Visible-light-promoted α-methoxymethylation and aminomethylation of ketones with methanol as the C1 source

Article abstract of DOI:10.1039/d1ob00765c

Visible-light-promoted α-methoxymethylation and aminomethylation of ketones using methanol as a sustainable C1 source have been developed. With rose bengal as the photosensitizer and air as the green oxidant, the methoxymethylation reactions proceeded smoothly under visible light irradiation at ambient temperature. Additionally, a one-pot one-step α-aminomethylation of ketones was achieved by adding N-nucleophiles. Preliminary mechanism studies suggest that the reaction mainly proceedsviaa radical pathway.

Full text of DOI:10.1039/d1ob00765c

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Visible-light-promoted α-methoxymethylation
and aminomethylation of ketones with methanol
as the C1 source†
Cite this: Org. Biomol. Chem., 2021,
19, 5572
Jingya Yang, *a Cai Liu,a Hongyan Zhou,
Zheng Li
*
a,b Rundong Fan,a Ben Ma a and
a
Visible-light-promoted α-methoxymethylation and aminomethylation of ketones using methanol as a
sustainable C1 source have been developed. With rose bengal as the photosensitizer and air as the green
oxidant, the methoxymethylation reactions proceeded smoothly under visible light irradiation at ambient
temperature. Additionally, a one-pot one-step α-aminomethylation of ketones was achieved by adding
N-nucleophiles. Preliminary mechanism studies suggest that the reaction mainly proceeds via a radical
pathway.
Received 21st April 2021,
Accepted 27th May 2021
DOI: 10.1039/d1ob00765c
rsc.li/obc
TBHP as the external oxidant, we developed the cheap and bio-
compatible cobalt catalyzed α-methoxymethylation of ketones
Introduction
The construction of C–C bonds is one of the unremitting under thermal conditions (Scheme 1b).8 Furthermore, based
research topics in organic synthesis. C–H functionalization on the observed reversibility of the methoxy adduct to enone,
does not require pre-functionalization of the substrate and rep- the one-pot two-step α-aminomethylation of ketones was
resents the most direct, economical and effective strategy to achieved by adding N-nucleophiles in situ. Recently, Pan and
construct C–C bonds.1 However, C(sp3)–H functionalization is colleagues described an electrochemical method for the trans-
considerably challenging due to the low reactivity of the formations at high temperature (Scheme 1c).9 However, devel-
C(sp3)–H bond.2
oping a metal-free, sustainable and operationally simple proto-
Methanol is cheap, abundant, readily available and col for the α-methoxymethylation and aminomethylation of
biodegradable. Recently, methanol has emerged as a pro- ketones under mild conditions is still desirable.
mising and sustainable C1 source for valuable organic
transformations.3,4 In terms of α-C(sp3)–H functionalization of
ketones with methanol, significant achievements have been
made in α-methylation through a metal-catalyzed borrowing
hydrogen strategy.5 In addition, the reactions concerning
α-methoxylation6 and α-hydroxymethylation7 of ketones have
also been reported. Besides, methanol is also a promising can-
didate for α-methoxymethylation of ketones. However, only
three cases of α-methoxymethylation of ketones with methanol
as the C1 source have been documented.5d,8,9
In 2015, Donohoe and co-workers reported an iridium-cata-
lyzed methoxymethylation of ketones with methanol in the
presence of a phosphine ligand (Scheme 1a).5d In 2018, using
aCollege of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, China. E-mail: yangjy@nwnu.edu.cn, zhouhy@gsau.edu.cn
bCollege of Science, Gansu Agricultural University, Lanzhou 730070, China
†Electronic supplementary information (ESI) available: Details of experimental
procedures, mechanistic studies, characterization data and NMR spectra. See
DOI: 10.1039/d1ob00765c
Scheme 1 α-Methoxymethylation and aminomethylation of ketones
with methanol.
