Oxidation of Aromatic Aldehydes to Esters: A Sulfate Radical Redox System

Article abstract of DOI:10.1021/acs.joc.6b02775

A mild oxidative esterification of various aromatic aldehydes by sulfate radical redox system was presented. In the reaction pathway exploration, the transiency of MeOSO₃^- was disclosed, which was generated from esterification between the in situ generated HSO₄^- and MeOH, a rate-limiting step in the process. More importantly, the selectivity-controlling step was represented by the subsequent nucleophilic displacement between MeOSO₃^- and aldehydes. The ionic oxidant 1a ((NH₄)₂S₂O₈) with more N-H numbers in the cation, as compared with 1c ((n-Bu₄N)₂S₂O₈) and 1d ((PyH)₂S₂O₈), has better performance in the oxidative esterification of aldehydes.

Full text of DOI:10.1021/acs.joc.6b02775

Article
pubs.acs.org/joc
Oxidation of Aromatic Aldehydes to Esters: A Sulfate Radical Redox
System
Ya-Fei Guo,†,‡ Sajid Mahmood,†,‡ Bao-Hua Xu,*,† Xiao-Qian Yao,∥,† Hong-Yan He,∥,†
and Suo-Jiang Zhang*,†
†Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of
Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
‡College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, PR China
S
* Supporting Information
ABSTRACT: A mild oxidative esterification of various aromatic
aldehydes by sulfate radical redox system was presented. In the reaction
−
pathway exploration, the transiency of MeOSO3 was disclosed, which
−
was generated from esterification between the in situ generated HSO4
and MeOH, a rate-limiting step in the process. More importantly, the
selectivity-controlling step was represented by the subsequent
−
nucleophilic displacement between MeOSO3 and aldehydes. The
ionic oxidant 1a ((NH4)2S2O8) with more N−H numbers in the cation,
as compared with 1c ((n-Bu4N)2S2O8) and 1d ((PyH)2S2O8), has better
performance in the oxidative esterification of aldehydes.
Sulfate radical redox systems are involved in many oxidation
and electron transfer processes,13,14 which herein are used to
INTRODUCTION
■
The significance and omnipresence of carboxylic esters in
organic chemistry, pharmaceutical chemistry, and materials
science has forced scientists to develop efficient methods for
their preparation.1 Esters are generally prepared via activation
of acids or their carboxylic derivatives followed by nucleophilic
substitution.2 Instead of such two-steps operation containing
oxidation and C−O bond formation, direct oxidative
esterification of aldehydes with alcohols is a conceptually and
economically attractive approach and has received increasing
attention.3 Both carbonyl activation4−8 and direct formyl C−H
activation strategies9 have been extensively investigated.
However, challenges still remain, especially in the case of
radical oxidative cross-coupling. One of the key issues is the
relatively weak C−H bond in the formyl group of aldehydes is
sensitive to electron transfer reagents and the undesired
decarboxylation often happens beyond the dehydrogenative
C−O cross coupling.10 Moreover, making the situation worse,
alkanols are easily transformed to the corresponding aldehydes
(further to acids) or ketones through radical mechanisms.11 Up
to now, the expected C−O cross-coupling of aldehydes with
alkanols could be classified into two general types: (a)
nucleophilic addition of alkoxyl moiety (-OR) to aldehydes
directly from alkanols mediated by various acids;5b,f (b)
through a chelating ligand of the M-OR (M = Pd, Ni et al.)
complexes.5a,c,9a In comparison, the other two potential models,
radical cross coupling and alkoxylation of acyl cation with
liberation of proton, were scarcely reported.12
study the dehydrogenative coupling of aromatic aldehydes and
−·
alcohols. The reactivity of SRA (sulfate radical anion, SO4
)
toward aromatics15 and alkanols16 may render the desired
oxidative esterification of aromatic aldehydes unexpected
pathways. For instance, one electron oxidation of the aromatics
by SRA was reported to generate aromatic radical cations (even
in the case of electron poor aromatic compounds, such as
benzoic acid and aromatic ketones).15 In the case of aromatics
substituted by formyl group under study, it is prone to liberate
one proton in the subsequent step when no other reaction
paths are available, with generating the corresponding acyl
radicals.10a,17 Thus formed acyl radicals were expected to
undergo a second one-electron oxidation by SRA with
conversion to acyl cations in the light of Lin and Sen’s study
on radical carboxylation of methane,18 therefore, making the
model of alcoholysis of acyl cation probable (path 1, Scheme
1). Independently, the radical cross-coupling (path 2, Scheme
1) may also be possible since the oxygen centered radicals were
once detected in the spin-trap experiments of reactions of
peroxydisulfates with alkanols.19 The unresolved question for
path 2 is the O−H group in alkanols is a poor hydrogen atom
donor and H-abstraction generally occurs on the alkyl moiety
rather than the O−H group (BDE: CH3OH, 96.06 0.15 kcal/
mol; CH3OH, 104.6 0.7 kcal/mol).16,20 The interpretation of
Received: November 20, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.6b02775
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Scheme 1. Supposed Reaction Pathway and the One Disclosed by This Work
formation and fate of alkoxyl radicals in the oxidation process
warrant further study.
