Tuning the redox chemistry of 4-benzoyl-N-methylpyridinium cations through para substitution. Hammett linear free energy relationships and the relative aptitude of the two-electron reduced forms for H-bonding

Article abstract of DOI:10.1021/jo020489+

In anhydrous CH₃CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: -OCH₃, -CH₃, -H, -SCH₃, -Br, -C≡CH, -CHO, -NO₂, and -⁺S(CH₃)₂) demonstrate two chemically reversible, well-separated one-electron (1-e) reductions in the same potential range as other main stream redox catalysts such as quinones and viologens. Hammett linear free energy plots yield excellent correlation between the E_1/2 values of both waves and the substituent constants σ_p-X. The reaction constants for the two 1-e reductions are ρ₁ = 2.60 and ρ₂ = 3.31. The lower ρ₁ value is associated with neutralization of the pyridinium ring, and the higher ρ₂ value with the negative charge developing during the 2nd-e reduction. Structure-function correlations point to a purely inductive role for substitution in both 1-e reductions. The case of the 4-(4-nitrobenzoyl)-N-methylpyridinium cation is particularly noteworthy, because the 4-nitrobenzoyl moiety undergoes reduction before the 2nd reduction of the 4-benzoyl-N-methylpyridinium system. Correlation of the third wave of this compound with the 2nd-e reduction of the others yields σ_p-NO₂^- = -0.97 ± 0.02, thus placing the -NO₂^- group among the strongest electron donors. Solvent deuterium isotope effects and maps of the electrostatic potential (via PM3 calculations) as a function of substitution support that 2-e reduced forms develop H-bonding with proton donors (e.g., CH₃-OH) via the O-atom. The average number of CH₃OH molecules entering the H-bonding association increases with e-donating substituents. H-bonding shifts the 2nd reduction wave closer to the first one. This has important practical implications, because it increases the equilibrium concentration of the 2-e reduced form from disproportionation of the 1-e reduced form.

Full text of DOI:10.1021/jo020489+

Tu n in g th e Red ox Ch em istr y of 4-Ben zoyl-N-m eth ylp yr id in iu m
Ca tion s th r ou gh P a r a Su bstitu tion . Ha m m ett Lin ea r F r ee En er gy
Rela tion sh ip s a n d th e Rela tive Ap titu d e of th e Tw o-Electr on
Red u ced F or m s for H-Bon d in g
Nicholas Leventis,* Abdel-Monem M. Rawaswdeh, Guohui Zhang, Ian A. Elder, and
Chariklia Sotiriou-Leventis*
Department of Chemistry, University of Missouri-Rolla, Rolla, Missouri 65409
leventis@umr.edu; cslevent@umr.edu
Received J uly 23, 2002
In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations
(substituent: -OCH3, -CH3, -H, -SCH3, -Br, -CtCH, -CHO, -NO2, and -+S(CH3)2) demon-
strate two chemically reversible, well-separated one-electron (1-e) reductions in the same potential
range as other main stream redox catalysts such as quinones and viologens. Hammett linear free
energy plots yield excellent correlation between the E1/2 values of both waves and the substituent
constants σp-X. The reaction constants for the two 1-e reductions are F1 ) 2.60 and F2 ) 3.31. The
lower F1 value is associated with neutralization of the pyridinium ring, and the higher F2 value
with the negative charge developing during the 2nd-e reduction. Structure-function correlations
point to a purely inductive role for substitution in both 1-e reductions. The case of the
4-(4-nitrobenzoyl)-N-methylpyridinium cation is particularly noteworthy, because the 4-nitrobenzoyl
moiety undergoes reduction before the 2nd reduction of the 4-benzoyl-N-methylpyridinium system.
•-
Correlation of the third wave of this compound with the 2nd-e reduction of the others yields σp-NO
2
•-
) -0.97 ( 0.02, thus placing the -NO2 group among the strongest electron donors. Solvent
deuterium isotope effects and maps of the electrostatic potential (via PM3 calculations) as a function
of substitution support that 2-e reduced forms develop H-bonding with proton donors (e.g., CH3-
OH) via the O-atom. The average number of CH3OH molecules entering the H-bonding association
increases with e-donating substituents. H-bonding shifts the 2nd reduction wave closer to the first
one. This has important practical implications, because it increases the equilibrium concentration
of the 2-e reduced form from disproportionation of the 1-e reduced form.
In tr od u ction
probes in self-assembled monolayers,7 dendrimers,8 sili-
cates,9 zeolites,10 and semiconductors.11
Several of the best-known chemically reversible redox-
active species contain the quaternized pyridinium sys-
tem. Notable examples include the NAD+/NADH couple
that functions as an electron-transfer catalyst in the
respiratory chain,1 and the various viologens (N,N′-
diquaternized 4,4′-bipyridinium salts),2 which have been
employed widely as redox mediators,3 electrochromic
materials,4,5 electron-transfer quenchers,6 and as redox
Similarly, it has been found that 4-acyl-, 4-carboxy-,
4-carbomethoxy-, 4-cyano-, and 4-benzoyl-N-alkylpyri-
dinium cations may also undergo reversible one electron
(6) (a) Konishi, T.; Fujitsuka, M.; Ito, O.; Toba, Y.; Usui, Y. J . Phys.
Chem. A 1999, 103, 9938. (b) Zahavy, E.; Seiler, M.; Marx-Tibbon, S.;
J oselevich, E.; Willner, I.; Du¨rr, H.; O’Connor, D.; Harriman, A. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1005. (c) Inada, T. N.; Miyazawa, C.
S.; Kikuchi, K.; Yamauchi, M.; Nagata, T.; Takahashi, Y.; Ikeda, H.;
Miyahi, T. J . Am. Chem. Soc. 1999, 121, 7211. (d) Warren, J . T.; Chen,
W.; J ohnston, D. H.; Turro, C. Inorg. Chem. 1999, 38, 6187. (e)
Borsarelli, C. D.; Braslavsky, S. E. J . Phys. Chem. A 1999, 103, 1719.
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Electrochem. Soc. 1997, 144, L305. (b) Li, J .; Chen, G.; Dong, S.
Electroanalysis 1997, 9, 834. (c) Tang, X.; Schneider, T. W.; Walker,
J . W.; Buttry, D. A. Langmuir 1996, 12, 5921. (d) Tang, X.; Schneider,
T.; Buttry, D. A. Langmuir 1994, 10, 2235. (e) Katz, E.; Itzhak, N.;
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Langmuir 1990, 6, 1617. (h) Lee, K. A. B. Langmuir 1990, 6, 709. (i)
De Long, H. C.; Buttry, D. A. Langmuir 1990, 6, 1319.
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Chem. B 1999, 103, 5832.
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10.1021/jo020489+ CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/25/2002
J . Org. Chem. 2002, 67, 7501-7510
7501

Leventis et al.
(1-e) reduction.12 In that context, the reduced forms of
4-acyl- and 4-benzoylpyridinium cations are stable spe-
cies, for which ESR spectroscopy, dipole moment studies,
and chemical trapping point toward a pyridinium-based
radical.13,14 On the contrary, it is noteworthy that the 1-e
reduction of nonquaternized 4-acetylpyridine is followed
by dimerization to the corresponding pinacol.15
those of their free bases (BP-X) both in anhydrous
CH3CN and in the presence of a typical proton donor,
such as CH3OH. The results confirm that in general, the
4-benzoyl-N-methylpyridinium system demonstrates
two reversible 1-e reductions. Structure-function cor-
relations via semiempirical (PM3) calculations and
Hammett Linear Free Energy relationships point toward
a purely inductive interaction between the substit-
uents and the reduction sites. e-Donating substituents
increase the e-density at the oxygen of the 2-e reduced
form, hence its ability to associate with CH3OH via
H-bonding. In the course of these studies, it was noticed
that the p-nitrobenzoyl moiety of 1-NO2 is reduced before
the 4-benzoyl-N-methylpyridinium system uptakes its
2nd-e. Thus, by correlating the E1/2(3) value of 1-NO2 with
the E1/2(2) values of the other 1-X’s, we have been able
Recently, we reported that in anhydrous CH3CN, the
4-benzoyl-N-methylpyridinium cation undergoes two suc-
cessive, well-separated (∆E1/2 > 0.6 V), chemically revers-
ible 1-e reductions, generally in the same potential range
as quinones and main stream viologens.16 It was discov-
ered further that the 2-e reduced form develops hydrogen
(H-) bonding with weak protic acids. At time scales much
longer (e.g., 400 s) than the time scale of a typical cyclic
voltammetric scan at 0.1 V s-1 (20-30 s), H-bonded
adducts with alcohols or water undergo proton transfer
and yield carbinols. Meanwhile, owing to disproportion-
ation of the red 1-e reduced form, very small concentra-
tions (<10-10 M) of the yellow 2-e reduced form are
always present in equilibrium with the former.14,16 There-
fore, under rigorous exclusion of potential proton donors
the 4-benzoyl-N-methylpyridinium cation may be suitable
as a redox catalyst,16 a photoinduced electron-transfer
acceptor,12,17 or an electrochromic material,18 in applica-
tions where the 1-e reduced form must be generated and
survive millions of cycles over a very long period of time.19
•-
•-
to report for the first time the σp-NO value of -NO2
.
