C(sp3)-H bond functionalization with styrenesviahydrogen-atom transfer to an aqueous hydroxyl radical under photocatalysis

Article abstract of DOI:10.1039/d1gc00753j

The redox-neutral addition of α-C-H bonds of acetonitrile and acetone to styrenes was enabledviathe hydrogen-atom transfer from relatively acidic and water-miscible C(sp³)-H bonds to an aqueous hydroxyl radical generated cleanly and iteratively

Full text of DOI:10.1039/d1gc00753j

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C(sp3)–H bond functionalization with styrenes via
hydrogen-atom transfer to an aqueous hydroxyl
radical under photocatalysis†
a,b
Cite this: DOI: 10.1039/d1gc00753j
Received 28th February 2021,
Accepted 7th April 2021
Shogo Mori a and Susumu Saito
*
DOI: 10.1039/d1gc00753j
rsc.li/greenchem
The redox-neutral addition of α-C–H bonds of acetonitrile and the cleavage of the relatively polarized and acidic C(sp3)–H
acetone to styrenes was enabled via the hydrogen-atom transfer bonds is the formation of strong Ar–H (BDE ≈ 113 kcal mol−1
)
from relatively acidic and water-miscible C(sp3)–H bonds to an and RCOO-H (C6H5COO-H, BDE = 105 kcal mol−1) bonds,
aqueous hydroxyl radical generated cleanly and iteratively by the which offsets the difficulty of the homolytic cleavage of
oxidation of water under silver-nanoparticle-loaded titania photo- NCCH2-H (pKa = 31 in DMSO; BDE = 93 kcal mol−1) and
catalysis without using stoichiometric oxidation agents.
CH3COCH2-H (pKa = 27 in DMSO; BDE = 95 kcal mol−1
)
bonds.19–23 However, in these strategies, the Ar•, RCOO•, or
other radical species24–26 were used as sacrificial agents.
Therefore, the development of a straightforward catalytic
method to avoid the generation of stoichiometric organic and/
or inorganic waste remains a significant challenge.
Herein, we report that the α-C–H bonds of acetonitrile
(CH3CN, 2a) and acetone [(CH3)2CO, 2b] can, despite their rela-
tively high acidity, undergo addition to the CvC double bonds
One of the most important and fundamental chemical steps in
both thermal and photocatalytic organic synthesis is the sim-
ultaneous movement of a proton (H+) and an electron; this
process is known as hydrogen-atom transfer (HAT).1 Molecular
photoredox HAT catalysis has emerged as a powerful strategy
for the discovery and development of numerous unique and
valuable transformations under mild reaction conditions
through open-shell pathways.2,3 Most of those reported to date
involve the cleavage of hydridic to neutral C(sp3)–H bonds
with electrophilic HAT catalysts generated by the photo-
excitation of [Bu4N]4[W10O32] (TBADT),4 aromatic ketones,5 or
the oxidation of organocatalysts using appropriate photoredox
catalysts.2,3 Despite these substantial advancements, HAT from
acidic α-C(sp3)–H bonds adjacent to an electron-withdrawing
group, such as a cyano or acetyl group, under light still
remains elusive (Fig. 1a), probably due to the mismatched
polarity and kinetically disfavored transition state; in fact, the
β-C(sp3)–H bonds of alkylnitriles and alkylketones are more
smoothly functionalized by well-established electrophilic HAT
catalysts than α-C(sp3)–H bonds.6,7 The removal of this
obstacle would pave the way for new radical chemistry that
would diversify organic synthesis.8–14 Very recently, photo-
catalytic HAT was demonstrated to occur from acidic C(sp3)–H
bonds to aryl radicals (Ar•)15–17 or oxygen-centered carboxy rad-
icals (RCOO•)18 (Fig. 1b). The thermodynamic driving force for
aGraduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan.
E-mail: saito.susumu@f.mbox.nagoya-u.ac.jp
bResearch Center for Materials Science, Nagoya University, Chikusa,
Nagoya 464-8602, Japan
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00753j
Fig. 1 Inspiration for the development for the hydrogen-atom transfer
(HAT) from acidic C(sp3)–H bonds.
