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J. Am. Chem. Soc. 2000, 122, 10244-10245
Scheme 1. Generation of Alkoxycarbenium Ion
Electrooxidative Generation and Accumulation of
Alkoxycarbenium Ions and Their Reactions with
Carbon Nucleophiles
Seiji Suga, Shinkiti Suzuki, Atsushi Yamamoto, and
Jun-ichi Yoshida*
Department of Synthetic Chemistry and
Biological Chemistry
Graduate School of Engineering
Kyoto UniVersity, Kyoto 606-8501, Japan
Scheme 2. Generation of Alkoxycarbenium Ion Pool from
R-Silyl Ether and Its Reaction with Allyltrimethylsilane
ReceiVed June 13, 2000
Alkoxycarbenium ions are carbenium ions stabilized by a
neighboring alkoxy group and are important reactive intermediates
in organic synthesis.1 For example, the Lewis acid promoted
reactions of acetals and related compounds with carbon nucleo-
philes such as allylsilanes and enol silyl ethers are considered to
proceed through alkoxycarbenium ion intermediates (Scheme 1
(a)).
Although highly stabilized alkoxycarbenium ions, such as
benzylic alkoxycarbenium ions and di- and tri(alkoxy)carbenium
ions, are well-characterized spectroscopically,2 it is difficult to
characterize simple alkylalkolxycarbenium ions. Extensive NMR
studies on the mechanism of the reaction of acetals with Lewis
acids revealed the presence of Lewis acid-acetal complexes, but
failed to detect alkoxycarbenium ions.3
Although alkoxycarbenium ions in superacid solution have been
investigated extensively,4 to our knowledge, there is no report
on the characterization of simple alkylalkoxycarbenium ions in
reaction media that are normally used in organic synthesis. Thus,
we initiated a project to study alkoxycarbenium ions using the
“cation pool” method. This technique involves the irreVersible
electrooxidative generation (Scheme 1 (b)) and accumulation of
carbocations.5
In the “cation pool” method, anodic oxidations are used to
generate and accumulate relatively high concentrations of car-
bocations at low temperature in the absence of nucleophiles. In
the next step the carbocations are then allowed to react with
nucleophiles. This one-pot method has an advantage over the
conventional processes because nucleophiles that might be
otherwise oxidized during an in situ process can be used without
any difficulty.
the reaction media.7 The regioselectivity is another problem,
because two regioisomeric alkoxycarbenium ions are generally
formed from unsymmetrical dialkyl ethers.
The pre-introduciton of a silyl group solves these problems.8
The oxidation potentials of R-silyl ethers (Scheme 1 (b), M )
Si) are much less positive than the corresponding dialkyl ethers,9
and their anodic oxidation takes place smoothly, giving rise to
selective cleavage of the C-Si bond which eventually leads to
the generation of an isomerically pure alkoxycarbenium ion.
Thus, R-silyl ether (1) was oxidized in a divided cell equipped
with a carbon felt anode and a platinum plate cathode in deuterated
dichloromethane in the presence of tetrabutylammonium tet-
rafluoroborate as electrolyte at -72 °C (Scheme 2).
After 2.5 F/mol of electricity was consumed, the solution thus
obtained was analyzed by NMR spectroscopy at -80 °C. 1H NMR
exhibited a signal at 9.55 ppm due to the methine proton. 13C
NMR exhibited a signal at 231.0 ppm due to the methine carbon.10
These chemical shifts are consistent with those of alkoxycarbe-
nium ions generated in superacid.4 These values also suggest the
presence of a strong positive charge at the carbon, indicating the
formation of a solution of an ionic species.
The pool of the alkoxycarbenium ion 2, thus generated by the
low temperature electrolysis, was then allowed to react with
allyltrimethylsilane as a carbon nucleophile. The corresponding
allylated product 3 was obtained in 80% yield. Noteworthy is
that this reaction is extremely fast even at -72 °C. Even under
these conditions, the reaction was complete within a few
minutes.11 Other electrolytes such as tetrabutylammonium per-
chlorate, triflate, hexafluorophosphate in electrolysis gave us poor
yields of 3, probably because of inefficiency in the accumulation
of 2.12
As precursors of alkoxycarbenium ions in the cation pool
method, dialkyl ethers should be the first choice (Scheme 1 (b),
M ) H)6 in the analogy with the reported oxidative generation
of iminium cation pool from amine derivatives. The oxidation
potentials of dialkyl ethers, however, are very positive, and hence,
it is rather difficult to oxidize ethers selectively without affecting
(1) For example, Santelli, M.; Pons, J.-M. Lewis Acids and SelectiVity in
Organic Synthesis; CRC Press: Boca Raton, 1995; Chapter 4.