2
Resu lts a n d Discu ssion
4-Benzoylpyridines have been investigated for their
potential pharmacological properties.20 So, most of our
free bases have been described before,20-24 while most
-
BF4 quaternary salts and the free bases of 1-SCH3,
1-CHO, and 1-+S(CH3)2 are new compounds. The syn-
thetic tree of all 1-X’s is summarized in Scheme S.1 of
the Supporting Information.
All redox properties (Table 1) were determined by cyclic
voltammetry at 0.1 V s-1 in anhydrous CH3CN/0.1 M
tetrabutylammonium perchlorate (TBAP), vs ferrocene
as internal standard. With the exception of 1-NO2 (see
below), all other 1-X’s show two well-separated redox
waves in analogy to 1-H and 1-CtCH.16 (A similar
observation is made with the 4-acetyl-N-methylpyri-
dinium cation, a typical nonbenzoyl 4-carbonylpyridinium
system: see Table 1.) The cathodic-to-anodic peak current
ratios, ip,c/ip,a, of both waves were determined by using
the extrapolation of the decaying current of the previous
wave as baseline. With the exception of the second wave
of 1-+S(CH3)2, both waves of all other 1-X’s (and all three
waves of 1-NO2) show chemical reversibility (ip,c/ip,a ∼ 1.0;
Table 1). E1/2 values for each wave were calculated as
the middle points between the cathodic and anodic peak
current potentials. The peak-to-peak potential separa-
tions, ∆Ep-p, are comparable to the value found for the
ferrocenium/ferrocene (Fc•+/Fc) couple (68 ( 3 mV).
Free bases BP-X (Table 1) generally show two reduc-
tion waves as well. With the exception of BP-Br and
BP-CtCH, all 1st waves behave as chemically revers-
The similar redox behavior of the 4-benzoyl-N-meth-
ylpyridinium system to those of quinones and viologens
notwithstanding, the former class of compounds offers
direct synthetic access to the strategic para position of
the benzoyl group. Thus, nine different 4-(p-substituted-
benzoyl)-N-methypyridinium cations (1-X) were prepared,
and their redox properties were studied in parallel to
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7502 J . Org. Chem., Vol. 67, No. 21, 2002

Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations
TABLE 1. Da ta for th e Red u ction of 1-X’s, Th eir F r ee Ba ses [BP -X’s], a n d Com p ou n d s Used a s Con tr ols, in CH3CN/0.1
M TBAP a t 0.1 V s-1 (Br a ck eted Qu a n tities Cor r esp on d to th e F r ee Ba ses)a
1-X
[BP-X]
E1/2(2)
[E1/2(1)]
∆Ep-p(2) ip,c/ip,a(2) )
[∆Ep-p(1)] [ip,c/ip,a(1
E1/2(3)
[E1/2(2)]
∆Ep-p(3)
ip,c/ip,a(3)
b
σp-X
E1/2(1)
∆Ep-p(1) ip,c/ip,a(1)
[∆Ep-p(2)] [ip,c/ip,a(2)]
1-OCH3
[BP-OCH3]
1-CH3
[BP-CH3]
1-H
[BP-H]
-0.27 -1.128 ( 0.003 71 ( 2
-0.14 -1.089 ( 0.001 66 ( 1
0.00 -1.068 ( 0.001 65 ( 2
0.00 -1.097 ( 0.001 69 ( 2
0.23 -1.035 ( 0.001 68 ( 2
0.26 -1.037 ( 0.001 68 ( 1
0.47 -0.994 ( 0.002 66 ( 1
0.78 -0.955 ( 0.001 66 ( 1
1.00
1.00
1.03
1.06
1.04
1.01
1.02
1.00
-1.754 ( 0.008
[-1.982 (0.002]
-1.725 ( 0.002
[-1.939 (0.002]
-1.697 ( 0.001
72 ( 2
[72 ( 2]
71 ( 1
[70 ( 2]
74 ( 3
1.02
[1.07]
1.00
[1.08]
1.03
[1.04]
1.06
[1.06]
1.05
[-2.56]c
[d]
[d]
[d]
[d]
[d]
[-2.53]c
[-2.52]c
[-2.38]c
[-2.27]c
[-1.905 ( 0.005] [70 ( 2]
-1.722 ( 0.007 75 ( 2
[-1.908 ( 0.001] [70 ( 1]
1-SCH3
]
[BP-SCH3
1-CtCH ]
[BP-CtCH
1-Br
[BP-Br]
1-CHO
-1.659 ( 0.001
76 ( 2
[d]
75 ( 3
[d]
75 ( 1
[65 ( 2]
71 ( 2
[-1.81]c
[1.95]
1.07
-1.668 ( 0.003
[-1.82]c
[-2.45]c
-2.48c
[d]
-1.590 ( 0.001
[-1.598 (0.001]
-1.367 ( 0.003
1.02
[0.98]
1.00
[1.04]
d
d
[BP-CHO]
1-NO2
[-1.992 ( 0.001] [77 ( 2]
-1.893 ( 0.001 83 ( 1
[1.00]
1.04
[1.04]
d
[BP-NO2]
1-+S(CH3)2
N-MeAP+ e
[AP]e
[-1.242 ( 0.002] [66 ( 2]
[-1.613 ( 0.001] [72 ( 3]
0.90 -0.946 ( 0.001 72 ( 1
-1.110 ( 0.001 67 ( 1
1.12
1.04
-1.53c
-2.25c
-1.830 ( 0.001
[-2.05]c
74 ( 2
[d]
70 ( 2
1.06
HV2+ e
-0.826 ( 0.002 67 ( 1
-2.22c
1.02
d
1.00
d
-1.250 ( 0.002
1.12
Ph-CHO
Ph-NO2
-1.548 ( 0.003 73 ( 1
Ph-+S(CH3)2
-2.03c
a
b
E1/2 values reported vs the ferrocenium/ferrocene couple used as internal standard. Taken from ref 25a, except σp-SCH which was
3,
taken from ref 26. c The E1/2 values of chemically irreversible waves were approximated by adding 28 mV to the corresponding peak
potentials. Irreversible. e N-MeAP+: 4-acetyl-N-methylpyridinium tetrafluoroborate. AP: 4-acetylpyridine. HV2+: N,N’-diheptyl-4,4’-
d
bipyridinium tetrafluoroborate. Ph-CHO: benzaldehyde. Ph-NO2: nitrobenzene., Ph-+S(CH3)2: dimethylphenylsulfonium tertrafuoroborate.
ible 1-e reductions, at least within the time scale of cyclic
voltammetry at 0.1 V s-1. With the exception of BP-CHO
and BP-NO2, the 2nd waves of the free bases are
generally chemically irreversible and smaller (about half)
compared with the 1st-e waves. This observation is
consistent with the fact that as the 2-e reduced form of
a free base diffuses out of the electrode, it reacts with
free base diffusing toward the electrode yielding the
pinacol dianion.27 Meanwhile, as will become apparent
by the ensuing discussion, the reversible reduction waves
of BP-CHO and BP-NO2 do not have the same origin
as the waves of the rest of the free bases, as they involve
reduction of the substituents. By comparison, 4-acetylpy-
ridine shows only one chemically irreversible reduction
wave.15 Overall, it seems that the redox behavior of 1-X’s
is less idiosyncratic than that of their free bases.
Figure 1A compares the voltammograms of 1-H, its free
base, and a typical viologen (N,N′-diheptyl-4,4′-bipyri-
dinium tetrafluoroborate; HV2+), which due to symmetry
has only one possible reduction site, the pyridinium.
