LiBr-promoted photoredox neutral Minisci hydroxyalkylations of quinolines with aldehydes

Article abstract of DOI:10.1039/d0gc01872d

Photoredox-neutral hydroxyalkylations of quinolines with aldehydes, induced by sustainable visible light under mild conditions, are described. Non-toxic and inexpensive LiBr is found to be the key for the success of the atom-economical Minisci method. Combined with a highly oxidative photocatalyst and visible light irradiation, the bromide additive mediates the H abstraction/acyl radical formation directly from aldehydes. The present mild photoredox neutral protocol provides an important alternative, especially for the challenging Minisci hydroalkylations, as well as a promising approach for atom-economical Minisci reactions with broader N-heterocycle spectra.

Full text of DOI:10.1039/d0gc01872d

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DOI: 10.1039/D0GC01872D
Green Chemistry
COMMUNICATION
LiBr-promoted photoredox neutral Minisci hydroxyalkylations
of quinolines with aldehydes
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaochen Ji,* Qiong Liu, Zhongzhen Wang, Pu Wang,* Guo-Jun Deng and Huawen Huang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Photoredox-neutral hydroxyalkylations of quinolines with
aldehydes, induced by sustainable visible light under mild
conditions, is described. Non-toxic and inexpensive LiBr is found
to be the key for the success of the atom-economic Minisci
method. Combined with a highly oxidative photocatalyst and
visible light irradiation, the bromide additive mediates the H
abstraction/acyl radical formation directly from aldehydes. The
present mild photoredox neutral protocol provides an important
compounds II and III (Fig. 1), for example, found to feature
high antibacterial activities (EC50 = 11.1 and 9.6 μM,
respectively).5
Because of the high pharmaceutical importance of quinoline
derivatives, the synthetic chemistry of this structural motif has
accordingly attracted considerable attention. While transition-
metal-catalyzed cross couplings provide powerful approaches
for functionalized heterocycles, to date the access to
hydroxyalkylated quinoline derivatives mainly relies on
stepwise Grignard chemistry,6 where formally heteroaryl
magnesium halides add to corresponding carbonyls. Minisci
reaction refers generally to radical-type C-C couplings of aza-
heteroaromatics that give broad and versatile opportunities for
heterocycle functionalizations including alkylation, arylation,
and acylation.7 Among numerous viable Minisci reaction
systems, however, few of them are suitable for
hydroxyalkylations, probably because of the variability of
hydroxyalkyl radicals and over-oxidation of the otherwise
resultant hydroxyalkylated products under oxidative
conditions.8 In 2011, Li group demonstrated a PdCl2/BINAP-
alternative
especially
for
the
challenging
Minisci
hydroalkylations, as well as a promising approach for atom-
economic Minisci reactions with broader N-heterocycle spectra.
Nitrogen-containing heterocycles, such as quinoline and its
analogs, are of prominence as key structural units of numerous
bioactive natural compounds and pharmaceuticals. Many FDA
approved drugs contain this privileged quinoline ring structure.1
Of them, hydroxyalkylated quinoline derivatives display a
unique ability to selectively disrupt biofilm formation,2 which
is growingly being recognized as a major component of
bacterial pathogenesis.3 Given the efficacy of initial lead
compounds such as quinine (I), the scientific community
synthesized and tested a vast array of quinoline derivatives
possessing a β-amino alcohol motif.4 Representatively, the 2-
aryl quinoline
based
catalytic
oxidative
system
that
enables
hydroxyalkylations of quinolines and analog aza-
heteroaromatics with alcohols (Fig. 2a).9 The potent photoredox
catalysis10 has proved in recent years to be an important
alternative for Minisci reactions,11 of which hydroxyalkylations
was realized with methanol under mild conditions (Fig. 2b).12
Instead of hydroxyalkyl radical formation,13 recent results in
photocatalysis reveal that acyl radicals attacking may lead to
non-acylated products,14 which poses a possible access to
hydroxyalkylations. Very recently, Melchiorre and co-workers
HN
HN
HO
HO
HO
N
H
HCl
MeO
N
N
N
CF3
III
( )
I
II
NSC 13480 ( )
Quinine ( )
discovered
the
first
photoredox
neutral
Minisci
Fig. 1 Highly valuable hydroxyalkylated quinoline derivatives.
