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\begin{document}

\title{Synthesis of C2,3-vicinal diaminoindoles via a copper-iodine
co-catalytic strategy}



\author[1]{Wenhua Yu}%
\author[1]{Yingying Zhang}%
\author[1]{Xiaoxiang Zhang}%
\author[1]{Xiaoting Gu}%
\author[1]{Xirui Qing}%
\author[1]{Wanxing Wei}%
\author[1]{Zhuan Zhang}%
\author[1]{Guanghua Li}%
\author[1]{Taoyuan Liang}%
\affil[1]{Guangxi University}%


\vspace{-1em}



  
  \date{June 05, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
A one-pot synthesis of vicinal diamines using indoles, pyrazoles, and
phenothiazines in a tandem multi-component reaction is developed. The
utilization of a copper-iodine co-catalytic system enables the
generation of a diverse range of vicinal diaminoindoles with good
selectivity and moderate to good yields. An attractive aspect of this
method is that it can be conducted under mild and environmentally
friendly conditions, showcasing its potential as an alternative approach
for synthesizing vicinal diamines. Moreover, the use of a multicomponent
tandem reaction highlights the power and versatility of such strategies
in synthetic chemistry.%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2023} , \emph{41}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202300XXX}

Synthesis of C2,3-vicinal diaminoindoles \emph{via} a copper-iodine
co-catalytic strategy

Wenhua Yu,\textsuperscript{++,\emph{a}} Yingying
Zhang,\textsuperscript{++,\emph{a}} Xiaoxiang
Zhang,\emph{\textsuperscript{a}} Xiaoting
Gu,\emph{\textsuperscript{a}}Xirui Qing,\emph{\textsuperscript{a}}
Wanxing Wei,\emph{\textsuperscript{a}} Zhuan
Zhang,\emph{\textsuperscript{a,b,c}} Guanghua
Li,*\textsuperscript{,\emph{a,c,d}} and Taoyuan
Liang*\textsuperscript{,\emph{a,b,c}}

\textsuperscript{a} School of Chemistry and Chemical Engineering,
Guangxi University, Nanning, Guangxi 530004, P. R.
China\textsuperscript{b} Guangxi Key Laboratory of Electrochemical
Energy Materials, Nanning, Guangxi 530004, P. R.
China\textsuperscript{c} Guangxi Colleges and Universities Key
Laboratory of Applied Chemistry Technology and Resource Development,
Nanning, Guangxi 530004, P. R. China

\textsuperscript{d} Guangxi Key Laboratory of Petrochemical Resource
Processing \& Process Intensification Technology, Nanning, Guangxi
530004, P. R. China.\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Indoles \textbar{} C-H bonds diamination \textbar{} Vicinal diamines
\textbar{} Copper-iodine co-catalysis \textbar{} Multi-component
reaction\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
A one-pot synthesis of vicinal diamines using indoles, pyrazoles, and
phenothiazines in a tandem multi-component reaction is developed. The
utilization of a copper-iodine co-catalytic system enables the
generation of a diverse range of vicinal diaminoindoles with good
selectivity and moderate to good yields. An attractive aspect of this
method is that it can be conducted under mild and environmentally
friendly conditions, showcasing its potential as an alternative approach
for synthesizing vicinal diamines. Moreover, the use of a multicomponent
tandem reaction highlights the power and versatility of such strategies
in synthetic chemistry.\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Background and Originality Content

Vicinal diamines are highly important structural motifs found in a broad
range of biologically active compounds, natural products,
pharmaceuticals, and materials science.\textsuperscript{{[}1{]}} They
also serve as privileged ligands in organic and transition metal
catalysis, highlighting their
significance.\textsuperscript{{[}2{]}}Therefore, extensive research
efforts have been dedicated to developing synthetic strategies and
functionalizing these distinctive scaffolds.\textsuperscript{{[}3{]}}
Moreover, vicinal diamine-containing compounds, exemplified in Figure 1,
have demonstrated remarkable effects on biological
activities.\textsuperscript{{[}4{]}} For instance, compound (A) exhibits
exceptional antiemetic properties, while compounds (B) and (C) possess
potent inhibitory effects with respect to anti-arrhythmic and
antidepressant applications, respectively.

