Nucleophilic Reactivities of Bleach Reagents

Article abstract of DOI:10.1021/acs.orglett.8b00645

The nucleophilicities of the oxidants hydroperoxide, hypochlorite, hypobromite, bromite, and peroxymonosulfate were determined by following the kinetics of their reactions with a series of benzhydrylium ions of known electrophilicity (E) in alkaline, aque

Full text of DOI:10.1021/acs.orglett.8b00645

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nucleophilic Reactivities of Bleach Reagents
Robert J. Mayer and Armin R. Ofial*
Department Chemie, Ludwig-Maximilians-Universitat Munchen, Butenandtstraße 5−13, 81377 Munchen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: The nucleophilicities of the oxidants hydroperoxide, hypochlor-
ite, hypobromite, bromite, and peroxymonosulfate were determined by
following the kinetics of their reactions with a series of benzhydrylium ions
of known electrophilicity (E) in alkaline, aqueous solutions at 20 °C. The
reactivities of the oxidants correlate only weakly with their basicities. Analyzing
the rate constants by using the relationship log k2 = sN(N + E) gave the
parameters N (and sN), which were applied to predict the rates of Weitz−Scheffer epoxidations.
ydrogen peroxide, hypochlorite, or persulfates are low-
H
cost oxygen-nucleophiles and are often the oxidants of
choice for industrial applications. They are used for the transfer
of oxygen atoms in organic synthesis1 and also at large scale for
the bleaching of paper pulp and textiles, in disinfectants, or as
ingredients in cosmetics.2 It is generally recognized that the
hydroperoxide anion, hypochlorite, or persulfate are nucleo-
philic species that undergo analogous reactions with organic
electrophiles. Although several of these reactions had been
studied kinetically3 and reactions of bleach reagents with
cationic organic food dyes are used in tutorials for introducing
kinetic methods to students,4 a direct comparison of reactivities
of different oxygen-transfer reagents is not available to date.
We now set out to investigate the nucleophilic reactivity of
the hydroperoxide anion (1, originating from H2O2−urea,
H2O2−polyvinylpyrrolidone, or H2O2−sodium carbonate com-
plexes), hypochlorite (2), hypobromite (3), peroxymonosulfate
(4), and bromite (5) in alkaline, aqueous solution by following
the kinetics of their reactions with the benzhydrylium ions 6a−
6e as reference electrophiles (Figure 1). The second-order rate
constants k2(20 °C) of these reactions and the known
electrophilicities E of 6a−6e5 were then applied in the linear
free-energy relationship (see eq 1)6 to derive the nucleophilicity
parameters N and the susceptibilities sN of the oxidants 1−5.
Figure 1. Nucleophiles and reference electrophiles included in this
study. Electrophilicity (E) values are taken from ref 5.
bis(benzhydryl)peroxide 8 was detected, which can be
rationalized by the subsequent reaction of the initially formed
benzhydryl hydroperoxide 7 with a second equivalent of 6a.
Isolating the precipitate by filtration of the aqueous reaction
mixture gave 8 in a yield of 58% (see Scheme 1).
As in the solid-state structure of bis(triphenylmethyl)-
peroxide,9,10 8 has a center of inversion in its crystal structure
and a C−O−O−C dihedral angle of 180°. Steric bulk around
the COOC units is compensated differently in 8 and
Ph3COOCPh3, however: bis(triphenylmethyl)peroxide has a
“normal” O−O bond length (148.0 pm),9,11 but elongated C−
O (146.1 pm) and C−Car distances (153.3 pm). In contrast,
the solid-state structure of 8 is characterized by normal C−O
(140.6 pm) and C−Car bond lengths (152.5 pm), but an
unusually long O−O bond (151.3 pm), which is significantly
longer than the usually observed O−O bond lengths in other
dialkyl peroxides (145−146 pm).10,11 Packing effects may
log k2(20°C) = sN(N + E)
(1)
Thus, the anionic oxygen nucleophiles studied in this work
can be integrated into Mayr’s comprehensive nucleophilicity
scale,7 which enables the direct comparison of the nucleophiles
1−5 with previously investigated organic peroxide anions8 and
more than 1000 other nucleophilic species whose reactivities
were investigated by the benzhydrylium method.6c
We observed that the decoloration of the deeply blue
aqueous solution of 6a upon treatment with an alkaline solution
of hydrogen peroxide (2 equiv H2O2, from 1UHP) was
accompanied by the formation of a colorless precipitate.