Since the 1st wave of 1-H is pyridinium based,13,14 it
correlates well and is bracketed by the two waves of
HV2+. The relative position of the 1st wave of 1-H and
the two waves of HV2+ is the result of the relative group
electronegativities: pyridinium > benzoyl > reduced
pyridinium. The 2nd wave of 1-H is in the same region
F IGURE 1. (A) Cyclic voltammetry of 1-H (s; 2.877 mM),
N,N′-diheptyl viologen (- - -; 2.878 mM), and 4-benzoylpyridine
(25) Isaacs, N. Physical Organic Chemistry, 2nd ed.; Longman
(‚ ‚ ‚; 2.620 mM) in CH3CN/0.1 M TBAP with a Au-disk
electrode (0.0201 cm2) at 0.1 V s-1. (B) Cyclic voltammetry of
1-NO2 (s; 3.393 mM) and of nitrobenzene (PhNO2, - - -; 6.490
mM) under the same conditions as in part A. (For PhNO2, the
current scale should be multiplied by a factor of “2”.)
Scientific and Technical: Essex, U.K., 1995: (a) p 152; (b) p 161.
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1969, 34, 2810. (b) Rudd, E. J .; Conway, B. E. Trans. Faraday Soc.
1971, 67, 440. (c) Nadjo, L.; Saveant, J . M. J . Electroanal. Chem. 1973,
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as the carbonyl-based 1st-e reduction of 4-benzoylpyri-
dine. This is a general observation among all 1-X’s and
their free bases (Table 1).
J . Org. Chem, Vol. 67, No. 21, 2002 7503

Leventis et al.
Compound 1-NO2 comprises an interesting deviation
from the other 1-X’s in that it shows three reversible
waves (Figure 1B, solid line). The 1st wave of 1-NO2 falls
in the same potential range as the 1st waves of the other
1-X’s; its middle wave is associated with the redox wave
of nitrobenzene (Figure 1B, dashed line) and is attributed
to the independent reduction of the p-nitrobenzoyl moi-
ety. As expected then, BP-NO2 shows two reversible
waves that correlate with the 2nd and 3rd waves of 1-NO2
(Table 1). (Note that third waves are also demonstrated
by 1-CHO and 1-+S(CH3)2. By comparison to the reduc-
tion of benzaldehyde and of dimethylphenylsulfonium
tetrafluoroborate (Table 1), those waves are assigned to
the reduction of the substituents; however, since those
waves are more negative than the reduction waves of the
4-benzoyl-N-methylpyridinium system, they do not in-
terfere with it.)
In view of possible applications, Figure 1 suggests that
1-X’s may be used in lieu of viologen. With that prospect,
a better insight into the redox chemistry of 1-X’s is
obtained via Hammett Linear Free Energy (HLFE)
relationships in conjunction with structure/function con-
siderations. For that analysis, it is understood that from
a thermodynamic perspective, the electrode reactions of
Scheme 1, referenced vs the Fc‚+/Fc couple, can be written
equivalently as:
K1
\
[1-X ]+ + Fc {
} [1-X]• + Fc•+
(1)
(2)
K2
[1-X]• + Fc { } [1-X]- + Fc•+
\
where K1 and K2 are the equilibrium constants of eqs 1
and 2, respectively. HLFE relationships for these reac-
tions are expressed via eq 3, where F1 and F2 are the
corresponding reaction constants, and σp-X is the sub-
On the basis of the above, Scheme 1 summarizes the
redox processes of all 1-X’s, except 1-NO2. Use of [1-X]+,
[1-X]•, and [1-X]- emphasizes the charge/spin properties
of the various redox states of the 4-benzoyl-N-methylpy-
ridiium system.
log [K1or2/K1or2(X)H)] ) (F1or2)σp-X
(3)
stituent constant of group X in the para position. Since
RT ln(K1or2) ) -∆G° ) nFEorxn, eq 3 is rearranged into
eq 4, where n is the number of electrons involved in the
SCHEME 1. Th e Red ox P r ocesses of th e
4-Ben zoyl-N-m eth ylp yr id in iu m System
E
1/2(1 or 2) ) [2.303RT/nF]F1or2σp-X
+
E1/2(1 or 2 for X ) H) (4)
redox process, F and R are the Faraday and gas con-
stants, respectively, and T is the absolute temperature.
By definition (eqs 1 and 2), the reaction standard
potential, Eorxn, is the standard potential, E°, of the
[1-X]+/[1-X]• couple, or the [1-X]•/[1-X]- couple vs the
The pyridinium-based radical, [1-X]•, is consistent with
literature reports on the 1-e reduced forms of other
4-substituted pyridinium salts.13 The alternative zwit-
terionic species 2 for the 1-e reduced forms of 1-X’s cannot
reference couple (Fc•+/Fc), and in general E° ∼ E1/2
.
Deriving the electrochemical version of the Hammett
equation may appear redundant, however, eq 4 is not the
typical HLFE relationship used in the literature, where
customarily sensitivity of redox reactions to substitution
29,30
is expressed simply in terms of the slope, dE1/2/dσp-X
.
On the basis of eq 4, if those slopes are divided by
[2.303RT/nF], they yield the dimensionless F-values (eq
3), which allow correlation of electrode reactions with any
other reaction (redox or otherwise) susceptible to substi-
tution effects.
With the exception of the 2nd and 3rd waves of 1-NO2,
both E1/2 values of all other 1-X’s show good correlation
with the substituent constants, σp-X values (Figure 2).
Dividing the experimental slopes (Figure 2, legend) by
0.05916 V (the value of 2.303RT/nF for n ) 1 at 25 °C)
yields F1 ) 2.60 and F2 ) 3.31. Both F values are positive,
consistent with the fact that a positive charge is neutral-
ized and a negative charge is developing during the 1st-e
and the 2nd-e reduction, respectively. On the other hand,
the E1/2(1) values of the free bases give poor correlation
with the σp-X values (r2 ) 0.908; FFB ) 7). Introducing
the σ- value for -CHO (1.0326) yields a much better
be derived from [1-X]•, consistent with the rules of
resonance.28a The pyridinum ring of 2 is aromatic and
planar, but there is no such guarantee for [1-X]•, for
which PM3 calculations show that the geometry of N is
pyramidal (Scheme 2). Species 2 conforms to the typical
1-e reduced form of a ketone,27 and should lead to
pinacolization, which has not been observed.16
SCHEME 2. P M3 Str u ctu r es of [1-H]• a n d 2
(29) Zuman, P. Substituent Effects in Organic Polarography; Plenum
Press: New York, 1967; Chapters 1 and 2.
(30) For more recent examples see: (a) Sauro, V. A.; Workentin, M.
S. J . Org. Chem. 2001, 66, 831. (b) Connelly, N. G.; Davis, P. R. G.;
Harry, E. E.; Klangsinsirikul, P.; Venter, M. J . Chem. Soc., Dalton
Trans. 2000, 2272. (c) Aguilar-Martinez, M.; Cuevas, G.; J imenez-
Estrada, M.; Conzalez, I.; Lotina-Hennsen, B.; Macias-Ruvalcaba, N.
J . Org. Chem. 1999, 64, 3684.
(28) Smith, M. B.; March, J . March’s Advanced Organic Chemistry,
Reactions, Mechanisms and Structure, 5th ed.; J ohn Wiley and Sons:
New York, 2001:. (a) p 41; (b) p 99.
7504 J . Org. Chem., Vol. 67, No. 21, 2002

Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations
Interestingly, the E1/2 values of neither the 2nd nor the
3rd wave of 1-NO2 correlate with the E1/2 values of the
2nd waves of the other 1-X’s (see Figure 2). This is not
difficult to reconcile: the 1st-e reduction product of 1-NO2
is 3, while the 2nd-e reduction takes place at the
p-nitrobenzoyl moiety giving 4.34 Therefore, the 3rd-e
process reduces the carbonyl group of a species where
the substituent of the benzoyl group is -NO2•-. Since
there is no stereoelectronic reason (Table 2) for the
reduction of 4 not to conform with the reduction of the
F IGURE 2. Hammett plots for the 1st-e reduction (dark
circles; slope ) 0.154 V; intercept ) -1.077 V; correlation (r2)
) 0.971) and the 2nd-e reduction (open circles; slope ) 0.196
V; intercept ) -1.704; correlation (r2) ) 0.971) of the p-sub-
stituted-4-benzoyl-N-methylpyridinium cations (1-X’s) of Table
1. The dark triangle corresponds to the 1-e reduction of the
p-nitrobenzoyl moiety of 1-NO2 (refer to the middle wave in
Figure 1B).
