C70Fullerene Catalyzed Photoinduced Aerobic Oxidation of Benzylamines to Imines and Aldehydes

Benzylamines to Imines and Aldehydes

Article

C Fullerene Catalyzed Photoinduced Aerobic Oxidation of

Inder Kumar, Rakesh Kumar, Shiv Shankar Gupta, and Upendra Sharma*

Cite This: J. Org. Chem. 2021, 86, 6449 −6457

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sı Supporting Information

ABSTRACT: C₇₀fullerene catalyzed photoinduced oxidation of

benzylic amines at ambient conditions has been explored here. The

developed strategy’s main feature includes the additive/oxidant-

free conversion of benzylic amine to corresponding imine and

aldehydes. The reaction manifests broad substrate scope with

excellent function group leniency and is applicable up to the gram

scale. Further, symmetrical secondary amines can also be

synthesized from benzylic amine in a one-pot two-step process. Various experiments and density functional theory studies revealed

that the current reaction involves the generation of reactive oxygen species, single electron transfer reaction, and benzyl radical

formation as key steps under photocatalytic conditions.

INTRODUCTION

Catalytic oxidation is always a fascinating and useful strategy in

the ﬁeld of organic synthesis. Considering the increased

Scheme 1. Oxidative Conversion of Primary Benzylic

Amines to Imines

■

environmental concern, researchers are continuously focusing

on greener synthetic approaches. A wide variety of metal-based

oxidants and organic oxidants such as H O , NaOCl, PhIO,

and t-BuOOH have been used continuously; however, most

of these are associated with harsh reaction conditions, limited

selectivity, and lower competence that bounded their practical

applications. Consequently, in recent time, catalytic aerobic

oxidations remain an eventual technique for producing

valuable chemicals using O as the solitary oxidant. Recently,

photocatalyzed emergence of reactive oxygen species (ROS)

from O /air for chemical oxidation is being explored.

Among the various chemical oxidations, the oxidative

coupling of primary benzylic amines to imine derivatives

received much consideration from chemists due to the

prevalence of this building block in pharmaceuticals and ﬁne

chemists. Hence, various research groups have developed

photocatalytic methodologies to synthesize imines via

oxidative coupling of primary benzylamines. Jiang and Lei’s

group independently reported MOF photocatalyzed oxidative

chemicals. Moreover, the conversion of benzylic amines to

8c,16

coupling of benzylic amines,

whereas similar trans-

benzaldehydes via C−N bond cleavage also represents a

formation is reported by other groups using modiﬁed

paramount functional group transformation in organic syn-

carbon-based photocatalytic systems such as silver-doped

thesis. Accordingly, researchers have developed diﬀerent

graphene oxide and carbon nitride. Metal-based photo-

catalytic oxidation of benzylic amines to imines or aldehydes is

methods toward the oxidation of benzylamines to commensu-

rate imines and benzaldehydes. Leading examples comprise

also disclosed. However, these methods involve either a

metal/metal-free dehydrogenative coupling, metal−organic

complex catalytic system or harsh reaction conditions, thus

^lⁱ^mⁱ^tⁱⁿ^g^t^h^eⁱ^r^u^tⁱ^lⁱ^t^y^.^H^e^r^eⁱⁿ^,^w^e^r^e^p^o^r^t^p^rⁱ^s^tⁱⁿ^e^C70 fullerene

framework (MOF) catalyzed oxidative coupling, vitamin/

enzymatic amine oxidation, ionic liquid assisted oxidation,

naphthoquinone catalyzed aerobic dehydrogenation, gra-

phene oxide catalyzed coupling of amines to imines, ball

milling oxidation, graphene/graphite oxide catalyzed oxida-

Received: February 6, 2021

Published: April 22, 2021

2,14

tion,

Scheme 1).

Currently, photocatalysis is being explored as a greener

approach in organic synthesis and is a handy tool for synthetic

and zinc dichromate catalyzed oxidative deamination

(

2021 American Chemical Society

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catalytic conditions to form the analogous oxidized products.

Article

catalyzed oxidation of primary benzylamines to corresponding

imines and aldehydes in a state of metal/oxidant-free

conditions. Fullerenes are known as exceptional photo-

sensitizers for decades and exhibit prominent potential to

produce ROS under aerobic conditions. Using this property,

fullerenes were explored for secondary benzylamine oxidation

(Table 1, entries 2−4). The reduction in the product yield

with green or white light may be due to the absorption maxima

of the C fullerene catalyst, which lies in the blue light region.

Very low product emergence or no product was observed

either in the absence of C70 catalyst or light irradiation,

respectively, thus manifesting the developed strategy’s photo-

catalytic disposition (Table 1, entries 5 and 6). Only 12%

NMR yield of 3a was observed, when the reaction was carried

under argon atmosphere (Table 1, entry 7), thus indicating the

crucial role of molecular oxygen in the course of the reaction.

Further, acetonitrile replacement with other solvents had a

negative e ﬀ ect on product yield (Table 1, entry 8; see

Supporting Information ). The use of air instead of molecular

oxygen was not adequate (Table 1, entry 9). Additionally, the

reduction in reaction time, catalyst amount, and the amount of

solvent was harmful to the reaction outcome (Table 1, entries

to imines but only in an intramolecular fashion.

During our previous study on the photocatalytic conversion

of phenyl boronic acid to corresponding phenols, we

perceived the hydrolysis of diisopropylethylamine to diisopro-

pylamine (Figure 1). Building on this observation, we

envisaged the benzylic C−N bond cleavage under photo-

0−12).

Next, we explored the substrate scope by reacting various

primary benzylic amines under the developed reaction

condition (Table 2). Benzylamines with −Me and −Cl

Figure 1. ROS-assisted photocatalytic cleavage of the C−N bond.

Initially, benzylamine, 1a (0.1 mmol), was reacted with

phenylboronic acid, 2a (0.1 mmol), in the presence of C₇₀

Table 2. Substrate Scope of Benzylic Amines for the

Synthesis of Corresponding Imines

fullerene (0.05 mol %) in CHCl (0.05 M) under blue light-

emitting diode (LED) irradiation. Gas chromatography−mass

spectrometry analysis of this reaction revealed the formation of

benzaldehyde (4a) along with phenol. Formation of 4a was

observed even without 2a (Table 1, entry 1), thus conﬁrming

Table 1. Screening and Optimization of Conditions

entry variation from standard condition 3a yield (%)

4a yield (%)

c e

none

99 (98)

traces

60 (55)

C₆₀fullerene catalyst

white light instead of blue light

green light instead of blue light

without catalyst

without light

under Ar

1,4-dioxane instead of CH CN

air instead of O₂

12 h instead of 24 h

0.025 mol % of catalyst

0.2 M CH CN

traces

Reagents and conditions: 1a (0.2 mmol), C₇₀fullerene catalyst (0.05

mol %), CH CN (0.05 M), 34 W blue LED light, 24 h. Yield

determined by NMR analysis of crude reaction mixture using 1,1,2,2-

tetrachloroethane as an internal standard. Isolated yield in

^R^e^a^g^eⁿ^t^s^aⁿ^d^c^oⁿ^dⁱ^tⁱ^oⁿ^s^:¹^a⁽⁰^.²^m^m^o^l⁾^,^C70 fullerene catalyst (0.05

parentheses. Reaction solvent was CH CN. Reaction solvent was

mol %), CH CN (0.05 M), 34 W blue LED, 24 h.

