Reactions of a N-acyliminium ion pool with benzylsilanes. Implication of a radical/cation/radical cation chain mechanism involving oxidative C-Si bond cleavage

Article abstract of DOI:10.1021/ja068589a

The reaction of a N-acyliminium ion pool with benzylsilane proceeds by a chain mechanism involving oxidative C-Si bond cleavage. A radical/cation/radical cation crossover mechanism involving single electron transfer has been proposed. The effective binary

Full text of DOI:10.1021/ja068589a

Published on Web 01/30/2007
Reactions of a N-Acyliminium Ion Pool with Benzylsilanes. Implication of a
Radical/Cation/Radical Cation Chain Mechanism Involving Oxidative C-Si
Bond Cleavage
Tomokazu Maruyama, Yusuke Mizuno, Ikuo Shimizu, 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
Received November 30, 2006; E-mail: yoshida@sbchem.kyoto-u.ac.jp
Table 1. Reactions of Benzylsilanes 2 and Benzenystannanes
Mechanistic studies on the cleavage of the carbon-metal bond
with a Pool of N-Acyliminium Ion 1a
by the action of electrophiles have been carried out extensively so
far, and the importance of a single electron transfer (SET) process1
has been recognized not only for transition metal compounds but
also for main group organometallics.2 We have recently developed
the “cation pool” method,3 in which highly reactive organic cations
are generated and accumulated in the absence of nucleophiles.
Cation pools serve as powerful electrophiles toward various
nucleophiles, including organosilicon and organotin compounds.
Thus, we initiated our study to examine the possibility of a SET
mechanism for the reaction of cation pools with such organometallic
compounds.
We focused on the reactions of N-acyliminium ion pools3a,c-e
because their reactions with various nucleophiles have been studied
so far. Benzylsilanes were chosen to study as nucleophilic orga-
nometallics because benzylsilanes of various oxidation potentials
can be easily synthesized by introducing electron-withdrawing or
electron-donating groups on the benzene ring. Thus, several
benzylsilanes were synthesized, and they were allowed to react with
a pool of N-acyliminium ion 1, which was generated by low-
temperature electrochemical oxidation of the corresponding car-
bamate in Bu4NBF4/CH2Cl2 (Scheme 1).
Benzylsilanes 2a-c did not react with N-acyliminium ion 1 as
shown in Table 1 (runs 1, 4, and 5). Benzylsilanes of lower
oxidation potentials, however, reacted with 1 to give the corre-
sponding coupling products 3 (runs 6-7, 9-10, and 13), and 1.5
V seems to be the threshold point. Generally, benzylstannanes have
lower oxidation potentials than the corresponding benzylsilanes.
Thus, the reactions of benzylstannanes 8b and 8e with 1 were
examined. As expected from the oxidation potentials, the reactions
did take place to give 3 (runs 14 and 15).
In order to test the possibility of a SET mechanism, the effect
of a 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical was
examined for the reaction of 1 with 2g. The reaction was inhibited
completely in the presence 1 equiv of TEMPO (run 11). It is also
noteworthy that 0.2 equiv of TEMPO was effective to inhibit the
reaction completely (run 12). These observations imply that the
reaction proceeds by a chain mechanism involving a free radical
intermediate, although the possibility that TEMPO might retard
other processes in the chain mechanism, such as a redox process,
cannot be ruled out.
a Reactions were carried out with 0.2 mmol of benzylsilane 2 (or
benzylstannane 8) and 1.5 equiv of a pool of 1 at -50 °C for 1.5 h. b OSWV,
in Bu4NBF4/CH2Cl2 using SCE as a reference electrode. c Slow addition.
d The yield was based on 2e. The yield based on the total amount of 2e and
8e was 88%.
