Photoelectrochemical cross-dehydrogenative coupling of benzothiazoles with strong aliphatic C-H bonds

Article abstract of DOI:10.1039/d1cc01012c

A photoelectrochemical strategy for the cross-dehydrogenative coupling of unactivated aliphatic hydrogen donors (e.g.alkanes) with benzothiazoles is reported. We used tetrabutylammonium decatungstate as the photocatalyst to activate strong C(sp³)-H bonds in the chosen substrates, while electrochemistry scavenged the extra electrons.

Full text of DOI:10.1039/d1cc01012c

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Photoelectrochemical cross-dehydrogenative
coupling of benzothiazoles with strong aliphatic
C–H bonds†
Cite this: Chem. Commun., 2021,
7, 4424
5
Received 22nd February 2021,
Accepted 24th March 2021
Luca Capaldo, ‡ Lorenzo L. Quadri, ‡ Daniele Merli
and Davide Ravelli
*
DOI: 10.1039/d1cc01012c
rsc.li/chemcomm
ꢁ
A photoelectrochemical strategy for the cross-dehydrogenative coupling underwent homolysis upon irradiation, affording Cl that was in
16
of unactivated aliphatic hydrogen donors (e.g. alkanes) with benzothia- turn responsible for C–H bond cleavage (Scheme 1b).
zoles is reported. We used tetrabutylammonium decatungstate as the
We hereby report our results on the development of a PEC
3
photocatalyst to activate strong C(sp )–H bonds in the chosen sub- strategy enabling the CDC of benzothiazoles with strong ali-
strates, while electrochemistry scavenged the extra electrons.
3
phatic C(sp )–H bonds in the presence of tetrabutylammonium
1
7,18
decatungstate (TBADT, (Bu
4
N)
4
[W10O
32]; Scheme 1c).
Our
1
2,3
A successful combination of photocatalysis and electrochemistry
approach exploits the excited state of TBADT to trigger a mild
4–7
has recently opened new avenues in synthesis, offering unpar- and selective C–H bond activation via a hydrogen-atom transfer
1
9
alleled mild operative conditions and contributing to addressing (HAT) step. The obtained products contain the benzothiazole
8
the urgent need of developing sustainable synthetic protocols.
ring, an important structural motif in several bioactive
2
0
A seminal example dealt with the photoelectrochemical (PEC) compounds. In particular, 2-alkyl substituted benzothiazoles,
alkylation of (hetero)arenes by a variety of alkyltrifluoroborates among the other applications, have been adopted as anti-
9
+
(
Scheme 1a). In this process, a mesitylacridinium (Mes-Acr ) cancer, antimicrobial, antitubercular, antidiabetic and antide-
ꢀ
20a
photocatalyst activated R–BF
3
substrates (potassium salts) via pressant agents.
an oxidative single electron transfer (SET), delivering a C-centered
Building upon our previous studies on the TBADT-photo-
ꢁ
radical (R ). On the other hand, electrochemistry took care of catalyzed Minisci-type CDC of H-donors with heteroarenes, which
10
photocatalyst recovery and of adjusting the redox state of the took place in the presence of potassium persulfate (K S O , 2 equiv.)
2
2 8
21
involved intermediates, enabling this net-oxidative transforma- as the terminal oxidant, we started off by investigating the
tion in the absence of any chemical redox agent. More recently, oxidative coupling of cyclohexane (1a) and benzothiazole (2a) to
11
12
the PEC trifluoromethylation and carbamoylation of (hetero)-
arenes have been likewise described.
A much more convenient and straightforward approach would
require starting from substrates containing a C–H bond, in an
6
c,13,14
overall cross-dehydrogenative coupling (CDC) process.
In
+
one instance, a trisaminocyclopropenium (TAC ) catalyst has been
used to activate the targeted C–H bond in ethers. In particular,
electrochemical oxidation of TAC afforded TAC , which func-
+
ꢁ
2+
15
tioned as the actual photocatalyst.