CHCl₃.

the direct oxidation of benzylic amines to benzaldehyde.

However, when the same reaction was carried out in MeCN as

a solvent, we got (E)-N-benzylidene-1-phenylmethanamine 3a

as a major product in quantitative yield instead of 4a (Table 1,

entry 1). Further, any divergence from these reaction

conditions has a detrimental eﬀect on the yield of product

substituent at the C-2 position resulted in oxidative imine

products 3b and 3c in 82 and 86% yield, respectively. Benzylic

amines with various functional groups at the C-3 position were

successfully converted to corresponding imine product with

good to plenteous yields (3d−g). Imines with methyl (3d) and

triﬂuoromethyl (3e) substituents were obtained in 96 and 86%

isolated yields, respectively. Similarly, −F- and −Cl-substituted

benzylic amines resulted in 98 and 74% isolated yields of

a. Changing the catalyst to C fullerene or changing the light

source to white or green was ineﬃcient for the current reaction

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requisite products 3f and 3g, respectively. A variety of

substituents were well-tolerated at the C-4 position of benzylic

amines. Amines with alkyl substituents such as methyl and

tertiary butyl led to the formation of imine products (3h−i) in

carbonyl products (Table 4). N-Methyl, N-butylbenzylamines,

and N-allylbenzylamine (5a−c) brought forth the benzalde-

Table 4. Scope of Secondary and Tertiary Benzylic

a,b

4−98% yield. 4-Triﬂuoromethyl benzylamine provided the

corresponding imine 3j in 80% isolated yield. We obtained a

8% isolated yield of 4-phenoxy-substituted imine 3k. The

Amines

halogen substituents at C-4 positions of primary benzylic

amine were well-tolerated (3l−o). 4-Fluoro-, 4-chloro-, 4-

bromo-, and 4-iodo-substituted amine generated products 3l−

o in 84−96% isolated yields.

A triﬂuoromethylthio substituent at the C-4 position was

also well-tolerated under the current reaction condition, and as

expected, product 3p was obtained in 67% yield. 2,4-

Dichlorobenzylamine also reacted successfully under the

developed reaction condition (3q).

Further, photocatalytic oxidation of various benzylic amines

to corresponding benzaldehyde was explored (Table 3).

Reagents and conditions: 1a (0.2 mmol), C₇₀fullerene catalyst (0.05

mol %), CHCl (0.05 M), 34 W blue LED, 24 h. Percentage NMR

yields only.

Table 3. Substrate Scope of Benzylic Amines for the

Synthesis of Corresponding Benzaldehydes

hyde formation in good yields. Dibenzylamine (5d) also

displayed good reactivity under the devised conditions and

rendered the carbonyl product in excellent yield. Conversely,

N-phenylbenzylamine (5e) bestowed only 10% yield of

oxidized product, which might be attributed to the

involvement of the nitrogen lone pair in resonance with the

benzene ring, making it less readily available for donation.

Alternatively, N,N-dimethylbenzylamine (5f) dispensed the

anticipated benzaldehyde in good yield. Tertiary benzylic

amines with substituents such as 4-tert-butyl (5g) and 4-chloro

(

5h) led to the formation of corresponding benzaldehydes in

0 and 62% NMR yield, respectively. Likewise N,N-dimethyl-

-(4-((triﬂuoromethyl)thio)phenyl)methanamine (5j) fur-

nished the expected benzaldehyde in 36% NMR yield (Table

4).

Also, we endeavored the scale-up synthesis with 4-

phenoxybenzylamine (Scheme 2) and successfully isolated

Reagents and conditions: 1a (0.2 mmol), C₇₀fullerene catalyst (0.05

mol %), CHCl (0.05 M), 34 W blue LED, 24 h.

1.81 g of the imine product while attempting the reaction at 10

Benzylic amine with a methoxy and ethoxy substituent at the

C-2 position converted to corresponding carbonyl products

mmol scales.

(

4b,c) in 63−78% yields. Chloro-substituted benzylamine at

Scheme 2. Gram-Scale Synthesis of Imine

the C-2 position gave only 37% isolated yield of oxidized

carbonyl product 4d. Photocatalytic oxidation of primary

benzylic amine having methoxy and chloro substituents at the

C-3 position furnished the corresponding benzaldehydes (4e,f)

in 69−92% yield. Benzylic amines comprising alkyl sub-

stituents such as methyl and tertiary butyl at the C-4 position

were also well-tolerated under the developed conditions and

provided the expected products 4g,h in good yield. Similarly,

we isolated 4-methoxybenzaldehyde (4i) in 60% yield.

Halogen-substituted benzylic amines such as 4-chloro and 4-

bromo aﬀorded the desired products 4j and 4k in good yield.

Di- and trisubstituted benzylic amines were also found to be

compatible, and 2,4-dimethoxybenzaldehyde (4l) was isolated

in 44% yield, whereas 3,4,5-trimethoxybenzaldehyde (4n) was

obtained in 46% isolated yield. Furthermore, 3-chloro-4-

methoxybenzylamine gave 96% isolated yield of expected

product 4m.

Furthermore, we brought in a one-pot two-step conversion

of benzylic primary amine to symmetrical secondary amine

(Scheme 3). Here, we used a combination of polyethylene

glycol and NaBH for the reduction of oxidized dimer

product.

Quenching experiments with 1,4-diazabicyclo[2.2.2]octane

(DABCO), benzoquinone and 2,2,6,6-Tetramethylpiperidine

1-oxyl (TEMPO) resulted in diminished or extremely low

product yield, endorsing the ubiquity of radical species (ROS)

in the course of the present reaction (Scheme 4).

In addition, we performed the oxidation of benzylamine to

After the successful oxidation of primary benzylic amines to

corresponding benzaldehydes, we explored the oxidation of a

few secondary and tertiary benzylic amines to corresponding

analogous aldehyde in the presence of O (Scheme 5).

Liquid chromatograph−mass spectrometry (LC-MS) analysis

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reaction in that interval of time.

Article

Scheme 3. One-Pot Two-Step Synthesis of Symmetrical

Secondary Amines

Scheme 4. ROS Quenching Experiments

Figure 3. UV−visible absorption spectra for the detection of H O .

Based on these experiments, we propose that the current

reaction involves the generation of O from O via energy

^t^r^aⁿ^s^f^e^r^f^r^o^m^p^h^o^t^o^a^c^tⁱ^v^a^t^e^d^C70 fullerene (Scheme 6). O

of the crude reaction mixture evinced the incorporation of O

in the oxidation product (corresponding benzaldehyde).

Scheme 6. Proposed Mechanism for the Photocatalytic

Oxidation of Benzylic Amine to Imine and Aldehyde

Scheme 5. ¹⁸O-Labeling Experiment

As the reaction proceeds through the free radical

mechanism, we disproved the possibility of a chain reaction

mechanism through light on−oﬀ experiments (Figure 2).

