Copper(II) Tetrafluoroborate

Cu(BF4)2

[38465-60-0]  · B2CuF8  · Copper(II) Tetrafluoroborate  · (MW 237.15)

(Lewis acid capable of promoting Diels-Alder reactions;1 promotes decomposition of diazo compounds;2 functions as an oxidizing agent;3 reacts with siloxycyclopropanes to generate b-acylalkylcopper(II) species;4 in combination with hydrosilanes is a syn-selective reducing agent of alkynic sulfones5)

Physical Data: d 2.175 g cm-3 for the hexahydrate.6

Solubility: sol water, THF, MeOH, EtOH; insol ether.

Form Supplied in: blue powder. Drying: copper(II) tetrafluoroborate (Cu(BF4)2.xH2O), irrespective of the number of crystalline water, has a highly hygroscopic nature. Therefore, prior to use, it should be dried and stored in a dry nitrogen atmosphere (e.g. over KOH at 25 °C for 5 d;1 P2O5 at 50 °C/1 mmHg overnight;4,5 at 100 °C, under high vacuum for 5 h7). Previous work has suggested that dehydration of the hexahydrate to give a tetrahydrate8 takes place easily at reduced pressure at rt, while the anhydrous salt appears to be unknown. Accordingly, note that the procedures so far examined for drying are unlikely to give a pure anhydrous salt.

Handling, Storage, and Precautions: because of its highly hygroscopic nature, copper(II) tetrafluoroborate should be stored so as to preclude contact with moisture. Lumps should be crushed only in a glove bag or dry box filled with N2. Use in a fume hood.

Catalyst of Diels-Alder Reactions.

Copper(II) tetrafluoroborate has been found to promote Diels-Alder reaction of 5-methoxymethyl-1,3-cyclopentadiene and a-chloroacrylonitrile (eq 1).1 This dienophile serves as a ketene synthon; thus, when subjected to alkaline hydrolysis, the product gives the bicyclic ketone. A similar acceleration of Diels-Alder reactions by copper tetrafluoroborate is recognized in the reaction of a-methylthioacrylonitrile and cyclopentadiene (eq 2).9 The thermal process requires heating to 140 °C without solvent, whereas the copper-catalyzed reaction can be conducted at 20 °C in benzene solution. With furan as a dienophile, copper tetrafluoroborate in combination with hydroquinone catalyzes the [4 + 2] cycloaddition (eqs 3 and 4).10,11 In this reaction system, a copper(I) tetrafluoroborate complex, formed through the reduction of copper(II) by hydroquinone, is believed to play a key role.10

Decomposition of Diazo Compounds.

Like other copper salts, copper(II) tetrafluoroborate can serve as the catalyst for the decomposition of diazo compounds to generate copper carbenoids.2 The preference of cyclopropanation by Ethyl Diazoacetate (EDA) for less-substituted alkenes has been observed specifically with copper(II) tetrafluoroborate (also with Copper(II) Trifluoromethanesulfonate), showing different selectivity with other copper catalysts (Copper(II) Acetylacetonate, Copper(I) Iodide-Trimethyl Phosphite, (MeO)3P.CuCl, Copper(II) Sulfate, etc.) (eq 5). The active catalyst may be alkene coordinated copper(I) tetrafluoroborate,12 which might be generated by the reduction of copper(II) tetrafluoroborate by diazo compounds.

The copper(II) tetrafluoroborate-catalyzed reaction of EDA with N-methylpyrrole gives a mixture of 2- and 3-acetate adducts (eq 6).13 Thermal decomposition of a pyrazoline to give a cyclopropane ring can be performed efficiently with a catalytic amount of copper tetrafluoroborate in refluxing benzene (eq 7).14

Oxidations.

Irradiation of a solution of benzyltrimethylsilane and 2 equiv of copper tetrafluoroborate in acetonitrile-methanol (3:1) with a 300 W high-pressure mercury lamp through a quartz filter gives benzyl methyl ether in 70% yield. Tetrakis(acetonitrile)copper(I) tetrafluoroborate is formed as a byproduct (eq 8).3 The oxidation sequence can be applied to benzylgermanes and benzylstannanes. The reaction mechanism may involve (i) one-electron transfer from photoexcited singlet organometallic compounds to CuII to form the cation radical and CuI, (ii) dissociation of the cation radical to an arylmethyl radical, and (iii) the second oxidation of the radical by CuII to give the benzyl cation and CuI. It has also been reported that copper(II) tetrafluoroborate can be used for the effective oxidation of 5-exo-alkyl-5H-phlorin derivatives to give the corresponding 5-alkylporphyrins.15 The oxidation ability of copper tetrafluoroborate has been utilized for the iodofunctionalization of alkenes, where Cu2+ ion oxidizes iodine to form iodonium cation.16 In this reaction, water, alcohol, carboxylic acid, lithium chloride, sodium iodide, etc. can be used as the nucleophile.

