Heat-or light-induced acylarylation of unactivated alkenes towards 3-(α-acyl) indolines

Article abstract of DOI:10.1039/d0ob01105c

A heat-or photoredox/iron dual catalysis-enabled dehydrogenative acylarylation of N-allyl anilines leading to 2-substituted 3-(α-acyl) indolines with a quaternary stereogenic center is presented, with unactivated alkenic bonds as radical acceptors and simple aldehydes as radical precursors. This reaction features high yields, a broad substrate scope, and a great exo selectivity, and gram-scale syntheses could be readily carried out.

Full text of DOI:10.1039/d0ob01105c

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Heat- or light-induced acylarylation of unactivated
alkenes towards 3-(α-acyl) indolines†
Cite this: DOI: 10.1039/d0ob01105c
Yanni Li,a Fengyuan Ying,a Tingfeng Fu,a Ruihan Yang,a Ying Dong,b Liqing Lin,a,c
Yinghui Han,a,c Deqiang Liang *a and Xianhao Long*a
A heat- or photoredox/iron dual catalysis-enabled dehydrogenative acylarylation of N-allyl anilines
leading to 2-substituted 3-(α-acyl) indolines with a quaternary stereogenic center is presented, with unac-
tivated alkenic bonds as radical acceptors and simple aldehydes as radical precursors. This reaction fea-
tures high yields, a broad substrate scope, and a great exo selectivity, and gram-scale syntheses could be
readily carried out.
Received 29th May 2020,
Accepted 7th July 2020
DOI: 10.1039/d0ob01105c
rsc.li/obc
dehydrogenative aldehydic C–H functionalization,7 which are
atom- and step-economic, and categorized into three types: (1)
Introduction
Indolines are found in the skeletons of many alkaloids and umpolung reactions catalysed by N-heterocyclic carbenes
clinical drugs, and exhibit a broad range of important (NHCs)8 or cyanides,9 (2) aldehydic C–H activations enabled by
bioactivities.1,2 However, in the indole chemistry the construc- noble metal catalysis,10,11 (3) radical couplings of acyl radicals
tion of structurally diverse indoline architectures might be produced via hydrogen atom transfer (HAT).12–17 In recent
challenging, as compared to other indolic variants. For years, alkene difunctionalization has emerged as a powerful
example, functionalized indoles3 or oxindoles4 could be tool to access molecule complexity,18 since two functional
accessed either from acyclic materials or through derivatiza- groups are installed across a double bond in one step. In this
tion of their parent heterocycles, whereas entries to indoline regard, great progress has been made towards radical acylation
frameworks are mainly restricted to indole dearomatization of alkenes with aldehydes, yet the olefins used are mainly
reactions,2 leading to 2-substituted indoline derivatives. restricted to activated ones.12–15 It is probably because the acti-
2-Unsubstituted indolines might be prepared via the reduction vating groups, such as aryl,12 alkoxyacyl,13 carbamoyl14 and
of their indole or oxindole counterparts, yet such a transform- sulfamoyl functionalities,15 could significantly stabilize in situ
ation suffers from severe functional group-intolerance, generated radical intermediates. Li and co-workers reported
especially when a reductant-sensitive motif (e.g., the carbonyl the heat-induced radical annulation of aldehydes with olefins
group) is involved. Therefore, it is still urgent to develop towards indolines and dihydropyrans,13e and only alkoxyacyl-
general and efficient paradigms for the synthesis of functiona- activated olefins were tested. Acyl radicals and unactivated
lized 2-unsubstituted indolines from readily available acyclic alkenes are electrophiles and nucleophiles, respectively, and
materials.
