Organophotoredox-Catalyzed Cascade Radical Annulation of 2-(Allyloxy)arylaldehydes with N-(acyloxy)phthalimides: Towards Alkylated Chroman-4-one Derivatives

Article abstract of DOI:10.1002/asia.201901735

An organophotoredox catalyzed efficient and robust approach for the synthesis of highly important 3-alkyl substituted chroman-4-one scaffold is developed using visible light induced radical cascade cyclization strategy. The reaction is initiated through t

Full text of DOI:10.1002/asia.201901735

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Organophotoredox Catalyzed Cascade Radical Annulation of 2-
(Allyloxy)arylaldehydes with N-(acyloxy)phthalimides: Towards
Alkylated Chroman-4-one Derivatives
Authors: Sanju Das, Sushanta Kumar Parida, Tanumoy Mandal,
Laxmikanta Sing, Suman De Sarkar, and SANDIP MURARKA
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Organophotoredox Catalyzed Cascade Radical Annulation of 2-
(Allyloxy)arylaldehydes with N-(acyloxy)phthalimides: Towards
Alkylated Chroman-4-one Derivatives
Sanju Das,[a] Sushanta Kumar Parida+,[b] Tanumoy Mandal+,[a] Laxmikanta Sing,[a] Suman De Sarkar*[a]
and Sandip Murarka*[b]
Dedication ((optional))
[a]
[b]
[+]
Sanju Das, Tanumoy Mandal, Laxmikanta Sing, Dr. Suman De Sarkar
Department of Chemical Sciences
Indian Institute of Science Education and Research Kolkata
Mohanpur-741246, West Bengal, India
E-mail: sds@iiserkol.ac.in
Sushanta Kumar Parida, Dr. Sandip Murarka
Department of Chemistry
Indian Institute of Technology Jodhpur
Karwar-342037, Rajasthan, India
E-mail: sandipmurarka@iitj.ac.in
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: An organophotoredox catalyzed efficient and robust
approach for the synthesis of highly important 3-alkyl substituted
chroman-4-one scaffold is developed using visible light induced
radical cascade cyclization strategy. The reaction is initiated through
the generation of alkyl radicals from N-(acyloxy)phthalimides under
photoredox conditions, which subsequently undergo intermolecular
cascade radical cyclization on 2-(allyloxy)arylaldehydes to afford
chroman-4-one scaffolds. The presented strategy is attractive with
regard to mild reaction conditions, operational simplicity, high
functional group tolerance and broad substrate scope.
containing 1,3-dicarbonyls as radical precursors.[3f] More
recently, Liu group developed cross-coupling of 2-
a
(allyloxy)arylaldehydes with cyclopropanols to afford carbonyl
containing alkyl substituted chroman-4-ones (Scheme 1b).[3b]
Despite these important contributions the existing methods
require multistep synthesis of organocatalysts, usage of higher
loading of metal catalysts, higher reaction temperature and
superstoichiometric amount of strong oxidant. Considering
significant applications of chroman-4-ones, it is needless to
mention that the development of more straightforward, milder
and environmentally benign approaches for the synthesis of this
scaffold is highly desired.
The chroman-4-one framework belongs to the privileged class of
heterocycles and constitutes an integral part of numerous
natural products, pharmaceuticals and biologically active
compounds.[1] They exhibit a variety of physiological and
biological activities, such as antibacterial, anti-HIV, anti-tobacco
mosaic virus and SIRT2 inhibitors, to name a few (Figure 1).
Consequently, significant efforts have been made towards the
development of efficient, atom-economic and robust approaches
to access a range of functionally orchestrated chorman-4-
ones.[2] For example, a base promoted and microwave-assisted
condensation of 2-hydroxyacetophenones with aliphatic
aldehydes, and N-heterocyclic carbene (NHC) catalyzed
intramolecular Stetter reactions are among the representative
Figure 1. Selective examples of biologically active chroman-4-one scaffolds.