\textbf{Figure 1} Representative bioactive vicinal diamines.

Efficient synthetic methods for tandem C-N bond formation have been
extensively studied due to the widespread importance of compounds
containing such bonds. Traditional strategies involve nucleophilic
substitution reactions and metal-catalyzed C-N cross-coupling reactions
such as Buchwald-Hartwig, Ullmann-Goldberg, and Chan-Lam
reactions.\textsuperscript{{[}5{]}} However, these methods typically
require pre-functionalization of aromatic substrates into aryl halides
or aryl boric acids as coupling reagents. Catalytic diamination of
carbon-carbon double bonds has emerged as one of the most efficient and
straightforward approaches for synthesizing vicinal diamines, making it
a popular topic in synthetic organic
chemistry.\textsuperscript{{[}6{]}}Despite significant progress in
developing catalytic diamination methods, there are still notable
challenges and limitations that need to be addressed. Firstly, many of
these methods rely on the use of stoichiometric amounts of oxidants and
metal reagents, which can lead to cost, environmental, and safety
concerns.\textsuperscript{{[}7{]}}Additionally, most reported
diamination strategies are limited to symmetrical diamination reactions,
where two identical diamination reagents are incorporated into the same
molecule. Alternatively, specific diamination reagents must be
pre-synthesized in some cases.\textsuperscript{{[}8{]}} While there have
been advancements in asymmetric diamination reactions of alkenes,
achieving regioselective diamination with two readily available and
different diamination reagents is still a highly desirable goal.

In 2021, Zhao and colleagues reported a novel method for the
regioselective C-N bond formation between anilines and 1,2-diols (as a
mixture of diastereomers), enabling the efficient and diastereoselective
synthesis of vicinal diamines. This was accomplished through cooperative
ruthenium/Lewis acid catalysis or an effective chiral iridium/phosphoric
acid co-catalyzed process.\textsuperscript{{[}9{]}} Glorius' group later
presented a metal-free, photosensitized bifunctional diamination reagent
that exhibited excellent regio- and diastereoselectivity in the
synthesis of a range of previously inaccessible vicinal diamines (Scheme
1a).\textsuperscript{{[}10{]}} Additionally, a novel class of
bifunctional nitrogen-free radical precursors was developed, which
generated two \emph{N} -centered radicals with different reactivities
via an energy transfer process. However, intermolecular regioselective
diamination reactions of indole carbon-carbon double bonds have yet to
be explored.

As part of our continuous efforts to develop innovative methods for the
synthesis and functionalization of indoles,\textsuperscript{{[}11{]}} we
present our unexpected findings on a one-step, three-component
intermolecular C-H diamination reaction for the highly regioselective
synthesis of C2,3-diaminoindole. This reaction utilizes a copper-iodine
co-catalytic system (Scheme 1b). Notable features of this method
include:

1. The mild diamination reaction employs an inexpensive and readily
available copper catalyst, making it highly practical for broader
applications.

2. The approach exhibits excellent compatibility with various indoles
and demonstrates exceptional regioselectivity based on the different
reactivity of the utilized amines.

\textbf{Scheme 1} Previous work and our new observation.

