Copper(II) Trifluoromethanesulfonate


[34946-82-2]  · C2CuF6O6S2  · Copper(II) Trifluoromethanesulfonate  · (MW 361.68)

(dimerization of ketone enolates and TMS enol ethers;2a,2b cyclization of dienolates2c and unsaturated silyl enol ethers;4 allylation of ketones;3 reactions of diazo compounds;5-10 dehydration of alcohols11)

Alternate Name: copper(II) triflate.

Physical Data: dec at 530 °C (no definite mp).

Solubility: sol MeOH, EtOH, DMF, MeCN, and formamide; also sol i-PrCN and acetone.

Form Supplied in: white powder, commercially available. Blue powder when freshly prepared (see below).

Preparative Methods: most conveniently prepared from copper(II) carbonate and triflic acid (Trifluoromethanesulfonic Acid) in MeCN.1 The freshly prepared salt precipitated from Et2O is pale blue.

Handling, Storage, and Precautions: moisture sensitive; can be handled in air for quick transfers; pure samples are only mildly corrosive. Appears to be indefinitely stable in the absence of air, moisture, and light.

Oxidative Coupling.

Both intermolecular and intramolecular oxidative coupling reactions can be effected using Cu(OTf)2. Examples of dimerization include one-pot syntheses of 1,4-diketones from ketone enolates or from silyl enol ethers (eqs 1 and 2),2a,2b and coupling of allylstannanes with TMS-enol ethers to give g,d-unsaturated ketones in good to moderate yields (eq 3). Other copper(II) or tin(IV) catalysts can also be used with allylstannanes. The regiochemistry depends on both the substrate and the catalyst.

Examples of intramolecular oxidative cyclizations promoted by Cu(OTf)2 include cyclization of enolates of diketones and diesters (eq 4)2c and oxidative cyclization of hydrolytically resistant d,ε- and ε,ζ-unsaturated silyl enol ethers.4 For instance, (1) reacts with excess Cu2O/Cu(OTf)2 in MeCN to give a 90% yield of a 20:1 mixture of the trans-fused and cis-fused tricyclic ketones (2) and (3) (eq 5).4

Reactions of Diazo Compounds.

These include various cyclopropanations where the metal carbenoid generated from the diazoacetic acid esters/Cu(OTf)2 system reacts with alkenes (eq 6) or carbodiimides (eq 7).5,6

Cu(OTf)2 is the reagent of choice for intramolecular cyclization of b,g-unsaturated diazo ketones to cyclopentenones (eq 8) and for intramolecular cyclopropanation of g,d-unsaturated diazo ketones (eq 9).7

It is a useful reagent for orthoester homologation via dialkoxycarbenium ions and for oxazole formation by reaction of ketocarbenes (via diazo esters/Cu(OTf)2) with nitriles (eq 10).8 With unsaturated nitriles, the nitrile group is selectively attacked. Kinetic and ESR evidence shows that CuII -> CuI reduction is the key step.9

The regiochemistry of the monoacetate adducts formed from N-methylpyrrole with the carbenoid derived from N2CHCO2Et/Cu(OTf)2 is indicative of a reactive and less discriminating intermediate compared to carbenoids generated from N2CHCO2Et with other CuII reagents (eq 11).10

Angularly functionalized polycyclic systems may be prepared from b,g-unsaturated diazo ketones by a vinylogous Wolff rearrangement in the presence of copper(II) triflate. Dry Copper(II) Acetylacetonate is equally suitable.10

Dehydration of Alcohols.

Various tertiary, secondary, and primary alcohols are dehydrated with Cu(OTf)2 to alkenes.11 Preferred formation of Zaitsev orientation products and (E)-alkenes are indicative of a carbocationic mechanism. Zn(OTf)2 and Mg(OTf)2 are ineffective. In selected examples, yields are superior to those of H2SO4 and POCl3/pyridine dehydrations.11 See also Copper(II) Sulfate.

Elimination Reactions.

A Cu(OTf)2/Hünig base combination provides a useful method for converting 1,1-bis(phenylthio)cyclobutanes to 1-(phenylthio)cyclobutenes (eq 12).12

Reduction of Alkynyl Sulfones.

The HSiEt2Me/Cu(OTf)2 system reduces alkynyl sulfones to cis vinylic sulfones (eq 13).13 The yields with Copper(II) Tetrafluoroborate are higher than those with Cu(OTf)2. Dimeric side products are formed in some cases.

Oxazoles from Ketones.

Aliphatic ketones react with nitriles in the presence of Cu(OTf)2 and catalytic amounts of p-Toluenesulfonic Acid in refluxing MeCN to give oxazoles.14 The oxazole produced in this way is isomeric with that formed from a ketone and an amide (eq 14).

Oxidation of Alkyl Radicals.

Various alkyl radicals are oxidized with CuII triflate or perchlorate to carbenium ions whose reactivities are similar to solvolytically formed cations (eq 15).1 The synthetic utility of such Cu(OTf)2-catalyzed oxidations remains to be explored.

Related Reagents.

Copper(II) Sulfate; Copper(I) Trifluoromethanesulfonate.

1. Jenkins, C. L.; Kochi, J. K. JACS 1972, 94, 843.
2. (a) Kobayashi, Y.; Taguchi, T.; Tokuno, E. TL 1977, 3741. (b) Kobayashi, Y.; Taguchi, T.; Morikawa, T.; Tokuno, E.; Sekiguchi, S. CPB 1980, 28, 262. (c) Kobayashi, Y.; Taguchi, T.; Morikawa, T. TL 1978, 3555.
3. Takeda, T.; Ogawa, S.; Koyama, M.; Kato, T.; Fujiwara, T. CL 1989, 1257.
4. Snider, B. B.; Kwon, T. JOC 1992, 57, 2399.
5. (a) Andrist, A. H.; Agnello, R. M.; Wolfe, D. C. JOC 1978, 43, 3422. (b) Doyle, M. P.; Dorow, R. L.; Buhro, W. E.; Griffin, J. H.; Tamblyn, W. H.; Trudell, M. L. OM 1984, 3, 44.
6. Hubert, A. J.; Feron, A.; Warin, R.; Teyssie, P. TL 1976, 1317.
7. Doyle, M. P.; Trudell, M. L. JOC 1984, 49, 1196.
8. Moniotte, Ph. G.; Hubert, A. J.; Teyssie, Ph. JOM 1975, 88, 115.
9. Maryanoff, B. E. JOC 1979, 44, 4410.
10. Saha, B.; Bhattacharjee, G.; Ghatak, U. R. TL 1986, 27, 3913.
11. Laali, K.; Gerzina, R. J.; Flajnik, C. M.; Geric, C. M.; Dombroski, A. M. HCA 1987, 70, 607.
12. Kwon, T. W.; Smith, M. B. SC 1992, 22, 2273.
13. Ryu, I.; Kusumoto, N.; Ogawa, A.; Kambe, N.; Sonoda, N. OM 1989, 8, 2279.
14. Nagayoshi, K.; Sato, T. CL 1983, 1355.

Kenneth K. Laali

Kent State University, OH, USA

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