2c, 2d]
methods.[2a,
Recently, cascade radical cyclization of 2-
(allyloxy)arylaldehydes triggered by diverse radicals, such as
phosphoryl, acyl, and alkyl have evolved as an alternate and
efficient approach allowing access to chroman-4-one
derivatives.[3] Among these methodologies, the cascades those
are initiated by alkyl radicals captivated our attention. In 2018,
In this regard, visible light-mediated photoredox catalysis
represents a powerful alternative and environment-friendly tool
in fostering of modern synthetic chemistry.[4] Additionally, it has
recently
been
revealed
that
redox-active
N-
Chen, Yu and coworkers documented
a silver-catalyzed
(acyloxy)phthalimides (NHPI esters) participate in an exciting
range of single electron transfer (SET)-based cross-coupling
reactions under photolytic conditions.[5] NHPI esters are bench
stable and readily available feedstock that can easily be
prepared from the corresponding carboxylic acids and N-
decarboxylative radical cyclization of 2-(allyloxy)arylaldehydes
using tert-carboxylic acids as the reaction partner at 80 °C
(Scheme 1a).[3a] Subsequently, authors reported another alkyl
radical initiated cyclization employing active methylene
1
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hydroxyphthalimide under standard coupling conditions.[6]
Following Okada’s initial discovery[7] of redox properties of NHPI
esters and their ability to serve as a precursor of alkyl radicals
under photoredox catalysis; the recent years have witnessed an
upsurge in the applications of NHPI esters in a multitude of
transformations, including decarboxylative alkylations[8] and
chemoselective tandem radical cyclizations.[9] We were intrigued
by the possibility of reacting alkyl radical generated from NHPI
the reaction did work in the presence of inorganic base K3PO4,
albeit lower yield was obtained (entry 8). Among the various
solvents tested, CH3CN remained to be the optimal one (entries
9-11). Lowering of Eosin Y loading from 5 mol% to 2.5 mol%
(entry 12) or DIPEA loading from 3 equiv to 1.5 equiv (entry 13)
led to inferior results. Notably, the yield plummeted to 42% upon
reducing 2a loading from 3 equiv to 1.5 equiv (entry 14).
Subsequent control experiments showed that Eosin Y, DIPEA
and visible light irradiation are indispensable to the success of
this cascade process (entries 15-17). Importantly, the argon
atmosphere was crucial to the positive outcome of the reaction
as the yield substantially reduced to 37% upon carrying out the
reaction under the air atmosphere (entry 18).
esters
under
visible
light
irradiation
with
2-
(allyloxy)arylaldehydes to construct the alkyl-substituted
chroman-4-one scaffold. Such a visible photoredox catalysis
driven tandem radical strategy would be highly efficient and
fascinating from the perspective of green chemistry. As part of
our ongoing program on photoredox catalysis[10] and driven by
our interest in N-(acyloxy)phthalimides as redox-active esters,[5]
we herein disclose a hitherto unknown Eosin Y catalyzed
cascade radical annulation of NHPI esters and 2-
(allyloxy)arylaldehydes to yield 3-alkyl substituted chroman-4-
ones under visible light irradiation (Scheme 1c).
Table 1. Optimization of the reaction conditions[a]
Entry
Catalyst
Base
Solvent
Yield (%)[b]
1
Eosin Y
Rose bengal
Fluorescein
[Ru(bpy)3]Cl2
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
-……
DIPEA
DIPEA
DIPEA
DIPEA
DABCO
Et3N
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
1,4-Dioxane
DCM
71
2
44
3
33
4
59
5
ND
29
6
7
2,6-Lutidine
K3PO4
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
-…..
67
8
32
9
55
Scheme 1. Synthesis of alkyl substituted chroman-4-one derivatives.
10
11
12[c]
13[d]
14[e]
15
16
17[f]
18[g]
70
DMF
67
Recently, Eosin Y has gained popularity as a photocatalyst
enabling an array of photochemical reactions.[4g, 4h] König and
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
65
coworkers documented an elegant Eosin
Y
catalyzed
45
decarboxylative alkylation of acrylates using NHPI esters as the
alkyl radical precursors under visible light irradiation.[8p]
Accordingly, we began our studies through reacting 1 equiv of 2-
(allyloxy)arylaldehyde 1a with 3 equiv of NHPI ester 2a in the
presence of 5 mol% of photoredox catalyst Eosin Y using 3
equiv of N,N-diisopropylethylamine (DIPEA) as the base and
CH3CN as solvent under blue LED irradiation (440 nm) at room
temperature under argon atmosphere (Table 1, see the SI for
detailed optimization table). Gratifyingly, the desired product 3aa
was obtained in 71% yield (entry 1). Subsequent screening of
various photocatalysts established Eosin Y as the most effective
catalyst with all other catalysts leading to decrement of chemical
yield. The organo-photocatalysts, such as Rose Bengal,
Fluorescein and commonly employed metallic photocatalyst
[Ru(bpy)3]Cl2 failed to improve the yield (entries 2-4). Switching
of base from DIPEA to DABCO (1,4-diazabicyclo[2.2.