There was no increase in the yield of the product when the

light was turned oﬀ, thus insinuating the cessation of the

then undergoes single electron transfer with 1a to form O ^•

−

and amine radical cation. The two products thus generated

undergo hydrogen transfer to generate imine I and H O .

Further, pathway a involves the nucleophilic addition of 1a and

intermediate I to give II, which then gives the product 3a with

the elimination of NH . Additionally, 3a may undergo

hydrolysis to give 4a (see Supporting Information). Pathway

b involves the direct hydrolysis of I to give benzaldehyde 4a.

Moreover, we observed exclusively imine product 3 while using

CH CN solvent, whereas aldehyde 4 as the major product with

CHCl solvent. This selectivity of product due to solvent might

be attributed to the acidic/basic nature of solvent being used

for the synthesis of these compounds. Chloroform being

weakly acidic in nature may favor the hydrolysis of imine I to

a, whereas acetonitrile being weakly basic in nature, may be

endorsing the nucleophilic addition reaction among 1a and I,

thus yielding product 3a (Supporting Information, Figure S6).

Further, the formation of intermediate I from an imine

radical cation may occur through two possible intermediates, A

Figure 2. Disproved radical chain reactions through light on−oﬀ

experiments.

Furthermore, in addition to the anticipated dimerized imine,

we also presume the formation of other side products such as

H O and NH in the reaction. The addition of concentrated

or B (Figure 4), as proposed by Jiang’s group. Here, we

performed density functional theory (DFT) energy calcu-

lations of these possible intermediates of the reaction at

B3LYP/6-31G level. As shown in the energy proﬁle diagram,

benzyl-free-radical B’s energy is lower than that of amine-free-

radical A. The energy diﬀerence between the two was 19.44

kcal/mol, thus indicating I’s formation from an amine radical

cation through benzylic radical intermediate B (Supporting

Information, Table S3).

HCl drops into the reaction vial after completion of the

reaction led to the formation of dense white fumes, conﬁrming

ammonia formation. A standard spectrophotometric method

was used to detect in situ generated H O . Appearance of two

absorption maxima at 510 and 551 nm (Figure 3)

substantiated the presence of H O in the reaction.

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J. Org. Chem. 2021, 86, 6449−6457

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dried over a rotary evaporator. To this were added primary benzylic

amine (21.9 μL, 0.2 mmol) and acetonitrile (4 mL). The reaction vial

was closed with the screw cap, purged with oxygen, and kept for

vigorous stirring under blue LED light at room temperature for 24 h.

After completion, the reaction mixture was dried over a rotary

evaporator and puriﬁed by column chromatography using silica gel

(

60−120 mesh size) as the stationary phase and n-hexane/EtOAc as

the eluent. Also, triethylamine (5%) was added to the eluent to avoid

any kind of degradation of the imine product.

General Procedure for Synthesis of Benzaldehydes (4) from

Corresponding Benzylic Amines. To an oven-dried screw cap

reaction vial charged with a spin vane magnetic stir-bar was added 168

μL of C₇₀solution (1 mg/2 mL in toluene) (0.05 mol %) and dried

over a rotary evaporator. To this were added primary benzylic amine

(21.9 μL, 0.2 mmol) and chloroform (4 mL). The reaction vial was

closed with a screw cap, purged with oxygen, and kept for vigorous

stirring under blue LED light at room temperature for 24 h. After

completion, the reaction mixture was dried over a rotary evaporator

and puriﬁed by ﬂash chromatography using silica gel (230−400 mesh

size) as the stationary phase and n-hexane/EtOAc as the eluent.

Characterization Data. (E)-N-Benzylidene-1-phenylmethan-

amine (Table 2, entry 3a): Yellow liquid (19.1 mg, 98%); isolated

Figure 4. DFT energy proﬁle diagram for reaction intermediates.

We also calculated the green metrics parameters for the

developed oxidative reaction. We found imine synthesis

more endorsing in terms of atom economy (91%) and atom

eﬃciency (89.3) compared to that of benzaldehyde synthesis

from column chromatography (5% EtOAc/n-hexane); H NMR (600

MHz, chloroform-d) δ 8.42 (s, 1H), 7.82−7.83 (m, 2H), 7.43−7.46

(

m, 3H), 7.38−7.39 (m, 4H), 7.28−7.32 (m, 1H), 4.86 (s, 2H);

(

49% atom economy and 27.2 atom eﬃciency). Oxidation to

C{ H} NMR (151 MHz, CDCl ) δ 162.1, 139.4, 136.3, 130.9,

imine was carbon eﬃcient (98%), as well, compared to that of

oxidation to benzaldehyde (55% carbon eﬃciency). Moreover,

we also computed other green metrics parameters, viz. reaction

mass eﬃciency, optimum eﬃciency, mass intensity, process

mass intensity and e-factor, and solvent intensity, for both

oxidation reactions (Supporting Information, Table S2).

In summary, we uncovered photoinduced synthesis of

imines and aldehydes using a pristine C₇₀fullerene catalyst

via oxygenous oxidation of primary benzylamines. The solvent

plays a vital role in deciding the outcome of the reaction as

28.7, 128.6, 128.4, 128.1, 127.1, 65.1; HRMS (EI) m/z [M + H]

calcd for C H N 196.1121; found 196.1125.

(

E)-N-(2-Methylbenzylidene)-1-(o-tolyl)methanamine (Table 2,

entry 3b): Yellow liquid (18.3 mg, 82%); isolated from column

chromatography (3% EtOAc/n-hexane); H NMR (600 MHz,

chloroform-d) δ 8.72 (s, 1H), 7.98 (dd, J = 7.8, 1.2 Hz, 1H), 7.35

(m, 2H), 7.29 (m, 1H), 7.24 (m, 4H), 4.88 (s, 2H), 2.56 (s, 3H), 2.45

(

s, 3H); C{ H} NMR (151 MHz, CDCl ) δ 160.7, 137.8, 137.7,

36.2, 134.4, 130.9, 130.4, 130.2, 128.4, 127.8, 127.2, 126.3, 126.2,

3.4, 19.5, 19.4; HRMS (EI) m/z [M + H] calcd for C H N

24.1434; found 224.1430.

E)-N-(2-Chlorobenzylidene)-1-(2-chlorophenyl)methanamine

Table 2, entry 3c): Yellow liquid (22.6 mg, 86%); isolated from

MeCN leads to the formation of imines, whereas CHCl leads

(

to aldehydes, thus stipulating the endorsement of basic solvent

to imine.

(

column chromatography (10% EtOAc/n-hexane); H NMR (600

MHz, chloroform-d) δ 8.88 (s, 1H), 8.13 (dd, J = 7.8, 1.8 Hz, 1H),

7.43−7.44 (m, 1H), 7.39−7.40 (m, 2H), 7.35−7.37 (m, 1H), 7.30−

EXPERIMENTAL SECTION

■

.32 (m, 1H), 7.25−7.28 (m, 1H), 7.21−7.24 (m, 1H), 4.96 (s, 2H);

Unless otherwise stated, all reactions were carried out under an

oxygen atmosphere in screw cap reaction vials. All solvents were

bought from Sigma-Aldrich in sure-seal bottles and used as such.

