Atom-transfer radical cyclization of α-bromocarboxamides under organophotocatalytic conditions

Article abstract of DOI:10.1016/j.tetlet.2021.152952

The atom-transfer radical cyclization (ATRC) reaction gives halogenated heterocycles from the corresponding halocarbonyls that possess a C–C double bond. In contrast, the reductive ATRC reaction gives non-halogenated heterocycles in the presence of a redu

Full text of DOI:10.1016/j.tetlet.2021.152952

Tetrahedron Letters 69 (2021) 152952
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Atom-transfer radical cyclization of
organophotocatalytic conditions
a-bromocarboxamides under
⇑
Naoki Tsuchiya, Yusei Nakashima, Goki Hirata, Takashi Nishikata
Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The atom-transfer radical cyclization (ATRC) reaction gives halogenated heterocycles from the corre-
sponding halocarbonyls that possess a C–C double bond. In contrast, the reductive ATRC reaction gives
non-halogenated heterocycles in the presence of a reductant. In this research, we successfully control
Received 18 January 2021
Revised 15 February 2021
Accepted 18 February 2021
Available online 23 February 2021
the ATRC and reductive ATRC reactions of N-allyl-
a
-haloamides under visible-light irradiation in the
presence/absence of the Hantzsch ester as a reductant.
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Radical
Photocatalyst
Cyclization
Reduction
Introduction
Despite such progress in this area, no organo-photoredox-catalyzed
PC) ATRC reaction of N-allyl- -haloamides has been established
(Fig. 1C).
We recently reported the organo-photoredox-catalyzed atom-
transfer radical substitution reaction (ATRS) [17] between styrenes
(
a
One of the most useful synthetic methodologies for the
preparation of cyclic compounds is the atom-transfer radical
cyclization (ATRC) reaction [1–3], which is a modification of the
Kharasch-type atom-transfer radical addition (ATRA) reaction [4].
and a-halocarbonyls as the tertiary alkyl source to produce tertiary
Among these processes, the ATRC reactions of N-allyl-
to synthesize -lactams have been investigated in detail due to the
number of bioactive compounds and synthetic intermediates that
contain substituted -lactams [5]. More specifically, N-allyl-
haloamides can be cyclized to give -lactams in the presence of
organotin reagents [6], Lewis acids [7], Cu [8], Ni [9], Fe [10], Pd
a
-haloamides
alkyl-substituted styrenes [18]. We found that an organophotore-
dox catalyst, N-Ph-phenothiazine (PTH), enabled the ATRC reaction
of N-allyl-a-haloamide to take place to yield a c-lactam. The corre-
sponding reductive ATRC reaction [19] was also observed in the
presence of a reductant. Thus, we herein report the ATRC and
reductive ATRC reactions of N-allyl-a-haloamides under visible-
c
c
a-
c
[
11], or Ru [12].
light irradiation.
Recently, visible-light photocatalysts have enabled a new
organic reaction methodology to be developed via a single electron
transfer process [13], which achieves greener organic reactions
without the requirement for heating. In ATRC chemistry, Martin
and co-workers were the first to report the visible light-promoted
ATRC reactions of unactivated alkyl iodides (Fig. 1A) [14,15]. Under
irradiation from blue light-emitting diodes (LEDs), alkyl iodides
possessing C–C multiple bonds underwent the ATRC reaction in
the presence of an Ir catalyst. Recent progress in the context of
Results and discussion
We initially investigated the ATRC of
as a standard substrate to optimize the reaction parameters
Table 1, and see also the Tables in the SI). Upon the evaluation
a-bromocarboxamide 1a
(
of a variety of organophotocatalysts that are typically employed
in photoredox reactions (entries 1–5), PTH was found to give the
desired ATRC product 2a in 71% yield, along with the correspond-
ing reductive ATRC product 3a in 6% yield (entry 1). Although
the ATRC reaction of N-allyl-a-haloamides includes the use of the
ball-milling technology (Fig. 1B) [16], whereby Cu (and a Ba salt)
initiate the ATRC reaction under mechanochemical conditions.
These reactions are generally completed within a few hours.
2
DMSO, MeOH, and H O/iPrOH provided good results for this
reaction (entries 1, 6, and 7), the reductive ATRC product 3 was
predominantly generated when NMP (N-methylpyrrolidinone)
was used as the solvent. It is possible that NMP acts as a hydrogen
donor in this reaction system. Thus, in an attempt to improve the
⇑
Corresponding author.
E-mail address: nisikata@yamaguchi-u.ac.jp (T. Nishikata).
2
ATRC/reductive ATRC product ratio, MgBr was employed as a
https://doi.org/10.1016/j.tetlet.2021.152952
040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
0