Table 1. Oxidative Esterification of Aromatic Aldehydes with
Alcohols by 1a
a
To better understand the reactivity of SRA, we have
2−
synthesized and characterized several organic salts of S2O8
with different cationic structure. Interestingly, their perform-
ances in the oxidative esterification of aromatic aldehydes vary
significantly. The reaction pathway was explored and, to our
surprise, neither of the supposed was tenable (two supposed
pathways in Scheme 1). Both of the kinetic selectivity-
controlling and rate-determining steps were freshly pointed out.
b
entry substrate 2 (R1) substrate 3 (R2) time (h) 4 (yield (%))
1
2a (H)
3a (Me)
3a (Me)
3a (Me)
3a (Me)
3a (Me)
3a (Me)
3b (n-Et)
3c (n-Bu)
3d (i-Pr)
3e (t-Bu)
3f (PhCH2)
3g (Ph)
4.0
4aa (97%)
4ba (99%)
2
2b (p-NO2)
2c (p-OMe)
2d (p-Me)
2e (m-Cl)
2f (m-Me)
2a (H)
2.5
c
3
16.0
2.5
4ca (14%)
4da (97%)
4ea (94%)
4fa (87%)
4ab (98%)
4ac (99%)
4ad (85%)
4ae (21%)
4af (82%)
RESULTS AND DISCUSSION
4
■
5
2.5
Our study initiated with oxidative esterification of benzaldehyde
(PhCHO, 2a) in the presence of two commercially available
oxidants, respectively, (NH4)2S2O8 (1a) and K2S2O8 (1b). 1a
performed much better with a high aldehyde conversion and
ester selectivity, which resulted in a 97% yield of 4aa (Table
S1). In sharp contrast, only traces of 4aa were obtained in the
case of 1b as the oxidant. It might be attributed to the
differences in the solubility of oxidants as reported21 (the
aqueous solubility at 20 °C: 1a 522 g/L22 vs 1b 47 g/L23).
Based on this point, (n-Bu4N)2S2O8 (1c), which was supposed
to perform better than or at least comparable as 1a with an
increased solubility in MeOH by incorporating four butyl
chains into the cation unit, was prepared.24 However, the
reaction in the case of 1c as the oxidant preferably generated
carboxylic acid 5aa in a selectivity of 32%, nearly four times
higher than that of desired ester formed. The next efforts on
synthesis of ammonium oxidants (nBumH4‑mN)2S2O8 (m = 1, 2
or 3) were proven failures, while their pyridinium analogue
(PyH)2S2O8 (1d) was finally obtained through ion exchange
between pyridine hydrochloride (PyHCl) and 1b. Compound
1d features a typical pyridinium NMR resonance in DMSO-d6
6
3.0
7
24.0
48.0
24.0
16.0
8.0
8
2a (H)
9
2a (H)
10
11
12
2a (H)
2a (H)
d
2a (H)
8.0
4ag (8%)
a
General conditions: 2 (1 mmol), 3 (16 mmol), 1a (1.5 mmol), 60
°C. Isolated yields. 4-Methoxyphenol was mainly obtained in a yield
of 80%. Yields determined by GC-MS through using areas of peak
b
c
d
normalization method.
most cases (entry 1−2, 4−9). Peculiarly, those bearing
electron-rich groups such as p-anisaldehyde provided 4-
methoxyphenol as the major product in 80% yields (entry 3).
Besides, the bulky alcohol also performed poorly (entry 10).
Noteworthy, the reaction with phenol (3g) was nearly
suppressed (entry 12) probably because it is a radical scavenger.
As posited in Scheme 1, both the alkoxylation of acyl cation
(path 1) and radical cross coupling (path 2) experience the
same transiency of acyl radical. In experiments, the obtained
kinetic isotope value (kH/kD ≈ 1.0) from the independent
esterification of benzoic aldehyde versus benzoic aldehyde−d1
indicates the cleavage of formyl C−H bond is not the turnover-
limiting step (Scheme 2A). However, efforts on trapping benzyl
acyl or its derived benzyl radical by 2,2,6,6-tetramethylpiper-
idinooxy (TEMPO) from the reaction of 2a and 3a25 under
standard reaction conditions were proven unsuccessful
(Scheme 2B). The formation of ester proceeded quantitatively
with the use of methyl acrylate as the potential trapper of acyl
radical (Scheme 2C), thus disfavoring an acyl radical pathway.
1
[δ H (Py): 8.93 (2H), 8.60 (1H), and 8.07 (2H) ppm, δ
13C{1H}: 147.0, 142.1, 127.7 ppm]. The X-ray single crystal
diffraction displays a layered ionic arrangement with one central
S2O82− dianion and two separated protonated pyridine (PyH+)
as the counterion (Figure S2). The reaction by using 1d as an
oxidant resulted in a moderate ester selectivity of 78%
compared with those obtained by the other oxidants screened
(1a: 97% and 1c: 9%).
Various aromatic aldehydes were then reacted, by using 1.5
mol equiv of 1a as the oxidant, with 16 equiv of alkanols at 60
°C (Table 1). It offered excellent yield of the expected ester in
B
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mass spectrometry (UPLC-ESI-MS) technique. Besides the
·DMPO-OMe eluted at 0.7 min in an oxidized form with a
mass signal at m/z 144.10 (calcd HRMS (ESI) exact mass for
[M+H]+ (C7H14NO2): m/z 144.10, Figure 2B), the peak
Scheme 2. Mechanistic Studies
·
assigned to the DMPO-hemiacetal was also observed. ESI
analysis of the elution at 12.6 min indicates it is again an
oxidized form with molecular weight at m/z 250.14 (calcd
HRMS (ESI) exact mass for [M+H]+ (C14H20NO3): m/z
250.14, Figure 2C), while the fragment peak at m/z 129.09 was
attributed to the breaking C−O bond of hemiacetal unit and
generation of the oxidized DMPO moiety (calcd HRMS (ESI)
exact mass for (C6H11NO2): m/z 129.08, Figure 2C).
To declare if the hydrogen abstraction had ever occurred on
the methyl group as expected, a 1:1 mixture of CD3OH and 2a
was then treated with 1a. However, neither D/H exchange nor
kinetic isotope effect (KIE; kH/kD = 0.94 (≈1.0)) was obtained
(Figure S8). Independent to this, the negative mode ESI
analysis of the reaction solution of 1a/1d and MeOH under the
standard condition shows peaks at m/z 96.96 (assigned to
To elucidate the oxidative esterification pathway, the spin-
trapping technique was introduced to detect the involved but
short-lived radicals, wherein the dimethylpyridine N-oxide
(DMPO) was used as the spin-trap reagent. By in situ electron
spin resonance (ESR) study of a MeOH solution of 1a and 2a
−
−
HSO4 ) and 110.98 (assigned to MeOSO3 , a monomethyl
−
sulfate analogue from reaction of HSO4 and MeOH). These
newly formed methyl sulfate salts (6a and 6d) were finally
isolated and the solid state of the anion moiety was confirmed
by X-ray single crystal diffraction study in the pyridinium case
·
(under air, see SI for the detail), it features a typical DMPO-
OMe resonance26 at aN = 13.3 G, aH = 9.4 G (Figure 1A)
β
·
(6d, Figure 3). Importantly, a mass signal of DMPO-OMe in
an oxidized form at m/z 144.10 was detected in the reaction of
6d with 1a at 60 °C in THF, suggesting the methyl sulfate
species are precursors of MeO· trapped in ESR experiments.
