Trichloromethylative Olefin Cycloamination by Photoredox Catalysis

Article abstract of DOI:10.1002/ejoc.202100898

The trichloromethyl group is an important functionality. It is a useful synthetic moiety and appears in the structures of natural and biologically active compounds; hence, developing synthetic methods for its introduction into structural motifs is an important objective. Herein, we report an efficient chemistry for the trichloromethylations and cycloaminations of olefin. These reactions are realized in a single step and triggered by the addition of the trichloromethyl radical, generated from bromotrichloromethane by photoredox catalysis, to a double bond, with the resulting intermediate trapped through intramolecular reaction with an amino group. This process facilitates the construction of N-heterocycles having trichloromethyl units and tetrasubstituted carbons from simple compounds under mild conditions.

Full text of DOI:10.1002/ejoc.202100898

Communications
doi.org/10.1002/ejoc.202100898
Trichloromethylative Olefin Cycloamination by Photoredox
Catalysis
Shinji Harada,*[a, b] Ryuya Masuda,[a] Takahiro Morikawa,[a] and Atsushi Nishida[a]
The trichloromethyl group is an important functionality. It is a
useful synthetic moiety and appears in the structures of natural
and biologically active compounds; hence, developing synthetic
methods for its introduction into structural motifs is an
important objective. Herein, we report an efficient chemistry for
the trichloromethylations and cycloaminations of olefin. These
reactions are realized in a single step and triggered by the
addition of the trichloromethyl radical, generated from bromo-
trichloromethane by photoredox catalysis, to a double bond,
with the resulting intermediate trapped through intramolecular
reaction with an amino group. This process facilitates the
construction of N-heterocycles having trichloromethyl units and
tetrasubstituted carbons from simple compounds under mild
conditions.
The carbon atom in the trichloromethyl (CCl3) functional group
is in oxidation state of a carboxylic acid; hence, CCl3 is the
synthetic equivalent of a carboxylic acid or its derivatives. The
unique characteristics of CCl3 make it attractive for the develop-
ment of new reactions,[1–4] with CCl3 having been used in
intermediates in the syntheses of natural products.[5–6] The
trichloromethyl group itself has also been observed in the
structures of natural and biologically active compounds;[7–8]
consequently, developing methods for the introduction of the
trichloromethyl unit is a significant objective. Hence, several
methodologies have been reported,[9–10] including ones that use
*
trichloromethyl radicals ( CCl3) generated from bromotrichloro-
methane (BrCCl3) by visible-light photoredox catalysis.[11]
On the basis of prior work, introducing a trichloromethyl
group onto one carbon of a double bond with an amino group
Scheme 1. Introducing a trichloromethyl group and an N-functional group
tethered to the other carbon is expected to be an efficient and
practical synthetic strategy for the preparation of diverse amine
derivatives from simple compounds.[12] Azidotrichlorometh-
ylation is the example of such a reaction (Scheme 1).[13] While
azide is a synthetically useful nitrogen functional group, the
substrate scope of this reaction remains limited, requiring high
temperature conditions or a strong oxidant; these factors
to a double bond.
narrow the applicability of the reaction. Moreover, to the best
of our knowledge, there is only one report on the direct
introduction of an amino group,[14] and there still remains issues
such as the use of two equivalents of diazonium salt and the
need for a solvent amount of chloroform as a trichloromethyl
source. Thus, we envisaged the development of a new method
for the trichloromethylative amination of an olefin that
especially focused on the simultaneous construction of a
nitrogen heterocycle, which presents new methodology for the
synthesis of useful molecular skeletons.[15] Herein we report
aminotrichloromethylation chemistry that constructs nitrogen-
containing heterocycles and is triggered by the addition of
[a] Prof. Dr. S. Harada, R. Masuda, Dr. T. Morikawa, Prof. Dr. A. Nishida
Graduate School of Pharmaceutical Sciences
Chiba University
1-8-1 Inohana, Chuo-ku, Chiba 2608675, Japan
E-mail: s.harada@faculty.chiba-u.jp
[b] Prof. Dr. S. Harada
Molecular Chirality Research Center
Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 2638522, Japan
Supporting information for this article is available on the WWW under
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Table 1. Screening conditions.