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Over the past few decades, visible-light-promoted reactions not effective (Table 1, entries 2–7). Then, using rose bengal as
provide a sustainable and powerful strategy for organic syn- the optimal photosensitizer, some inorganic and organic
thesis.10 In 2018, using methanol as the C1 source and air as bases were examined, and they led to inferior results (Table 1,
the green oxidant, our group developed the visible-light- entries 8–15). Notably, when K2CO3, which proved to be the
mediated, rose bengal-catalyzed α-hydroxymethylation of best choice for the previous hydroxymethylation7 of ketones,
ketones.7 Therein, water was added to prevent the dehydration was used, 2a was obtained in only 40% yield (Table 1, entry 8).
of the target products. Based on this finding, we speculate that The investigation of the amount of Cs2CO3 showed that
enone could be generated via dehydration of the hydroxy- increasing the amount of Cs2CO3 to 2.5 equivalents did not
methylated intermediate to undergo further conjugate improve the reaction, while reducing the amount of Cs2CO3 to
addition of methanol or N-nucleophiles, thereby enabling the 1.5 equivalents resulted in a significant decrease in the yield
visible-light-promoted α-methoxymethylation and amino- of 2a (Table 1, entry 16). In addition, when a 6 W green LED or
methylation of ketones under metal-free and mild conditions. a 6 W blue LED was employed as the light source, 2a was
Here, we report the successful implementation of this novel obtained in 60% or 57% yield, respectively (Table 1, entry 17).
and sustainable strategy (Scheme 1d).
Furthermore, shortening the reaction time resulted in a sig-
nificant decrease in the yield of 2a, while prolonging the reac-
tion time did not obviously improve the reaction (Table 1,
entry 18). Finally, the control experiments showed that light,
photosensitizer and base are indispensable for this reaction
(Table 1, entries 19–21).
With the optimized reaction conditions in hand, we then
investigated the substrate scope of ketones to verify the appli-
cability of the established method (Scheme 2). The evaluated
Results and discussion
At the beginning of the study, we chose propiophenone (1a) as
the template substrate to optimize the reaction conditions for
the visible-light-promoted α-methoxymethylation of ketones
(Table 1). Initially, using rose bengal as the photosensitizer,
the reaction was performed in methanol under 23 W compact
fluorescent lamp (CFL) irradiation in the presence of Cs2CO3
(Table 1, entry 1). To our delight, the desired product 2a was
obtained in 74% yield. Next, with Cs2CO3 as the base, several
commonly used photocatalysts were screened, but they were
Table 1 Optimization of the reaction conditionsa
Entry
Photosensitizer
Base
Yieldb (%)
1
2
3
4
5
6
7
8
Rose bengal
DDQ
Eosin B
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
NaHCO3
K2HPO4
KOH
74
6
10
28
19
0
8
40
Trace
0
0
40
43
0
Eosin Y
Ru(bpy)3Cl2·6H2O
Rhodamine B
Fluorescein
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
Rose bengal
—
9
10
11
12
13
14
15
16
17
18
19
20
21
KOtBu
DABCO
DBU
11
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
—
28,c 73d
60,e 57f
49,g 77h
0i
0
Rose bengal
0
a Reaction conditions: 1a (0.3 mmol), photosensitizer (0.015 mmol,
5 mol%), base (0.6 mmol, 2 equiv.), MeOH (3 mL), air, 23 W CFL, rt,
48 h. b Isolated yields. c Cs2CO3 (1.5 equiv.). d Cs2CO3 (2.5 equiv.). e 6 W
green LED. f 6 W blue LED. g 36 h. h 60 h. i In the dark.
Scheme 2 Scope of α-methoxymethylation of ketones. Reaction con-
ditions: 1 (0.3 mmol), rose bengal (0.015 mmol, 5 mol%), Cs2CO3
(0.6 mmol, 2 equiv.), MeOH (3 mL), air, 23 W CFL, rt, 48 h. Isolated yields
are shown.