The subsequently trapped hemiacetal radical was then
formally attributed to the nucleophilic addition of MeO· at
the formyl cabon of PhCHO (Scheme 3B). However, by DFT
calculations with the consideration of solvent effect (Figure S9),
the hydrogen abstraction of PhCHO by MeO· was investigated
to proceed easily with a comparatively lower energy barrier
(calcd 3.47 kcal/mol, Scheme 3A). Although the ester
formation by radical cross-coupling was proven to be
spontaneous, neither the benzyl acyl radical nor its decarboxy-
lated derivative was trapped by radical inhibitors (DMPO/
TEMPO/methyl acrylate) in our experiments. In fact,
monitored by in situ NMR, the reaction of 6a with 2a in the
absence of oxidants proceeded immediately in methanol even at
room temperature, while directly and qualitatively generating
the corresponding acetal (PhCH(OMe)2, Figure S11). It was
reasonably attributed to two sequential steps of electrophilic
alkylation as depicted in Scheme 3. The intermediate 7 was
formed by nucleophilic displacement between methyl sulfate
salt 6 and aldehyde Scheme 3C), which was prone to release
the corresponding hemiacetal radical (detected by spin-trap
experiments) upon reaction with SRA. However, at this point,
the alternative pathway leading to 7 from stepwise nucleophilic
substitution could not be completely ruled out (Scheme 3D).
A key insight into ester formation is revealed by monitoring
the selectivity for C−H esterification of a series of para-
substituted benzaldehydes via competition experiments, in
which an equimolar quantity of a para-substituted benzalde-
hyde and unsubstituted benzaldehyde, were added in the 1a/
MeOH system. The resulted products distribution was
measured by gas chromatography (GC) at early reaction
times. Electron-donating groups greatly enhance the selectivity
for C−H esterification, with generating a negative Hammett
slop (ρ = −1.5, R2 = 0.99, Figure 4). This moderate value is in
line with a free-radical mechanism rather than considerable
positive charge development on the aromatic hydrocarbons in
Figure 1. ESR spectra of spin-trap experiment of 1a and 2a in MeOH.
(A) 4 min, with the simulation spectra (bottom, aN = 13.3G, aH
=
β
9.4G); (B) 12 min; (C) 16 min, with the simulation spectra (bottom,
aN = 14.5G, aH = 16.2G).
β
other than the carbon centered adduct of DMPO (·DMPO−
CH2OH) after reaction at 60 °C for 4 min. Subsequently,
another signal at aN = 14.5 G, aH = 16.2 G quickly emerged
β
and gradually surpassed (Figure 1B, C). The spectral
parameters quoted for the second, obtained by the simulation
of ESR spectra (Figure 1C), were assigned to an unreported
·DMPO-hemiacetal. Compared with the hyperfine coupling
·
constants of DMPO-OMe, it has a larger aH parameter at
β
16.2G, which might be attributed to a perpendicular β-H to
NO· unit of DMPO caused by steric forces of the i-OMe
substituent.
The structure assumption for the radical adducts detected by
in situ ESR was further confirmed by ultraperformance liquid
chromatography coupled with electrospray ionization tandem
C
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Figure 2. UPLC-ESI-MS analysis of spin-trap experiment of 1a and 2a in MeOH. (A) UV trace, (B) reconstituted ion chromatograms for m/z
144.10, and (C) 250.14. Other signals in A can be attributed to UV-chromatography of the substrates, DMPO (*) and to 2a (•).
Figure 3. ORTEP of the intermediate 6d (PyHMeOSO3) represented
at the 30% probability level from the X-ray single crystal structure.
Hydrogen atoms are included but unlabeled for clarity.
Figure 4. Relative reactivities of para-substituted benzaldehydes in 1a/
MeOH system.
Scheme 3. Proposed Key Steps for the Oxidative
Esterification of Aldehydes
Scheme 4. Control Experiments
the transition state. It can be rationalized by preferential
formation of 7 derived from the electron-rich aldehydes since
the cleavage of C−H bond is not the turnover-limiting step
(indicated by the kinetic isotope experiment). Therefore,
electrophilic alkylation between 6 and aldehydes sounds more
probable for the route to 7, thereby kinetically controlling the
selectivity.
Before the reaction pathway well settled, it is necessary to
understand the origin for the peculiar Dakin-type product from
p-OMe-PhCHO (2c) in Table 1. As it demonstrated, even
under argon, 4-methoxyphenol was mainly formed (Scheme
4A). Nucleophilic substitution of the potentially generated
aromatic radical cations (formed via one electron oxidation of
the aromatics by SRA) was immediately excluded since no 1,4-
dimethoxybenzene was detected in the presence of abundant
MeOH as nucleophiles (more stronger than H2O). The well-
known pathway derived from H2SO5 (Caro’s acid) is unlikely
since the system under study is a neutral to weak acidic
oxidation process,27−29 not up to the requirements for
2−
generation of H2SO5 from S2O8
.
D
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We then conducted the reaction in the presence of 9,10-
dihydroanthracene, a potential trap for alkoxyl radical (BDE:
C(sp3)−H(9,10‑dihydroanthracene): 76.3 kcal/mol; MeO−H: 104.6
0.7 kcal/mol),20 which forms only traces of anthracene
(Scheme 4C), while the reaction leading to 4-methoxyphenol
was not affected at all. It demonstrates either methoxyl or
hemiacetal radical, if formed, would be quickly trapped by SRA
via radical cross-coupling, while leading to a new type of
peroxosulfate product 8 (Scheme 5). A similar termination by
than it does first order (note that the decomposition at 79.8 °C
in aqueous solution at pH 8 is strictly of the first order).30 Most
importantly, the difference between the oxidants (ca. 1a: 1.48;
1d: 0.38 (M−1/2·min−1)) was found remarkable in the presence
of a solvent scale of H-bond donors, such as a mixture of
methanol and water (both at a completely dissolved status).
