Palladium-catalyzed cascade 5-exo-trigradical cyclization/aromatic C-H alkylation with unactivated alkyl iodides

Article abstract of DOI:10.1039/d1ob00346a

A novel and convenient palladium-catalyzed cascade 5-exo-trigradical cyclization/aromatic C-H alkylation with unactivated alkyl iodides has been described. This strategy provides an efficient access to a variety of 3a-methyl-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrole derivatives, which facilitate access to a series of medically important heterocyclic bioactive molecules. This protocol involves mild catalytic reaction conditions and shows high functional group tolerance with high stereoselectivity. Mechanistic investigations reveal that an alkyl radical pathway is involved in this reaction.

Full text of DOI:10.1039/d1ob00346a

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Palladium-catalyzed cascade 5-exo-trig radical
cyclization/aromatic C–H alkylation with
unactivated alkyl iodides†
Cite this: Org. Biomol. Chem., 2021,
19, 2676
Received 24th February 2021,
Accepted 5th March 2021
Xi Sun,‡ Xu Dong, ‡ Yi Yang, Jiangchen Fu, Yan Wang, Zirui Li, Yuying Liu and
DOI: 10.1039/d1ob00346a
Hui Liu
*
rsc.li/obc
A novel and convenient palladium-catalyzed cascade 5-exo-trig ters.1h Despite the growing interest in molecules containing
radical cyclization/aromatic C–H alkylation with unactivated alkyl this skeleton, their synthesis and modification remain a chal-
iodides has been described. This strategy provides an efficient lenging task.
access to a variety of 3a-methyl-1,2,3,3a,4,8b-hexahydroindeno[1,2-
Scheme 2 summarizes the retrosynthetic analysis that led to
b]pyrrole derivatives, which facilitate access to a series of medically the successful strategy for the synthesis of hexahydroindeno
important heterocyclic bioactive molecules. This protocol involves pyrroles. Hexahydroindeno pyrrole A was retrosynthetically
mild catalytic reaction conditions and shows high functional group traced back to precursor B whose transformation to A would
tolerance with high stereoselectivity. Mechanistic investigations rely on alkylation of the aryl group.3 Precursor B was obtained
reveal that an alkyl radical pathway is involved in this reaction.
from the key radical intermediate C via 5-exo-trig cyclization.
On the basis of our research interest and previous work on
alkyl halides,4 we supposed that alkyl halides could react via
the single electron transfer process. With this conception in
mind, we designed alkyl halides D which might undergo
cascade radical cyclization to construct the desired skeleton
(Scheme 2).
In the past few years, transition-metal-catalyzed C–H alkyl-
ation has emerged as a versatile and important synthetic tool
throughout organic chemistry in preparing polycyclic carbo-
cycles and heterocycles.5 Recently, there has been emerging
interest in Pd radical involved reactions.6 In particular, there
has been great progress in palladium-catalyzed selective alkyl-
ation and numerous excellent research works have been
reported via the radical process.7 In 2015, Alexanian and co-
Functionalized hexahydroindeno pyrroles are important motifs
in many bioactive natural products and medicines, such as
alkyl substituted hexahydroindeno[1,2-b]pyrroles (for the treat-
ment of hyperglycemia), 8-carbaphysostigmine (a series of
potent acetylcholinesterase inhibitors), strigolactam GR-24 (for
seed germination stimulators and plant growth regulators),
γ-lactam derivatives, isatisine A (for the treatment of viral dis-
eases) and potential drugs from GlaxoSmithKline and other
companies (Scheme 1).1 Because of the special features listed
above, it is necessary to develop efficient protocols for the con-
struction of hexahydroindeno pyrroles. In the past few
decades, some elegant strategies have been described to con-
struct the hexahydroindeno pyrrole motif.2 In 1973, De and co-
workers developed a total synthesis of hexahydroindeno[1,2-b]
pyrroles via multi-synthetic steps.1b In 1992, Chen and co-
workers described a new synthesis of 1,2,3,3a,8,8a-hexahy-
droindeno[2,1-b]pyrrole 5-alkylcarbamates from an indene
derivate.1c In 1998, Harling and co-workers developed a syn-
thesis of 3-substituted hexahydroindeno[2,1-b]pyrroles in a
stereoselective manner in a high yield via an intermolecular
azomethine ylide cycloaddition.1d In 2020, Ogoshi and co-
workers reported a strategy to synthesize γ-lactam motifs that
contained more than two contiguous stereogenic carbon cen-
School of Chemical Engineering, Shandong University of Technology, 266 West
Xincun Road, Zibo, 255049, P. R. China. E-mail: huiliu1030@sdut.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00346a
Scheme 1 Hexahydroindeno pyrrole structural skeleton containing
‡Co-first authors.
bioactive molecules.