In another instance, a different strategy for the CDC of
heteroarenes with aliphatic C–H bonds was developed. In such
a case, Cl was electrogenerated in situ (from excess HCl) and
2
Department of Chemistry, University of Pavia, viale Taramelli 12, Pavia 27100,
Italy. E-mail: davide.ravelli@unipv.it
†
Electronic supplementary information (ESI) available: Experimental details
about the used materials, sample preparation, electrochemical measurements,
laser flash photolysis, kinetic analysis and copy of NMR spectra. See DOI: 10.1039/
d1cc01012c
Scheme 1 Selected strategies for the functionalization of benzothiazoles
‡
These authors contributed equally to this work.
encompassing the combination of electrochemistry and light.
4
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a
ꢀ1
Table 1 Survey of reaction conditions
1.9 F mol ; entry 1). The reaction was rather sensitive to the
nature of the electrolyte used, with yields failing to reach 55%
when using TBAClO or different Li-based salts (entries 2–4).
4
Similarly, different photocatalyst loadings were found to be
detrimental (entries 5 and 6). Next, we considered the influence
of different electrochemical parameters on the reaction out-
come and modified the applied potential DEWE–RE, as well as
adopted an amperostatic operation mode (2 mA for 20 h),
however a lower yield of 3 was observed in all cases (entries
7–9). Next, we screened alternative materials for the WE,
including carbon cloth, boron-doped diamond (BDD) and Pt-
gauze electrodes, finding that 3 could be formed in 80% yield
when using the noble metal electrode (entries 10–12). We also
attempted to run the model reaction in a round bottom flask
(undivided cell conditions), however a very poor performance
was obtained (entry 13). Control experiments proved the essen-
tial role of light, photocatalyst and electricity (entries 14–16).
Notably, under purely photocatalytic conditions, a small amount
b
2
a consump-
3, Yield
(%)
F
mol
ꢀ1
Entry Introduced variation
tion (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
None
490
83
73
66
44
81
85
75
80
83
55
490
23
80
43
42
53
35
46
62
55
66
64
43
80
16
1.9
0.8
0.9
1.2
0.6
1.1
1.9
1.5
2
2
0.9
2
1.6
c
4
TBACIO as the electrolyte
LiOTf as the electrolyte
LiCIO as the electrolyte
TBADT (1 mol%)
TBADT (8 mol%)
4
(
9% yield) of the redox-neutral adduct 2-cyclohexylbenzothiazoline
d
e
0
(3 ) was observed besides traces of product 3 (entry 16). Finally, we
DEWE–RE = 0 mV
replaced the potentiostat with two 1.5 V batteries (AAA-type; B3 V
applied) in a 2-electrode setup, observing the formation of product
3 in 65% yield (entry 17; see the ESI† for further details).
With the optimized conditions in hand (Table 1, entries
DEWE–RE = +300 mV
Amperostatic mode (2 mA)
Carbon cloth as the anode
BDD as the anode
Pt gauze as the anode
Undivided cell, Pt gauze as
the anode
0
1
2
3
1
and 12), we next evaluated the scope of the reaction by
reacting parent 2a with a library of selected H-donors
Table 2, upper part). We consistently adopted Pt gauze as the
1
1
1
4
5
6
7
No light, Pt gauze as the
anode
No TBADT, Pt gauze as the
anode
No electricity (electrodes
omitted)
2 ꢂ 1.5 V AAA batteries
0
n.d.
n.d.
0
(
f
anode for its robustness and ease of cleaning, while selected
entries were run with a GC electrode (see Table S1 in the ESI†).
Thus, the model reaction allowed the isolation of product 3 in
0
0
g
21
89
Traces
—
—
78% yield; no improvement was observed if the excess of 1a was
1
a
65
increased to 10 equiv. (75% isolated yield). On the other hand,
with cyclopentane (1b) a higher excess of H-donor (up to
Reactions performed on a 0.75 mmol scale. Anolyte: 1a (0.25 M, 5 equiv.),
a (0.05 M), TBADT (4 mol%), LiNTf (0.05 M) in MeCN/H O 10:1 (15 mL).
