Debenzylative Sulfonylation of Tertiary Benzylamines Promoted by Visible Light

Article abstract of DOI:10.1002/ejoc.202100144

An efficient, general, inexpensive, and environmentally friendly photosynthesis of sulfonamides via visible light promoted debenzylative sulfonylation of tertiary benzylamines is described. Compared to the traditional S?N coupling reactions, which are promoted by oxidative C?N bond cleavage of symmetrical tertiary alkylamines, this strategy provides a selective C?N bond cleavage protocol and avoids the use of transition-metal, explosive oxidants, and ligands.

Full text of DOI:10.1002/ejoc.202100144

Communications
doi.org/10.1002/ejoc.202100144
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Debenzylative Sulfonylation of Tertiary Benzylamines
Promoted by Visible Light
Ying Fu,*[a] Qing-Kui Wu,[a] and Zhengyin Du*[a]
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An efficient, general, inexpensive, and environmentally friendly
photosynthesis of sulfonamides via visible light promoted
debenzylative sulfonylation of tertiary benzylamines is de-
scribed. Compared to the traditional SÀ N coupling reactions,
which are promoted by oxidative CÀ N bond cleavage of
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symmetrical tertiary alkylamines, this strategy provides
a
selective CÀ N bond cleavage protocol and avoids the use of
Figure 1. Selected drugs and bioactive compounds encompassing a tertiary
sulfonamide moiety.
transition-metal, explosive oxidants, and ligands.
The tertiary sulfonamide structural motif represents a funda-
mental class of compounds that play a pivotal role in both
synthetic[1] and medicinal chemistry.[2] For examples (Figure 1),
Sildenafil (I) is a potent and selective inhibitor of type 5 cGMP
phosphodiesterase.[3] Probenecid (II) is an uricosuric agent used
for the treatment of hyperuricemia associated with gout and
gouty arthritis.[4] Compound (III) was observed strong selectivity
toward tumor-associated inhibitors of human carbonic anhy-
drases hCA IX.[5] The classic tertiary sulfonamide synthesis
method is the coupling reaction between a secondary amine
and a sulfonyl chloride.[6] Other methods including the metal-
catalyzed N-alkylation/arylation of secondary sulfonamides,[7]
oxidative coupling reactions between sulfonyl hydrazides and
secondary amines,[8] oxidative coupling reactions between
sulfinate salts and amines,[9] and the electrochemical oxidative
coupling of amines with thiols,[10] etc. are continually developed.
Although these procedures are effective in a way, limitations
such as the employments of expensive and toxic metal
catalysts, explosive oxidants, and air-sensitive secondary amines
are still involved in these procedures.
Tertiary alkylamines are readily available amine species and
are commonly more stable than their primary and secondary
siblings. However, these compounds, due to the absence of
active NÀ H groups as well as the tough CÀ N bonds,[11] are rarely
employed as the feedstock in organic syntheses. Recently, with
the quick developments on CÀ H[12] and CÀ N[13] bonds activation
of tertiary amines, the synthetic application of tertiary alkyl-
amines, commonly with CÀ N bond cleavage, have attracted
extensive attention. However, previous investigations mainly
focused on transition-metal catalytic processes which are
largely limited to symmetric tertiary alkylamines.[14] Thus, a
selective transitional metal-free catalysis for CÀ N bond dissocia-
tion of tertiary aliphatic amines is still in highly desirable.
In recent years, photochemical rebuild of chemical bond
has achieved impetus in organic synthesis. In this scenario, we
recently demonstrated that the adjacent functional groups
presented at aliphatic tertiary amines have significant effects on
CÀ N bond activation and future dissociation.[15] In view that
benzylamine is frequently used as a nitrogen source in many
natural products syntheses where the benzyl group, being an
amine protecting group,[16] could be selectively removed via a
variety
of
methods
including
palladium-catalyzed
hydrogenolysis,[17] lithium base,[18] bromo radical,[19] or metals in
protic solvents,[20] together with our recent achievements on
radical CÀ N bond cleavage of tertiary aliphatic amines,[15] we
thus envision that the benzylic CÀ N bonds in tertiary benzyl-
amines could be cleaved via a visible light induced, oxidative
pathway.