other [1-X]•’s, Figure 2 leads directly into σp-NO ) -0.97
•-
2
( 0.02 (for the 95% confidence limit the standard
deviation should be multiplied by a factor of 2). The fact
that the σp-NO value is negative is not surprising:
•-
2
•-
-NO2 is an electron-rich group and therefore electron
•-
donating. Actually, σp-NO is the most negative substitu-
2
ent constant we are aware of after σp-S () -1.21).26 It
-
should be noted further that interference by reduction
of the -NO2 group in the same potential range as the
main redox-active moiety is not something unique to
these experiments. For example, it was reported recently
that reduction of the -NO2 group was also observed in
the case of 2-(p-nitrophenylamine)-1,4-naphthalenedione.30c
However, the redox potentials were not well-separated,
and therefore an analysis similar to ours was not pos-
sible. By the same token, it should be noted also that
the same analysis for BP-NO2 is not possible either;
although the -NO2 group of that compound is reduced
first (Table 1), PM3 calculations show that the p-(NO2•-)-
phenyl ring and the reduced carbonyl, C•-O-, are almost
coplanar (dehedral ) 4.48°). That supports a direct
correlation (r2 ) 0.993; FFB ) 5.04), although the 2nd
wave of BP-NO2 never correlates well with the other free
bases (see Figure S.1 in the Supporting Information). The
F values reflect the ability of the core to transmit
electronic effects to the reaction site, and may have both
inductive and resonance contributions.25b Overall, (a) the
good correlation of 1-X’s with the “primitive” σp-X values,
(b) the improved Hammett correlation of BP-X’s by
introducing σ- values, and (c) the fact that FFB > F1, F2
all corroborate for enhanced resonance effects in the free
bases and their lack thereof from 1-X’s. A satisfactory
justification for this conclusion relies on structural
considerations.
According to semiempirical (PM3) calculations (Table
2), the dihedral angle between the carbonyl and the
p-substituted-phenyl group of the free bases suggests a
variable degree of possible resonance interaction between
the substituent and the carbonyl group (i.e., the site of
the 1st-e reduction). On the contrary, the carbonyl group
of all 1-X’s is nearly orthogonal to the pyridinium group.
Furthermore, after the 1st-e reduction (Scheme 2) or after
the 2nd-e reduction (Table 2) both the surviving carbonyl
group and the resulting enolate, respectively, are copla-
nar with the reduced pyridinium ring and orthogonal to
the p-substituted-phenyl group. Therefore, in both reduc-
tion steps of 1-X’s the substitution effect is only inductive,
and the fact that F2 > F1 reflects the relative distance of
the reduction site from the point of substitution.31,32
(Further supporting evidence for the proximity effect in
the relative magnitude of F values is found in the IR
stretching frequency of CdO, which is more sensitive to
the value of σp-X than the frequency of N+-CH3.33 See
Table S.1 and Figure S.2 in the Supporting Information.)
resonance interaction between -NO2 and the C•-O-
•-
•-
group (see 5 and 6). Therefore, although -NO2 is
e-donating by induction, in the case of the 1-e reduced
benzoyl moiety of BP-NO2, it interacts directly and
delocalizes the carbonyl radical. Similar arguments can
be made for [BP-CHO]2-
.
The strong inductive interactionssuggested by the
high F2 valuesbetween the substituents and the negative
charge developing upon the 2nd-e reduction of 1-X’s finds
further support in Scheme 3, which shows a greatly
(33) For example see: Berthelot, M.; Laurence, C.; Wojtkowiak, B.
Bull. Soc. Chim. Fr. 1973, 662.
(31) Wells, P. R. Linear Free Energy Relationships; Academic
Press: New York, 1968; p 12.
(32) Carey, F. A.; Sundberg, R. J . Advanced Organic Chemistry, 4th
ed.; Part A: Structure and Mechanisms; Kluwer Academic/Plenum
Publishers: New York, 2000; p 209.
(34) Comparison of the redox potentials of nitrobenzene (Table 1),
1-nitronaphthalene (-1.486 V vs Fc•+/Fc) and 9-nitroanthracene (ca.
-1.40 V vs Fc•+/Fc; irreversible) indicates that the dominant canonical
•-
form of reduced nitro-aromatic species contains the -NO2 group.
J . Org. Chem, Vol. 67, No. 21, 2002 7505

Leventis et al.
TABLE 2. Dih ed r a l An gles of th e Ca r bon yl Gr ou p of 1-X’s a n d Th eir F r ee Ba ses, w ith th e P yr id in iu m a n d th e P h en yl
Rin gsa
1-X’sb
free bases
C3-C4-C7-O
pyridinium, deg
O-C7-C8-C9
phenyl, deg
C3-C4-C7-O
pyridinium, deg
O-C7-C8-C9
phenyl, deg
substituent (X)
OCH3
CH3
H
SCH3
CtCH
Br
87.5 (1.2 )
81.4 (1.2)
81.4 (1.1)
85.8 (1.2)
81.9 (1.1)
79.4 (1.1)
80.2 (1.1)
81.3 (0.4)
86.5
2.7 (89.3 )
15.9 (89.2)
17.0 (89.2)
6.7 (89.2)
16.5 (89.1)
22.9 (89.1)
23.0 (89.1)
37.4 (73.9)
39.5
40.5
79.4
73.4
82.8
66.0
82.9
45.1
40.5
86.0
27.6
32.0
27.9
33.1
34.4
56.1
86.0
CHO
NO2
+S(CH3)2
a
b
The energy minimization was carried out with PM3 semiempirical calculations. The dihedral angle data after the 2nd-e reduction
of the 4-benzoyl-N-methylpyridinium system are in parentheses. (In the case of 1-NO2, this refers to [1-NO2•-]-.)
SCHEME 3. Nega tive Electr osta tic P oten tia l
Su r fa ces (via P M3 ca lcu la tion s) of [1-NO2•-]-,
[1-H]-, a n d [1-CHO]-, a t -200 a u
enhanced negative charge density at the enolate oxygen
•- -
of [1-NO2
]
relative to [1-H]-, which in turn has a
higher negative charge density than [1-CHO]- in the
same region. The implications of these findings are
significant. According to recent evidence, weak protic
acids interact selectively with the 2-e reduced forms of
1-H and 1-CtCH.16 On the basis of an analogous behav-
ior demonstrated by quinones,35 and spectroelectrochemi-
cal data from the reduction of 1-H in the presence of
1-butanol,16 the interaction between [1-H]- and weak
protic acids was attributed to H-bonding. As it turns out,
H-bonding is the preamble of an irreversible proton
transfer leading to carbinols,16 hence its study has
practical significance. H-bonding has been assumed to
develop at the O-atom of [1-X]-.16 But since para sub-
stitution modifies the negative charge density of the
CdC-O- group, it should be also able to tune its ability
to develop H-bonding. That should be considered as
further evidence not only for the development of H-
bonding, but also for its location.
F IGURE 3. The effect of methanol on the cyclic voltammetry
of 1-CH3 (3.310 mM) in CH3CN/0.1 M TBAP at 0.1 V s-1, using
a Au-disk electrode (0.0201 cm2). [CH3OH]: (a) 0.0 M; (b) 0.098
M; (c) 0.244 M; (d) 0.719 M; (e) 1.615 M. Inset: Dark circles:
|∆E1/2| of the two waves vs log[CH3OH]; slope ) 159 mV;
intercept ) 403 mV; correlation (r2) ) 0.982. Open circles:
|∆E1/2| of the two waves vs log[CD3OD]; slope ) 140 mV;
intercept ) 431 mV; correlation (r2) ) 0.993.
effect, the fact that the first wave is not affected by CH3-
OH means that [1-CH3]• and methanol do not interact.
Also, the absence of any prewaves indicates that any
possible interaction between [1-X]+’s and CH3OH can be
omitted safely from a mechanistic interpretation of the
data of Figure 3.37 These phenomena can be explained
only by an interaction between the 2-e reduced form,
[1-X]-, and the proton donor (Scheme 4).16 For large
equilibrium constants, KH, the mechanism of Scheme 4
The most dramatic effect of H-bonding between [1-X]-’s
and weak protic acids is a shift of the second cyclic
voltammetric wave to more positive potentials.16 The first
wave remains unaffected.36 This is a common observation
among all 1-X’s and is demonstrated in Figure 3 by 1-CH3
and CH3OH. These results cannot be attributed to a
matrix effect introduced by, e.g., the small change in the
SCHEME 4. Red u ction of 1-X’s in th e P r esen ce of
Wea k Acid P r oton Don or s (HA) ([1-X]- ‚‚‚m (HA)
H-Bon d ed Ad d u ct)
dielectric constant after the addition of methanol (ꢀCH
3CN
) 36, ꢀCH OH ) 33), because that change would also affect
3
the position of the first wave. Having excluded a matrix
(35) Gupta, N.; Linschitz, H. J . Am. Chem. Soc. 1997, 119, 6384.
(36) As expected, in the presence of methanol (e.g., 50 mM) the free
bases of all 1-X’s, even of those that show reversible voltammograms
in anhydrous CH3CN (Table 1), move to more positive potentials and
become chemically irreversible.