Results and Discussion

\textbf{Table 1} Screening for optimal reaction
conditions.\emph{\textsuperscript{a}}\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Entry & Catalyst & Solvent & Additive & \textbf{4aaa}, Yield
(\%)\emph{\textsuperscript{b}}\tabularnewline
\midrule
\endhead
1 & CuI & DMF & & 10\tabularnewline
2 & CuCl\textsubscript{2} & DMF & & 0\tabularnewline
3 & Cu(OAc)\textsubscript{2} & DMF & & trace\tabularnewline
4 & Fe(OTf)\textsubscript{3} & DMF & & trace\tabularnewline
5 & Cu(OTf)\textsubscript{2} & DMF & & 52\tabularnewline
6 & Cu(OTf)\textsubscript{2} & DMSO & & 26\tabularnewline
7 & Cu(OTf)\textsubscript{2} & PhCl & & 30\tabularnewline
8 & Cu(OTf)\textsubscript{2} & DMA & & 34\tabularnewline
9 & Cu(OTf)\textsubscript{2} & 1,4-dioxane & & 0\tabularnewline
10 & Cu(OTf)\textsubscript{2} & p-xylene & & 15\tabularnewline
11 & Cu(OTf)\textsubscript{2} & DMF & & (38,
48)\emph{\textsuperscript{c}}\tabularnewline
12 & Cu(OTf)\textsubscript{2} & DMF & & (43,
58)\emph{\textsuperscript{d}}\tabularnewline
13 & Cu(OTf)\textsubscript{2} & DMF & AlCl\textsubscript{3} &
0\tabularnewline
14 & Cu(OTf)\textsubscript{2} & DMF & Zn(OTf)\textsubscript{2} &
41\tabularnewline
15 & Cu(OTf)\textsubscript{2} & DMF & NaOTf & 62\tabularnewline
\textbf{16} & \textbf{Cu(OTf)\textsubscript{2}} & \textbf{DMF/TFE} &
\textbf{NaOTf} & (57, \textbf{73},
12)\emph{\textsuperscript{e}}\tabularnewline
17 & Cu(OTf)\textsubscript{2} & DMF/1,4-dioxane & NaOTf &
57\tabularnewline
18 & Cu(OTf)\textsubscript{2} & DMF/HFIP & NaOTf & 60\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{\emph{a}} Reaction conditions, unless specified
otherwise: \textbf{1a} (0.25 mmol), \textbf{2a} (0.375 mmol),\textbf{3a}
(0.375 mmol), catalyst (30 mol\%), I\textsubscript{2} (20 mol\%),
additive (60 mol\%) and solvent (2.0 mL) were stirred at 130 \selectlanguage{ngerman}°C under
O\textsubscript{2} for 24 h. \emph{\textsuperscript{b}} Isolated yield.
\emph{\textsuperscript{c}} 10 mol\% and 30 mol\% of I\textsubscript{2}
were used respectively. \emph{\textsuperscript{d}} 20 mol\% and 60 mol\%
of Cu(OTf)\textsubscript{2} were used
respectively.\emph{\textsuperscript{e}} The mixed solutions of DMF and
TFE (v/v = 4:1, 3:1, 1:1).

Expanding upon our previous investigations, we embarked on an
exploration of the model substrates 1-methyl-1\emph{H} -indole
(\textbf{1a} ), pyrazole (\textbf{2a} ), and 10\emph{H}
-phenothiazine-2-carbonitrile (\textbf{3a} ) to investigate various
reaction parameters (Table 1). In our initial screening, several
catalysts were evaluated in the presence of \emph{N,N}
-dimethylformamide (DMF) as the solvent, with the reaction exposed to
O\textsubscript{2} at 130 °C for 24 hours. However, all the tested
catalysts demonstrated inferior performance compared to
Cu(OTf)\textsubscript{2}, which furnished a yield of 52\% (entries 1-5).
Consequently, we selected a combination of Cu(OTf)\textsubscript{2} and
I\textsubscript{2} as our preferred catalyst system to examine the
influence of different solvents (entries 6-10). Our screening endeavors
revealed DMF to be the optimal solvent among the options explored.
Furthermore, additional screening experiments indicated that altering
the amount of I\textsubscript{2} had no beneficial effects on the
reaction outcome (entry 11). Although the use of 60 mol\%
Cu(OTf)\textsubscript{2} slightly enhanced the isolated yield to 58\%
(entry 12), this effect was not statistically significant.
Encouragingly, the inclusion of 60 mol\% NaOTf as an additive resulted
in an improved yield of 62\% (entries 13-15). Further screening
experiments involving mixed solvents unveiled that a combination of DMF
and trifluoroethanol (v/v = 3:1) yielded a significant enhancement in
the overall yield, with a remarkable 73\% achieved (entries 16-18).
Finally, we established the optimal conditions for maximizing the yield.
The identified standard conditions are as follows: \textbf{1} (0.25
mmol), \textbf{2} (0.375 mmol), \textbf{3} (0.375 mmol),
Cu(OTf)\textsubscript{2} (30 mol\%), I\textsubscript{2} (20 mol\%),
NaOTf (60 mol\%), DMF (1.5 mL), and TFE (0.5 mL) were stirred at 130 °C
under O\textsubscript{2} for 24 h (entry 16).