Entry
Catalyst
Base
Solvent
Time
Yield [%]
1
2
3
Ir[(dtbbpy)(ppy)2](PF6)
Ir(ppy)3
(Ir[dF(CF3)(ppy)]2
Li2CO3
Li2CO3
Li2CO3
CH2Cl2/H2O[a]
CH2Cl2/H2O[a]
CH2Cl2/H2O[a]
15 min
25 min
25 min
66
68
70
(dtbpy))(PF6)
4
5
6
7
8
9
10
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Li2CO3
K2CO3
K2HPO4
–
Li2CO3
Li2CO3
Li2CO3
CH2Cl2/H2O[a]
CH2Cl2/H2O[a]
CH2Cl2/H2O[a]
CH2Cl2/H2O[a]
DMF
15 min
30 min
30 min
24 h
15 min
15 min
30 min
73
71
74
69
68
71
50
DMSO
MeCN
[a] A 1:1 CH2Cl2:H2O mixture was used.
Table 2. Substrate scope of the aromatic ring.[a]
trichloromethyl radicals under mild reaction conditions by
visible-light photoredox catalysis.
The reaction conditions were screened using styrene
derivative 1a, which contains an amino group in its side chain
(Table 1). Using Ir complexes as photoredox catalysts and Li2CO3
as the Brønsted base, 1a was mixed with BrCCl3 and irradiated
with visible light from blue LEDs; 1a disappeared in 15 min to
give the desired product 2a in 66% yield (entry 1), with no
other isolable product obtained. All screened photoredox
catalysts provided comparable results (entries 2–4); however,
Ru(bpy)3(PF6)2 exhibited slightly better reactivity (and is less
expensive) than the other catalysts; hence, chose to screen this
Ru complex further.
The base was next investigated (entries 5 and 6), with
comparable results obtained for the bases examined. Li2CO3
was chosen as optimal because it led to the shortest amount of
time for 1a to disappear. This reaction also proceeded in the
absence of a base to give 2a in 69% yield, although a
significantly longer reaction time was required (entry 7). We
finally examined the solvent (entries 8–10). The use of DMF or
DMSO led to slightly lower yields of 2a (entries 8 and 9 vs.
entry 4), while the use of MeCN led to a low 50% yield
(entry 10). Based on these results, the conditions in entry 4
were determined to be optimal. In all cases, we have stopped
the reaction when the substrate disappeared, setting 24 h as
the maximum. It is noteworthy that the solvent needs to be
degassed otherwise the double bond isomerizes, oxygenation
occurs, and a lower yield of the desired product is obtained.[16]
To examine substrate scope, we first investigated the
substituents on the benzene ring of the styrene unit. The target
product 2b was obtained in 38% yield when the unsubstituted
1b was subjected to the reaction conditions (Table 2), while 1c
bearing a CF3 group at the para-position gave the target
product 2c in 24% yield. These results suggest that the electron
density on the benzene ring affects the reactivity. Meta-meth-
oxy-substituted 1d afforded 2d in 55% yield. The reaction also
[a] 0.3 mmol of 1 were used. [b] 500 mg (1.78 mmol) of 1a were used.
proceeded using naphthyl derivative 1e and furan 1f to afford
2e and 2f in yields of 46% and 38%, respectively. We
confirmed that a subgram-scale reaction using 1a also gave
comparable results with a little longer reaction time.
We then examined substrates with various substituents on
nitrogen (Table 3). When benzyl-substituted 1g was subjected
to the reaction conditions, 1g disappeared in 2 h to give 2g in
66% yield. Sulfonylated (Ts and Ms) derivatives 1h and 1i
required significantly extended reaction times to afford the
target products 2h and 2i in yields of 72% and 16%,
respectively, while the Dpp-substituted substrate 1j gave the
target product 2j in 51% yield. When substrates with acyl
protecting groups were examined, the target products were
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49% yield; however, a longer reaction time was required in this
Table 3. Scope of the substituent on nitrogen.
case, as the formation of the seven-membered ring is entropi-
cally less-favored.[18–19]
The construction of the morpholine ring was examined
using substrate 1q, which had not disappeared even after
reacting for 7 h. At that point, a further 1.3 equiv. of BrCCl3 was
added, which led to the disappearance of 1q within 15 min.
