Oxidative generation of diarylcarbenium ion pools

Article abstract of DOI:10.1021/ol061647c

(Figure Presented) "Cation pools" of diarylcarbenium ions have been generated by the oxidative C-H bond dissociation of diarylmethanes using anodic oxidation. "Diarylcarbenium ion pools" thus generated react with various nucleophiles, such as allylsilanes

Full text of DOI:10.1021/ol061647c

ORGANIC
LETTERS
2
006
Vol. 8, No. 22
005-5007
Oxidative Generation of Diarylcarbenium
Ion Pools
5
Masayuki Okajima, Kazuya Soga, Toshiki Nokami, Seiji Suga, and
Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan
yoshida@sbchem.kyoto-u.ac.jp
Received July 5, 2006
ABSTRACT
“Cation pools” of diarylcarbenium ions have been generated by the oxidative C−H bond dissociation of diarylmethanes using anodic oxidation.
“Diarylcarbenium ion pools” thus generated react with various nucleophiles, such as allylsilanes, ketene silyl acetals, and aromatic compounds.
The reductive homocoupling of the “diarylcarbenium ion pool” has been achieved. The dimer thus obtained also serves as a precursor of the
diarylcarbenium ion pool” via oxidative C C bond dissociation.
“
−
Diarylcarbenium ions have received significant research
interest from a mechanistic viewpoint. Some diarylcarbenium
Laser flash photolysis has also been utilized as a method
for the generation of diarylcarbenium ions. Although some
4
1
ions are rather stable, and their reactivities toward nucleo-
stable diarylcarbenium ions can be prepared using such
2
5
philes have been extensively studied. It is also worth noting
methods, a general and convenient method for the generation
that diarylcarbenium ions may serve as versatile intermediates
for the synthesis of aryl-substituted compounds of interesting
functions.
and accumulation of reactive diarylcarbenium ions is still
needed.
Recently, we have developed the “cation pool” method
that involves irreversible oxidative generation and accumula-
tion of carbocations in the absence of nucleophiles at low
Diarylcarbenium ions are often generated from the cor-
1b,3
responding alcohols and halides by ionization in superacid.
6
(1) (a) Deno, C.; Schriesheim, A. J. Am. Chem. Soc. 1955, 77, 3051. (b)
temperature. N-Acylminium ion pools and alkoxycarbenium
Arnett, E. M.; Hofelich, C. J. Am. Chem. Soc. 1983, 105, 2889. (c) Schade,
C.; Mayr, H.; Arnett, E. M. J. Am. Chem. Soc. 1988, 110, 567.
ion pools can be easily generated by the oxidative cleavage
of C-H, C-Si, and C-C bonds. Thus, it is appropriate to
ask if diarylcarbenium ions can be generated by the “cation
pool” method. We report herein that the idea works. This is
the first example of the generation of “cation pools” without
(2) (a) Mayr, H.; Schneider, R.; Schade, C.; Bartl, J.; Bederke, R. J.
Am. Chem. Soc. 1990, 112, 4446. (b) Mayr, H.; Schneider, R.; Irrgang, B.;
Schade, C. J. Am. Chem. Soc. 1990, 112, 4454. (c) Mayr, H.; Schneider,
R.; Grabis, U. J. Am. Chem. Soc. 1990, 112, 4460. (d) Hagen, G.; Mayr,
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Mayr, H. J. Am. Chem. Soc. 1993, 115, 6025. (g) Mayr, H.; Kempt, B.;
Ofial, A. R. Acc. Chem. Res. 2003, 36, 66. (h) Minegishi, S.; Mayr, H. J.
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2
003, 68, 6880. (j) Ofial, A.; Ohkubo, K.; Fukuzumi, S.; Lucius, R.; Mayr,
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Soc. 2003, 125, 12980. (l) Loots, R.; Kobayashi, S.; Mayr, H. J. Am. Chem.
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Chem. Soc. 2004, 126, 5174. (n) Bug, T.; Lemek, T.; Mayr, H. J. Org.
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Kobayashi, S.; Mayr, H. J. Am. Chem. Soc. 2005, 127, 2641. (q) Denekamp,
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(5) (a) Jutz, C. Chem. Ber. 1958, 91, 850. (b) Mayr, H.; Bug, T.; Gotta,
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1
0.1021/ol061647c CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/30/2006

