Nucleophilic Reactivities of Bis-Acceptor-Substituted Benzyl Anions

Article abstract of DOI:10.1002/ejoc.201601513

The kinetics of the reactions of bis-acceptor-substituted benzyl anions (PhCXY^–, X,Y = CN, CO₂Et, COPh, SO₂Ph) with benzhydrylium ions and quinone methides (reference electrophiles) have been determined in dimethyl sulfoxide solution at 20 °C. The reactions follow second-order kinetics, first order with respect to the electrophile and first order with respect to the carbanion. The addition of 18-crown-6 ether, which efficiently coordinates to the anions' counter ions K⁺, did not affect the kinetics, which indicates that the measured rate constants refer to the reactivities of the nonpaired carbanions. Comparison with the reactivities of the structurally analogous secondary carbanions HCXY^– shows that replacement of H at the carbanionic center by Ph reduces the nucleophilic reactivities towards a reference benzhydrylium ion by factors in the range of only 1.2 (X,Y = SO₂Ph) to 6 (X,Y = CO₂Et). The plots of lg k₂ versus the electrophilicity parameters E of the reference electrophiles are linear, thereby indicating that the correlation lg k₂(20 °C) = s_N(E + N), which characterizes nucleophiles by the two solvent-dependent parameters s_N and N and electrophiles by the parameter E, is applicable. In this way, it becomes possible to integrate these carbanions into our comprehensive nucleophilicity scale, which provides a direct comparison of the nucleophilic reactivities of different families of compounds.

Full text of DOI:10.1002/ejoc.201601513

DOI: 10.1002/ejoc.201601513
Full Paper
Reactivity
Nucleophilic Reactivities of Bis-Acceptor-Substituted Benzyl
Anions
Ángel Puente,[a] Armin R. Ofial,[a] and Herbert Mayr*[a]
Abstract: The kinetics of the reactions of bis-acceptor-substi- anionic center by Ph reduces the nucleophilic reactivities to-
tuted benzyl anions (PhCXY–, X,Y = CN, CO2Et, COPh, SO2Ph) wards a reference benzhydrylium ion by factors in the range of
with benzhydrylium ions and quinone methides (reference elec- only 1.2 (X,Y = SO2Ph) to 6 (X,Y = CO2Et). The plots of lg k2
trophiles) have been determined in dimethyl sulfoxide solution versus the electrophilicity parameters E of the reference electro-
at 20 °C. The reactions follow second-order kinetics, first order philes are linear, thereby indicating that the correlation
with respect to the electrophile and first order with respect to lg k2(20 °C) = sN(E + N), which characterizes nucleophiles by the
the carbanion. The addition of 18-crown-6 ether, which effi- two solvent-dependent parameters sN and N and electrophiles
ciently coordinates to the anions' counter ions K+, did not affect by the parameter E, is applicable. In this way, it becomes possi-
the kinetics, which indicates that the measured rate constants ble to integrate these carbanions into our comprehensive
refer to the reactivities of the nonpaired carbanions. Compari- nucleophilicity scale, which provides a direct comparison of the
son with the reactivities of the structurally analogous secondary nucleophilic reactivities of different families of compounds.
carbanions HCXY– shows that replacement of H at the carb-
Introduction
determinations of heterolysis kinetics without disturbing proton
shifts.
Acceptor-substituted carbanions, commonly generated by the
deprotonation of CH-acidic compounds, are important reagents
in organic synthesis due to their ability to act as nucleophiles
in numerous synthetic transformations.[1] Furthermore, relation-
ships between the rate and equilibrium constants for their for-
mation and for their reactions with electrophiles have contrib-
uted significantly to the development of our present under-
standing of organic reactivity.[2,3] In previous work, we have al-
ready quantified the nucleophilicities of several secondary
anions bearing CN, CO2Et, COPh, SO2R, and NO2 groups,[4,5c]
and demonstrated that their additions to benzhydryl cations
and structurally related quinone methides followed the linear
free-energy relationship given in Equation (1), in which E is an
electrophile-specific parameter and N and sN are solvent-
dependent nucleophile-specific parameters.[5]
Scheme 1. Tertiary benzyl anions 1 studied in this work. [a] Previously re-
ported in ref.[6c]
Table 1. Benzhydrylium ions 2a–e and quinone methides 2f–h employed as
reference electrophiles in this study.
