Comparison between electron transfer and nucleophilic reactivities of ketene silyl acetals with cationic electrophiles

Article abstract of DOI:10.1021/jo005733g

The products and kinetics for the reactions of ketone silyl acetals with a series of p-methoxy-substituted trityl cations have been examined, and they are compared with those of outer-sphere electron transfer reactions from 10,10′-dimethyl-9,9′, 10, 10′- tetrahydro-9,9′-biacridine [(AcrH)₂] to the same series of trityl cations as well as other electron acceptors. The C-C bond formation in the reaction of β,β-dimethyl-substituted ketene silyl acetal (1: (Me₂C=C(OMe)OSiMe₃) with trityl cation salt (Ph₃C+ClO₄-) takes place between 1 and the carbon of para-positon of phenyl group of Ph₃C⁺, whereas a much less sterically hindered ketene silyl acetal (3: H₂C=C(OEt)OSiEt₃) reacts with Ph₃C⁺ at the central carbon of Ph₃C⁺. The kinetic comparison indicates that the nucleophilic reactivities of ketene silyl acetals are well correlated with the electron transfer reactivities provided that the steric demand at the reaction center for the C-C bond formation remains constant.

Full text of DOI:10.1021/jo005733g

1450
J . Org. Chem. 2001, 66, 1450-1454
Com p a r ison betw een Electr on Tr a n sfer a n d Nu cleop h ilic
Rea ctivities of Keten e Silyl Aceta ls w ith Ca tion ic Electr op h iles
Shunichi Fukuzumi,*,† Kei Ohkubo,† and J unzo Otera*,‡
Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST,
J apan Science and Technology Corporation, Suita, Osaka 565-0871, J apan, and Department of Applied
Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, J apan
fukuzumi@ap.chem.eng.osaka-u.ac.jp
Received November 14, 2000
The products and kinetics for the reactions of ketone silyl acetals with a series of p-methoxy-
substituted trityl cations have been examined, and they are compared with those of outer-sphere
electron transfer reactions from 10,10′-dimethyl-9,9′, 10, 10′- tetrahydro-9,9′-biacridine [(AcrH)2]
to the same series of trityl cations as well as other electron acceptors. The C-C bond formation in
the reaction of â,â-dimethyl-substituted ketene silyl acetal (1: (Me2CdC(OMe)OSiMe3) with trityl
cation salt (Ph3C+ClO4-) takes place between 1 and the carbon of para-positon of phenyl group of
Ph3C+, whereas a much less sterically hindered ketene silyl acetal (3: H2CdC(OEt)OSiEt3) reacts
with Ph3C+ at the central carbon of Ph3C+. The kinetic comparison indicates that the nucleophilic
reactivities of ketene silyl acetals are well correlated with the electron transfer reactivities provided
that the steric demand at the reaction center for the C-C bond formation remains constant.
Carbon-carbon bond formation reactions of organo-
are one of the most important factors to determine the
electron-transfer reactivities.14 However, the nucleophilic
reactivity is often affected by the steric demand at the
reaction center.8 In the case of ketene silyl acetals,
â-methyl-substitution increases the electron-transfer re-
activity whereas an increase in the steric demand would
affect the nucleophilic reactivity.8 Thus, an exact com-
parison between the nucleophilic and electron-transfer
reactivities of ketene silyl acetals should be made by
using the same series of electrophiles or electron accep-
tors in order to keep the steric demand at the reaction
center constant.