Concentration of a dichloromethane extract of the reaction
mixture yielded colorless needles (mp 137 °C), which were
analyzed by single-crystal X-ray diffraction (XRD). Instead of
the expected 1:1-adduct, benzhydryl hydroperoxide 7, the
Received: February 22, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00645
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
exponential function At = A0 exp(−kobst) + C to the decaying
absorbances.
Scheme 1. Bis(benzhydryl)peroxide 8 from the Reaction of
6a with Hydrogen Peroxide in Alkaline Solution
As expressed by eq 2 and Figure 2, the observed first-order
rate constants kobs reflect the reactions of cations 6 with water,
hydroxide ions (HO−), and the anions of the bleach reagents
(XO−).
kobs = kW + kOH[HO−] + k2[XO−]
(2)
The basicities of the investigated anions (pKaH in Table 1)13
were used to calculate the concentrations [OH−] and [XO−].
Rate constants for the reactions of the cations 6a−6e with
water (kW) and hydroxide ions (kOH) had previously been
reported.3h Thus, the second-order rate constants k2 (Table 1)
for the attack of the nucleophiles 1−5 at the benzhydrylium
ions 6b−6e were obtained from the slopes of the linear
correlations of k1 (= kobs − kOH[HO−]) with the nucleophile
concentrations (Figure 2C). Table 1 lists the nucleophilicity
parameters for 1 (generated from different H2O2 formulations),
along with the N and sN values for anions 2−5. The linear
correlations of the second-order rate constants k2 with the
electrophilicity E of the benzhydrylium ions (Figure 3)
furnished the nucleophilicity parameter N of the anions 1−5
as the intercept with the abscissa (equal to −E at log k2 = 0).
The individual susceptibilities of the nucleophiles toward
changes in the electrophiles’ reactivity are reflected by the
different slopes (sN) of the log k2 vs E correlations.
aMolecular structure of 8 in the crystal (thermal ellipsoids are drawn at
a 50% probability level at T = 100 K).
account for these structural differences, because the solid-state
structure of 8 shows motifs of intermolecular π−π stacking,
which are not found in the crystal structure of bis-
(triphenylmethyl)peroxide [see the Supporting Information
(SI)].
The kinetics of the reactions of 1−5 with the benzhydrylium
ions 6b−6e as reference electrophiles were measured in
alkaline, aqueous solution by using the previously established
procedure (see Figure 2).8,12 In all kinetic experiments,
Table 1 shows that the nucleophilic reactivity of HOO− (1)
toward the reference electrophiles 6 is almost independent of
the source of the reactant: hydrogen peroxide anions liberated
from H2O2−urea (UHP), H2O2−polyvinylpyrrolidone (PVP),
or sodium percarbonate (SPC) are only slightly less reactive
than HOO− generated from aqueous hydrogen peroxide, for
which N = 15.40 (sN = 0.55) was reported in ref 3h.
−
Bromite (BrO2 , 5) is significantly less reactive than
hypobromite (BrO−, 3). Depending on the electrophilic
reaction partner, the rate constants for the reactions of 6
with 5 are lower by a factor 10−20 than the k2 values for the
reactions of 3 with the same set of electrophiles. The relative
−
reactivities thus follow the order of basicities because BrO2
(pKaH = 3.43) is a weaker base than BrO− (pKaH = 8.8). In
contrast, the reactivities of 1−4 toward electrophile 6c (Table
1) are almost identical, although their basicities (pKaH) differ by
4 orders of magnitude.14 Figure 4 is, thereby, giving another
example for the fact that the Brønsted basicities cannot be used
as a guideline for predicting the reactivities of nucleophiles,
even when the reacting atom remains the same as in the bleach
reagents XO−.
Figure 2. (A) Reaction of benzhydrylium ions 6 with nucleophiles in
alkaline, aqueous solution of bleach reagents XOH. (B) Absorbance A
(at 610 nm) vs time for the reaction of 1UHP (c = 2.46 × 10−4 M) with
6b (c = 1.05 × 10−5 M) at 20 °C. (C) The slope of the linear plot of
the first-order rate constant k1 (= kobs − kOH[HO−]) versus
nucleophile concentration was used to derive the second-order rate
constant k2 for the attack of the anion 1 at the benzhydrylium ion 6b.
