Recycling triflate

Triflates of boron, aluminum, and gallium were found to be efficient catalysts in Friedel-Crafts acylations.46 However, these are water-sensitive materials and were required to be used in equimolar quantities to be effective. More recently various water-tolerant and recyclable triflate salts, which were also tested in alkylation, were found to exhibit similar good characteristics in Friedel-Crafts acylations. Although benzene cannot be acylated, Sc triflate,47 48 lanthanum triflates,48-51 and Hf triflate52 usually give high yields of aryl ketones in acylation with acid anhydrides. In many cases, Li perchlorate was found to accelerate the reactions.48 52... 

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

As mentioned several times Lewis acids are highly valuable catalysts but the most commonly used ones such as aluminium chloride and boron trifluoride are highly water sensitive and are not usually recovered at the end of a reaction, leading to a significant source of waste. In recent years there has been much research interest in lanthanide triflates (trifluoro-methanesulfonates) as water stable, recyclable Lewis acid catalysts. This unusual water stability opens up the possibility for either carrying out reactions in water or using water to extract and recover the catalyst from the reaction medium. 

The multi-component procedure is also effective for the chromium-catalyzed addition of organic halides to aldehydes (the Nozaki-Hiyama-Kishi reaction) [73]. The active Cr(II) species is recycled by redox interaction with Mn powder as the stoichiometric co-reductant in the presence of MesSiCl (Scheme 34), which mainly liberates the chromium catalyst from the alkoxide adduct. The chemo- and diastereo-selective addition reaction is performed with a variety of organic halides and alkenyl triflates. In the case of crotyl bromide, the addition is highly stereoconvergent, i.e., the respective anti-... 

To avoid excessive acid waste, lanthanide(III) triflates are used as recyclable catalysts for economic aromatic nitration. Among a range of lanthanide(III) triflates examined, the ytterbium salt is the most effective. A catalytic quantity (1-10 mol%) of ytterbium(III) triflate catalyzes the nitration of simple aromatics with excellent conversions using an equivalent of 69% nitric acid in refluxing 1,2-dichloromethane for 12 h. The only by-product of the reaction is water, and the catalyst can be recovered by simple evaporation of the separated aqueous phase and reused repeatedly for further nitration.12... 

However, this catalyst is not effective for less reactive aromatics such as o-nitrotoluene. In such cases, hafnium(IV) and zirconium(IV) triflates are excellent catalysts (10 mol%) for mononitration of less reactive aromatics. The catalysts are readily recycled from the aqueous phase and reused (Eqs. 2.3 and 2.4).12... 

A clean, Strecker-type synthesis of a-aminonitriles has been developed amine, aldehyde, tributyltin cyanide, and scandium(III) triflate (as catalyst) are mixed together at room temperature. Yields for a range of aliphatic and aromatic aldehydes are typically ca 90%, the solvent can be organic or aqueous, the 10% catalyst loading is recoverable and reusable, and the tin reagent is similarly recyclable. 

Vanadium(V) oxytrinitrate [VO(N03)3] has been established as a powerful but moisture-sensitive reagent which nitrates a range of substituted aromatic compounds in good yield in dichloromethane at room temperature. The hydrated triflates Hf(OTf)4 and Zr(OTf)4 are excellent recyclable catalysts for the mononitration of o-nitrotoluene with 1 equiv. of concentrated nitric acid. ... 

Attempts to recycle these catalysts showed that the chloride complexes deactivated faster than their triflate analogues, which showed very little deactivation (although there is a small drop in the e.e. values and the cis/trans ratios of the products). 

Metal triflate Lewis acids can also be dispersed in ionic liquids for catalytic applications. Acetylation of alcohols with acetic anhydride and acetic acid has been reported with Cu(OTf)2, Yb(OTf)3, Sc(OTf)3, In(OTf)3, HfClq. (THF)2, and InCl3 in ionic liquids that consist of [BMIM] and the anions BF4, PF, or SbF 166). With lmol% acid, all the catalysts in [BMIMJPF showed >99% acetylation products in acetyl anhydride acetylation of benzyl alcohol. Sc(OTf)3 showed the best yield with recycling, with a 25% drop in yield after two cycles. A relatively long reaction period was needed to obtain a high yield (95-98%) for the acetylation of benzyl alcohol with acetic acid, indicating that the activities of the catalysts were... 

The green chemistry advantages of using triflates are not only that it helps avoid the use of hazardous catalysts, but also that the triflates themselves can be recovered and reused rather easily. In one series of reactions involving the reaction between phenethyl alcohol and acetic acid, for example, Barrett s team found that yields of 95-98 percent were possible, with catalyst recovery amounting to no less than 98 percent and, in most cases, nearly 100 percent. The team concluded one of their reports on the use of triflate catalysts by suggesting that the catalysts they used were "readily recyclable and we believe this to be a major step forward in the area of clean technology for aromatic nitration (Francis J. Waller, et ah, "Lanthanide(III)... 

