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TRIFLUOROPERACETIC ACID 1 Trifluoroperacetic Acid1 Original Commentary

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TRIFLUOROPERACETIC ACID 1 Trifluoroperacetic Acid1 Original Commentary TRIFLUOROPERACETIC ACID 1 Trifluoroperacetic Acid1 Original Commentary Kenneth C. Caster O Union Carbide Corporation, South Charleston, WV, USA F3C H OO A. Somasekar Rao & H. Rama Mohan Indian Institute of Chemical Technology, Hyderabad, India [359-48-8] C2HF3O3 (MW 130.03) General Considerations. InChI = 1S/C2HF3O3/c3-2(4,5)1(6)8-7/h7H Trifluoroperacetic acid oxidizes InChIKey = XYPISWUKQGWYGX-UHFFFAOYSA-N simple alkenes, alkenes carrying a variety of functional groups (such as ethers, alcohols, esters, ketones, and amides), aromatic compounds, alkanes,11 amines and N-heterocycles. Ketones un- dergo oxygen insertion reactions (Baeyer–Villiger oxidation). (electrophilic reagent capable of reacting with many functional groups; delivers oxygen to alkenes, arenes, and amines;1 useful Epoxidations of Alkenes. Due to the presence of the strongly 27,44 reagent for Baeyer–Villiger oxidation of ketones ) electron withdrawing CF3 group, TFPAA is the most powerful organic peroxy acid and as such is more reactive than performic21 Alternative Names: TFPAA; peroxytrifluoroacetic acid. or 3,5-dinitroperbenzoic acids.41 It reacts readily even with Solubility: sol CH Cl , dichloroethane, ether, sulfolane, 2 2 electron-poor alkenes to furnish the corresponding epoxides (see acetonitrile. m-Chloroperbenzoic Acid). Form Supplied in: not available commercially. Trifluoroacetic acid is a strong acid which opens epoxides Analysis of Reagent Purity: assay using iodometry.2 readily.12,44 Since TFPAA is a much weaker acid than trifluo- Preparative Methods: the preparation and handling of TFPAA roacetic acid (pK 3.7 vs. 0.3), the latter reagent can be selectively should be carried out behind a safety shield. A mixture of a neutralized with Na CO or Na HPO , leading to the isolation of Trifluoroacetic Anhydride (46.2 g; 0.22 mole) and CH Cl 2 3 2 4 2 2 epoxides in high yields. When the substrate is highly reactive, (50 mL) is cooled with stirring in an ice bath. 90% H O 2 2 Na CO is used as buffer; when the substrate reacts sluggishly, (caution: for hazards see Hydrogen Peroxide) (5.40 mL, 0.20 2 3 Na HPO is used as buffer.12 The TFPAA reagent is rapidly de- mol) is added in 1 mL portions over a period of 10 min. When 2 4 composed by Na CO . the mixture has become homogeneous, it is allowed to warm 2 3 Since monosubstituted alkenes are not electron rich, they react to rt and then again cooled to 0 ◦C.3 TFPAA prepared from sluggishly with the standard organic peroxy acids. By contrast, the 30% aqueous H O and Trifluoroacetic Acid has been used for 2 2 monosubstituted alkene 1-pentene (1) is epoxidized efficiently by some reactions.4–6 Hydrogen peroxide of high concentration TFPAA (eq 1).12 TFPAA prepared from 0.3 mol of 90% H O and (70%) is not widely available due to hazards involved in han- 2 2 0.36 mol of trifluoroacetic anhydride in CH Cl is added during dling, storage, and transportation. The commercially available 2 2 30 min to a stirred mixture of (1) (0.2 mol), Na CO (0.9 mol), Hydrogen Peroxide–Urea (UHP) system, which is safe to han- 2 3 and CH Cl (200 mL). Since the alkene is volatile the reaction dle, has been introduced recently as a substitute for anhydrous 2 2 flask is fitted with an efficient ice water-cooled condenser. The H O in the preparation of TFPAA.2,7,8 2 2 reaction mixture boils during the addition of the peracid. After all Purification: in the preparation of TFPAA, a slight excess of tri- the reagent has been added, the reaction mixture is heated under fluoroacetic anhydride is used to ensure that no water is present reflux for 30 min, cooled, and the insoluble salts are removed in the reagent. The reaction between H O and trifluoroacetic 2 2 by centrifugation. The salt is thoroughly washed with CH Cl . anhydride is very fast; the reagent is ready for use after the 2 2 Fractional distillation of the combined CH Cl extracts furnishes reactants have been mixed and the solution has become homo- 2 2 the epoxide 2 in 81% yield. geneous. No special purification steps are employed. Suitable buffers (Na2CO3,Na2HPO4) are used to neutralize the highly O 1.5 equiv TFPAA, CH2Cl2 reactive and strongly acidic trifluoroacetic acid which is present (1) 4.5 equiv Na2CO3, reflux, 30 min along with TFPAA in the reagent. (1)81% (2) Handling, Storage, and Precautions: the reagent can be stored at − ◦ 9 20 C for several weeks and exhibits no loss in active oxygen The alkene (3), which is resistant to epoxidation by m-CPBA 40 