(0.05 M) in H O (15 mL). NMR yield, CH Br used as
2
2
2
b
20 equiv.) was required to push the reaction to full conversion
Catholyte: LiNTf
an external standard. TBAClO
2
2
2
2
c
d
4
was poorly soluble. Complex mixture. with adduct 4 isolated in 73% yield. Cycloheptane (1c; 5 equiv.
e
ꢀ1
2
mA applied to transfer 2 F mol within 20 h. The formation of a thick
deposit on the electrode surface was observed. The experiment was
stopped after 30 min since a very low current was observed (B1 mA). The
f
used) was scarcely soluble under our conditions, however the
expected arylated adduct 5 and the dimerization product 5A
redox-neutral adduct 2-cyclohexylbenzothiazoline (3 ) was also formed in were formed in 42 and 21% isolated yield, respectively. Simi-
g
0
9
% yield. GC: glassy carbon; Pt : Pt gauze, BDD: boron-doped diamond
larly, norbornane (1d) was smoothly functionalized to give the
expected product 6 in 61% isolated yield (77% brsm) as an exo/
endo 8 : 1 mixture, along with a minor amount of 2-(cyclopen-
(see ESI for further information).
2
2
give adduct 3 (Table 1). The reaction setup consisted of a conven- tylmethyl)benzothiazole (6A; 8% yield).§ Finally, the selective
s
17a
tional H-type electrochemical cell (separator: Nafion N-117 poly- functionalization of isocapronitrile (1e; at the methine posi-
meric membrane) equipped with a three-electrode system, while tion) and cyclopentanone (1f; at the b-position) occurred in
irradiation was performed with an LED lamp (lem = 390 nm; 40 W) high isolated yields (480%) to give products 7 and 8 when
under rigorous oxygen-free conditions (see the ESI† for details). The using 10 equiv. of the H-donors, respectively.
anolyte was composed of a LiNTf
2
2
(0.05 M) MeCN/H O 10:1
Next, we turned our attention to the scope in terms of
solution (15 mL) containing 2a (0.05 M), 1a (5 equiv.) and TBADT benzothiazoles (Table 2, lower part) and selected 1a as the
(
4 mol%), while a LiNTf
2
(0.05 M) water solution (15 mL) was used model H-donor (10 equiv. used). Halogenated derivatives in
the 6-position offered different reactivity profiles depending on
for the catholyte.
Gratifyingly, when the cell was operated in a potentiostatic the substituent nature, leading to products 9–11 in modest to
mode by setting a fixed potential between the working (WE) and excellent isolated yields (26–80%). Strong electron-withdrawing
the reference (RE) electrode (DEWE–RE = +150 mV; reference or donating substituents offered similar results, as testified by
electrode: Ag/AgCl, Sat’d NaCl; anode: glassy carbon; cathode: adducts 12 and 13, that were both isolated in 50% yield
Pt gauze), we found that product 3 was formed in 80% NMR (the former with a better mass balance). The biologically-
2
3
yield (at 490% starting materials conversion; total charge: relevant trifluoromethoxy group
was tolerated as well,
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Table 2 Reaction scope
Scheme 2 Proposed reaction mechanism.
+
5ꢀ
+
decatungstate H [W10
O
32
]
(Wred (H )) to give the redox-
0
neutral benzothiazoline derivative 3 , also restoring the original
form of the photocatalyst (path a). Eventually, as mentioned
before, 3 is converted to 3 via a photoelectrochemical
0
sequence, wherein wO triggers an oxidative SET event to deliver
ꢁ
intermediate III upon deprotonation (see Section 1.4 in the
ESI†).