This study reports products and kinetics for the reac-
tions of â,â-dimethyl-substituted ketene silyl acetal and
a much less sterically hindered ketene silyl acetal with
a series of p-methoxy-substituted trityl cations [(Me-
OC6H4)x(C6H5)(3-x)C+] (x ) 0-3), and these data are
directly compared with those of outer-sphere electron-
transfer reactions from 10,10′-dimethyl-9,9′,10,10′-tet-
rahydro-9,9′-biacridine [(AcrH)2]15 to the same series of
trityl cations as well as other electron acceptors. The
present study provides valuable insight into the electron
transfer vs nucleophilic reactivities of ketene silyl acetals.
silanes have found considerable interest in organic
synthesis.1-4 The nucleophilic reactivities of organo-
silanes in polar reactions have been evaluated extensively
based on the rate constants for the reactions with
carbocations.5-7 On the other hand, the electron-transfer
reactivities of organosilanes have been determined based
on the rate constants for the electron-transfer reactions
with one-electron oxidants.8-10 The importance of electron-
transfer processes has now been well recognized in many
areas of chemistry.11-13 It has previously been shown that
nucleophilic reactivities are largely correlated with the
one-electron oxidation potentials of nucleophiles, which
* To whom correspondence should be addressed. S. Fukuzumi:
Phone: Int. code + (6)6879-7368, Fax: Int. code + (6)6879-7370. J .
Otera: Phone: Int. code + (86)256-9533, Fax: Int. code + (86)256-
4292. E-mail: otera@dac.ous.ac.jp.
† Osaka University.
‡ Okayama University of Science.
(1) (a) Mukaiyama, T.; Murakami, M. Synthesis 1987, 1043. (b)
Reetz, M. T. Angew. Chem., Int. Ed. Engl. 1982, 21, 96; 1984, 23, 556.
(b) Yamamoto, H. In Lewis Acid Chemistry, A Practical Approach;
Oxford University Press: Oxford, 1999.
(2) (a) Hosomi, A. Acc. Chem. Res. 1988, 21, 200-206. (b) Yamamoto,
Y. Acc. Chem. Res. 1987, 20, 243.
(3) Fleming, I. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 2, pp 563-593.
(4) Santelli, M.; Pons, J .-M. Lewis Acids and Selectivity in Organic
Synthesis; CRC Press: Boca Raton, FL, 1995.
(5) (a) Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33,
938-957. (b) Bartl, J .; Steenken, S.; Mayr, H. J . Am. Chem. Soc. 1991,
113, 7710.
Resu lts a n d Discu ssion
The reactions of â,â-dimethyl-substituted ketene silyl
acetal (1) with less sterically hindered benzhydryl cation
(6) Patz, M.; Mayr, H. Tetrahedron Lett. 1993, 34, 3393-3396.
(7) Burfeindt, J .; Patz, M.; Mu¨ller, M.; Mayr, H. J . Am. Chem. Soc.
1998, 120, 3629.
(8) (a) Sato, T.; Wakahara, Y.; Otera, J .; Nozaki, H.; Fukuzumi, S.
J . Am. Chem. Soc. 1991, 113, 4028. (b) Fukuzumi, S.; Fujita, M.; Otera,
J .; Fujita, Y. J . Am. Chem. Soc. 1992, 114, 10271. (c) Otera, J .; Fujita,
Y.; Sato, T.; Nozaki, H.; Fukuzumi, S.; Fujita, M. J . Org. Chem. 1992,
57, 5054.
(9) Fukuzumi, S.; Itoh, S. In Advances in Photochemistry; Neckers,
D. C., Volaman, D. H., von Bu¨nau, G., Eds.; Wiley: New York, 1998;
Vol. 25, pp 107-172.
(10) (a) Fukuzumi, S.; Fujita, M.; Otera, J . J . Org. Chem. 1993, 58,
5405. (b) Fukuzumi, S.; Okamoto, T.; Otera, J . J . Am. Chem. Soc. 1994,
116, 5503.
(11) (a) Photoinduced Electron Transfer; Fox, M. A., Chanon, M.,
Eds.; Elsevier: Amsterdam, 1988; Parts A-D. (b) Advances in Electron
Transfer Chemistry; Mariano, P. S., Ed.; J AI Press: Greenwich, CT,
1991, 1992, 1994-1996, 1999; Vols. 1-6.
(12) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, 2000; Vols. 1-5.
(13) (a) Kochi, J . K. Angew. Chem., Int. Ed. Engl. 1988, 27, 1227.