Reaction with Siloxycyclopropanes.

Copper tetrafluoroborate causes ring-opening of siloxycyclopropanes to provide useful synthetic reactions.4 Treatment of a siloxycyclopropane with Cu(BF4)2 results in desilylative dimerization to give a 1,6-diketone in good yield (eq 9).17 The ring-opening takes place regioselectively across the bond between the methylene and siloxy carbons. The reaction is reasonably interpreted by assuming electrophilic ring-opening by Cu2+ ion to form the b-copper(II) ketone, followed by dimerization.

With Dimethyl Acetylenedicarboxylate and water, b-(acyl)alkyls can be trapped to give dimethyl 2-(3-oxoalkyl)maleates with a high degree of stereoselectivity (eq 10).17 The selectivity arises from syn addition of b-copper(II) ketone across the triple bond followed by in situ protonation of the resulting vinylcopper species with retention of configuration. The stereoselective transfer of b-(acyl)alkyls to alkynic sulfones, which gives b-(acyl)alkylated vinylic sulfones, is also successful (eq 11). Intramolecular cyclization with 1-phenyl-2-trimethylsilyloxy-2-(3-hexenyl)cyclopropane results in modest amounts of the 5-exo cyclization product.18

Reaction with Hydrosilanes.

Copper(II) tetrafluoroborate reacts readily with hydrosilanes to give hydrogen and trialkylfluorosilanes (eq 12). When an alkynic sulfone is present in this system, highly stereoselective syn reduction takes place to give a vinylic sulfone (eq 13).5 The mechanism involving the syn addition of HCuII seems likely.

Related Reagents.

Silver(I) Tetrafluoroborate; Tetrafluoroboric Acid.


1. Corey, E. J.; Koelliker, U.; Neuffer, J. JACS 1971, 93, 1489.
2. Salomon, R. G.; Kochi, J. K. JACS 1973, 95, 3300.
3. Mizuno, K.; Yasueda, M.; Otsuji, Y. CL 1988, 229.
4. Ryu, I.; Ando, M.; Ogawa, A.; Murai, S.; Sonoda, N. JACS 1983, 105, 7192.
5. Ryu, I.; Kusumoto, N.; Ogawa, A.; Kambe, N.; Sonoda, N. OM 1989, 8, 2279.
6. Sharp, D. W. A. Adv. Fluorine Chem. 1960, 1, 68.
7. Maryanoff, B. E. JOC 1979, 44, 4410.
8. El'Kenbard, A. G. Sbornik Statei po Obshchei Khim., Akad. Nauk SSSR 1953, 2, 1239 (CA 1955, 49, 2931e).
9. (a) Stella, L.; Boucher, J.-L. TL 1982, 23, 953. (b) Goering, H. L.; Chang, C.-S. JOC 1975, 40, 2565.
10. Moore, J. A.; Partain III, E. M. JOC 1983, 48, 1105.
11. Vieira, E.; Vogel, P. HCA 1982, 65, 1700.
12. Sharp, D. W. A.; Sharpe, A. G. JCS 1956, 1858.
13. Maryanoff, B. E. JOC 1979, 44, 4410.
14. Wulfman, D. S.; McDaniel, Jr., R. S. TL 1975, 4523.
15. Setsune, J.; Yamaji, H.; Kitao, T. TL 1990, 31, 5057.
16. Barluenga, J.; Rodriguez, M. A.; Campos, P. J. JCS(P1) 1990, 2807.
17. Ryu, I.; Matsumoto, K.; Kameyama, Y.; Ando, M.; Kusumoto, N.; Ogawa, A.; Kambe, N.; Murai, S.; Sonoda, N. JACS 1993, 115, 12330.
18. Snider, B. B.; Kwon, T. JOC 1992, 57, 2399.

Ilhyong Ryu & Noboru Sonoda

Osaka University, Japan



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