Aerobic oxidations with N-hydroxyphthalimide in trifluoroacetic acid

Article abstract of DOI:10.1016/j.mcat.2017.12.017

The potential of highly polar trifluoroacetic acid as a media for the metal-free aerobic oxidations propagated by phthalimide N-oxyl- (PINO) radical was demonstrated experimentally utilizing N-hydroxyphthalimide (NHPI) as a catalyst and HNO₃ or NaNO₂ as promoters. The oxidations of toluene, cyclohexane, adamantane, diamantane, and 3-oxadiamantane gave, respectively, benzaldehyde, adipic acid, 1-hydroxyadamantane, 1-hydroxydiamantane, and 9-hydroxy-3-oxadiamantane with up to 90% selectivities under high conversions of starting material. From the density functional theory computations at the M06-2X and B3LYP-D3 levels the polarized transition structures (TS) for the hydrogen abstraction from toluene and benzyl alcohol with highly electrophilic PINO-radical are substantially stabilized by non-covalent interactions (π–π stacking). Such additional stabilization is not present in the TS for the H-abstraction from benzaldehyde thus retarding its over-oxidation to benzoic acid.

Full text of DOI:10.1016/j.mcat.2017.12.017

Molecular Catalysis 447 (2018) 72–79
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Aerobic oxidations with N-hydroxyphthalimide in trifluoroacetic acid
Pavel A. Gunchenkoa,b, Jing Li , Bifu Liu , Hongyan Chen , Alexander E. Pashenko ,
a
a
a
b
b
b
b,∗
Vladyslav V. Bakhonsky , Tatyana S. Zhuk , Andrey A. Fokin
School of Chemistry and Material Engineering, Huizhou University, Huizhou, 516007, PR China
a
b
Department of Organic Chemistry, Igor Sikorsky Kiev Polytechnic Institute, pr. Pobedy, 37, 03056, Kiev, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
The potential of highly polar trifluoroacetic acid as a media for the metal-free aerobic oxidations
propagated by phthalimide N-oxyl- (PINO) radical was demonstrated experimentally utilizing N-
hydroxyphthalimide (NHPI) as a catalyst and HNO3 or NaNO2 as promoters. The oxidations of toluene,
cyclohexane, adamantane, diamantane, and 3-oxadiamantane gave, respectively, benzaldehyde, adipic
acid, 1-hydroxyadamantane, 1-hydroxydiamantane, and 9-hydroxy-3-oxadiamantane with up to 90%
selectivities under high conversions of starting material. From the density functional theory compu-
tations at the M06-2X and B3LYP-D3 levels the polarized transition structures (TS) for the hydrogen
abstraction from toluene and benzyl alcohol with highly electrophilic PINO-radical are substantially sta-
bilized by non-covalent interactions (␲–␲ stacking). Such additional stabilization is not present in the
TS for the H-abstraction from benzaldehyde thus retarding its over-oxidation to benzoic acid.
© 2018 Elsevier B.V. All rights reserved.
Received 12 October 2017
Received in revised form 3 December 2017
Accepted 12 December 2017
Keywords:
Oxidation
Metal free
Toluene
Benzaldehyde
Cyclohexane
Adipic acid
3
-oxadiamantane
NHPI
1
. Introduction
Partial oxidation of toluene to benzaldehyde represents a
of starting material were observed only for enzymatic oxidations
of toluene with Cu-laccase [19–22] or oxidation with NHPI and
II
Co -salts in HFIP [23].
formidable challenge both for the laboratory and industry as over-
oxidation to benzoic acid dominates already at the moderate
conversions of starting material. Aerobic oxidations [1] of toluene
are particularly unselective: While the oxidation with O2 in the
Powerful “green” [24] organocatalyst N-hydroxyphthalimide
(NHPI, Scheme 1) occupies an outstanding position in aerobic oxi-
dations [25–29] as it is effective both in the presence of transition
metals [30] and under metal-free conditions [31–35] operating for
CH-bonds in alkylaromatics and even in unactivated alkanes. At
the initiation step (typically with metal-peroxo species or with
II
presence of Co -complexes gave benzaldehyde and benzyl alco-
hol only in 60% selectivity even at very low conversions (8.9%), [2]
benzaldehyde forms in 68% preparative yield from the oxidation
of toluene with tert-butyl hydroperoxide [3]. In many other aero-
bic transition-metal-based systems [4] benzaldehyde was found
as a major product only under moderate conversions (<20%) of
toluene [5]. The oxidation in various solvents [6], with additives
the NO -radical, A, Scheme 1) NHPI generates highly electrophilic
2
phthalimide N-oxyl- (PINO) radical, which is involved into the CH-
abstraction step (B) from hydrocarbon (RCH ). This recovers the
3
•
catalyst NHPI [36] and thus formed hydrocarbon radicals RCH2
are trapped (C) with molecular oxygen to form peroxyl-radicals
•
[7], in presence of copper binary oxides, [8,9] MOFs, [10,11] porous
RCH OO that propagate the oxidation by generating PINO from
2
carbon nitride C N [12,13], Au-Pd alloy carbon nanoparticles,[14]
NHPI (D) [37]. Hydroperoxides RCH OOH thus formed undergo
3
4
2
cerium nanocubes,[15] CuCr O spinel nanoparticles [16], and zeo-
dehydrations/exchanges/oxidations typical for hydrocarbon aero-
2
4
lites [17] led to little improvements. Remarkably, benzaldehyde is
the main product of toluene photocatalytic aerobic oxidation on the
Ru-doped TiO2 nanotubes at 650 C where the formation of ben-
bic chemistry (E).
