Br?nsted Base Assisted Photoredox Catalysis: Proton Coupled Electron Transfer for Remote C?C Bond Formation via Amidyl Radicals

Article abstract of DOI:10.1002/chem.201802907

The synthesis of alkyne- and alkene-decorated lactams has been achieved through a photoredox-initiated radical cascade reaction. The developed Br?nsted base assisted, photoredox-catalyzed, intramolecular 5-exo-trig cyclization/intermolecular radical addit

Full text of DOI:10.1002/chem.201802907

A Journal of
Accepted Article
Title: Brønsted base assisted photoredox catalysis: Proton coupled
electron transfer for remote C-C bond formations via amidyl
radicals
Authors: Jiaqi Jia, Yee Ann Ho, Raoul F. Bülow, and Magnus Rueping
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To be cited as: Chem. Eur. J. 10.1002/chem.201802907
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COMMUNICATION
Brønsted base assisted photoredox catalysis: Proton coupled
electron transfer for remote C-C bond formations via amidyl
radicals
[a]
[a]
[a]
[a,b]
Jiaqi Jia, Yee Ann Ho, Raoul F. Bülow and Magnus Rueping*
Abstract: The synthesis of alkyne and alkene decorated lactams
has been achieved through a photoredox initiated radical cascade
reaction. The developed Brønsted base assisted photoredox
catalyzed intramolecular 5-exo-trig cyclization/intermolecular radical
addition/elimination reaction provides facile access to functionalized
γ-lactams, with good functional group tolerance and high yields.
substrate is activated through a photoredox catalyst (Scheme 1
[
7-9]
top). The amidyl radicals
obtained thereby have been
successfully applied in 1,5-hydrogen atom transfer reactions
[
7d-f]
with subsequent functionalization of the γ-position
intramolecular cyclization reactions.
and in
[
7a-c]
The cyclization reactions
designed to date, involve reduction of the resulting radical
intermediate or addition to a radical acceptor. This however
inevitably decreases the number of functional groups on the final
product that could serve as site for subsequent functionalization.
In order to retain the functional group introduced with the radical
acceptor, a suitable radical leaving group on the radical acceptor
might be a feasible solution. For example, the presence of a
γ-Lactams (pyrrolidin-2-ones) constitute an important class
of biologically active compounds which are particularly relevant
[1]
for the pharmaceutical and medicinal chemistry. The presence
of the γ-lactam core structure in a wide range of naturally
occurring as well as a multitude of synthetic bioactive molecules,
has stimulated the development of improved methods which
sulfonyl group in the acceptor, would lead to the product while
[10,11]
releasing a sulfonyl radical,
which would continue its role for
[2]
provide access to this valuable structural motif. Ring expansion
of β-lactams, intramolecular cyclizations, cycloaddition reactions
and intermolecular cascade or sequential reactions constitute
[11h]
the photocatalyst regeneration,
while retaining the functional
group of the reactant (Scheme 1 bottom).
[
2]
the main general approaches for the synthesis of γ-lactams.
With these considerations in mind, we started our
[
12]
Well-established cyclization methods for the synthesis of this
important class of N-heterocycles include intramolecular
cyclization of amino acid derivatives via amide bond formation
and intramolecular cyclization of amides either via C-C bond
formation or via N-alkylation with N-C bond formation.
Approaches based on N-C bond formation comprise basically
the intramolecular N-alkylation of substrates bearing a suitably
located leaving group and the addition of amidyl radicals to
research,
aiming at the synthesis of functionalized γ-lactam
derivatives. Herein, we report our successful development of
Brønsted base assisted visible light photoredox catalyzed
intramolecular 5-exo-trig cyclization / intermolecular radical
addition / elimination cascade reaction.
[
3]
pendant alkenes. Amidyl radicals have attracted increasing
attention, in particular as highly reactive intermediates for the
synthesis of N-heterocyclic structures. Traditionally, amidyl
radicals can be accessed via reductive cleavage of N-X (X = Cl,
Br, I), N-N, N-C, N-O or N-S bonds under harsh reaction
conditions or via oxidative cleavage of N-H amide bonds in the
presence of strong oxidants. Nevertheless, with the recent
[
4]
increasing advances in photoredox catalyzed reactions,
a
more convenient and practical alternative for the homolytic
cleavage of N-H bonds in amides has appeared. Neutral, N-
centered amidyl radicals have been elegantly obtained through a
[
5,6]
proton-coupled electron transfer (PCET)
process in which the
Scheme 1. Proton-coupled Electron Transfer (PCET) as promoter for cascade
reactions.
[a]
M. Sc.J. Jia, M. Sc. Y. A. Ho, M. Sc. R. F. Bülow, Prof. Dr. M.
Rueping
Initial experiments were conducted with substrates 1a and
Institute of Organic Chemistry, RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
E-mail: magnus.rueping@rwth-aachen.de
Prof. Dr. M. Rueping
King Abdullah University of Science and Technology (KAUST)
KAUST Catalysis Center (KCC), Thuwal, 23955-6900 (Saudi
Arabia);
E-mail: magnus.rueping@Kaust.edu.sa
Supporting information for this article is given via a link at the end of
the document.
2
a, catalyst 4a and NBu
different polarity (Table 1, entries 1-4) and the desired product
a could be obtained in up to 62% when the reaction was
performed in toluene (entry 1). However, in all these cases
entries 1-4) side reactions resulted in impurities that were
4 2 2
(OMe) PO as base in solvents of
3
[
b]
(
inseparable from the desired product. When switching to DMF
as solvent, no reaction occurred (Table 1, entry 5). When
trifluorotoluene was used instead,
[]
a clean reaction was
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observed, giving the desired product 3a in 66% yield (Table 1,
entry 6). Thus, further optimization was performed in
trifluorotoluene as solvent.
give any conversion (Table 1, entries 7-8). The reaction hardly
occurred after switching to organic bases, as both pyridine and
2,6-lutidine gave no conversion (Table 1, entries 9-10), and
DMAP provided a low yield of 34% (Table 1, entry 11). This
confirmed the importance of the Brønsted bases for the
homolytic N-H bond activation. Different photocatalysts were
evaluated as well in order to improve the yield further.