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Table 1 Optimization of the reaction conditions and control
of styrenes via HAT from the α-C–H bonds to a hydroxyl radical
(•OH) generated using Ag/TiO2 and water under LED
irradiation (λ = 365 nm) without generating stoichiometric
waste (Fig. 1c). While the generation of the α-carbon-centered
radicals of 2a and 2b catalyzed by the photo-generated, electro-
philic OH radical in water [•OH⋯(H2O)n: •OH randomly deloca-
lized in bulk water] is kinetically relatively unfavorable, it is
thermodynamically more favorable, as the major driving force
of the reaction is the formation of an extremely strong HO-H
bond (BDE = 119 kcal mol−1).20,21,27,28 Although there have
been several reports of oxidative reactions of olefins via the
experimentsa
Changes from standard
conditions
Yieldb
(%)
Entry
Photocatalyst
c
1
TiO2(P25)
—
—
—
—
—
2d
3
M(5 wt%)/TiO2
Ag(5 wt%)/TiO2
Ag(1 wt%)/TiO2
—
Ag(1 wt%)/TiO2
Ag(1 wt%)/TiO2
Ag(1 wt%)/TiO2
Ag(1 wt%)/TiO2
Ag(1 wt%)/TiO2
≤20
69
•
•
addition of CH2CN or CH2COCH3,8–12,15,16,18,24 examples for
4
5
6
7
86 (85)
an overall redox-efficient catalysis remains scarce.29,30 The
c
—
—
c
•
In the dark
Without KOH
Without H2O
Without KOH, H2O
Under air
—
reactions using CH2CN under Pd/TiO2 or Pt/TiO2 photocataly-
sis has been reported, albeit that the productivity was low.31–33
The semiconductor photocatalyst TiO2 is photostable, ther-
mally stable, and tolerant of radicals and anionic nucleo-
philes, which often degrade other catalysts. TiO2 has a
bandgap suitable for the oxidation and reduction of many
cheap, practical, and stable chemicals, especially the oxi-
15
c
8e
9e
10f
—
c
—
c
—
a Standard conditions: 1a (0.2 mmol), 2a (1 mL, 19.0 mmol), Ag
(1 wt%)/TiO2 (10.0 mg), H2O (1 mL), and KOH (0.02 mmol) in the pres-
ence of LED light (λ = 365 nm) at room temperature under N2 for 24 h.
b 1H NMR yield of 3aa. Isolated yield in parentheses. c Not detected.
d M = Co, Ni, Cu, Rh, Pd, Ir, Pt, or Au. e Dehydrated 2a (2 mL) was
used. f 4-Hydroxy-4-phenylpentanenitrile (3aa′, 16%) was isolated
(Scheme S1†). See ESI† for experimental details.
•
dation of water to OH.34,35 The OH radicals easily undergo
hydrogen abstraction from, and addition to, organic frame-
works, which have been used extensively for the so-called
‘cold combustion’ of organic waste/contaminants through
their degradation to CO2.36–38 In contrast, transformations
involving the controlled cleavage and formation of chemical tively poor (≤20%; Table 1 entries 1 and 2; Table S1†). The
•
bonds mediated by OH are scarce, and those that have been loading of noble transition metals, including Ag, on TiO2 can
reported are impractical in terms of productivity and selecti- facilitate charge [hole (h+) and electron (e)] separation and effec-
vity.39 In other words, the use of •OH for selective organic tively prevent h+/e recombination.46 Unlike other noble metal
synthesis has so far been far beyond the scope of existing M/TiO2 catalysts, hydrogen atoms generated via the reduction of
catalytic approaches. Considering the higher reactivity of OH H+ on the metal nanoparticles do not readily recombine to give
•
in HAT in water than in organic solvents,40 and the undesired dihydrogen over Ag/TiO2 (Table S1†).43,46 In addition, pristine
•
addition of OH to aromatic compounds,41,42 we envisioned TiO2 cannot resupply a hydrogen atom to the benzylic radical,
that the selective HAT from highly water-miscible C–H bonds which may be an intermediate formed by the addition of
to •OH in the aqueous phase favorably occurs against •CH2CN to 1a.47 These facts may partially account for the
decomposition of poorly water-miscible styrenes and the efficiency of Ag/TiO2 in this transformation. The yield of 3aa
coupling products, both being favorably deployed in the was further improved by reducing the loading of Ag on TiO2,
organic phase. With the aim of making such a ground-break- and the exclusive formation of 3aa was observed with Ag
ing achievement based on our continuous development of the (1 wt%)/TiO2 (85% isolated yield; Table 1 entry 4; Table S2†).
semiconductor-based photocatalysis of alcohols for practical The turnover number (TON: mol/mol) based on Ag is 204.