The question pending in the route to reach 6 was if the
−
esterification between MeOH and HSO4 is fast enough to be
ignored. Where possible, rate constants were independently
evaluated and determined. Dry MeOH was mixed with three
−
different HSO4 salts in CD3CN and heated to 60 °C.
Scheme 5. Formation of Peroxosulfate 8 and Two Reaction
Modes in the Case of 8b
Equilibrium was not reached within 80 min in three cases, all
−
−
resulting in a limited conversion of HSO4 to MeOSO3 salts
−
+
(<40%). The MeOSO3 peak integral (CH3, NH4 : 3.62 ppm;
PyH+: 3.63 ppm; n-Bu4N+: 3.56 ppm) was normalized to the
methyl peak resonance of the MeOH solvent and the initial
second-order forward rate constants were calculated from the
peak-integral data. Results demonstrated that these reactions
proceeded with rate constants of the order of 10−4 M−1·min−1
and n-Bu4NHSO4 is relatively poor agent in the esterification
(Figure 5B, k (M−1·min−1), NH4 : 1.64 × 10−3; PyH+: 8.41 ×
+
10−4; n-Bu4N+: 1.98 × 10−4). Nearly 104 times gap in the
2−
calculated rate constant between the decomposition of S2O8
SRA was discovered for HO·, which subsequently produced
HSO4− and O2.29 Following the Baeyer−Villiger reaction rules,
either the corresponding ester or phenol could be produced
from 8b depending on the electrophilicity of the R group
inside. For instance, substituted by electron donating
substituent, the aromatic benzyl rather than the hydrogen
would migrate, thereby generating phenol as the major product.
Cross experiments further indicated the generation of methyl
sulfate salts 6 (Scheme 3) proceeded comparatively slower
(Figure S11−14) and was therefore regarded as the rate-
determining. To understand the reaction route further,
especially the ways that different oxidant operate, we then
turned our attention to the details leading to 6 under a neutral
oxidation process. Previous studies demonstrated that the effect
of organic substances on the rate of decomposition of S2O82− is
striking. For instance, Bartlett and Cotman disclosed the
decomposition rate of K2S2O8 was increased by 25-fold in the
presence of MeOH, with quantitatively transforming methanol
to formaldehyde.30 In this study, the rate constant of
decomposition of 1a and 1d (0.015 M) in the presence of
MeOH (0.5 M) at 80 °C were evaluated, respectively. The
kinetics more closely approximate the 2/3 order (Figure 5A)
and subsequent esterification renders the later a rate-limiting
step.
CONCLUSION
■
In this study, we presented mild oxidative esterification of
various aromatic aldehydes by sulfate radical redox system. The
reaction pathways, especially the kinetic selectivity-controlling
and rate-determining steps, have been fully discussed. The
supposed step of converting aromatic aldehydes to acyl radical
via aromatic radical cation may occur at the initial stage, with
−
generating HSO4 for the oxidation process. But, it was not
absolutely essential to use aldehydes as the sacrifice. Neither of
the expected alkoxylation of acyl cation or radical cross-
coupling was tenable in the formation of ester. Instead, the
−
transiency of MeOSO3 was disclosed, which was generated
−
from esterification between HSO4 and MeOH, a rate-limiting
step in the oxidative cross-coupling process. It readily generates
−
methoxyl radical upon reacting MeOSO3 with SRA, thereby
the contradiction between theoretic analysis and spin-trap
experiments on the origin of alkoxy radical from reaction of
SRA and alkanols was well answered. Moreover, the important
selectivity-controlling step was attributed to the subsequent
Figure 5. (A) Decomposition of the persulfate 1a and 1d (0.015 M) at 80 °C in the presence of 0.5 M methanol aqueous solution monitored by
titration, plotted as a 3/2 order reaction (left), and the reaction rate constant were calculated as twice the slope. (B) The esterification between
MeOH (0.29M) and HSO4− (0.015M) at 60 °C monitored by in situ NMR (right), and the initial reaction rate constant were calculated accordingly.
E
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−
Preparation of 1d. An acetonitrile (100 mL) suspension of PyHCl
(4.72 g, 0.04 mol) and K2S2O8 (5.43 g, 0.02 mol) was stirred at room
temperature for 24 h and then filtered. The solvent of filtrate was
removed under rotate machine and the residue was recrystallized from
methanol at −15 °C to give white crystals (5.29 g, 75%). Crystals
suitable for single X-ray diffraction were obtained from recrystallization
in methanol at −15 °C. mp 57.5 °C. Anal. Calcd for C10H12N2O8S2: C,
34.09; H, 3.43; N, 7.95; S, 18.20. Found: C, 34.13; H, 3.36; N, 7.99; S,
17.80. IR (KBr) [cm−1] = 3423 (s); 3134 (vw); 3064 (w); 2956 (vw);
2801 (vw); 1636 (w); 1611 (w); 1537 (vw); 1486 (m); 1299 (vs);
1271 (s); 1199 (vw);1117 (w); 1062 (s); 728 (w); 703 (m); 681 (m);
619 (vw); 592 (w); 562 (m). 1H NMR (600 MHz, DMSO-d6, 298 K):
nucleophilic displacement between MeOSO3 and aldehyde in
this study. Although the structure-effect relationship in terms of
the oxidants was not formulated, the ionic oxidant 1a with
more N−H numbers in the cation, as compared with 1c and
1d, proceeded better in the oxidative esterification of aldehydes.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were done under air and the
solvents were not dried unless otherwise noted. The task specific
oxidant (Bu4N)2S2O8 (1c) was prepared according to the literature
(Yang, S. G.; Hwang, J. P.; Park, M. Y.; Lee, K.; Kim, Y. H. Tetrahedron
2007, 63, pp 5184−5188). NMR spectra were recorded on a 600
3
4
3
δ = 8.93 (dd, JHH = 6.4 Hz, JHH = 1.3 Hz, 2H, Py), 8.60 (tt, JHH
=
7.8 Hz, 4JHH = 1.3, 1H, p-Py), 8.07 (tm, 3JHH = 7.8 Hz, 2H, Py), N.O.
(br s, 1H, N−H). 13C{1H} NMR (151 MHz, DMSO-d6, 298 K):
147.0, 142.1, 127.7 (Py).
