Lanthanide trifluoromethanesulfonates
Lanthanide triflates are triflate salts of the lanthanides. These salts have been investigated for application in organic synthesis as Lewis acid catalysts. These catalysts function similarly to aluminium chloride or ferric chloride, but are stable in water. Commonly written as Ln(OTf)·(HO) the nine waters are bound to the lanthanide and the triflates are counteranions, so more accurately lanthanide triflate nonahydrate is written as [Ln(HO)](OTf). Synthesis. Lanthanide triflates are synthesized from lanthanide oxide and aqueous triflic acid: Anhydrous lanthanide triflates can be produced by dehydrating their hydrated counterparts by heating between 180 and 200 °C under reduced pressure : Example reactions. Friedel-Crafts reactions. Lanthanide triflates are proposed for Friedel-Crafts acylations and alkylations, which are usually carried out with AlCl as the catalyst in an organic solvent. The nature of the Friedel-Craft reaction, especially the acylation, forces the AlCl to irreversibly complex with any oxygen-containing group in the product, with the only way of decomplexing it being to destroy the AlCl part with water altogether. An estimated 0.9 kg of AlCl is wasted per kilogram of typical product- it is hydrolysed into AlO and the extremely corrosive HCl. In contrast, lanthanide triflates' complexes with the product  are easily separated by water, and the lanthanide triflate hydrate thus formed can be simply heated to boil the water away (This does not work for alumimium chloride due to loss of HCl; same goes for the lanthanide chlorides, hence the necessity of the triflate counterion). This avoids the need to use organic solvents- one can just use water as the solvent. Ln(OTf) catalysts can also reduce the number of processing steps and use greener reagents; Walker et al. reported successful acylation yields using carboxylic acid directly, rather than acyl chloride. Their process generates only a small volume of aqueous  sodium bicarbonate waste. Similar results have been cited for the direct acetylation of alcohols. Other C-C bond-forming reactions. La(OTf) catalysts have been used for Diels-Alder, aldol, and allylation reactions. Some reactions require a mixed solvent, such as aqueous formaldehyde, although Kobayashi et al. have developed alternative surfactant-water systems. Michael additions are another very important industrial method for creating new carbon-carbon bonds, often with particular functional groups attached. Addition reactions are inherently atom efficient, so are preferred synthesis pathways. La(OTf) catalysts not only enable these reactions to be carried out in water, but can also achieve  asymmetric catalysis, yielding a desired enantio-specific or diastereo-specific product. C-N bond-forming reactions. Lewis acids are also used to catalyse many C-N bond-forming reactions. Pyridine compounds are common in biology and have many applications. Normally, pyridine is synthesized from acetaldehyde, formaldehyde and ammonia under high temperatures and pressures. Lanthanide triflates can be used to synthesize pyridine by catalysing either the condensation of aldehydes and amines, or the aza Diels-Alder reaction catalytic synthesis. Again, water can be used as a solvent, and high yields can be achieved under mild conditions. Nitro compounds are common in  pharmaceuticals, explosives, dyes, and plastics. As for carbon compounds, catalysed Michael additions and aldol reactions can be used. For aromatic nitro compounds, synthesis is via a substitution reaction. The standard synthesis is carried out in a solution of nitric acid, mixed with excess sulfuric acid to create nitronium ions. These are then substituted on to the aromatic species. Often, the para-isomer is the desired product, but standard systems have poor selectivity. As for acylation, the reaction is normally quenched with water, and creates copious acidic waste. Using a La(OTf) catalyst in place of sulfuric acid reduces this waste considerably . Clark et al. report 90% conversion using just 1 mol% of ytterbium triflate in weak nitric acid, generating only a small volume of acidic waste. La(OTf) catalysts have also been used for cyanations, and three-component reactions of aldehydes, amines and nucleophiles. Advantages. The substitution of organic solvents by water reduces the amount of waste and the metals are recoverable and hence reusable. Generally, the benefits of these catalysts include: Green catalysts. Lanthanide triflates are one of the most promising green chemistry catalysts. Unlike most conventional catalysts, these compounds are stable in water, so avoid the need for organic solvents , and can be recovered for reuse. Since leading researcher Kobayashi's 1991 paper on their catalytic effect in water, the range of researched applications for La(OTf) catalysts has exploded. The commercialisation of these techniques has the potential to significantly reduce the environmental impact of the chemical industries. Disadvantages. The main disadvantages of these new catalysts compared with conventional ones are less industrial experience, reduced availability and increased purchase cost. As they contain rare metals and sulfonate ions, the production of these catalysts may itself be a polluting or hazardous process. For example, metal extraction usually requires large quantities of sulfuric  acid. Since the catalyst is recoverable, these disadvantages would be less over time, and the cost savings from reduced waste treatment and better product separation may be substantially greater. The toxicity of individual lanthanides vary. One vendor MSDS lists safety considerations including dermal/eye/respiratory/GI burns on contact. It also lists possible hazardous decomposition products including CO, CO, HF and SO. The compounds are hygroscopic, so care is required for storage and handling. However, these considerations also apply to the more common catalysts. These possible disadvantages are difficult to quantify, as essentially all public domain publications on their use  are by research chemists, and do not include Life Cycle Analysis or budgetary considerations. Future work in these areas would greatly encourage their uptake by industry. Recent developments. Researchers are continually finding new applications where it can replace other less efficient, more toxic Lewis acids. Recently it has been tested in synthesizing epoxies and other polymerisation reactions, and in polysaccharide synthesis. It has also been trialled in green solvents other than water, such as ionic liquids and supercritical carbon dioxide. To enhance recovery, researchers have developed La(OTf) catalysts stabilised by ion exchange resin or polymer backbones, which can be separated  by ultrafiltration. Solvent-free systems are also possible with solid-supported catalysts.