and selective N-alkylation reactions,43–45 we screened various Photosensitized TBADT or benzophenone used in place of Ag/
metal nanoparticle-loaded TiO2 (M/TiO2) materials for their TiO2 under otherwise identical reaction conditions failed to cat-
ability to functionalize relatively acidic α-C–H bonds in a alyze this transformation (Table S3†).4,5 Although a base addi-
water-based photocatalytic system.
tive was not required to achieve the coupling (Table 1 entry 7),
Initially, a series of reactions using α-methyl styrene (1a) was the addition of KOH (10 mol%) significantly accelerated the
carried out using an aqueous mixture of 2a and the photo- reaction, leading to superb reactivity (Table S4†). Control experi-
catalyst under LED-light irradiation (λ = 365 nm) and N2 at ments verified the necessity of Ag(1 wt%)/TiO2, LED-light
ambient temperature to optimize the reaction conditions irradiation, KOH, water, and an N2 atmosphere for an efficient
(Table 1), since, unlike aliphatic alkenes, styrenes were poor transformation (Table 1, entries 5–10). Gratifyingly, the desired
coupling partners in relevant radical chain reactions, in which a reaction occurred effectively using the optimal, standard con-
stoichiometric oxidant is even needed.30 The metal (M) loading ditions (Table 1, footnote a).
of TiO2 was found to be critical for the effective addition of 2a
With the standard conditions in hand, we examined the
to 1a. Among the various M/TiO2 combinations tested, only Ag substrate scope with respect to the styrene component
(5 wt%)/TiO2 gave the desired product (3aa) in acceptable yield (Table 2). A scalable production of 3ba from a 1 mmol of 1b
(69%; Table 1, entry 3); the efficiency of the product formation was also successful (Scheme S2†). Both electron-deficient and
using pristine TiO2 (P25) and other M/TiO2 catalysts was rela- electron-rich α-methyl styrenes underwent the reaction (3ba:
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Table 2 Addition of 2a and 2b to styrenesa
a Isolated yield. Using 1 (0.2 mmol), 2 (1 mL), Ag(1 wt%)/TiO2 (10.0 mg), H2O (1 mL), and KOH (0.02 mmol) in the presence of LED light (λ =
365 nm) at room temperature under N2 for 24 h. b Used 1b (1 mmol). c Ag(1 wt%)/TiO2 (20.0 mg). d 1 (0.1 mmol), Ag(1 wt%)/TiO2 (20.0 mg). e Ag
(1 wt%)/TiO2 (50.0 mg). See ESI† for experimental details.
92%; 3ca: 83%; 3da–3fa: 64–84%). An ortho-substituent (3ga:
To gain further insight into the catalytic cycle, control experi-
44%), the halogens F and Cl (3ha and 3ia: ∼70%), and pyri- ments were carried out (Fig. 2). First, the formation of •OH in the
dines (3ja–3la: 50–74%) were well tolerated. Styrenes that do presence of irradiated Ag(1 wt%)/TiO2 in water was confirmed by
•
not bear an α-methyl group also afforded the desired products the fluorescence change when the aqueous OH was trapped by
(3ma–3qa: 68–87%), suggesting that the transient stabilization water-miscible coumarin (Fig. S2†).49 Secondly, the radical sca-
of benzylic radicals by substitution is not mandatory. Likewise, venger (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO; 4 equiv.
2b underwent addition of its α-carbon to a wide variety of sty- relative to 1a) was added to a 2a/H2O–KOH system under the
renes (3ab, 3bb, 3db, 3gb, 3mb and 3qb: up to 96%).
standard conditions (irradiation time: 6 h) (Fig. 2a; Table S6†). As
Versatile commodity organic solvents were also tested as coup- a result, the C–C coupling step was significantly inhibited;
•
ling partners for styrenes (Table 3). Highly water-miscible solvents instead, 5a was obtained as a result of the coupling of CH2CN
such as THF (2c), 1,4-dioxane (2d), DMF (2e), and DMA (2f) gave with TEMPO, which is not miscible with water (the water phase
the desired C–C coupling products in good to excellent yield. Even was not colored red by TEMPO). These results suggest a possi-
the α-carbon of the unactivated amide DMA was slightly alkylated. bility of a free radical coupling process at the interface of the
Surprisingly, C–C coupling of 4-methylstyrene (1m) did not take water and organic phase (‘on water’).50 A similar result was
place with poorly water-miscible 2g–2j, which are structurally ana- obtained using the 2b/H2O–KOH system (Table S7†). Thirdly,
logous to 2a–2d, nor did the reaction proceeded with 2k, which deuterium (D)-labelling experiments were performed under the
has a neutral and relatively weak C(sp3)–H bond (pKa = 43 in standard conditions. When previously synthesized 3aa was added
DMSO; BDE = 90 kcal mol−1).19,22 Oil/water-biphasic mixtures to 2a-d3/D2O–KOH and irradiated, H–D exchange barely occurred
were readily formed with 2g–2k, suggesting that the formation at the benzylic position or α-position of 3aa, suggesting that 3aa
and reaction of •OH are only viable in water,40,48 as the Ag/TiO2 is effectively escaped from the aqueous phase containing HO− and
present in the aqueous phase (Table S5†).
