Visible-Light-Enabled C?H Functionalization by a Direct Hydrogen Atom Transfer Uranyl Photocatalyst

Article abstract of DOI:10.1002/chem.202003431

The development of the uranyl cation as a powerful photocatalyst is seriously delayed in comparison with the advances in its fundamental and structural chemistry. However, its characteristic high oxidative capability in the excited state ([UO₂]²⁺* (+2.6 V vs. SHE; SHE=standard hydrogen electrode) combined with blue-light absorption (hv=380–500 nm) and a long-lived fluorescence lifetime up to microseconds have reveals that the uranyl cation approaches an ideal photocatalyst for visible-light-driven organic transformations. Described herein is the successful use of uranyl nitrate as a photocatalyst to enable C(sp³)?H activation and C?C bond formation through hydrogen atom transfer (HAT) under blue-light irradiation. In particular, this operationally simple strategy provides an appropriate approach to the synthesis of diverse and valuable diarylmethane motifs. Mechanistic studies and DFT calculations have provided insights into the detailed mechanism of the photoinduced HAT pathway. This research suggests a general platform that could popularize promising uranyl photocatalytic performance.

Full text of DOI:10.1002/chem.202003431

Accepted Article
Accepted Article
Title: Visible-Light-Enabled C-H Functionalization by a Direct Hydrogen
Atom Transfer Uranyl Photocatalyst
Authors: Jipan Yu, Chongyang Zhao, Rong Zhou, Wenchao Gao,
Shuai Wang, Kang Liu, Si-yu Chen, Kongqiu Hu, Lei Mei,
Liyong Yuan, Zhifang Chai, Hanshi Hu, and Wei-Qun Shi
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003431
Link to VoR: https://doi.org/10.1002/chem.202003431
0
1/2020

Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
Uranyl Photocatalysis
Visible-Light-Enabled C-H Functionalization by a Direct Hydrogen
Atom Transfer Uranyl Photocatalyst
+
[a]
+ [b]
[c]
[c]
[a]
[a]
[a]
Jipan Yu , Chongyang Zhao , Rong Zhou, Wenchao Gao, Shuai Wang, Kang Liu, Siyu Chen,
[a]
[a]
[a]
[a,d]
[b]
[a]
Kongqiu Hu, Lei Mei,* Liyong Yuan, Zhifang Chai,
Hanshi Hu and Weiqun Shi*
[
5]
3
Abstract: Uranyl cation, as a powerful photocatalyst, at present,
is seriously delayed compared to the development of its fundamental
and structural chemistry. However, the characteristic of highly
photocatalysis. In general, photoinduced C(sp )-H bond activation
strategy gives rise to advanced opportunities for organic
[
6]
functionalization, which involves HAT,
Transfer), PCET (Proton-Coupled Electron Transfer) or energy
reactive routes. However, direct HAT catalyst, which
directly abstract a hydrogen atom from substrate with the control of
SET (Single Electron
2+*
[7]
[8]
oxidative capability in the excited state {[UO
2
]
(+ 2.6 V vs. SHE)
[
3a, 9]
combined with blue light (hv 380-500 nm) absorption and a long-lived
fluorescence lifetime up to microseconds have revealed that uranyl
cation approaches an ideal photocatalyst for visible-light-drvien
organic transformations. Described herein is a successful utilization of
transfer
[
10]
redox potentials, has been rarely investigated.
Additionally, a set
system with relatively more positive oxidation potential and good
compatibility suitable for operationally simple protocol by photo-
promoted direct HAT process is more scarce. Thus, uranyl cation
2
, a relatively underdeveloped photocatalyst, endowed with
visible light absorption, readily available and highly oxidizing excited
3
uranyl nitrate photocatalyst which enables C(sp )-H activation and C-
C bond formation through hydrogen atom transfer (HAT) under blue
light irradiation. In particular, this operationally simple scenario
provides an appropriate approach to synthesize diverse and valuable
diarylmethane motifs. Mechanistic studies and DFT calculations
provide insights into detailed mechanism of the photoinduced HAT
pathway. The current research is suggestive of a general platform that
could popularize promising uranyl photocatalytic performance.
2
+
UO
[
11]
state performance, arrested our attention.
