Transition state imbalances in the deprotonation of picrylacetophenones by carboxylate and phenoxide bases

Article abstract of DOI:10.1021/jo9518617

The kinetics of the reversible deprotonation of 4-X-substituted picrylacetophenones 3a-c (X ) NO₂, H, MeO) by a variety of bases have been measured in 50% H₂O-50%Me₂SO (v/v) at 25 °C. Comparison of Bronsted β_B v

Full text of DOI:10.1021/jo9518617

1978
J . Org. Chem. 1996, 61, 1978-1985
Tr a n sition Sta te Im ba la n ces in th e Dep r oton a tion of
P icr yla cetop h en on es by Ca r boxyla te a n d P h en oxid e Ba ses
G. Moutiers,* B. El Fahid, R. Goumont, A. P. Chatrousse, and F. Terrier*
Laboratoire SIRCOB, EP CNRS 102, Department of Chemistry, The University of Versailles,
45 Avenue des Etats-Unis, 78035 Versailles Cedex, France
Received October 16, 1995X
The kinetics of the reversible deprotonation of 4-X-substituted picrylacetophenones 3a -c (X ) NO2,
H, MeO) by a variety of bases have been measured in 50% H2O-50%Me2SO (v/v) at 25 °C.
Comparison of Bronsted âB values for the ionization of each carbon acid by phenoxide and carboxylate
bases and RCH values for deprotonation of 3a -c by individual buffers bases indicates that the
reaction proceeds through strongly imbalanced transition states. The intrinsic reactivities of 3a -
c, as determined from the Bronsted plots for phenoxide ion reactions, are typical for the formation
of resonance-stabilized polynitrobenzyl-type carbanions, but the intrinsic rate constant k0 decreases
regularly on going from the less acidic p-methoxyacetophenone derivative 3c to the more acidic
p-nitroacetophenone derivative 3a . This trend is attributed to the fact that the contribution of the
benzoyl moiety to the resonance stabilization of the resulting carbanion C-3 is completely negligible
for the p-methoxy compound but not for the p-nitro compound. An extensive 1H and 13C NMR
study of the ionization of 3a -c confirms this proposal, with steric hindrance to rotation of the
picryl ring around the CR-Cipso linkage being observed at the probe temperature in the p-methoxy-
substituted carbanion C-3c, at -40 °C for the unsubstituted carbanion C-3b, but not at all for the
p-nitro carbanion C-3a . A major finding, however, is that the three carbanions undergo protonation
at the p-nitro group of the picryl ring to form nitronic acids in acidic media. This behavior clearly
shows that charge delocalization through the 2,4,6-trinitrophenyl moiety is predominant in the
three systems, including C-3a .
In tr od u ction
1b with a variety of bases of 50% H2O-50% Me2SO (v/
v). These reactions actually proceed through strongly
imbalanced transition states, exhibiting intrinsic energy
barriers which are among the highest so far measured
for ionization of carbon acids in aqueous solution.5,6 The
results were accounted for on the basis of the planar
structure of the resulting carbanions C-1a and C-1b,
respectively, which allow extensive delocalization of the
negative charge over the two phenyl rings, as shown in
structures A-D.6 Low to extremely low intrinsic reac-
tivities were also found for the ionization of the penta-
and hexanitrodiphenylmethanes 1c and 1d as well as for
that of the series of polynitrotriphenylmethanes 2a -
2d .5b,c Although steric factors prevent mutual coplanar-
ity of the phenyl rings in the conjugate carbanions C-2a -
d , all the results obtained emphasized that nitro-
substituted phenyl moieties are very effective at resonance
stabilization of these species.
It has now been clearly demonstrated that the intrinsic
reactivity (in the Marcus sense) of a carbon acid is closely
related to the extent of the structural and solvational
reorganization that is needed to form the conjugate
carbanion.1 The greater the importance of the resonance
stabilization of the carbanion is, the greater the lag with
which it develops behind proton transfer along the
reaction coordinate and the lower the intrinsic reactivity
of the carbon acid. On these grounds, determination of
the intrinsic reactivities of carbon acids of various
structural types can help clarify the electronic mode of
action of substituents or moieties whose resonance
capability is still subject to discussion.2-4
In this context, we have recently studied benzylic-type
structures whose ionization affords conjugate carbanions
with a high potentiality of delocalization of the negative
charge through remote electron-withdrawing groups.5
Illustrative systems are the ionization reactions of 2,4,4′-
trinitro- and 2,2′,4,4′-tetranitrodiphenylmethanes 1a and
In this paper, we report a structural and kinetic study
of the ionization of the three 4-X-substituted picryl-
acetophenones 3a -c to give the carbanions C-3a -c in
50% H2O-50% Me2SO (v/v) or Me2SO solutions. It will
be shown that the strong π-acceptor capability of the
2,4,6-trinitrophenyl moiety is the major factor determin-
ing the low intrinsic reactivity of 3a -c but also that the
X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) (a) Bernasconi, C. F. Tetrahedron 1985, 41, 3219. (b) Bernasconi,
C. F. Acc. Chem. Res. 1987, 20, 301. (c) Bernasconi, C. F. Adv. Phys.
Org. Chem. 1992, 27, 119.
(2) (a) Bernasconi, C. F.; Stronach, M. W. J . Am. Chem. Soc. 1990,
112, 8448. (b) Bernasconi, C. F.; Ohlberg, D. A. A.; Stronach, M. W.
J . Org. Chem. 1991, 56, 3016. (c) Bernasconi, C. F.; Wierserma, D.;
Stronach, M. W. Ibid. 1993, 58, 217.
(3) (a) Stefanidis, D.; Bunting, J . W. J . Am. Chem. Soc. 1991, 113,
991. (b) Bunting, J . W.; Stefanidis, D. Ibid. 1990, 112, 779. (c)
Stefanidis, D.; Bunting, J . W. Ibid. 1990, 112, 3163. (d) Wodzinski,
S.; Bunting, J . W. Ibid. 1994, 116, 6910.
(6) Simonin, M. P.; Xie, H. Q.; Terrier, F.; Lelievre, J .; Farrell, P.
G. J . Chem. Soc., Perkin Trans. 2 1989, 1553.
(7) Bernasconi, C. F. J . Org. Chem. 1971, 36, 1671.
(8) (a) Buncel, E.; Norris, A. R.; Russell, K. E.; Tucker, R. J . Am.
Chem. Soc. 1972, 94, 1646. (b) Buncel, E.; Venkatachalam, T. K.;
Menon, B. C. J . Org. Chem. 1984, 49, 413.
(9) Crampton, M. R.; Brooke, D. N. J . Chem. Res. Synop. 1980, 340;
J . Chem. Res., Miniprint 4401.
(4) Terrier, F.; Croisat, D.; Chatrousse, A. P.; Pouet, M. J .; Halle´, J .