Scheme 1
On the basis of the observations described above, we proposed
a mechanism shown in Scheme 2. The initial SET from benzylsilane
2 to N-acyliminium ion 1 gives radical cation 4 and radical 5.4
The interaction of the C-Si σ orbital with the benzene π orbital
increases the HOMO level to facilitate the electron transfer.5
Because this process seems to be still energetically unfavorable, a
possibility of photoinduced SET cannot be ruled out.6 Another
possibility to be considered is that the SET is promoted by the
exoergonic follow-up reaction.7 Radical cation 4 then collapses to
benzyl radical 6 and a formal silyl cation. Such oxidative cleavage
of C-Si bonds has been extensively studied in photochemical,8
electrochemical,9 and chemical10 electron-transfer reactions. Because
such cleavage is known to be promoted by nucleophilic attack on
9
1902
J. AM. CHEM. SOC. 2007, 129, 1902-1903
10.1021/ja068589a CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
In conclusion, we have revealed that the reaction of a N-
acyliminium ion pool with a benzylsilane proceeds by a chain
mechanism involving oxidative C-Si bond cleavage. The success
of the binary system consisting of a stoichiometric amount of a
benzylsilane of high oxidation potential and a catalytic amount of
a more easily oxidized benzylstannane opens a new possibility of
promoting organometallic reactions that are otherwise difficult to
achieve. Further work aiming at elucidation of the detailed
mechanism and development of synthetic applications is in progress.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research.
Supporting Information Available: Experimental procedures and
spectroscopic data of compounds (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
Scheme 3
References
(1) For example: (a) Kochi, J. K. Acc. Chem. Res. 1974, 7, 351. (b) Kochi,
J. K. Organometallic Mechanism and Catalysis; Academic Press: New
York. 1978.
(2) Early examples of electron-transfer driven cleavage of a C-Si bond: (a)
Tamao, K.; Yoshida, J.; Murata, M.; Kumada, M. J. Am. Chem. Soc. 1980,
102, 3267. (b) Yoshida, J.; Tamao, K.; Kumada, M.; Kawamura, T. J.
Am. Chem. Soc. 1980, 102, 3269.
(3) (a) Yoshida, J.; Suga, S.; Suzuki, S.; Kinomura, N.; Yamamoto, A.;
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.; Suzuki, S.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 30. (d)
Yoshida, J.; Suga, S. Chem.sEur. J. 2002, 8, 2650. (e) Suga, S.; Watanabe,
M.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 14824. (f) Suga, S.; Nishida,
T.; Yamada, D.; Nagaki, A.; Yoshida, J. J. Am. Chem. Soc. 2004, 126,
14338. (g) Okajima, M.; Suga, S.; Itami, K.; Yoshida, J. J. Am. Chem.
Soc. 2005, 127, 6930. (h) Suga, S.; Matsumoto, K.; Ueoka, K.; Yoshida,
J. J. Am. Chem. Soc. 2006, 128, 7710.
(4) Reduction of iminium ions: (a) Conant, J. B.; Sloan, A. W. J. Am. Chem.
Soc. 1923, 45, 2466. (b) Andrieux, C. P.; Save´ant, J.-M. J. Electroanal.
Chem. 1970, 26, 223.
(5) (a) Traylor, T. G.; Hanstein, W.; Berwin, H. J.; Clinton, N. A.; Brown,
R. S. J. Am. Chem. Soc. 1971, 93, 5715. (b) Brown, R. S.; Eaton, D. F.;
Hosomi, A.; Traylor, T. G.; Wright, J. M. J. Organomet. Chem. 1974,
66, 249. (c) Giordan, J. C. J. Am. Chem. Soc. 1983, 105, 6544. (d) Bock,
H. Angew. Chem., Int. Ed. Engl. 1989, 28, 1627. (e) Yoshida, J.; Nishiwaki,
K. J. Chem. Soc., Dalton Trans. 1998, 2589.
(6) Photoinduced, electron transfer between iminium ion and benzylsilane:
Lan, A. J. Y.; Heuckeroth, R. O.; Mariano, P. S. J. Am. Chem. Soc. 1987,
109, 2738.
(7) Initiation of a chain process by concerted electron transfer-bond breaking
has been postulated in thermal SRN1 reactions. Costentin, C.; Hapiot, P.;
Me´debielle, M.; Save´ant, J.-M. J. Am. Chem. Soc. 1999, 121, 4451.
(8) (a) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Ioele, M. J. Org. Chem. 1995,
60, 7974. (b) Cermenati, L.; Freccero, M.; Venturello, P.; Albini, A. J.
Am. Chem. Soc. 1995, 117, 7869. (c) Fukuzumi, S.; Fujita, M.; Noura,
S.; Ohkubo, K.; Suenobu, T.; Araki, Y.; Ito, O. J. Phys. Chem. A 2001,
105, 1857. (d) Fukuzumi, S.; Satoh, N.; Okamoto, T.; Yasui, K.; Suenobu,
T.; Seko, Y.; Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 7756.