MHz; spectrometer. H NMR and 13C NMR, chemical shift δ was
1
given relative to TMS and referenced to the solvent signal. Column
chromatography was performed using silica gel. Analytical TLC was
done using precoated silica gel 60 F254 plates. Melting points were
determined from differential scanning calorimetry (DSC) data, which
were obtained in sealed aluminum pans with a heating rate of 10 °C/
min. Fourier transform infrared (FTIR) data were collected with
anhydrous KBr as standard. Thermogravimetric (TG) analysis was
operated at a heating rate of 5 °C/min under N2 atmosphere using an
empty crucible as the reference. The ultraperformance liquid
chromatography system was equipped with TUV detector and a
ACQUITY UPLUPLC BEH C18 column (2.1 × 50 mm, 1.7 μm).
ESI-MS analysis was performed on a time-of-flight mass spectrometer
using an electrospray ionization (ESI) source.
Preparation of 6a (NH4MeOSO3). A methanol (50 mL) suspension
of (NH4)2S2O8 (4.56 g, 0.02 mol) was stirred and refluxed under
argon for 24 h and then filtered. The filtrate was crystallized at −15 °C
and the crystals obtained were recrystallized from methanol for two
times to give white crystals (0.45 g, 8%). The HSO− signal was no
longer present in the ESI-MS spectrum with negative mode. Anal.
Calcd for CH7NO4S: C, 9.30; H, 5.46; N, 10.85; O, 49.56; S, 24.83.
1
Found: C, C, 9.32; H, 5.45; N, 10.88; O, 49.58; S, 24.85. H NMR
(600 MHz, DMSO-d6, 298 K): 7.09 (t, 3JHH = 51 Hz, 4H, NH4). 3.40
(s, 3H, CH3). 13C{1H} NMR (151 MHz, DMSO-d6, 298 K) 53.1
(CH3).
Preparation of 6d (PyHMeOSO3). A methanol (100 mL)
suspension of PyHCl (4.62 g, 0.04 mol) and K2S2O8 (5.41 g, 0.02
mol) was stirred and refluxed under argon for 24 h and then filtered.
The filtrate was crystallized at −15 °C and the crystals obtained were
recrystallized from methanol to give white crystals (1.58 g, 23%).
Crystals suitable for single X-ray diffraction were obtained from
recrystallization in methanol at −15 °C. Anal. Calcd for C6H9NO4S:
C, 37.69; H, 4.74; N, 7.33; O, 33.47; S, 16.77. Found: C, 37.71; H,
4.77; N, 7.30; O, 33.43; S, 16.79. 1H NMR (600 MHz, DMSO-d6, 298
K): δ = 8.95 (dd, 3JHH = 6.6 Hz, 4JHH = 1.6 Hz, 2H, Py), 8.65 (tt, 3JHH
The crystal structure analysis was performed on a CCD X-ray
diffractometer equipped with an area detector, and the crystals were
irradiated using graphite monochromated Mo Kα radiation (λ =
0.7103 Å). Data collection was performed and the unit cell was initially
refined using APEX2 (Bruker, APEX2 v2010.3−0 ed., Bruker AXS
Inc., Madison, 2010). Data reduction was performed using SAINT
(Bruker, SAINT v7.68A ed., Bruker AXS Inc., Madison, 2009) and
XPREP (Bruker, XPREP v2008/2 ed., Bruker AXS Inc., Madison,
2008). Absorption corrections were applied using SADABS (Bruker1,
SADABS v2008/1 ed., Bruker AXS Inc., Madison, 2008). The
structure was solved and refined with the aid of the SHELXTL
software package (Bruker4, SHELXTL v2008/4 ed., Bruker AXS Inc.,
Madison, 2008). The full-matrix least-squares refinement on F2
included atomic coordinates and anisotropic thermal parameters for
all non-hydrogen atoms. The hydrogen atoms were included using a
riding model. CCDC 1013691 (1a, (NH4)2S2O8), 1014063 (1d,
(PyH)2S2O8), and 1013692 (6d, MeOSO3PyH) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.
html.
4
3
= 7.8 Hz, JHH = 1.5, 1H, p-Py), 8.11 (tm, JHH = 7.21 Hz, 2H, Py),
N.O. (br s, 1H, N−H), 3.41 (s, 3H, CH3). 13C{1H} NMR (151 MHz,
DMSO-d6, 298 K): 146.7, 142.2, 127.4 (Py), 53.1 (CH3).
Decomposition of the Oxidants in the Presence of Methanol.
Peroxodisulfate (1a, 1d) was titrated by indirect indometry. The
sample to be analyzed was made 1 M in potassium iodide and one
gram of sodium bicarbonate was dissolved in it. To this sample was
added 20−25 mL of 10% sulfuric acid. During and after the liberation
of the carbon dioxide, which provided a convenient means of sweeping
out the oxygen, the solution stood in a glass-stoppered flask. After
standing for 30 min in the dark at room temperature, the liberated
iodine was titrated with standard thiosulfate. The reaction was carried
out using a 150 mL stainless steel vessel with a sampling pipe. Solution
of peroxodisulfate (150 mL, 0.015 M) in the presence of 0.5 M
methanol was prepared. The first sample was took before heating and
closing the vessel. And samples (7 mL) were taken every 4 min after
heating. The rate constant was calculated according to the following
eqs 1 and 2:
Recrystallization of 1a ((NH4)2S2O8). A saturated aqueous solution
of commercial 1a at 50 °C was cooled down to room temperature,
from which crystals of 1a suitable for X-ray structure analysis were
slowly obtained. mp 187 °C (dec). IR (KBr) [cm−1] = 3159 (vs); ν =
2352 (vw); 1402 (s); 1298 (s); 1264 (s); 1057 (s); 689 (s); 621 (m);
593 (w); 559 (s); 432 (vw).