lg k2 (20 °C) = sN(N + E)
(1)
Because only a few examples of tertiary carbanions[6] have
previously been studied by our group, we have now investi-
gated the nucleophilic reactivity of different bis-acceptor-sub-
stituted benzyl anions 1 (Scheme 1). These carbanions are ideal
candidates for mechanistic studies, because the absence of pro-
tons at the α carbon enables equilibrium studies and systematic
[a] Department Chemie, Ludwig-Maximilians-Universität München,
Butenandtstraße 5–13 (Haus F), 81377 München, Germany
E-mail: Herbert.Mayr@cup.uni-muenchen.de
http://www.cup.uni-muenchen.de/oc/mayr/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201601513.
[a] Counter ions for 2a–e: BF4–. [b] From refs.[5b,c,f,j]
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The colored reference electrophiles 2, benzhydrylium ions color of the solution (commonly within 2 min at ambient tem-
and structurally related quinone methides, that served as refer- perature), water was added to precipitate adducts 3. Recrystalli-
ence electrophiles in this work are listed in Table 1.
zation from ethyl acetate/pentane mixtures yielded of 61–70 %
of the pure products 3, which were characterized by NMR spec-
troscopy and mass spectrometry (Scheme 3).
Results
Synthesis of the Potassium Salts of the Carbanions 1
The CH acids [1-(COPh)2]-H, [1-(CN)2]-H, and [1-(CN,SO2Ph)]-H
were prepared by CuI-catalyzed reaction of iodobenzene with
the corresponding bis-acceptor-substituted methane deriva-
tives H2C(Acc)2 in DMSO following a literature procedure.[7] The
bis(sulfone) [1-(SO2Ph)2]-H was obtained by oxidation of the
corresponding dithioacetal.[8] Treatment of the corresponding
CH acids [1]-H with an equimolar amount of potassium tert-
butoxide (KOtBu) in ethanol, subsequent concentration of the
solutions, and washing of the precipitated salts with diethyl
ether furnished good yields of the potassium salts [1]-K
(Scheme 2), which were characterized by NMR techniques.
Scheme 3. Reactions of the potassium methanides [1]-K with the benzhydryl-
ium tetrafluoroborate 2d-BF4. [a] Yield of isolated products after crystalliza-
tion. [b] From ref.[6c]
When the potassium salts [1-(COPh)2]-K and [1-(SO2Ph)2]-K
were combined with the benzhydrylium tetrafluoroborate 2d in
DMSO solution, the blue color of the electrophile disappeared
within a few minutes. The corresponding adducts could not be
isolated by aqueous work-up, however, because in the polar
solvent water a fast retroaddition led to the regeneration of the
starting compounds.
Kinetic Investigations
Scheme 2. Synthesis of the potassium salts [1]-K.
The kinetic investigations of the reactions of the anions 1 with
the reference electrophiles 2 were performed in DMSO solu-
tions at 20 °C and monitored by UV/Vis spectroscopy at the
absorption maxima of the electrophiles (Table 1) using
stopped-flow techniques. Owing to their stability, stock solu-
tions of the potassium salts [1]-K in DMSO could be employed.
As the nucleophiles 1 were employed in large excess (typically
10–100 equiv.) over the electrophiles 2, their concentrations can
be considered to be almost constant throughout the reactions,
resulting in first-order kinetics with an exponential decay of
the concentration of 2. The first-order rate constants kobs were
derived by least-squares fitting of the exponential function
Quantitative deprotonation of the CH acids [1]-H with
1 equivalent of base was confirmed photometrically by adding
solutions of the CH acids [1]-H in DMSO to solutions of potas-
sium tert-butoxide in the same solvent. Addition of further
equivalents of KOtBu did not lead to a further increase in ab-
sorbance of the carbanions. The detected absorption maxima
λmax and molar absorption coefficients (lg εmax) for the potas-
sium methanides [1]-K are summarized in Table 2.