(b) Rathore, R.; Kochi, J . K. Adv. Phys. Org. Chem. 2000, 35, 193-
318.
(14) Patz, M.; Mayr, H.; Maruta, J .; Fukuzumi, S. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1225.
(15) (a) Fukuzumi, S.; Kitano, T.; Ishikawa, M. J . Am. Chem. Soc.
1990, 112, 5631. (b) Fukuzumi, S.; Tokuda, Y.; Fujita, M. J . Phys.
Chem. 1992, 96, 8413.
10.1021/jo005733g CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/26/2001

Reaction of Ketene Silyl Acetals with Cationic Electrophiles
J . Org. Chem., Vol. 66, No. 4, 2001 1451
Sch em e 1
salts Ar2CH+X- (X ) BF4 or OTf) as compared to trityl
cation are known to produce the corresponding esters in
which the central carbon of benzhydryl cation is con-
nected to 1 via nucleophilic addition of 1 to benzhydryl
cation followed by the facile desilylation of the intermedi-
ate siloxy-substituted carbenium ion as shown in Scheme
1.5,6 In contrast, we have found that C-C bond formation
in the reaction of 1 with trityl cation salt (Ph3C+ClO4
)
-
takes place between 1 and the carbon of para-position of
phenyl group of Ph3C+ (see Experimental Section) rather
than the central carbon to yield a different type of ester
2 (eq 1).
F igu r e 1. Cyclic voltammograms of (a) Ph3C+ClO4-, (b)
(MeOC6H4)Ph2C+ClO4-,(c)(MeOC6H4)2PhC+ClO4-,(d)(MeOC6H4)3-
C+ClO4 (2.0 × 10-2 M) in deaerated MeCN containing
-
Bu4NClO4- (0.10 M) with a gold microelectrode at 298 K; sweep
rate 100 V s-1
.
When 1 is replaced by a much less sterically hindered
ketene silyl acetal (3: H2CdC(OEt)OSiEt3), C-C bond
formation occurs at the central carbon of Ph3C+ (eq 2).8
Thus, the steric demand at the reaction center for the
C-C bond formation results in a change in the type of
products. To examine the contribution of an electron-
transfer pathway in the reactions of ketene silyl acetals
with trityl cations, the rates were determined by using
a stopped-flow technique, and the observed rate constants
are compared with those of outer-sphere electron-transfer
reactions from (AcrH)2 to the same series of trityl cations
as well as other electron acceptors.
358 nm, ꢀmax ) 18000 M-1 cm-1) as reported previously.15
The rates obey second-order kinetics, showing first-order
dependence on each reactant concentration. The observed
second-order rate constants (kobs) of electron transfer from
(AcrH)2 to a variety of one-electron oxidants are listed
in Table 1. The free energy change of electron transfer
(∆G0 in eV) was determined from the one-electron
et
oxidation potential of (AcrH)2 (E0 ) 0.62 V vs SCE)15
ox
and the one-electron reduction potentials (E0 vs SCE)
red
of a series of trityl cations. The E0 values of a series of
trityl cations are determined by the fast scanning cyclic
voltammograms which show reversible redox waves as
shown in Figure 1.
red
The reactions of (AcrH)2 with various electron acceptors
including trityl cations are known to occur via an outer-
sphere electron transfer from (AcrH)2 to the acceptors,
followed by the facile C-C bond cleavage to produce
AcrH• which can rapidly transfer an electron to the
acceptors to yield two equivalents of AcrH+ and trityl
radical (eq 3).15
The observed rate constants (log kobs) are plotted
against ∆G0 as shown in Figure 2a, which exhibits
et
typical dependence of log kobs vs ∆G0 for an endergonic
et
outer-sphere electron transfer, i.e., log kobs increases
Trityl radical produced was detected by the ESR
spectrum. The electron-transfer rates were determined
linearly with increasing ∆G0 with a slope of -1/2.3kT
et
from the appearance of absorbance due to AcrH+ (λmax
)
() -16.9 at 298 K, k is the Boltzmann constant).