hydroxyalkylations through basically visible-light-induced acyl
radical formation from 4-acyl-1,4-dihydropyridines (acyl-DHPs)
(Fig. 2c).15 With the insight obtained from our recent
experimental results in the reductive Minisci alkylations with
aldehydes wherein acyl radicals were directly generated
through deprotonated electron transfer (DPET) with bromo
radical,16 we speculate that hydroxyalkylations of
heteroaromatics would be selectively accomplished from
Key Laboratory for Green Organic Synthesis and Application of Hunan Province,
Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry
of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China.
Fax: (+86)0731-5829-2251; Tel: (+86)0731-5829-8601; E-mail:xcji@xtu.edu.cn;
90wangpu@xtu.edu.cn; hwhuang@xtu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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COMMUNICATION
Journal Name
4CzIPN (0.5 mol%)
LiBr (0.5 equiv)
TfOH (1.0 equiv)
HO
Ph
readily available aldehyde feedstocks in a redox-neutral manner
(Fig. 2d).
Ph
View Article Online
O
DOI: 10.1039/D0GC01872D
+
+
Ph
H
PhCl/H2O, 60 oC, 48 h
N2, 36 W blue LED
N
Ph
N
Ph
N
Ph
(a) Thermal Minisci hydroxyalkylation
H
"standard conditions"
1a
2a
3a
3a'
(2.0 equiv)
(0.2 mmol)
PdCl2, rac-BINAP
Het
OH
oxidant, 120 o
C
OH
OH
Yieldb
R
Entry
Deviation from standard conditions
R
Het
R
- H+, - e-
- H+, - e-
3a
3a’
6
50
(b) Photoredox oxidative Minisci hydroxyalkylation
1
2
None
77 (72)
34
H
With additional (PhO)2PO2H (1.0
equiv)
Ir[dF(CF3)ppy]2(dtbbpy)PF6 instead
of 4CzIPN
NaBr instead of LiBr
Bu4NBr instead of LiBr
LiCl instead of LiBr
NaI or KI instead of LiBr
Using 1.5 equiv of 2a
At 25 oC, 50 oC
Blue LEDs
PC,
OH
Het
oxidant
OH
MeOH
Het
3
66
8
- H+, - e-
- H+, - e-
(c) Photoredox neutral Minisci hydroxyalkylation with acyl-DHPs
4
5
6
7
8
32
52
22
n.d
50
<5
24
11
n.d
5
O
R
H
OH
Het
EtO2C
Me
CO2Et
Me
Blue LEDs
O
R
Het
- PyH+, - e-
R
+ H+, + e-
N
H
(d) Photoredox neutral Minisci hydroxyalkylation with aldehydes (this work)
9
10
42, 62
n.d
<5
n.d
In dark or no PC or no LiBr or no
acid
No water
N
H
Blue LEDs
NC
CN
N
Het
4CzIPN, acid
LiBr
OH
11
16
<5
O
a
R
CHO
N
R
Reaction condition: 1a (0.2 mmol), 2a (0.4 mmol),
R
+ H+, + e-
- H+, - e-
Het
N
photocatalyst (PC, 0.5 mol%), halide additive (0.5 equiv), acid
(1.0 equiv), water (50 equiv), PhCl (1 mL), N2, 36 W blue LED,
4CzIPN
b
48 h. [Ir]PF6
=
Ir[dF(CF3)ppy]2(dtbbpy)PF6. Yields were
determined by crude 1H NMR with CH2Br2 as internal standard
Fig. 2 Minisci hydroxyalkylations
and isolated yield was given in parentheses.