\textbf{Scheme 2} Scope of indoles

\emph{\textsuperscript{a}} Reaction conditions: \textbf{1} (0.25
mmol),\textbf{2a} (0.375 mmol), \textbf{3a} (0.375 mmol),
Cu(OTf)\textsubscript{2} (30 mol\%), I\textsubscript{2} (20 mol\%),
NaOTf (60 mol\%), DMF (1.5 mL) and TFE (0.5 mL) were stirred at 130 °C
under O\textsubscript{2} for 24 h.

With the optimized conditions established, the scope and limitations of
the synthetic protocol were carefully examined. Initially,
pyrazole\textbf{2a} and 10\emph{H} -phenothiazine-2-carbonitrile
\textbf{3a} were tested in conjunction with various indoles \textbf{1} .
As depicted in Scheme 2, the majority of the reactions proceeded
smoothly, affording the desired C2,3-diaminoindoles in moderate yields
upon isolation (\textbf{4aaa-4oaa} ). Notably, diverse functional groups
(\selectlanguage{english}-Me, \selectlanguage{english}-Et, cyclopropylmethyl, alkyl, \selectlanguage{english}-Bn, \selectlanguage{english}-OMe, \selectlanguage{english}-Bpin, \selectlanguage{english}-F, \selectlanguage{english}-Cl, and \selectlanguage{english}-Br)
on the N1 pyrrolyl ring as well as on the C4-7 aryl ring were well
tolerated. However, the presence of bulky substituents and the
electronic properties of these substituents exerted some influence on
the product yield. Specifically, indoles bearing electron-donating
groups exhibited relatively high yields (\textbf{4aaa-4caa} ,
\textbf{4faa-4jaa} ). In the case of halogen substituents (\selectlanguage{english}-F, \selectlanguage{english}-Cl, and
\selectlanguage{english}-Br) on the aryl ring, the corresponding products were obtained in
acceptable to good yields (\textbf{4kaa-4maa} , 31-60\%), thereby
showcasing the potential for further chemical transformations and the
introduction of molecular complexity. Remarkably, despite the
significant steric hindrance imposed by the -Bpin group at the C4
position of indole, the corresponding product \textbf{4naa} was obtained
in a yield of 40\%. Furthermore, 1-methyl-1\emph{H}
-pyrrolo{[}2,3-\emph{b} {]}pyridine \textbf{1o} was subjected to the
reaction conditions, affording the cross-coupling product \textbf{4oaa}
in a yield of 43\%. Regrettably, indoles with strong
electron-withdrawing groups (e.g., \selectlanguage{english}-CN, \selectlanguage{english}-NO\textsubscript{2}) on the
aryl ring did not successfully yield the desired products (not depicted
in Scheme 2), presumably due to the comparatively lower reactivity of
these electron-deficient indole substrates towards the intermolecular
amination process. It is noteworthy that our experimental findings
indicated the N1 unprotected indole to be unsuitable as a starting
material for the synthesis of the target compound, as decomposition was
observed under the reaction conditions (not shown in Scheme 2).

