Terminal-oxidant-free photocatalytic C-H alkylations of heteroarenes with alkylsilicates as alkyl radical precursors

Article abstract of DOI:10.1039/d0cc03286g

We report the photocatalytic C-H alkylations of heteroarenes with alkylsilicates bearing C,O-bidentate ligands under acidic conditions. Irradiation of heteroaromatics in the presence of the silicates and trifluoroacetic acid produced the corresponding alkylated compounds. The present reaction system does not require any terminal oxidant although the reaction seems to be a formal oxidation reaction. This study demonstrates that alkylsilicates can be used in photocatalytic radical chemistry under acidic conditions.

Full text of DOI:10.1039/d0cc03286g

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Terminal-oxidant-free photocatalytic C–H alkylations of
heteroarenes with alkylsilicates as alkyl radical precursors†
Gun Ikarashi,a,b Tatsuya Morofuji*,a and Naokazu Kano*,a
__Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report the photocatalytic C–H alkylations of heteroarenes with
alkylsilicates bearing C,O-bidentate ligands under acidic conditions.
Irradiation of heteroaromatic compounds in the presence of the
silicates and trifluoroacetic acid produced the corresponding
alkylated heteroaromatic compounds. Notably, the present
reaction system does not require any terminal oxidant although the
reaction seems to be a formal oxidation reaction. This study clearly
demonstrates that alkylsilicates can be used in photocatalytic
radical chemistry under acidic conditions.
Heteroarenes are important components of natural products
and active pharmaceutical ingredients.1 The radical substitution
of a C–H bond of a protonated heteroarene, the Minisci
reaction, is a well-known versatile method for derivatizing
heteroarenes,2 and many Minisci-type reactions have been
developed over the past several decades. Carboxylic acids,3
alkyl halides,4 activated esters,5 peroxides,6 alcohols,7 boronic
acids,8 sulfinate salts,9 alkenes,10 alkyltrifluoroborates,11 and
1,4-dihydropyridines12 have all been used as radical precursors
in these reactions. In addition, although most Minisci-type
reactions require stoichiometric amounts of an oxidant,
oxidant-free versions, which are expected to serve as mild and
clean methods for alkylating heteroarenes, have been
developed recently.3d-3f,11d Despite great effort, there are only a
few examples of Minisci-type reactions that use organosilicon
compounds.13 Only benzylsilanes and silicon compounds
bearing single chalcogen atoms at their α-positions have been
used as radical precursors in Minisci-type reactions (Fig. 1a)
because most silicon compounds, whose carbon–silicon
bonding orbitals do not interact with π electrons or lone pairs,
have very high oxidation potentials, making it difficult to
generate alkyl radicals through oxidation.14 In this decade,
Fig 1. (a) Organosilicon compounds as radical precursors. (b) Synthesis of
alkylsilicates 2. (c) This work: C–H alkylation of heteroarenes with alkylsilicates
as alkyl radical precursors.
pentacoordinated alkylsilicates 1 bearing pairs of catecholate
ligands have turned out to be alkyl radical precursors with low
oxidation potentials.15 Because of lower oxidation potentials,
alkysilicates 1 have been used in place of alkyltrifluoroborates
that produce toxic boron fluoride byproducts and are poorly
soluble in most organic solvents. As one example, desulfonative
alkylation
of
N-heteroaryl
sulfones
using
bis(catecholato)silicates under non-acidic conditions was
reported.15d However, such alkylsilicates, which are unstable to
acid, have not been used in reactions under acidic conditions,
such as Minisci-type C–H alkylations of heteroarenes.
a. Department of Chemistry, Faculty of Science, Gakushuin University
1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan.
b. Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-
3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
† Electronic Supplementary Information (ESI) available: Experimental procedures
and NMR data. For ESI see DOI: 10.1039/x0xx00000x
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With this background in mind, we focused our attention on C–H alkylated product 5 in 81% yield (entry 1). Photocatalyst
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alkylsilicates
2
bearing pairs of [-C6H4-2-C(CF3)2O-] C,O- was essential for achieving the alkylatDioOnI: 1(0e.n10tr3i9e/sD02CCa0n3d2836G).
can be directly Surprisingly, the reaction proceeded well in the absence of
bidentate ligands.16,17 Alkylsilicates
2
synthesized by the reaction of an alkyllithium or (NH4)2S2O8 (entry 4), in contrast to the Minisci-type reactions
alkylmagnesium bromide with Martin’s spirosilane 3 (Fig. with alkyltrifluoroborates. We considered the possibility that a
1b).16a,b Beneficially, alkylsilicates 2 are stable to water and trace amount of oxygen present in the solution acted as the
dissolve well in common organic solvents. Moreover, 2 remain oxidant, but the desired product was hardly obtained in an
intact under acidic conditions.16c Hence, we envisaged that 2 oxygen atmosphere (entry 5). Irradiation with blue light, the
would provide opportunities for the development of new presence of an acid, and the photocatalyst were all essential for
photoredox reactions using organosilicon reagents as radical the reaction to progress (entries 6–8). Using CH2Cl2 as the
precursors in acidic media. Herein, we report the photocatalytic solvent improved the yield of 5 (entry 9). As expected,
C–H alkylation of heteroarenes using alkylsilicates 2 (Fig. 1c). cyclohexylsilicate 1a, which bears two catecholate ligands,
Notably, the present reaction system does not require any failed to give the desired product (entry 10). The reaction with
terminal oxidant although the reaction seems to be a formal 4CzIPN as the photocatalyst proceeded, albeit with a slightly
oxidation reaction.
lower yield of 5 (entry 11).