Based on this result, we concluded that the reaction would
proceed smoothly through the slow addition of BrCCl3;
consequently, we dripped a solution of BrCCl3 in CH2Cl2 into the
irradiated reaction mixture with a syringe pump over 2 h. Under
these conditions, 1q was completely absent at the end of the
slow addition, and 2q was obtained in 52% yield.
Two catalytic cycles operate in redox catalysis. One is the
oxidative quenching cycle, in which the catalyst undergoes
one-electron oxidation, and the other is the reductive quench-
ing cycle, in which the catalyst undergoes one-electron
reduction. A catalytic cycle involving Ru(bpy)3(PF6)2 (=Ru(II)) in
this reaction was proposed based on the redox potentials of
the compounds involved (Figure 1). In this reaction, the
ground-state Ru(II) catalyst is excited by irradiation with visible
light to form Ru(II)* (E1/2II*/III=À 0.81 V vs. SCE),[20] which is
capable of reducing BrCCl3 (E1/2 =À 0.18 V vs. SCE).[21] On the
other hand, the electrochemical potentials of secondary amines
range between +0.89 and +0.94 V vs. SCE (or higher),[22] which
are very close to the redox potential of Ru(II)* (E1/2II*/I= +
0.77 V vs. SCE);[20] consequently, the reductive quenching path
becomes difficult. By comparing the differences in redox
potentials, we conclude that the single-electron reduction of
BrCCl3 by Ru(II)* is preferred; therefore Ru(II)* is involved in the
oxidative quenching cycle.[11e,23–24]
[a] The corresponding ammonium chloride 1m was used as the substrate
and 3.0 equiv. of Li2CO3 was used.
obtained in yields of 59% (2k, Bz group) and 41% (2l, Ac
group). The reaction was also carried out using ammonium salt
1m devoid of a formal protecting group; target product 2m
was obtained in 59% yield when the amount of base was
increased to 3.0 equiv.
We next investigated the construction of nitrogen-contain-
ing rings other than simple pyrrolidines (Table 4). Substrate 1n
bearing two geminal Me groups was used in the expectation
that the reaction rate would be accelerated through the
Thorpe-Ingold effect,[17] however, no such acceleration was
observed, with the target product 2n obtained in 75% yield in
2 h. We also used 1o, the one-carbon homologue of 1a, which
led to the formation of piperidine 2o in 66% yield. By analogy,
the two-carbon homologue 1p led to azepane product 2p in
Based on the results presented above, we propose a
complete mechanism for the trichloromethylative cycloamina-
tion of an olefin that proceeds through an oxidative quenching
cycle (Figure 2). First, Ru(II) is excited by irradiation with blue
light to Ru(II)*. The single-electron reduction of BrCCl3 by Ru(II)*
*
generates the trichloromethyl radical ( CCl3), which is intermo-
lecularly trapped by substrate 1 to form benzyl radical 3, after
Table 4. Scope of the heterocycles.
[a]
BrCCl3 (1.3 equiv., 0.5 M in CH2Cl2) was added dropwise by syringe
Figure 1. Redox potentials and quenching paths for an activated ruthenium
pump for 2 h.
complex.
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Figure 2. Proposed mechanism for the trichloromethylative cycloamination
of an olefin.
which single-electron oxidation by Ru(III) generates ground-
state Ru(II) and benzyl cation 4.[11e] Facile intramolecular
nucleophilic attack (cyclization) by the nitrogen atom delivers
product 2.
Scheme 2. Derivatizing the product.
Finally, we investigated converting the trichloromethyl Conflict of Interest
moieties in the products (Scheme 2). Elimination of two
chlorides in 2a by two-cycles of KHMDS treatment afforded
chloroalkyne 5 in quantitative yield.[9j] Chloroalkyne 5 was then
lithiated with n-butyl lithium, and the resulting lithium acetylide
6 was protonated to form terminal alkyne 7 in 72% yield.
Intermediate 6 was also converted into tertiary alcohol 8 in
64% yield (from 5) by the addition to benzophenone.
The authors declare no conflict of interest.
Keywords: Alkenes · Cyclization · Heterocycles · Photocatalysis ·
Trichloromethyl
In summary, we developed
a
photoredox-catalyzed
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Manuscript received: July 28, 2021
Revised manuscript received: August 6, 2021
Accepted manuscript online: August 9, 2021
Eur. J. Org. Chem. 2021, 4531–4535
www.eurjoc.org
4535
© 2021 Wiley-VCH GmbH