7506 J . Org. Chem., Vol. 67, No. 21, 2002

Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations
TABLE 3. Slop es, In ter cep ts, a n d Oth er Da ta Rela ted to th e H-Bon d ed Ad d u cts [1-X]-‚‚‚m (CH3OH) a n d
[1-X]-‚‚‚m (CD3OD) Der ived fr om th e P lots of ∆E1/2 vs log [CH3OH] a n d vs log [CD3OD], Resp ectively (see F igu r e 3, In set)a
d∆E1/2/d log [methanol]b
interceptb
H (D)
r2
H (D)
KH or D × 10-3 e
∆G° f
H (D)
c
d
[1-X]-
σp-X
H (D)
m
m
H (D)
H
D
•-
-
[1-NO2
]
-0.97
-0.27
-0.14
0.00
0.23
0.26
243 (230)
172 (152)
159 (140)
159 (135)
141 (127)
141 (125)
139 (117)
178 (166)
4.11 (4) 3.89 (4) -569 (-615) 0.998 (0.999)
2.91 (3) 2.57 (3) -387 (-407) 0.993 (0.996)
2.69 (3) 2.37 (2) -403 (-431) 0.982 (0.989)
2.69 (3) 2.28 (2) -417 (-446) 0.987 (0.985)
2.38 (2) 2.15 (2) -415 (-429) 0.993 (0.989)
2.38 (2) 2.11 (2) -415 (-433) 0.990 (0.993)
2.35 (2) 1.98 (2) -393 (-418) 0.993 (0.991)
1800 (300)
11.2 (5.15)
8.90 (2.98)
3.92 (1.26)
3.49 (2.02)
4.58 (2.27)
2.76 (1.04)
11.4 (8.30)
-8.53 (-7.46)
-5.51 (-5.06)
-5.38 (-4.74)
-4.90 (-4.23)
-4.83 (-4.51)
-4.99 (-4.57)
-4.69 (-4.11)
-5.55 (-5.21)
[1-OCH3]-
[1-CH3]-
[1-H]-
[1-CtCH]-
[1-Br]-
[1-CHO]-
N-MeAP+ g
0.47
3.01 (3) 2.88 (3)
-480 (-488) 0.986 (0.986)
a
∆E1/2 ) E1/2(2) - E1/2(1). ∆E1/2 reports the position of the second wave, E1/2(2), with respect to the fixed position of the first one,
b
E
1/2(1). In mV. c <m>H ) slope of the ∆E1/2 vs log [CH3OH] curve divided by 59.16 mV (see eq 6); in parentheses are the first significant
d
figure approximations for the average (apparent) number of CH3OH molecules participating in the H-bonded adducts. As in footnote c
for CD3OD as “proton” donor. e In M-m
tetrafluoroborate.
.
f In kcal/mol. Corresponding data for the 2nd-e reduction wave of N-methyl-4-acetylpyridinium
g
is the classic mechanism (albeit in reverse) where
e-transfer is coupled with a fast equilibrium that keeps
the redox-active species ([1-X]-) in short supply.38
According to Scheme 4, the relationship between the
electrode potential, E, and the ratio of the two electrode-
•
-
surface concentrations, C[1-X] /C[1-X] ‚‚‚m(HA), along the
second wave is given at 25 °C by eq 5, where E1/2°(2) is
E ) E1/2°(2) + 0.059 log KH + m(0.059 log[HA]) +
0.059 log (C[1-X]•/C[1-X]-‚‚‚m(HA)) (5)
the half-wave potential of the second wave in the absence
of HA. Consequently,27b the value of E1/2(2) of the second
wave in the presence of HA is given by eq 6. Therefore,
E
1/2(2) ) E1/2°(2) + 0.059 log KH + m(0.059 log[HA])
(6)
F IGURE 4. Plot of the slopes d∆E1/2/d log [CH3OH] (dark
circles) and d∆E1/2/d log [CD3OH] (open circles) of the 1-X’s
against Hammett substituent constants σp-X. (Data for this
figure are from Table 3; refer also to the inset of Figure 3.)
the absolute slopes of the E1/2(2) vs log[HA] curves (e.g.,
Figure 3, inset), expressed in multiples of 0.059 V,
represent the apparent (see below) number
m of HA
molecules associated with [1-X]- in [1-X]-‚‚‚m(HA). The
corresponding intercepts yield the equilibrium constants
KH (eq 6). All pertinent data for several 1-X’s with CH3-
OH and CD3OD as proton donors are summarized in
Table 3. Note that despite the rather high correlation
coefficients (r2), all data like those in inset in Figure 3
show distinct concave-down curvatures and slopes in-
creasing with log [CH3OH(D)]. This suggests that as the
concentration of CH3OH(D) increases, more methanol
molecules associate with [1-X]-.39 In other words, there
is no single value of m to describe the situation with each
1-X, so the calculated m values should be considered only
as apparent (average) values obtained by assuming only
one type of [1-X]-‚‚‚m(methanol) species. Thereby, the
varies from 2 for species bearing electron-withdrawing
susbstituents such as [1-CHO]- or [1-CtCH]-, to 3 for
[1-CH3]- and [1-OCH3]-, and up to 4 for [1-NO2•-]-.40
Importantly, |dE1/2(2)/d log [CH3OH]| is always greater
than |dE1/2(2)/d log [CD3OD]| for all 1-X’s (see Table 3
and Figure 4), meaning (eq 6) that at any given concen-
tration m D is less than m H. This correlation underlines
the direct involvement of the -OH(D) group in the results
of Figure 3, and is attributed to the steric effect resulting
from the fact that the O-D bond is shorter than O-H.41
The equilibrium constants, KH (Scheme 4), are cited
in Table 3 together with the corresponding standard
Gibbs free energy changes, ∆G°, calculated via ∆G° )
-RT ln KH. In all cases the equilibrium favors the
H-bonded adduct. The ∆G° values are in the typical range
for H-bonding (3-8 kcal/mol),28b providing independent
evidence for this type of interaction. It should be noted
that the energy that was calculated spectroscopically for
[1-H]-‚‚‚(m ) 1)(butanol) was 5.4 kcal/mol.16 The high
utility of
m values relies mostly with their relative
values, and in this context, Figure 4 illustrates that there
is strong correlation between the slopes dE1/2(2)/d log
[CH3OH(D)] and the σp-X values. Those slopes, and
therefore m H(D), increase as the negative charge density
on the enolate oxygen increases (Scheme 3). Thus, m
H(D)
(40) The 2nd reduction wave of 1-NO2 (corresponding to the reduc-
tion of the -NO2 group) is also sensitive to CCH OH. Analysis according
to eqs 5 and 6 yields a slope of 116 mV (r2 ) 30.943), indicating that
(37) (a) Paduszek-Kwiatek, B.; Kalinowski, M. K. Electrochim. Acta
1984, 29, 1439. (b) Kwiatek, B.; Kalinowski, M. K. Aust. J . Chem. 1988,
41, 1963.
(38) Bard, A. J .; Faulkner, L. R. Electrochemical Methods Funda-
mentals and Applications, 2nd ed.; J ohn Wiley and Sons: New York,
2001; p 36.
•-
the -NO2 group associates with ∼2 CH3OH molecules. (The corre-
sponding values for CD3OD are slope ) 105 mV and r2 ) 0.962.)
(41) (a) Whiddon, C.; Soderman, O. Langmuir 2001, 17, 1803. (b)
Abildgaard, J .; Bolvig, S.; Hansen, P. E. J . Am. Chem. Soc. 1998, 120,
9063.
(39) We thank the reviewer who brought this to our attention.
J . Org. Chem, Vol. 67, No. 21, 2002 7507

Leventis et al.
equilibrium constants for the [1-NO2•-]-‚‚‚(m ) 4)-
(methanol) adducts are attributed to the high electro-
static charge density on the enolate oxygen (Scheme 3).
then was poured into 100 g of ice. The resulting solution was
pH adjusted to 8 with concentrated NH4OH and extracted with
diethyl ether. The combined organic layers were dried with
anhydrous Na2SO4, the solvent was removed in vacuo, and the
crude product was recrystallized from ethyl acetate to give
4-(4-nitrobenzyl)pyridine as off-white crystals. Yield 2.85 g
(27%); mp 72-74 °C (lit.23 mp 74 °C); 1H NMR (CDCl3, 400
MHz) δ 4.08 (2H, s, CH2), 7.10 (2H, dd, J 2,6 ) J 3,5 ) 1.6 Hz,
J 2,3 ) J 5,6 ) 4.4 Hz, H-2,6), 7.35 (2H, d, J 9,10 ) J 12,13 ) 8.8 Hz,
H-9,13), 8.18 (2H, d, J 9,10 ) J 12,13 ) 8.8 Hz, H-10,12), 8.55 (2H,
dd, J 2,6 ) J 3,5 ) 1.6 Hz, J 2,3 ) J 5,6 ) 4.4 Hz, H-3,5).