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catalyzed radical-mediated highly 5-exo-trig-selective cycliza-
tion/aromatic C–H alkylation with unactivated alkyl iodides to
efficiently deliver 3a-methyl-1,2,3,3a,4,8b-hexahydroindeno
[1,2-b]pyrrole derivatives, amenable to a diverse array of simple
unactivated aryl haloalkanes (Scheme 3d). Based on previous
work, 5-exo-trig cyclization is much more predisposed than 6-
endo-trig cyclization, which easily forms relatively stable sec-
ondary or tertiary radical intermediates.13 This also poses a
more severe challenge to our free radical mechanism.
To test our hypothesis, we initially investigated the radical
cascade cyclization of unactivated iodide 1a (Table 1). In the
beginning, we tested numerous conditions to obtain the
desired cyclization product 2a. Gratifyingly, the desired
product 2a was obtained in 60% yield using Pd(PPh3)4
(10 mol%) as a catalyst and Cs2CO3 (2.0 equiv.) as a base in
toluene (entry 1). However, the use of different palladium cata-
lysts, such as PdCl2(PPh3)2 and PdCl2(dppf), did not give posi-
tive results (entries 2 and 3). Following these results, we con-
tinued with further exploration of the catalyst. To our delight,
when Pd2(dba)3 was used, the yield was dramatically increased
to 81% (entry 4). Then a series of monodentate ligands and
bidentate ligands were examined for this protocol (entries
5–9). However, the use of ligands, such as Binap, PPh3, dppf,
PCy3 and Xantphos, could not promote the yield of this radical
cyclization.
Scheme 2 Design plan for the hexahydroindeno pyrroles.
workers developed a palladium-catalyzed intramolecular C–H
alkylation of arenes with unactivated alkyl halides to deliver a
number of valuable carbocyclic and heterocyclic products
(Scheme 3a).8 In 2014, Zhou and co-workers reported a palla-
dium-catalyzed intermolecular alkylation of heterocycles with
unactivated alkyl halides, amenable to a series of secondary
and tertiary alkyl halides (Scheme 3b).9 In 2018, Zhou and co-
workers described a palladium-catalyzed intermolecular para-
selective alkylation of electron-deficient arenes (Scheme 3c).10
Our laboratory is focusing on developing novel palladium
radical involved reactions with unactivated alkyl iodides.11 In
2012, Jiang et al. developed a palladium-catalyzed atom trans-
fer radical cyclization of unactivated alkyl iodides.12 In 2016,
our group described the first palladium-catalyzed 6-endo-selec-
tive alkyl-Heck radical cyclization of unactivated alkyl iodi-
des.11a In 2017, a palladium-catalyzed 5-exo-selective inter-
molecular radical hydrocarbonation of unactivated alkyl
iodides was also realized.11b In continuation of our interest in
developing new methodologies for the construction of hexahy-
droindeno pyrrole scaffolds, herein, we reported a palladium-
Subsequently, various organic bases and inorganic bases
were evaluated (entries 10–13), indicating that Cs2CO3 was the
best choice. Finally, a series of solvents, such as THF, MeCN
and dioxane, were screened; however, no positive results were
obtained (entries 14–16). On the basis of these results, the
optimal conditions were found to include Pd2(dba)3 combined
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst
Ligand
Base
Solvent
Yieldb
1c
2c
3c
4
5
6
7
8
9
10
11
12
13
14
15
16
Pd(PPh3)4
PdCl2(PPh3)2
PdCl2(dppf)
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
—
—
—
Ph2PCy
Binap
PPh3
dppf
PCy3
Xantphos
Ph2PCy
Ph2PCy
Ph2PCy
Ph2PCy
Ph2PCy
Ph2PCy
Ph2PCy
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
KOtBu
Cy2NMe
DIPEA
Cs2CO3
Cs2CO3
Cs2CO3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
60%
NR
NR
81%
44%
75%
20%
57%
ND
57%
60%
30%
17%
55%
42%
53%
MeCN
1,4-Dioxane
a Reaction conditions: 1a (0.2 mmol), catalyst (5 mol%), ligand
(30 mol%), Cs2CO3 (0.4 mmol), toluene (2 ml), nitrogen atmosphere,
12 h, 120 °C. b Isolated yields. c [Pd] (10 mol%, 0.02 mmol).