Taking advantage of our experiences in tertiary alkylamine
sulfonylation, sulfonyl chlorides were again selected as the
electrophile to evaluate the efficiency of our photopromoted
debenzylation protocol (Scheme 1). At the outset of the
research, the reaction of tosyl chloride (1a) and N,N-dimeth-
ylbenzylamine (2a) was selected as a model to explore the
optimal reaction conditions, including the bases, photocatalysts,
[a] Dr. Prof. Y. Fu, Q.-K. Wu, Dr. Prof. Z. Du
Key Laboratory of Eco-functional Polymer Materials of the Ministry of
Education,
College of Chemistry and Chemical Engineering,
Northwest Normal University
Lanzhou, 730070, China
E-mail: fu_yingmail@126.com
zhengyin_du@nwnu.edu.cn
https://chem.nwnu.edu.cn/2012/0420/c373a20188/page.htm
Scheme 1. Visible-light-mediated debenzylative sulfonylation of tertiary
benzylamines.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100144
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and solvents under air and irradiation with visible light using a
in 85% isolated yield (entries 2–5). Obviously, the photosensi-
tizer played a crucial role to initiate this debenzylative coupling
reaction as shown in entry 6, without the presence of a
photosensitizer, no reaction occurred (entry 6). Then a variety of
bases, including inorganic bases, i.e. anhydrous Na2CO3, K2CO3,
Cs2CO3, and organic bases such as pyridine, triethylamine, N-
methylmorpholine (NMM), and N-methylpiperidine (NMP) were
validated, and the results revealed that none of these bases was
superior to CaH2 (entries 7–13). A screen of solvents showed
that the debenzylative cross-coupling of 1a and 2a could be
performed in CH2Cl2 (82%), toluene (66%), but could not be
carried out in THF (n.r.) and 1,4-dioxane (n.r.). Thus acetonitrile
was chosen to be the optimal reaction media.
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3 W blue LED bulb. At first, Ru(bpy)3Cl2, a powerful metal
photosensitizer that succeeds in many photoredox reactions,
was checked and failed to promote such as a coupling reaction
(Table 1, entry 1). To our delight, when the reaction was
proceeded in the presence of 2 mol% of Eosin Y in MeCN under
a 3 W blue LED irradiation, sulfonamide 3a was indeed
produced in 33% yield. Further screening of photosensitizers
suggested that methyl violet was the best choice, affording 3a
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Table 1. Screening of the Reaction Conditions.[a,b]
Having the optimal reaction conditions in hand (Table 1,
entry 5), the scope of tertiary benzylamines for this trans-
formation was examined with TsCl (1a) (Table 2). First, N,N-
diethylbenzylamine, and N,N-di-n-propylbenzylamine pro-
ceeded this reaction quite well and yielded the desired
sulfonamide products 3b (86%) and 3c (84%) in high yield. The
sterically hindered branched tertiary benzylamines such as N,N-
diisopropylbenzylamine and N,N-diisobutylbenzylamine could
participate in this transformation with slightly lower yields of
products obtained, indicating the broad feasibility of this
approach. Other dialkylbenzylamines screened, i.e. N-methyl-N-
ethylbenzylamine, N-ethyl-N-propylbenzylamine, and N-benzyl-
piperidine participated in this transformation very well and
yielded the corresponding sulfonamides in high yields (3f–3h).
This process could tolerate a variety of functional groups such
as allyl, ester, and ketone (3i–3k), which provided the
possibility for further functionalization. Finally, N-benzyl-N-
methylanilines were also investigated under the standard
conditions, affording the desired products in 74–77% yields (3l
and 3m).
Next, we sought to examine the scope with respect to
various sulfonyl chlorides (Table 3). Aromatic sulfonyl chlorides
bearing both electron-withdrawing and electron-donating sub-
stituents readily underwent the desired debenzylative coupling
reactions, providing sulfonamides in good to high yields (4a–
4j). Functional groups, including halogens (F, Cl, Br),
trifluoromethyl, trifluoromethoxyl were well tolerated and trans-
ferred into desired products (4b–i, 64–81% yields). Fortunately,
the highly steric hindered aromatic sulfonyl chloride i.e. 2,4,6-
trimethylbenzenesulfonyl chloride could also participate this
transformation to yield the desired sulfonamides (4g) in
moderate yield. Heterocyclic sulfonyl chlorides represented
with 8-quinolinesulfonyl chloride and 2-thiophenesulfonyl
chloride readily underwent this debenzylative coupling reac-
tions to form desired sulfonamide products in moderate to
good yields (Table 3, 4k–4m).