1452 J . Org. Chem., Vol. 66, No. 4, 2001
Fukuzumi et al.
Ta ble 1. On e-Electr on Red u ction P oten tia ls of Va r iou s Oxid a n ts, F r ee En er gy Ch a n ges of Electr on Tr a n sfer (∆G0et),
a n d Ra te Con sta n ts (kobs) in th e Rea ction of Tr ityl Ca tion Der iva tives w ith (Acr H)2 in Dea er a ted MeCN a t 298 K
no.
oxidant
E0 vs SCE, V
reductant
∆G0et, eV
kobs, M-1 s-1
red
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
11a
12a
13a
14a
15a
16a
10b
12b
15b
16b
15c
16c
10c
16c
[Fe(C5H5)2]+
0.37a
0.36a
0.35a
0.32a
0.31a
0.28a
0.26a
0.25a
0.22a
0.21a
0.19a
0.05
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
(AcrH)2
1
1
1
1
3
3
4
4
0.25
0.26
0.27
0.30
0.31
0.34
0.36
0.37
0.40
0.41
0.43
0.57
0.61
0.62
0.69
0.83
0.69
0.85
0.97
1.11
1.37
1.51
1.07
1.49
2.3 × 105 a
2.8 × 105 a
1.8 × 105 a
1.1 × 105 a
8.1 × 104 a
1.1 × 104 a
2.1 × 104 a
2.0 × 104 a
3.8 × 103 a
4.2 × 103
3.8 × 103 a
1.9
[Fe(C5H5)(HgClC5H4)]+
[(TPP)Co]+
[Fe(C5H5)(tert-amylC5H4)]+
[Fe(C5H5)(BuC5H4)]+
2,3-dicyano-p-benzoquinone
[Fe(MeC5H4)2]+
[Fe(BuC5H4)2]+
TCNE
Ph3C+
TCNQ
(p-MeOC6H4)Ph2C+
p-chloranil
0.01a
0.00a
-0.07
-0.21
0.21
2.4a
p-bromanil
2.3a
(p-MeOC6H4)2PhC+
(p-MeOC6H4)3C+
Ph3C+
9.0 × 10-2
<10-3
4.2 × 104
3.3 × 103
7.9
(p-MeOC6H4)Ph2C+
(p-MeOC6H4)2PhC+
(p-MeOC6H4)3C+
(p-MeOC6H4)2PhC+
(p-MeOC6H4)3C+
Ph3C+
0.05
-0.07
-0.21
-0.07
-0.21
0.21
6.1 × 10-2
5.0 × 102
1.9 × 10
3.4 × 104
2.1 × 10
(p-MeOC6H4)3C+
-0.21
a
Taken from ref 15b.
F igu r e 2. Plots of log kobs vs ∆G0 for the reaction of (a)
et
(AcrH)2 (O), (b) (CH3)2CdC(OMe)OSiMe3 (b), (c) CH2dC(OEt)-
OSiEt3 (4), CH2dC(OEt)OSiMe2But (2) with the various
oxidants at 298 K. 1 Fc+, 2 (1-HgCl)Fc+, 3 [(TPP)Co]+, 4 (1-
tert-amyl)Fc+, 5 1-BuFc+, 6 2,3-dicyano-p-benzoquinone, 7 1,1′-
Me2Fc+, 8 1,1′-Bu2Fc+, 9 tetracyanoethylene, 10 Ph3C+, 11
7,7′,8,8′-tetracyano-p-quinodimethane, 12 (MeOC6H4)(C6H5)2C+,
13 p-chloranil, 14 p-bromanil, 15 (MeOC6H4)2(C6H5)C+, 16
(MeOC6H4)3C+. Fc ) ferrocene, TPP2- ) the dianion of
tetraphenylporphyrin.