The mild aerobic oxidations of toluene with NHPI in the pres-
◦
II
ence of Co -salts occur at room temperature either in CH CN or
3
zoic acid is completely suppressed, though, no preparative yields
were reported [18]. Satisfactory selectivities at high conversions
CH COOH, however even at ca. 50% of toluene conversion 45% of
3
benzoic acid forms with only traces of benzaldehyde. The catalytic
system based on the combination of NHPI and VO(acac)2 oxidizes
toluene with conversion of 14.4% in 93/7 benzaldehyde/benzyl
alcohol ratio [38]. Heterogeneous transformations usually require
harsher conditions. The NHPI immobilized on mesoporous molec-
∗ Corresponding author.
E-mail address: aaf@xtf.kpi.ua (A.A. Fokin).
https://doi.org/10.1016/j.mcat.2017.12.017
2
468-8231/© 2018 Elsevier B.V. All rights reserved.

P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
73
Scheme 1. The aerobic oxidations of the methyl group carried by the phthalimide N-oxyl- (PINO) radical generated from N-hydroxyphthalimide (NHPI) and initiated by the
•
•
NO2 or metal-peroxo (M-OO ) radicals.
ular sieves catalyzes an aerobic liquid-phase oxidation of toluene
in acetonitrile at 190 C that gives benzaldehyde as a main prod-
Detailed spectroscopic characterization of new compounds are pro-
vided in the Supplementary Information.
◦
uct only at very low conversions still together with benzoic acid
and benzyl alcohol [39]. The oxidation in acetic acid at 100 C
◦
2
.1.1. Oxidation of adamantane (9) in the TFA/HNO /O system
3 2
on SiO -supported Co(II)/NHPI heterogeneous catalyst yields only
2
(
1 eq. of HNO3)
.6 mL (0.8 g, 7.5 mmol) of 56% HNO was added to a stirred mix-
1
8% of oxidation products [40]. The aerobic oxidation on polymer-
0
II
◦
3
supported NHPI in the presence of Co and AIBN at 80 C even at
ture of 1 g (7.4 mmol) of adamantane (9) in 11 mL of CF COOH.
The mixture was heated to 50 C and stirred for 4 h, strong
nitrous oxides evolution occurred and starting material dissolved
during the course of reaction. The GC–MS analysis of reaction
mixture identified 53% of 1-adamantyl-trifluoroacetate, 44% of
3
2
0% conversion produces a comparable amounts of benzyl alcohol,
◦
◦
aldehyde, and acid [41]. The oxidation on MOFs at 110 C under
conversions <10% leads to benzaldehyde and benzyl alcohol in
ca. 5/1 ratio [10]. The o-phenanthroline-mediated, metal-free cat-
alytic system with Br2 produces benzaldehyde and benzoic acid
at 1:4 ratio already at ca. 20% toluene conversion [42]. An effi-
cient oxidation of methylarenes to corresponding aldehydes in
hexafluoroisopropan-2-ol in presence of NHPI was recently sug-
1
-nitroadamantane (11) and 3% of 1-hydroxy adamantane (10). 1-
Adamantyl-trifluoroacetate was identified by comparative GC–MS
analysis with standard sample. Then CF COOH was removed in
3
vacuum and 8 mL of 10% ethanol solution of KOH was added. The
residue after evaporation of ethanol was dissolved in ether (20 mL),
washed with water (2 × 5 mL) and dried. Evaporation followed by
column chromatography on silica gel (ethyl acetate/hexane = 1/9)
gave 0.56 g (42%) of 1-nitroadamantane [43] (11) and 0.61 g (54%)
of 1-hydroxyadamantane (10) identical to the standard samples.
II
gested but still requires the presence of the Co -salts [23].
The general conclusion is that, despite enormous efforts, the
preparative synthesis of aldehydes by direct metal-free aerobic
oxidations of methylaromatics, in particular toluene conversion
to benzaldehyde, still is problematic even with such remarkable
catalyst as NHPI. Herein we first model the aerobic oxidation
of toluene with and without NHPI computationally at the den-
sity functional theory (DFT) level and then study the oxidation
of toluene experimentally. We then apply our new reactive sys-
tem for the preparative CH-oxidations of cyclohexane, adamantane,
and diamantane as well as otherwise extremely unreactive 3-
oxadiamantane.
2.1.2. Oxidation of adamantane (9) in the TFA/HNO3/O2 system
(cat. HNO3)
The same as above, but 0.06 mL (0.08 g, 0.74 mmol, 0.1 eq) of
56% HNO3 was used. Evaporation of the extract after ethanol/KOH
hydrolysis gave 1.05 g (94%) of 1-hydroxyadamantane (10) identi-
cal to the standard sample.
2
. Experimental
2
.1.3. Oxidation of adamantane (9) in the TFA/NHPI/HNO /O
3 2
2.1. General
system (cat. HNO3)
The same as above, but 0.121 g (0.74 mmol, 0.1 eq.) of NHPI was
added, 1.05 g (94%) of 1-hydroxyadamantane (10) identical to the
standard sample was isolated as above.
All computations were performed with the GAUSSIAN09 pro-
gram suite utilizing analytical first and second energy derivatives.