Con clu sion s
The facile synthesis of 1-X’s renders the 4-benzoylpy-
ridinium system an attractive redox molecular building
block. However, from a practical standpoint, chemical
reversibility is of paramount importance for any redox
system. In that regard, comparison of 1-X’s with viologens
such as HV2+ has risen naturally (Figure 1A). In its most
economically significant application (solution-phase elec-
trochromic systems), HV2+ is cycled between the dica-
tionic and the 1-e reduced form, HV•+.19 But chemical
stability of HV•+ or [1-X]• is not sufficient, because
disproportionation (eq 7 ) produces a very low concentra-
4-(4-Nitr oben zoyl)p yr id in e (BP-NO2) was prepared via
KMnO4 oxidation of 4-(4-nitrobenzyl)pyridine according to the
method by Muth et al.24 Mp 123-124 °C (lit.24 mp 118-121
1
°C); H NMR (CDCl3, 400 MHz) δ 7.57 (2H, dd, J 2,6 ) J 3,5
)
1.9 Hz, J 2,3 ) J 5,6 ) 4.4 Hz, H-2,6), 7.94-7.98 (2H, m, AA′XX′,
H-9,13), 8.34-8.37 (2H, m, AA′XX′ H-10,12), 8.86 (2H, dd, J 2,6
) J 3,5 ) 1.9 Hz, J 2,3 ) J 5,6 ) 4.4 Hz, H-3,5); 13C NMR (CDCl3,
100 MHz) δ 122.6, 123.8, 130.8, 140.7, 142.7, 150.3, 150.7,
193.4 (CdO).
2[1-X]• a [1-X]- + [1-X]+
(7)
4-(4-Meth ylben zoyl)p yr id in e (BP-CH3) was prepared
from isonicotinic acid (15 g, 0.12 mol) and SOCl2 (50 mL),
followed by reaction of the isonicotinoyl chloride with toluene
(80 mL) in the presence of AlCl3 (30 g), according to the method
used for BP-Br.16 The crude product was recrystallized from
hexane to yield BP-CH3 as a white solid. Yield 9.8 g (42%);
mp 94-95 °C (lit.21b mp 96-97 °C); 1H NMR (CDCl3, 400 MHz)
δ 2.44 (3H, s, CH3), 7.27-7.31 (2H, m, AA′XX′, H-10,12), 7.54
(2H, dd, J 2,3 ) J 5,6 ) 4.3 Hz, J 3,5 ) J 2,6 ) 1.6 Hz, H-2,6), 7.69-
7.73 (2H, m, AA ′XX′, H-9,13), 8.78 (2H, dd, J 2,3 ) J 5,6 ) 4.3
Hz, J 3,5 ) J 2,6 ) 1.6 Hz, H-3,5); 13C NMR (CDCl3, 100 MHz) δ
21.7 (Me), 122.8, 129.3, 130.3, 133.2, 144.6, 144.7, 150.2, 194.7
(CdO).
tion of HV° or of [1-X]-. And, although [1-X]• and HV•+
are stable species, it is also well-documented that in
water both [1-X]- and the 2-e reduced form of viologen
are not stable.2,16 Since ∆E1/2 of 1-X’s is much wider than
that of HV2+ (Figure 1A), the equilibrium concentration
of HV° is calculated at about 104 times higher than that
of [1-X]-, under the same conditions. Therefore, based
on thermodynamics alone, [1-X]•’s should be able to
tolerate traces of water better than HV•+, and 1-X’s may
be well-suited for applications analogous to those of HV2+
.
4-(4-F or m ylben zoyl)p yr id in e (BP-CHO) was prepared
via a modification of the synthesis of p-nitrobenzaldehyde.22
BP-CH3 (2.0 g, 0.012 mol) was added to glacial acetic acid (15
mL) and acetic anhydride (15 mL). The solution was cooled in
an ice bath and concentrated H2SO4 (4 mL) was added slowly.
While the flask was still in the ice bath, CrO3 (3 g, 0.03 mol)
was added in portions to ensure that the temperature of the
mixture never exceeded 10 °C. The cold mixture was stirred
for another 30 min and was poured into ice (50 g). The pH of
the resulting solution was raised to 8-9 with a 30% (w/v) K2-
CO3 solution, followed by extraction with CH2Cl2. The organic
layers were collected, dried (Na2SO4), and evaporated to
dryness. The residue was recrystallized from EtOH and dried
under vacuum to give 4-[4-bis(acetyloxy)methylbenzoyl]py-
ridine as off-white crystals. Yield 1.18 g (37%); mp 120-122
A second important ramification of this study stems
from 1-NO2. Since -NO2•- is a strong electron donor, by
reducing nitro-aromatic systems one may invert their
reactivity. Currently, we are investigating Diels-Alder
reactions of reduced nitrobenzene and 1-nitronaphtha-
lene.
Exp er im en ta l Section
Meth od s. All electrochemical experiments were carried out
with an Au disk working electrode (1.6 mm in diameter, from
Bioanalytical Systems, Inc., West Lafayette, IN), an aqueous
Ag/AgCl reference electrode (also from BAS), and an Au foil
(2.5 cm2) as a counter electrode, in Ar-degassed anhydrous
CH3CN/0.1 M TBAP solutions at room temperature (23 (1 °C).
All voltammograms have been 80% compensated for solution
resistance. For experiments involving addition of CD3OD vs
CH3OH (Figures 3 and 4) we used a common stock solution of
1-X, separated in two halves. All reactions were carried out
under N2 in flame-dried glassware. Melting points were
uncorrected. Elemental analyses were performed by Oneida
Research Services, Inc., Whiteboro, NY. Semiempirical calcu-
lations (PM3) were conducted with the Spartan Version 5.0.3
for SiliconGraphics software package (Wavefunction, Inc.,
Irvine, CA).
Ma ter ia ls. All starting materials, reagents, and solvents
were commercially available and were used as received unless
noted otherwise. Fc and BP-H were sublimed. TBAP was
prepared as described before.16 The synthesis of the free bases
of 1-Br, and 1-CtCH (i.e., BP-Br and BP-CtCH) also has been
described before.16 The remaining free bases were synthesized
as follows.
1
°C; H NMR (CDCl3, 400 MHz) δ 2.14 (6H, s, CH3), 7.56 (2H,
dd, J 2,3 ) J 5,6 ) 4.4 Hz, J 3,5 ) J 2,6 ) 1.6 Hz, H-2,6), 7.64 (2H,
br d, J 9,10 ) J 12,13 ) 8.3 Hz, H-10,12), 7.71 (1H, s, CH), 7.83
(2H, br d, J 9,10 ) J 12,13 ) 8.3 Hz, H-9,13), 8.80 (2H, dd, J 2,3
)
J 5,6 ) 4.4 Hz, J 3,5 ) J 2,6 ) 1.6 Hz, H-3,5); 13C NMR (CDCl3,
100 MHz) δ 20.7, 88.9, 122.7, 127.0, 130.3, 136.9, 140.4, 143.8,
150.4, 168.6 (CdO, acetyloxy), 194.4 (CdO, benzoyl). Anal.
Calcd for C17H15NO5: C, 65.17; H, 4.83; N, 4.47. Found: C,
64.88; H, 4.85; N, 4.34. Subsequently, 4-[4-bis(acetyloxy)-
methylbenzoyl]pyridine (1 g, 3.19 mmol) was dissolved in
EtOH/H2O/concentrated H2SO4 (21 mL, 10:10:1, v/v/v) and was
refluxed for 30 min. At the end of the period, the reaction
mixture was allowed to cool to room temperature, the pH was
adjusted to 8-9 with 30% (w/v) K2CO3, and the mixture was
cooled in an ice bath. The precipitate was filtered, washed with
water, and dried in vacuo to give BP-CHO as a white solid.
Yield 0.51 g (76%); mp 122-123 °C; 1H NMR (CDCl3, 400 MHz)
δ 7.58 (2H, dd, J 2,3 ) J 5,6 ) 4.4 Hz, J 3,5 ) J 2,6 ) 1.6 Hz, H-2,6),
7.92-7.96 (2H, m, AA ′XX′, H-10,12), 8.00-8.03 (2H, m,
AA′XX′, H-9,13), 8.84 (2H, dd, J 2,3 ) J 5,6 ) 4.4 Hz, J 3,5 ) J 2,6
) 1.6 Hz, H-3,5), 10.13 (1H, s, CHO); 13C NMR (CDCl3, 100
MHz) δ 122.7, 129.7, 130.5, 139.2, 140.5, 143.3, 150.6, 191.3
(CHO), 194.4 (CdO). Anal. Calcd for C13H9NO2: C, 73.92; H,
4.29; N, 6.63. Found: C, 73.39; H, 3.74; N, 6.47.