Chemicals were bought from Sigma-Aldrich, Alfa-Aesar, and TCI. For

column chromatography, silica gel (230−400 mesh) from Merck was

used. A gradient elution using n-hexane and ethyl acetate was used

based on Merck aluminum TLC sheets (silica gel 60F₂₅₄). The

melting points were recorded on a Barnstead Electrothermal 9100

C{ H} NMR (151 MHz, CDCl

) δ 159.9, 137.0, 135.5, 133.6,

133.3, 131.9, 130.0, 129.8, 129.5, 128.6, 128.5, 127.2, 127.1, 62.3;

HRMS (EI) m/z [M + H] calcd for C14H12Cl N 264.0341; found

264.0346.

(E)-N-(3-Methylbenzylidene)-1-(m-tolyl)methanamine (Table 2,

entry 3d): Yellow liquid (21.4 mg, 96%); isolated from column

chromatography (5% EtOAc/n-hexane); H NMR (600 MHz,

instrument. All isolated compounds were characterized by H NMR,

chloroform-d) δ 8.39 (s, 1H), 7.69 (s, 1H), 7.58 (d, J = 7.8 Hz,

1H), 7.33−7.35 (m, 1H), 7.28 (m, 2H), 7.18 (m, 2H), 7.11 (d, J = 7.2

C{ H} NMR, and LC-MS. In addition, all of the compounds were

Hz, 1H), 4.82 (s, 2H), 2.42 (s, 3H), 2.39 (s, 3H); C{ H} NMR

further characterized by high-resolution mass spectrometry (HRMS).

Mass spectra were recorded on Water Q-ToF-Micro Micromass

(

151 MHz, CDCl ) δ 162.2, 139.3, 138.4, 138.2, 136.2, 131.7, 128.9,

instrument. Copies of H and C{ H} NMR are included in the

NMR (Supporting Information ). Nuclear magnetic resonance spectra

were recorded either on a Bruker Avance 600 or 300 MHz

128.58, 128.56, 128.5, 126.0, 125.2, 65.2, 21.5, 21.4; HRMS (EI) m/z

[M + H] calcd for C16H18N 224.1434; found 224.1434.

(E)-N-(3-(Triﬂuoromethyl)benzylidene)-1-(3-(triﬂuoromethyl)-

phenyl)methanamine (Table 2, entry 3e): Yellow liquid (28.4 mg,

instrument. All H NMR experiments are reported in units of parts

6%); isolated from column chromatography (5% EtOAc/n-hexane);

per million (ppm) and were measured relative to the signals for

residual chloroform (7.26), acetone (2.05), pyridine (7.22), and

H NMR (600 MHz, chloroform-d) δ 8.47 (s, 1H), 8.08 (s, 1H), 7.98

methanol (3.31) in the deuterated solvents. All C NMR spectra

were reported in ppm relative to deuterated chloroform (77.16),

acetone (29.84), pyridine (135.91), and methanol (49.00), and all

(d, J = 7.8 Hz, 1H), 7.70 (d, J = 7.8 Hz, 1H), 7.62 (s, 1H), 7.55−7.58

(m, 3H), 7.49 (t, J = 7.8 Hz, 1H), 4.89 (s, 2H); C{ H} NMR (151

MHz, CDCl

33.0 Hz), 130.98 (q, JC−F = 31.5 Hz), 129.4, 129.2, 127.6 (q, JC−F

) δ 161.1, 140.0, 136.7, 131.6, 131.5, 131.39 (q, JC−F

were obtained with H decoupling. Optimization studies were done

by H NMR, and NMR yield was calculated using tetrachloroethane

3.0 Hz), 125.2 (q, JC−F = 4.5 Hz), 124.8 (q, JC−F = 3.0 Hz), 124.3 (q,

C−F = 271.5 Hz), 124.2 (q, JC−F = 4.5 Hz), 124.0 (d, JC−F = 271.5

as an internal standard.

General Procedure for Synthesis of Imines from Corre-

sponding Primary Benzylic Amines (3). To an oven-dried screw

cap reaction vial charged with a spin vane magnetic stir-bar was added

Hz), 64.6; F NMR (565 MHz, CDCl ) δ −62.53, −62.73; HRMS

(EI) m/z [M + H] calcd for C H F N 332.0868; found 332.0871.

12 6

(E)-N-(3-Fluorobenzylidene)-1-(3-ﬂuorophenyl)methanamine

(Table 2, entry 3f): Yellow liquid (22.6 mg, 98%); isolated from

168 μL of C₇₀solution (1 mg/2 mL in toluene) (0.05 mol %) and

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column chromatography (3% EtOAc/n-hexane); ¹H NMR (600

MHz, chloroform-d) δ 8.36 (d, J = 1.2 Hz, 1H), 7.54−7.56 (m, 1H),

115.9 (d, J_C₋_F= 21.0 Hz) 115.5 (d, J

= 21.0 Hz), 64.3; HRMS

C−F

(EI) m/z [M + H] calcd for C H F N 232.0932; found 232.0935.

12 2

.51−7.53 (m, 1H), 7.38−7.41 (m, 1H), 7.29−7.33 (m, 1H), 7.11−

(E)-N-(4-Chlorobenzylidene)-1-(4-chlorophenyl)methanamine

(

Table 2, entry 3m): Yellow solid (23.7 mg, 90%); mp = 62−63 °C;

.16 (m, 2H), 7.05−7.07 (m, 1H), 6.96 (td, J = 8.4, 2.4 Hz, 1H), 4.81

s, 2H); ¹³C{ H} NMR (151 MHz, CDCl ) δ 163.1 (d, J

= 244.5

isolated from column chromatography (15% EtOAc/n-hexane); H

NMR (600 MHz, chloroform-d) δ 8.34 (s, 1H), 7.71 (d, J = 8.4 Hz,

(

C−F

Hz), 163.1 (d, J_C₋_F= 244.5 Hz), 161.2 (d, J_C₋_F= 3.0 Hz), 141.7 (d,

J_C₋_F= 7.5 Hz), 138.3 (d, J_C₋_F= 7.5 Hz), 130.3 (d, J_C₋_F= 9.0 Hz),

H), 7.40 (d, J = 8.4 Hz, 2H), 7.32 (d, J = 8.4 Hz, 2H), 7.27 (d, J =

.8 Hz, 2H), 4.77 (s, 2H); C{ H} NMR (151 MHz, CDCl ) δ

30.1 (d, J_C₋_F= 7.5 Hz), 124.5 (d, J_C₋_F= 3.0 Hz), 124.5 (d, J_C₋_F= 3.0

Hz), 118.0 (d, J_C₋_F= 21.0 Hz), 114.9 (d, J_C₋_F= 21.0 Hz), 114.4 (d,

J_C₋_F= 21.0 Hz), 114.0 (d, J = 21.0 Hz), 64.3 (d, J_C₋_F= 3.0 Hz);

61.0, 137.8, 137.1, 134.6, 133.0, 129.6, 129.4, 129.1, 128.8, 64.3;

HRMS (EI) m/z [M + H] calcd for C H Cl N 264.0341; found

14 12

C−F

264.0346.