Table 2. Absorption maxima of the potassium methanides [1]-K generated by
deprotonation of the corresponding CH acids [1]-H by KOtBu in DMSO at
20 °C.
obst
At = A0e–k + C to the time-dependent absorbances At of the
[1-(X,Y)]-K
λmax [nm]
lg εmax
electrophiles. Second-order rate constants k2 (Table 3) were ob-
tained as the slopes of the linear correlations of kobs versus
typically five different concentrations of the nucleophiles (Fig-
ure 1).
The coordination of 1,3-dicarbonyl-substituted carbanions to
potassium ions was previously reported to reduce their nucleo-
philic reactivities in DMSO by up to 60 %.[4h] However, because
the first-order rate constants kobs measured in this study in the
presence or absence of 18-crown-6 ether are on the same kobs
versus [1] correlation line (see Figure 1b), one can conclude that
the measured rate constants reflect the reactivities of the free
carbanions.
[1-(COPh)2]-K
[1-(SO2Ph)2]-K
[1-(CN)2]-K
[1-(CN,CO2Et)]-K
[1-(CN,SO2Ph)]-K
258
268
304
308
292
3.84
3.39
4.44
4.37
4.22
Product Studies
Benzhydrylium ion 2d was selected as a representative electro-
phile to examine the course of the kinetically studied reactions.
For this purpose DMSO solutions of the potassium salts [1]-K
were combined with the benzhydrylium tetrafluoroborate 2d at
All the reactions of the carbanions 1 (used in excess) with
room temperature. After the disappearance of the deep-blue the benzhydrylium tetrafluoroborates 2a–e proceeded with
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Figure 1. a) Decay of the absorbance A (at 620 nm) of the benzhydrylium ion 2a during its reaction with 1-(SO2Ph)2 {[1-(SO2Ph)2]0 = 1.51 × 10–4
M, [2a]0 =
–1
7.65 × 10–6
M
, kobs = 1.42 × 102 s–1} in DMSO at 20 °C. b) Determination of the second-order rate constant k2 = 9.69 × 105
M
s–1 for this reaction by plotting
kobs against the concentration of 1-(SO2Ph)2 {empty circles: in the presence of 1.05 equiv. of 18-crown-6 with respect to [1-(SO2Ph)2]-K}.
Table 3. Second-order rate constants for the reactions of the carbanions 1
(counter ion: K+) with the reference electrophiles 2 in DMSO at 20 °C.
versus the concentration of the potassium salts [1]-K were neg-
ligible [exception 1-(CN)2 + 2d], as shown in Figure 1 and the
Supporting Information. Rate constants for the reactions of 1-
(CN)2 with the more electrophilic benzhydrylium ions 2a–c
could not be measured, because these reactions proceeded too
fast to be followed with our stopped-flow instruments.
We have also investigated the kinetics of the reactions of the
carbanions 1-(CO2Et)2[6c] and 1-(CN,CO2Et) with the less electro-
philic quinone methides 2f–h. The consumption of these elec-
trophiles was almost complete and the final concentrations of
the quinone methides 2f–h were negligibly small, leading to
small intercepts in the linear correlations of kobs versus the con-
centration of 1 [except for 1-(CN,CO2Et) + 2g]. In contrast, the
degree of conversion was very small in the reactions of the
phenylmalononitrile anion 1-(CN)2 with the quinone methides
2g and 2h, which impeded the measurement of the corre-
sponding addition rate constants. Full conversion of 2g and 2h
with 1-(CN)2 was achieved, however, when the reactions with
1-(CN)2 were carried out in the presence of triethylammonium
chloride as proton source. The protonation of the initially
formed phenolates
4 favored the consumption of 2g,h
(Scheme 4). The amount of Et3NH+Cl– with respect to the quin-
one methides was optimized to 6 equiv. (Figure 2) with a zero-
order of reaction, which implies that the presence of Et3NH+Cl–
[a] The nucleophile-specific parameters N and sN were determined by correla-
tion analysis using Equation (1) as described below. [b] The listed second-
order rate constants (k2) were obtained from typically five experiments with
different anion concentrations (see the Supporting Information for details).
[c] From ref.[6c] [d] Data obtained in the presence of 6 equiv. of triethylammo-
nium chloride relative to the electrophiles.
complete decolorization of the solutions, that is, quantitative
consumption of the benzhydrylium ions. Accordingly, the inter-
cepts in the plots of the pseudo-first-order rate constants kobs
Scheme 4. Reversible addition of phenylmalonitrile 1-(CN)2 to the quinone
methides 2g,h.