To this end, we began our study with choosing 2-
phenylquinoline (1a) and benzaldehyde (2a) as the model
substrates and LiBr as the key moderator. As expected, the
main challenge was to inhibit the over-reacted reductive
benzylation reaction. Systematic screening of reaction
parameters afforded the optimal result that the desired 3a was
obtained in good yield (77%) and satisfactory chemoselectivity
(~13:1) (Table 1, entry 1). Similarly to our previous results,
additional (PhO)2PO2H (1.0 equiv) benefited the benzylation
reaction (50%),16b together with the hydroxyalkylated quinoline
product 3a generated in a considerable yield (34%) (entry 2).
4CzIPN proved to be the best choice of photocatalysts (PCs),
and among others only Ir[dF(CF3)ppy]2(dtbbpy)PF6, possessing
similar photoredox potential, could be an option of PCs (entry
3). Moreover, LiBr was superior to other bromide or halide
additives (entries 4-7). Notably, reducing the equivalents of 2a
(1.5 equiv) did not improve the selectivity, but instead both
alkylation products were formed in diminished yields with the
quinoline reactant 1a incompletely consumed (entry 8). A
similar result was observed when the reaction was carried out at
room temperature (entry 9). Control experiments revealed that
visible light irradiation, photocatalyst, bromide, and acid were
all necessary (entry 10). The addition of water, which increased
the solubilities of bromide additive and protonated quinolines,
dramatically enhanced the efficiency of the target alkylation,
while only 16% yield was obtained when removing the
additional water (entry 11).
With the optimized reaction conditions in hand, we next
probed the substrate scope and generality of the LiBr-promoted
photoredox neutral Minisci hydroxyalkylations. A broad range
of substituted benzaldehydes were smoothly employed to react
with 2-phenylquinoline 1a (Fig. 3), generating the
corresponding C4 functionalized quinoline derivatives in
moderate to excellent yields (3a-3p, 49-93%). The present
photocatalytic system found to well tolerate such ubiquitous
functionalities as alkyl, alkoxy, and halogen (F, Cl, Br).
Notably, in this protocol, both electronic and steric effects of
the benzaldehyde substrates were elusive for the final
productivity. In this regard, for example, the best efficiency was
arisen from the coupling with 2-bromo-5-fluorobenzaldehyde
(3p, 93%), while some other ortho-substituted benzaldehydes
gave the desired products only in modest yields. Among
heteroaromatic aldehydes tested, thiophene-2-carbaldehyde was
productive, giving the hydroxyalkylated product bearing a
thiophene moiety in good yield (3q, 73%). Other aromatic
aldehydes including 2-naphthaldehyde worked, albeit in low
yield (3r, 73%). Unfortunately, aliphatic aldehydes could not
participate into this kind of Minisci reaction, in which major
quinoline substrate was recovered. For example, only trace
amount of product was detected by TLC and GC-MS when
cyclopropanecarbaldehyde was used (3s), while no alkylation
product by ring opening of cyclopropane was observed.
Subsequently, a number of quinoline derivatives were
subjected to our LiBr-promoted photoredox system (Fig. 4).
The experimental results revealed that the reaction efficiency is
Table 1. Optimization of reaction conditionsa
2 | J. Name., 2012, 00, 1-3
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Green Chemistry
Journal Name
COMMUNICATION
4CzIPN (0.5 mol%)
LiBr (0.5 equiv)
TfOH (1.0 equiv)
highly dependent upon the substituent attached on the quinoline
unit. Unexpectedly, 2-methylquinoline featured modest
reactivity with poor chemoselectivity. By contrast, 2-
arylquinolines worked smoothly to afford the target molecules
in moderate to excellent yields (4a-4h, 51-89%), as well as that
containing a thiophene moiety (4i). For a purpose of quinoline
C2 functionalization, C4-occupied heterocycles were employed.