2]octane) completely ceased the photochemical reaction (entry
5), whereas yield deteriorated upon replacement of DIPEA with
bases such as Et3N and 2,6-Lutidine (entries 6-7). Interestingly,
42
Trace
ND
ND
37
Eosin Y
Eosin Y
Eosin Y
DIPEA
DIPEA
[a] Reaction Conditions: 0.30 mmol scale using 1a (1 equiv), 2a (3 equiv),
catalyst (5 mol%), base (3 equiv) and solvent (3 mL) under argon atmosphere.
[b] Isolated yields. [c] Using 2.5 mol% Eosin Y. [d] Using 1.5 equiv DIPEA. [e]
Using 1.5 equiv of 2a. [f] In the absence of light. [g] Air atmosphere. ND = not
detected.
With optimized conditions in hand, we embarked on the
exploration of scope of the alkyl radical triggered cascade
manifold through reacting
a range of electronically and
structurally diverse 2-(allyloxy)arylaldehydes 1 with an array of
NHPI esters 2 derived from the corresponding carboxylic acids.
2
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First we studied the effect of electronically distinct substitution
patterns on the phenyl ring of 2-(allyloxy)arylaldehydes 1
(Scheme 2). Pleasingly, diverse electron-rich substrates bearing
alkyl groups (1a-c), dialkyl group (1d) and methoxy group (1e)
underwent smooth transformation to yield the corresponding
cross-coupled products 3aa-3ea in moderate to good yields
(35%-71%). Delightfully, the cascade tolerates the presence of
several electron-withdrawing groups on the phenyl ring
(-F, -Cl, -Br, -I) and similar or better reactivities than those with
electron-donating counterpart were observed to afford the
respective products 3fa-3ia in good yields. Importantly, di-iodo
substituted substrate 1j participated in the process to furnish
product 3ja in 55% yield. We were pleased to find that N-allyl-N-
(2-formylphenyl)-4-methylbenzenesulfonamide 1k demonstrated
good reactivity and the corresponding heteroaryl moiety 3ka was
obtained in 63% yield.
good yields. Notably, NHPI ester derived from secondary
cyclohexane carboxylic acid was well accommodated in the
cascade process giving 3ae in an acceptable chemical yield. In
general, NHPI esters derived from secondary carboxylic acids
were less reactive and primary NHPI esters were not suitable
substrates for this radical cascade process. This could
presumably be due to the inherent less stability of the
corresponding primary and secondary alkyl radicals.
[a] Reaction Conditions: 0.30 mmol scale using 1a (1 equiv), 2a (3 equiv),
catalyst (5 mol%), base (3 equiv) and solvent (3 mL) under argon atmosphere.
Scheme 3. Scope of N-(acyloxy)phthalimides[a]
To gain further insight into this photocatalytic cascade process,
a few control experiments were carried out (Scheme 4). The
yield of 3aa drastically reduced to 7% when 1a and 2a were
allowed to react under optimized conditions in the presence of 3
equiv of TEMPO (2,2,6,6-tetramethyl-1-piperidine-1-oxyl) as the
radical scavenger (Scheme 4a). On the other hand, carrying out
the same reaction in the presence of another radical scavenger
BHT (2,6-di-tert-butyl-4-methylphenol) led to the formation of
3aa (25%) and an adduct 4 (35%) resulting from the trapping of
initially generated tert-butyl radical by BHT (Scheme 4b). These
experiments explicitly established the intermediacy of radicals in
the presented cascade process. Interestingly, the reaction of
ketone 5 with 2b under optimized conditions led to the formation
of 2-hydroxy acetophenone 6, allylated adamantane 7 and
adamantane 8, which were detected by GC-MS and HRMS
(Scheme 4c) (see the SI for details). Presumably, the radical
intermediate generated through the addition of adamantyl radical
on the C=C double bond could not undergo intramolecular
cyclization owing to the less reactivity of pendant ketone
functionality, which ultimately led to the fragmentation and
formation of 6 and 7. This interesting control experiment does
not only reaffirm the intermediacy of alkyl radicals in this process
but also unequivocally established the requirement of high
reactivity of carbonyl functionality towards the success of the
annulation process.