Scheme 3 Palladium-catalyzed C–H alkylations of aromatic substrates
with unactivated alkyl halides.
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with Ph2PCy and Cs2CO3 in toluene at 120 °C (entry 4). With yields (Table 2, entries 2–6). However, the substrates bearing
the optimized conditions in hand, we then turned our atten- electron-withdrawing substituents on the phenyl ring exhibited
tion to the radical cyclization reaction scope and the results relatively poor reactivity, delivering the desired products in
are presented in Table 2. In general, the palladium-catalyzed relatively low yields (Table 2, entries 7–9). Furthermore, para-
radical cascade 5-exo-trig cyclization/aromatic C–H alkylation and ortho-substituted substrates could react smoothly to afford
can be efficiently achieved with a considerably wide range of the final cyclization products. We then attempted the reaction
N-(2-iodoethyl)-4-methyl-N-(2-methyl-1-phenylallyl)benzenesul- of meta-substituted substrates under the standard conditions.
fonamides, delivering a series of hexahydroindeno pyrrole However, the corresponding cyclization product could not be
derivatives 2 in moderate to high yields. The substrates obtained in this reaction. Finally, the oxygen tethered product
bearing electron-donating groups on the phenyl ring, such as 2k was synthesized and examined, while we could only isolate
methyl, ethyl, methoxy, etc., were well tolerated in this trans- a complex including multiple compounds which could not be
formation and provided the desired products in excellent determined by NMR and GC-MS (Table 2, entry 11).
In order to gain insight into the mechanism of this proto-
col, we examined this cyclization in the presence of TEMPO
(2,2,6,6-tetramethylpiperidine 1-oxyl) (Scheme 4). When
Table 2 Substrate scope of the 5-exo-trig cyclization/aromatic C–H
alkylationa
TEMPO (3.0 equiv.) was subjected to the standard reaction
conditions, the reaction was totally inhibited, indicating that a
radical cyclization might be involved. Unfortunately, the
TEMPO adduct 6a was not isolated. On the other hand, BHT
(2,6-di-tert-butyl-4-methylphenol) was also subjected to the
standard conditions, and the cyclization was partially inhib-
ited, which delivered product 2a in 11% yield without the
detection of 7a.
Entry
1
Substrate 1
Product 2
Yieldb (%)
81
On the basis of the experimental result and previous
work,14 a possible radical mechanism for this cyclization is
proposed as shown in Scheme 5. Firstly, alkyl radical inter-
mediate 3 and Pd(I) species are generated via a single electron
transfer (SET) between 1a and Pd(0). Then, the addition of the
alkyl radical to the double bond of intermediate 3 generates
alkyl radical intermediate 4, which then adds to the aromatic
ring to deliver radical intermediate 5. Finally, radical inter-
mediate 5 undergoes single-electron oxidation and loss of one
proton to afford the final product 2a and regenerate the Pd(0)
catalyst.
In conclusion, we have developed a palladium-catalyzed
radical cascade 5-exo-trig cyclization/aromatic C–H alkylation
with unactivated alkyl iodides. This protocol can proceed
smoothly with a series of alkyl iodides and efficiently deliver
various 3a-methyl-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrole
derivatives, which is difficult to achieve via other methods. A
2
77
3
81
4
80
5
82
6
72
7
37
8
54
9
40
10
11
66
Complex
a Reaction conditions:
1 (0.2 mmol), catalyst (5 mol%), ligand
(30 mol%), Cs2CO3 (0.4 mmol), toluene (2 ml), nitrogen atmosphere,
12 h, 120 °C. b Isolated yield.
Scheme 4 Radical experiments for the mechanism.
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Shandong
Provincial
Natural
Science
Foundation
(ZR2018MB008 and ZR2018MB012), the Youth Innovative
Talents Attracting and Cultivating Plan of Colleges and
Universities in Shandong Province, and the Shandong
Provincial Key Research and Development Program of China
(2017GGX20131).
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