Triflates as Recyclable Catalysts for Atom Economic Aromatic Nitration," Chemical Communications [1997] 613). 

Triflate salts, mainly scandium triflate273 and lanthanide(III) triflates,274,275 have found increasing use in various aromatic substitutions. They have some unusual properties specifically, they may be used in catalytic quantity and active even in the presence of water. They are also easily recyclable via aqueous workup. 

A cheap and efficient enantioselective aza-Henry reaction of nitromethane with a variety of A-protected arylaldimines has been reported.73 Using zinc triflate and (-)-A-methylephedrine at -20 °C, yields and ees of up to 99% have been achieved with wide tolerance of aryl substituent in terms of both electronic nature and position. The auxiliary is also easily recycled. 

A chiral pyridine-bisoxazoline ( PYBOX ) ligand has been combined with indium (III) triflate to produce an enantioselective catalyst for allylation of a wide variety of aldehydes in ionic liquids 183 ees of >90% were obtained, and extraction and reuse of the catalyst-ionic liquid combination saw this figure hold up to >80% on the fourth recycle. 

Reactions of the bound alkoxyphosphoniura triflate (formed in an identical manner to that of the unbound analog 3a), with the nucleophiles shown in Table II were followed by ir and the yields determined by gc. Our few preliminary results demonstrate that their alkylating abilities are comparable to that of the unanchored alkoxyphosphoniums. The triphenylphosphine oxide polymer produced can be recycled back to the ditriflate, with triflic anhydride. 

Waller, F. J., Barrett, A. G. M., Braddock, D. C. and Ramprasad, D. Lanthanide(III) triflates as recyclable catalysts for atom economic aromatic nitration, Chem. Commun., 1997, 613-614. 

Alleti, R., W.S. Oh, M. Perambuduru, Z. Afrasiabi, E. Sinn and V.P. Reddy, Gadolinium Triflate Immobilized in Imidazolium Based Ionic Liquids A Recyclable Catalyst and Green Solvent for Acetylation of Alcohols and Amines, Green Chemistry, 7, 203-206 (2005). 

Waller, F.J., A.G.M. Barrett, D.C. Braddock and D. Ramprasad, Lanthanide(III) Triflates as Recyclable Catalysts for Atom Economic Aromatic Nitration, Chemical Communications, 613-614 (1997). 

The use of solid supported, recyclable catalysts, is a well-assessed technique in classic organic chemistry, and many exhaustive reviews dealing with this subject are available [105, 115]. The use of solid supported catalysts for library synthesis in solution has also been reported. Among others, Kobayashi et al. presented the use of a new supported scandium catalyst for 3CC reactions leading to solution libraries of amino ketones, esters, and nitriles (24-member model discrete library) [116], or to quinolines (15-member model discrete library) [117], and Jang [118] presented a polymer bound Pd-catalyzed Suzuki coupling of organoboron compounds with halides and triflates. This area was also briefly reviewed recently [119]. 

Unfortunately ZnCl2/Si02 cannot be recycled after reaction, due to irreversible catalyst deactivation which is believed to occur via hydrolysis of the Zn-Cl bond. Indeed chlorinated campholenic aldehyde was observed to form during the reaction, which would occur through reaction with HC1 from hydrolysed ZnCl2. It was anticipated that the enhanced water stability of the metal triflate would thus be advantageous for this reaction. 

A number of Lewis acidic metal salts have been tested in the acetylation of alcohols with acetic anhydride and acetic acid.[67] Of these, copper(II) triflate showed by far the highest activity, but the recycling potential was low. The recyclability was found to be much better with either Sc(OTf)3 or Yb(OTf)3. The reaction between benzyl alcohol and acetic anhydride proceeded to completion within one hour, whereas with acetic acid two days were required under identical reaction conditions. Of the ionic liquids tested, the best activities were obtained in [C4Ciim][PF6]. 

Carbonyl compounds as well as their 0,0-acetals undergo thioacetalisation and ram-thioacetalisation in ionic liquids with scandium(III) triflate as catalyst under facile reaction conditions.1681 Both hydrophilic and hydrophobic ionic liquids give good results and the catalyst could be recycled at least three times without change in activity after extraction of the product with diethyl ether. Higher reaction rates were observed relative to the reaction in dichloromethane. The procedure is highly chemoselective in that in the presence of both an aldehyde and a ketone only the aldehyde reacts to form the corresponding thioacetal. 

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