4-(4-Nitr oben zyl)p yr id in e.23 4-Benzylpyridine (8.0 mL,
0.05 mol) was added slowly to a cooled (∼10 °C) mixture of
concentrated HNO3 (17 mL) and concentrated H2SO4 (17 mL),
so that the temperature of the reaction mixture does not rise
above 20 °C. The reaction mixture was stirred at 10-20 °C
for 1 h, subsequently was heated to ∼50 °C for 5 min, and
7508 J . Org. Chem., Vol. 67, No. 21, 2002

Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations
4-(4-Meth oxyben zoyl)p yr id in e (BP-OCH3) was prepared
from isonicotinic acid (5 g, 0.040 mol) and SOCl2 (20 mL),
followed by reaction of the isonicotinoyl chloride with anisole
(30 mL) in the presence of AlCl3 (10 g) according to the method
used for BP-CH3 above. Yield 3.2 g (38%); mp 119-121 °C
J 2,3 ) J 5,6 ) 6.6 Hz, H-3,5), 10.17 (1H, s, CHO); 13C NMR
(DMSO-d6, 100 MHz) δ 48.2 (CH3N+), 126.7, 129.7, 130.8,
138.4, 139.5, 146.6, 150.4, 191.8 (CdO), 193.0 (CHO); UV-
vis (CH3CN) λmax, nm (ꢀ, M-1 cm-1) 233 (11 200), 273 (18 300).
Anal. Calcd for C14H12NO2BF4: C, 53.71; H, 3.86; N, 4.47.
Found: C, 53.46; H, 3.39; N, 4.42.
1
(lit.21b mp 124 °C); H NMR (CDCl3, 400 MHz) δ 3.88 (3H, s,
OCH3), 6.95-6.98 (2H m, AA′XX′, H-10,12), 7.52 (2H, dd, J 2,3
) J 5,6 ) 6.0 Hz, J 3,5 ) J 2,6 ) 1.7 Hz, H-2,6), 7.79-7.83 (2H, m,
AA′XX′, H-9,13), 8.77 (2H, dd, J 2,3 ) J 5,6 ) 6.0 Hz, J 3,5 ) J 2,6
) 1.7 Hz, H-3,5); 13C NMR (CDCl3, 100 MHz) δ 55.6 (OCH3),
113.9, 122.7, 128.6, 132.7, 145.2, 150.2, 164.0, 193.7 (CdO).
4-(4-Meth ylth ioben zoyl)p yr id in e (BP-SCH3) was pre-
pared from isonicotinic acid (5 g, 0.040 mol) and SOCl2 (20
mL), followed by reaction of the isonicotinoyl chloride with
thioanisole (30 mL) in the presence of AlCl3 (10 g) according
to the method used for BP-CH3 above. The crude product was
recrystallized from CH3OH to give off-white crystals. Yield 2.6
4-(4-Meth oxylben zoyl)-N-m eth ylp yr id in iu m tetr a flu o-
r obor a te (1-OCH3) was prepared from BP-OCH3 (0.80 g, 3.76
mmol). Yield 0.70 g (59%); mp 142-143 °C;1H NMR (DMSO-
d6, 400 MHz) δ 3.89 (3H, s, OCH3), 4.41 (3H, s, CH3N+), 7.13-
7.17 (2H, m, AA′XX′, H-10,12), 7.78-7.82 (2H, m, AA′XX′,
H-9,13), 8.30 (2H, d, J 2,3 ) J 5,6 ) 6.6 Hz, H-2,6), 9.15 (2H, d,
J 2,3 ) J 5,6 ) 6.6 Hz, H-3,5); 13C NMR (DMSO-d6, 100 MHz) δ
48.1 (CH3N+), 55.9 (OCH3), 114.5, 126.2, 127.0, 132.8, 146.4,
152.1, 164.6, 190.5 (CdO); UV-vis (CH3CN) λmax, nm (ꢀ, M-1
cm-1) 208 (18 300), 258 (10 500), 304 (7 ,900). Anal. Calcd for
C
14H14NO2BF4: C, 53.37; H, 4.48; N, 4.45. Found: C, 53.20;
1
g (28%); mp 134-136 °C; H NMR (CDCl3, 400 MHz) δ 2.53
H, 4.04; N, 4.40.
(3H, s, SCH3), 7.27-7.31 (2H, m, AA′XX′, H-10,12), 7.53 (2H,
br d, J 2,3 ) J 5,6 ) 5.7 Hz, H-2,6), 7.70-7.74 (2H, m, AA′XX′,
H-9,13), 8.78 (2H, br d, J 2,3 ) J 5,6 ) 5.7 Hz, H-3,5); 13C NMR
(CDCl3, 100 MHz) δ 14.7 (SCH3), 122,7, 124.9, 130.6, 131.9,
144.7, 147.1, 150.3, 194.1 (CdO). Anal. Calcd for C13H11NOS:
C, 68.09; H, 4.84; N, 6.11. Found: C, 68.19; H, 4.77; N, 5.98.
Gen er a l P r oced u r e for th e Syn th esis of 1-X Qu a ter -
n a r y Sa lts. (CH3)3OBF4 (1.0-1.8 equiv) dissolved in CH3NO2
(10 mL) was added dropwise to a solution of the corresponding
free base (1 equiv) and the resulting solution was stirred at
room temperature for 20 min. The crude product was precipi-
tated by addition of diethyl ether, collected by vacuum filtra-
tion, and recrystallized from water to yield 1-X as a white solid,
unless indicated otherwise.
4-(4-Nit r ob en zoyl)-N-m et h ylp yr id in iu m t et r a flu or o-
bor a te (1-NO2) was prepared from BP-NO2 (0.90 g, 3.95
mmol). Yield 1.12 g (86%); mp 171-172 °C; 1H NMR (CD3CN,
400 MHz) δ 4.39 (3H, s, CH3N+), 7.97-8.01 (2H, m, AA′XX′,
H-9,13), 8.20 (2H, br d, J 2,3 ) J 5,6 ) 6.4 Hz, H-2,6), 8.36-8.40
(2H, m, AA′XX′, H-10,12), 8.82 (2H, br d, J 2,3 ) J 5,6 ) 6.4 Hz,
H-3,5); 13C NMR (DMSO-d6, 100 MHz) δ 48.2 (CH3N+), 123.9,
126.9, 131.5, 139.2, 146.6, 149.8, 150.5, 191.1 (CdO); UV-vis
(CH3CN) λmax, nm (ꢀ, M-1 cm-1) 227 (11 000), 271 (18 600).
Anal. Calcd for C13H11N2O3BF4: C, 47.31; H, 3.36; N, 8.49.
Found: C, 47.30; H, 3.00; N, 8.61.
4-(4-Met h ylt h iob en zoyl)-N-m et h ylp yr id in iu m t et r a -
flu or obor a te (1-SCH3) was prepared as a yellow solid from
BP-SCH3 (0.51 g, 0.22 mol) and exactly 1 equiv of (CH3)3OBF4.
Yield 0.61 g (85%); mp 168-169 °C;1H NMR (DMSO-d6, 400
MHz) δ 2.57 (3H, s, SCH3), 4.41 (3H, s, CH3N+), 7.42-7.48
(2H, m, AA′XX′, H10,12), 7.70-7.75 (2H, m, AA′XX′, H9,13),
8.31 (2H, d, J 2,3 ) J 5,6 ) 6.6 Hz, H2,6), 9.17 (2H, d, J 2,3 ) J 5,6
) 6.6 Hz, H3,5); 13C NMR (CD3CN, 100 MHz) δ 14.7 (SCH3),
49.5 (CH3N+), 118.3, 126.0, 128.0, 131.3, 131.6, 147.2, 150.1,
153.5, 191.6 (CdO); UV-vis (CH3CN) λmax, nm (ꢀ, M-1 cm-1
)
-
219 (13 500), 275 (10 700), 341 (10 400). Anal. Calcd for C14H14
NOSBF4: C, 50.78; H, 4.26; N, 4.23. Found: C, 50.54; H, 4.20;
N, 4.17.