C.; J acob, G. J . Org. Chem. 1992, 57, 3684.
(5) (a) Terrier, F.; Lelievre, J .; Chatrousse, A. P.; Farrell, P. G. J .
Chem. Soc., Perkin Trans. 2 1985, 1479. (b) Terrier, F.; Xie, H. Q.;
Farrell, P. G. J . Org. Chem. 1990, 55, 2610. (c) Terrier, F.; Boubaker,
T.; Xiao, L.; Farrell, P. G. Ibid. 1992, 57, 3924.
(10) Fyfe, C. A.; Malkiewich, C. D.; Damji, S. W. H.; Norris, A. R. J .
Am. Chem. Soc. 1976, 98, 6983.
(11) See the supporting information paragraph at the end of this
paper.
(12) Terrier, F.; Lelievre, J .; Chatrousse, A. P.; Schaal, R.; Farrell,
P. G. Can. J . Chem. 1987, 65, 1980.
0022-3263/96/1961-1978$12.00/0 © 1996 American Chemical Society

Deprotonation of Picrylacetophenones
J . Org. Chem., Vol. 61, No. 6, 1996 1979
study of the ionization reactions in Me2SO-d6 and aceto-
nitrile-d3. The most pertinent data are summarized in
Tables 1 and 2. A major finding is that there is a
nonequivalence of the H3,5 protons as well as of the C2,6
and C3,5 carbons of the picryl ring of the p-methoxy-
substituted carbanion C-3c at probe temperature. Simi-
larly, a nonequivalence of the H3,5 protons is observed in
X
NO2
X
NO2
–
CH2
CH
Y′
Y ′
O2N
Y
NO2 O2N
Y
NO2
1
C-1
(a) X = Y = Y′ = H
(b) X = NO2, Y = Y′ = H
(c) X = Y = NO2, Y′ = H
(d) X = Y = Y′ = NO2
1
the H spectra of the monosubstituted carbanion C-3b
recorded at -40 °C in acetonitrile. As will be elaborated
further in the discussion, NMR data favor structures
E-G as the main resonance contributors for C-3a -c.
–
X
H
C
NO2
X
H
NO2
C
–
–
X
X
O2N
NO2
O2N
NO2
H
C
NO2
H
C
NO2
B
A
C
O
C
O
–
–
O2N
NO2
O2N
NO2
X
H
NO2
X
H
NO2
E
F
C
C
–
O2N
NO2
O2N
NO2
D
C
X
X
NO2
H
C
NO2
H
C
NO2
NO2
C
O
H
C
O
G
–
–
X′′
X
X
X′′
O2N
NO2
O2N
NO2
–
CH
C
X′
X′
O2N
NO2 O2N
NO2
Kin etic a n d Equ ilibiu m Mea su r em en ts. All rate
and equilibrium measurements pertaining to the ioniza-
tion of 3a -c (eq 1) were made in 50-50 (v/v) H2O-Me2-
SO at 25 °C and constant ionic strength of 0.5 M
maintained with NMe4Cl. Pseudo-first-order conditions
were used throughout with a large excess of the acid
(HCl), base (NMe4OH), or buffer (carboxylic acids, phe-
nols) reagents over the picrylacetophenone concentration
([3] ≈ 3 × 10-5 M).
2
C-2
(a) X = X′ = X′′ = H
(b) X = NO2, X′ = X′′ = H
(c) X = X′ = NO2, X′′ = H
(d) X = X′ = X′′ = NO2
role of the X-substituted benzoyl moiety is important in
governing some particular aspects of the acid-base
behavior of 3a -c, especially the thermodynamic acidity.
CH
The pKa values of 3a -c were first determined from
X
X
observed absorbance variations at λmax of C-3a -c ob-
tained at equilibrium as a function of pH. These de-
scribed clear acid-base equilibrations, as evidenced by
Figure S111 which shows that excellent straight lines with
unit slopes were obtained on plotting the log values of
the ratio of the concentrations of ionized to nonionized
acetophenones as a function of pH (eq 2). We thus readily
3′
6′
NO2
NO2
2′
1′
4′
5′
–
CH2
CH
2
1
C
O
C
O
3
4
(α)
6
5
O2N
NO2
O2N
NO2
3
C-3
(a) X = NO2; (b) X = H; (c) X = OCH3
CH
CH
obtained the following: pKa ) 5.71 for 3a ; pKa ) 7.80
CH
for 3b; pKa ) 10.19 for 3c (Table 4). Depending upon
Resu lts
H
O
-
]
kp
+ kpB[B] + kpOH[OH
2
Str u ctu r a l Stu d ies. Addition of dilute tetramethyl-
ammonium hydroxide to ∼3 × 10-5 M solutions of 3a -c
in 50% H2O-50% Me2SO results in the complete forma-
tion of purple species exhibiting intense absorption
maxima at 528, 517, and 495 nm, respectively. These
3a -c {
} C-3a -c
(1)
(2)
k-pH[H+] + k-pBH[BH] + k-p
H
O
2
CH
log [C-3]/[3] ) pH - pKa
H2O
λ
max are typical for the formation of polynitrobenzyl-type
kobsd ) kp
+ kpB[B] + kpOH[OH-] +
carbanions.5,7-10 Unambiguous evidence that base ad-
dition to 3a -c affords the carbanions C-3a , C-3b, and
C-3c was obtained by carrying out a 1H and 13C NMR
H2O
k-p
+ k-pBH[BH] + k-pH[H+] (3)
the pH studied, the kinetics of the interconversion of 3
Ta ble 1. 1H Ch em ica l Sh ifts for P icr yla cetop h en on es 3a -c a n d Rela ted Ca r ba n ion s C-3a -c in Me2SO-d 6 a n d
a
Aceton itr ile-d 3
substituent X
NO2
compd
δH3,5
δHR
δH2′,6′
δH3′,5′
δH4′
δOCH3
3a
C-3a
3b
C-3b
7c
C-3c
9.13 (9.02)
8.52 (8.56)
9.12 (9.23)
8.46 (8.54)b
9.10
5.17 (5.05)
7.13 (7.11)
5.07 (5.00)
7.26 (7.24)
4.99
8.31 (8.24)
8.00 (8.02)
8.07 (8.06)
7.80 (7.84)
8.04
8.44 (8.39)
8.29 (8.23)
7.61 (7.59)
7.47 (7.46)
7.11
H
7.71 (7.73)
7.47 (7.46)
OCH3
3.88
3.81
8.31
7.30
7.78
7.00
8.57
a
b
Internal reference Me4Si; δ in ppm; values in parentheses refer to [2H3]-MeCN. A large broadening of this resonance occurs at -40
°C in acetonitrile.

1980 J . Org. Chem., Vol. 61, No. 6, 1996
Moutiers et al.