F igu r e 3. Visible spectral change observed in the reaction of
(p-MeOC6H4)2PhC+ClO4- (3.0 × 10-5 M) with (CH3)2CdC(OMe)-
OSiMe3 (1: 2.4 × 10-3 M) in deaerated MeCN at 298 K (20 s
interval). Inset: Plots of time dependence of the decay in
absorbance at 496 nm due to (p-MeOC6H4)2PhC+.
values of ketene silyl acetals8b,16 and the E0 values of
red
trityl cations. A plot of log kobs vs ∆G0 for the reactions
et
of 1 with trityl cations shown in Figure 2b has a good
parallel linear correlation with the plot for outer-sphere
electron-transfer reactions from (AcrH)2 to trityl cations
(Figure 2a). The slope is almost identical between these
two different types of reactions: one is the addition of 1
to trityl cations, and the other is the outer-sphere electron
transfer. However, it is unlikely that the addition of 1 to
trityl cations proceeds via an outer-sphere electron
transfer, since the kobs values are at least by a five orders
magnitude larger than those expected from the outer-
sphere electron-transfer reactions. Thus, there should be
an orbital interaction (the difference in 105 in kobs
The rates of reactions of ketene silyl acetals with trityl
cations were determined from the disappearance of
absorbance due to trityl cations (e.g., Ph3C+ClO4-; λmax
) 400 nm, ꢀmax ) 3800 M-1 cm-1) as shown in Figure 3.
The disappearance rate obey pseudo-first-order kinetics
in the presence of large excess ketene silyl acetal (see
inset in Figure 3) and the pseudo-first-order rate con-
stants increases linearly with increasing ketene silyl
acetal concentration. The observed second-order rate
constants determined from the slope of plots of the
pseudo-first-order rate constant vs ketene silyl acetal
concentration are also listed in Table 1.
(16) The E0 values have been estimated based on free energy
ox
The ∆G0 values for electron transfer from ketene silyl
et
relationsships for outer-sphere electron-transfer reactions of ketene
acetals to trityl cations were determined from the E0
silyl acetals.8b
ox

Reaction of Ketene Silyl Acetals with Cationic Electrophiles
J . Org. Chem., Vol. 66, No. 4, 2001 1453
(m, 12H). Methyl trimethylsilyl dimethylketene acetal (Me2-
CdC(OMe)OSiMe3, 1) was were obtained commercially from
Aldrich. 2-Ethoxy-2-butenyldimethyl-tert-butylsilane (CH2dC-
(OEt)OSiMe2But and 2-ethoxy-2-butenyltriethylsilane (CH2d
C(OEt)OSiEt3, 3) were prepared as described in the litera-
ture.18 The silicon reagents were purified by vacuum distilla-
tion. 10,10′-Dimethyl-9,9′,10,10′-tetrahydro-9,9′-biacridine
[(AcrH)2] was prepared from the reduction of 10-methylacri-
dinium perchlorate (AcrH+ClO4-) with Me3SnSnMe3 in MeCN
at 333 K and purified by recrystallization from the mixture of
acetonitrile and chloroform. Anal. Calcd for C28H24N2; C, 86.56;
Sch em e 2
1
H, 6.23; N, 7.21. Found: C, 86.56; H, 6.29; N, 7.15. H NMR
(CDCl3, 400 MHz) δ (Me4Si, ppm) 3.08 (s, 6H), 3.95 (s, 2H),
6.5-7.3 (m, 16H). 10-methylacridinium iodide (AcrH+I-) was
prepared by the reaction of acridine with methyl iodide in
acetone,19 and it was converted to the perchlorate salt
(AcrH+ClO4-) by the addition of magnesium perchlorate to the
iodide salt and purified by recrystallization from methanol.20
Acetonitrile and dichloromethane used as solvent were pur-
chased from Wako Pure Chemical Ind. Ltd., J apan, and
distilled over P2O5 prior to use.21 p-Benzoquinones were
purchased from Tokyo Kasei Organic Chemicals and purified
by the standard methods.21 Acetonitrile-d3 and chloroform-d
were obtained from EURI SO-TOP, CEA, France. Trifluoro-
acetic acid was also obtained commercially. Tetrabutylammo-
nium perchlorate (TBAP), obtained from Fluka Fine Chemical,
was recrystallized from ethanol and dried in vacuo prior to
use.