content after 24 h in refluxing CH2Cl2. However, since it can or Peracetic Acid, has been epoxidized with TFPAA to furnish in be prepared in a short time, the usual practice is to prepare the 83% yield a mixture of esters (4) and (5) (eq 2).13 Esters (4) and reagent when needed. Note that solutions of TFPAA in CH2Cl2 (5) undergo facile deacylation when chromatographed on silica 41 can lose activity by evaporation of the volatile peracid. Since gel to furnish alcohols (6) and (7). peroxy acids are potentially explosive, care is required while carrying out the reactions and also during workup of the reac- O O O tion mixture. Solvent removal from excess H O –CF CO H OR OR OR 2 2 3 2 O O O experiments can result in explosions; the peroxide must be de- 5 equiv TFPAA stroyed by addition of MnO (until a potassium iodide test is O + O (2) 2 Na HPO , rt, 4 h 10a Br 2 4 Br Br negative) before solvent removal. For a further discussion of 83% 10b safety, see Luxon. This reagent should only be handled in a (3) (4) R = COCF3 (5) R = COCF3 fume hood. R = H (6) R = H (7) R = H 2 TRIFLUOROPERACETIC ACID Epoxidation of allyldiphenylphosphine oxide (8) with TFPAA this selectivity is due to the formation of the hydrogen bond of furnishes in quantitative yield the corresponding epoxide, 2- the type shown in (14). The stereoselectivity in the epoxidation (diphenylphosphinoylmethyl)oxirane; m-CPBA epoxidation of of (15) is solvent dependent. When (15) is epoxidized in THF (8) furnishes the epoxide in only 56% yield.14 Epoxide (9)is (which disrupts hydrogen bonding) the ratio of syn:anti epoxides obtained in 80% yield through regio- and stereoselective epoxi- obtained is 1:12. The epoxidation of the allyl alcohol (16) with dation of the corresponding alkene with TFPAA in CH2Cl2 in the TFPAA is highly syn selective (syn:anti epoxidation = 100:1); the 15 presence of Na2HPO4 buffer. syn selectivity in the epoxidation of (16) with m-CPBA is much less (syn:anti epoxidation = 5.2:1). MeO O OAc O N H K2HPO4, TFPAA, CH2Cl2 H O O 40 °C, 30 min Ph O O H OMe 75% P H R MeO O Ph O OAc (8) (12) (9) R = O O (5) O (Z) MeO O O H The tertiary amine of (10) is expected to react more readily than the disubstituted double bond on treatment with an organic R peracid. Selective epoxidation of the double bond in (10)was R achieved by initially treating it with CF CO H. This led to salt O O 3 2 O O H O R3 formation due to protonation of the amine. Epoxidation of the O O HO salt with TFPAA and subsequent workup furnished the epoxide H (11) (eq 3).16 R1 R2 R1 R2 (13) (14) H H O H H R H N N TFPAA, Na2HPO4 TFPAA, H2O2, CH2Cl2 (3) CH Cl , –40 °C t-Bu 2 2 23 °C, 3 h; 0 °C, 8 h 89% 76% (15) R = OTBDMS (16) R = OH OMe OMe R (10) (11) R (6) Alkenes have been epoxidized efficiently employing TFPAA O + O prepared by the UHP method (eq 4).2 t-Bu t-Bu syn:anti 2.5 equiv TFPAA, 10 equiv UHP C6H13 C6H13 (4) The diol (17) is epoxidized stereoselectively to furnish (18) 8.8 equiv Na HPO , CH Cl , reflux, 0.5 h 2 4 2 2 20 88% O (eq 7). OH OH α,β-Unsaturated esters and α,β-unsaturated ketones are resis- TFPAA, Na2HPO4 tant to epoxidation by organic peracids since the double bonds are OH OH (7) not electron rich; however, these compounds can be epoxidized by CH2Cl2 O 90% TFPAA. 1-Acetylcyclohexene17 and methyl methacrylate12 fur- NHTs NHTs nish the corresponding epoxides in 50% and 84% yields, respec- (17) (18) tively, when treated with TFPAA/Na2HPO4 in CH2Cl2 (reflux for about 0.5 h). The α,β-unsaturated ester (12) has been epoxidized stereoselectively by TFPAA (eq 5).18 With m-CPBA, this epoxi- Oxidation of Alkenes to Diols and Ketones. Alkenes react dation requires a higher reaction temperature which results in the readily with a CF3CO3H/CF3CO2H mixture to furnish hydroxy formation of a complex mixture. trifluoroacetates, e.g. (19) → (20) (eq 8).21 In this reaction, high With organic peracids, allyl alcohols form hydrogen bonds in- molecular weight byproducts are formed due to the condensa- volving the hydrogen of the alcohol, as in (13).19 Ganem has tion of hydroxy trifluoroacetates with the epoxides formed from suggested that, with TFPAA, allylic ethers form hydrogen bonds alkenes. The formation of the byproduct can be avoided by adding involving the hydrogen of the peracid (14). triethylammonium trifluoroacetate. After the formation of the gly- Epoxidation of (15) having an allylic ether substituent axially col ester is complete, the solvent is evaporated under reduced pres- oriented is syn selective (syn:anti epoxidation = 12.4:1) (eq 6);.19 sure and the crude ester is subjected to methanolysis to furnish the TRIFLUOROPERACETIC ACID 3 vicinal diol (21).
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