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does not affect the rate of the addition of the anion 1-(CN)2 to (Table 3). The similar values of sN listed in Table 3 (0.72 < sN
<
2g,h (Figure 3). Under these conditions second-order kinetics 1.04), reflected by the almost parallel correlation lines in Fig-
with complete disappearance of the quinone methides oc- ure 4, imply that the relative reactivities of these anions depend
curred.
only slightly on the nature of the electrophiles.
Figure 2. Decay of the absorbance A of the quinone methide 2g at 422 nm
during its reaction with [1-(CN)2]-K in DMSO at 20 °C in the presence of
variable concentrations of Et3NH+Cl– {[1-(CN)2]0 = 6.16 × 10–4
M, [2g]0 =
1.51 × 10–5
M}.
Figure 4. Correlation of the rate constants lg k2 for the reactions of the
benzylic anions 1 with the electrophiles 2 in DMSO at 20 °C with the corre-
sponding electrophilicity parameters E. [a] With rate constants from ref.[6c]
Benzhydrylium ion 2e is the electrophile for which most di-
rectly measured rate constants are available. For that reason,
we selected this electrophile to compare the reactivities of the
–
tertiary carbanions 1 [PhC(Acc)2 ] with those of the correspond-
–
ing secondary carbanions 6 [HC(Acc)2 ]. Figure 5 shows that the
reactivities all lie within a narrow range, and the rate constants
k2 for the reactions with 2e increase from the disulfonyl-substi-
tuted benzyl anion 1-(SO2Ph)2 (least reactive) to the diethyl
malonate anion 6-(CO2Et)2 as the most reactive species by a
factor of only 330. Replacement of the proton at the carban-
ionic center in 6-(X,Y) by phenyl to give 1-(X,Y) has an astonish-
ingly small effect. Thus, the disulfonyl-substituted methyl anion
6-(SO2Ph)2 and the corresponding benzyl anion 1-(SO2Ph)2
have almost equal reactivities. Replacement of the proton at
the carbanionic center by phenyl reduces the reactivities of the
cyano-stabilized carbanions by factors of 1.3–3, and in the case
of benzoyl- and ethoxycarbonyl-substituted carbanions by fac-
tors of 4–6. Although the phenyl effects are generally small, the
largest effects are found for the carbonyl-substituted systems,
which are stabilized by π conjugation, and the differences dis-
appear for the pyramidal sulfonyl-stabilized carbanions, which
are stabilized by polarization.
Figure 3. Dependence of the observed first-order rate constant kobs on the
concentration of added Et3NH+Cl– in the reaction of [1-(CN)2]-K with 2g
{[1-(CN)2]0 = 6.16 × 10–4 , [2g]0 = 1.51 × 10–5 , [Et3NH+Cl–]0/[2g]0 = 3–111}.
M M
As previously discussed,[6c] the correlation between the nu-
cleophilic reactivities of carbanions and the corresponding pKaH
values in DMSO is generally rather poor.[9] Figure 6 shows that
Correlation Analysis and Discussion
To determine the nucleophilicity parameters N and sN of the the large difference in the pKaH values of the malononitrile
carbanions 1, values of lg k2 for their reactions with the refer- anions 1-(CN)2 and 6-(CN)2 (ΔpKaH = 6.8) is not reflected by the
ence electrophiles 2 were plotted against the corresponding relative nucleophilic reactivities towards carbocation 2e, which
electrophilicity parameters E. As shown for some representative differ only by a factor 3.4. The comparison of the benzylic
examples in Figure 4, and for all the reactions in the Supporting anions 1-(CN)2 and 1-(CO2Et)2 is even more striking: They have
Information, linear correlations of lg k2 with E were found, almost the same reactivity towards 2e but differ in pKaH by 12
which indicates that Equation (1) is applicable. From the slopes orders of magnitude. On the other hand, the diethyl malonate
of these correlations, the nucleophile-specific parameters sN 6-(CO2Et)2 and the phenyl-substituted diethyl malonate anion
were derived, and the negative intercepts on the abscissa 1-(CO2Et)2, which differ by a factor of 6 in nucleophilic reactivity
(lg k2 = 0) correspond to the nucleophilicity parameters N towards 2e, have equal pKaH values.