The NMR spectra were recorded with a Bruker Advance II spectrom-
eter, the chemical shifts are given in ppm relative to TMS. GC–MS
analyses were performed with an HP5890 GC with an HP5971A
mass-selective detector. Products were purified by chromatogra-
phy on 100–160 mesh silica gel. All melting points were determined
without correction. Commercially available reagents and solvents
were used without further purification. No special inert conditions
were established and free access of the atmospheric oxygen was
2
.1.4. Oxidation of diamantane (12) in the TFA/HNO /O system
3 2
(
1 eq. of HNO3)
0
.43 mL (0.57 g, 5.31 mmol) of 56% HNO3 was added to a stirred
mixture of 1 g (5.31 mmol) of diamantane (12) in 8 mL of CF COOH.
3
Column chromatography on silica gel (ethyl acetate/hexane = 1/9)
gave 0.51 g (41%) of 1-nitrodiamantane [43] (14) and 0.56 g (52%)
of 1-hydroxydiamantane (13) identical to the standard samples.
allowed in the experiments where the presence of O is mentioned.
2

7
4
P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
Mp = 159–161 C. 1H NMR (400 MHz, CDCl ): 3.98 (bs, 1Н), 3.82
◦
2
.1.5. Oxidation of diamantane (12) in the TFA/HNO /O system
3 2
3
(
cat. HNO3)
(bs, 1Н), 2.2 (bs, 2Н), 2.11 (bs, 1Н), 2.05 (dd, 2Н), 1.9 (bs, 2Н), 1.79
(dd, 2Н), 1.72 (dd, 2Н), 1.7 (dd, 4Н), 1.58 (dd, 1Н). 13C NMR: 73.7
(CH), 66.8 (C), 65.4 (CH), 44.9 (CH ), 43.4 (CH ), 38.2 (CH), 38.2
The same as above, but 0.043 mL (0.057 g, 0.53 mmol, 0.1 eq.) of
6% HNO3 was used. Evaporation of the extract after ethanol/KOH
5
2
2
hydrolysis gave 1.05 g (97%) of 1-hydroxydiamantane (13) identical
(CH), 36.6 (CH ), 34.8 (CH). HR-MS: found 206.1329 (calculated for
2
to the standard sample [44].
C13H18O : 206.1317).
2
3
-Oxadiamantane-8-one (22)
◦
1
Mp = 204–206 C. H NMR (400 MHz, CDCl ): 4.1 (1Н, m), 4.0
2
.1.6. Oxidation of diamantane (12) in the TFA/NHPI/HNO /O
3
3
2
(
2
2
1Н, m), 2.65–2.6 (1Н, m), 2.45–2.4 (1Н, m), 2.25–2.2 (4Н, m),
system (cat. HNO3)
.2–2.15 (2Н, dd), 1.99–1.92 (4Н, m), 1.72–1.64 (2Н, dd). 13C NMR:
16.1 (C), 73.0 (CН), 65.2 (CН), 54.1 (CН), 42.5 (CН), 37.5 (CН), 36.4
The same as above, but 0.087 g (0.53 mmol, 0.1 eq.) of NHPI was
added, 1.05 g (97%) of 1-hydroxydiamantane (13) identical to the
standard sample [44].
(
CН ), 35.5 (CН), 34.9 (CН ). HR-MS: found 204.1219 (calculated
2
2
for C13H18O : 204.1206).
2
2
.1.7. Oxidation of cyclohexane (15) in the TFA/NHPI/HNO /O
3 2
2
.1.9. Toluene (1) oxidation to benzaldehyde (3), method A
Under oxygen atmosphere 6 mL of TFA was slowly added in
system
.194 g (1.19 mmol) of NHPI and 1.93 mL (2.55 g, 23.8 mmol)
5
1
ture was heated to 50 C and stirred for 4 h, strong nitrous oxides
evolution occurred during the course of reaction. Then CF COOH
0
portions to a stirred mixture of 2 mL (18.9 mmol) of toluene (1),
7
co-solvent (n-hexane or CH Cl ) at 0 C. The reaction mixture was
6%-HNO3 was added to a stirred solution of 1.28 mL (1 g,
50 mg (4.6 mmol) of NHPI, 525 mg (7.6 mmol) of NaNO in 6 mL of
2
1.9 mmol) of cyclohexane (15) and 18.2 mL of CF COOH. The mix-
3
◦
◦
2
2
stirred at room temperature for 13 h, quenched by the 10 mL of
water. Organic layer was separated and water part was extracted
with n-hexane (2 × 5 mL). Combined extracts were washed with
3
and 15 were removed in vacuum and 10% solution of NaOH (20 mL)
was added to the residue, stirred for 30 min, extracted with CH Cl2
2
1
0%-NaHCO solution (3 × 5 mL), water (2 × 5 mL), shaked with sat-
3
(
3 × 10 mL), the aqueous layer was refluxed with 0.2 g of activated
urated solution of NaHSO3 (10 mL) for 2 h and diluted with water
to dissolve the precipitate. Water layer was separated, washed
with n-hexane (2 × 5 mL) and 10 mL of 20%-NaOH solution was
added. A free aldehyde was extracted by pentane (3 × 5 mL), com-
carbon for 6 h and filtered. Residue was washed with small por-
tions of water (3 × 10 mL). Filtrate was acidified with diluted HCl
and crystals thus formed were filtered off and dried under vacuum
to give 800 mg (45%) of 16, identical to the standard sample.
bined extracts washed with water (2 × 5 mL), dried over MgSO and
4
evaporated to give 0.7 g (35%) of benzaldehyde (3) identical to the
2
.1.8. Oxidation of 3-oxadiamantane (17) in the
standard sample.