-3
Redox window of uranyl
-
-
2
1
-
1.73
Ir(III)*/Ir(IV)
-1.56
-
1.33
-1.4
-
1.06
-1.2
9-fluorenone
9-fluorenone
/
Ru(I)/Ru(II)
Eosin B*/Eosin B
Eosin Y /Eosin Y
-
0.6
-
0.16
Acr -Mes/
0
UO2+/ UO22+
[Acr+-Mes]
Introduction
+
0.77
+0.84
+0.83
Eosin Y*/Eosin Y
+0.6
Ir(IV)/Ir(III)
+
1
2
Ru(II)*/Ru(I)
+
1.35
Eosin B /Eosin B
Eosin B
2
.76 eV
[
1]
[2]
Ru(bpy)32+ Ir(ppy)33+ Eosin Y
4CzIPN
+1.52
9-fluorenone*/
9-fluorenone
Since the pioneering works of MacMillan
and Yoon,
+
+
+2.15
Acr+-Mes]*/
[
photocatalytic organic synthesis, especially the one stimulated by
visible light, has undergone a remarkable renaissance and witnessed
dramatic developments for their mild and highly-efficient
+2.6
2
+
+
Acr -Mes
+
9-fluorenone
3
U
O
2
*
/
U
O
2
[
Acr -Mes]
Figure 1. Redox potentials of uranyl with respect to commonly used photocatalysts
[
3]
characteristics over the past decade. While the functionalization of
3
C(sp )-H bonds, as Holy Grails in modern Chemistry, has
[
4]
The available redox window of uranyl confirmed that the high
oxidative ability for some redox-active substrates is greater than those
of widely applied transition metal complexes and organic dyes,
revolutionized the program of organic synthetic chemistry. In this
context, spectacular advances have been created in the direct
[
12]
[13]
3
functionalization of C(sp )-H bonds by visible light-enabled
2
+
such as Ru(bpy)
3
and eosin Y, etc. (Figure 1). The excited state,
2
+
[
UO
adopts
microseconds and possesses a high oxidizability (E = + 2.6 V vs
2
] *, with characteristic absorption of 380-500 nm blue right,
a
long-lived fluorescence lifetime with the domain of
o
[
*] Dr. J.-P. Yu, Dr. K.-Q. Hu, Dr. L. Mei, Dr. L.-Y. Yuan, Prof. Z.-F.
Chai, Prof. W.-Q. Shi
[
11, 14]
SHE).
In the uranyl-catalyzed photocatalytic process, carbon-
Laboratory of Nuclear Energy Chemistry, Institute of High Energy
Physics, Chinese Academy of Sciences, Beijing 100049 (China)
E-mail: shiwq@ihep.ac.cn; meil@ihep.ac.cn
centered radical is usually generated through C-H bond homolytic
cleavage (direct HAT process) by the oxygen site of the robust linear
2
+
[14a]
[UO
2
] *.
In view of this promising reactivity and abundance of
Dr. C.-Y. Zhao, Prof. H.-S. Hu
desirable characteristics, it appears that uranyl-catalyzed C-H
functionalization with HAT procedure by visible light irradiation is
anticipated. Among the few correlated reports, Sorensen’s group
developed a catalytic system for direct fluorination of unactivated
Department of Chemistry & Key Laboratory of Organic
Optoelectronics and Molecular Engineering of Ministry of
Education, Tsinghua University, Beijing 100084 (China)
Prof. R. Zhou, Prof. W.-C. Gao
3
[15]
C(sp )-H bonds by a uranyl photocatalyst. In addition, Ravelli and
co-workers described a practical protocol for the C-H to C-C bond
College of Chemistry and Chemical Engineering, Taiyuan
University of Technology, Taiyuan 030024 (China)
Prof. Z.-F. Chai
[
16]
conversion via uranyl photocatalysis upon visible-light irradiation.
Very recently, Jiang et al. investigated a selective and compatible
Engineering Laboratory of Advanced Energy Materials, Ningbo
Institute of Industrial Technology, Chinese Academy of Sciences,
oxidation of sulfides for the synthesis of sulfones and sulfoxides using
[17]
UO
2
(OAc)
2
·2H
2
O
photocatalyst
and
molecular
oxygen.
Ningbo 315201 (China)
[18]
[19]
Concurrently, Arnold
and Takao
demonstrated a photocatalytic
+
[
] These authors contributed equally to this work.
oxygenation of hydrocarbon in the presence of uranyl complex and
visible light irradiation, respectively. In general, these remarkable but
few contributions highlight the novelty and importance of unexplored
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.202xxxxxx.
1
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Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
Table 1. Optimization of conditions for alkylation of 4-benzylidene-2,6-di-tert-
uranyl photochemistry. Therefore, we attempted to utilize this
innovative scheme to activate the C-H bond through direct HAT
photocatalysis, providing a convenient pathway for the synthesis of
important bioactive molecules.
butylcyclohexa-2,5-dien-1-one
(1a)
leading
to
2,6-di-tert-butyl-4-
[
a]
(
phenyl(tetrahydrofuran-2-yl)methyl)phenol (3a).