Preparation of 1c ((Bu4N)2S2O8). Tetra-butylammonium hydro-
gensulfate (Bu4N)HSO4 (14.1 g, 0.04 mol) and K2S2O8 (5.43 g, 0.02
mol) were dissolved in 100 mL of distilled water and the solution was
stirred for 30 min at room temperature. The mixture was then
extracted with CH2Cl2 (20 mL × 3), and the combined organic layers
were washed with distilled water (20 mL × 3), dried over anhydrous
MgSO4, and filtered. Evaporation of the solvent in vacuo and
subsequent drying under high vacuum gave a white solid of 1c in 80%
yield. mp 115 °C (dec); Anal. Calcd for C32H72N2O8S2: C, 56.77; H,
10.72; N, 4.14; S, 9.47. Found: C, 56.42; H, 10.58; N, 4.16; S, 9.21. IR
(KBr) [cm−1] = 3424 (s); 2961 (s); 2940 (w); 2874 (w); 1624 (w);
1487 (w); 1467 (w); 1382 (vw); 1298 (s); 1278 (s); 1152 (vw); 1062
(m); 1041 (w); 884 (vw); 728 (w); 703 (m); 621 (w); 591 (vw); 562
(w); 1H NMR (600 MHz, CDCl3, 298 K): δ = 3.30 (m, 2H, N−CH2),
dc
−
= kc2/3
(1)
dt
c−1/2
k
−1/2
=
t + c0
(2)
2
Then the rate constant correspond to twice the slope of c−1/2 ∼ t plot.
Preparation of 4aa. A methanol suspension (0.65 mL, 16 mmol,
16 equiv) of benzaldehyde (0.11 g, 1 mmol, 1 equiv) and (NH4)2S2O8
(0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 4 h. After cooling
to room temperature, distilled water (10 mL) was used to dissolve the
solid and the product was extracted by ethyl acetate (3 × 10 mL). The
combined organic extract was concentrated and then purified by
column chromatography on silica gel (petroleum ether/ethyl acetate
3
1.63 (m, 2H, CH2), 1.44 (m, 2H, CH2), 0.97 (t, JHH = 7.4 Hz, 3H,
CH3). 13C{1H} NMR (151 MHz, CDCl3, 298 K) δ = 58.6 (N−CH2),
1
24.1 (CH2), 19.8 (CH2), 13.8 (CH3).
40:1) provided 4aa in a yield of 96%. H NMR (600 MHz, CDCl3,
F
DOI: 10.1021/acs.joc.6b02775
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
298 K): δ = 8.04 (dd, 3JHH = 8.3 Hz, 3JHH = 1.3 Hz, 2H, Ph), 7.54 (tt,
= 7.5 Hz, 1H, m′-Ph), 3.89 (s, 3H, OCH3), 2.37 (s, 3H, CH3).
13C{1H} NMR (151 MHz, CDCl3, 298 K): δ = 167.1 (CO), 138.0
(m-Ph), 133.6 (p-Ph), 130.1 (i-Ph and o-Ph), 128.2 (m′-Ph), 126.7
(o′-Ph), 51.9 (OCH3), 21.1 (CH3).
3
3
3JHH = 7.6 Hz, JHH = 1.3 Hz, 1H, p-Ph), 7.43 (ddm, JHH = 8.3 Hz,
3JHH = 7.6 Hz, 2H, Ph), 3.91 (s, 3H, OCH3). 13C{1H} NMR (151
MHz, CDCl3, 298 K): δ = 167.2 (CO), 133.0, 130.3, 129.7, 128.4
(Ph), 52.2 (OCH3).
Preparation of 4ab. An ethanol suspension (0.74 g, 16 mmol, 16
equiv) of benzaldehyde (0.11 g, 1 mmol, 1 equiv) and (NH4)2S2O8
(0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 24 h. After
cooling to room temperature, distilled water (10 mL) was used to
dissolve the solid and the product was extracted by ethyl acetate (3 ×
10 mL). The combined organic extract was concentrated and then
purified by column chromatography on silica gel (petroleum ether/
Preparation of 4ba. A methanol suspension (0.65 mL, 16 mmol,
16 equiv) of 4-nitrobenzaldehyde (0.15 g, 1 mmol, 1 equiv) and
(NH4)2S2O8 (0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 2.5
h. After cooling to room temperature, distilled water (10 mL) was
used to dissolve the solid and the product was extracted by ethyl
acetate (3 × 10 mL). The combined organic extract was concentrated
and then purified by column chromatography on silica gel (petroleum
1
ethyl acetate 30:1) provided 4ab in a yield of 98%. H NMR (600
1
MHz, CDCl3, 298 K): δ = 8.05 (dm, 3JHH = 8.4 Hz, 2H, Ph), 7.53 (tm,
ether/ethyl acetate 20:1) provided 4ba in a yield of 99%. H NMR
(600 MHz, CDCl3, 298 K): δ = 8.27 (d, 3JHH = 8.3 Hz, 2H, Ph), 8.19
(d, 3JHH = 8.3 Hz, 2H, Ph), 3.97 (s, 3H, OCH3). 13C{1H} NMR (151
MHz, CDCl3, 298 K): δ = 165.3 (CO), 150.6 (i-Ph), 135.60 (i-Ph),
130.8, 123.7 (Ph), 52.9 (OCH3).
