"Cation pool" method based on C-C bond dissociation. Effective generation of monocations and dications

Article abstract of DOI:10.1021/ja050414y

The "cation pools" of alkoxyarylcarbenium ions were effectively generated by the oxidative C-C bond dissociation using low temperature electrolysis. The present method is especially effective for the generation and accumulation of dications, which react with carbon nucleophiles. Copyright

Full text of DOI:10.1021/ja050414y

Published on Web 04/26/2005
“Cation Pool” Method Based on C-C Bond Dissociation. Effective Generation
of Monocations and Dications
Masayuki Okajima, Seiji Suga, Kenichiro Itami, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Kyoto 615-8510, Japan
Received January 21, 2005; E-mail: yoshida@sbchem.kyoto-u.ac.jp
We have recently developed the “cation pool” method,1 which
Scheme 1
involves the irreversible oxidative generation and accumulation of
highly reactive cations in the absence of nucleophiles. Heteroatom-
stabilized carbocations, such as N-acyliminium ion pools and
alkoxycarbenium ion pools, have been generated by this method.
In the first step of the cation pool method, a radical cation is
Scheme 2
generated by one-electron transfer. In the next step, a proton or a
formal silyl cation is eliminated to give the carbon radical, which
undergoes further one-electron oxidation to give the corresponding
carbocation (Scheme 1).
We have been interested in developing a new method for the
generation of cation pools, which involves the oxidative C-C bond
Scheme 3
dissociation, because such a process is commonly observed in the
decomposition of radical cations2-4 (Scheme 2). A major advantage
of this approach is easy generation of dications,5 if we employ cyclic
compounds as starting materials (Scheme 3).6 Such dications seem
to provide powerful intermediates for the construction of various
organic architectures. We report here the realization of this concept.
Scheme 4
The electrochemical oxidation of meso-1,2-dimethoxy-1,2-di-
phenylethane (1) was carried out in an undivided cell equipped
with a carbon felt anode and a platinum plate cathode using Bu4-
NBF4/CD2Cl2 at -48 °C (Scheme 4). After 2.5 F/mol of electricity
was consumed, the solution thus obtained (a pool of alkoxycarbe-
Scheme 5
nium ion 2) was analyzed by NMR spectroscopy at 0 °C. 1H NMR
exhibited a signal at 9.87 ppm due to the methine proton. 13C NMR
exhibited a signal at 207.8 ppm due to the methine carbon. These
chemical shifts are consistent with those of alkoxycarbenium ions
generated from acetals by the treatment with a Lewis acid.7 These
values also suggest the presence of a strong positive charge at the
carbon, indicating the formation of an ionic species.
Thus, a pool of 2 was generated at -48 °C in a preparative scale
in CH2Cl2 and was allowed to react with allyltrimethylsilane
(2.5 equiv) (Scheme 4). The corresponding allylated product 3 was
obtained in 90% yield based upon 1 (100% yield means that 2 mols
dissociations. Compounds 7-9 were also examined (runs 8-13).
They gave 3, but the yields were lower.
In the next step, we examined the use of cyclic compounds as
starting materials to generate dications. The anodic oxidation of
of 2 is produced from 1 mol of 1) (Table 1, run 2), indicating that
compound 10 (Eox ) 1.55 V) resulted in the formation of dication
cation 2 was effectively formed by the dissociation of the C-C
bond. Although the reaction at -25 °C gave a similar result (run
3), the reaction at -78 °C resulted in lower yield of 3, presumably
because the dissociation of the C-C bond is slow at this temperature
(run 1).
11, which exhibited 13C NMR signals at 74.3 ppm due to methyl
carbons and at 205.0 ppm due to the cationic carbons (Figure 1),
indicating that 11 has a symmetrical structure (Scheme 5). The
addition of allyltrimethylsilane to the solution gave rise to the
formation of diallylated compound 12 in 80% yield.
D,L-1,2-Dimethoxy-1,2-diphenylethane (4) was also effective as
a precursor, although the yield of 3 was slightly lower (run 4). It
is interesting to note that the electrochemical oxidation of meth-
oxyphenylmethane (5) also gave 3 despite its higher oxidation
potential, but the yield was somewhat lower (runs 5 and 6). The
use of a silyl group as an electroauxiliary8 decreased the oxidation
potential and led to effective formation of 3 (run 7). These
observations indicate that the C-C bond dissociation is comparable
to, or somewhat more effective than, the C-H and C-Si bond
The reaction with methylmagnesium bromide and a ketene silyl
acetal afforded 13 and 14, respectively, as a mixture of stereo-
isomers. It is important to note that the anodic oxidation of 15
(Eox ) 1.86 V, 5.0 F/mol) followed by the reaction with the ketene
silyl acetal gave only a trace amount of 14 (ca. 5% yield) together
with the monoalkylation product 16 (30% yield), indicating that
the dissociation of two C-H bonds to generate dication 11 did not
take place effectively. This observation demonstrates the advantage
of the present C-C bond dissociation approach to dications. The
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10.1021/ja050414y CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Generation of a Pool of 2a
Table 2. Generation of Dications and Their Reaction with
Nucleophilesa
a Reactions were usually carried out with 0.4 mmol of a precursor. After
the electrolysis (2.5 F/mol) at -78 °C, a nucleophile (5.0 equiv) was added
at the same temperature. b Isolated yields. c The electrolysis and the follow-
up reaction were carried out at -48 °C. d A mixture of isomers. e The
electrolysis was carried out at -15 °C, and the follow-up reaction was
carried out at -78 °C.
a Reactions were usually carried out with 0.4 mmol of substrate and 2.5
equiv of allyltrimethylsilane. b Yields were determined by GC.
Yoshida, J. Org. Lett. 2004, 6, 3755-3758. (l) Suga, S.; Suzuki, S.;
Maruyama, T.; Yoshida, J. Bull. Chem. Soc. Jpn. 2004, 77, 1545-1554.
(m) Suga, S.; Nishida, T.; Yamada, D.; Nagaki, A.; Yoshida, J. J. Am.
Chem. Soc. 2004, 126, 14338-14339. (n) Nagaki, A.; Kawamura, K.;
Suga, S.; Ando, T.; Sawamoto, M.; Yoshida, J. J. Am. Chem. Soc. 2004,
126, 14702-14703.
Figure 1. 13C NMR spectrum of 11 (in CD2Cl2, -80 °C).
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anodic oxidation of 17 (Eox ) 1.45 V) followed by the reaction
with allyltrimethylsilane also proceeded smoothly to give the desired
diallylated product 18 in good yield. In summary, the present
observations illustrate the potential of a new approach for the
generation of cation pools, which involves C-C bond dissociation
in the radial cation intermediates. It is also noteworthy that the
method opens a simple and straightforward approach to dications
under mild conditions. Further work is in progress to explore the
full range of applicability of the present approach.
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Acknowledgment. This work was partially supported by the
Grant-in-Aid for Scientific Research.
Supporting Information Available: Experimental procedures and
analytical and spectroscopic data (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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