Entry
Photocatalyst
Base
Solvent
3a Yield [%]
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2
3
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5
6
7
8
Ru(bpy)3Cl2
Eosin Y
Alizarine yellow R
Solvent red 49
Methyl violet
–
Methyl violet
Methyl violet
Methyl violet
Methyl violet
Methyl violet
Methyl violet
Methyl violet
Methyl violet
Methyl violet
Methyl violet
CaH2
CaH2
CaH2
CaH2
CaH2
CaH2
Na2CO3
K2CO3
Cs2CO3
pyridine
Et3N
NMM
NMP
CaH2
CaH2
CaH2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
Toluene
THF
trace
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21
85
N.r.
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42
48
72
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60
78
82
66
N.r.[c]
N.r.
1,4-dioxane
[a] Reaction conditions: TsCl 1a (0.5 mmol), N,N-dimethylbenzylamine 2a
(0.75 mmol), photocatalyst (0.01 mmol), base (1.0 mmol), for CaH2
(100 mg), solvent (4 mL), r.t., 24 h; [b] Isolated yields based on N,N-
dimethylbenzylamine 2a. [c] N.r.=No reaction.
Table 2. Scope of Tertiary Benzylamines.[a,b]
The efficiency of the present reaction system was further
highlighted by the synthesis of probenecid (7),
a well-
established drug for the treatment of gout (Scheme 2). Thus
ethyl 4-(chlorosulfonyl)benzoate 5 completed conversion of
N,N-di-n-propylbenzylamine (1.0 mmol) within 24 h to afford,
after in situ alcoholysis, compound 7 in 62% isolated yield.
To gain insights into the origination of radical species
involved in this debenzylative sulfonylation reaction, a series of
[a] A mixture of 1a (0.5 mmol), benzylamine (0.75 mmol), methyl violet
(0.01 mmol) and CaH2 (100 mg) in MeCN (4 mL) under air irradiation using
3 W blue LEDs at room temperature for 24 h. [b] Isolated yields.
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Table 3. Scope of Various Sulfonyl Chlorides.[a,b]
control experiments were then carried out (Scheme 3). At first,
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when the reaction of 1a with 2a was carried out under N2, the
desired product 3a was not detected (Scheme 3, (a)), indicating
that the reaction might involve an aerobic oxidative cross-
coupling. When the reaction was conducted in the dark, no
reaction occurred (Scheme 3, (b)), suggesting that visible light is
an indispensable reaction factor for this transformation. When
the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidine N-
oxyl) (2 equiv.) was employed under standard conditions, the
reaction was largely suppressed and adduct 8 was detected by
HRMS, indicating the existence of an N,N-dimethylbenzylamine
radical in the reaction system (Scheme 3c). In order to check if a
reactive singlet oxygen species were formed in our photo-
promoted oxidative cross-coupling reaction system, a singlet
oxygen quencher,[21] 1,4-diazabicyclo[2.2.2]octane (DABCO), was
added to the above-mentioned reaction system and that
proceeded smoothly to produce 3a in 72% isolated yield,
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indicating that O2 is not generated from O2 under our reaction
conditions. (Scheme 3d). The “light/dark” experiments showed
that the desired product 3a formed only under continuous
irradiation, thus excluding the possibility of a radical chain
propagation in our reaction system (Figure 2). To further probe
the nature of the radical species formed during the reaction,
electron paramagnetic resonance (EPR) studies were next
performed (Figure 3). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO)
was introduced as a spin-trap, as it is known to react with short-
living radicals to furnish more stable and EPR detectable
nitroxide radicals.[22] After irradiation for 30 s under blue LEDs at
room temperature, a quadruple EPR signal (g=2.01), which
might be ascribed to peroxide radical, was observed in the
mixture of photocatalyst with TsCl 1a, N,N-dimeth-
ylbenzylamine 2a, indicating that peroxide radical was gen-
erated therein. To verify if a charge-transfer complex, which has
been frequently observed in our previously reported reaction
systems,[15] relates to sulfonyl chlorides and aliphatic tertiary
amine, a UV-Vis experiment was then carried out. The UV-Vis
spectroscopic measurements of the combination of 1a and 2a
in MeCN showed that weak absorption bands ranging from 300
and 400 nm appeared (Figure 4), demonstrating the formation
[a] A mixture of sulfonyl chloride (0.5 mmol), benzylamine (0.75 mmol), and
2.0 mmol of CaH2 in MeCN (4 mL) under air irradiation using 3 W blue LEDs
at room temperature for 24 h. [b] Isolated yields.