according to polar effects19 and our experiences in radical
The introduction and application of the carbonyl motif, chemistry,20 the addition of acyl radicals to unactivated olefins
due to its versatility, is one of the pillars of classical synthetic is polarity-matched and thus might be viable. In addition, a
chemistry.5 On the other hand, simple aldehydes are funda- few pioneering works have been reported. Pan, Yu and co-
mental building blocks in organic synthesis and are bulk workers16a as well as Leng, Wu and co-workers16b disclosed
chemicals for the chemical industry.6 It proves that aldehydes 1,2-aryl migration-primed acylarylation of α-hydroxy-α,α-diaryl
could be an ideal acyl source owing to the development of propylenes, while Duan, Li and co-workers reported the syn-
thesis of dihydroisoquinolinones and indanones through the
acylation of N-allylbenzamides.17 However, light-induced acyla-
tion of alkenes, either activated21,22 or unactivated ones,22 was
rarely reported. In 2018, Salles et al. presented a visible-light-
driven hydroacylation of unactivated olefins with aromatic
aldehydes using methylene blue and K2S2O8 as the photo-
catalyst (PC) and the HAT agent, respectively, affording chain-
elongated ketone products.22 Herein, we wish to report an
iron-catalysed heat- or light-induced acylarylation of N-allyl
aSchool of Chemistry and Chemical Engineering, Kunming University, Kunming
650214, China. E-mail: liangdq695@nenu.edu.cn, longxh@kmu.edu.cn
bCollege of Chemistry, Chemical Engineering and Materials Science, Shandong
Normal University, Jinan 250014, China
cResearch Center on Life Sciences and Environmental Sciences, Harbin University of
Commerce, Harbin 150076, China
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, spectral data, and copies of NMR spectra. See DOI: 10.1039/d0ob01105c
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anilines, with unactivated alkenic bonds as radical acceptors (entry 4), CuI (entry 5) and AgNO3 (entry 6), all led to an
and simple aldehydes as radical precursors, affording 3- erosion of the yield. Oxidant DTBP proved to be the optimal
(α-acyl) indolines with a quaternary stereogenic center as HAT agent. While the use of tert-butyl hydroperoxide (TBHP,
value-added products. This reaction is associated with high entry 7) or dicumyl peroxide (DCP, entry 8) delivered indoline
yields, a broad substrate scope, and a great exo selectivity, and 3a1 in diminished yields, only trace amounts of 3a1 were
gram-scale syntheses could be readily carried out.
observed with benzoyl peroxide (BPO, entry 9) or K2S2O8 (entry
10). Then, a survey of a series of solvents was conducted. The
use of toluene furnished indoline 3a1 in only 39% yield, prob-
ably owing to the competitive HAT from the arylmethane
moiety, whereas reactions performed in a polar solvent like
1,2-dichloroethane (DCE, entry 12), CH3CN (entry 13) or di-
methylsulfoxide (DMSO, entry 14) were retarded.
Results and discussion
Our initial attempt was launched with the alkene acylation uti-
lizing N-(2-methylallyl) acetanilide 1a and 4-chlorobenzalde-
hyde 2a as model substrates (Table 1; see ESI† for full details).
Combining 1a and 3 equiv. of 2a together with 5 mol% of
FeCl2 and 4 equiv. of di-tert-butyl peroxide (DTBP) in chloro-
benzene solution at 120 °C furnished the acylated indoline
product 3a1 bearing a quaternary stereogenic center in 87%
yield after 6 h (entry 1). FeCl2 is a beneficial additive for this
transformation, probably due to the abilities of transition-
metal salts to stabilize radicals and to facilitate some single-
electron transfer (SET) processes, and a compromised yield
was obtained in the absence of FeCl2 (entry 2). While ben-
eficial effects were not associated with the inclusion of FeSO4
(entry 3), the use of other tested metal salts, such as FeCl3
Intrigued by the fact that photoredox catalysis-enabled reac-
tions generally proceed under mild conditions, we sub-
sequently sought to effect the title indoline synthesis in a
photocatalytic manner by switching from heating to the com-
bination of a PC and the light-emitting-diode (LED) irradiation
(see ESI† for full details). At room temperature rather than
120 °C and under blue-light irradiation, a range of photosensi-
tizers were assessed. To our delight, 3-(α-acyl) indoline product
3a1 was afforded in a moderate yield after 12 h using fac-Ir
(ppy)3 as the PC (entry 15), whereas Ru(bpy)3Cl2 (entry 16),
−
Mes-Acr+ClO4 (entry 17), eosin Y (entry 19) and rose bengal
(entry 20) all proved ineffective. When 1,2,3,5-tetrakis(carbazol-
9-yl)-4,6-dicyanobenzene (4CzIPN) was used, indoline 3a1 was
produced in 62% yield, identifying 4CzIPN as the optimal PC,
and the yield was further improved to 70% by prolonging the
reaction time to 24 h (entry 18). Interestingly, this photoredox
process benefits from the FeCl2 additive as well, and in the
absence of it indoline 3a1 was produced in a moderate yield
(entry 21).