corresponds to the energy of ca. 7 kcal mol-1) in the
radical pair which would be produced by the electron-
transfer reaction. Such a reaction may be best described
as an inner-sphere electron transfer from 1 to trityl
cation, followed by the C-C bond formation leading to
the formal nucleophilic addition of 1 to trityl cation as
shown in Scheme 2. It has not been strictly ruled out that
the reaction of 1 with trityl cation simply is an ionic
reaction at the least hindered position of the trityl cation,
since the charge may be delocalized into the aromatic
ring, In such a case, however, it would be difficult to
account for a good parallel linear correlation between the
reactions of 1 with trityl cations and the outer-sphere
electron-transfer reactions in Figure 2.
In the case of a less sterically hindered ketene silyl
acetal 3, however, the type of product in the reaction of
trityl cation is different from that obtained in the reaction
with a sterically hindered ketene silyl acetal (eq 1). The
kobs values of the reactions of 3 with trityl cations (Figure
2c) are by more than 1012 larger than those expected from
the outer-sphere electron-transfer correlation. The much
larger reactivity of 3 as compared to 1 in the nucleophilic
addition to trityl cations shows sharp contrast with the
reversed reactivity in the electron-transfer reactions in
which the reactivity of 3, which has the more positive
E0ox value (1.30 V),8b is expected to be much smaller than
1 (0.83 V).8b
Sp ectr a l a n d Kin etic Mea su r em en ts. The reactions of
the trityl cation derivatives with the various nucleophiles in
deaerated MeCN were monitored with a Hewlett-Packard 8452
diode array spectrophotometer when the rates were slow
enough to be determined accurately. The rates were deter-
mined from appearance of the absorbance due to AcrH+ (λmax
) 358 nm, ꢀmax ) 1.8 × 104 M-1 cm-1) or the trityl cation
derivatives (e.g., Ph3C+ClO4-, λmax ) 400 nm, ꢀmax ) 3.8 × 103
M-1 cm-1). The kinetic measurements for faster reactions such
-
as the reaction of Ph3C+ClO4 with (AcrH)2 were carried out
with a Union RA-103 stopped-flow spectrophotometer which
was thermostated at 298 K under deaerated conditions. The
concentration of the trityl cation derivatives or the various
nucleophiles was maintained at more than 15-fold excess of
the other reactant to attain pseudo-first-order conditions.
Pseudo-first-order rate constants were determined by a least-
squares curve fit using an NEC microcomputer. The first-order
plots of ln(A∞ - A) vs time (A∞ and A are the final absorbance
and the absorbance at the reaction time, respectively) were
linear for three or more half-lives with the correlation co-
efficient, F > 0.999. In each case, it was confirmed that the
rate constants derived from at least five independent measure-
ments agreed within an experimental error of (5%.
In conclusion, the nucleophilic reactivities of ketene
silyl acetals vary significantly depending on the steric
demand at the reaction center, but they are well cor-
related with the electron-transfer reactivities when the
steric demand at the reaction center for the C-C bond
formation remains constant (Figure 2).