Figure 2. Selected bioactive compounds with a diarylmethane skeleton.
para-Quinone methides (p-QMs), as representative electrophilic
acceptors, are unique feedstocks for the synthesis of value-added
diarylmethane
derivatives
with
multiple
biological
and
pharmacological properties, such as antimicrobial, antiviral and
[
20]
anticancer (Figure 2). Although 1,6-addition of p-QMs with various
[
b]
Entry
photocatalyst
Solvent
Yield (%)
0
nucleophiles provide facile access to biologically active diarymethane
[
21]
1
2
3
4
5
6
7
8
9
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CN
derivatives have been reported,
the investigations on the pathway
[
22]
A (5 mol%)
B (5 mol%)
C (5 mol%)
D (5 mol%)
E (5 mol%)
F (5 mol%)
G (5 mol%)
CN
CN
CN
CN
CN
CN
CN
trace
trace
56%
40%
trace
trace
trace
86%
51%
57%
66%
0
via visible light-promoted radical process of p-QMs are few. Based
on previous work, we reasoned that the radical alkylation of p-QMs
through photo-induced direct HAT strategy might facilitate the
delivery of diarylmethane derivatives with structural diversity.
Accordingly, to address the above referred topics, herein we describe
that uranyl ions can be employed as effective photocatalysts for the
direct alkylation of para-quinone methides (p-QMs) under visible light
irradiation.
UO
2
(NO
(OAc)
SO
(OTf)
3
)
2
·6H
·4H
2
O (5 mol%)
CH
3
CN
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
UO
UO
UO
UO
UO
UO
UO
UO
2
2
2
2
2
2
2
2
2
2
O (5 mol%)
CH
CH
CH
CH
CH
CH
CH
CH
3
CN
4
·4H
2
O (5 mol%)
3
3
3
3
3
3
3
CN
2
·6H
2
O (5 mol%)
CN
Results and Discussion
[c]
(NO
(NO
(NO
(NO
(NO
3
3
3
3
3
)
2
2
2
2
2
·6H
·6H
·6H
·6H
·6H
2
O (5 mol%)
O (1 mol%)
O (3 mol%)
O (10 mol%)
O (5 mol%)
CN
)
)
)
)
2
2
2
2
CN
30%
71%
84%
82%
73%
trace
79%
trace
60%
To better assess the catalytic feasibility of this process, reactant 1a
and 2a with various photosensitizers were employed as the benchmark
substrate and alternative catalysts for optimization. As shown in Table
CN
CN
COCH
3
+
-
1
, Ir(dF(CF
3
)ppy)
2
(dtbbpy)(PF
6
) (A), Mes-Ac ClO
(E), Eosin Y (F), Eosin B (G) and
CN
as the solvent under argon atmosphere with irradiation of 5 W blue
LED (entries 2-12), and we found that UO (NO ·6H O afforded the
highest catalytic efficiency to get the target product (3a) in 86%
isolated yield (entry 9). The outcome indicated that UO (NO ·6H
4
(B), 4CzIPN (C),
18
19
20
21
22
UO (NO ) ·6H O (5 mol%)
EtOAc
DMSO
o-DCB
HFIP
2
3
2
2
9-Fluorenone (D), [Ru(bpy)
3
]Cl
2
2 3 2 2
UO (NO ) ·6H O (5 mol%)
Uranyl Salt (H) were screened with THF as radical donor and CH
3
2 3 2 2
UO (NO ) ·6H O (5 mol%)
UO (NO ) ·6H O (5 mol%)
2
3
2
2
[
d]
2
3
)
2
2
UO
2
(NO
3
)
2
·6H
2
O (5 mol%)
3
CH CN
[a] Reaction conditions: under argon atmosphere, 4-benzylidene-2,6-di-tert-
butylcyclohexa-2,5-dien-1-one (1a) (0.2 mmol, 1.0 equiv), catalyst (0.05 equiv),
solvent (2.0 mL), room temperature (~ 25 C), time (24 h) in a sealed Schlenk tube. [b]
Isolated yield. [c] No light. [d] Under air condition. [e] DMSO = dimethylsulfoxide. o-
2
3
)
2
2
O
o
has an exclusive advantage over other photocatalysts for this catalytic
system. The reaction did not occur in the absence of photocatalyst or
DCB = 1,2-dichlorobenzene. HFIP = 1,1,1,3,3,3-Hexafluoro-2-propanol.
2 3 2 2
without light (entry 1, 13). When the amount of UO (NO ) ·6H O was
increased from 5.0 mol% to 10 mol% (entry 16), similar yield of 3a
was obtained (compare entries 9 and 16), but lower reaction efficiency
With the optimized visible-light-stimulated uranyl-catalyzed
alkylation conditions in hand, we then explored the structural
generality of p-QMs leading to diarylmethane derivatives under blue
LED irradiation at the room temperature. As shown in Table 2,
satisfactorily, the examined diverse substituents on p-QMs provided
moderate to excellent yields. No evident difference was observed due
to the electronic effect among the examined substrates. The substrates
containing neutral methyl and electron-donating methoxyl groups gave
the desired products in moderate to high yields (compounds 3b, 3c, 3d,
were observed with the decrease of amount of UO
2
(NO
3
)
2
·6H
2
O
(
entries 14 and 15). Next, effect of solvents was surveyed, and CH
3
CN
was found to be a suitable solvent (compare entries 9, 17-21).