TFA/NHPI/HNO /O2 system
3
The mixture of 0.57 g (3 mmol) 3-oxadiamantane (17), [45]
2.1.10. Toluene (1) oxidation to benzaldehyde (3), method B
The same as Method A, but without a co-solvent gave 0.76 g
(38%) of benzaldehyde (3) isolated as above.
0
.048 g (0.1 mmol) NHPI, 0.24 mL (0.32 g, 3 mmol) 56% nitric
◦
acid, in 5 mL of TFA. The mixture was heated to 50 C and
stirred for 4 h, strong nitrous oxides evolution occurred during
the course of reaction. The GC–MS analysis of reaction prod-
ucts identified mixture of 3-oxadiamantane trifluoroacetates and
2.1.11. Toluene (1) oxidation to benzaldehyde (3), method C
Under the oxygen atmosphere a solution of 525 mg (7.6 mmol)
ketones. Then CF COOH was removed in vacuum and 8 mL of
of NaNO in 1 mL of water was added in portions to a stirred mixture
3
2
1
0% ethanol solution of KOH was added. The residue after evap-
of 2 mL (18.9 mmol) of toluene (1), 750 mg (4.6 mmol) of NHPI, 6 mL
of TFA at room temperature over 1 h. Reaction mixture stirred at
room temperature for 3.5 h to give 0.74 g, (37%) of benzaldehyde
(3) isolated as above.
oration of ethanol was dissolved in ether (20 mL), washed with
water (2 × 5 mL) and dried. Evaporation followed by column chro-
matography on silica gel with CH Cl /Methanol = 9/1 gave 0.19 g,
2
2
(
34%) of starting 3-oxadiamantane (17) and 0.105 g, (17%) of 6-
hydroxy-3-oxadiamantane (19) identical to standard sample, [45]
3
. Results and discussion
0.05 g, (8%) of 1-hydroxy-3-oxadiamantane (18), 0.093 g, (15%) of
11-hydroxy-3-oxadiamantane (20), 0.062 g, (10%), of 9-hydroxy-
3-oxadiamantane (21), 0.037 g, (10%) of 3-oxadiamantane-8-one
3.1. Consecutive PINO-promoted oxidations of toluene from
theoretical viewpoint
(
22), and 0.061 g, (10%) of inseparable mixture identified (GC–MS)
as isomeric ketones.
-Hydroxy-3-oxadiamantane (18)
The high rates of the NHPI-catalyzed oxidations of toluene are
determined by pronounced electrophilicity of the PINO-radical.
This is seen from the accelerations of the H-abstractions from
toluene derivatives with electron-donating substituents (negative
Hammett ꢀ-values) [46],[47]. Due to high polarization of the tran-
sition structures for the hydrogen abstraction the reaction is highly
sensitive to the polarity of the solvent [48]. In order to estimate the
influence of PINO on the aerobic oxidations of toluene we first mod-
eled the transformations computationally [49]. As the non-covalent
interactions, in particular ␲–␲ stacking, may contribute substan-
tially to the H-abstractions with PINO from aromatics [50] and the
traditional DFT-models may be not relevant, [51] we employed the
M06-2X method, [52] which is parametrized to account dispersions
through the medium-range electron correlations. This method was
previously successfully applied for different classes of hydrogen-
bonded, van der Waals ␲–␲ stacked clusters [53–56]. We compare
the results with empirically dispersion-corrected method B3LYP-
D3 [57]. To account for the solvent effects we utilize a polarizable
1
Mp = 178–179 C. 1H NMR (400 MHz, CDCl ): 3.95 (bs, 1Н), 3.55
◦
3
(
bs, 1Н), 2.95 (s, 1Н), 2.46–2.39 (m, 1Н), 2.75 (s, 1Н), 2.11 (s, 1Н),
2
7
.4 (m, 1Н), 1.99–1.91 (m, 1Н), 1.9–1.79 (dd, 2Н), 1.73–1.55 (m,
Н), 1.45 (dd, 1Н). 13C NMR: 78.9 (CН), 68.2 (C), 65.5 (CН), 42.4
(
CH ), 41.9 (CH), 38.1 (CH), 37.8 (CH), 36.6 (CH ), 36.1 (CH ),
2 2 2
35.7 (CH ), 34.6 (CH), 31.0 (CH ), 28.9 (CH). HR-MS: found
2 2
2
06.1321 (calculated for C13H18O : 206.1311).
2
1
1-Hydroxy-3-oxadiamantane (20)
◦
1
Mp. = 264–266 C. H NMR (400 MHz, CDCl ): 3.9 (bs, 1Н), 3.68
3
(
1
(
bs, 1Н), 2.29 (bs, 2Н), 2.13 (dd, 2Н), 2.05 (m, 1Н), 1.8 (m, 3Н),
.75 (dd, 2Н), 1.65 (m, 4Н), 1.55 (dd, 2Н). 13C NMR: 73.5 (CH), 70.0
C), 65.6 (CH), 45.8 (CH ), 41.7 (CH), 39.1 (CH), 35.4 (CH ), 31.6
2
2
(
CH ), 29.6 (CH). HR-MS: found 206.1335 (calculated for C13H18O :
2
2
2
06.1302).