(e) Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould,
I. R.; Todd, W. P. J. Am. Chem. Soc. 1997, 119, 1876.
silicon,11 the attack of BF4-, which is the counteranion of 1, on
silicon seems to play a significant role. Benzyl radical 6 thus
generated adds to N-acyliminium ion 1 to give radical cation 7.12,13
We have already reported that an alkyl radical adds N-acyliminium
ion pools very rapidly.14 Radical cation 7 undergoes a SET reaction
with benzylsilane 2 to give the coupling product 3 and radical cation
4, which collapses to benzyl radical 6. It is interesting that the
present mechanism can be seen as the “umpolung” of the SRN1
mechanism.15
(9) (a) Yoshida, J.; Murata, T.; Isoe, S. Tetrahedron Lett. 1986, 27, 3373. (b)
Koizumi, T.; Fuchigami, T.; Nonaka, T. Bull. Chem. Soc. Jpn. 1989, 62,
219.
As we have reported previously, DFT calculations indicated that
the reduction of 7 to give 3 is thermodynamically more favorable
than the reduction of 1 to radical 5.14 In other words, the radical
cation 7 is a stronger oxidant than the cation 1. This idea prompted
us to examine the following hypothesis: The use of a catalytic
amount of benzylstannane 8, which is more easily oxidized than
benzylsilane 2, as an initiator might lead to an effective chain
reaction of benzylsilanes of high oxidation potentials. Radical cation
7, which is generated by the reaction of 8 and 1, might initiate the
propagation cycle shown in Scheme 3. The hypothesis works. For
example, the reaction of 2e with 1 in the presence of 0.1 equiv of
8e gave rise to the formation of coupling product 3e in high yield
(Table 1, run 8). Even p-fluorobenzylsilane 2a, which has the
highest oxidation potential among those examined, reacted with 1
in the presence of 8e under the slow addition condition to give the
desired coupling product 3a together with 3e (runs 2 and 3). It is
important to note that the success of the present binary system also
supports the proposed chain mechanism.
(10) (a) Baciocchi, E.; Crescenzi, M.; Fasella, E.; Mattioli, M. J. Org. Chem.
1992, 57, 4684. (b) Hirao, T.; Morimoto, C.; Takada, T.; Sakurai, H.
Tetrahedron 2001, 57, 5073.
(11) (a) Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R.; Todd, W.
P.; Mattes, S. L. J. Am. Chem. Soc. 1989, 111, 8973. (b) de Lijser, H. J.
P.; Snelgrove, D. W.; Dinnocenzo, J. P. J. Am. Chem. Soc. 2001, 123,
9698.
(12) (a) Minisci, F.; Fontana, F.; Pianese, G.; Yan, Y. M. J. Org. Chem. 1993,
58, 4207. (b) Clerici, A.; Porta, O. Tetrahedron Lett. 1990, 31, 2069. (c)
Cannella, R.; Clerici, A.; Pastori, N.; Regolini, E.; Porta, O. Org. Lett.
2005, 7, 645. (d) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 1140.
(13) Non-oxidative radical cation generation: Taxil, E.; Bagnol, L.; Horner,
J. H.; Newcomb, M. Org. Lett. 2003, 5, 827.
(14) (a) Maruyama, T.; Suga, S.; Yoshida, J. J. Am. Chem. Soc. 2005, 127,
7324. (b) Maruyama, T.; Suga, S.; Yoshida, J. Tetrahedron 2006, 62,
6519.
(15) (a) Kornblum, N. Angew. Chem., Int. Ed. Engl. 1975, 14, 734. (b) Bunnett,
J. F. Acc. Chem. Res. 1978, 11, 413. (c) Save´ant, J.-M. Acc. Chem. Res.
1980, 13, 323. (d) Bowman, W. R. Chem. Soc. ReV. 1988, 17, 283. (e)
Save´ant, J.-M. Acc. Chem. Res. 1993, 26, 455. (f) Austin, E.; Ferrayoli,
C. G.; Alonso, R. A.; Rossi, R. A. Tetrahedron 1993, 49, 4495. (g)
Save´ant, J.-M. J. Phys. Chem. 1994, 98, 3716. (h) Save´ant, J.-M.
Tetrahedron 1994, 50, 10117. (i) Galli, C.; Gentili, P.; Rappoport, Z. J.
Org. Chem. 1994, 59, 6786.
JA068589A
9
J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 1903