To optimize the reaction conditions, 4-methylquinoline (4)
With the optimal conditions in hand, the generality of this
and cyclohexylsilicate 2a were selected as the model substrate Minisci-type C–H alkylation reaction was evaluated. As shown
and radical precursor, respectively (Table 1). We first carried out in Table 2, a diverse range of heteroarenes was alkylated under
the reaction referring to the reaction conditions of the optimized conditions. The reaction also efficiently produced
photocatalytic
Minisci-type
reactions
using 5 on the gram scale. 2-Methylquinoline also gave the desired
alkyltrifluoroborates as radical precursors.11c Irradiation of 4, product 6 in high yield, while unsubstituted quinoline gave the
2a, (NH4)2S2O8, and 9-mesityl-10-methylacridinium perchlorate 2,4-dialkylated product 7 in 83% yield using 2.2 equivalents of
(Mes-Acr+) as the photocatalyst in CH3CN/H2O gave the desired cyclohexylsilicate 2a. Quinolines with a variety of substituents,
including phenyl, chloro, bromo, and methoxy, also gave the
corresponding products 8–11. Isoquinolines, as well as
unsubstituted phenanthridine and benzothiazole, were also
amenable to the protocol to produce 12–16, respectively. As
expected, the reaction with benzimidazole resulted in almost no
conversion to 17 because of the electron-rich nature of its C2
site,11c while quinazolinone and 3,6-dichloropyridazine afforded
the desired products 18 and 19. 4-(Trifluoromethyl)pyridine
was converted into the 2,6-dialkylated product 20.
Furthermore, this Minisci-type alkylation protocol was also
successfully used to functionalize complex bioactive molecules.
For instance, the fungicide quinoxyfen was alkylated at the C2
position of its quinoline ring to give 21 in 65% yield. Fasudil, a
potent vasodilator bearing a free NH group, was selectively
alkylated at the C1 position to give 22 in 43% yield, which
contrasts sharply with the previous photocatalytic Minisci-type
alkylation using persulfate as the terminal oxidant that required
protection of the NH group.3c Quinine gave the corresponding
product 23 in 49% yield.
The scope of the silicate was subsequently investigated.
Primary alkylsilicates could be used in this reaction to give 24–
26, although primary alkyl radicals are less stable than
secondary and tertiary alkyl radicals. Both acyclic and cyclic
secondary alkylsilicates as well as tertiary alkylsilicates were
applicable to the present reaction system to give 27–31.
The following experiments were performed to obtain
mechanistic insight into the present reaction. Stern–Volmer
quenching experiments revealed that 2a efficiently quenches
the excited state of the photocatalyst, whereas the
heteroarene, TFA, the protonated heteroarene, and spirosilane
3 did not (see Supporting Information). These results suggest
that oxidation of 2a is the key initiation step for the C–H
alkylation reaction. We next performed a radical clock
experiment. Alkylation of 4 with 5-hexenylsilicate 2b under the
Table 1. Optimizing the photocatalytic C–H alkylation reaction.
entry
1
silicate
2a
photocatalyst
Mes-Acr+
none
oxidant
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
none
solvent
yield [%]a
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH2Cl2
81
0
2
2a
3b
4
2a
none
0
2a
Mes-Acr+
Mes-Acr+
Mes-Acr+
Mes-Acr+
none
71
5
2a
O2 (1 atm)
none
trace
0
6b
7c
8
2a
2a
none
trace
trace
87
2a
none
9
2a
Mes-Acr+
Mes-Acr+
4CzIPN
none
10
11
1a
none
CH2Cl2
0
2a
none
CH2Cl2
73
4-Methylquinoline (4, 0.1 mmol), cyclohexylsilicate 1a or 2a (0.12 mmol),
oxidant (0.2 mmol), photocatalyst (5 mol%), and TFA (0.11 mmol) were stirred
at 23 °C in solvent (4 mL) for 24 h and irradiated with blue light. a Determined
by 1H NMR spectroscopy. b The reaction was carried out in the dark. c TFA was
not added.