Ta ble 2. 13C Ch em ica l Sh ifts for P icr yla cetop h en on es 3a -c a n d Rela ted Ca r ba n ion s C-3a -ca
X
compd
δC1
δC2,6
δC3,5
δC4
δCR
δCO
δC1′
δC2′,6′
δC3′,5′
δC4′
δOCH3
NO2
3a
C-3a
3b
C-3b
3c
C-3c
130.82
133.26
131.41
132.23
131.52
131.64
150.85
141.82
150.94
140.88
150.94
138.00
142.00
123.77
124.53
123.63
124.71
123.48
124.83c
146.72
129.56
146.6
127.74
146.52
127.08
39.59
92.95
39.11
95.32
38.63
96.33
193.00
180.97
193.51
184.53
191.69
184.13
139.90
146.82
135.34
140.88
128.18
133.35
129.82
128.16
128.4
127.05
130.82
129.14
124.13
123.66
129.4
128.33
114.23
113.60
150.49
148.36
134.16
130.88
163.89
161.67
H
OCH3
55.72
55.34
3a
RHR
) 132.5; J CC-3a ) 159; J C3b ) 132.3; J CC-3b ) 158.9; J 3Cc ) 132.7; J CC-3c ) 158.6.
RHR RHR RHR RHR RHR
a
b
Internal reference Me4Si; δ in ppm, J in Hz. J C
c Broad signal.
Ta ble 3. Ra te Con sta n ts for th e Ion iza tion of th e P icr yla cetop h en on es 3a -c in 50% H2O-50% Me2SO (v/v)a
3a
3b
3c
B
BH
B
BH
B
BH
kp
(M-1 s-1
k-p
(M-1 s-1
kp
(M-1 s-1
k-p
(M-1 s-1
kp
(M-1 s-1
k-p
(M-1 s-1
)
BHb
buffer (basic species) no. pKa
)
)
)
)
)
b
b
b
H2O
1
2
3
4
5
6
-1.44
3.71
4.45
4.65
5.78
7.40
7.21 × 10-4 10200c
1.73 × 10-5 30300c
1.64 × 10-7 70 175c
chloroacetate ion
formiate ion
methoxyacetate ion
acetate ion
2,6-dichlorophenoxide
ion
2-cyanophenoxide ion
4-cyanophenoxide ion
2-bromophenoxide ion
4-chlorophenoxide ion 10 10.18 10 600
phenoxide ion 11 11.21 25 000
0.61
2.04
1.95
6.30
7600
4570
2750
660
28.7d
48d
17 750
7
8
9
7.97
8.45
9.52
820
1250
4480
4.50d
2.27d
0.69d
0.36d
0.17d
218
158
63
7950
6020
1250
540
316
1000
2510
5000
5500
110d
19d
267
480
10.47d
1.94d
1.17d
1320
130
4-methoxyphenoxide 12 11.47
ion
OH-
13 17.34 55 600e
1.42 × 10-7 13 660e
4.31 × 10-6 2190e
1.69 × 10-4
f
f
f
a
BH
I ) 0.5 M NMe4Cl; t ) 25 °C; experimental error in the rate constants: (4% or better; in pKa and pKaCH: (0.05 pK units; in the
rate constants calculated from eq 4: (10%; pKa values from ref 5a,b. kp O/27.6 with kp
calculated from Ka
k
.
c k-p ) kp
calculated from kpOHKs/Ka with pKs ) 15.83 at 25 °C (see
.
BH
b
H2
H2
O
CH
H
BH
H
-p
d
B
BH
B
OH
H2
H2
O
CH
Calculated from kp or k-p via eq 4. e kp ) kp
.
f k-p O/27.6 with k-p
text).
Ta ble 4. p Ka Va lu es a n d Ta u tom er iza tion Con sta n ts KT
for P icr yla cetop h en on es 3a -c in 50% H2O-50% Me2SOa
and C-3 was studied by mixing a neutral solution of 3a -c
with the appropriate NMe4OH or buffer solutions (pH g
pKaCH) or by mixing a freshly prepared 0.01 M NMe4OH
solution of the carbanion C-3 with the appropriate buffer
or HCl solutions (pH e pKaCH). In agreement with a
direct equilibrium approach according to eq 1, oscilloscope
pictures obtained in stopped-flow experiments revealed
that only one relaxation time is associated with the
interconversion in buffer and NMe4OH solutions of pH
g 4.45. In these instances, the general expression for
the observed first-order rate constant pertaining to the
process is given by eq 3. In this equation, the rate
3a
3b
3c
CH
pKa
5.71
2.70
7.80
2.94
10.19
3.17
pK′ aci
a
KT
9.77 × 10-4
1.38 × 10-5
9.55 × 10-8
% aci-form
≈0.1
≈1.4 × 10-3
≈10-5
a
t ) 25 °C; I ) 0.5 M NMe4Cl.
H2
H2
O
O
H2O
k-p
k-p
) 3.92 × 10-6 s-1; k-p
) 1.19 × 10-4 s-1
;
(3a )
(3c)
(3b)
) 4.66 × 10-3 s-1
.
In the various phenol and carboxylic acid buffer solu-
tions employed to cover the pH range of 4.45-11.47, a
standard treatment of the rate data showed that only the
kpB[B] and k-pBH[BH] terms were important in determin-
H2
O
constants kpOH, kpB, and kp
refer to the deprotonation
of 3a -c by hydroxide ion, the buffer base species, and
H
the solvent, respectively, while the rate constants k-p
,
BH
ing the corresponding kobsd values. All kpB and k-p rate
H2
O
k-pBH, and k-p
refer to the reprotonation of the car-
constants determined for eq 1 are summarized in Table
banions C-3a -c by hydronium ion, the buffer acid
species, and the solvent, respectively.
BH
CH
BH
4. For buffer solutions with pKa g pKa + 1 (or pKa
< pKa - 1), the k-p (or kpB) values were calculated
CH
BH
All kobsd values for ionization of 3a -c according to eq
1 are summarized in Tables S1-S6 given as supporting
information.11 To be noted is that equilibrium (1) was
approached from both reactant and product sides in
from the measured kp (or k-pBH) values by means of eq
B
4:
B
BH
kp ) k-pBHKaCH/Ka
(4)
BH
buffer systems with pKa ≈ pKaCH, e.g., 2-cyanophenol
BH
BH
(pKa ) 7.97) and 4-cyanophenol (pKa ) 8.45) buffers
CH
BH
in the case of 3b (pKa ) 7.80) or 2-bromophenol (pKa
Oscilloscope pictures revealed that reprotonation of
BH
) 9.52), 4-chlorophenol (pKa
) 10.18), and phenol
C-3a , C-3b, and C-3c in dilute HCl solutions (5 × 10-4
-
BH
BH
(pKa ) 11.21) buffers in the case of 3c (pKa ) 10.19).