[a] Reaction Conditions: 0.30 mmol scale using 1a (1 equiv), 2a (3 equiv),
catalyst (5 mol%), base (3 equiv) and solvent (3 mL) under argon atmosphere.
Scheme 2. Scope of 2-(allyloxy)arylaldehydes[a]
We next explored the generality of the reaction by varying NHPI
esters 2 (Scheme 3). We resumed by evaluating the reactivity of
tertiary carbon-centered radicals and were glad to note that
sterically bulky adamantyl radical underwent planned radical
cyclization to provide the desired product 3ab in 48% yield.
Tertiary radical derived from NHPI ester of 1-methyl-1-
cyclohexane carboxylic acid furnished 3ac in 77% yield.
Moreover, acyclic tertiary alkyl radical generated from NHPI
ester of 2,2-dimethylbutanoic acid reacted with a range of
electronically diverse 2-(allyloxy)arylaldehydes (1a-b, 1f, 1h) to
afford the corresponding 4-alkylated chroman-4-one derivatives
(3ad, 3bd, 3fd, 3hd) in good yields (60%-75%). Pleasingly, the
N-tosyl substituted analogue 1k participated in the alkyl radical
cascade annulation through reaction with various cyclic and
acyclic tertiary radicals to provide the desired 2,3-
dihydroquinolin-4-one scaffolds (3kb, 3kc, 3kd) in moderate to
3
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Scheme 4. Control experiments.
Scheme 5. Proposed mechanism of the radical cascade cyclization.
To shed light on the mechanism of the cascade process,
fluorescence quenching studies were carried out. In Stern-
Volmer studies (see SI for details) significant quenching of
fluorescence emission of Eosin Y was observed by NHPI ester
2a, rather than with 2-(allyloxy)arylaldehyde 1a or DIPEA,
indicating that the reaction proceeded through an oxidative
quenching of Eosin Y (Figure 2).[9g]
In summary, we have developed
a straightforward and
convenient organophotoredox catalyzed decarboxylative radical
cascade annulation of 2-(allyloxy)arylaldehydes and N-
(acyloxy)phthalimides towards the construction of biologically
important 3-alkylated chroman-4-one scaffolds in moderate to
good yields. Importantly, the presented methodology also
allowed an efficient access to alkylated 2,3-dihydroquinolin-4-
ones in good yields. The protocol is significant as it does not
require any metal catalyst and the reactions were carried under
mild conditions. Further applications of NHPI esters in
combination with photocatalysis towards creating densely
functionalized molecular frameworks are currently ongoing in our
laboratory.
3.0
2a
DIPEA
2.5
2.0
1.5
1.0
1a
Acknowledgements
Generous support by the CSIR [Fellowship to SD, TM and LS],
IISER Kolkata for funding and infrastructure [to SDS] is gratefully
acknowledged. SM thanks IIT Jodhpur [fellowship to SKP and
for infrastructural facilities] and SERB for funding
[ECR/2017/001408].
0
50
100
150
200
250
300
350
Concentration (10-3 M)
Figure 2. Fluorescence quenching of Eosin Y by 1a, NHPI ester 2a and
DIPEA.
Keywords: Photocatalysis • Redox-active ester • N-
(acyloxy)phthalimide • Radicals • Chroman-4-one
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5
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COMMUNICATION
Entry for the Table of Contents
We have developed a mild, metal-free, operationally simple and convenient approach towards the construction of biologically
important chroman-4-one scaffolds through an organophotoredox catalyzed decarboxylative radical cascade annulation of 2-
(allyloxy)arylaldehydes and N-(acyloxy)phthalimides. The reaction proceeds through the intermediacy of radicals and the products
were obtained in moderate to good yields.
Institute and/or researcher Twitter usernames: @SM_IITJ
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