E)-N-(4-Bromobenzylidene)-1-(4-bromophenyl)methanamine

HRMS (EI) m/z [M + H] calcd for C H F N 232.0932; found

12 2

(

32.0935.

E)-N-(3-Chlorobenzylidene)-1-(3-chlorophenyl)methanamine

Table 2, entry 3g): Yellow liquid (19.5 mg, 74%); isolated from

(

Table 2, entry 3n): White solid (29.6, 84%); mp = 96−98 °C;

(

isolated from column chromatography (10% EtOAc/n-hexane); H

NMR (600 MHz, chloroform-d) δ 8.33 (s, 1H), 7.63−7.65 (m, 2H),

(

column chromatography (10% EtOAc/n-hexane); H NMR (600

.54−7.57 (m, 2H), 7.46−7.48 (m, 2H), 7.21 (d, J = 8.4 Hz, 2H),

MHz, chloroform-d) δ 8.33 (s, 1H), 7.82 (t, J = 1.8 Hz, 1H), 7.63

.75 (s, 2H); C{ H} NMR (151 MHz, CDCl ) δ 161.2, 138.2,

(

dd, J = 7.8, 1.2 Hz, 1H), 7.41 (ddd, J = 7.8, 2.4, 1.2 Hz, 1H), 7.34−

34.9, 132.0, 131.7, 129.81, 129.76, 125.5, 121.1, 64.4; HRMS (EI)

.37 (m, 2H), 7.28−7.30 (m, 1H), 7.22−7.27 (m, 2H), 4.79 (s, 2H);

m/z [M + H] calcd for C14

H12Br

N 351.9331; found 351.9337.

C{ H} NMR (151 MHz, CDCl ) δ 161.0, 141.1, 137.7, 134.9,

(

E)-N-(4-Iodobenzylidene)-1-(4-iodophenyl)methanamine (Table

34.5, 131.0, 130.0, 129.9, 128.1, 128.0, 127.4, 126.8, 126.1, 64.4;

, entry 3o): White solid (93.3 mg, 88%); mp = 146−148 °C; isolated

HRMS (EI) m/z [M + H] calcd for C H Cl N 264.0341; found

from column chromatography (15% EtOAc/n-hexane); H NMR

600 MHz, chloroform-d) δ 8.30 (s, 1H), 7.77 (d, J = 8.4 Hz, 2H),

64.0346.

(

(E)-N-(4-Methylbenzylidene)-1-(p-tolyl)methanamine (Table 2,

.67 (d, J = 7.8 Hz, 2H), 7.49 (d, J = 8.4 Hz, 2H), 7.08 (d, J = 7.8 Hz,

H), 4.73 (s, 2H); C{ H} NMR (151 MHz, CDCl ) δ 161.3, 138.9,

entry 3h): Yellow solid (21.0 mg, 94%); mp = 73−75 °C; isolated

from column chromatography (5% EtOAc/n-hexane); H NMR (600

38.0, 137.7, 135.5, 130.1, 129.9, 97.7, 92.5, 64.4; HRMS (EI) m/z

MHz, chloroform-d) δ 8.35 (s, 1H), 7.67 (d, J = 7.8 Hz, 2H), 7.22−

[M + H] calcd for C H I N 447.9054; found 447.9058.

12 2

.23 (m, 4H), 7.16 (d, J = 7.8 Hz, 2H), 4.78 (s, 2H), 2.39 (s, 3H),

(

E)-N-(4-((Triﬂuoromethyl)thio)benzylidene)-1-(4-

.34 (s, 3H); ¹³C{ H} NMR (151 MHz, CDCl ) δ 161.8, 141.1,

((triﬂuoromethyl)thio)phenyl)methanamine (Table 2, entry 3p):

Yellow solid (26.5 mg, 67%); mp = 42−43 °C; isolated from column

36.6, 136.5, 133.7, 129.4, 129.3, 128.4, 128.1, 64.9, 21.6, 21.2;

HRMS (EI) m/z [M + H] calcd for C H N 224.1434; found

chromatography (15% EtOAc/n-hexane); H NMR (600 MHz,

24.1432.

chloroform-d) δ 8.44 (s, 1H), 7.85 (d, J = 8.4 Hz, 2H), 7.72 (d, J =

(

E)-N-(4-(tert-Butyl)benzylidene)-1-(4-(tert-butyl)phenyl)-

.4 Hz, 2H), 7.65 (d, J = 8.4 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 4.87

methanamine (Table 2, entry 3i): Yellow solid (30 mg, 98%); meting

point: 56−58 °C; isolated from column chromatography (5% EtOAc/

(s, 2H); C{ H} NMR (151 MHz, CDCl ) δ 161.3, 142.4, 138.2,

36.7, 136.4, 129.8 (q, J = 306 Hz), 129.6 (q, J = 306 Hz), 129.2,

29.0, 127.4 (d, J = 3.0 Hz), 123.0 (d, J = 1.5 Hz), 64.5; F NMR

n-hexane); H NMR (600 MHz, chloroform-d) δ 8.42 (s, 1H), 7.77−

.80 (m, 2H), 7.48−7.50 (m, 2H), 7.41−7.43 (m, 2H), 7.33 (d, J =

13 1

.4 Hz, 2H), 4.84 (s, 2H), 1.39 (s, 9H), 1.37 (s, 9H); C{ H} NMR

(

151 MHz, CDCl ) δ 161.8, 154.2, 149.8, 136.5, 133.6, 128.2, 127.8,

25.6, 125.5, 64.9, 35.0, 34.5, 31.5, 31.3; HRMS (EI) m/z [M + H]

(E)-N-(2,4-Dichlorobenzylidene)-1-(2,4-dichlorophenyl)-

methanamine (Table 2, entry 3q): Yellow solid (23.3 mg, 70%); mp

= 68−69 °C; isolated from column chromatography (15% EtOAc/n-

calcd for C H N 308.2373; found 308.2376.

(

E)-N-(4-(Triﬂuoromethyl)benzylidene)-1-(4-(triﬂuoromethyl)-

phenyl)methanamine (Table 2, entry 3j): Yellow liquid (26.5 mg,

0%); isolated from column chromatography (5% EtOAc/n-hexane);

hexane); H NMR (600 MHz, chloroform-d) δ 8.80 (s, 1H), 8.05 (d,

J = 8.4 Hz, 1H), 7.42 (dd, J = 4.8, 1.8 Hz, 2H), 7.36 (d, J = 8.4 Hz,

1H), 7.29 (dd, J = 8.4, 1.8 Hz, 1H), 7.25 (dd, J = 8.4, 2.4 Hz, 1H),

H NMR (600 MHz, chloroform-d) δ 8.47 (s, 1H), 7.91 (d, J = 8.4

4.88 (s, 2H); C{ H} NMR (151 MHz, CDCl ) δ 159.0, 137.5,

Hz, 2H), 7.69 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 8.4 Hz, 2H), 7.48 (d, J

136.0, 135.4, 134.1, 133.6, 131.7, 130.6, 129.8, 129.5, 129.3, 127.7,

7.8 Hz, 2H), 4.90 (s, 2H); C{ H} NMR (151 MHz, CDCl ) δ

127.4, 61.7; HRMS (EI) m/z [M + H] calcd for C H Cl N

14 10

(

61.3, 143.1(d, J_C₋_F= 2.8 Hz), 139.1, 132.7 (q, J_C₋_F= 33.0 Hz), 129.5

d, J_C₋_F= 33.0 Hz), 128.7, 128.3, 125.8 (q, J_C₋_F= 3.0 Hz), 125.6 (q,

J_C₋_F= 3.0 Hz), 124.4 (d, J = 270.0 Hz), 124.0 (d, J_C₋_F= 271.5

331.9562; found 331.9565.