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mono-substituted propiophenone derivatives with either elec-
tron-donating or electron-withdrawing substituents reacted
smoothly under the optimal conditions to afford the desired
products 2a–2d and 2f–2j in moderate to high yields, although
para-fluorine and meta-trifluoromethyl substituted propiophe-
nones gave the desired products 2e and 2k in low yields. It is
worth noting that in the case of 1-(4-fluorophenyl)propan-1-
one (1e), the by-product, 1-(4-methoxyphenyl)propan-1-one, in
which para-fluorine is substituted by a methoxy group was
obtained in 20% yield, which is also the main reason for the
low yield of 2e. Exceptionally, 1-(4-nitrophenyl)propan-1-one
was inert under the standard reaction conditions. When 1-(3,4-
difluorophenyl)propan-1-one (1l) was subjected to the reac-
tion, the para-methoxyl substituted α-methoxymethylated
product 2l was obtained with a yield of 68%. In addition, a
series of ketones with longer alkyl chains (R2 = Et, Pr, Bu, and
phenethyl) were well tolerated and led to the desired products
2m–2r with good yields. Similarly, for 1-(4-fluorophenyl)butan-
1-one (1n), in addition to the desired product 2n, a by-product
in which para-fluorine is substituted by a methoxy group was
obtained in a yield of 12%. In other aspects, the reaction of
3-chloro-1-phenylpropan-1-one (1s) resulted in a complex
mixture, and only the product 2s in which the chlorine atom
was replaced by a methoxy group was isolated. Furthermore,
ketones containing heterocycles, such as 1-(benzo[d][1,3]
dioxol-5-yl)butan-1-one and 1-(thiophen-2-yl)propan-1-one,
also reacted readily and afforded the desired products 2t and
2u in satisfactory yields.
Scheme 4 One-pot α-aminomethylation of ketones with methanol
and N-nucleophiles. Reaction conditions: 1m (0.3 mmol), N-nucleophile
(0.36 mmol, 1.2 equiv.), rose bengal (0.015 mmol, 5 mol%), Cs2CO3
(0.6 mmol, 2 equiv.), MeOH (3 mL), air, 23 W CFL, rt, 48 h. Isolated yields
are shown.
To showcase the practicability of the developed method, a
gram-scale reaction of 1a (8.0 mmol) was conducted under
irradiation of two 32 W CFLs (Scheme 3). The reaction pro-
ceeded smoothly and was completed in 60 hours, affording the
desired product 2a in 62% yield (0.887 g).
In our previous research,8 we have developed the amino-
methylation of ketones with methanol and N-nucleophiles in a
one-pot two-step fashion based on the observed reversibility
between the enone and methoxymethylated product. Hence,
we envisioned whether α-aminomethylation of ketones can be
achieved through this photochemical method. Considering
the important application in natural products and medicines,
pyrazoles, indazoles, indoline, benzotriazole and p-tosylamide
were selected as the N-nucleophiles to evaluate the feasibility
by reaction with ketone 1m (Scheme 4). Excitingly, the corres-
ponding aminomethylated products 3ma–3mi were readily
obtained through a simple “one-pot one-step” procedure
under standard conditions.
Scheme 5 Control experiments.
To explore the mechanism of the reaction, several control 1a was carried out under argon, only a trace amount of 2a was
experiments were conducted (Scheme 5). When the reaction of detected (Scheme 5, eqn (1)), indicating that oxygen in air par-
ticipates in the reaction. When the radical inhibitor 2,2,6,6-tet-
ramethyl-1-piperidinyloxyl (TEMPO), 1,1-diphenylethylene or
2,6-di-tert-butyl-p-cresol (BHT) was added to the reaction
mixture, the reaction was remarkably inhibited (Scheme 5, eqn
(2)–(4)). Meanwhile, the carbon radical trapped by 1,1-
diphenylethylene, the hydroxymethyl radical trapped by BHT,
and the oxidation products BHT-OH and BHT-OOH were
Scheme 3 Gram-scale synthesis.
detected by high-resolution mass spectrometry (HRMS, see the
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ESI†). Furthermore, the radical signal was observed by electron process. Finally, enone 5a is formed by the dehydration of 4a,
paramagnetic resonance (EPR) spectroscopy with N-tert-butyl- which then reacts with methanol or N-nucleophiles through
α-phenylnitrone (PBN) as a radical trap (see the ESI†). These the oxa- or aza-Michael reaction, respectively, to afford the
results suggest that the reaction may proceed via a radical methoxymethylation product 2a or the corresponding amino-
pathway. Furthermore, the formation of the BHT oxidation methylation product 3.
products11 and the result of no reaction in the presence of
DABCO (Table 1, entry 14), a strong physical quencher of
singlet oxygen (1O2),12 indicate the generation of 1O2 during
the reaction. Nevertheless, without the addition of 1a, the for-
Conclusions
In summary, we have developed a photochemical method for
the α-methoxymethylation and aminomethylation of ketones
using methanol as the C1 source. With a catalytic amount of
rose bengal as the photosensitizer and upon visible light
irradiation, the ketones were successfully converted to the
methoxymethylated or aminomethylated products in air at
ambient temperature. Compared to the previous works, this
protocol has significant improvements, including avoiding the
use of transition metal catalysts and strong external oxidants,
mild reaction conditions, and simple operation.
mation of formaldehyde was also confirmed by a silver mirror
reaction (see the ESI†). In addition, the reaction of
α-hydroxymethylated propiophenone 4a in the presence of
Cs2CO3 led to 2a with a yield of 81%, indicating that
α-hydroxymethylated ketone may be the key intermediate.