9
-Hydroxy-3-oxadiamantane (21)

P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
75
Fig. 1. Consecutive H-abstractions from toluene (1), benzyl alcohol (2), and benzaldehyde (3) with PINO- (TS1–TS3), peroxybenzyl- (TS4–TS6) radicals, and from NHPI (TS7)
and toluene (TS8) with NO2-radical (relative ꢁG298 at PCM(H2O)-M06-2X/cc-pVDZ).
continuum model (PCM) as implemented in Gaussian09 program
58].
The oxidation proceeds as a consecutive H-abstractions from
an agreement with the experiment where 2 typically is a minor
component in the reaction mixtures of toluene oxidations [28,65].
The critical is that the barrier for the H-abstraction from ben-
[
=/
toluene (1), benzyl alcohol (2), and benzaldehyde (3, Fig. 1). The
first H-abstraction by PINO from toluene (1) to form benzyl radical
zaldehyde (3) through TS3 is substantial (ꢁG298 = 20.4 kcal/mol)
and is close to that for H-abstraction from 1 (TS1). Remarkably,
the ␲-␲ stacking is not present in TS3 due to distant loca-
tion of the aromatic moieties. Relatively high barrier and high
endergonicity (ꢁG298 = +10.1 kcal/mol) of 3 → 6 step facilitate the
termination of the reaction at the stage of aldehyde 3. The fact that
toluene is only slightly less reactive than benzaldehyde towards
=/
(
4) requires a barrier of ꢁG298 = 21.1 kcal/mol (M06-2X/cc-pVDZ,
TS1, Fig. 1) [59]. The TS1 in the cis-conformation is substantially sta-
bilized by ␲–␲-stacking as the respective trans-form (not shown)
is c.a. 4 kcal/mol higher in energy both at M06-2X and B3LYP-D3. As
the net NBO charge and spin on the hydrocarbon part of TS1 is sub-
stantial (+0.3e and 0.6, respectively) mechanistically the reaction
is best viewed as the proton-coupled electron transfer previously
suggested for the CH-activations of hydrocarbons with charged
electrophiles [60,61]. Such pronounced polarization makes the
reaction markedly solvent-dependent (vide infra) [62]. As the 1 → 4
•
=/
PINO (ꢁG298 = 21.1 vs 20.4 kcal/mol) is in accord with the
experiment which displays ktoluene/kbenzaldehyde = 0.67 (see SI for
details).
All above is in a sharp contrast with the reaction without
PINO where the peroxybenzyl- (BnOO) radicals are responsible for
the H-abstractions. The oxidation consequently proceeds through
TS4 → TS5 → TS6 without contribution of ␲–␲ stacking (Fig. 1). The
barrier for the H-abstraction from toluene is 2 kcal/mol higher for
transformation is substantially endergonic (ꢁG2 = +9.4 kcal/mol),
98
the H-abstraction with PINO is effective only in the presence
of molecular oxygen [63], which traps benzyl radical (4) and
makes the process irreversible (step C, Scheme 1). Recently the
•
•
BnOO than for PINO (cf. TS4 and TS1) that agrees well with the
experimental kinetics, which displays that the PINO-radical is much
more reactive than the peroxyl-radicals [62]. Additionally, toluene
=/
TSs for the H-abstraction from toluene (ꢁG298 = 20.6 kcal/mol)
with PINO were located at the B3LYP-DCP/6-31G(2d,2p) level of
theory and the contribution of ␲–␲ stacking was revealed [64].
The barrier agrees well with our computations at M06-2X/cc-pVDZ
•
=/
(1) reacts much slower with BnOO (ꢁG
than aldehyde 3 (ꢁG298 = 20.1 kcal/mol, TS6). Thus, the over-
oxidation to benzoic acid is more favored for BnOO than for the
= 23.1 kcal/mol, TS4)
298
=/
=/
•
(
ꢁG298 = 21.1 kcal/mol).
The next oxidation step, the H-abstraction from benzyl alco-
PINO-radical.
hol (2) with PINO through TS2, requires only moderate barrier
of 15.7 kcal/mol and this reaction, in contrast to the first step, is
thermoneutral. As in the case of TS1, the ␲–␲ stacking substan-
tially stabilizes TS2 and reduces the barrier even further. This is
We also estimated the barriers for the H-abstractions with
NO -radical from NHPI that is a primary initiation step under
2
the metal-free conditions (step A, Scheme 1). The barrier for
=/
this reaction (ꢁG298 = 21.3 kcal/mol, TS7) is much lower than

7
6
P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
Scheme 2. The NHPI-catalyzed and HNO3-promoted aerobic oxidations of hydrocarbons 1, 9 and 12 in trifluoroacetic acid (yields are preparative).
Scheme 3. The NHPI-catalyzed and HNO3-promoted aerobic oxidations of nonactivated substrates 15, and 17 in trifluoroacetic acid (yields are preparative).
the barrier for the H-abstraction from toluene with the NO -
nitrodiamantane (14) in 41% preparative yield in the HNO /TFA/O2
2
3
=/
radical (ꢁG298 = 27.2 kcal/mol, TS8). The latter cannot compete
for toluene with the PINO-radical due to much lower energy of TS1
system. In contrast, the nitration is suppressed in the presence of
catalytic amounts of HNO3 where alcohols 10 and 13 form exclu-
sively (Scheme 2). The formation of respective alcohols (10 and 13)
and nitro-derivatives (11 and 14) in the TFA/HNO /O system could
=/
(
ꢁG298 = 21.1 kcal/mol).