Subsequently, we attempted to perform the reaction under air
atmosphere, and the result showed that it was inferior to argon
(
compare entries 9 and 22). According to the results above, we believe
the option that UO (NO ·6H O as the photocatalyst in the presence of
CH CN solvent is more suitable for the formation of intermolecular
alkylated target adduct under visible light irradiation in Ar atmosphere.
2
3
)
2
2
3
e and 3f). In addition, methylsulfide substrate could be used in the
3
reaction to produce the corresponding product 3g in 83% yield.
Notably, naphthalene-substituted p-QMs as representative substrate
provided the corresponding desired product 3h with 38% yield. The
uranyl-catalyzed alkylated reactions could tolerate some functional
2
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Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
Table 2. Substrate scope for the visible-light-promoted uranyl-catalyzed alkylation of
constructed, especially isopropyl substituents 3w, albeit with low
yields. With respect to methyl- and H-substituted p-QMs, we found
that no desired products 3x and 3y were isolated, suggesting that
large steric hindrance group served as suitable precursors. Additionally,
it must be pointed out that 3e, 3f, 3h and 3m require
artificially prolonging reaction time to improve the reaction efficiency.
[
a]
2
,6-di-alkyl-4-(arylidene)cyclohexa-2,5-dien-1-one 1 with 2a.
O
OH
R
R
R
R
O
O
UO2(NO3)2 6H2O (5 mol%)
+
blue LED (5 W)
CH3CN, Ar, r.t.
Ar
R'
Ar
R'
1
2a
OH
3
Table 3. Substrate scope for the visible-light-promoted uranyl-catalyzed alkylation of
OH
OH
OH
[
a]
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
O
4-benzylidene-2,6-di-tert-butylcyclohexa-2,5-dien-1-one 1a with 2.
Me
O
O
O
MeO
O
OH
tBu
tBu
tBu
tBu
Me
3
a 86% (24 h)
3b, 77% (24 h)
OH
3c, 76% (24 h)
3d, 71% (24 h)
UO2(NO3)2 6H2O (5 mol%)
blue LED (5 W)
+
C
H
OH
OH
tBu
OH
tBu
tBu
O
tBu
tBu
tBu
tBu
tBu
3
CH CN, Ar, r.t.
C
1
a
2
3
MeO
MeO
MeO
O
O
O
OH
OH
OH
tBu
OH
MeS
tBu
tBu
tBu
tBu
O
tBu
tBu
tBu
OMe
e, 47% (48 h)
3
3f, 43% (48 h)
3g, 83% (24 h)
3h, 38% (48 h)
OH
OH
OH
OH
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
O
O
O
O
O
Br
O
O
O
O
3
z, 80% (24 h)
3aa, 79% (24 h)
3ab, 55% (24 h)
3ac, 78% (24 h)
F
Cl
Cl
3
i, 69% (24 h)
3j, 59% (24 h)
3k, 61% (24 h)
OH
3l, 64% (24 h)
OH
tBu
OH
tBu
OH
OH
tBu
tBu
tBu
tBu
N
tBu
tBu
N
OH
OH
OH
tBu
tBu
O
tBu
tBu
tBu
tBu
tBu
tBu
O
+
O
O
O
O
Br
O
O
O
O
Br
I
O2N
3m, 63% (60 h)
3n, 62% (24 h)
3o, 80% (24 h)
3p, 66% (24 h)
3ad, 90% (24 h)
3ae, 44% (24 h)
3af, 23% (24 h)
3ag, 55% (24 h)
OH
OH
OH
tBu
tBu
tBu
tBu
tBu
tBu
OH
OH
OH
tBu
OH
tBu
tBu
tBu
N
tBu
tBu
S
tBu
tBu
O
NC
O
O
F3C
NC
3q, 67% (24 h)
3r, 82% (24 h)
CCDC 2025995
3s, 65% (24 h)
O
O
OH
OH
OH
OH
tBu
tBu
tBu
tBu
tBu
tBu
iPr
iPr
3ah, 72% (24 h)
3ai, 70% (24 h)
3aj, 36% (60 h)
3ak, 73% (24 h)
O
O
[a] Reaction conditions: under argon atmosphere, 4-benzylidene-2,6-di-tert-
butylcyclohexa-2,5-dien-1-one (1a) (0.2 mmol, 1.0 equiv), UO (NO ·6H O (5 mg, 5
mol%), R-H (1.0 mol, 5.0 equiv), CH CN (2.0 mL), room temperature (~25 C), time
S
O
O
2
3
)
2
2
N
N
o
N
O
O
3
3t, 70% (24 h)
3u, 66% (24 h)
3v, 34% (24 h)
3w, 28% (24 h)
(24~60 h) in a sealed tube. [b] Isolated yield.