Benzaldehyde (Table 3, entry 4a): Colorless liquid (11.5 mg,

55%); isolated from column chromatography (5% EtOAc/n-hexane);

C−F

Hz), 64.5; F NMR (565 MHz, CDCl ) δ −62.42, −62.81; HRMS

H NMR (600 MHz, chloroform-d) δ 10.01 (s, 1H), 7.86−7.88 (m,

(EI) m/z [M + H] calcd for C H F N 332.0868; found 332.0870.

2H), 7.60−7.63 (m, 1H), 7.52 (t, J = 7.8 Hz, 2H); C{ H} NMR

(151 MHz, CDCl ) δ 192.5, 136.5, 134.6, 129.8, 129.1.

12 6

(

E)-N-(4-Phenoxybenzylidene)-1-(4-phenoxyphenyl)-

methanamine (Table 2, entry 3k): White solid (37.1 mg, 98%); mp

96−97 °C; isolated from column chromatography (10% EtOAc/n-

2-Methoxybenzaldehyde (Table 3, entry 4b): Yellow liquid

(21.2 mg, 78%); isolated from column chromatography (5% EtOAc/

hexane); H NMR (600 MHz, chloroform-d) δ 8.37 (s, 1H), 7.77

n-hexane); H NMR (600 MHz, chloroform-d) δ 10.47 (s, 1H), 7.82

(

dd, J = 10.8, 2.4 Hz 2H), 7.37−7.40 (m, 2H), 7.32−7.35 (m, 4H),

(dd, J = 7.8, 1.2 Hz, 1H), 7.53−7.56 (m, 1H), 7.01−7.03 (m, 1H),

.16−7.18 (m, 1H), 7.10 (t, J = 7.2 Hz, 1H), 7.05−7.07 (m, 3H),

6.98 (d, J = 8.4 Hz, 1H), 3.92 (s, 3H); C{ H} NMR (151 MHz,

.01−7.04 (m, 5H), 4.80 (s, 2H); C{ H} NMR (151 MHz, CDCl )

CDCl ) δ 190.0, 161.9, 136.1, 128.7, 124.9, 120.8, 111.7, 55.7.

δ 161.1, 159.9, 157.6, 156.5, 156.2, 134.5, 131.2, 130.04, 130.02,

29.8, 129.5, 124.1, 123.2, 119.7, 119.2, 118.8, 118.4, 64.6; HRMS

2-Ethoxybenzaldehyde (Table 3, entry 4c): Yellow liquid (18.9

mg, 63%); isolated from column chromatography (5% EtOAc/n-

hexane); H NMR (600 MHz, chloroform-d) δ 10.50 (s, 1H), 7.81

(

EI) m/z [M + H] calcd for C H NO 380.1645; found 380.1651.

(

E)-N-(4-Fluorobenzylidene)-1-(4-ﬂuorophenyl)methanamine

(dd, J = 7.8, 1.8 Hz, 1H), 7.50−7.52 (m, 1H), 6.98−7.00 (m, 1H),

(Table 2, entry 3l): Yellow liquid (22.2 mg, 96%); isolated from

6.96 (d, J = 8.4 Hz, 1H), 4.12−4.17 (m, 2H), 1.45−1.48 (m, 3H);

column chromatography (5% EtOAc/n-hexane); H NMR (600

C{ H} NMR (151 MHz, CDCl ) δ 190.0, 161.5, 136.0, 128.3,

MHz, chloroform-d) δ 8.35 (s, 1H), 7.76−7.78 (m, 2H), 7.30 (dd, J =

125.0, 120.6, 112.6, 64.2, 14.7.

2-Chlorobenzaldehyde (Table 3, entry 4d): Yellow solid (10.4

.4, 6.0 Hz, 2H), 7.09−7.12 (m, 2H), 7.02−7.05 (m, 2H), 4.77 (s,

H); ¹³C{ H} NMR (151 MHz, CDCl ) δ 164.6 (d, J = 249 Hz),

C−F

mg, 37%); mp = 131−133 °C; isolated from column chromatography

62.1 (d, J_C₋_F= 243 Hz), 160.7, 135.1 (d, J_C₋_F= 3.0 Hz), 132.5 (d,

(10% EtOAc/n-hexane); H NMR (600 MHz, chloroform-d) δ 10.48

J_C₋_F= 3.0 Hz), 130.3 (d, J_C₋_F= 9.0 Hz), 129.6 (d, J_C₋_F= 9.0 Hz),

(s, 1H), 7.92 (dd, J = 7.8, 1.8 Hz, 1H), 7.51−7.54 (m, 1H), 7.45 (m,

454

J. Org. Chem. 2021, 86, 6449−6457

The Journal of Organic Chemistry

Article

H), 7.37−7.40 (m, 1H); C{ H} NMR (151 MHz, CDCl ) δ 189.9,

for 24 h. After completion, the reaction mixture was dried over a

rotary evaporator and 2.0 equiv of distilled water was added along

with 0.5 mL of PEG (polyethylene glycol) 400. The reaction mixture

38.1, 135.3, 132.6, 130.7, 129.5, 127.4.

-Methoxybenzaldehyde (Table 3, entry 4e): Yellow liquid

18.8 mg, 69%); isolated from column chromatography (10% EtOAc/

(

was then stirred for 10 min. Next, 0.5 equiv of NaBH was then added

n-hexane); H NMR (600 MHz, chloroform-d) δ 9.97 (s, 1H), 7.43−

slowly, and the reaction mixture was heated to 60 °C for 4 h using an

.46 (m, 2H), 7.38−7.39 (m, 1H), 7.16−7.18 (m, 1H), 3.86 (s, 3H);

oil bath. After completion, the reaction mixture was washed with 30

mL of distilled water (3 × 10 mL) and extracted with ethyl acetate.

The organic layer was then subjected to column chromatography

using hexane/ethyl acetate (85:15) as an eluent to get the pure form

C{ H} NMR (151 MHz, CDCl ) δ 192.3, 160.2, 137.9, 130.2,

23.7, 121.7, 112.1, 55.6.