3
2
3
.2. Preparative aerobic oxidations with NHPI in trifluoroacetic
be attributed to the TFA-mediated electrophilic CH-substitution
[75–77]. Note, that the presence of NHPI has no influence on the
ratio of the products for cage substrates 9 and 12 as it would be
expected for non radical reaction. Changing the solvent from TFA
to acetic acid gave unchanged starting materials for all substrates
(1, 9 and 12), even in presence of nitric acid excess. We conclude
that TFA/HNO3/O2 system promotes electrophilic reactions and is
not suitable for toluene conversion to benzaldehyde.
acid initiated by HNO3
Exceptionally high polarizations of the transition structures for
the H-abstraction from toluene with PINO favor the oxidations
with NHPI in polar solvents. Acetic acid was first demonstrated
the efficiency as a solvent for aerobic NHPI-catalyzed oxidations
of cyclooctane [66] and then for many other hydrocarbons [28,48].
The fast equilibrium between PINO and ROO exists even in H-
bonding solvents and this, together with the high rate constants for
the H-abstractions by PINO, determine the propagation efficiency
of the NHPI-catalyzed oxidations in protic acids [67].
Trifluoroacetic acid (TFA) is among the most polar organic sol-
vents and is characterized by the highest known acceptor number
of 105.3 (cf. 52.9 for acetic acid) [68]. To the best of our knowl-
edge it has never been tested as a media for the NHPI-chemistry.
Considering the significance of “green” nitroxide-catalyzed aerobic
oxidations [69] we employ a standard transition-metal-free oxida-
tive system with NO2 as an initiator [48]. We first utilize the nitric
•
•
Nevertheless, this system appears to be very effective for the
oxidations of saturated hydrocarbons that are less reactive than
1, 9, and 12 towards nitronium electrophiles. In particular, we
were able to oxidize cyclohexane (15) to adipic acid (16) in
NHPI/HNO3/TFA/O2 system. To achieve satisfactory conversions
for such deep oxidations larger NHPI loadings (up to 10%) are
required. The reaction displays exceptional selectivity as it allows
to achieve 45% preparative yield of 16 at 50% conversion of starting
material. Note, that the most successful previous NHPI-catalyzed
◦
aerobic oxidation of 15 (with Mn(acac)2 in acetic acid at 100 C)
[78] reached 73% preparative yield of 16 at 73% conversion. Our
NHPI/HNO3/TFA/O2 system is preparatively convenient as TFA and
unreacted cyclohexane (15) are easily recovered from the reaction
mixture by distillation. All the attempts to oxidize 15 in acetic acid
or without NHPI gave no conversion of starting material, which
displays the key role of TFA in the PINO-promoted H-abstraction
step. From this point of view, the behavior of 3-oxadiamantane
(17), which is low reactive towards electrophiles [45] is instruc-
tive. It is noteworthy that the transformation of 17 occurs in the
HNO3/TFA/O2 system only in the presence of NHPI that is the
strong evidence for the radical process promoted by the PINO-
radical. While in the NHPI/HNO3/AcOH/O2 system 17 is unreactive,
acid as the NO -source, however, the transformation of toluene
2
in the NHPI/TFA/O2 system displays almost exclusive formation
of a mixture of ortho- (7) and para- (8) nitro toluenes as a result
of well-documented [70] electrophilic aromatic substitutions in
TFA (Scheme 2). The same product ratio was found in absence of
NHPI. Taking to account that relatively high reactivity of 9 towards
nitric acid or nitronium reagents have already been reported by
many [71–74], similar behavior for adamantane (9) and diaman-
tane (12) is expected. The ratio of the products depends on the
amounts of HNO employed. With one equivalent of HNO 9 forms
3
3
1
-nitro adamantane (11) in 42% preparative yield and 12 forms 1-

P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
77
Table 1
The oxidations of toluene (1) in trifluoroacetic acid (TFA).
#
Methoda
Temp.
RT
Time, h
Conv. of 1, %
(2), %
(3), %
(23), %
Isolated yield of 3, %
1
2
3
4
5
6
7
8
9
Ab
A
A
25
13
4.5
13
2
4.5
22
20
13
4
traces
2
0.5
7
4
3
traces
0
traces
0.1
38
0.6
48
31
42
1.4
0
0.3
0
0
5
11
3
3
0
0
–
35
–
38
–
37
–
RT
83
12
98
90
89
18
0
◦
◦
◦
10
35
45
RT
RT
RT
RT
C
C
C
c
B
c
B
d
C
e
A
f
A
–
–
g
A
7
1.1
a
b
c
d
e
f
Method A: 18.9 mmol of toluene, 4.6 mmol of NHPI, 6.2 mmol of NaNO2, 6 mL of hexane, 6 mL of TFA.
Acetic acid used instead of TFA.
Method B: Same as Method A, but without n-hexane.