\textbf{Scheme 3} Scope of azoles and phenothiazines

\emph{\textsuperscript{a}} Reaction conditions: \textbf{1a} (0.25
mmol),\textbf{2} (0.375 mmol), \textbf{3} (0.375 mmol),
Cu(OTf)\textsubscript{2} (30 mol\%), I\textsubscript{2} (20 mol\%),
NaOTf (60 mol\%), DMF (1.5 mL) and TFE (0.5 mL) were stirred at 130 degC
under O\textsubscript{2} for 24 h. \emph{\textsuperscript{b}} deviation:
at 80 degC.

In the subsequent investigations, we explored the reactions between
1-methyl-1\emph{H} -indole \textbf{1a} and a diverse array of
azoles\textbf{2} and phenothiazines \textbf{3} . As illustrated in
Scheme 3, azoles \textbf{2} possessing electron-donating groups or
halogen substituents demonstrated favorable reactivity, resulting in the
formation of \textbf{4aba-4aea} in yields ranging from 51\% to 79\%.
Conversely, lower yields were obtained when electron-withdrawing groups
and halogens were present on the azoles (\textbf{4afa-4aia} , 39-47\%),
suggesting a notable influence of the electronic effect on the reaction
outcome. Consistent with expectations, the reactions involving other
phenothiazines proceeded smoothly, furnishing the desired products in
acceptable yields (\textbf{4aab-4aaf} , 18-43\%). Furthermore, the
structure of 4aab was confidently determined \emph{via} X-ray
diffraction analysis (CCDC: 2283366).

To further investigate the reaction mechanism, several control
experiments were conducted (Scheme 4). Firstly, in order to observe the
reaction sequence, a reaction between the C2-aminated
product\textbf{1a-1} and phenothiazine \textbf{3b} was performed under
the standard conditions, yielding the desired C3-aminated
product\textbf{4aab} in 80\% yield (eq. (1)). However, when the
C3-aminated product \textbf{1a-2} was reacted with pyrazole \textbf{2a}
under the standard conditions, only 15\% yield of \textbf{4aab} was
obtained (eq. (2)). This observation suggests a preference for amination
at the C2 position over the C3 position. Additionally, a radical
trapping experiment was performed. When a stoichiometric amount of TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy) was added to the model reaction,
complete suppression of the diamination reaction was observed (eq. (3)),
indicating the occurrence of a free radical process during the reaction.

\textbf{Scheme 4} Control experiments

\textbf{Scheme 5} Plausible reaction mechanisms

Based on the observed results and in consideration of previous relevant
studies,\textsuperscript{{[}12{]}}a plausible mechanism is proposed, as
illustrated in Scheme 5. The reaction begins with the nucleophilic
addition of \textbf{1a} to I\textsubscript{2}, resulting in the
formation of iodonium species\textbf{I} . Subsequently, nucleophilic
attack by \textbf{2a} at the C2 position of I gives rise to intermediate
\textbf{II} , which undergoes elimination of HI to afford the 2-azolated
product \textbf{1a-1} . In the presence of the Cu(OTf)\textsubscript{2},
the 2-azolated product\textbf{1a-1} undergoes single-electron-transfer
(SET), yielding a radical cation. Deprotonation by NaOTf yields the
radical intermediate\textbf{III} . Likewise, an analogous process occurs
with phenothiazine\textbf{3} , generating intermediate \textbf{IV} .
Finally, a dual-radical cross-coupling between the C-centered radical
intermediate III and the N-centered radical intermediate \textbf{IV}
occurs, leading to the formation of the target molecule \textbf{4aaa} .

Conclusions

In summary, our research has led to the development of a novel and
efficient strategy for the regioselective synthesis of vicinal diamines
using indoles, pyrazoles, and phenothiazides in a three-component tandem
reaction. This methodology offers a straightforward route to
2,3-diaminoindoles and presents several notable advantages, which
include excellent tolerance towards functional groups, high
chemo-selectivity, outstanding step- and atom-economy, and the
elimination of the need for pre-installed specific coupling agents. The
results of our study not only provide an alternative approach for the
synthesis of vicinal diamines but also contribute to the further
advancement of multicomponent tandem reactions.