•OH after its formation (Fig. 2b, eqn (1); Fig. S3†), whereas D was
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Table 3 Water-miscibility and reactivity of C(sp3)–H bondsa
a Isolated yield. Using 1 (0.2 mmol), 2 (1 mL), Ag(1 wt%)/TiO2
(10.0 mg), H2O (1 mL), and KOH (0.02 mmol) in the presence of LED
light (λ = 365 nm) at room temperature under N2 for 24 h. b Ag(1 wt%)/
TiO2 (20.0 mg). See ESI† for experimental details.
incorporated exclusively at the benzylic position during the reac-
tion of 1a in 2a-d3/D2O–KOH (Fig. 2b, eqn (2); Fig. S4†). In sharp
contrast, D-incorporation at the same carbon atom scarcely
occurred when 1a was reacted in 2a-d3/H2O–KOH (Fig. 2b, eqn
(3); Fig. S5†). These results suggest that H2O is the exclusive
source of the hydrogen atoms (H•) supplied to the benzylic rad-
icals in the system; H• is generated upon the reduction of H+
by e on Ag and stabilized by the Ag/TiO2 surface.43 In the reac-
tion of 1a in 2a/D2O–KOH, D was not incorporated at the
α-position of the cyano group, while the benzylic position was
almost fully deuterated, suggesting that H–D exchange between
2a and D2O was so sluggish that the nitrile enolate is hardly
formed to participate in the coupling (Fig. 2b, eqn (4);
Fig. S6†). The deuteration frequency was easily altered, selec-
tively furnishing 3aa-d1, 3aa-d2, and 3aa-d3. An experiment
using a 1 : 1 (v/v) mixture of 2a and 2a-d3 with H2O under the
standard coupling conditions gave a mixture of 3aa and 3aa-d2
with a large kinetic isotope effect (kH/kD = 8.9), indicating that
Fig. 2 (a) Radical-trapping experiments. 1H NMR yields of 3aa and 5a.
(b) Labelling experiments. Conversion of 1a determined by GC-MS ana-
lysis. Isolated yields of 3aa, 3aa-d3, 3aa-d2, and 3aa-d1. D/H ratio deter-
mined by 1H NMR analysis of the isolated products. (c) Plausible mecha-
nism. h+ = hole, e = electron, CB = conduction band, VB = valence
band. See ESI† for experimental details.
the cleavage of the α-C(sp3)–H bond of 2a would be the rate- cible α-C–H bonds of 2b, 2b-d6, and 3ab underwent feasible H–
determining step (Fig. 2b, eqn (5); Fig. S7†). Such a large kH/kD D scrambling with D2O or H2O (Fig. S8–11†).
(>7) would imply that the HAT reaction resulted from the
Based on the experimental data, and interpretations in pre-
quantum mechanical tunnelling.51 Similar trends were observed ceding reports that the radical chain propagation via HAT to
in the coupling of 2b, although the more acidic and water-mis- benzylic radicals from substrate C–H bonds is energetically
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Table 4 Comparison of green chemistry metrics for the addition of
acetonitrile or acetone to alkenes using different protocols
Conflicts of interest
There are no conflicts to declare.
Entry
1
2
3
4
Reference number
AE (%)a
This work
100
7.7
29
30
25
100
1.4
70
70
—
100
1.2
86
100
0.9
121
121
2g
RME (%)b
Acknowledgements
PMI (reaction)c
PMI (reactant, reagent, catalyst)c
TONd
29
13
86
This work was supported by the Asahi Glass Foundation (Step-
up-grant to SS), and partially funded by a MEXT Grant-in-aid
for Scientific Research on Innovative Areas (18H04247 to SS)
and a NU-GTR fellowship (to SM).
171e
9f
a Atom economy. b Reaction mass efficiency. c Process mass intensity.
d Turnover number (mol/mol). e Based on Ag. f Based on CuI. g Based
on 4,4′-dichlorobenzophenone. See ESI (page S33†) for the reactions
and calculation details.
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