having an adjacent cation-stabilizing heteroatom such as
oxygen and nitrogen.
The anodic oxidation reactions of various diarylmethanes
1
4 4 2 2
in Bu NBF /CH Cl were carried out using a carbon felt
anode, and the resulting solutions were allowed to react with
allyltrimethylsilane to obtain the corresponding allylated
diarylmethane 5 (Scheme 1).
Scheme 1. Proposed Mechanism of Generation of
Diarylcarbenium Ion
Figure 1. Oxidation potentials of diarylmethanes 1 and yields of
allylated diarylmethanes 5. Oxidation potentials were determined
by RDE (rotating disk electrode) in LiClO /CH CN using an SCE
4 3
as a reference electrode. The electrochemical oxidation was carried
out in a divided cell equipped with a carbon felt anode in 0.3 M
4 4 2 2
Bu NBF /CH Cl at -78 °C unless otherwise stated under constant
current conditions. After the electrolysis (2.5 F/mol), the resulting
solution was allowed to react with allyltrimethylsilane at -78 °C
for 20 min. Yields of allylated product from 1i under various
temperatures were determined by NMR.
As depicted in Figure 1, the efficiency of the reaction
strongly depends on the nature of the diarylmethane. It was
surprising that simple diphenylmethane (1a) gave complex
mixture, and the allylated compound 5a was not detected,
although the detailed mechanism is not clear at present. The
introduction of substituents at para positions of the benzene
rings resulted in a smooth transformation (1b-g). Meta
substituents did not work (1h), and 84% of 1h was recovered
unchanged. Not only electron-donating substituents (1b-
e), but also weak electron-withdrawing substituents (1f-g)
were effective. It is interesting that the introduction of a para
substituent on only one of the two phenyl rings is effective
for the success of the reaction (1e). Successful reaction of
extended diarylmethane 1i, which implies the possibility for
the synthesis of various extended π-electron systems, is also
similar processes may take place to generate 2 (Scheme 1).
It is well-known that the benzylic C-H bond in such a
9
radical cation is readily cleaved. The overlap of C-H σ
orbital with the SOMO of the aromatic ring weakens the
10
C-H bond, which eventually leads to its cleavage to
generate benzylic radical 3. Another important factor is the
11
nature of the substituent on the aromatic ring. An electron-
donating group stabilizes the radical cation to suppress the
C-H bond cleavage. On the other hand, an electron-
withdrawing group facilitates the C-H bond cleavage,
although the first electron transfer is less favorable. Experi-
mental results indicate that the electronic nature of the
substituent is not so important for the success of the reaction.
Anyway, thus-generated carbon radical 3 is further oxidized
7
worth noting. Easy access to diarylmethanes is an advantage
of the present process from a synthetic point of view.
As to the mechanism, Kochi reported that the initial
electron transfer from di(pentamethylphenyl)methane leads
to the formation of the radical cation localized on a single
aromatic ring, which is converted to the radical cation
to give the corresponding diarylcarbenium ion 4 (Scheme
12
1
).
Temperature effect for the reaction of 1i is interesting
(Figure 1). The reaction at -78 °C did not afford the allylated
8
π-delocalized on two aromatic rings. In the present cases,
product 5i at all, and 64% of 1i was recovered unchanged.
(
6) (a) Yoshida, J.; Suga, S.; Suzuki, S.; Kinomura, N.; Yamamoto, A.;
(8) Sankararaman, S.; Lau, W.; Kochi, J. K. J. Chem. Soc., Chem.
Commun. 1991, 396.
(9) Reviews: (a) Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2550. (b) Baciocchi, E.; Bietti, M.; Lanzalunga, O. Acc.
Chem. Res. 2000, 33, 243.
Fujiwara, K. J. Am. Chem. Soc. 1999, 121, 9546. (b) Suga, S.; Suzuki, S.;
Yamamoto, A.; Yoshida, J. J. Am. Chem. Soc. 2000, 122, 10244. (c) Suga,
S.; Okajima, M.; Yoshida, J. Tetrahedron Lett. 2001, 42, 2173. (d) Suga,
S.; Okajima, M.; Fujiwara, K.; Yoshida, J. J. Am. Chem. Soc. 2001, 123,
7
3
941. (e) Suga, S.; Suzuki, S.; Yoshida, J. J. Am. Chem. Soc. 2002, 124,
0. (f) Yoshida, J.; Suga, S. Chem. Eur. J. 2002, 8, 2650. (g) Suga, S.;
(10) Bond dissociation energies of radical cations have been studiedex-
tensively. Zhang, X.; Bordwell, G. J. Org. Chem. 1992, 57, 4163.
(11) (a) Perrott, A. L.; Arnold, D. R. Can. J. Chem. 1992, 70, 272. (b)
Tolbert, L. M.; Khanna, R. K.; Popp, A. E.; Gelbaum, L.; Bottomley, L.
A. J. Am. Chem. Soc. 1990, 112, 2373. (c) Baciocchi, E.; Mattioli, M.;
Romano, R.; Ruzziconi, R. J. Org. Chem. 1991, 56, 7154. (d) Tolbert, L.
M.; Li, Z.; Sirimanne, S. R.; VanDerveer, D. G. J. Org. Chem. 1997, 62,
3927.
Watanabe, M.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 14824. (h) Suga,
S.; Suzuki, S.; Maruyama, T.; Yoshida, J. Bull. Chem. Soc. Jpn. 2004, 77,
1
545. (i) Suga, S.; Nishida, T.; Yamada, D.; Nagaki, A.; Yoshida, J. J. Am.
Chem. Soc. 2004, 126, 14338. (j) Maruyama, T.; Suga, S.; Yoshida, J. J.
Am. Chem. Soc. 2005, 127, 7324. (k) Okajima, M.; Suga, S.; Itami, K.;
Yoshida, J. J. Am. Chem. Soc. 2005, 127, 6930.
(
7) Itami, K.; Mineno, M.; Kamei, T.; Yoshida, J. Org. Lett. 2002, 4,
(12) The oxidation potentials of radicals have been studied. Wayner, D.
D. M.; McPhee, D. J.; Griller, D. J. Am. Chem. Soc. 1988, 110, 132.
3
635 and references cited therein.
5006
Org. Lett., Vol. 8, No. 22, 2006