In analogy to 2-methylquinoline, 4-methylquinoline gave also a
poor selectivity in current system. To our delight, the 4-butyl
and 4-chloro analogs were suitable substrates, delivering the
expected hydroxyalkylation products 4k and 4l in 76% and
58% yields, respectively. Finally, while simple isoquinoline
gave only trace amounts of target product, its bromo derivative
at C6 can be incorporated into the present photocatalysis (4n,
47%). Other related N-heterocycles such as quinoxalinone,
benzo[d]thiazole, and benzo[d]oxazole failed to work in the
present systems.
Notably, benzylation products by reduction were detected as
major side products by GC-MS in all cases of these reactions,
albeit with generally low yields (<20%). Some benzylation
products were formed in considerable yields that were then
isolated and characterized (3b’, 3c’, 3r’ and 4l’, See ESI for
details). Because of the same reason, the reaction yields did not
enhanced when increasing the catalyst loading or performed
within prolonged reaction time. Actually, in some cases, the use
of more catalyst diminished the efficiency of the target
transformation. We also tried to further screen the reaction
conditions when aliphatic aldehydes were used. But we failed
to obtain satisfactory results.
HO
View Article Online
O
Ph
DOI: 10.1039/D0GC01872D
H
+
PhCl/H2O, 60 oC, 48 h
N2, 36 W blue LED
N
N
2a
4
1
HO
Ph
HO
Ph
HO
Ph
Cl
Br
N
N
N
4g
R
, 69%
4h, 54%
4a
4b
4c
4d
4e
, R = Me, 51%
, R = F, 59%
, R = Cl, 54%
, R = Br, 73%
, R = CF3, 89%
, R = OCF3, 84%
HO
Ph
HO
Ph
Me
S
N
N
Ph
4f
4i
, 53%
4j
, 52%
Br
n-Bu
Cl
N
Cl
MeO
MeO
Ph
N
Ph
OH
Ph
OH
Ph
N
N
OH
OH
a
4k
, 76%
4l
, 58% (10%)
4m
, 47%
4n
, 47%
a
Fig. 4 Scope of quinolines. Yields of benzylation products
were given in parentheses.
To gain more mechanistic insight into the hydroalkylation
reaction with aldehydes, we carried out some control
experiments (Fig. 5). First, when we used deuterated
benzaldehyde (D-2a) instead of 2a, no D was incorporated into
the final product 3a (Fig. 5a), while the additional deuterium
oxide led to 76% D incorporation (Fig. 5b). These results rule
out the ketyl radical pathway and instead direct C-H scission of
aldehyde would occur to form acyl radical. The radical trapping
experiment further proved our speculation, in which benzoyl
radical was captured by TEMPO and the generation of 3a was
completely prohibited (Fig. 5c). The Stern-Volmer quenching
experiments of reactants and reagents to 4CzIPN were in line
with those results to Ir[dF(CF3)ppy]2(dtbbpy)PF6.16b Hence,
only the bromide featured obvious luminescence quenching
effect to photocatalyst (Fig. 6), revealing the feasibility of the
bromide oxidation in the initial step. Finally, we carried out a
switched light-on/off experiment. The yield did not increase in
dark, which therefore rule out a radical chain pathway.
4CzIPN (0.5 mol%)
LiBr (0.5 equiv)
TfOH (1 equiv)
R
O
HO
H
R
+
PhCl/H2O, 60 oC, 48 h
N2, 36 W blue LED
N
Ph
N
Ph
1a
2
3
3a
, R = H, 72%
a
R
3b
3c
3d
3e
3f
, R = CH3, 49% (15%)
, R = F, 50% (18%)
, R = Cl, 78%
, R = Br, 61%
, R = OCF3, 62%
HO
HO
R
a
HO
R
N
Ph
N
Ph
3i
3j
3k
, R = CH3, 69%
, R = Cl, 55%
, R = CH3, 68%
3g
3h
, R = OMe, 68%
, R = Ph, 68%
N
Ph
3l
, R = F, 61%
3m
, R = Br, 57%
F
Br
Cl
S
HO
HO
HO
HO
F
Cl
Ph
Cl
N
Ph
N
N
Ph
N
Ph
3n
, 69%
3o
, 89%
3p
, 93%
3q
, 73%
HO
HO
N
Ph
N
Ph
a
3r
3s
, trace
, 34% (20%)
a
Fig. 3 Scope of aldehydes. Yields of benzylation products
were given in parentheses.