Table 1 Optimization of the reaction conditionsa
Thus, we established both the heat-induced (conditions A)
and the photoredox conditions (conditions B) for the synthesis
of 3-(α-acyl) indolines, and the two protocols are complemen-
tary. Both sets of optimal conditions were selected to probe
the substrate scope, and the reactions were first extended to a
Entry Catalyst Oxidant PCb (mol%)
Solvent
Yield (%)
1
2
3
4
5
6
7
8
FeCl2
—
FeSO4
FeCl3
CuI
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBHPc
DCP
—
—
—
—
—
—
—
—
—
—
—
—
—
—
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
Toluene 39
DCE
CH3CN
DMSO
PhCl
87
68
69
57
41
18
53
71
broad collection of allylated anilines
4-Chlorobenzaldehyde 2a reacted smoothly with N-(2-methyl-
allyl) acetanilides bearing methyl, tert-butyl, methoxy,
1
(Table 2).
a
bromo, chloro, trifluoromethyl, ethoxycarbonyl, acetyl, or
cyano group at the para position of the N-aryl group, affording
the corresponding 3-(α-acyl) indolines 3a2-10 in moderate to
excellent yields. In general, the yields of the heat-induced
syntheses are higher than the ones of photocatalysed reac-
tions, and the substrates with an electron-deficient N-aryl
group are more reactive. In contrast, 5-phenyl indoline 3a11
was furnished in only 45% yield under the heating conditions
from allylated biphenyl-4-amine, the origin of which remains
unclear at this time. While propionyl- or pivaloyl-protected N-
(2-methylallyl) anilines were transformed efficiently into the
corresponding indoline products 3b1,2, the tert-butyloxy-car-
bonyl (Boc, 3c1,2), ethanesulfonyl (3d1,2), methanesulfonyl
(3e1), tosyl (3e2) and o-tolylsulfonyl N-protecting groups (PGs,
3e3) are all compatible with this transformation.
Unfortunately, we failed to isolate any pure product from the
complex reaction mixtures of allyl anilines protected by an
AgNO3
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
—
9
BPO
Trace
Trace
10
11
12
13
14
15
16
17
18
19
20
21
K2S2O8
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
11
0
0
52
fac-Ir(ppy)3 (1)
Ru(bpy)3Cl2 (1)
PhCl
Nr
Mes-Acr+ClO4− (1) PhCl
Trace
62 (70)d
Nr
Nr
51
4CzIPN (1)
PhCl
PhCl
PhCl
PhCl
Eosin Y (5)
Rose bengal (5)
4CzIPN (1)
a Reaction conditions: 1a (0.5 mmol), 2a (3.0 equiv), catalyst (5 mol%),
oxidant (4.0 equiv.), solvent (2.0 mL), Ar, 120 °C (for entries 1–14) or
room temperature (for entries 15–21), 6 h (for entries 1–14) or 12 h
(for entries 15–21). b 6 W Blue LEDs were used to excite the PCs.
c 5.0–6.0 mol L−1 in decane. d The reaction time was prolonged to 24 h.
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Table 2 Scope of N-allyl anilinesa,b
selectivity might be associated with the intermediate stability.
Variations of the R1 branch have also been performed, and the
non- or methoxycarbonyl-branched substrates participated in
this transformation as well, delivering the corresponding indo-
line products 3i1,2 albeit with a depreciation in yield.