Based on the above observations and our previous research
studies,7,8
a plausible reaction mechanism is proposed
(Scheme 6). Initially, upon visible light irradiation, rose bengal
(RB) is excited to its excited state RB*. Then, RB* interacts with
oxygen via energy transfer to provide 1O2. The subsequent con-
version of ketone 1a to the key intermediate,
α-hydroxymethylated ketone 4a, possibly proceeds through two
pathways. In path a, first, the enolization of 1a forms enol 1a′ Experimental
in the presence of a base. Subsequently, 1a′ reacts rapidly with
General procedure for α-methoxymethylation of ketones
1O2 to generate α-carbon radical A and hydroperoxide radical
B. The latter abstracts a hydrogen atom from methanol to
produce hydroxymethyl radical C. Then, the combination of
α-carbon radical A and hydroxymethyl radical C provides the
α-hydroxymethylated ketone 4a. In path b, methanol is first
oxidized to formaldehyde, which then reacts with 1a to provide
4a. According to the results of the radical inhibiting experi-
ments (Scheme 5, eqn (2)–(4)), path a may be the main
Ketones 1 (0.3 mmol), rose bengal (15.3 mg, 0.015 mmol,
5.0 mol%), Cs2CO3 (195.2 mg, 0.6 mmol, 2.0 equiv.) and
MeOH (3 mL) were added into a 10 mL borosilicate glass tube.
The reaction mixture was stirred at room temperature under 23
W CFL irradiation and ambient air for 48 h. After completion
of the reaction (monitored by TLC), the reaction solution was
concentrated under reduced pressure and the residue was puri-
fied by column chromatography on silica gel (PE : EtOAc =
10 : 1–100 : 1 or PE : acetone = 100 : 1) to afford the pure
product 2.
Procedure for gram-scale synthesis of 2a
Propiophenone 1a (1.073 g, 8 mmol), rose bengal (0.529 g,
0.4 mmol, 5 mol%), Cs2CO3 (5.213 g, 16 mmol, 2.0 equiv.) and
MeOH (50 mL) were added into a 80 mL borosilicate glass
tube. The reaction mixture was stirred at room temperature
under irradiation of two 32 W compact fluorescent lamps
(CFLs) and ambient air for 60 h. After completion of the reac-
tion (monitored by TLC), the reaction solution was concen-
trated under reduced pressure and the residue was purified by
column chromatography on silica gel (PE : EtOAc = 10 : 1) to
afford the pure product 2a (0.887 g, 62%).
General procedure for α-aminomethylation of ketones
Ketone 1m (44.5 mg, 0.3 mmol), rose bengal (15.3 mg,
0.015 mmol, 5.0 mol%), Cs2CO3 (195.2 mg, 0.6 mmol, 2.0
equiv.), N-nucleophiles (0.36 mmol, 1.2 equiv.) and MeOH
(3 mL) were added into a 10 mL borosilicate glass tube. The
reaction mixture was stirred at room temperature under 23 W
CFL irradiation and ambient air for 48 h. After completion of
the reaction (monitored by TLC), the reaction solution was
concentrated under reduced pressure and the residue was puri-
Scheme 6 Proposed reaction mechanism.
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fied by column chromatography on silica gel (PE : EtOAc =
5 : 1–30 : 1) to afford the pure product 3.
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Conflicts of interest
5 (a) L. K. M. Chan, D. L. Poole, D. Shen, M. P. Healy and
T. J. Donohoe, Angew. Chem., Int. Ed., 2014, 53, 761–765;
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We thank the National Natural Science Foundation of China
(21961035 and 22061001), the Natural Science Foundation of
Gansu Province (20JR5RA520), the Program for Improving
Scientific Research Ability of Young Teachers of Northwest
Normal University (NWNU-LKQN-17-3), and the Gansu
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support.
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