OH
OH
Me
Then, apart from THF, we diverted our attention to other
oxygenated radical precursors, including pyran, ether, diglyme,
dioxane, 1,3-dioxolane and benzo[d][1,3]dioxole. Notably, the
Me
O
O
3
functionalization of C(sp )-H bonds in these oxygenated substrate
N
O
occurred smoothly in moderate to excellent yields ranging 44-90% via
the visible-light-driven generation of α-oxy radicals as well as α,α-
dioxy radical (compounds 3z-3ae). Dimethylformamide was chosen as
H-donor to survey the reactive situation, whereas C-H activation was
not regioselective, and produced the acylated and alkylated adducts 3af
and 3ag with 23% and 55% yields, respectively. Interestingly, the
protocol starting from N,N-dimethylacetamide (DMA) could be easily
adapted to perform amidoalkyl rather than acylalkyl radical addition
onto 2a, which afforded the adduct 3ah in 72% yield. And
tetrahydrothiophene was functionalized toward the sulfuralkyl radical
site with high regioselectivity to afford the desired product 3ai in 70%
yield. However, the functionalization of unactivated cyclalkane, such
as hexane, occurred sluggishly with adduct 3aj in only 36% yield even
with longer reaction time. Importantly, adduct 3ak was obtained in
3
x, 0% (24 h)
3y, 0% (24 h)
[
2
(
a] Reaction conditions: under argon atmosphere, 2,6-di-alkyl-4-(arylidene)cyclohexa-
,5-dien-1-one (1) (0.2 mmol, 1.0 equiv), UO (NO ·6H O (5 mg, 5 mol%), THF
2a) (1.0 mol, 5.0 equiv), CH CN (2.0 mL), room temperature (~25 C), time (24~60
2
3
)
2
2
o
3
h) in a sealed tube. [b] Isolated yield.
groups in the substrates, including C-F bond (compound 3i), C-Cl
bonds (compounds 3i and 3k), C-Br bonds (compounds 3l, 3m and 3n),
C-I bond (compound 3o), C-NO
2 3
bond (compound 3p), C-CF bond
(
compound 3q) and C-CN bonds (compounds 3r and 3s). The structure
of 3r was further confirmed by X-ray single crystal diffraction.
Halogen moieties, as the classical cross-coupling reagent for further
structural transformation, were compatible with the reaction system
along with the target products in satisfactory yields (3i-3o). In the
meanwhile, heterocycles such as thiophene- and indole-derivative
compounds 3t and 3u were also successfully obtained in decent yields
73% yield with a complete regioselectivity via acyl radical procedure.
According to this, we firmly believe that the present uranyl-catalyzed
reactive pattern will be applicable extensively in the synthesis of
(
70% and 66%). To our delight, multi-substituted indolin-2-one
derivatives 3v and 3w with the carbon quaternary center were
3
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Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
pharmaceutically active molecules and/or organic functional materials
containing diarylmethane moieties.
markedly formed and trapped by TEMPO in situ and detected by ESI-
MS analysis. This observation severs as a compelling proof for the
intermediate of THF radical be involved in this transformation process
(Figure S8). Moreover, To further illustrate the C-H bond cleavage of
THF in HAT process, the control deuterium isotopic labeling
experiments were performed to verify the rate-determining step. The
To demonstrate the synthetic practicability, a scale-up synthesis of
3
1
3
a was investigated as the example. As shown in Scheme 1, reaction of
a (3.74 mmol, 1.1 g) and THF under the standard conditions provided
a in 77% yield (1.05 g). Therefore, this method presents the effective
applicability for the gram-scale synthesis of desired diarylmethane
derivatives.
reaction was carried out in the present of 1:1 ratio of THF and D
as the partner, the experimental results showed that 3a and [D
were obtained with a significant kinetic isotope effect (KIE) value of
.0 ± 0.3, revealing that the C-H bond cleavage occurred in the rate-
8
-THF
7
]-3a
3
determining step (Scheme 2b, see Figure S9 for details).
Scheme 1. Scale synthesis of 2,6-di-tert-butyl-4-(phenyl-tetrahydrofuran-2-
yl)methyl)phenol.
Figure 3. Profile of the formation of 1a with the light turned on or off at regular
1
intervals. Yields were determined by H NMR spectroscopy with CH
2 2
Br as an
internal standard.
Scheme 2. (a) Radical trapping experiment. (b) Mechanistic studies on the kinetic
isotope effect.
