In(OTf)3-Catalyzed Cascade Cyclization for Construction of Oxatricyclic Compounds
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In(OTf)3-catalysed easy access to dihydropyranocoumarin and dihydropyranochromone derivatives
N. Boufroua, E. Dunach, F. Fontaine-Vive, S. Achouche-Bouzroura and S. Poulain-Martini, New J. Chem., 2020, 44, 6042 DOI: 10.1039/D0NJ00080A
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Triflate, also known by the systematic name trifluoromethanesulfonate, is a functional group with the formula CF3SO3−. The triflate group is often represented by −OTf, as opposed to −Tf (triflyl). For example, n-butyl triflate can be written as CH3CH2CH2CH2OTf.
The corresponding triflate anion, CF
3, is an extremely stable polyatomic ion; this comes from the fact that triflic acid (CF3SO3H) is a superacid; i.e. it is more acidic than pure sulfuric acid, already one of the strongest acids known.
A triflate group is an excellent leaving group used in certain organic reactions such as nucleophilic substitution, Suzuki couplings and Heck reactions. Since alkyl triflates are extremely reactive in SN2 reactions, they must be stored in conditions free of nucleophiles (such as water). The anion owes its stability to resonance stabilization which causes the negative charge to be spread symmetrically over the three oxygen atoms. An additional stabilization is achieved by the trifluoromethyl group, which acts as a strong electron-withdrawing group using the sulfur atom as a bridge.
Triflates have also been applied as ligands for group 11 and 13 metals along with lanthanides.
Lithium triflates are used in some lithium ion batteries as a component of the electrolyte.
A mild triflating reagent is phenyl triflimide or N,N-bis(trifluoromethanesulfonyl)aniline, where the by-product is [CF3SO2N−Ph]−.
Triflate salts are thermally very stable with melting points up to 350 °C for sodium, boron and silver salts especially in water-free form. They can be obtained directly from triflic acid and the metal hydroxide or metal carbonate in water. Alternatively, they can be obtained from reacting metal chlorides with neat triflic acid or silver triflate, or from reacting barium triflate with metal sulfates in water:
- MCln + n HOTf → M(OTf)n + n HCl
- MCln + n AgOTf → M(OTf)n + n AgCl ↓
- M(SO4)n + n Ba(OTf)2 → M(OTf)2n + n BaSO4 ↓
Metal triflates are used as Lewis acid catalysts in organic chemistry. Especially useful are the lanthanide triflates of the type Ln(OTf)3 (where Ln is a lanthanoid). A related popular catalyst scandium triflate is used in such reactions as aldol reactions and Diels–Alder reactions. An example is the Mukaiyama aldol addition reaction between benzaldehyde and the silyl enol ether of cyclohexanone with an 81% chemical yield. The corresponding reaction with the yttrium salt fails:
Triflate is a commonly used weakly coordinating anion.
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Lanthanide triflates are triflate salts of the lanthanides. These salts have been investigated for application in organic synthesis as Lewis acidcatalysts. These catalysts function similarly to aluminium chloride or ferric chloride, but are stable in water. Commonly written as Ln(OTf)3·(H2O)9 the nine waters are bound to the lanthanide and the triflates are counteranions, so more accurately lanthanide triflate nonahydrate is written as [Ln(H2O)9](OTf)3.
Lanthanide triflates are synthesized from lanthanide oxide and aqueous triflic acid:
- Ln2O3 + 6HOTf + 18H2O → 2[Ln(H2O)9](OTf)3 + 3H2O
Anhydrous lanthanide triflates can be produced by dehydrating their hydrated counterparts by heating between 180 and 200 °C under reduced pressure:
- [Ln(H2O)9](OTf)3 → Ln(OTf)3 + 9H2O
Lanthanide triflates are proposed for Friedel-Crafts acylations and alkylations, which are usually carried out with AlCl3 as the catalyst in an organic solvent. The nature of the Friedel-Craft reaction, especially the acylation, forces the AlCl3 to irreversibly complex with any oxygen-containing group in the product, with the only way of decomplexing it being to destroy the AlCl3 part with water altogether. An estimated 0.9 kg of AlCl3 is wasted per kilogram of typical product- it is hydrolysed into Al2O3 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)3 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)3 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)3 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)3 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)3 catalysts have also been used for cyanations, and three-component reactions of aldehydes, amines and nucleophiles.
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:
- Selective, often producing fewer by-products than standard methods
- Asymmetric catalysts: chiral forms can be highly diastereo- and enantio-selective
- Some reactions can use greener non-chlorinated reagents, and reduce the number of synthesis steps
- Less toxic and not corrosive, so safer and easier to handle
- Mild reaction conditions are safer and reduce energy consumption.
Lanthanide triflates are one of the most promising green chemistrycatalysts. 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)3 catalysts has exploded. The commercialisation of these techniques has the potential to significantly reduce the environmental impact of the chemical industries.
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, CO2, HF and SOx. 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.
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)3 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.
- ^Harrowfield, J. M.; Keppert, D. L.; Patrick, J. M.; White, A. H. (1983). "Structure and stereochemistry in "f-block" complexes of high coordination number. VIII. The [M(unidentate)9] system. Crystal structures of [M(OH2)9] [CF3SO3]3, M = lanthanum, gadolinium, lutetium, or yttrium". Australian Journal of Chemistry. 36 (3): 483–492. doi:10.1071/CH9830483.
- ^Kobayashi, S.; Hachiya, I. (1994). "Lanthanide Triflates as Water-Tolerant Lewis Acids. Activation of Commercial Formaldehyde Solution and Use in the Aldol Reaction of Silyl Enol Ethers with Aldehydes in Aqueous Media". J. Org. Chem.59 (13): 3590–6. doi:10.1021/jo00092a017.
- ^ abClark, J.; Macquarie, D. (2002). Handbook of Green Chemistry & Technology. Oxford, UK: Blackwell Science. ISBN .
- ^Walker, M., Balshi, M., Lauster, A., & Birmingham, P. 2000, “An Environmentally Benign Process for Friedel-Crafts Acylation”, 4th Annual Green Chemistry Conference & Proceedings, National Academy of Sciences, Washington US
- ^Barrett, A.; Braddock, D. (1997). "Scandium(III) or Lanthanide(III) Triflates as Recyclable Catalysts for the Direct Acetylation of Alcohols with Acetic Acid". Chem. Commun.1997 (4): 351–352. doi:10.1039/a606484a.
- ^ abEngberts, J., Feringa, B., Keller, E. & Otto, S. 1996, “Lewis-acid Catalysis of Carbon Carbon Bond Forming Reactions in Water”, Recuil des Travaux Chimiques des Pays-Bas 115(11-12), 457-464
- ^ abKobayashi, S.; Manabe, K. (2000). "Green Lewis Acid Catalysts in Organic Synthesis". Pure Appl. Chem.72 (7): 1373–1380. doi:10.1351/pac200072071373.
- ^Wenhua Xie; Yafei Jin; Peng George Wang (1999). "Lanthanide triflates as unique Lewis acids". Chemtech. 29 (2): 23–29.
- ^Kobayashi, S.; Uchiro, H.; Fujishita, Y.; Shiina, I.; Mukaiyama, T. (1991). "Asymmetric Aldol Reaction between Achiral Silyl Enol Ethers and Achiral Aldehydes by use of a Chiral Promoter System". J. Am. Chem. Soc.113 (11): 4247–4252. doi:10.1021/ja00011a030.
- ^Fisher Scientific 2006, Acros Organics Catalog, Fisher Scientific International
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