From the kobsd values obtained in NMe4OH solutions,
excellent straight lines fitting the reduced equation kobsd
5 × 10-2 M) as well as that of C3b in chloroacetic acid
buffers (pH ) 3.41-4.01) does not proceed directly
according to eq 1 but involves the instantaneous forma-
tion of an intermediate species X which subsequently
decomposes to 3a , 3b, or 3c. The UV-vis absorption
spectra of these species, whose formation is pH-depend-
ent and essentially complete in 5 × 10-2 M HCl, could
be recorded by stopped-flow spectrophotometry (Figure
1). On the grounds of analogy with the protonation
behavior of the carbanion C-1b of 2,2′,4,4′-tetranitrodi-
) kpOH[OH-] were obtained in the three systems (not
OH
shown), which afforded the following: kp
) 55 600
(3a )
M-1 s-1; kp
) 13 660 M-1 s-1; kp
) 2190 M-1
OH
OH
(3b)
(3c)
s-1. This allowed the calculation of the k-p
rate con-
) kp Ks/Ka where Ks is the auto-
H2
O
H2
O
OH
CH
stants from k-p
protolysis constant of the 50% H2O-50% DMSO mixture
containing 0.5 M NMe4Cl (pKs ) 15.83 at 25 °C):5a

Deprotonation of Picrylacetophenones
J . Org. Chem., Vol. 61, No. 6, 1996 1981
F igu r e 1. UV-vis absorption spectra of the carbanion C-3a
and the related nitronic acid C-3c,H in a 5 × 10-2 M HCl
solution in 50% H2O-50% Me2SO (v/v).
F igu r e 2. Inversion plots according to eq 8 for protonation of
the carbanions C-3a -c in dilute HCl solutions in 50% H2O-
50% Me2SO (v/v): T ) 25 °C, I ) 0.5 M NMe4Cl.
phenylmethane in methanol12 and some other features
to be discussed later, there is little doubt that the X
species arise from protonation at a NO2 group of C-3a -
c, most likely at the 4-position, and are therefore the
nitronic acids C-3H. Thus, the appropriate scheme for
the conversion of C-3 to 3 at low pH is shown in eq 5
and the measured kobsd values for this process are
expected to obey the general equation (6) in buffer
solutions (Table S2) and the reduced equation (7) in HCl
solutions (Table S6):11,12
X
H
C
NO2
C
O
HO2N
NO2
C-3H
+
H
O
BH
k-pH[H ] + k-p
+ k-p
C-3H {K′
\
a} C-3 + H+
[BH]8 3 (5)
2
F igu r e 3. Effect of buffer concentration and pH on the
observed rate constant, kobsd, for protonation of the carbanion
C-3b in chloroacetic acid buffers in 50% H2O-50% Me2SO (v/
v): T ) 25 °C, I ) 0.5 M NMe4Cl.
K′
H+
H2O
a
kobsd ) (k-p [H+] + k-p
)
+
K′ + [H+]
a
K′
a
k-pBH[BH]
(6)
and 3.17 for C-3cH. From the k-p values, the kpH O rate
H
2
K′ + [H+]
a
constants for deprotonation of 3a -c by the solvent were
also calculated as kpH O ) Ka
k
(kp3a H O ) 1.99 × 10-2
CH
H
2
2
-p
K′
H+
H2O
a
kobsd ) (k-p [H+] + k-p
)
(7)
(8)
s-1; kp3b
) 4.77 × 10-4 s-1, and kp3c
) 4.52 × 10-6
H2
O
H2O
K′ + [H+]
s-1).
a
Consideration of the reprotonation of C-3b in chloro-
acetic acid buffers, eq 6 predicts that linear plots of kobsd
vs [BH] should be obtained at constant pH but that the
slopes of these plots should decrease with increasing the
H+ concentration. As shown in Figure 3, the data
obtained at the three pH studied confirmed these expec-
tations. In addition, the values of the intercepts agreed
well with estimates made from eq 7. After appropriate
correction for the K′a/(K′a + [H+]) terms, the slopes of the
lines in Figure 4 afforded three consistent values for the
1
kobsd
1
1
)
+
k-pH[H+] k-pHK′
a
In agreement with eq 7, curvilinear kobsd vs [H+] plots
were obtained from measurements in HCl solutions (not
shown, see Table S6, supporting information). Taking
into account that the solvent contribution (k-p O) is
H2
negligible in the pH range at hand, eq 7 may be rewritten
in the form of eq 8. In accordance with this equation,
plots of 1/kobsd vs 1/[H+] afforded satisfactory straight
BH
k-p rate constant pertaining to chloroacetic acid (Table
4). The corresponding kpB value was calculated from eq 4.
H
H
lines (Figure 2), from which the following k-p and k-p
K′a values were determined: k-p ) 10 200 M-1 s-1, k-p
H
H
K′a ) 20.4 s-1 for 3a , k-p ) 30 300 M-1 s-1, k-p K′a )
H
H
Discu ssion
34.8 s-1 for 3b, k-p ) 70 175 M-1 s-1, k-p K′a ) 47.45
H
H
s-1 for 3c. Interestingly, the above k-p K′a values agree
Str u ctu r e of th e Ca r ba n ion s. As can be seen in
H
well with the plateaus observed in Table S6 (supporting
Tables 1 and 2, the resonances of the exocyclic CR and
HR atoms of 3a , 3b, and 3c suffer large downfield shifts
upon ionization: the ∆δ(HR) and ∆δ(CR) values lie in the
ranges 1.96-2.31 and 53.46-57.70 ppm, respectively.
H
H
information). Combination of k-p and k-p K′a afforded
the following pK′a values for ionization of the three
nitronic acids: pK′a ) 2.70 for C-3a H, 2.94 for C-3bH,

1982 J . Org. Chem., Vol. 61, No. 6, 1996
Moutiers et al.
Although such ∆δ values are mainly the result of two
opposing effects,6,13,14 namely a high-field shift caused by
increased electron density at CR and a low-field shift
caused by the sp3 f sp2 rehybridization of this carbon,
the importance of the observed downfield variations
shows that the rehybridization factor is largely predomi-
nant. The finding that the ionization induces large
variations in 1J CRHR (∆J ) 27) together with the high
values of these coupling constants for C-3a , C-3b, and
C-3c are also consistent with a strong olefinic character
of the CR carbon.15 This implies a concomitant and
important delocalization of the negative charge of the
carbanion either through the picryl ring (structures E-G)
or through the benzoyl moiety (structure H).