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the nucleophilic reactivities of carbanions. We have now found,
however, that the nucleophilicities of carbanions stabilized by
two acceptor groups (CN, CO2Et, COPh, SO2Ph) are barely re-
duced by an additional phenyl group. Although steric effects
can be expected to augment the electronic effect by also reduc-
ing the nucleophilic reactivities of the carbanions [PhC(Acc)2]–
(1) relative to their secondary analogues [HC(Acc)2]– (6), we
have now found that both classes of carbanions have almost
the same nucleophilic reactivities. It should be noted, however,
that this analysis refers to reactions with an electrophile of E =
–10 and that the relative reactivities of two nucleophiles de-
pend on the nature of the electrophile if they differ in sN.
To compare nucleophiles of widely differing reactivity, we
usually neglect sN and consider the magnitudes of N, as shown
in Figure 7. The nucleophilicity parameter N reflects relative re-
activities towards an electrophile, which reacts with an average
rate constant of 1 M
–1 s–1 (lg k = 0) with the nucleophiles under
consideration (floating scale of reference electrophiles).[11] In
this way, a rough estimation of relative reactivities over a wide
range of reactivity can be obtained.
Figure 5. Second-order rate constants lg k2 for the reactions of the tertiary
benzyl anions 1 (right) and the structurally related secondary anions 6 (left)
with the benzhydrylium ion 2e in DMSO at 20 °C. [a] Rate constants from
refs.[4h,5c,6d] [b] Rate constants from Table 3. [c] The kinetic data in ref.[4a] give
N = 16.73 and sN = 0.66 for 6-(CN,SO2Ph), which were used to calculate lg k2
for the reaction with 2e by using Equation (1).
Figure 6. Brønsted correlation for the reactions of the acceptor-substituted
carbanions 1, 6, and 7 with the benzhydrylium ion 2e (DMSO, 20 °C, pKaH
from ref.[10]); for references and further details see the Supporting Informa-
tion.
Figure 7. Structure–reactivity relationships for benzyl anions 7, secondary
anions 6, and tertiary anions 1, including their nucleophilicity parameters N
(and sN). [a] From ref.[4h] [b] From ref.[4g] [c] From ref.[5c] [d] From ref.[6d]
[e] From ref.[6c]
Conclusions
Phenyl groups are known to stabilize charge by the mesomeric
Figure 7 illustrates the limitations of the neglect of sN, how-
effect. Thus, phenyl substitution at the reaction center generally ever. Owing to the similar magnitude of their sN values, carb-
reduces the electrophilic reactivities of carbocations as well as anions 1-(SO2Ph)2 and 6-(SO2Ph)2, which differ by only one unit
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available ethyl 2-cyano-2-phenylacetate [1-(CN,CO2Et)]-H (1.0 mL,
5.8 mmol) and KOtBu (628 mg, 5.60 mmol). Yield: 1.19 g (93 %);
white solid. H NMR (400 MHz, [D6]DMSO): δ = 7.55 (d, J = 7.4 Hz,
2 H, Ar), 6.99–6.95 (m, 2 H, Ar), 6.55–6.51 (m, 1 H, Ar), 3.89 (q, J =
7.1 Hz, 2 H, OCH2), 1.12 (t, J = 7.1 Hz, 3 H, CH3) ppm. 13C NMR
(101 MHz, [D6]DMSO): δ = 167.5 (C, CO2), 141.8 (C, Ar), 127.6 (C, CN),
127.2 (CH, Ar), 120.9 (CH, Ar), 117.3 (CH, Ar), 55.8 (CH2, OCH2CH3),
54.6 (C, C–), 15.5 (CH3, OCH2CH3) ppm.
in N (Figure 7), also react with almost the same rates with elec-
trophile 2e (Figure 5). Carbanions 1-(CO2Et)2 and 6-(CO2Et)2, on
the other hand, which differ by four units of N according to
Figure 7, differ in the rates of their reactions with 2e by only a
factor of 6 because of the different magnitude of sN. Despite
these deviations, in light of the fact that the N scale presently
covers almost 40 orders of magnitude,[5g–5i,12] inspection of the
N scale gives valuable first information about the relative react-
ivities of different nucleophiles, which has to be refined by sN
if more accurate comparisons are needed.