3
3
3JHH = 7.5 Hz, 1H, p-Ph), 7.42 (ddm, JHH = 8.4 Hz, JHH = 7.5 Hz,
2H, Ph), 4.37 (q, JHH = 7.1 Hz, 2H, CH2Et), 1.39 (t, JHH = 7.1 Hz,
3H, CH3Et). 13C{1H} NMR (151 MHz, CDCl3, 298 K): δ = 166.7
(CO), 132.9 (Ph), 130.6 (i-Ph), 129.6, 128.4 (Ph), 61.0 (OCH2),
14.4 (CH3Et).
3
3
Preparation of 4ca. A methanol suspension (0.65 mL, 16 mmol, 16
equiv) of 4-methoxybenzaldehyde (0.14 g, 1 mmol, 1 equiv) and
(NH4)2S2O8 (0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 16
h. GC-MS analysis of the resulting solution indicated 4ca and 4-
methoxyphenol in a ratio of 1.0:6.0 was formed. After cooling to room
temperature, distilled water (10 mL) was used to dissolve the solid and
the product was extracted by ethyl acetate (3 × 10 mL). The
combined organic extract was concentrated and then purified by
column chromatography on silica gel (petroleum ether/ethyl acetate
2:1) provided 4ca in a yield of 14% and the major product 4-
Preparation of 4ac. A butanol suspension (1.19 g, 16 mmol, 16
equiv) of benzaldehyde (0.11 g, 1 mmol, 1 equiv) and (NH4)2S2O8
(0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 48 h. After
cooling to room temperature, distilled water (10 mL) was used to
dissolve the solid and the product was extracted by ethyl acetate (3 ×
10 mL). The combined organic extract was concentrated and then
purified by column chromatography on silica gel (petroleum ether/
1
ethyl acetate 30:1) provided 4ac in a yield of 99%. H NMR (600
MHz, CDCl3, 298 K): δ = 8.05 (dm, 3JHH = 7.5 Hz, 2H, Ph), 7.54 (tm,
1
methoxyphenol in a yield of 80%. For 4ca: H NMR (600 MHz,
3
3
3JHH = 7.4 Hz, 1H, p-Ph), 7.43 (ddm, JHH = 7.5 Hz, JHH = 7.4 Hz,
CDCl3, 298 K): δ = 8.00 (dm, 3JHH = 8.9 Hz, 2H, Ph), 6.92 (dm, 3JHH
= 8.9 Hz, 2H, Ph), 3.89, 3.86 (each s, each 3H, OCH3). 13C{1H} NMR
(151 MHz, CDCl3) δ = 167.0 (CO), 163.5, 122.8 (each i-Ph),
131.7, 113.8 (each Ph), 55.6, 52.0 (each OCH3). For 4-
methoxyphenol: 1H NMR (600 MHz, CDCl3, 298 K): δ = 6.80
2H, Ph), 4.33 (t, JHH = 6.6 Hz, 2H, OCH2Bu), 1.75 (m, 2H, CH2Bu),
3
1.48 (m, 2H, CH2Bu), 0.98 (3JHH = 7.3 Hz, 3H, CH3Bu). 13C{1H} NMR
(151 MHz, CDCl3, 298 K): δ = 166.7 (CO), 132.8 (Ph), 130.6 (i-
Ph), 129.6, 128.4 (Ph), 64.9 (OCH2), 30.9, 19.4 (CH2Bu), 13.8
(CH3Bu).
3
(dm, 3JHH = 9.3 Hz, 2H, Ph), 6.77 (dm, JHH = 9.3 Hz, 2H, Ph), 4.38
Preparation of 4ad. An isopropanol suspension (0.96 g, 16 mmol,
16 equiv) of benzaldehyde (0.11 g, 1 mmol, 1 equiv) and (NH4)2S2O8
(0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 24 h. After
cooling to room temperature, distilled water (10 mL) was used to
dissolve the solid and the product was extracted by ethyl acetate (3 ×
10 mL). The combined organic extract was concentrated and then
purified by column chromatography on silica gel (petroleum ether/
(br, 1H, OH), 3.77 (s, 3H, OCH3). 13C{1H} NMR (151 MHz,
CDCl3) δ = 153.9, 149.6 (each i-Ph), 116.2, 115.0 (Ph), 56.0 (OCH3).
Preparation of 4da. A methanol suspension (0.65 mL, 16 mmol,
16 equiv) of 4-methylbenzaldehyde (0.12 g, 1 mmol, 1 equiv) and
(NH4)2S2O8 (0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 2.5
h. After cooling to room temperature, distilled water (10 mL) was
used to dissolve the solid and the product was extracted by ethyl
acetate (3 × 10 mL). The combined organic extract was concentrated
and then purified by column chromatography on silica gel (petroleum
1
ethyl acetate 30:1) provided 4ad in a yield of 85%. H NMR (600
MHz, CDCl3, 298 K): δ = 8.05 (dm, 3JHH = 7.6 Hz, 2H, Ph), 7.53 (tm,
3
3
3JHH = 7.4 Hz, 1H, p-Ph), 7.42 (ddm, JHH = 7.6 Hz, JHH = 7.4 Hz,
1
ether/ethyl acetate 20:1) provided 4da in a yield of 97%. H NMR
2H, Ph), 5.26 (hept, JHH = 6.3 Hz, 1H, CHi‑Pr), 1.37 (d, JHH = 6.3
Hz, 6H, CH3i‑Pr). 13C{1H} NMR (151 MHz, CDCl3, 298 K): δ =
166.2 (CO), 132.7 (Ph), 131.0 (i-Ph), 129.6, 128.3 (Ph), 68.4
(CHi‑Pr), 22.0 (CH3i‑Pr).
3
3
(600 MHz, CDCl3, 298 K): δ = 7.93 (dm, 3JHH = 8.2 Hz, 2H, Ph), 7.22
(dm, 3JHH = 8.2 Hz, 2H, Ph), 3.89 (s, 3H, OCH3), 2.39 (s, 3H, CH3).
13C{1H} NMR (151 MHz, CDCl3, 298 K): δ = 167.2 (CO), 143.6
(i-Ph), 129.7, 129.1 (Ph), 127.51 (i-Ph), 52.0 (OCH3), 21.7 (CH3).