N. Tsuchiya, Y. Nakashima, G. Hirata et al.
Tetrahedron Letters 69 (2021) 152952
Scheme 1. Sunlight-induced ATRC of a-bromoamides.
Using the optimized conditions (Table 1, entry 9), the substrate
scopes for the ATRC and reductive ATRC reactions were examined
(Table 2). In the case of the ATRC, a-bromocarboxamides possess-
ing halogen atoms (i.e., 1b, 1c, and 1d) and electron-donating sub-
stituents (1e–1j) on their aryl groups were examined. Overall,
yields ranging from 67 to 80% were achieved, although iodoarene
1
d gave a lower 57% yield. In this case, unidentified products were
detected, which suggested that C–I cleavage may take place to pro-
mote radical side-reactions. In addition, the presence of N-benzyl
and tosyl groups (1k and 1l) resulted in smooth reactions under
the optimized conditions, although sterically-hindered bromides
(
1m–1o) underwent the ATRC reaction to give only low to good
yields of 2m–2o. For example, 1n, which possesses long alkyl
chains at the position to the carbonyl group resulted in a 52%
a
Fig. 1. ATRC reactions.
yield of 2n, while diethyl-substituted 1m gave only a 39% yield
of the corresponding product. In addition, the sterically-congested
N-methallyl-substituted 1o smoothly underwent the ATRC to give
Table 1
Visible light-induced ATRC of
a
-bromoamides.a
2o in 82% yield, while
methyl-crotyl group gave the corresponding elimination product
5) in 62% yield (Scheme 2). Moreover, an -bromocarboxamide
possessing a sec-alkyl moiety at the carbonyl -position (1p) gave
a-bromocarboxamide (4) possessing an N-3-
(
a
a
the expected ATRC product 2p in 52% yield with 2:98 (cis:trans)
selectivity.
We then examined the functional-group compatibility of the
Entry Variation from the standard conditions Yield of 2a/3a Ratio of
reductive ATRC process (Table 3). More specifically, a-bromocar-
b
(wavelength)
(%)
2a/3a
1
2
3
4
5
6
7
8
9
1
1
1
None
71/6
n.r.
n.r.
92/8
–
–
Table 2
d
a
PC = PDI , 450 nm
PC = Eosin Y, 525 nm
PC = Rose bengal, 525 nm
PC = BDNe
In MeOH
2
In H O/iPrOH = 9:1
In NMP
2
With MgBr (2 equiv)
Without PC
In the dark
With Hantzsch ester (1.5 equiv)
Substrate scope for the ATRC reaction.
n.r.
–
64/8
63/18
61/8
16/49
82/6 (72/2)c
n.r.
89/11
78/22
88/12
25/75
93/7
–
0
1
2
n.r.
–
8/80% (8/72)c 9/91
a
A mixture of 1a (0.50 mmol), 0.5 mol% of PTH, and DMSO (2 mL), was stirred at
room temperature with 365 nm LEDs for 24 h under N
2
.
b
The NMR yield was determined using 1,1,2,2-tetrachloroethane as an internal
standard.
c
d
e
Isolated yield.
0
PDI = N,N -Bis(2,6-diisopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide.
BDN = 1,4-bis(diphenylamino)naphthalene.
bromine source, although other bromine sources were also
examined, including ammonium bromide. However, it was found
that the ratio of products remained constant, although the overall
yield was slightly improved (entry 9). In this reaction, both the PC
process and the employed irradiation therefore appeared to be key
in determining the outcome. Indeed, in the absence of either of
these two components, the reaction did not proceed (entries 10
and 11). Moreover, when the Hantzsch ester was used as a
reductant, the reductive ATRC product was predominantly
obtained (entry 12). Interestingly, in this reaction, the use of sun-
light instead of LEDs was found to be effective, with 2a being
obtained in 72% yield after carrying out the ATRC for 48 h under
solar irradiation (Scheme 1).
a
2
A mixture of a-bromoamide 1 (0.50 mmol), PTH (0.5 mol%), MgBr (2.0 equiv),
and DMSO (2.0 mL), was stirred at room temperature with 365 nm LEDs for 24 h
under N
2
.
Scheme 2. Reaction using 4.
2