C-3a , C-3b, and C-3c; (4) there is a nonequivalence of
the ortho and meta carbons, as well as of the H3,5 protons
of the picryl ring of the p-methoxybenzoyl carbanion C-3c
at probe temperature and of the unsubstituted analogue
C-3b at -40 °C. These last findings are clearly consis-
tent with the high double bond character of the Cipso-CR
bond, as shown in structures E-G. That such non-
equivalence could not be observed for the carbanion C-3a
suggests that the contribution of the enolate structure
H to its stabilization is not totally negligible and therefore
that the p-nitrobenzoyl moiety is capable to compete to
some extent with the picryl ring in attracting the nega-
tive charge.
That charge delocalization occurs preferentially through
the 2,4,6-trinitrophenyl moiety, as depicted in structures
E-G, is supported by the following observations: (1) the
visible spectra of the carbanions C-3a , C-3b, and C-3c
show an intense absorption maximum at wavelengths
(528, 517, and 518 nm, respectively) similar to that of
the 2,4,6-trinitrotoluene anion C-4 (514 nm);7-10 (2) the
ionization of 3a , 3b, and 3c induces a strong shielding
of the H3,5 protons of the picryl ring. The ∆δ(H) values
lie in the range -0.61, -0.79 ppm, comparing well with
the proton shifts observed in the ionization of various
2,4,6-trinitrobenzyl derivatives, including 2,4,6-trinitro-
toluene (TNT) (∆δ(H3,5) ) -0.74 ppm),6,10 (3) the reso-
nances of the C2,6 and C4 carbons, especially the latter,
move markedly to high field, whereas those of the C3,5
carbons move slightly to low field on going from 3a -c to
C-3a -c: ∆δ(C2,6) ≈ -10; ∆δ(C4) ≈ -18; ∆δ(C3,5) ≈ +1.
Interestingly, the situation is reminiscent of the one
reported in 13C studies of the formation of the related
carbanion C-5 of 2,4,6-trinitrophenylacetone as well as
of that of picryl σ-adducts of general structure 5 according
to eq 9.16-21 In these latter instances, SCFMO calcula-
tions have indicated that the σ-complexation results in
increases in π-electron density at the 2, 4, and 6-ring
positions as well as on the NO2 substituents and in
decreases at the 3- and 5-positions, accounting well for
the observed variations in the carbon chemical shifts of
the cyclohexadienyl ring: ∆δ(C2,6) ≈ -17, ∆δ(C4) ≈ -29,
∆δ(C3,5) ≈ 2.22,25 In addition, the calculations confirmed
the major role of the paraquinoid structure 6B in the
delocalization of the negative charge of the adducts.24
Analogously, there is little doubt that the variations
suffered by the carbon resonances of the picryl ring in
reactions (1) suggest a major contribution of the nitronate
representations E-G, especially F, to the structure of
CH3
O2N
NO2
NO2
4
R
R
Nu
R
Nu
–
R
Nu
–
–
O2N
NO2 O2N
+ Nu–
NO2 O2N
NO2 O2N
NO2
1
4
1
4
2
3
2
3
6
5
6
5
(9)
NO2
NO2
NO2
NO2
6A
6B
6C
H3CO
OH,
,
5: R = H, OCH3 Nu = OCH3, H, CH2COCH3,
H3CO
Nitr on ic Acid F or m a tion . It follows from the pre-
dominance of the resonance structures E-G over H that
the fast reversible protonation of C-3a -c in acidic
solutions can only occur at a nitro group, and hence, the
intermediate protonated species X observed at low pH
must be nitronic acids. Furthermore, it is well known
that a p-NO2 group is significantly more effective at
resonance stabilization of charge than is a o-NO2
group.24-26 On this basis, one can understand, as it is
borne out by NMR results, that the negative charge of
C-3a -c will lie preferentially on the 4-NO2 group and
that it is this group which will be preferentially proto-
nated, allowing the identification of the X-species as the
nitronic acids C-3,H.
X
X
H
C
H
C
NO2
NO2
C
O
C
O
+ AH
+ A–
–
O2N
NO2
HO2N
NO2
(13) Stothers, J . P. Carbon-13 NMR Spectroscopy; Academic Press:
New York, 1972; p 210.
(14) Bradamante, S.; Pagani, G. A. J . Chem. Soc., Perkin Trans. 2
1986, 1035.
C-3
Nu
C-3H
R
RO OR′
O2N
NO2
Y
O
(15) Terrier, F.; Vichard, D.; Chatrousse, A. P.; Tope, S.; Mac
Glinchey, M. J . Organometallics 1994, 13, 690.
(16) Kovar, K. A.; Breitmaier, E. Chem. Ber. 1978, 111, 1646.
(17) Olah, G. A.; Mayr, H. J . Org. Chem. 1976, 41, 3448.
(18) Renfrow, R. A.; Strauss, M. J .; Terrier, F. J . Org. Chem. 1980,
45, 471.
(19) Simonnin, M. P.; Pouet, M. J .; Terrier, F. J . Org. Chem. 1978,
43, 855.
(20) Chudek, J . A.; Foster, R.; Marr, A. W. J . Chem. Soc., Perkin
Trans. 2 1987, 1341.
N
NO2H
6,H
NO2H
7,H
H3CO
OH,
H3CO
(a) Y = N, R = R′ = CH3
(b) Y = N O, R = R′ = CH3
(c) Y = N, R---R′ = –CH2CH2–
R = H; Nu = H,
(d) Y = N
O, R---R′ = –CH2CH2–
(21) Machacek, V.; Sterba, V.; Lycka, A.; Snobl, D. J . Chem. Soc.,
Perkin Trans. 2 1982, 355.
(22) Wennerstro¨m, H.; Wennerstro¨m, O. Acta Chem. Scand. 1972,
26, 2883.
NO2
NO2
CH
(23) Hosoya, H.; Hosoya, S.; Nakagura, S. Theor. Chim. Acta 1968,
12, 117.
(24) Terrier, F. In Nucleophilic Aromatic Displacement; Feuer, H.,
Ed.; VCH Publishers: New York, 1991; pp 93-95.
(25) Caveng, P.; Fischer, P. B.; Heilbronner, E.; Miller, A. L.;
Zollinger, H. Helv. Chem. Acta 1967, 50, 848.
HO2N
NO2
C-1b,H
(26) Terrier, F. Chem. Rev. 1982, 82, 77.