Interestingly, apart from photocatalyst 4a all other catalysts
proved inactive (Table 1, entries 12-15). This could be due to the
redox potentials of the photocatalysts in which only
photocatalyst 4a is a perfect match for the intermediary
amide/base pair. Subsequently the influence of both reaction
time and amount of base on the reaction outcome was
evaluated and the results are summarized in Table 2.
Decreasing the amount of base from 0.4 to 0.2 equiv., led to a
decrease in the reaction efficiency and the product was obtained
in a lower yield of 43% (Table 2 entry 2 vs. entry 1). The
efficiency increased dramatically upon extending the reaction
time from 16 to 24 hours, providing the product in 82% yield
[
a]
Table 1. Optimization of reaction condition.
(
Table 2, entry 3). The yield increased slightly to 88% and 89%
with increasing the amount of base to 0.6 and 1.0 equivalent
respectively (Table 2, entries 4-5). Furthermore, when 1.1 equiv.
of phosphate base was used, the yield increased to 92% (entry
6). The model reaction ran smoothly also on a 0.2 mmol scale,
providing the desired product 3a in 88% yield (entry 7).
[
a]
Table 2. Effect of base amount and reaction time on reaction efficiency.
Entry
Solvent
Base (x equiv.)
PC 4
Yield
[b]
(
%)
1
2
3
4
5
6
7
8
9
Toluene
THF
NBu
NBu
NBu
NBu
NBu
NBu
4
4
4
4
4
4
(OMe)
(OMe)
(OMe)
(OMe)
(OMe)
(OMe)
2
2
2
2
2
2
PO
PO
PO
PO
PO
PO
2
2
2
2
2
2
(0.4)
(0.4)
(0.4)
(0.4)
(0.4)
(0.4)
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4e
62
38
27
56
0
[
b]
Entry
NBu
4
(OMe)
2
PO
2
(x equiv)
Time (h)
Yield (%)
1
2
3
4
5
6
0.4
0.2
0.4
0.6
1.0
1.1
1.1
16
16
24
24
24
24
24
66
43
82
88
89
DCM
Acetone
DMF
PhCF
PhCF
PhCF
PhCF
PhCF
PhCF
PhCF
PhCF
PhCF
PhCF
3
3
3
3
3
3
3
3
3
3
66
0
K
3
PO
4
(2.0)
(2.0)
[
c]
92
2
Cs CO
3
0
[
d]
[c]
7
88
Pyridine (0.4)
0
[
a] Standard conditions: 1a (0.1 mmol), 2a (2 equiv) photocatalyst 4a (1
mol%), NBu OP(O)(OMe) (x equiv), PhCF (1 mL), 24 h, under Ar at 25 °C,
irradiation with 12 W blue LEDs. [b] NMR yield of 3a with CH Br as internal
standard. [c] Yield after purification. [d] Reaction with 0.2 mmol 1a.
10
11
12
13
14
15
2,6-Lutidine (0.4)
DMAP (0.4)
0
4
2
3
34
0
2
2
NBu
NBu
NBu
NBu
4
4
4
4
(OMe)
(OMe)
(OMe)
(OMe)
2
2
2
2
PO
PO
PO
PO
2
2
2
2
(0.4)
(0.4)
(0.4)
(0.4)
With the optimized conditions in hand, the scope of the
reaction with respect to the structure of both substrates was
investigated (Table 3). Good yields were obtained for a variety of
different substrates 1b-i upon varying the aryl substituent of the
amide group. Various substrates bearing either an electron-
withdrawing or an electron-donating group in the para-position of
the aryl group are tolerated under the reaction conditions. It is
noteworthy that functional groups such as Br, ester, ketone and
ether could be tolerated under the reaction conditions and gave
products also in good yields.
0
0
0
[
(
a] Standard conditions: 1a (0.1 mmol), 2a (2 equiv.), photocatalyst (PC) 4
1 mol%), base (x equiv.), solvent (1 mL), 16 h, under Ar at 25 °C,
2 2
irradiation with 12 W blue LEDs. [b] NMR yield of 3a with CH Br as
internal standard.
Next, the effect of various bases on the reaction outcome was
evaluated. Inorganic bases such as K PO and Cs CO did not
3
4
2
3
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[
a],[b]
[13]
Table 3. Substrate scope.
carbamates proved also to be compatible for such a reaction.
As shown in Table 3, good to high yields could be obtained (3p-
r) for the reaction with terminal and internal alkenes. An urea
derivative was also tested and gave the product 3s. These
examples emphasize the applicability of this method to diverse
substrates, leading to products with multiple functional groups,
which could be further manipulated for different purposes.
Scheme 2. Extension of substrate scope: alkenes as radical acceptor.
Furthermore, the reaction is applicable to alkenes as radical
acceptor. When E-alkene 5a was used, the product 6a was
obtained as a mixture of Z- and E-alkene in a ratio of 9:1, as E/Z
alkene isomerism could readily occur under photocatalytic
[
10]
conditions (Scheme 2a). When terminal alkene 5b was used
as radical acceptor instead, the reaction was clean, giving only
product 6b (Scheme 2b). These examples show that alkenes
can also be employed as a radical acceptor for this methodology.
With regard to the whole substrate scope a wide variety of
products bearing different functional groups could be obtained in
good to high yields. The method gives access to different
lactams, cyclic carbamates and urea derivatives that have not
been reported before and which are the potential for further
derivatization.
[
a] Standard conditions: 1 (0.2 mmol), 2 (2 equiv), photocatalyst (PC) 4a (1
mol%), NBu OP(O)(OMe) (1.1 equiv), PhCF (2 mL), 24 h, under Ar at 25 °C,
4
2
3
irradiation with 12 W blue LEDs. [b] Yield after purification.
The reaction also proceeded well when the terminal alkene
was changed to an internal alkene (1j), providing the
corresponding product 3j in 80% yield. Good to high yields were
obtained also by switching to other alkyne substrates. Alkyne 2b
having a methyl group in the meta-position of the aryl group,
afforded the product 3k in 64% yield. For alkynes 2c-f bearing
aromatic groups with substituents in the para-position, the yields
were high (3l-o, 67-88%) for both electron-rich as well as
electron-poor radical acceptors. In addition to amide substrates,
Scheme 3. Proposed mechanism of the visible light photoredox catalyzed
intramolecular 5-exo-trig cyclization
elimination reaction.
/ intermolecular radical addition /
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Regarding the reaction mechanism, a plausible proposal is
depicted in Scheme 3. With the aid of a suitable Brønsted base,
the amide 1a can undergo SET, cleaving the N–H bond
homolytically to generate an amidyl radical. Intramolecular
addition to the pendant olefin results in the formation of a γ-
lactam bearing an alkyl radical. Alkyne 2a can intercept this
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radical
addition/elimination mechanism extruding •SO
the •SO Ph radical can accept one electron from [Ir ],
regenerating the catalyst. The resulting phenyl sulfonyl anion
intermediate to form
product
3a
via an
2
Ph. Subsequently,
[7]
For examples of cleavage of amide N−H bonds by PCET, see: a) G. J.
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2