Both the heat-induced and the photoredox conditions are
also applicable to a range of aldehydes (Table 3). 2- or
3-Chlorobenzaldehyde are both usable acyl radical precursors,
and their reactions with N-(4-chlorophenyl)-N-(2-methylallyl)
acetamide 1b under either conditions afforded the indoline
products 3j,k in moderate to high yields. Whereas high yields
were obtained using 4-bromo (3l1) or 4-fluoro benzaldehyde
(3l2), aldehydes with a strongly electron-withdrawing substitu-
ent (e.g., trifluoromethyl or methoxycarbonyl group) reacted
with 1b to afford the indolines 3l3,4 in diminished yields, indi-
cating that the use of electron-deficient aromatic aldehydes
might have adverse effects on the title reaction. Nonetheless,
the reactions of pentafluorobenzaldehyde with a long-chain
acyl-protected allyl aniline proceeded to give the 3-(α-acyl)
indoline 3m in high yields. While non-substituted (3n) or
2-methyl-substituted benzaldehydes (3o) are both excellent
acyl sources for this transformation, electron-donating substi-
tuents, such as the methyl (3p1), acetoxy (3p2), and methoxy
groups (3p3), at the para-position of the aldehydic aryl group
were well-tolerated as well. Noteworthy is the compatibility of
Table 3 Scope of aldehydesa,b
a Reaction conditions A:
1 (0.5 mmol), 2a (1.5 mmol), FeCl2
(0.025 mmol), DTBP (2.0 mmol), degassed chlorobenzene (2.0 mL), Ar,
120 °C, 6 h; reaction conditions B: 1 (0.5 mmol), 2a (1.5 mmol), FeCl2
(0.025 mmol), 4CzIPN (0.005 mmol), DTBP (2.0 mmol), chlorobenzene
(2.0 mL), Ar, 6 W Blue LEDs, 24 h. b The yields of reactions under con-
ditions B are shown in parenthesis.
aroyl substituent, probably because both phenyl rings might
serve as the inbuilt radical trap during reaction. Allylated ben-
zamides, however, could undergo a similar acylarylation reac-
tion with 2a to afford the 3,4-dihydroisoquinolin-1-ones 4a,b,
albeit in modest yields, and in the case of N-benzyl-3-chloro-N-
(2-methylallyl)benzamide, the cyclizative arylation process
occurred on the benzoyl ring as well and at the less hindered
position (4b). Substantial steric effects were observed in the
reaction of the ortho-substituted anilide, and the 7-chloro
indoline 3f was obtained in only poor yields. Whereas a 3,5-
dichloro anilide reacted with aldehyde 2a to give the 4,6-
dichloro indoline 3g in high yields, the 4-chloroindoline 3h
and regioisomeric 6-chloroindoline 3h′ were produced simul-
taneously from a 3-chloro aniline substrate. Interestingly, more
encumbered isomer 3h is the major product, and such a regio-
a Reaction conditions A: 1b (0.5 mmol),
2 (1.5 mmol), FeCl2
(0.025 mmol), DTBP (2.0 mmol), degassed chlorobenzene (2.0 mL), Ar,
120 °C, 6 h; reaction conditions B: 1b (0.5 mmol), 2 (1.5 mmol), FeCl2
(0.025 mmol), 4CzIPN (0.005 mmol), DTBP (2.0 mmol), chlorobenzene
(2.0 mL), Ar, 6 W Blue LEDs, 24 h. b The yields of reactions under con-
ditions B are shown in parenthesis.
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2-thenaldehyde, with the corresponding product 3q isolated in
70% (conditions A) or 61% yield (conditions B). Interestingly,
when cyclohexanecarbaldehyde and 1b were subjected to the
heating conditions, decarbonylation occurred, furnishing the
cyclohexylated indoline 3r′ albeit in a poor yield. Under the
mild irradiation conditions, however, both the acylated indo-
line 3r and the decarbonylative product 3r′ were produced. It is
worthy of notice that an exo selectivity was achieved in all of
the above reactions, and a formal 6-endo-trig product was not
observed.
The present syntheses of 3-(α-acyl) indolines under either
the heating or the photoredox conditions could be readily
carried out on a gram-scale with only a slight loss in yield
(Scheme 1), rendering them practical protocols. The relatively
modest yield obtained under the photoredox conditions might
reflect the insufficient irradiation as a result of the micro-
photoreaction setup we used (see ESI† for full details).