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3
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COMMUNICATION
Journal Name
H/D exchange:
hv
View Article Online
R1
no D
Ph
H
DOI: 10.1039/D0GC01872D
4CzIPN (0.5 mol%)
LiBr (0.5 equiv)
TfOH (1.0 equiv)
HO
R
H+
2
N
PC
*PC
SET
O
Br-
H+
SET
H
(a)
RCHO
OH
+
F
PhCl/H2O, 60 oC, 48 h
N2, 36 W blue LED
Ph
D
N
Ph
DPET
N
Ph
O
D-2a
1a
3a
(56%)
Br
A
R1
R
R
-
N
B
PC
Ph
H/D
76% D
H
4CzIPN (0.5 mol%)
LiBr (0.5 equiv)
TfOH (1.0 equiv)
HO
OH
E
+
PC
= 4CzIPN)
(
1
[ + H]
O
(b)
+
PhCl/D2O, 60 oC, 48 h
N2, 36 W blue LED
Ph
2a
H
N
Ph
N
Ph
R1
R1
1a
3a + D-3a
(60%)
R
R
SCS
N
N
H+
H
H
H+
O
O
Radical trapping:
C
D
4CzIPN (0.5 mol%)
LiBr (0.5 equiv)
TfOH (1.0 equiv)
Fig. 7 Possible reaction mechanism.
O
N
O
(c)
+
PhCl/H2O, 60 oC, 48 h
N2, 36 W blue LED
TEMPO (2.0 equiv)
Ph
O
Ph
2a
H
N
Ph
The synthetic application of this protocol was further
demonstrated by the versatile late-stage manipulation of the
free hydroxyl group in the resultant products. Hence,
dehydrogenative oxidation, bromination, and esterification
were all readily realized via simple treatment under mild
1a
detected by GC-MS
3a
(
, 0%)
Fig. 5 Key findings of mechanistic studies.
conditions, delivering
a
broader access to diversely
functionalized quinolines (Fig. 8).
HO
Ph
oxidation
N
Ph
esterification
(c)
(a)
3a
AcO
Ph
O
Ph
(b)
Br
bromination
Fig. 6 Stern-Volmer quenching of LiBr to 4CzIPN.
N
Ph
N
Ph
Ph
5c
, 77%
5a
, 84%
Based on the experimental results and previous reports, a
plausible reaction mechanism was proposed (Fig. 7). The
photocatalyst 4CzIPN absorbs photons to form its exited state
(*PC, E1/2(*PC/PC-) = +1.35 V vs SCE),17 which undergoes a
N
Ph
5b
, 53%
red
Fig. 8 Derivation of 3a. Reaction conditions: (a) 3a (0.2 mmol),
Dess-Martin reagent (1.5 equiv), CH2Cl2 (0.2 M), 0 C-rt, 4 h;
(b) 3a (0.2 mmol), PPh3 (1.2 equiv), CBr4 (1.5 equiv), CH2Cl2
(0.2 M), 0 C-rt, 4 h; (c) 3a (0.36 mmol), Ac2O (2.0 equiv),
DMAP (5 mol%), toluene (0.36 M), 0 oC-rt, 4 h.