2 | J. Name., 2012, 00, 1-3
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standard conditions gave the ring-closed product 32 without dehydrogenated product 16 was observed in 38% yield by H
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DOI: 10.1039/D0CC03286G
the formation of the linear product 32’ (Scheme 1a), confirming NMR spectroscopy after irradiation with blue light in the
the radical pathway.
presence of Mes-Acr+. Furthermore, irradiation of 16’ in the
The present method does not require any terminal oxidant, presence of both the photocatalyst and TFA produced 16 in
whereas previous oxidant-free Minisci-type reactions include quantitative yield, strongly suggesting the intermediacy of 16’
electrochemical oxidation3f,11d or hydrogen evolution.3d,3e To during the formation of 16.
gain some insight into the alternative oxidative process
We propose a possible mechanism for the photocatalytic C–H
operating in the present system, we examined the crude alkylation reaction (Scheme 1d). First, Mes-Acr+ is excited by
products from the reactions of alkylsilicates 2a with visible light and oxidizes alkylsilicate 2, releasing alkyl radical R•
1
benzothiazole as a substrate. H NMR spectroscopy revealed and spirosilane 3. This process is energetically feasible because
the formation of 2-cyclohexyl-2,3-dihydrobenzo[d]thiazole (16’) the reduction potential of Mes-Acr+* (Ered = +2.06 V vs. SCE)18 is
in addition to the desired 2-cyclohexylbenzothiazole (16) more positive than the oxidation potential of cyclohexylsilicate
(Scheme 1b), which suggests that the hydrogenated form of the 2a (Ered = +1.47 V vs. SCE). The generated alkyl radical R• reacts
desired product is a reaction intermediate. To address this with the protonated heteroarene to form radical cation I. Single
possibility, we investigated reaction conditions for the electron transfer (SET) from Mes-Acr• to radical cation I then
conversion of 16’ into 16 (Scheme 1c). The addition of forms intermediate II. Quenching experiments indicate that the
alkylsilicate 2a, spirosilane 3, TFA, or Mes-Acr+ to a CD2Cl2
solution of 16’ resulted in no reaction. However, the
Table 2. Substrate scope for the photocatalytic C–H alkylation reaction.
5: 72, 72 %a
6: 98%
7: 83%b
8: 76%
9: 88%
10: 64%
15: 83%
11: 52%
16: 47%c
12: 71%
17: < 5%
13: 67%
18: 64%d
14: 51%
19: 33%
20: 43%b
24: 58%
21: 65%
22: 43%e
23: 49%f
27: 71%
25: 41%
26: 67%
28: 81%
29: 63%
30: 66%
31: 60%
Heteroarene (0.2 mmol), alkylsilicate 2 (0.24 mmol), Mes-AcrClO4 (5 mol%),
and TFA (0.22 mmol) were stirred at 23 °C in CH2Cl2 (4 mL) for 24 h while
a
irradiated with blue light. Yields are isolated yields. The reaction was
b
c
performed on a gram scale. 2.2 equiv. of 2a was used; performed in a
d
e
CH2Cl2/H2O mixture (1/1). performed in a CH3CN/H2O mixture (1/1); 2.1
equiv. of TFA was used and the product was isolated in N-Boc protected form.
f 2.1 equiv. of TFA was used and 1,1,1,3,3,3-hexafluoro-2-propanol was used as
the solvent.
Scheme 1. Mechanistic studies. (a) A radical clock experiment. (b) Observing
the intermediate. (c) The dehydrogenation reaction. (d) Proposed reaction
mechanism.
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photocatalyst would generate I (see Supporting Information).
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and protonated desired product, which is deprotonated to give
the final product. Protonation of III gives I again.
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6
7
In summary, we developed
a new protocol for the
photocatalytic Minisci-type C–H alkylations of heteroarenes
using alkylsilicates as alkyl radical precursors. This method does
not require any terminal oxidant, which makes it possible to
functionalize various heteroarenes in a mild and clean reaction
system. A variety of primary, secondary, and tertiary alkyl
groups was directly incorporated into various electron-deficient
heteroarenes in an efficient manner. Mechanistic studies
suggest that this terminal-oxidant-free alkylation involves the
photocatalytic formation of the hydrogenated form of the
desired product followed by photocatalytic dehydrogenation.
This study clearly demonstrates that alkylsilicates can be used
in photocatalytic radical chemistry under acidic conditions.
This research was partially supported by a MEXT-supported
program for Strategic Research Foundations at Private
Universities, JSPS KAKENHI Grant Number JP19K15570, the
Tokyo Ohka Foundation for The Promotion of Science and
Technology, Merck & Co., Inc and the Fukuoka Naohiko
Foundation. This research was also supported by crowdfunding
(academist). The authors thank Yumi Hayashi, Subaru Kawasaki,
Yuki Miyabayashi, Kazufumi Nakano, Nobu, Kenta Tanaka,
Yoshiyuki Yagi, and Jun Yamaoka for funding. The authors also
thank Prof. Koichi Iwata and Dr. Akira Takakado for their kind
assistance with the Stern–Volmer quenching experiments. The
authors thank Yu Matsui for her assistance with measuring
melting points and redox property of alkylsilicates. DFT
calculations were performed using Research Center for
Computational Science, Okazaki, Japan.
8
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Conflicts of interest
There are no conflicts to declare.
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