4-(4-Dim e t h ylsu lfon iu m b e n zoyl)-N -m e t h ylp yr id in -
iu m bis(tetr a flu or obor a te) (1-+S(CH3)2) was prepared from
1-SCH3 (0.27 g, 0.81 mmol) and 1.5 equiv of (CH3)3OBF4. At
the end of the reaction period, CH3OH (1 mL) was added to
quench the excess of (CH3)3OBF4 and the solution was evapo-
rated to dryness to give a pale yellow gum-like solid. Yield
0.31 g (88%); 1H NMR (DMSO-d6, 400 MHz) δ 3.32 (6H, s,
S(CH3)2), 4.44 (3H, s, CH3N+), 8.05-8.08 (2H, m, AA′XX′,
H-10,12), 8.25-8.28 (2H, m, AA′XX′, H-9,13), 8.33 (2H, d, J 2,3
) J 5,6 ) 6.7 Hz, H-2,6), 9.19 (2H, d, J 2,3 ) J 5,6 ) 6.7 Hz, H-3,5);
13C NMR (CD3CN, 100 MHz) δ 28.9 (CH3S+), 49.7 (CH3N+),
118.9, 128.3, 131.4, 132.8, 139.7, 147.7, 151.0, 192.1 (CdO);
UV-vis (CH3CN) λmax, nm (ꢀ, M-1 cm-1) 228 (13 000), 260
(11 600). Anal. Calcd for C15H17NOSB2F8: C, 41.61; H, 3.96;
N, 3.23. Found: C, 41.11; H, 3.84; N, 3.24.
4-(4-Met h ylb en zoyl)-N-m et h ylp yr id in iu m t et r a flu o-
r obor a te (1-CH3) was prepared from BP-CH3 (1.00 g, 5.08
mmol). Yield 1.28 g (84%); mp 162-164 °C; 1H NMR (DMSO-
d6, 400 MHz) δ 2.43 (3H, s, CH3), 4.42 (3H, s, CH3N+), 7.42-
7.46 (2H, m, AA′XX′, H-10,12), 7.70-7.74 (2H, m, AA′XX′,
H-9,13), 8.31 (2H, d, J 2,3 ) J 5,6 ) 6.6 Hz, H-2,6), 9.16 (2H, d,
J 2,3 ) J 5,6 ) 6.6 Hz, H-3,5); 13C NMR (DMSO-d6, 100 MHz) δ
21.3 (Me), 48.1 (CH3N+), 126.4, 129.7, 130.4, 131.7, 145.8,
146.4, 151.7, 191.7 (CdO); UV-vis (CH3CN) λmax, nm (ꢀ, M-1
4-(4-Br om ob en zoyl)-N-m et h ylp yr id in iu m t et r a flu o-
r obor a te (1-Br) was prepared from BP-Br (2.35 g, 8.97
mmol).16 Recrystallized from ethanol to white needles. Yield
1
2.52 g (77%); mp 147-148 °C; H NMR (CD3CN, 400 MHz) δ
4.40 (3H, s, CH3N+), 7.69-7.73 (2H, m, AA′XX′, H-10,12), 7.79
(2H, m, AA′XX′, H-9,13), 8.16 (2H, d, J 2,3 ) J 5,6 ) 6.4 Hz,
H-2,6), 8.82 (2H, d, J 2,3 ) J 5,6 ) 6.4 Hz, H-3,5); 13C NMR (CD3-
CN, 100 MHz) δ 49.6, 128.2, 130.6, 132.9, 133.4, 134.4, 147.5,
cm-1) 232 (10 100), 227 (9 300). Anal. Calcd for C14H14
-
NOBF4: C, 56.22; H, 4.68; N, 4.68. Found: C, 56.11; H, 4.71;
N, 4.60.
152.5, 192.0 (CdO); UV-vis (CH3CN) λmax, nm (ꢀ, M-1 cm-1
228 (13 500), 278 (12 300).
)
4-Ben zoyl-N-m eth ylp yr id in iu m tetr a flu or obor a te (1-
H) was prepared from BP-H (Aldrich, 1.00 g, 5.46 mmol). Yield
0.91 g (58%); mp 170-171 °C (lit.42 mp 168 °C); 1H NMR
(DMSO-d6, 400 MHz) δ 4.43 (3H, s, CH3N+), 7.61-7.66 (2H,
4-(4-Eth yn ylben zoyl)-N-m eth ylp yr id in iu m tetr a flu o-
r obor a te (1-CtCH) was prepared from BP-CtCH (1.00 g,
4.83 mmol).16 Yield 1.25 g (84%); mp 229-231 °C dec; 1H NMR
(CD3CN, 400 MHz) δ 3.73 (1H, s, acetylenic), 4.40 (3H, s,
CH3N+), 7.68-7.72 (2H, m, AA′XX′, H-10,12), 7.78-7.82 (2H,
m, AA′XX′, H-9,13), 8.17 (2H, d, J 2,3 ) J 5,6 ) 6.4, H-2,6), 8.81
(2H, d, J 2,3 ) J 5,6 ) 6.4, H-3,5); 13C NMR (CD3CN, 100 MHz)
δ 49.7, 83.0, 83.5, 128.3, 129.3, 131.4, 133.6, 135.3, 147.4, 152.6,
192.1 (CdO); UV-vis (CH3CN) λmax, nm (ꢀ, M-1 cm-1) 249
(10 900), 288 (12 300).
4-Acetyl-N-m eth ylp yr id iu m tetr a flu or obor a te (N-
MeAP+) was prepared from 4-acetylpyridine (AP, 1.03 g, 0.84
mol). Yield 1.70 g (90%); mp 107-109 °C (lit.42 mp 105-107
°C); 1H NMR (DMSO-d6, 400 MHz) δ 2.73 (3H, s, COCH3), 4.40
(3H, s, CH3N+), 8.45 (2H, d, J 2,3 ) J 5,6 ) 6.6, H-2,6), 9.16 (2H,
d, J 2,3 ) J 5,6 ) 6.6, H-3,5); 13C NMR (DMSO-d6, 100 MHz) δ
m, H-10,12), 7.77-7.84 (3H, m, H-9,11,13), 8.33 (2H, d, J 2,3
)
J 5,6 ) 6.6 Hz, H-2,6), 9.17 (2H, d, J 2,3 ) J 5,6 ) 6.6 Hz, H-3,5);
13C NMR (DMSO-d6, 100 MHz) δ 48.1 (CH3N+), 126.5, 129.1,
130.2, 134.2, 134.8, 146.5, 151.3, 192.2 (CdO); UV-vis (CH3-
CN) λmax, nm (ꢀ, M-1 cm-1) 225 (13 500), 270 (11 300).
4-(4-F or m ylben zoyl)-N-m et h ylp yr id in iu m t et r a flu o-
r obor a te (1-CHO) was prepared from BP-CHO (0.48 g, 2.27
mmol). Yield 0.61 g (86%); mp 181-182 °C; 1H NMR (DMSO-
d6, 400 MHz) δ 4.44 (3H, s, CH3N+), 8.00 (2H, br d, J 9,10
J 12,13 ) 8.2 Hz, H-10,12), 8.13 (2H, br d, J 9,10 ) J 12,13 ) 8.2 Hz,
H-9,13), 8.36 (2H, d, J 2,3 ) J 5,6 ) 6.6 Hz, H-2,6), 9.19 (2H, d,
)
(42) Burke, M. R.; Brown, T. L. J . Am. Chem. Soc. 1989, 111, 5185.
J . Org. Chem, Vol. 67, No. 21, 2002 7509

Leventis et al.
27.3, 48.2, 125.5, 147.0, 148.1, 195.7 (CdO); UV-vis (CH3CN)
Su p p or tin g In for m a tion Ava ila ble: Scheme S.1, Syn-
thesis of the 4-(p-substituted benzoyl)-N-methylpyridinium
tetrafluoroborate salts (1-X); Figure S.1, HLFE plot for the
4-benzoylpyridine free bases; Table S.1, IR stretching frequen-
cies of CdO and N+-CH3 as a function of substitution; Figure
S.2, correlation of the IR stretching frequencies of 1-X’s with
the σp-X values. This material is available free of charge via
the Internet at http://pubs.acs.org.
λ
max, nm (ꢀ, M-1 cm-1) 224 (9 350), 275 (3 900), 336 (sh); IR
(KBr) (ν, cm-1) 1698 (CdO), 1640 (CH3-N+).
Ack n ow led gm en t. We gratefully acknowledge sup-
port from the Petroleum Research Fund, administered
by the ACS (Grant No. 35154-AC5), and from the
National Cancer Institute (Grant No.1 R15 CA82141-
01A2 to C.S.-L.).
J O020489+
7510 J . Org. Chem., Vol. 67, No. 21, 2002