+
(
2
PhSO ) is protonated by the [B-H] to yield benzene sulfinic
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[
8]
9]
In summary, an efficient method for the synthesis of alkyne
and alkene decorated lactams by employing a Brønsted base
assisted visible light photoredox catalyzed intramolecular 5-exo-
trig
cyclization/intermolecular
radical
addition/elimination
2017, 2108; c) K. Wu, Y. Du, T. Wang, Org. Lett. 2017, 19, 5669.
reaction was developed. The feasibility to retain functional
groups on the radical acceptor by introducing a sulfonyl group is
successfully proven by the products obtained. The method
developed is widely applicable, affording products in good to
high yields for a broad range of substrates, with high functional
group tolerance starting from readily available amides. This
provides the first reported access to these core structures, which
will be useful for further functionalization into valuable fine
chemicals. Further attempts of exploring such concepts are
currently ongoing in our laboratories and will be reported in due
course.
[
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Y. A. Ho would like to thank Deutscher Akademischer
Austauschdienst (DAAD) for
a doctoral scholarship. The
research leading to these results has received funding from the
European Research Council under the European Union's
Seventh Framework Programme (FP/2007-2013) / ERC Grant
Agreement no. 617044 (SunCatChem).
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Keywords: photoredox catalysis • Brønsted base • lactam
derivatives • sulfonyl radical • PCET
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[
5757.
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Entry for the Table of Contents (Please choose one layout)
FULL PAPER
J. Jia, Y. A. Ho, R. F. Bülow, M. Rueping
Brønsted base assisted photoredox
catalysis: Proton coupled electron
3
transfer for remote sp -alkylations via
amidyl radicals
Text for Table of Contents
Page No. – Page No.
A Brønsted base assisted photoredox catalyzed intramolecular 5-exo-trig cyclization / intermolecular
radical addition / elimination cascade reaction which provides facile access to a broad range of
functionalized γ-lactams, with good functional group tolerance and good to high yields, was developed.
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