1
Potassium
Cyano(phenyl)(phenylsulfonyl)methanide
[1-
(CN,SO2Ph)]-K: Obtained according to GP A from 2-phenyl-2-
(phenylsulfonyl)acetonitrile [1-(CN,SO2Ph)]-H (950 mg, 3.69 mmol)
and KOtBu (394 mg, 3.51 mmol). Yield: 944 mg (91 %); white solid.
1H NMR (400 MHz, [D6]DMSO): δ = 7.74–7.72 (m, 2 H, Ar), 7.44–7.39
(m, 3 H, Ar), 7.09–7.07 (m, 2 H, Ar), 6.98–6.94 (m, 2 H, Ar), 6.56–6.52
(m, 1 H, Ar) ppm. 13C NMR (101 MHz, [D6]DMSO): δ = 148.0 (C, Ar),
138.5 (C, Ar), 130.0 (CH, Ar), 128.3 (CH, Ar), 127.7 (CH, Ar), 125.2 (C,
CN), 124.8 (CH, Ar), 119.5 (CH, Ar), 117.9 (CH, Ar), 59.9 (C, C–) ppm.
13C NMR spectroscopic data agree with those reported for [1-
(CN,SO2Ph)]-Na in DMSO.[13]
Experimental Section
Materials: All solvents were of p.a. quality. Unless otherwise speci-
fied, materials were obtained from commercial sources and used
without purification. The reference electrophiles (benzhydrylium
tetrafluoroborates 2a–e and quinone methides 2f–h) used in this
work were prepared as described before.[5c,5f] Bis(phenylsulfonyl)-
methane was prepared by oxidation of bis(phenylthio)methane
with hydrogen peroxide.[8b] Procedures for the synthesis of the CH
acids [1]-H that are not commercially available are given in the
Supporting Information.
Reactions of Highly Stabilized Carbanions 1 with 2d: A solution
of 2d-BF4 (1.00 equiv.) in DMSO (5 mL) was added to a solution of
[1]-K (1.05 equiv.) in DMSO (5 mL). After the disappearance of the
deep-blue color (usually within 2–5 min), H2O (5 mL) was added
and a precipitate formed. The mixture was filtered and the collected
precipitate was redissolved in CH2Cl2. The solution was washed with
water (2 × 10 mL), dried with MgSO4, and evaporated under re-
duced pressure to afford a crude product, which was recrystallized
in EtOAc/n-pentane mixtures. See the Supporting Information for
details of the product characterizations of 3-(CN)2, 3-(CN,CO2Et),
and 3-(CN,SO2Ph).
General Procedure for the Synthesis of Potassium Salts 1-K
(GP A): In a dried round-bottomed Schlenk flask, the corresponding
CH acid [1]-H (1.00 equiv.) was added to a solution of KOtBu
(0.95 equiv.) in ethanol (10 mL) under argon. The resulting solution
was stirred at room temperature for 30 min and then concentrated
under reduced pressure to give a solid residue, which was further
washed with freshly distilled Et2O (3 × 10 mL) under argon. The
solid residue was dried under vacuum for an additional 2 h to afford
the corresponding potassium salt [1]-K, which was stored in a glove
box under argon.
Determination of Rate Constants: The rates of all investigated
reactions were determined photometrically. For the evaluation of
the kinetics, stopped-flow spectrophotometer systems (Hi-Tech SF-
61DX2 or Applied Photophysics SX.18MV-R) were used. All kinetic
measurements were carried out in DMSO (Acros Organics, H2O con-
tent <50 ppm) under exclusion of moisture. The temperature of the
solutions during all kinetic studies was kept constant (20.0 0.1 °C)
by using a circulating bath thermostat. The nucleophiles were al-
ways employed as the major component (typically at least
10 equiv.) in the reactions with the electrophiles to ensure first-
order conditions with kobs = k2[Nu]0 + k0. In a few cases, plots of
Potassium 1,3-Dioxo-1,2,3-triphenylpropan-2-ide [1-(COPh)2]-K:
Obtained according to GP A from 1,2,3-triphenylpropane-1,3-dione
[1-(COPh)2]-H (500 mg, 1.66 mmol) and KOtBu (180 mg, 1.60 mmol).