Method C: The same as Method B but NaNO2 was added in 1 mL of water dropwise over 1 h.
without NHPI.
without NaNO2.
under argon atmosphere.
g
the oxidations in TFA gave 1-hydroxy-3-oxadiamantane (18),
-hydroxy-3-oxadiamantane (19), 11-hydroxy-3-oxadiamantane
20), 9-hydroxy-3-oxadiamantane (21) and ketone (22) (Scheme 3,
for the structure assignments see SI). Performing the reaction of
-oxadiamantane (17) in this system followed by alkaline hydrol-
ysis allowed us to synthesize and isolate previously inaccessible
alcohols (18, 20, and 21) and ketone (22). Note, that the sub-
stitution with electrophiles [45] give exclusively the products of
medial substitution in position “6”. Apically substituted 9-hydroxy-
saturated NaHSO3 solution. The GC/MS analyses identified the for-
mation of nitro-toluenes 7 and 8 as main by-products together
with isomeric nitro-benzaldehydes. We compared two different
procedures, namely the addition of solid NaNO2 with (Method A,
Table 1, Entries 2 and 3) or without (Method B, Entry 4) co-solvent
n-hexane and did not find any substantial differences. The signs
of the over-oxidation were observed only under slight heating,
which substantially accelerates the reaction but lowers its selec-
tivity due to formation of substantial amounts of benzoic acid
(Table 1, Entry 5). As some previous NHPI-oxidations in bipha-
sic systems [90,91] or in the aqueous organic solvents [92] were
successful, we also utilize NaNO2 as a saturated water solution
(Method C, Entry 6). All three procedures (A–C) gave benzalde-
hyde (3) in 35–38% preparative yield. Note that the transformations
require relatively large amounts of NHPI (up to 25%) to complete;
6
(
3
3
-oxadiamantane (21) obtained in the NHPI/TFA/HNO /O2 system
3
is useful for surface depositions as the apical diamondoid deriva-
tives are characterized by superior surface affinities [79].
We conclude, that while the TFA/HNO3 system demonstrates
great potential as a media both for nitronium reagents mediated
electrophilic reactions and deep NHPI-catalyzed aerobic oxida-
tions, but, again, it is not applicable for target selective oxidation of
methyl group in toluene due to fast electrophilic substitutions.
reducing the amounts of NaNO also substantially slower the reac-
2
tion. The fact that only minor amounts of benzyl alcohol (2) and
benzoic acid (23) were found in the reaction mixtures at room
temperature agrees well with the computations where the bar-
rier for the H-abstraction with PINO-radical is much higher from
3.3. Aerobic oxidations of toluene with NHPI in trifluoroacetic
acid initiated by NaNO2
3
(Fig. 1, TS3) than from 2 (TS2). Only very slow reaction was
observed in the absence of NHPI (Table 1, Entry 7), where ben-
zoic acid dominates over 3 that again is in agreement with the
computational picture of the reaction carried on by BnOO-radical
In order to avoid undesired electrophilic aromatic substitutions,
the next logical step is to replace the nitric acid by alternative source
of the NO -radical. Sodium nitrite (NaNO ) is an environmentally
2
2
(Fig. 1). The reaction is completely suppressed in the absence of
friendly inexpensive transition-metal-free “green” [80–84] catalyst
and produces NO under the mildly acidic conditions. It is useful for
the aerobic oxidations of alcohols to aldehydes and ketones in the
presence of 2,2,6,6-tetramethyl-piperidyl-1-oxyl (TEMPO) in the
acetic acid [85] and TFA [86]. In contrast, TEMPO acts as a radical
promotor NaNO2 (Table 1, Entry 8) and only traces of oxidation
products form under argon atmosphere (Table 1, Entry 9) demon-
strating the true aerobic nature of the reaction. In the NaNO /NHPI
2
system the main by-products resulted from the electrophilic nitra-
tion and are identical to those obtained with HNO3 (7 and 8,
Scheme 2).
3
scavenger and inhibits the aerobic oxidations of the sp C-H bonds:
In the NaNO /TFA system [87] cyclohexanol is oxidized to 16 quan-
2
Relatively unstable nitrous acid HNO formed from the reaction
titatively with 2 eq. of NaNO , whereas cyclohexane (15) is inert
2
2
of NaNO2 with TFA produces both radical (NO) and electrophilic
under these conditions [88]. The NaNO -promoted aerobic oxida-
2
+
(
NO ) [93] species (note that acetic acid is not effective, presum-
tions in aqueous HCl, were recently applied for activated benzylic
positions [89]. Previously, NaNO2 in trifluoroacetic acid was used
for selective nitration of aromatic compounds [75–77]; remarkably,
no oxidation products of methylarenes were observed.
ably due to its low acidity) [77,94]. Consequently, two different
reaction paths, namely radical and electrophilic were previously
identified for alkylbenzenes in the NaNO /TFA system [77]. The first
2
path led to the conventional aromatic nitration at room tempera-
ture, while the free-radical side-chain oxidations require heating
We first tested the acetic acid as a media for the oxidations of 1
with NaNO /NHPI and found a negligible conversion even at pro-
2
[77]. In our case, due to the presence of radical source (NHPI)
longed reaction times (Table 1, Entry 1). In contrast, in TFA at room
temperature in the presence of catalytic amounts of NaNO2 the
reaction proceeds smoothly displaying high conversions of 1. To
our surprise even at 83% conversion of 1 benzaldehyde (3) still
dominates in the reaction mixture and benzoic acid (23) forms
only in trace amounts (Table 1, Entry 2). The by-products were
separated after 3 was washed out from the reaction mixture with
and oxidant (O ), radical processes (Paths A–C, Scheme 4) dom-
2
inate over electrophilic substitutions. Additionally, electrophilic
+
nitrosonium cation NO , which is present in the reaction media
[93] may facilitate the side-chain reactions through the oxidation of
intermediately-formed radicals (Path D, Scheme 4). Indeed, while
•
•
the H-abstraction from toluene with PINO to give PhCH2 (4) is

7
8
P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
Scheme 4. The aerobic oxidation of toluene (PhCH3) carried by the phthalimide N-oxyl (PINO) radical generated from N-hydroxyphthalimide (NHPI) in the NaNO2/TFA
+
system with involvement of the nitrosonium cation (NO ).