-Chlorobenzaldehyde (Table 3, entry 4f): White solid (25.9

mg, 92%), mp = 136−139 °C; isolated from column chromatography

of the desired product: white solid (50.0 mg, 81%); mp = 142−144

(

10% EtOAc/n-hexane); H NMR (600 MHz, chloroform-d) δ 9.95

C, H NMR (600 MHz, chloroform-d) δ 7.36 (d, J = 8.2 Hz, 1H),

s, 1H), 7.82 (s, 1H), 7.74 (d, J = 7.8 Hz, 1H), 7.56−7.58 (m, 1H),

.29 (d, J = 8.2 Hz, 1H), 3.82 (s, 1H), 1.30 (s, 4H); C{ H} NMR

.46 (t, J = 7.8 Hz, 1H); C{ H} NMR (151 MHz, CDCl ) δ 190.9,

(151 MHz, CDCl ) δ 150.86, 144.63, 128.69, 125.70, 51.17, 34.67,

37.9, 135.5, 134.4, 130.5, 129.3, 128.1.

-Methylbenzaldehyde (Table 3, entry 4g): Yellow liquid (11.8

mg, 49%); isolated from column chromatography (5% EtOAc/n-

hexane); H NMR (600 MHz, chloroform-d) δ 9.96 (s, 1H), 7.77 (d,

J = 8.4 Hz, 2H), 7.33 (d, J = 7.8 Hz, 2H), 2.44 (s, 3H); C{ H}

NMR (151 MHz, CDCl ) δ 192.1, 145.7, 134.3, 130.0, 129.8, 22.0.

-(tert-Butyl)benzaldehyde (Table 3, entry 4h): White solid (21

mg, 65%);mp = 147−149 °C; isolated from column chromatography

5% EtOAc/n-hexane); H NMR (600 MHz, chloroform-d) δ 9.97 (s,

H), 7.81 (d, J = 8.4 Hz, 2H), 7.54 (d, J = 8.4 Hz, 2H), 1.34 (s, 9H);

C{ H} NMR (151 MHz, CDCl ) δ 192.1, 158.5, 134.2, 129.8,

31.45.

Reactive Oxygen Species Quenching Experiments. To the oven-

dried screw cap reaction vials charged with a spin vane magnetic stir-

bar was added 84 μL of C₇₀solution (1 mg/2 mL in toluene) (0.05

mol %) in two diﬀerent vials and dried over a rotary evaporator. To

this werd added primary benzylic amine (10.9 μL, 0.1 mmol) and

acetonitrile (2 mL). Then to this standard reaction were added

separately ROS quenchers (DABCO, benzoquinone). The reaction

vials were closed with screw caps, purged with oxygen, and kept for

vigorous stirring under blue LED light at room temperature for 24 h.

After completion, the reactions were dried over a rotary evaporator,

and crude reactions were analyzed with the help of NMR using

tetrachloroethane as an internal standard.

(

26.1, 35.4, 31.1.

-Methoxybenzaldehyde (Table 3, entry 4i): Yellow liquid (16.3

mg, 60%); isolated from column chromatography (10% EtOAc/n-

hexane); H NMR (600 MHz, chloroform-d) δ 9.88 (s, 1H), 7.84

dd, J = 10.8, 2.4 Hz, 2H), 7.00 (d, J = 9.0 Hz, 2H), 3.89 (s, 3H);

Radical Quenching Experiments. To the oven-dried screw cap

reaction vial charged with a spin vane magnetic stir-bar was added 84

^μ^L^o^f^C70 solution (1 mg/2 mL in toluene) (0.05 mol %) in vial and

dried over a rotary evaporator. To this were added primary benzylic

amine (10.9 μL, 0.1 mmol) and acetonitrile (2 mL). Then to this

standard reaction was added a radical quencher (2,2,6,6-tetramethyl-

piperidin-1-yl)oxyl. The reaction vial was closed with a screw cap,

purged with oxygen, and kept for vigorous stirring under blue LED

light at room temperature for 24 h. After completion, the reaction was

dried over a rotary evaporator, and the crude reaction was analyzed

with the help of NMR using tetrachloroethane as an internal standard.

(

C{ H} NMR (151 MHz, CDCl ) δ 191.0, 164.7, 132.1, 130.0,

14.4, 55.7.

-Chlorobenzaldehyde (Table 3, entry 4j): White solid (18.3

mg, 65%); mp = 175−177 °C; isolated from column chromatography

5% EtOAc/n-hexane); H NMR (600 MHz, chloroform-d) δ 9.96 (s,

(

H), 7.79−7.81 (m, 2H), 7.49 (d, J = 8.4 Hz, 2H); C{ H} NMR

151 MHz, CDCl ) δ 190.9, 141.0, 134.8, 131.0, 129.5.

-Bromobenzaldehyde (Table 3, entry 4k): White solid (21.8

mg, 59%); mp = 150−152 °C; isolated from column chromatography

5% EtOAc/n-hexane); H NMR (600 MHz, chloroform-d) δ 9.96 (s,

H), 7.73 (dd, J = 6.6 Hz, J = 1.8 Hz, 2H), 7.67 (d, J = 8.4 Hz, 2H);

C{ H} NMR (151 MHz, CDCl ) δ 191.2, 135.2, 132.5, 131.1,

O-Labeling Experiment. To the oven-dried screw cap reaction

(

vial charged with a spin vane magnetic stir-bar was added 84 μL of C₇₀

solution (1 mg/2 mL in toluene) (0.05 mol %) in vial and dried over

a rotary evaporator. To this were added primary benzylic amine (13.1

μL, 0.1 mmol) and chloroform (2 mL). The reaction vial was closed

29.9.

,4-Dimethoxybenzaldehyde (Table 3, entry 4l): White solid

with a screw cap, purged with O, and kept for vigorous stirring

under blue LED light at room temperature for 24 h. After completion,

the crude reaction was analyzed by LC-MS.

(

14.6 mg, 44%), mp = 65−67 °C; isolated from column

chromatography (10% EtOAc/n-hexane); H NMR (600 MHz,

chloroform-d) δ 10.27 (s, 1H), 7.79 (d, J = 8.4 Hz, 1H), 6.52−6.54

Light On−Oﬀ Experiments. To the four oven-dried screw cap

reaction vials charged with a spin vane magnetic stir-bar was added 84

μL of C₇₀solution (1 mg/2 mL in toluene) (0.05 mol %) in each vial

and dried over rotary evaporator. To this were added primary benzylic

amine 1a (10.9 μL, 0.1 mmol) and acetonitrile (2 mL). The reaction

vials were closed with a screw cap, purged with oxygen, and kept for

vigorous stirring under blue LED light at room temperature. First, the

reactions were irradiated for 4 h; the LED light was turned oﬀ, and

the NMR yield of one reaction was calculated. Second, the reaction

was analyzed after 8 h, and light was then turned on. Further, again,

the light was turned oﬀ after 12 h, and third reaction was analyzed

with NMR after 12 h and the fourth one after 16 h.

(

m, 1H), 6.43 (d, J = 2.4 Hz, 1H), 3.89 (s, 3H), 3.86 (s, 3H);

C{ H} NMR (151 MHz, CDCl ) δ 188.5, 166.3, 163.7, 130.8,

19.1, 105.8, 98.0, 55.7, 55.7.