How can the weak correlation of basicity and reactivity (r2 =
0.4614) in the series HOO−, ClO−, BrO−, [SO5]2− be
explained? It has recently been shown that early transition
states in reactions of the anions of organic peroxides ROO−
with benzhydrylium ions put desolvation of the nucleophile
into a key position for understanding the counterintuitive
nucleophilicity ordering.8 It is likely that analogous solvation
effects also control the nucleophilic reactivities of anions 1−4 in
aqueous solution.15
To assess the applicability of N and sN of 1−5, we next
studied the kinetics of the reactions of Malachite Green (MG)
with 1, 2, and hydroxide (see Table 2). The second-order rate
constants that we determined experimentally agreed within a
factor of 10 with those predicted by applying eq 1, the
electrophilicity parameter for MG (E = −10.29, from ref 3h),
and the nucleophile-dependent parameters N and sN from
acetonitrile [<0.5% (v/v)] was used as a co-solvent to solubilize
the benzhydrylium tetrafluoroborates. Applying a high excess of
the anions 1−5 over the electrophiles 6b−6e caused the
nucleophile concentrations to remain almost constant during
the kinetic runs. As [1]0 ≫ [6]0, electrophiles 6 were
quantitatively consumed by the excess of HOO− ions under
the conditions of the kinetic experiments, and the subsequent
reaction of 7 with 6 could be neglected in the kinetic analysis.
We followed the progress of the reactions by time-resolved
stopped-flow UV/vis photometry, which showed a mono-
exponential decay of the absorption of the electrophile in all
investigated reactions (Figure 2B). First-order rate constants
k
obs (s−1) were obtained from least-squares fitting of the single-
B
DOI: 10.1021/acs.orglett.8b00645
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Table 1. Basicities (pKaH) and Second-Order Rate Constants k2 for the Reactions of the Nucleophiles 1−5 with Reference
Electrophiles 6a−6e in Alkaline, Aqueous Solution at 20 °C
k2 (M−1 s−1
)
a
XO−
pKaH
6e
6d
6c
6b
N (sN)
1UHP
1PVP
1SPC
2
11.75
11.75
11.75
7.53
8.8
8.18 × 102
6.94 × 102
6.84 × 102
5.00 × 102
1.09 × 103
5.01 × 102
4.67 × 101
1.28 × 103
1.11 × 103
1.00 × 103
6.71 × 102
1.94 × 103
6.53 × 102
6.63 × 101
3.89 × 103
3.23 × 103
2.96 × 103
2.03 × 103
4.05 × 103
2.79 × 103
2.72 × 102
1.49 × 104
1.45 × 104
1.15 × 104
1.03 × 104
1.27 × 104
1.06 × 104
9.71 × 102
15.20 (0.55)
14.86 (0.58)
15.16 (0.54)
14.50 (0.58)
16.69 (0.46)
14.41 (0.60)
12.75 (0.59)
3
4
9.4
5
3.43
a
Basicities pKaH are taken from ref 13.
Scheffer-Weitz epoxidations of arylidenemalononitriles (9)
with various oxygen-transfer reagents had been described
previously.8,17 We used these reactions to test whether the
nucleophilicity parameters N (and sN) for 1−5 (Table 1) also
hold for the reactions of 1−5 with neutral electrophiles and,
therefore, determined the rate constants for the nucleophilic
epoxidations of 9a−9c (see Table 3).
Table 3. Second-Order Rate Constants k2 for the Reactions
of the Oxygen Nucleophiles 1−5 with the Neutral
Electrophiles 9a−c in Alkaline, Aqueous Solution at 20 °C
Figure 3. Plot of log k2 (Table 1) for the reactions of anions 1UHP, 2, 3,
and 5 with electrophiles 6b−6e in alkaline aqueous solution at 20 °C
versus the electrophilicity parameters E of 6b−6e (see the SI for
individual plots for all nucleophiles).
Rate Constants (M−1 s−1
)
a
b
nucleophile
9 (E)
ke2xp
kc2alcd
ke2xp/kc2alcd
HOO− (1UHP
)
9a (−9.42)
9b (−10.80)
9c (−13.30)
3.92 × 104
7.97 × 103
4.25 × 102
1.51 × 103
2.63 × 102
1.11 × 101
26
30
38
ClO− (2)
BrO− (3)
[SO5]2− (4)
9a (−9.42)
9b (−10.80)
6.63 × 103
1.39 × 103
8.84 × 102
1.40 × 102
7.5
9.9
9a (−9.42)
9b (−10.80)
1.10 × 104
2.40 × 103
2.21 × 103
5.12 × 102
5.0
4.7
Figure 4. Correlation of the reactivities of the nucleophiles 1−5
(toward 6c) with their basicities (pKaH).