-
Rea ction P r oced u r e. Typically, Ph3C+ClO4 (1.0 × 10-2
M) and Me2CdC(OMe)OSiMe3 (1.0 × 10-2 M) were added to
an NMR tube which contained deaerated CD3CN solution (0.60
cm3) under an atmospheric pressure of argon. The products of
was identified by the 1H NMR spectra by comparing with those
of authentic samples. The 1H NMR measurements were
performed using a J NM-GSX-400 (400 MHz) NMR spectrom-
eter. 2: 1H NMR (CD3CN, 298 K, 400 MHz); δ(Me4Si, ppm):
1.17 (s, 6H), 3.50 (tt, 1H, J ) 4.4 and 2.2 Hz), 3.67 (s, 3H),
5.72 (td, 2H, J ) 10.6 and 1.8 Hz), 6.54 (qd, 2H, J ) 10.6 and
1.8 Hz), 7.11-7.36 (m, 10H). The assignment was confirmed
by an NOE experiment in which the signal at 3.50 ppm is
coupled with that at 1.17 ppm. p-Methoxy-substituted 2: 1.16
Exp er im en ta l Section
Ma ter ia ls. Trityl cation derivatives were prepared by the
corresponding triphenylmethyl chloride or triphenylmethanol
with perchloric acid in acetic anhydride.17 The purity of trityl
cation derivatives thus obtained was checked by elemental
1
analysis and 1H NMR spectra. H NMR measurements were
performed with a J NM-GSX-400 (400 MHz) NMR spectrom-
eter. Anal. Calcd for C19H15O4Cl, Ph3C+ClO4-; C, 66.58; H, 4.41.
Found: C, 65.78; H, 7.15. Anal. Calcd for
C20H17O5Cl,
(MeOC6H4)Ph2C+ClO4-; C, 64.44; H, 4.60. Found: C, 64.15;
H, 4.54. Anal. Calcd for C21H19O6Cl, (MeOC6H4)2PhC+ClO4
;
-
C, 62.62; H, 4.75. Found: C, 61.20; H, 4.68. Anal. Calcd for
(18) (a) Ireland, R. E.; Wipf, P.; Armstrong, J . D. J . Org. Chem. 1991,
56, 680. (b) Gennari, C.; Beretta, M. G.; Bernardi, A.; Moro, G.;
Scolastico, C.; Todeschini, R. Tetrahedron 1986, 42, 893. (c) Hosomi,
A.; Shirahata, A.; Sakurai, H. Chem. Lett. 1978, 901.
(19) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. Tetrahedron 1986, 42,
1021.
C
22H21O7Cl, (MeOC6H4)3C+ClO4-; C, 61.05; H, 4.89. Found: C,
60.52; H, 4.81. 1H NMR (CD3CN, 400 MHz) δ (Me4Si, ppm)
Ph3C+ClO4-, 7.2-8.3 (m, 15H); (MeOC6H4)Ph2C+ClO4-, 4.22
(s, 3H), 7.4-8.1 (m, 14H); (MeOC6H4)2PhC+ClO4-, 4.12 (s, 6H),
7.3-8.1 (m, 13H); (MeOC6H4)3C+ClO4-, 4.07 (s, 9H), 7.2-8.1
(20) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J . Am.
Chem. Soc. 1987, 109, 305.
(17) Dauben, H. J .; Honnen, L. R.; Harmon, K. M. J . Org. Chem.
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Chemicals; Butterworth-Heinemann: Oxford, 1988.

1454 J . Org. Chem., Vol. 66, No. 4, 2001
Fukuzumi et al.
(s, 6H), 3.46 (m, 1H), 3.76 (s, 3H), 4.21 (s, 3H), 6.37-6.56 (m,
4H), 6.80-7.38 (m, 9H).
The E0 values (vs Ag/Ag+) are converted to those vs SCE by
red
adding 0.29 V.22
Electr och em ica l Mea su r em en ts. Electrochemical mea-
surements of trityl cation derivatives were performed on a BAS
100B electrochemical analyzer in deaerated acetonitrile con-
taining 0.10 M Bun4NClO4 as a supporting electrolyte at 298
K. The gold working microelectrode (i.d. ) 10 µm, BAS) was
polished with BAS polishing alumina suspension and rinsed
with acetone before use. The counter electrode was a platinum
wire (BAS). The measured potentials were recorded with
respect to the Ag/AgNO3 (1.0 × 10-2 M) reference electrode.
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for Scientific Research Priority
Area (No. 11228205) from the Ministry of Education,
Science, Culture and Sports, J apan.
J O005733G
(22) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in
Nonaqueous Systems; Marcel Dekker: New York, 1990.