Table 2
a
The aerobic oxidations of substituted toluenes in trifluoroacetic acid (TFA) .
◦
Isolated yield of aldehyde,b
%
#
Substrate
Temp.,
C
Time, h
Conv., %
%
%
%
1
2
3
4
5
6
7
8
p-CH3
p-Cl
m-Cl
o-Cl
p-Br
m-Br
o-Br
RT
45
45
45
45
45
45
45
1.5
3
3
3
3
3
3
3
100
67
65
46
60
37
37
0
10
2
3
3
4
1
2
0
46
49
27
18
36
21
12
0
–
35
45
16
12
22
13
10
–
traces
17
14
traces
traces
traces
0
p-NO2
a
Method C (18.8 mmol of toluene derivative, 4.6 mmol of NHPI, 6.2 mmol of NaNO2 in 1 mL of water, 8 mL of TFA).
b
For the 1H- and C NMR spectra of the isolated aldehydes see SI.
13
9
.4 kcal/mol endergonic (Fig. 1 and Path B, Scheme 4) the reac-
4. Conclusions
•
+
+
•
tion with electrophile PhCH2 + NO = PhCH2 + NO is 22.5 kcal/mol
exergonic (M06-2X/cc-pVDZ, Path D).
Highly polar trifluoroacetic acid demonstrates its efficiency as
a solvent for the metal-free aerobic NHPI-catalyzed oxidations of
toluenes as well as saturated systems such as cyclohexane and
3-oxadiamantane. In contract, with moderately reactive adaman-
tane and diamantane NHPI is inactive as faster electrophilic
H-substitutions take place. While with the HNO3 as the initiator
the reaction is effective for the oxidations of nonactivated saturated
substrates, the electrophilic aromatic substitutions take place for
toluene. Markedly, the utilization of catalytic amounts of sodium
nitrite NaNO2 allows to achieve unprecedentedly high selectiv-
ities for toluene conversion to benzaldehyde almost completely
avoiding over-oxidations. Besides the ability to generate HNO2,
trifluoroacetic acid encourages the formation of low-electrophilic
nitrozonium cation, which is able to oxidize the radical intermedi-
ates and shift the otherwise unfavorable radical equilibria en route
to benzaldehyde.
Thus formed NO-radical (or N O3 formed from the decomposi-
2
tion of HNO , Path C) is oxidized with molecular oxygen to give
2
•
the NO -radical, which generates PINO from NHPI (Path A), while
2
PhCH2+is trapped with water to give benzyl alcohol (Path E). Note
that the NO-radical is a very poor hydrogen abstractor, and its
reaction with NHPI is not operative under the reaction conditions
(
the reaction NHPI + NO = PINO + HNO is ca. 32 kcal/mol endergonic
at M06-2X/cc-pVDZ; in contrast’ the hydrogen abstraction from
NHPI with NO2 (Path A) is slightly exergonic). The involvement of
the electrophilic PINO-radical into the H-abstraction step (Path B)
determines the high sensitivity of the reaction towards the sub-
stituents present in the phenyl ring. With p-xylene (Table 2, Entry
1
) the reaction proceeds slightly faster than for 1 as the 100% con-
version was achieved after 1.5 h. The preparative yield of p-methyl
benzaldehyde is close to that of toluene (35%).
In contrast, the oxidation of isomeric chloro- (Table 2, Entries
2
–4) and bromo- (Entries 5–7) toluenes requires slight heating
Acknowledgements
due to the presence of electron-withdrawing groups in the ben-
zene ring. Substantial amounts of corresponding chlorobenzoic
acids form at this temperature and the yields of aldehydes are
reduced. The highest preparative yields of the respective halo-
aldehydes was achieved for the oxidation of p-chloro (49%, Entry
The work in P.R. China was supported by Sino-Ukraine Tech-
nology Park Platform Construction (Grant # 2014C050012001) and
Otava Ltd. Company. A.A.F. is grateful to the Institute of Applied Sys-
tem Analysis (Kiev, Ukraine) for computing facilities. We thank the
Institute of Organic Chemistry of University Giessen (Germany) for
NMR and MS facilities and Professor Peter R. Schreiner for fruitful
discussions.
2
) and p-bromo toluenes (36%, Entry 5). This is in full agreement
with the previous kinetic studies of the reactions with PINO in
acetic acid,[46] where para-substituted toluenes are the most and
ortho- are the least reactive. Due to strong electron withdraw-
ing effects of the NO -group p-nitro toluene remains unchanged
2
under this reaction conditions (Table 2, Entry 8) that is also a char-
acteristic for the NHPI-catalyzed reaction in the acetic acid [6].
All above data evidence strongly for the exclusive participation of
the PINO-radical in the H-abstractions from the methyl group in
TFA.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.12.
017.

P.A. Gunchenko et al. / Molecular Catalysis 447 (2018) 72–79
79
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