(2) Benzylic alkoxycarbenium ions: (a) Rabinovitz, M.; Bruck, D.
Tetrahedron Lett. 1971, 245. (b) Amyes, T. L.; Jencks, W. P. J. Am. Chem.
Soc. 1989, 111, 7888. (c) Jagannadham, V.; Amyes, T. L.; Richard, J. P. J.
Am. Chem. Soc. 1993, 115, 8465. (d) Mayr, H.; Gorath, G. J. Am. Chem.
Soc. 1995, 117, 7862. Di- and trialkoxycarbenium ions: (e) Ramsey, B. G.;
Taft, R. W. J. Am. Chem. Soc. 1966, 88, 3058. (f) Steenken, S.; Buschek, J.;
McClelland, R. A. J. Am. Chem. Soc. 1986, 108, 2808. (g) McClelland, R.
A.; Steenken, S. J. Am. Chem. Soc. 1988, 110, 5860. (h) Steenken, S.;
McClelland, R. A. J. Am. Chem. Soc. 1989, 111, 4967.
(7) For example, Shono, T. In The Chemistry of Ethers, Crown Ethers,
Hydroxyl groups and Their Sulfur Analoges, Part 1; Patai, S., Ed.; Wiley:
Chichester, 1980; Chapter 8.
(8) (a) Yoshida, J.; Murata, T.; Isoe, S. J. Organomet. Chem. 1988, 345,
C23. (b) Yoshida, J.; Matsunaga, S.; Murata, T.; Isoe, S. Tetrahedron 1991,
47, 615.
(9) (a) Yoshida, J.; Maekawa, T.; Murata, T.; Matsunaga, S.; Isoe, S. J.
Am. Chem. Soc. 1990, 112, 1962. (b) Yoshida, J.; Nishiwaki, K. J. Chem.
Soc., Dalton Trans. 1998, 2589.
(10) There are no signals which could be assigned to species having C-F
covalent bond, although tetrafluoroborate is well-known as fluorination reagent.
NMR spectra of compounds having O-C-F units are reported: See, Rozov,
L. A.; Rafalko, P. W.; Evans, S. M.; Brockunier, L.; Ramig, K. J. Org. Chem.
1995, 60, 1319.
(3) Denmark, S. E.; Willson, T. M. In SelectiVities in Lewis Acid Promoted
Reactions; Schinzer, D. Ed.; Kluwer Academic Publishers: Dordrecht, 1989;
p 247.
(4) (a) Olah, G. A.; Bollinger, J. M. J. Am. Chem. Soc. 1967, 89, 2993. (b)
Olah, G. A.; Sommer, J. J. Am. Chem. Soc. 1968, 90, 4323. (c) Forsyth, D.
A.; Osterman, V. M.; DeMember, J. R. J. Am. Chem. Soc. 1985, 107, 818.
(5) Yoshida, J.; Suga, S.; Suzuki, S.; Kinomura, N.; Yamamoto, A.;
Fujiwara, K. J. Am. Chem. Soc. 1999, 121, 9546.
(6) Generation of alkoxycarbenium ions by radiation of ethers: See refs
2f and 2g.
(11) The Lewis acid promoted reactions of acetals with carbon nucleophiles
such as allylsilanes and enol silyl ethers are relatively slower (usually several
hours at -78 °C) than our reaction, see: (a) Mukaiyama, T.; Hayashi, M.
Chem. Lett. 1974, 15. (b) Hosomi, A.; Endo, M.; Sakurai, H. Chem. Lett.
1976, 941. (c) Murata, S.; Suzuki, M.; Noyori, R. J. Am. Chem. Soc. 1980,
102, 3248. (d) Demmark, S. E.; Almstead, N. G. J. Am. Chem. Soc. 1991,
113, 8089.
10.1021/ja002123p CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/30/2000