Next, an on-off visible light irradiation experiment at regular
intervals on the profile of the uranyl-catalyzed reaction yield was
performed. The results implied that the continuous visible light
irradiation was crucial for the transformation (Figure 3). In addition,
the apparent quantum yield of the reaction was calculated to be 8.8%.
It suggested that the radical program present a closed photocatalytic
process without involvement of radical chain propagation (see details
in the Supporting Information).
To elucidate the possible mechanism of visible-light-mediated
uranyl-catalyzed alkylated reaction, Stern-Volmer fluorescence
quenching experiments were performed (see Figures S2-S7 in the
Supporting Information for the details). At first, we investigated the
emission and excitation spectra of the photocatalyst UO
2 3 2 2
(NO ) ·6H O.
A solution of UO (NO ·6H O (1.0 mM) in CH CN was chosen as the
2
3
)
2
2
3
model. The fluorescence excitation spectrum was obtained with the
detection wavelength of 480 nm (Figure S2), and the fluorescence
emission spectrum was excited at 420 nm (excitation maximum of
UO
nm, 505 nm, 528 nm and 554 nm finger peaks of fluorescence emission
spectrum of UO (NO ·6H O was observed when it was excited at 420
2 3 2 2
(NO ) ·6H O) (Figure S3). As shown in Figure S2a, 467 nm, 484
2
3
)
2
2
nm, and the fluorescence intensity dramatically decrease with the
addition of THF. Moreover, an energy transfer process between
UO
experiments (see Figures S4-S5 for the details). Meanwhile,
fluorescence emission spectra of UO (NO ·6H O with different
2 3 2 2
(NO ) ·6H O and THF was validated by fluorescence quenching
2
)
3 2
2
concentration of 4-benzylidene-2,6-di-tert-butylcyclohexa-2,5-dien-1-
one (1a) excited at 420 nm was carried out (Figure S2b). The outcome
turns out that the excited state of uranyl nitrate could be quenched by
…
the generation of the collision chelate complex through the C=O U
[
23]
coordination interaction (see Figures S6-S7 for details).
In order to survey the reaction mechanism for the visible-light-
enable uranyl-catalyzed alkylation process, a control experiment with
the treament of 4-benzylidene-2,6-di-tert-butylcyclohexa-2,5-dien-1-
one (1a) and THF (2a) in the presence of radical scavenger TEMPO (5
equiv) was conducted (Scheme 2a). The formation of 3a was
completely suppressed, and the radical adduct of THF-TEMPO was
Figure 4. Experimental evidence for U(V)-oxo radical character in the U(V)-oxo
moiety. The EPR spectra of UO (NO ) ·6H O dissolved in acetonitrile are recorded
2 3 2 2
under the blue light at 2K and 40K respectively. The signals are magnified at the
narrower range of the magnetic field in the inset.
4
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Chemistry - A European Journal
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RESEARCH ARTICLE
In the origins of catalysis, the U(VI)-oxo valence tautomer is
suggested as an important contributor to the excited state of U(V)-oxo
solution of UO
2
(NO
3
)
2
·6H
2
O (cat; 5 mol%), THF (2a, 50 mM) and 1a
(10 mM) were reacted under the in-situ irradiation. The g-value (g =
2.0028) and hyperfine splitting constants (A = 0.4103 mT, A = 0.1606
mT) of the reaction intermediate were derived by simulation. The
smaller constant A may be originated from four hydrogen nuclei
poccessing identical hyperfine splitting constants.
The lager hyperfine splitting constant A may be originated from
the hydrogen nucleus from the methyne at the ‘p’ position of the
phenol ring. From the DFT calculation result (see below), the π
molecular orbitals are found to contain the 1.09% contribution from the
atom orbitals for hydrogen of methyne but the contribution no more
than 0.12% from other H atoms, indicating the overlap between the C-
H bond of methyne and the π molecular orbitals and the delocalization
of π-spin on this hydrogen atom. Furthermore, according to the formula
in supplementary information and the spin density (21.77%) of C1
from DFT calculation, the dihedral angle between the C-H bond of
[
14a, 23]
radical intermediates under visible light irradiation.
However,
1
2
experimental evidence for validating the proposals of such U(V)-oxo
[
17]
radical species has never been reported. Thus, we sought to trap the
reactive intermediate U(V)-oxo by EPR (Electron Paramagnetic
Resonance) technique assisted by superfluid liquid helium at 2K. The
EPR spectrum at 2K shows the characteristic signal of U(V) ion, which
2
1
is produced from the excitation of UO
2
(NO
3
)2·6H
2
O by the irradiation
of blue light (Figure 4). The g-values (g
⊥
= 0.570 and g = 2.085)
Ⅱ
indicate the axial symmetry for the coordination geometry of U(V) ion.