ꢁ
Alternatively, the formation of the final product 3 from II
can occur via path b: we suggest that II undergoes a spin-
center shift (SCS),
medium, to give III . Notably, the intermediacy of this species
is corroborated by the formation of 5A in the reaction between
cycloheptane 1c and benzothiazole 2a via the dimerization of
ꢁ
2
5,26
possibly mediated by the protic
2
7
ꢁ
a
A minor amount (8% yield) of 2-(cyclopentylmethyl)benzothiazole (6A)
was also formed.
ꢁ
the corresponding III species. To evaluate the feasibility of the
ꢁ
product 14 being obtained in 75% yield. Finally, we varied the conversion of III to 3, following a previously reported
number and position of the substituents on the starting ben- approach we estimated its oxidation potential by measuring
9
+
zothiazole, and products 15 and 16 were formed in 67% (87% the reduction potential of the protonated product 3-H ; a value
red
+
ꢁ
brsm) and 47% yields, respectively.
of Ep/2(3-H /III ) = ꢀ0.68 V vs. SCE has been found (see Section
To have insights into the mechanism, we monitored over 1.5 in the ESI† for additional details). Indeed, due to the
ꢁ
time the reaction between 1a and 2a and found that the redox- transient nature of III , we consider its direct oxidation by the
neutral adduct 2-cyclohexylbenzothiazoline (3 ) was formed at anode unlikely and postulate that the decatungstate anion
0
short reaction times, along with the desired product 3 (see might serve as an electrocatalyst to promote this step. This
kinetic analysis in Section 1.2 in the ESI†). When we subjected possibility is fully supported by cyclic voltammetry experiments,
0
red
adduct 3 to optimized PEC conditions, we discovered that it indicating a reduction event at Ep/2(W/Wred) = ꢀ0.52 V vs. SCE in
28
could be easily converted to the final product 3 in 60% NMR our conditions,¶ confirming the above mentioned hypothesis.
yield. In contrast, the conversion of 3 to 3 did not occur under The electrochemical reaction in the anolyte was balanced by the
0
purely electrochemical conditions (irradiation omitted; see cathodic reduction of protons to molecular hydrogen.
Section 1.3 in the ESI† for further information).
The hereby reported PEC protocol, wherein electricity func-
Scheme 2 depicts the proposed reaction mechanism, based tions as the terminal oxidant, compares favourably with our
on the data reported above, as well as the laser flash photolysis previous work based on the use of a chemical oxidant (excess
2
1
(LFP; Section 1.4) and electrochemical experiments (Section
K
2
S
2
O
8
). In particular, compound 3 was prepared in 60%
1
.5) reported in the ESI.† Thus, upon light absorption the isolated yield adopting the latter method, while photoelectro-
1
7d,24
reactive excited state of TBADT (wO) is generated,
is responsible for aliphatic C(sp )–H bond activation (e.g. in 1a) with the adoption of a lower H-donor excess (5 equiv. vs.
to afford organoradical I . The latter then adds onto the 20 equiv.).
which chemistry allowed the improvement of this value (up to 78%)
3
ꢁ
2
-position of benzothiazole (e.g. 2a) to afford radical adduct
Overall, the present work unlocks the use of aliphatic
ꢁ
ꢁ
3
II . At this stage, II may follow two different pathways to be substrates featuring strong C(sp )–H bonds in photoelectro-
ultimately converted to product 3. One possibility is that II
ꢁ
chemical manifolds and enables their arylation with benzothia-
undergoes a back-HAT (b-HAT) from the reduced form of zoles in a cross-dehydrogenative coupling protocol. Notably,
4
426
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the decatungstate anion shows a chameleonic attitude in the
reported transformation and plays a three-fold role: HAT
photocatalyst, photoredox catalyst and electrocatalyst. The pro-
cess occurs under extremely mild conditions and with a very
low applied potential, resulting in a perfect matching with the
reactivity profile offered by TBADT, that is turned over after
each catalytic cycle with an excellent faradaic efficiency.
This work received financial support from Fondazione CAR-
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¶
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