Benzoyl Trifluoromethanesulfonate

[36967-85-8]  · C8H5F3O4S  · Benzoyl Trifluoromethanesulfonate  · (MW 254.18)

(reagent for protection of hindered hydroxy groups as their benzoyl esters;1 reagent effecting ring expansion of bridgehead carbaldehydes;2,3 a highly reactive benzoylating reagent for Friedel-Crafts acylations4)

Alternate Name: benzoyl triflate.

Physical Data: bp 92-94 °C/2.2 mmHg; d 1.51 g cm-3

Solubility: generated and used in inert solvents such as dichloromethane, carbon tetrachloride, nitromethane, or carbon disulfide.

Preparative Methods: synthesized from Benzoyl Chloride and Trifluoromethanesulfonic Acid.1,5 Generation in situ has been achieved with benzoyl chloride and silver triflate,5a,6 benzoic anyhdride and triflic acid,5b,7 and reaction of S-methyl thiobenzoate with methyl triflate.8

Purification: the reagent can be distilled under reduced pressure.

Handling, Storage, and Precautions: very moisture sensitive and corrosive. Darkens on exposure to air, but can be stored at rt under dry nitrogen for several months without appreciable decomposition.

Benzoylation of Sterically Hindered Alcohols.

Benzoyl triflate is a reagent that has successfully been used for protecting sterically hindered alcohols as their benzoyl esters.1 The reaction proceeds without base but in this case simple ketals and acetals undergo deprotection during the aqueous workup.1a,b If other Lewis acid-sensitive functional groups are present in the substrate, e.g. epoxides and spiroacetals, addition of pyridine is essential to prevent substantial decomposition.1b Nevertheless, some epoxide functions have been observed to undergo clean ring opening when the carbocation generated by the epoxide opening can be quenched by rearrangement (eq 1).1a,b

Ring Expansion of Bridgehead Carbaldehydes.

Bridgehead carbaldehydes react with benzoyl triflate and triflic acid to give ring-expanded 1,2-diol monobenzoyl esters (eq 2),2,3 but other strong acids in place of triflic acid give greatly reduced yields.2b Unsymmetrical carbaldehydes react to give a mixture of the possible rearranged products,2 and quenching the reaction with Tetra-n-butylammonium Iodide in place of water gives the corresponding tertiary iodo benzoate.3

Friedel-Crafts Benzoylation.

Mixed carboxylic-triflic acid anhydrides are highly active Friedel-Crafts acylating agents.4 Benzoyl triflate can benzoylate benzene and deactivated chlorobenzene without the addition of a Friedel-Crafts catalyst,5,6 whereas other sulfonic-carboxylic acid anhydrides fail to react at higher temperatures, even with activated arenes such as anisole. Friedel-Crafts benzoylation is possible in a catalytic cycle5b,6,9 using benzoyl chloride or benzoic anhydride and 1-10 mol% of triflic acid, which is far superior to other Brønsted or Lewis acids.5b,9 Substituted benzoyl triflates have been made also; for example, 4-nitro-,5,6,8,10 4-methyl-,8 and 2,4,6-trimethylbenzoyl triflates.8 Benzoyl triflate has also been used to benzoylate less common heteroaromatic substrates such as 1,3,5,2,4-trithiadiazepine.7 Although this was low yielding, conventional Friedel-Crafts chemistry had failed completely.

Reaction mixtures of 2-(trifluoromethylsulfonyloxy)pyridine, benzoic acid, and trifluoroacetic acid have also been used to benzoylate arenes, and benzoyl triflate is one of the proposed acylating species in this mixture.11

Benzoylation of alkynes12 has been reported using benzoyl triflate. Both cis- and trans-vinyl triflates as well as indenones are produced, with the distribution depending on ring and alkyne substituents (eq 3).

Reaction with a-diazo ketones13 was shown to proceed via O-benzoylation. Quenching of the reaction with an amine produced triazoles, whereas warming resulted in loss of nitrogen and formation of 1,3-dioxolium salts (eq 4).

Mixed carboxylic-triflic acid anydrides for Friedel-Crafts acylations are not just restricted to aromatic carboxylic acids, and these can have advantages over conventional Friedel-Crafts systems when applied to sensitive substrates.14 Aliphatic carboxyl triflates4-8 and trifluoroacetyl triflate15 have been made, and intramolecular Friedel-Crafts acylation has been achieved using an aromatic acid chloride with triflic acid catalysis.16

Related Reagents.

Acetyl Fluoride; Benzoyl Chloride.

1. (a) Koreeda, M.; Brown, L. CC 1983, 1113. (b) Brown, L.; Koreeda, M. JOC 1984, 49, 3875. (c) Kim, B. H.; Jacobs, P. B.; Elliott, R. L.; Curran, D. P. T 1988, 44, 3079.
2. (a) Takeuchi, K.; Kitagawa, I.; Akiyama, F.; Shibata, T.; Kato, M.; Okamoto, K. S 1987, 612. (b) Takeuchi, K.; Ikai, K.; Yoshida, M.; Tsugeno, A. T 1988, 44, 5681.
3. Takeuchi, K.; Ohga, Y.; Munakata, M.; Kitagawa, T.; Kinoshita, T. TL 1992, 33, 3335.
4. (a) Effenberger, F. AG(E) 1980, 19, 151. (b) Howells, R. D.; McCown, J. D. CRV 1977, 77, 69. (c) Stang, P. J.; White, M. R. Aldrichim. Acta 1983, 16, 15.
5. (a) Effenberger, F.; Epple, G. AG(E) 1972, 11, 299. (b) Effenberger, F.; Epple, G. AG(E) 1972, 11, 300.
6. Effenberger, F.; Sohn, E.; Epple, G. CB 1983, 116, 1195.
7. Rees, C. W.; Surtees, J. R. J. JCS(P1) 1991, 2945.
8. Minato, H.; Miura, T.; Kobayashi, M. CL 1977, 609.
9. Butler, I. R.; Morley, J. O. JCR(S) 1980, 358.
10. Makino, T.; Orfanopoulos, M.; You, T.-P.; Wu, B.; Mosher, C. W.; Mosher, H. S. JOC 1985, 50, 5357.
11. Keumi, T.; Yoshimura, K.; Shimada, M.; Kitajima, H. BCJ 1988, 61, 455.
12. Martens, H.; Janssens, F.; Hoornaert, G. T 1975, 31, 177.
13. Lorenz, W.; Maas, G. JOC 1987, 52, 375.
14. Comins, D. L.; Myoung, Y. C. JOC 1990, 55, 292.
15. Forbus, Jr., T. R.; Martin, J. C. JOC 1979, 44, 313.
16. Hulin, B.; Koreeda, M. JOC 1984, 49, 207.

Andrew N. Boa & Paul R. Jenkins

Leicester University, UK

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