On increasing the temperature to 40 K, the EPR signals are reduced
except the one at g = 2.0038 which is growing independently and is not
ascribed to the U(V) ion as a consequence. It is known that the 5f
orbitals are more extended than 4f ones, leading to the covalent U-O
2
+
2
bonds and the linear type of the form O-U-O for uranyl ion (UO ) ,
[
24]
1
[28]
which has no unpaired electron and is therefore EPR silent. 5f U(V)
center with an oxygen radical species was formed via homolysis of
U=O bonds under the irradiation of blue light. Taking the nitrate
ligands coordinated in the equatorial plane into consideration, the
symmetry of the crystal field about the O-U-O axis may be changed
methyne and the normal to the phenol ring is estimated as 71°.
Contrary to the C-H bond of methyne, the double of C-H bonds on ‘m’
position are almost parallel to the phenol ring, leading to a relatively
small hyperfine splitting constant (A
polarization of the C2 atom. The other two hydrogen atoms with the
constants of A may be the H atoms of furan ring and of the phenyl
2
) originated from the spin
from C
∞
for uranyl ion to threefold or less for this compound in
2
solution. In addition, there is possible structure deviation from linear
O-U-O, caused by the reduction reaction under light. Both may mix the
substituted to methyne. Pratically, the relatively large hyperfine
splitting constant for the hydrogen of methyne is also found for
galvinoxyl radical, an analogue to the phenoxyl radical in this work.
The π-spin character and the two identical H atoms at ‘m’ position of
phenol ring illustrate the resonance form II (see below, Scheme 3) is
the largest contributor among the three possible resonance forms. In
2
2
[28]
first excited states F7/2 to the ground doublet F5/2, activating the EPR
[
24-25]
lines.
For the U(V) species published in literatures, the g-value are
2+
1
found between 0.7 and 2.1, while for neptunyl ion ((NpO
g-value are g
2
) , 5f ) the
The above EPR results
[
26]
⊥
= 0.205 and g
Ⅱ
= 3.405.
confirmed that the generation of U(V)-oxo species from U(VI)-oxo
precursor is considered as the initial step of photo-induced reaction.
other work, the crystal structure of the 2,4,6-tri-tert-butylphenoxyl
t
radical ( Bu
3
ArO·) proves to be the form with the radical on the carbon
[
29]
atom linking to the p-tert-butyl.
2 3 2 2
Figure 5. The X-band EPR spectra of a mixture of UO (NO ) ·6H O (cat; 5 mol%),
THF (2a, 50 mM) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO; 50 mM) before and
after the irradiation of UV-Vis light. The blue line at bottom represents the EPR
2 3 2 2
spectrum of the phenoxyl radical produced in the reaction of UO (NO ) ·6H O (cat; 5
mol %), THF (2a, 50 mM) and 1a (10 mM) under the irradiation. The g-value and
hyperfine splitting constants from the adduct of the THF radical and DMPO and the
[
27]
phenoxyl radical are obtained by simulation with Easyspin.
Meanwhile, we investigated types of the radicals produced during
different conditions by electron paramagnetic resonance (EPR)
technique (Figure 5). UO
mM) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO, ca. 100 mM after
mixing) as the radical trapper were mixed in solvent CH CN and
transferred to capillaries. Then the EPR spectra of the mixture were
recorded under different conditions. No radical signal was observed
without irradiation of light; in contrast, a sextet signal with a g value of
2 3 2 2
(NO ) ·6H O (cat; 5 mol %), THF (2a, 50
2 3 2 2
Figure 6. The orbital levels for UO (NO ) ·6H O calculated at PBE0/6-31G(d,p)/SDD
level in combination with continuum solvation model IEFPCM. The unit of orbital
energy level gaps is kcal/mol.
3
2.006 and hyperfine splitting constants (A
N
= 1.445 mT and A
H
= 1.963
To explore mechanism of the visible light-mediated uranyl-
mT) was obtained under irradiation with UV-Vis light, indicating a
product of carbon-centered radical which is probably ascribed to the
adduct of DMPO by THF 2a (Figure 5). Considering that the phenoxyl
radical as the possible intermediate is stable enough for the observation
catalyzed alkylated reaction,
exhaustive detail about the reaction intermediates and mechanisms.
Refer to the photophysics process studied by Chen , the uranyl of
triplet is responsible for the abstraction of hydrogen atom in THF. It
a DFT calculation could provide
[
23]
3
of EPR signal, the EPR spectra were recorded when the CH
3
CN
has the electronic configuration (σ
u
fδ1). The DFT calculations
5
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Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
3
3
3
2+
reproduce the energic order (σ
u
f
δ1) < (σ
u
f
δ2) < (σ
u
fφ1) of triplet uranyl
excited state UO
2
*
under irradiation of visible light. But
[
23]
calculated by CASPT2 . This indicates the reliability of our DFT
calculations. The nearly equal two axial U-O bond with the length
difference by 0.001Å in triplet LMCT uranyl indicates the averaged
arrangement of spin densities on the two oxo ligand. Indeed, the spin
densities of two oxo ligand are 0.205 and 0.209. THF encounters the
uranyl and then forms the complex by hydrogen bound between the
proton in one of equatorial water molecules and oxygen atom in THF.