9a (−9.42)
9b (−10.80)
7.99 × 103
1.49 × 103
9.86 × 102
1.47 × 102
5.4
10
Table 2. Second-Order Rate Constants k2 for the Reactions
of MG with Nucleophiles in Aqueous Solution (at 20 °C)
Rate Constants (M−1 s−1
)
2.91 × 101
b
9.35 × 101
0.31
−
BrO2 (5)
9a (−9.42)
a
XO−
HO−
HOO− (1)
ClO− (2)
kt2his work
kc2alcd
kl2it
a
Electrophilicities E from ref 18. Second-order rate constant k2 by
applying eq 1, E of 9 (this table) and N and sN of 1−5 (from Table 1).
b
9.25 × 10−1
1.13 × 103
2.59 × 103
1.25
2.18 (25 °C)
c
6.46 × 102
2.76 × 102
1.3 × 104 (30 °C)
5.0 × 101 (30 °C)
1.0 × 103 (23 °C)
c
Table 3 shows that the predicted rate constants (kc2alcd) for
the epoxidations of the electron-deficient π-systems of 9 with
the nucleophiles 2−5 match the experimentally obtained rate
constants (k2exp) with a maximum deviation of a factor of 10, in
the case of the hydroperoxide anion (1UHP) within a factor of
40. Hence, all determined rate constants for the epoxidations of
9 by 1−5 are within the usual prediction accuracy of eq 1 (that
is, an error in the predicted rate constant within a factor of 10−
100).6b,c For 2−5, the small deviations between measured
(reactions with 9a and 9b) and predicted rate constants (based
on reactions with benzhydrylium ions 6) suggest that analogous
reaction mechanisms are operating, in which the first C−O
d
a
Calculated based on eq 1, the electrophilicity of MG (E = −10.29,
from ref 3d) and the N (and sN) parameters in Table 1 (for 1, 2) and
b
c
ref 3d (for OH−). Data taken from ref 16. Data taken from ref 3c.
d
Data taken from ref 3d.
Table 1. The significantly lower reactivity of ClO− (2) toward
MG, compared to HOO− (1), described by Dixon and Bruice,3c
was not observed in our measurements, which agreed better
with Ritchie’s data.3d
C
DOI: 10.1021/acs.orglett.8b00645
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
bond formation between nucleophile and electrophile (e.g., the
formation of 10) is rate-determining.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ofial@lmu.de
ORCID
Because of the different susceptibility of BrO− (3, sN = 0.46),
the relative reactivity of 3, if compared with other oxidants in
Figure 5, will be dependent on the choice of the electrophile.
Armin R. Ofial: 0000-0002-9600-2793
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Nathalie Hampel (LMU, preparation of 6 and 9), Dr.
Peter Mayer (LMU, X-ray crystallographic analyses of 8 and
[Ba(BrO2)2·(H2O)]), and Professor Herbert Mayr (LMU) for
valuable support. The authors are grateful to Dr. Jer
and Dr. Stephane Sabelle (L’Oreal) for helpful discussions.
Financial support by L’Oreal Research & Innovation (Aulnay
́ ̂
ome Gomar
́
́
́
Figure 5. Comparison of nucleophilicity parameters N of anions 1−5
with those of other peroxide anions (from ref 8). [Superscript “a”
denotes data taken from ref 3h.]
sous Bois) and the Deutsche Forschungsgemeinschaft (SFB
749, project B1) is gratefully acknowledged.
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In conclusion, the reactivity parameters of the oxidants 1−5
(in H2O) derived in this work from the rates of their reactions
with benzhydrylium ions may be used for the design of oxygen-
transfer reactions toward various types of C-centered electro-
philes (e.g., Michael acceptors).19 The comparable reactivities
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interchanging these bleach reagents will not change the
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00645.
Synthetic procedures, analytical data, X-ray structure
determinations, details of the kinetic measurements, and
copies of NMR spectra (PDF)
Accession Codes
(9) Glidewell, C.; Liles, D. C.; Walton, D. J.; Sheldrick, G. M. Acta
Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1979, 35, 500−502.
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Peroxides; Patai, S.; Ed.; Wiley: Chichester, U.K., 1983; pp 382.
(11) Hartung, J.; Svoboda, L. In The Chemistry of Peroxides, Vol. 2;
Rappoport, Z., Ed.; Wiley: Hoboken, NJ, 2006; Chapter 2, pp 93−144.
(12) For details on the preparation of the nucleophiles and the
kinetic experiments, see the Supporting Information. Bromite ions (5)
CCDC 1819620−1819621 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
D
DOI: 10.1021/acs.orglett.8b00645
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Letter
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E
DOI: 10.1021/acs.orglett.8b00645
Org. Lett. XXXX, XXX, XXX−XXX