Preparation of 4ea. A methanol suspension (0.65 mL, 16 mmol, 16
equiv) of 4-chlorobenzaldehyde (0.14 g, 1 mmol, 1 equiv) and
(NH4)2S2O8 (0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 2.5
h. After cooling to room temperature, distilled water (10 mL) was
used to dissolve the solid and the product was extracted by ethyl
acetate (3 × 10 mL). The combined organic extract was concentrated
and then purified by column chromatography on silica gel (petroleum
Preparation of 4ae. A tert-butanol suspension (1.19 g, 16 mmol, 16
equiv) of benzaldehyde (0.11 g, 1 mmol, 1 equiv) and (NH4)2S2O8
(0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 16 h. After
cooling to room temperature, distilled water (10 mL) was used to
dissolve the solid and the product was extracted by ethyl acetate (3 ×
10 mL). The combined organic extract was concentrated and then
purified by preparative TLC (petroleum ether/ethyl acetate 30:1)
provided 4ad in a yield of 21%. 1H NMR (600 MHz, CDCl3, 298 K):
δ = 7.99 (dm, 3JHH = 7.2 Hz, 2H, Ph), 7.52 (tm, 3JHH = 7.4 Hz, 1H, p-
Ph), 7.41 (ddm, 3JHH = 7.6 Hz, 3JHH = 7.4 Hz, 2H, Ph), 1.60 (s, 9H, 3
× CH3t‑Bu). 13C{1H} NMR (151 MHz, CDCl3, 298 K): δ = 165.9
(CO), 132.5 (Ph), 132.2 (i-Ph), 129.5, 128.3 (o,m-Ph), 81.1
(Cqt‑Bu), 28.3 (CH3t‑Bu).
1
ether/ethyl acetate 20:1) provided 4da in a yield of 94%. H NMR
(600 MHz, CDCl3, 298 K): δ = 7.95 (dm, 3JHH = 8.5 Hz, 2H, Ph), 7.39
3
(dm, JHH = 8.5 Hz, 2H, Ph), 3.89 (s, 3H, OCH3). 13C{1H} NMR
(151 MHz, CDCl3, 298 K): δ = 166.3 (CO), 139.4 (i-Ph), 131.1,
128.8 (Ph), 128.7 (i-Ph), 52.3 (OCH3).
Preparation of 4fa. A methanol suspension (0.65 mL, 16 mmol, 16
equiv) of 3-methylbenzaldehyde (0.12 g, 1 mmol, 1 equiv) and
(NH4)2S2O8 (0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 3.0
h. After cooling to room temperature, distilled water (10 mL) was
used to dissolve the solid and the product was extracted by ethyl
acetate (3 × 10 mL). The combined organic extract was concentrated
and then purified by column chromatography on silica gel (petroleum
Preparation of 4af. A benzyl alcohol suspension (1.73 g, 16 mmol,
16 equiv) of benzaldehyde (0.11 g, 1 mmol, 1 equiv) and (NH4)2S2O8
(0.35 g, 1.5 mmol, 1.5 equiv) was stirred at 60 °C for 8 h. After cooling
to room temperature, distilled water (10 mL) was used to dissolve the
solid and the product was extracted by ethyl acetate (3 × 10 mL). The
combined organic extract was concentrated and then purified by
preparative TLC (petroleum ether/ethyl acetate 30:1) provided 4af in
1
ether/ethyl acetate 20:1) provided 4fa in a yield of 87%. H NMR
1
(600 MHz, CDCl3, 298 K) δ = 7.85 (br s, 1H, o-Ph), 7.83 (dm, 3JHH
=
a yield of 82%. H NMR (600 MHz, CDCl3, 298 K): δ = 8.09 (dm,
7.5 Hz, 1H, o′-Ph), 7.33 (dm, 3JHH = 7.8 Hz, 1H, p-Ph), 7.30 (tm, 3JHH
3JHH = 7.1 Hz, 2H, o-Ph), 7.56 (m, 1H, p-Ph), 7.46 (dm, 3JHH = 7.5 Hz,
G
DOI: 10.1021/acs.joc.6b02775
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
2H, o-Ph), 7.44 (m, 2H, m-Ph), 7.40 (m, 2H, m-Ph), 7.36 (m, 1H, p-
Ph), 5.38 (s, 2H, CH2). 13C{1H} NMR (151 MHz, CDCl3, 298 K): δ
= 166.6 (CO), 136.2 (i-Ph), 133.2 (p-Ph), 130.3 (i-Ph), 129.8 (o-/
m-Ph), 128.7 (o-/m-Ph), 128.5 (o-/m-Ph), 128.4 (p-Ph), 128.3 (o-/m-
Ph), 66.8 (CH2).
2013, 15, 500−503. (e) Feng, J.-B.; Gong, J.-L.; Li, Q.; Wu, X.-F.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.6b02775.
■
S
Characterization and comparison of the ionic oxidants
and intermediate, oxidants screening, Hammett studies,
radical spin-trap experiment, experiments trapping the
−
transient MeOSO3 , NMR study of the reaction rate of
1
MeOH with 1a or 6, theoretic calculations, H and 13C
NMR spectra for all synthesized new compounds (PDF)
Single-crystal X-ray data for 1a, 1d, and 6d (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-Mail: bhxu@ipe.ac.cn
*E-Mail: sjzhang@ipe.ac.cn
ORCID
Bao-Hua Xu: 0000-0002-7222-4383
Suo-Jiang Zhang: 0000-0002-9397-954X
Author Contributions
∥X.-Q.Y. and H.-Y.H. contributed equally to the quantum
chemical calculations
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Yukio Mizuta and Dr. Yue-Qi Ye at JEOL
RESONANCE Inc. for the kind simulation of ESR spectra. And
financial supports by 973 Program (No. 2015CB251401), Key
Program of National Natural Science Foundation of China
(No. 91434203), National Natural Science Foundation of
China (No. 21476240 and 21406230), Instrument Developing
Project of the Chinese Academy of Sciences (No. YZ201521)
and CAS 100-Talent Program (2014), are gratefully acknowl-
edged.
REFERENCES
■
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