Figure 6 shows the molecular orbital of the uranyl of singlet state. In
the HAT transition state, the Mulliken spin density of carbon in THF is
accompanying generates active oxygen radical U(V) species with high
oxidative potential through ligand to metal charge transfer (LMCT).
Next, a carbon-centered radical I was generated by activated radical
U(V) species through a HAT process, and radical U(V) species abstract
2+
a H atom to produce U(V)O(OH) . In the alkylation process,
nucleophilic addition of radical to 4-benzylidene-2,6-di-tert-
I
butylcyclohexa-2,5-dien-1-one 1a provide a transient radical adduct II.
Since the largest resonance contribution, II could be transformed into
phenoxy radical species III, which is capable of oxidizing the reduced
2+
0.069, the C-H bond length is 1.121Å slightly longer than its usually
form of U(V)O(OH) to deliver anion IV while regenerating the
2
+
state, which reflects this transition state resembles the reactant complex. photocatalyst UO
2
. Deuterium labeling experiments indicated that
However, the arrangement of the spin densities on the oxo ligands are
not averaged, the oxo abstracting hydrogen atom in THF is centralized
more spin density than the other oxo by 0.270. The potential energy
curve for the reaction is shown in Figure 7, HAT process is accessible
phenoxyl anion IV would most likely abstract a free hydrogen proton
from the trace water in the reaction system. Finally, protonation of
intermediate IV affords the target molecule 3a. The process of path a
exhibited a low free energy barrier based on the DFT calculation
at room temperature due to the low free energy barrier of 3.10 kcal/mol. (Figure 7). Meanwhile, radical III and another THF molecule might
undergo a short cut to deliver the desired alkylation product 3a, while
the result based on the DFT calculation showed a high free energy
barrier (26.5 kcal mol-1) (path b). Therefore, we were able to safely
exclude the possibility of path b at current stage (Figure 7).
Scheme 3. A possible mechanism for the uranyl-catalyzed alkylation of 4-arylidene-
cyclohexa-2,5-dien-1-one derivatives under visible light irradiation.
Conclusion
In summary, we have developed an efficient and reliable visible
light-mediated uranyl-catalyzed C-H alkylation reaction through HAT
process at room temperature. A variety of diarylmethane derivatives
with important potential pharmaceutical value were obtained in
reasonable yields by using our method. The transformation
accommodates a broad substrate scope, available raw feedstocks,
operational simplicity, green protocol and gratified scale-up synthesis.
Mechanistic studies and DFT calculations reveal that the HAT process
is triggered by virtue of the high oxidative nature of the oxygen of
uranyl species upon light excitation, generating a radical intermediate,
Figure 7. The potential energy profile of HAT process of THF by LMCT uranyl
species and phenoxy radical species III.
which undergoes addition/isomerization/hydrogen migration process in
a synchronously concerted manner. Furthermore, we believe that
available photocatalyst uranyl cation induced radical reaction via HAT
According to the control experiments above and previous
process represents a forceful and advanced strategy and will find wide
applications for C-H activation. Other challenging organic
[
17]
references,
a possible mechanism for the visible-light-mediated
2
+
uranyl-catalyzed alkylated pathway is proposed in Scheme 3. At first,
transformation catalyzed by excellent candidate UO
under investigated in our lab.
2
, are currently
2+
photosensitizer, uranyl cation (UO
2
), is transformed into its triplet
6
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Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
1
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2
019, 21, 4480-4485.
reagent (1.0 mmol, 5.0 equiv.), UO
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2 3 2 2 3
(NO ) ·6H O (5.0 mol %), CH CN
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Chemistry - A European Journal
10.1002/chem.202003431
RESEARCH ARTICLE
Entry for the Table of Contents
O
OH
R
R
R
R
UO2(NO3)2 6H2O (5 mol%)
+
C
H
5
W blue LED
CH3CN, Ar, r.t.
Ar
R'
R'
C
Ar
novel uranyl photocatalyst
mild reaction condition and atomic economy 28-90% yields
moderate to high yields
35 examples
broad substrate scope and functional tolerance
scale reaction without yield loss
An efficient and convenient C-H activation strategy through direct HAT process for synthesis of highly value-added diarylmethane derivatives
from p-QMs was realized by combining a uranyl photocatalyst with visible light irradiation. EPR experiments and DFT calculations revealed that
·
the formation of U(V)-O species by reduction of U(VI)=O under visible light is responsible for the initial step of photo-induced reaction.
9
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