Scheme 2. Synthesis of probenecid.
Scheme 3. Control experiments.
Figure 2. “Light/dark” experiments over time.
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Scheme 4. Proposed reaction mechanism.
Figure 3. EPR spectra generated as follows: TsCl 1a (0.1 mmol), methyl violet
(2 mol%) and N,N-dimethylbenzylamine 2a (0.1 mmol) in MeCN were stirred
at room temperature under 3 W blue LED irradiation. After 30 s, the mixture
was directly measured by EPR.
of tertiary benzylamine 2a. Sulfonamide 3a was thus formed
and an unstable 3-phenyldioxirane intermediate V was gen-
erated simultaneously. Intermediate V could be further oxidized
to benzoic acid under photocatalysis.[25] The existence of
benzoic acid was verified in our 13C NMR analysis of the reaction
mixture.
In summary, we have developed a new photochemical CÀ N
bond cleavage protocol of tertiary benzylamines. Compared
with existing literature methods to CÀ N bond cleavage
protocols, the present approach demonstrates several attractive
features, including using air as the ideal and green oxidant,
photocatalysis, high efficiency, and simple operation.
Experimental Section
General synthetic procedure: To a 10 mL Schlenk-tube equipped
with a stirring bar, sulfonyl chlorides (0.5 mmol), tertiary benzyl-
amines (0.75 mmol), methyl violet (4 mg, 2 mmol%), and MeCN
(4 mL) were added. After stirred and irradiated with a 3 W blue LED
for 24 hours at room temperature, the reaction mixtures were
quenched with aqueous NaHCO3 solution and was then dissolved
in ethyl acetate (10 mL), washed successively with water (2×
10 mL), and then brine (10 mL). The aqueous phase was further
extracted from ethyl acetate (10 mL) and washed as previously. The
organic phase was combined, dried over MgSO4, and concentrated.
Purification by silica gel column chromatography gave the desired
products.
Figure 4. UV-Vis absorption spectra of 1a (TsCl, 0.2 mM), 2a (N,N-dimeth-
ylbenzylamine 0.2 mM), and their mixture (1a+2a, 0.2 mM) in MeCN.
of the charge-transfer complex by the reaction between amine
and sulfonyl chloride which may play a role in the oxidative
CÀ N bond cleavage of aliphatic tertiary benzylamines.
On the basis of the above-mentioned results and related
documents on oxidative radical CÀ N bond cleavages,[23]
a
plausible radical mechanism of this photopromoted oxidative
coupling protocols, which was exemplified by the reactions of
1a and 2a for the formations of 3a is depicted in Scheme 4.
Upon visible light irradiation, photocatalyst (PC) was excited Acknowledgements
(PC*) and was reductively quenched with N,N-dimeth-
*
À
ylbenzylamine (2a) to PC , concomitantly, a nitrogen-centered
The authors are grateful for financial support from the National
*
À
radical cation (I) was generated.[24] The reduced catalyst (PC )
Natural Science Foundation of China (No. 21762040) and the
Natural Science Foundation of Gansu Province, China (No.
20JR10RA079).
was then oxidized by dioxygen to its normal state and
*
generated a reactive perhydroxyl radical anion (HO2 À ). The
carbon-centered radical (II), generated from radical cation (I) via
hydronium ion extrusion, cross-coupled with the perhydroxyl
*
radical anion (HO2 À ) to produce the peroxide III. Subsequently, Conflict of Interest
a charge-transfer complex (IV) between sulfonyl chloride 1a
and intermediate III was formed to facilitate CÀ N bond cleavage
The authors declare no conflict of interest.
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Keywords: Debenzylation
photocatalysts · Sulfonamide · Synthetic methods
·
Photocatalysis
·
Organic
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Manuscript received: February 6, 2021
Revised manuscript received: March 5, 2021
Accepted manuscript online: March 9, 2021
Eur. J. Org. Chem. 2021, 1896–1900
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