A R T I C L E S
Uchiyama et al.
Figure 1. Generalized reaction pathways for ate complexes.
Table 1. Oxidation Potential Data in THF at 0 °Ca
Entry
Metal reagent
E° (V)
Entry
Metal reagent
E° (V)
1
2
3
4
FeCl2
MeFeCl
Me2Fe
+0.15
-1.38
-1.93
-2.50
5
6
7
8
Me3MnLi
Me3CoLi
SmI2
-2.56
-2.60
-2.33
-3.05
Me3FeLi
Mg
a Measured in 0.1 M n-Bu4N+ClO4--THF solution vs Ag/AgCl.
Figure 2. Working model for the catalytic ET system.
Results and Discussion
complexes also exists in the reaction system, a catalytic ET ate
complex cycle should theoretically proceed (Figure 2). The high
oxidation potential of Mg raises the possibility that Mg would
serve as a reducing agent of the trivalent ET complexes.
Magnesium has various advantages as an electron store, such
as low cost, easy operation, and latent high oxidation potential.
Therefore, the development of a catalytic ET system using
magnesium as the rereductor would provide a safe and simple
catalytic electron transfer system. Moreover, in such a system,
electron transfer ability (oxidation potential) and reaction
selectivity could in principle be controlled by the choice of
mediator. To confirm that the chemical reactivities were
consistent with the estimated ET abilities, we applied the present
ET system to several chemical reactions.
Although sulfonamides are one of the most important and
stable nitrogen protective groups, a general procedure for depro-
tection that consistently gives high yields is not available.8 The
sulfonamides can be divided into three types based on the
strength of the N-S bond, i.e., less basic arylamine, more basic
arylamine, and more basic alkylamine. Sulfonamides of the more
basic aryl- and alkylamines are much harder to cleave than those
of the less basic amines such as indoles, pyrroles, and imida-
zoles.8 Most sulfonamides are stable to alkaline hydrolysis and
to catalytic reduction. They are cleaved under more severe con-
ditions such as Na/NH3, Na/butanol, sodium naphthalenide, or
sodium anthracenide, and by refluxing in acid (48% HBr). How-
ever, their usefulness has been limited because various functional
groups do not remain intact under such severe deprotection con-
ditions.9 Thus, the development of a simple and practical method
for the deprotection of the N-phenylsulfonyl moiety would be
very useful. Initial attempts focused on the development of
catalytic reductive desulfonylation of various sulfonamides.10
As model substrates, we chose N-phenylsulfonylindole (less
Catalytic ET Systems Based on Transition Metal Ate
Complexes and Magnesium. Recently, C-C bond-forming
reactions using some transition metal ate complexes such as
Mn-ates, Fe-ates, and Co-ates have been reported, and ET
processes are proposed to be involved in some cases (Figure
1).6 We first compared the ET abilities of Mn-ates, Fe-ates,
and Co-ates by using in situ attenuated total reflectance (ATR)
IR spectroscopy. The transformation of benzophenone to the
corresponding ketyl radical by the transition metal ate complexes
(Me3Mn(II)Li, Me3Fe(II)Li, and Me3Co(II)Li) or Na was
monitored with in situ IR. As the signal at 1663 cm-1, which
can be assigned to the C-O stretching vibration of the carbonyl
moiety of benzophenone, diminished in intensity, a new band
(at 1561 cm-1 (Me3Co(II)Li), 1555 cm-1 (Me3Mn(II)Li), 1559
cm-1 (Me3Fe(II)Li), or 1561 cm-1 (Na)) increased in intensity
in all cases (Chart 1). This new absorption can be assigned to
the C-O stretching vibration of the newly generated ketyl
species.7 Therefore, the present in situ IR data strongly support
the occurrence of electron-release reactions from Me3Mn(II)-
Li, Me3Fe(II)Li, and Me3Co(II)Li.
Next, for quantitative estimation of the ET abilities of these
ET ate complexes, we applied electrochemical measurements
of various kinds of transition metal reagents based on differential
pulse voltammograms (DPV). Electrochemical data in THF at
0 °C for transition metal complexes are summarized in Table
1. The FeII/III oxidation potentials of Fe(II)Cl2, MeFe(II)Cl,
Me2Fe(II), and Me3Fe(II)Li in THF were +0.15, -1.38, -1.93,
and -2.50 V, respectively. Similarly, the oxidation potentials
of other ET ate complexes, such as Me3Mn(II)Li (-2.56 V)
and Me3Co(II)Li (-2.60 V), were much more negative than
those of the corresponding dichloride derivatives (MnCl2, +1.10
V; CoCl2, +1.37 V). The oxidative potentials of SmI2 and Mg
were evaluated as -2.33 and -3.05 V, respectively.4 Therefore,
the present ET ate complexes have adequately high oxidation
potentials intermediate between those of SmI2 and Mg. Fur-
thermore, the MII/III oxidation and reduction couples of the ET
ate complexes were electrochemically reversible in all cases.
Therefore, if a reduction pathway of trivalent to divalent ate
(8) Greene, T. W.; Wuts, P. G. M. In ProtectiVe groups in organic synthesis,
2nd ed.; John Wiley & Sons: New York, 1991; Part 7.
(9) Recently, the potential of the sulfonamide functionality as a protective group
has been suggested, as exemplified by Fukuyama’s (di)nitrophenylsulfona-
mide protecting group, which undergoes facile cleavage under basic
conditions. (a) Fukuyama, T.; Jow, C.; Cheung, M. Tetrahedron Lett. 1995,
36, 6373-6374. (b) Fukuyama, T.; Cheung, M.; Jow, C.; Hidai, Y.; Kan,
T. Tetrahedron Lett. 1997, 38, 5831-5834. (c) Fukuyama, T.; Cheung,
M.; Kan, T. Synlett 1999, 8, 1301-1303.
(10) The reductive deprotection of the N-sulfonyl moiety is also known to
proceed via a ET process from alkali metals: (a) Heathcock, C. H.; Smith,
K. M.; Blumenkopf, A. B. J. Am. Chem. Soc. 1986, 108, 5022-5024. (b)
Schultz, A. G.; McCloskey, P. J.; Court, J. J. J. Am. Chem. Soc. 1987,
109, 6493-6502. (c) Yamazaki, N.; Kibayashi, C. J. Am. Chem. Soc. 1989,
111, 1396-1408. (d) Ji, S.; Gortler, L. B.; Waring, A.; Battisti, A.; Bank,
S.; Closson, W. D. J. Am. Chem. Soc. 1967, 89, 5311-5312. Low-valent
lanthanides: (e) Vedejs, E. J. Org. Chem. 1994, 59, 1602-1603.
(6) For reviews, see: (a) Kauffmann, T. Angew. Chem., Int. Ed. Engl. 1996,
35, 386-403 and references therein. (b) Shinokubo, H.; Oshima, K. J. Synth.
Org. Chem. Jpn. 1999, 57, 27-37.
(7) The absorptions of these ketyl species are consistent with the values
previously reported; see: (a) Pons, S.; Davidson, T.; Bewick, A. J. Am.
Chem. Soc. 1983, 105, 1802-1805. (b) Bewick, A.; Jones, V. W.; Kalaji,
M. Electrochim. Acta 1996, 41, 1961-1970 and references therein.
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8756 J. AM. CHEM. SOC. VOL. 126, NO. 28, 2004