To gain further insight into the mechanisms, a series of
mechanistic investigations were performed (Scheme 2). Upon
addition of either 2 equiv. of 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO, Scheme 2a1) or 1.2 equiv. of butylated hydroxyto-
luene (BHT, Scheme 2a2) to the model reactions induced by
heat, both reactions were completely suppressed, and the alde-
hyde-TEMPO and the aldehyde-BHT adducts 5a,b were isolated
in 35% and 52% yields, respectively. The model reactions
under the photoredox conditions doped with 0.3 equiv. of
TEMPO or 1.2 equiv. of BHT did not proceed either, with the
aldehyde-TEMPO adduct 5a furnished in 67% yield in the
TEMPO experiment (Scheme 2a3). These results suggest that
radical processes might be involved in this transformation,
and acyl radicals might be the key open-shell intermediates.
The kinetic isotope effect (KIE) experiments between 1a
and the aryl-deuterated substrate 1a-d5 under both sets of
optimal conditions were conducted (Scheme 2b), giving minor
KIE values of 1.5 (conditions A) and 1.3 (conditions B), respect-
ively. These results indicate that the C–H bond cleavage of the
N-aryl moiety is probably not involved in the rate-limiting step.
Finally, light on–off experiments showed that during light off-
cycles the photoredox reaction was retarded, and that a con-
stant irradiation is necessary for it to reach completion
(Scheme 2c). A radical chain might exist,23 yet the chain length
is short. These were further supported by measuring the
quantum yield, which was found to be 0.24 (see ESI† for full
details). Interestingly, there seems to be an induction period,
which might be explained by the requirement to access an Fe
(III) species from the Fe(II) catalyst, facilitating the key SET
process from the dearomatized aryl radical B.
Scheme 2 Mechanistic investigations.
Based on the above experimental results and literature
reports, plausible reaction mechanisms were proposed
(Scheme 3). Under the photoredox conditions, the PC, 4CzIPN,
is transferred to its highly oxidizing excited state (PC*) by
absorbing photons, and the excited-state species PC* would
then be reductively quenched by an Fe(II) species, affording an
Scheme 1 Gram-scale syntheses.
Scheme 3 Proposed mechanisms.
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Fe(III) complex and the reduced photosensitizer PC•−. The later
reduces the oxidant DTBP to produce the tert-butanolate anion
and a tert-butoxyl radical, as well as the regenerated PC. HAT
from the aldehydic C–H bond to the tert-butoxyl radical
affords an acyl radical, the addition of which across the teth-
ered double bond leads to the radical intermediate A. A is
highly active, and the rapid intramolecular radical trapping by
the N-aryl moiety ensues, delivering the dearomatized aryl
radical intermediate B. Subsequent SET from B to an Fe(III)
complex gives the dearomatized cationic intermediate C and
regenerates the Fe(II) catalyst. Finally, deprotonation of C by
the tert-butanolate anion furnishes the 3-(α-acyl) indoline 3a1.
As for the heat-induced reaction, the tert-butoxyl radical was
produced by the thermal decomposition of DTBP, and this
radical could oxidize the Fe(II) catalyst to give an Fe(III)
complex. The direct SET from aryl radical B to DTBP might
occur as well,23 enabling a propagation process as a minor
reaction pathway observed in the light on–off experiments.
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Conclusions
To conclude, a heat- or photoredox/iron dual catalysis-enabled
synthesis of 3-(α-acyl) indolines bearing a quaternary stereo-
genic center has been developed, with unactivated alkenic
bonds as radical acceptors and simple aldehydes as radical
precursors. This reaction is associated with high yields, a
broad substrate scope, and a great exo selectivity, and gram-
scale syntheses could be readily carried out.
8 For NHC-catalysed alkene acylation using aldehydes, see:
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Conflicts of interest
There are no conflicts to declare.
9 (a) E. Shurdha, H. A. Miller, R. E. Johnson, G. J. Balaich
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Acknowledgements
We gratefully acknowledge the financial support from the
Basic Research Programs of Yunnan Province (2018FH001-018,
2018FH001-002, 2019FGF01/2019-1-C-25318000002188), the 10 For noble metal-catalysed acylation of activated alkenes
Scientific Research Funds of Yunnan Education Department
(2018JS394), the National Natural Science Foundation of China
(21702083), the Yunnan Ten Thousand Talent Program for
Young Top-Notch Talents, and the Program for Innovative
Research Team (in Science and Technology) in Universities of
Yunnan Province.
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