Deprotonation of Picrylacetophenones
J . Org. Chem., Vol. 61, No. 6, 1996 1983
A number of nitronic acids of type 6,H and 7,H arising
from protonation of anionic nitrocyclohexadienylide struc-
tures have been characterized in the literature.27-29 Also,
the formation of the nitronic acid C-1b,H upon protona-
tion of the (2,2′,4,4′-tetranitrodiphenyl)methyl carbanion
C-1b has been reported in methanol.12 In all cases, the
protonation results in a marked hypsochromic shift of the
absorption maxima in the electronic spectra, as it is
actually observed for the conversion of C-3 to C-3,H:
tors like 8 and 9 is also consistent with recent conclusions
that have been drawn from theoretical calculations of the
electron density distributions in carbonyl-containing
functional groups.33,34
+
OMe
NO2
X
+
CH2
CH2
C
O
C
Y
O
NO2
O2N
C-3a
C-3a ,H
C-3b
λmax
λmax
) 528 nm, λmax
) 382 nm; λmax
) 517 nm,
C-3c,H
NO2
–
–
C-3b,H
C-3c
) 404 nm; λmax
) 495 nm, λmax
) 438
9 (Y = N, NR+)
8
nm. On the other hand, pK′a values of the order 4.2-
4.5 have been reported for formation of the nitronic acids
7H and C-1b,H in methanol.12,29 Since the acidity of
oxygen acids is usually increased on going from metha-
nolic to aqueous solutions but decreased upon addition
of a dipolar aprotic solvent like Me2SO,30 our pK′a values
measured for the oxygen protonation of C-3a -c in 50%
H2O-50% Me2SO compare well with previous results,
giving additional support to our assignment of C-3,H as
nitronic acids. However, the fact that the pK′a values
increase in the order C-3a ,H < C-3b,H < C-3c,H is
consistent with NMR evidence that charge delocalization
through the picryl moiety decreases to some extent on
going from the p-methoxybenzoyl derivative to the p-
nitrobenzoyl derivative, as expected from the electronic
effects of the X substituent.
CH2COC6H5
+
N
COCH3
X
Me
10
11
(a) X = NO2
(b) X = H
(c) X = OCH3
H2
O
pKa
values of 16.70, 18.24, and 19 have been
reported for the parent acetophenones 10a -c of 3a -
c.35-37 Neglecting the enhancing effect of Me2SO on the
pKa values measured for 3a -c in 50% H2O-50% Me2-
SO,30 the acidifying effect due to the substitution of a
methyl hydrogen of 10a -c for a picryl ring appears to
be of about 10 pK units. This makes the acidity of 3a -c
of the same order as that of phenacylpyridinium deriva-
tives of type 9 (Y ) NR+)3 as well as of compounds like
11 (pKa ) 7.02). â-Diketone derivatives like acetylac-
Equ ilibr iu m a n d Kin etic Acid ity of 3a -c. In
agreement with the general observation that compounds
yielding highly delocalized anions show decreased pKa
values in Me2SO relative to water,30 the acidity of TNT
) 13.6)31 to Me2-
H2
O
etone (pKHa O ) 9.11 and pK5a0%Me SO ) 9.12 at 20 °C)38 or
2
2
increases on going from water (pKa
SO (pKa
) 10.5),32 making reasonable to estimate a
2
Me2SO
dibenzoylmethane (pK5a0%Me SO ) 8.72 at 20 °C)26 also
pKa value of ca. 12.5 for this carbon acid in 50% H2O-
50% Me2SO. On this basis, Table 3 shows that substitu-
tion of one methyl hydrogen of TNT for an unsubstituted
benzoyl group markedly increases the acidity: ∆pKa ≈
4.7. As expected, introduction of the electron-donating
OCH3 group in the para position of the phenyl ring
decreases the acidifying effect of the benzoyl moiety,
whereas that of the electron-withdrawing NO2 group
increases it. A noteworthy result, however, is that the
pKa variations define a satisfactory Hammett relation-
ship (F ) 2.87), providing that σp+ rather than σp be used
for the methoxy group (Figure S2).11 This implies that
the electronic influence of the X substituent on the acidity
of 3a -c is of much greater importance at the ground
state levelsresonance structure 8 is obviously essential
in determining the stability of 3csthan at the product
carbanion level, where the role of the picryl ring in
delocalizing the negative charge of the three carbanions
C-3a -c is very dominant. Bunting and Stefanidis have
recently suggested that ground state stabilization through
structures of type 9 is responsible for the finding that
substituents in the phenyl ring induce a large effect upon
the acidity of 4-phenacylpyridines and 4-phenacylpyri-
dinium cations.3c The importance of resonance contribu-
CH
have similar pKa values in aqueous solution.
CH
From the pKa
and pK′ values pertaining, respec-
a
tively, to the deprotonation of 3a -c and nitronic acid
formation from the conjugate carbanions, the equilibrium
constants measuring the extent of the conversion of the
three picrylacetophenones into their aci-forms in 50%
H2O-50% Me2SO (eq 10) can be calculated. The data are
NO2
NO2
CH CO
NO2
X
CH2 CO
NO2
X
KT
(10)
HO2N
O2N
summarized in Table 4. As can be seen, KT and hence
the aci content is very small, even for the more acidic
p-nitroacetophenone substrate. This is in agreement
with the failure to detect the presence of C-3,H in 1H
and 13C NMR spectra recorded under common experi-
mental conditions in Me2SO-d6.
Figure 4 shows statistically corrected Bro¨nsted plots
for the deprotonation of 3b by carboxylate and phenoxide
ions while Figure S311 shows the same relationships for
deprotonation of 3a and 3c by phenoxide ions. For a
given ∆pK + log(p/q) value, Figure 4 exemplifies the
reactivity order ArO- > RCOO- ions, consistent with the
general observation that aryl oxide ions are less solvated
and therefore more efficient catalysts than carboxylate
ions of the same pKa’s in proton transfer reactions at
(27) (a) Wennerstro¨m, O. Acta. Chem. Scand. 1971, 25, 2871. (b)
Mo¨berg, C.; Wennerstro¨m, O. Ibid. 1971, 25, 2871.
(28) Buncel, E.; Eggimann, W. Can. J . Chem. 1976, 54, 2436.
(29) Terrier, F.; Ah-Kow, G.; Chatrousse, A. P. J . Org. Chem. 1985,
50, 4583.
(30) Buncel, E.; Wilson, H. Adv. Phys. Org. Chem. 1977, 14, 133.
(31) Lelievre, J .; Farrell, P. G.; Terrier, F. J . Chem. Soc., Perkin
Trans. 2 1986, 333.
(32) Farrell, P. G.; Terrier, F.; Schaal, R. Tetrahedron Lett. 1985,
26, 2435.
(34) Wiberg, K. B.; Laidig, K. E. J . Am. Chem. Soc. 1988, 110, 1872.
(35) Chiang, Y.; Kresge, A. J .; Wirz, J . J . Am. Chem. Soc. 1984, 106,
6392.
(36) Keeffe, J . R.; Kresge, A. J .; Wirz, J . Can. J . Chem. 1986, 64,
1224.
(33) (a) Siggel, M. R. F.; Streitwieser, A., J r.; Thomas, T. D. J . Am.
Chem. Soc. 1988, 110, 8022. (b) Siggel, M. R. F.; Thomas, T. D. Ibid.
1986, 108, 4360.