-Chloro-4-methoxybenzaldehyde (Table 3, entry 4m): White

solid (32.7 mg, 96%), mp = 48−50 °C; isolated from column

chromatography (10% EtOAc/n-hexane); H NMR (600 MHz,

chloroform-d) δ 9.81 (s, 1H), 7.86 (d, J = 2.4 Hz, 1H), 7.73 (dd, J =

.4, 2.4 Hz, 1H), 7.01 (d, J = 8.4 Hz, 1H), 3.96 (s, 3H); C{ H}

NMR (151 MHz, CDCl ) δ 189.8, 159.8, 131.1, 130.7, 130.3, 123.7,

111.7, 56.6.

,4,5-Trimethoxybenzaldehyde (Table 3, entry 4n): White solid

18.0 mg, 46%); mp = 65−66 °C; isolated from column

chromatography (10% EtOAc/n-hexane); H NMR (600 MHz,

(

Detection of Hydrogen Peroxide (H O ). To the oven-dried screw

2 2

chloroform-d) δ 9.84 (s, 1H), 7.11 (s, 2H), 3.92 (s, 3H), 3.91 (s, 6H).

cap reaction vials charged with a spin vane magnetic stir-bar was

added 84 μL of C70 solution (1 mg/2 mL in toluene) (0.05 mol %) in

vial and dried over a rotary evaporator. To this were added primary

benzylic amine (10.9 μL, 0.1 mmol) and acetonitrile (2 mL). The

reaction vial was closed with a screw cap, purged with oxygen, and

kept for vigorous stirring under blue LED light at room temperature

for 24 h. After completion, the reaction mixture was added to 20 mL

of water and then extracted three times with 30 mL of ethyl acetate

(10 × 3). The aqueous layer was then used for the determination of

C NMR (151 MHz, CDCl ) δ 191.2, 153.7, 143.6, 131.8, 106.8,

1.1, 56.3.

Bis(4-(tert-butyl)benzyl)amine (Scheme 3, 6a). To an oven-

dried screw cap reaction vial charged with a spin vane magnetic stir-

bar was added 168 μL of C₇₀solution (1 mg/2 mL in toluene) (0.05

mol %) and dried over rotary evaporator. To this were added primary

benzylic amine (21.9 μL, 0.2 mmol) and acetonitrile (4 mL). The

reaction vial was closed with a screw cap, purged with oxygen, and

kept for vigorous stirring under blue LED light at room temperature

H O by following the standard procedure reported by Bader et al.

455

J. Org. Chem. 2021, 86, 6449−6457

The Journal of Organic Chemistry

Preparation of Unsymmetric Benzils from β -Ketoaldehydes. Org. Lett.

Article

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ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

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AUTHOR INFORMATION

Corresponding Author

■

(d) Ushakov, D. B.; Plutschack, M. B.; Gilmore, K.; Seeberger, P.

Upendra Sharma − Chemical Technology Division, CSIR-

Institute of Himalayan Bioresource and Technology,

Palampur 176061, India; Academy of Scientiﬁc and

Innovative Research (AcSIR), Ghaziabad 201002, India;

orcid.org/0000-0002-7693-8690 ; Email: upendra@

ihbt.res.in , upendraihbt@gmail.com

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Asymmetric Secondary Amines with Singlet Oxygen. Chem. - Eur. J.

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Chem. 2013, 2013, 5225−5235.

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Catalyzed Cleavage of C-N Single Bonds. Chem. Rev. 2015, 115 (21),

12045 −12090. (b) Stanek, F.; Pawlowski, R.; Mlynarski, J.; Stodulski,

M. Visible- Light-Mediated α -Oxygenation of 3-(N ,N -Dimethylami-

nomethyl)-Indoles to Aldehydes. Eur. J. Org. Chem. 2018, 2018,

Authors

Inder Kumar − Chemical Technology Division, CSIR-Institute

of Himalayan Bioresource and Technology, Palampur

76061, India; Academy of Scientiﬁc and Innovative

Research (AcSIR), Ghaziabad 201002, India; orcid.org/

000-0003-3285-701X

(

624−6628.

Rakesh Kumar − Chemical Technology Division, CSIR-

Institute of Himalayan Bioresource and Technology,

Palampur 176061, India; orcid.org/0000-0002-8588-

7) (a) Ainembabazi, D.; An, N.; Manayil, J. C.; Wilson, K.; Lee, A.

F.; Voutchkova-Kostal, A. M. Acceptorless Amine Dehydrogenation

and Transamination Using Pd-Doped Hydrotalcites. ACS Catal. 2019,

615

(2), 1055−1065. (b) Aman, M.; Tremmel, J.; Dostál, L.; Erben, M.;

Shiv Shankar Gupta − Chemical Technology Division, CSIR-

Institute of Himalayan Bioresource and Technology,

Palampur 176061, India; Academy of Scientiﬁc and

Innovative Research (AcSIR), Ghaziabad 201002, India;

orcid.org/0000-0002-9607-9301

Tydlitát, J.; Jansa, J.; Jambor, R. Highly Active and Selective Ru-PNH

Catalyst in Aerobic Oxidation of Benzyl Amines. ChemCatChem 2019,

11 (18), 4624 −4630. (c) Bottaro, F.; Takallou, A.; Chehaiber, A.;

Madsen, R. Cobalt-Catalyzed Dehydrogenative Coupling of Amines

into Imines. Eur. J. Org. Chem. 2019, 2019 (42), 7164 −7168.

(d) Chen, B.; Wang, L.; Dai, W.; Shang, S.; Lv, Y.; Gao, S. Metal-Free

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.joc.1c00297

and Solvent-Free Oxidative Coupling of Amines to Imines with

Mesoporous Carbon from Macrocyclic Compounds. ACS Catal.

015 , 5 (5), 2788 −2794. (f) Deb, M.; Hazra, S.; Dolui, P.; Elias, A. J.

Notes

Ferrocenium Promoted Oxidation of Benzyl Amines to Imines Using

Water as the Solvent and Air as the Oxidant. ACS Sustainable Chem.

Eng. 2019, 7 (1), 479 −486. (g) Li, Y.; Shang, S.; Wang, L.; Lv, Y.;

Niu, J.; Gao, S. Selectivity-tunable amine aerobic oxidation catalysed

by metal-free N,O-doped carbons. Chem. Commun. 2019 , 55 (81),

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The authors are thankful to the Director, CSIR-IHBT,

Palampur. This work is supported by CSIR (MLP0159).

S.S.G. thanks CSIR, New Delhi, for the research fellowship.

CSIR-IHBT Communication No. 4752.

2251 −12254. (h) Mukhi, P.; Roy, S. Bimetallic Pd-Sn Nanocatalysts

for Selective Synthesis of Amines and Imines in Water. ChemistrySelect

020, 5 (3), 1000 −1006. (i) Yu, H.; Zhai, Y.; Dai, G.; Ru, S.; Han, S.;

Wei, Y. Transition-Metal-Controlled Inorganic Ligand-Supported

Non-Precious Metal Catalysts for the Aerobic Oxidation of Amines

to Imines. Chem. - Eur. J. 2017, 23 (56), 13883 −13887.

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