N. Tsuchiya, Y. Nakashima, G. Hirata et al.
Tetrahedron Letters 69 (2021) 152952
Table 3
a
Substrate scope for the reductive ATRC reaction.
Scheme 4. Derivatization of 2a.
Finally, we carried out a number of control experiments to
understand the reaction mechanism involved in this process. Inter-
estingly, we found that the reductive ATRC product was not
directly generated from the reaction of 1a in the presence of the
Hantzsch ester (Scheme 5A). This result indicated that no pri-
mary-alkyl radical species was generated from 2a under the reac-
tion conditions examined herein, and that an intermediate radical
was directly captured by the reductant. To confirm the involve-
ment of a single-electron transfer process, the reaction of 1a was
carried out in the presence of 1,4-dinitrobenzene (Scheme 5B).
No trace of product 2a was obtained, and instead, substrate 1a
was recovered in 55% yield (Other products were not able to iden-
tified), along with the 1,4-dinitrobenzene reagent (>99% yield, <1%
conversion). This reaction was also carried out in the presence of
the Hantzsch ester (Scheme 5C), and a similar result was obtained
a
A mixture of
a-bromoamide 1 (0.50 mmol), PTH (0.5 mol%), Hantzch ester
(
1.5 equiv), and NMP (2.0 mL), was stirred at room temperature with 365 nm LEDs
for 24 h under N
2
.
boxamides possessing halogen atoms (1c, 1d, 1r, and 1q) or elec-
tron donating substituents (1e–1j) on their aryl groups were exam-
ined. Overall, yields ranging from 70 to 78% were obtained,
although the presence of Cl- and methyl-substituted aryl groups
1e, 1f, and 1q) gave low yields. Similar to the case of the ATRA pro-
cess, -bromocarboxamides bearing N-benzyl and tosyl groups (1k
and 1l) smoothly reacted under the optimized conditions. We also
examined -bromocarboxamides possessing various lengths of
alkyl chains. More specifically, when the n-ethyl-substituted 1m
was used, 3m was produced in a low 34% yield. In contrast, n-pro-
pyl-substituted 1n gave the corresponding product in 86% yield.
Subsequently, to produce reductive ATRC products possessing vic-
(
a
a
(
i.e., 90% 1a, no product). But 1,4-dinitrobenzene was reduced to
the corresponding amine, and Hantzsch ester was converted to
the pyridine derivatives. These results supported the fact that the
single-electron reduction of 1a could proceed via a PC process. To
verify the effect of photoirradiation, the ATRC reaction of 1a was
monitored over time (Fig. 2). The reaction was found to be partic-
ularly efficient, with 56% of product 2a forming within 1 h. Impor-
tantly, when the LED light was switched off, the reaction stopped,
and when it was switched back on again, the reaction restarted.
This observation strongly suggests that PC plays a critical role in
this ATRC reaction, and that no radical chain transfer is involved.
inal quaternary carbon centers, we tested an a-bromocarboxamide
possessing a sterically-congested N-methallyl group (1o), and the
expected product 3o was obtained in 46% yield. When the reaction
was carried out using 1p, which possesses a sec-alkyl moiety at the
carbonyl
a-position, lactam 3p was obtained in 66% yield with a
high stereoselectivity. Interestingly, the reactivity of 2-bromoallyl
substituted 6 differed from those of substrate 1. More specifically,
this substrate underwent the ATRC reaction, but the subsequent
HBr elimination was rapid, resulting in the formation of 7 in 77%
yield (Scheme 3).
We then moved on to demonstrate that 2a can undergo three
different chemical transformations, as outlined in Scheme 4. More
specifically, the reaction of 2a in the presence of morpholine
resulted in the formation of aminated product 8 in 88% yield,
while the elimination reaction of 2a with DBU gave the corre-
sponding olefin 7 in 88% yield. Furthermore, the reaction of 2a
with arylboronic acid in the presence of a Ni catalyst resulted
in a primary-alkylative cross-coupling reaction to afford 9 in
4
4% yield.
Scheme 3. Reaction of 6.
Scheme 5. Control experiments.
3

N. Tsuchiya, Y. Nakashima, G. Hirata et al.
Tetrahedron Letters 69 (2021) 152952
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Declaration of Competing Interest
(
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
(
(
Acknowledgments
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o) M. Pattarozzi, F. Roncaglia, L. Accorsi, A.F. Parsons, F. Ghelfi, Tetrahedron 66
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We thank JST CREST (Grant Number JPMJCR18R4), JSPS
KAKENHI Grant Number JP20H04823 in Hybrid Catalysis for
Enabling Molecular Synthesis on Demand, Tokuyama Science
Foundation, the Naito Foundation and Yamaguchi University for
financial support.
2
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3
: M. Benedetti, L. Forti, F. Ghelfi, U.M. Pagnoni, R. Ronzoni Tetrahedron 53
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Appendix A. Supplementary data
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