(37) Guthrie, J . P.; Cossar, J .; Klym, A. Can. J . Chem. 1987, 65,
2154.
(38) Bernasconi, C. F.; Bunnell, R. D. Isr. J . Chem. 1985, 26, 420.

1984 J . Org. Chem., Vol. 61, No. 6, 1996
Moutiers et al.
Ta ble 5. Br on sted Coefficien ts a n d In tr in sic Ra te
Con sta n ts for th e Ion iza tion of th e P icr yla cetop h en on es
3a -c by Ca r boxyla te a n d P h en oxid e Ba ses in 50%
H2O-50% Me2SOa
3a
3b
3c
RCH
0.28
0.49
0.28
0.39
0.46
2.25
1.55
0.28
0.44
âArO
âRCOO
log k0
log k0
ArO
1.80
2.73
RCOO
a
t ) 25 °C; I ) 0.5 M NMe4Cl.
the ionization reactions of the three picrylacetophenones
3a -c proceed through imbalanced transition states in
which the development of resonance of the carbanions
lags behind charge transfer.1-5,40-42 Measuring the ex-
tent of the imbalance as I ) RCH - âB,1 then negative
values of the order of -0.17 ( 0.03 are calculated.
Although much greater imbalances have been reported
in various carbon acid systems,1c such I values clearly
indicate that the deprotonation of 3a -c gives rise to
strongly resonance-stabilized carbanions, supporting a
predominant role of the picryl moiety in the delocalization
of the negative charge.
F igu r e 4. Bro¨nsted plots for ionization of 3b by carboxylate
and phenoxide ions in 50% H2O-50% Me2SO (v/v): T ) 25
°C, I ) 0.5 M NMe4Cl. The numbering of the various catalysts
is given in Table 3.
In the last decade it has been clearly recognized that
imbalanced transition states are characteristic for the
ionization of carbon acids exhibiting a low intrinsic
reactivity.1 Values of the intrinsic rate constants k0 (in
the Marcus sense)43 for 3a -c, as determined from the
Bro¨nsted plots of Figures 4 and S3 (supporting informa-
BH
CH
tion) as k0 ) kpB/q when pKa + log(p/q) ) pKa are
given in Table 5. As can be seen, the intrinsic reactivity
of 3a -c increases on going from the p-nitroacetophenone
ArO
derivative 3a (log k0
analogue 3b (log k0
) 1.80) to the unsubstituted
ArO
) 2.25) to the p-methoxyacetophe-
none compound (log k0
ArO
) 2.73). Thus, in agreement
with our finding of smaller RCH than âB values and
therefore of negative I values, it is the less acidic
compound which has the highest intrinsic reactivity.
Taking into account that log k0 values for aryl oxide ion
catalysts are generally greater than log k0 values for
carboxylate ions and primary amines by about 0.8 and
1.7 log unit, respectively,5 it appears from Table 6 that
the intrinsic reactivities of 3a -c are about 2 orders of
magnitude lower than those of â-dicarbonyl compounds
of similar pKa’s, like acetylacetone and 1,3-indandione.38,41b
This result can be accounted for only if the picryl ring
contributes dominantly to the delocalization of the nega-
tive charge of C-3a -c, as shown in the resonance
structures E-G. However, the finding of a lower k0 value
for the p-nitroacetophenone compound confirms our
previous suggestion that in this case the enolate con-
tributor H does not have a negligible influence in the
F igu r e 5. Effect of the acidity of the carbon acids 3a -c on
the rate constant of deprotonation by various phenoxide bases
(entries 7-11 in Table 3) and OH- (entry 13 in Table 3).
carbon.5 All Bronsted âB reported in Table 5 are in the
range 0.5 ( 0.1 commonly found for the ionization of
carbon acids, suggesting that proton transfer is about
half-complete at the transition states of reactions (1). To
be noted, however, is that âArO does not decrease regularly
on going from the less thermodynamically favored pro-
tonation reaction (3c) to the more thermodynamically
favored system (3a ). Failure of the reactivity-selectivity
principle in accounting for data pertaining to the ioniza-
tion of carbon acids has often been noted by various
authors.1,39
delocalization of the negative charge of C-3a . Interest-
Our kinetic data also allow a determination of the
Bro¨nsted RCH values measuring how the rate of depro-
tonation by a given base depends on the acidity of the
carbon acid. Although they are only based on the three
ArO
ingly, the log k0
value for 3a is essentially the same
as that for the carbanion C-1d derived from (2,2′,4,4′,6,6′-
hexanitrodiphenyl)methane in which steric factors have
been shown to allow conjugation of the sp2-hybridized
B
points pertaining to 3a , 3b, and 3c, the various log kp
CH
vs pKa plots are satisfactorily linear with slopes (RCH
)
in the range 0.28 ( 0.03 (Figure 5). Clearly, such low
RCH values do not agree with the transition state picture
provided by the âB values, a situation which suggests that
(41) Bernasconi, C. F.; Hibdon, S. A. J . Am. Chem. Soc. 1983, 105,
4343. (b) Bernasconi, C. F.; Paschalis, P. Ibid. 1986, 108, 2969. (c)
Bernasconi, C. F.; Kliner, D. A. V.; Mullin, A. S.; Ni, J . X. J . Org. Chem.
1988, 53, 3342. (d) Bernasconi, C. F.; Bunnell, R. D.; Terrier, F. J .
Am. Chem. Soc. 1988, 110, 6514.
(39) (a) Pross, A. Adv. Phys. Org. Chem. 1977, 14, 69. (b) Giese, B.
Angew. Chem., Int. Ed. Engl. 1976, 16, 125.
(40) (a) Bordwell, F. G.; Boyle, W. J ., J r. J . Am. Chem. Soc. 1972,
94, 3907 and references therein. (b) Bordwell, F. G.; Bartmess, J . E.;
Hautala, J . A. J . Org. Chem. 1978, 43, 3107, 3113. (c) Bordwell, F.
G.; Drucker, G. E.; Mc Collum, G. J . Ibid. 1982, 47, 2504.
(42) (a) Bernasconi, C. F.; Wiersema, D.; Stronach, M. W. J . Org.
Chem. 1993, 58, 217. (b) Bernasconi, C. F.; Wenzel, P. J . J . Am. Chem.
Soc. 1994, 116, 5405. (c) Gandler, J . R.; Bernasconi, C. F. Ibid. 1992,
114, 631. (d) Bernasconi, C. F.; Ni, J . X. Ibid. 1993, 115, 5060.
(43) (a) Marcus, R. A. J . Phys. Chem. 1968, 72, 891. (b) Cohen, A.
O.; Marcus, R. A. Ibid. 1968, 72, 4249.