Yield: 223 mg (41 %); white solid. 1H NMR (400 MHz, [D6]DMSO):
δ = 8.06–8.04 (m, 2 H, Ar), 7.85–7.83 (m, 3 H, Ar), 7.66–7.62 (m, 1 H,
Ar), 7.55–7.51 (m, 2 H, Ar), 7.42–7.31 (m, 2 H, Ar), 7.27–7.24 (m, 5 H,
Ar) ppm. 13C NMR (101 MHz, [D6]DMSO): δ = 195.0 (CO), 168.4 (Ar),
135.5 (Ar), 133.7 (Ar), 130.0 (Ar), 129.0 (Ar), 128.9 (Ar), 128.5 (Ar),
127.0 (Ar), 61.3 (C–) ppm.
kobs versus [Nu]0 showed negative intercepts, which were explained
by the presence of unknown contaminations. Pseudo-first rate con-
stants kobs [s–1] were obtained by fitting the monoexponential func-
tion At = A0exp(–kobst) + C to the observed time-dependent absorb-
ance (average from 10 kinetic runs for each nucleophile concentra-
Potassium Phenyl[bis(phenylsulfonyl)]methanide [1-(SO2Ph)2]-
K: Obtained according to GP A from (phenylmethylenedisulfonyl)di-
benzene [1-(SO2Ph)2]-H (550 mg, 1.48 mmol) and KOtBu (157 mg,
1.40 mmol). Yield: 412 mg (72 %); white solid. 1H NMR (400 MHz, tion). To obtain the second-order rate constants k2
[M
–1 s–1], each
[D6]DMSO): δ = 7.81–7.80 (m, 4 H, Ar), 7.39 (br. s, 6 H, Ar), 7.05–7.04
electrophile/nucleophile combination was measured at three to five
different concentrations of the nucleophile. N and sN parameters for
(m, 4 H, Ar), 6.93 (br. s, 1 H, Ar) ppm. 13C NMR (101 MHz, [D6]DMSO):
δ = 150.6 (Ar), 137.4 (Ar), 132.3 (Ar), 129.2 (Ar), 127.8 (Ar), 126.7 (Ar), benzylic carbanions 1 were determined from Equation (1).
125.7 (Ar), 123.5 (Ar), 76.4 (C–) ppm.
Supporting Information (see footnote on the first page of this
article): Synthetic procedures, product characterization, detailed
kinetic data, and NMR spectra of new compounds.
Potassium Dicyano(phenyl)methanide [1-(CN)2]-K: Obtained ac-
cording to GP A from 2-phenylmalononitrile [1-(CN)2]-H (1.00 g,
7.03 mmol) and KOtBu (750 mg, 6.68 mmol). Yield: 1.03 g (85 %);
white solid. 1H NMR (300 MHz, [D6]DMSO): δ = 7.03–6.97 (m, 2 H,
Ar), 6.74–6.70 (m, 2 H, Ar), 6.52–6.47 (m, 1 H, Ar) ppm. 13C NMR
(75.5 MHz, [D6]DMSO): δ = 141.4 (C, Ar), 128.1 (CH, Ar), 126.3 (C,
CN), 118.0 (CH, Ar), 116.8 (CH, Ar), 27.2 (C, C–) ppm. 13C NMR spec-
troscopic data agree with those reported for [1-(CN)2]-Na in
DMSO.[13]
Acknowledgments
We thank the Deutsche Forschungsgemeinschaft (SFB 749,
Project B1) for support of this work, Nathalie Hampel for the
synthesis of the reference electrophiles, and Dr. Francisco Cor-
ral-Bautista for his help during the preparation of this manu-
script. Á. P. thanks the Fundación Ikerbasque and the Basque
Potassium 1-Cyano-2-ethoxy-2-oxo-1-phenylethan-1-ide [1-
(CN,CO2Et)-K]: Obtained according to GP A from commercially Government for a postdoctoral fellowship.
Eur. J. Org. Chem. 2017, 1196–1202
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Received: November 29, 2016
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