single electron transfer (SET) process with bromo anion (E1/2
o
= +0.80 V vs SCE)18 to generate bromo radical A and 4CzIPN
anion (PC-). The bromo radical would abstract H atom from
aldehyde 2 via deprotonated electron transfer (DPET) (a
o
comparable HBr Br–H bond dissociation energy
= 87
kcal/mol,19 benzaldehyde C(O)-H bond dissociation energy =
89 kcal/mol20) to access the acyl radical B, driven by the polar
effect in the transition state.21 Then, the radical addition of B to
the charged N-heterocycle, deprotonation, and spin-centre shift
(SCS) process15 sequentially occur to generate the hydroalkyl
radical E. Ultimately, the SET process from the reductive PC-
(E1/2(PC/PC-) = −1.21 V vs SCE)17 to the intermediate E
furnishes the protonated product F, with late-stage workup to
afford the final products. Our previous work revealed that the
further photoreductive process of F by aldehyde led to the
formation of 3a’,16b although the exact mechanism is not clear
in current stage.
Conclusions
In summary, we have developed a photoredox neutral
Minisci hydroalkylation reaction of quinolines and related
nitrogen-containing heterocycles. In this protocol, non-toxic
and cheap LiBr plays an exceptional role as a moderator in the
plausibly proposed SET/HAT cycle. Hence, acyl radicals were
readily generated via directly endergonic C–H abstractions of
aldehydes, which therefore commence the Minisci-type C-H
functionalizations. Although there appear limitations with
respect to substrate scope in this system, the major features
regarding atom economy and mild photoredox neutral
conditions make it an important and attractive alternative
especially for the challenging Minisci hydroalkylations, as well
as a promising approach for atom-economic Minisci reactions
with broader N-heterocycle spectra.
4 | J. Name., 2012, 00, 1-3
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Green Chemistry
Journal Name
COMMUNICATION
Chem., 2019, 21, 3858-3863; (n) L.-Y. Xie, L.-L. Jiang, J.-X.
Conflicts of interest
View Article Online
Tan, Y. Wang, X.-Q. Xu, B. Zhang, Z. Cao and W.-M. He, ACS
DOI: 10.1039/D0GC01872D
Sustainable Chem. Eng., 2019, 7, 14153-14160; (o) J. Dong, Z.
Wang, X. Wang, H. Song, Y. Liu and Q. Wang, Sci. Adv., 2019,
5, eaax9955; (p) L.-Y. Xie, Y.-L. Chen, L. Qin, Y. Wen, J.-W.
Xie, J.-X. Tan, Y. Huang, Z. Cao and W.-M. He, Org. Chem.
Front., 2019, 6, 3950-3955; (q) L.-Y. Xie, Y.-S. Bai, X.-Q. Xu,
X. Peng, H.-S. Tang, Y. Huang, Y.-W. Lin, Z. Cao and W.-M.
He, Green Chem., 2020, 22, 1720-1725; (r) L.-Y. Xie, Y.-S. Liu,
H.-R. Ding, S.-F. Gong, J.-X. Tan, J.-Y. He, Z. Cao and W.-M.
He, Chin. J. Catal., 2020, 41, 1168-1173.
There are no conflicts of interest to declare.
Acknowledgements
Support by the Science and Technology Planning Project of Hunan
Province (2019RS2039), the National Natural Science Foundation of
China (21502161, 21602187), and the Collaborative Innovation
Center of New Chemical Technologies for Environmental Benignity
and Efficient Resource Utilization is gratefully acknowledged.
12 C. A. Huff, R. D. Cohen, K. D. Dykstra, E. Streckfuss, D. A.
DiRocco and S. W. Krska, J. Org. Chem., 2016, 81, 6980-6987.
13 (a) M. Szostak, N. J. Fazakerley, D. Parmar and D. J. Procter,
Chem. Rev., 2014, 114, 5959-6039; (b) Q. Xia, J. Dong, H. Song
and Q. Wang, Chem. Eur. J., 2019, 25, 2949-2961.
14 A. Banerjee, Z. Lei and M. Y. Ngai, Synthesis, 2019, 51, 303-
333.
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