer catalysts may be beneficial in either a liquid: liquid or liquid: solid system, in increasing the rate of oxidation of catalyst residue and reducing down time.

The process preferably includes the addition to the reaction solution, after the oxidising agent has oxidised the transition metal species, of a solution of a quenching agent to prevent the oxidising agent from oxidising the intended reaction product. Preferably, the quenching agent is also added as an aqueous solution. The quenching agent preferably comprises a neutral inorganic salt which is inert to oxidising and reducing agents; sodium sulphite is an example of a quenching agent suitable for use with a hydrogen peroxide oxidising agent.

Catalyst residues removed from reaction solutions by a process according to the invention may be recoverable for re-use.

Embodiments of the invention will now be described by way of example, in which reaction solutions from an olefin metathesis reaction using a ruthenium-based complex (Grubbs catalyst) as catalyst were subjected to oxidation in air (as a control experiment) and using a hydrogen peroxide solution (see Samples 1 and 2 in Table 1 below). Samples 3, 4 and 5

relate to further experiments on ruthenium olefin metathesis reaction solutions at different catalyst loadings, Sample 3 being a low-loading and Samples 4 and 5 being a high-loading solution. Sample 4 was following one treatment with peroxide solution; Sample 5 concerned a second treatment.

In all experiments, a dichloromethane solution of the metathesis product was washed with a 15% solution of hydrogen peroxide, quenched of any excess peroxides from the organic phase by addition of an aqueous solution of sodium sulphite and the product filtered through silica gel. Visual inspection of the products indicated removal of all coloured by-products and no indication of oxidation of the reaction product was observed by ¹ H NMR analysis, the product being an olefin which, potentially, is reactive towards peroxides. For analysis purposes, the product was digested with aqua regia and analysed by ICPMS (Inductively Coupled Mass Spectrometry).

As can be seen, a straight comparison of air oxidation with peroxide on the same reaction sample produced an improvement, in terms of residual ruthenium present in the reaction solution, of a factor of approximately 100. In Samples 3 to 5, the product of Sample 3 had ruthenium below those recommended for oral doses for pharmaceutical products according to the EMA regulations; a similar result was obtained for the high-loading Sample 5 after the second treatment with peroxide.

Table 1

In carrying out the experiments, it was observed that the aqueous solution of peroxide effervesced after the work-up procedure, indicating peroxide decomposition by autocatalytic quenching by ruthenium oxide products.

The invention is preferably only for the treatment of contaminated organic solutions of products and may not be suitable for the purification of polymeric materials.

Further preliminary experiments using a representative range of transition metal complexes have shown the method to be equally effective for the removal of contamination by palladium, platinum, iridium, nickel, copper, silver, rhodium and rhenium derived from catalytic species from a range of organic products. Levels as low as those achieved in the ruthenium removal experiments have been reached.

A final feature of the peroxide method is that various other transition metal catalyst species such as phosphine ligands, so often a constituent part of this type of catalyst, are also rapidly oxidised to the corresponding phosphine oxides. These are much more polar than the initial trivalent phosphines and are also removed, usually during the final filtration through silica gel.