Deprotonation of Picrylacetophenones
J . Org. Chem., Vol. 61, No. 6, 1996 1985
Ta ble 6. Com p a r ison of In tr in sic Ra te Con sta n ts for
Dep r oton a tion of Som e Rep r esen ta tive Ca r bon Acid s in
50% H2O-50% Me2SO a t 25 °C
CH2CO
NO2
X
H
O2N
(NH4)2Ce(NO3)6
+
, NHEt3
CH acid
base
log k0
–
CH3CN/H2O
RCH(CN)2
RR′NH
RCOO-
ArO-
≈7a
CH3COCH2COCH3
1,3-indandione
3.80b
(4.60)b,c
3.18d
4d
NO2
12: (a) X = NO2; (b) X = H; (c) X = OCH3
RCOO-
ArO-
CH2CO
X
3c
3b
ArO-
2.73e
RCOO-
ArO-
1.55e
O2N
NO2
2.25e
(11)
3a
1d
ArO-
1.80e
RCOO-
ArO-
0.41f
1.75f
NO2
CH3NO2
RR′NH
0.73b,g
(1.5)b,c
-0.60f
0.50f
ArO-
few hours at room temperature (1 h 30 min for 3a , 2 h 30 min
for 3b, and 24 h for 3c). Then, removal of part of the solvent
resulted in a precipitation of crude products which were
collected by filtration and washed with water. Recrystalliza-
tion in a pentane-ethyl acetate mixture (3a , 3b) or a pentane-
methanol mixture (3c) afforded analytical examples of 3a , 3b,
and 3c in 60%, 69.5%, and 40% yields, respectively.
3a : mp 180°C; mass spectrum m/z ) 377 (M + 1)+. Anal.
Calcd for C14H8N4O9: C, 44.72; H, 2.14; N, 14.9. Found: C,
45.07; H, 1.83; N, 14.63.
1b
RCOO-
ArO-
C6H5CH2NO2
RCOO-
ArO-
-0.59b,g
(0.10)b,c
-1.10f
0f
1a
RCOO-
ArO-
In water, ref 1c. T ) 20 °C, ref 38. c Values in parentheses
a
b
are estimated on the basis that log k0 values for both RCOO-
anions and secondary amines are generally lower than those for
3b: mp 135 °C; mass spectrum m/z ) 332 (M + 1)+. Anal.
Calcd for C14H9N3O7: C, 50.8; H, 2.74; N, 12.69. Found: C,
51.43; H, 2.75; N, 12.55.
d
ArO- ions by about 0.8 ( 0.2 log k unit.1c T ) 20 °C, ref 41b.
e This work. f Ref 5b. T ) 20 °C, ref 41c.
g
3c: mp 112°C; mass spectrum m/z ) 362 (M + 1)+. Anal.
Calcd for C15H11N3O8: C, 49.86; H, 3.07; N, 11.64. Found: C,
49.97; H, 3.03; N, 11.61.
exocyclic carbon with only one aromatic rings; i.e., the
conjugation is restricted to one picryl ring at a given
time.5b
Mea su r em en ts. Kinetic measurements were carried out
with a Durrum stopped-flow spectrophotometer equipped with
a thermostated cell compartment (25 °C ( 0.2). Some slow
rates were measured with a conventional UV-vis Kontron-
Uvikon spectrophotometer. All rates were reproducible to
within (5%. pH and pKa determinations in the 50:50 (v/v)
H2O-Me2SO mixture containing 0.5 M NMe4Cl were carried
out at 25 °C using the same procedures as those previously
described.5 A Tacussel Isis 20000 electronic pH meter was ued
for this purpose. The autoprotolysis constant of this solvent
mixture was known from previous studies: pKs ) 15.83 at 25
°C.5
On the other hand, Table 6 shows that the log k0 values
for 3a -c are much higher than those associated with the
ionization of (2,4,4′-trinitrodiphenyl)methane (1a ) and
(2,2′,4,4′-tetranitrodiphenyl)methane (1b ), two com-
pounds which give rise to planar carbanions (C-1a and
C-1b) with a much higher potentiality of delocalization
of negative charge than C-3a -c. Also, the intrinsic
reactivities of 3a --c are greater than those of nitroal-
kanes like nitromethane and phenylnitromethane. In
these two latter compounds, the structural reorganization
required to form the conjugate nitronate carbanions is
reduced compared to that involved in the formation of
C-3a -c but the concentration of charge onto the oxygen
atoms of the adjacent NO2 group is more favorable to
hydrogen-bonding solvation. It is probably this factor
which accounts for the lower log k0 values for ni-
tromethane and phenylnitromethane.41c
1H and 13C NMR spectra were recorded on a Brucker AC300
spectrometer. Chemical shifts are reported in ppm (J values
in Hz) with tetramethylsilane (Me4Si) as the internal refer-
ence.
Mass spectra (EI, 70 eV) were obtained using a NERMAG
R10-10C.
Ack n ow led gm en t. The authors are very indebted
to Mrs. Marie J ose´ Pouet for her help in carrying out
NMR experiments and to Dr. Nicole Sellier for mass
spectra determinations.
Su p p or tin g In for m a tion Ava ila ble: Observed first-order
rate constants, kobsd, for the ionization of picrylacetophenones
3a -c in phenol and carboxylic acid buffer solutions (Table S1-
S4) as well as in tetramethylammonium hydroxide and
hydrochloric acid solutions (Tables S5 and S6) in 50:50 (v/v)
H2O-Me2SO at 25 °C. Variation of the ratio of ionized to
unionized picrylacetophenones 3a -c as a function of pH, effect
of the X substituent in the benzoyl moiety on the acidity of
the picrylacetophenones 3a -c, and Bro¨nsted plots for the
ionization of 3a and 3c by phenoxide ions in 50:50 (v/v) H2O-
Me2SO at 25 °C (12 pages). This material is contained in
libraries on microfiche, immediately follows this article in the
microfilm version of the journal, and can be ordered from the
ACS; see any current masthead page for ordering information.
Exp er im en ta l Section
Ma ter ia ls. Solvents were purified and solutions made up
as described previously.5 Buffers were purified commercial
products.
Picrylacetophenone (3b) and the related p-nitro and p-
methoxy derivatives 3a and 3c were prepared by oxidation of
the σ-adducts 12a -c according to eq 11.18 These adducts were
readily obtained as crystalline triethylammonium salts from
the reaction of 1,3,5-trinitrobenzene with the appropriate
acetophenone in the presence of triethylamine, as described
previously.18
The oxidation of each of the three adducts 12a -c was
carried out as follows: 1 g of the appropriate triethylammo-
nium salt was dissolved in 30 mL of a 2:1 (v/v) H2O-CH3CN
mixture. To this stirred solution was added dropwise a
solution of 2 equiv of ammonium cerium(IV) nitrate in 10 mL
of water. The resulting solutions were allowed to stand for a
J O9518617