With the increase of the electrolysis temperature, the yield
increased. It seems to be reasonable to consider that the
deprotonation of the initially formed radical cation 2i requires
higher temperature because the charge may be delocalized
through the extended π-system.
Another important transformation of the diarylcarbenium
ion pool is reductive homocoupling (Scheme 3). For example,
Scheme 3. Reductive Homocoupling of Diarylcarbenium Ion
and Oxidative Cleavage of Tetraarylethane
The accumulation of diarylcarbenium ion was confirmed
by NMR spectroscopy. For example, a solution of 4f
generated from 1f exhibited a signal at 192.6 ppm due the
carbenium carbon. This chemical shift is very close to that
13
2
(193.3 ppm) of 4f generated in a superacid media (SO ClF).
CSI-MS analysis (spray temperature: -20 °C) also supported
the formation of 4f (m/z ) 203).
In order to demonstrate the synthetic utility of diarylcar-
benium ion pools, several transformations have been exam-
ined. As exemplified in Scheme 2, the dicarylcarbenium ion
Scheme 2. Reactions of Diarylcarbenium Ions with
Nucleophiles
1
f was anodically oxidized at -78 °C to generate 4f, and
the resulting solution was cathodically reduced at -78 °C.
The homocoupling product 10f was obtained in 81% yield,
14
presumably via radical 3f. It should be noted that the reverse
transformation can also be accomplished electrochemically.
The anodic oxidation of 10f at -78 °C generated 4f via
6
k
oxidative C-C bond dissociation, which can be trapped
by allyltrimethylsilane to give 5f (55% yield).
In summary, the “cation pool” method was found to be
effective for the generation of diarylcarbenium ions, although
the efficiency depends on the nature of the starting diaryl-
methanes. The present method serves as powerful tools for
the construction of various organic compounds having
diarylmethane structures. Further work is in progress to
explore the scope and limitations of the present method and
to develop its applications to the synthesis of organic
architectures containing aromatic rings.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Scientific Research.
pool 4f reacted with various nucleophiles including allylsi-
lanes and ketene silyl acetals. Friedel-Crafts-type reactions
with aromatic and heteroatomatic compounds also took place
smoothly. Such reactions serve as powerful tools for
construction of various organic compounds containing
aromatic rings. When diethylzinc was used as a nucleophile,
the yield of product was not high.
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL061647C
(
14) Reductive coupling of “cation pool”: Suga, S.; Suzuki, S.;
Maruyama, T.; Yoshida, J. Bull. Chem. Soc. Jpn. 2004, 77, 1545. See also
ref 6e.
(13) Kelly, D. P.; Jenkins, M. J. J. Org. Chem. 1984, 49, 409.
Org. Lett., Vol. 8, No. 22, 2006
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