Experimental

All the obtained products were characterized by melting points
(m.p.),\textsuperscript{1}H-NMR,
\textsuperscript{13}C-NMR,\textsuperscript{19}F-NMR, high resolution
mass spectra (HRMS). Melting points were measured on an Electrothemal
SGW-X4 microscopy digital melting point apparatus and are uncorrected;
\textsuperscript{1}H-NMR,\textsuperscript{13}C-NMR,
\textsuperscript{19}F-NMR spectra were obtained on Bruker-600. All the
new compounds were further characterized by high resolution mass spectra
(HRMS). Chemical shifts were reported in parts per million (ppm, \selectlanguage{greek}δ\selectlanguage{english})
downfield from tetramethylsilane. Proton coupling patterns are described
as singlet (s), doublet (d), triplet (t), multiplet (m); TLC was
performed using commercially prepared 100-400 mesh silica gel plates
(GF254), and visualization was effected at 254 nm; All the reagents were
purchased from commercial sources, and used without further
purification.

\textbf{Typical procedure for the synthesis of 4aaa.} The mixture of
1-methylindole \textbf{1a} (33.0 mg, 0.25 mmol), \textbf{2a} (27.0 mg,
0.375 mmol), \textbf{3a} (84.0 mg, 0.375 mmol), and
Cu(OTf)\textsubscript{2} (30 mol\%), I\textsubscript{2} (20 mol\%),
NaOTf (60 mol\%), DMF (1.5 mL) and TFE (0.5 mL) were stirred at 130 \selectlanguage{ngerman}°C
under O\textsubscript{2} for 24 h. The resulting mixture was
concentrated by removing the solvent under vacuum, and the residue was
purified by preparative TLC on silica gel by using petroleum ether/ethyl
acetate (v/v = 8:1) as the eluent to give \textbf{4aaa} as yellow solid
(76.0 mg, 73\% yield).

Supporting Information

The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.

Acknowledgement

The authors thank the National Natural Science Foundation of China
(22201048), the Natural Science Foundation of Guangxi Province
(2022GXNSFBA035480), the Specific Research Project of Guangxi for
Research Bases and Talents (2022AC21099) and the Start-up Fund of
Guangxi University (A3040051013) for financial support.

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\begin{longtable}[]{@{}l@{}}
\toprule
Manuscript received: XXXX, 2023 Manuscript revised: XXXX, 2023
Manuscript accepted: XXXX, 2023 Accepted manuscript online: XXXX, 2023
Version of record online: XXXX, 2023\tabularnewline
\bottomrule
\end{longtable}

The Authors\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
& After acceptance, please insert a group photo of the authors taken
recently. &\tabularnewline
& \textbf{Left to Right:} Authors Names &\tabularnewline
\bottomrule
\end{longtable}

\textbf{Entry for the Table of Contents}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Synthesis of C2,3-vicinal diaminoindoles via a copper-iodine
co-catalytic strategy} Wenhua Yu,\textsuperscript{++,\emph{a}} Yingying
Zhang,\textsuperscript{++,\emph{a}} Xiaoxiang
Zhang,\emph{\textsuperscript{a}} Xiaoting Gu,\emph{\textsuperscript{a}}
Xirui Qing,\emph{\textsuperscript{a}} Wanxing
Wei,\emph{\textsuperscript{a}} Zhuan
Zhang,\emph{\textsuperscript{a,b,c}} Guanghua
Li,\emph{\textsuperscript{a,c,d}} and Taoyuan
Liang*\emph{\textsuperscript{a,b,c}} \emph{Chin. J. Chem.}
\textbf{2023}, \emph{41}, XXX---XXX. \textbf{DOI:
10.1002/cjoc.202300XXX}\tabularnewline
A one-pot synthesis of vicinal diamines using indoles, pyrazoles, and
phenothiazines in a tandem multi-component reaction is
developed.\tabularnewline
\bottomrule
\end{longtable}

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