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\begin{document}

\title{Phosphine-catalyzed {[}4+3{]} annulation reaction of indole deriva-tives
with MBH carbonates: A facile access to indole-1,2-fused
1,4-diazepinones and azepines}



\author[1]{Yannan Zhu}%
\author[1]{Haoran Jiang}%
\author[1]{Yi-Ning Wang}%
\affil[1]{Yancheng Institute of Technology}%


\vspace{-1em}



  
  \date{June 14, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
A phosphine-catalyzed {[}4+3{]} annulation between dinucleophilic indole
derivatives and Morita-Baylis-Hillman (MBH) carbonates was discovered by
using the N1 and N4' or C4' nucleophilicities of indole precursors,
which provides an efficient and facile access to in-dole-1,2-fused
1,4-diazepinones and azepines in good to high yields in one step, in
which indoles act as four atom synthons. Various transformations of
products illustrate promising applications of the given protocols.%
\end{abstract}%



\sloppy


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

Phosphine-catalyzed {[}4+3{]} annulation reaction of indole derivatives
with MBH carbonates: A facile access to indole-1,2-fused
1,4-diazepinones and azepines

Yannan Zhu,\emph{\textsuperscript{a}} Haoran
Jiang,\emph{\textsuperscript{a}} and Yi-Ning Wang
*\textsuperscript{,\emph{a}}

\textsuperscript{a} Faculty of Chemistry and Chemical Engineering,
Yancheng Institute of Technology, Yancheng 224051,
China\textsuperscript{}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Phosphine catalysis \textbar{} {[}4+3{]} Annulation \textbar{} Indole
derivatives \textbar{} Nitrogen heterocycles\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
A phosphine-catalyzed {[}4+3{]} annulation between dinucleophilic indole
derivatives and Morita-Baylis-Hillman (MBH) carbonates was discovered by
using the N1 and N4' or C4' nucleophilicities of indole precursors,
which provides an efficient and facile access to indole-1,2-fused
1,4-diazepinones and azepines in good to high yields in one step, in
which indoles act as four atom synthons. Various transformations of
products illustrate promising applications of the given
protocols.\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Background and Originality Content

Indole-fused polyheterocycles are prevalent in pharmaceuticals and
complex natural products. Among them, indole-1,2-fused 1,4-diazepinones
and azepines, comprising an indole skeleton and a seven-membered ring
scaffold, make up a highly attractive class of structural element in
drug discovery due to their endow significant biological activities
(Fig. 1), such as antihypertensive
activity,\textsuperscript{{[}1{]}}ribosomal S6 kinase (RSK)
inhibitor,\textsuperscript{{[}2{]}} HCV NS5B
inhibitor\textsuperscript{{[}3{]}} and prostaglandin D2 receptor
agonist,\textsuperscript{{[}4{]}} etc.\textsuperscript{{[}5-6{]}} So
far, the reported approaches to these complex polycyclic frameworks were
mainly relied on Lewis acid\textsuperscript{{[}7{]}} or Br\selectlanguage{ngerman}ønsted
acid\textsuperscript{{[}8{]}}-induced {[}5+2{]} annulations,
transition-metal-activated intramolecular\textsuperscript{{[}9{]}} or
intermolecular\textsuperscript{{[}10{]}} oxidative C-H coupling
reactions or NHC-involved {[}4+3{]}
annulations.\textsuperscript{{[}11{]}} Unfortunately, some limitations
of these methodologies are ineluctable, such as harsh reaction
conditions, costly catalysts, limited substrate scopes and tedious
manipulation to generate the necessary precursors. Thus, the development
of more versatile and efficient synthetic strategies for the
polyheterocycles fused by these two cores are still highly desirable.

\textbf{Figure 1} Bioactive molecules containing indole-1,2-fused
1,4-diazepinone and azepine frameworks.

Lewis base-catalyzed cyclization reactions is one of the most important
tools for the construction of diverse
heterocycles.\textsuperscript{{[}12{]}} However, the probes of
annulation for the assemblies of indole-fused polyheterocycles were
still quite limited. In 2019, the groups of
Lu\textsuperscript{{[}13{]}}and Zhang\textsuperscript{{[}14{]}}
independently published the dearomative {[}3+2{]} annulation of
3-nitroindoles with allenoates facilitated by nucleophilic phosphines to
access 2,3-fused cyclopentaindolines (Scheme 1a). However, the
construction of indole-1,2-fused polycycles by nucleophilic phosphine
catalysis remains challenging.\textsuperscript{{[}15{]}} Considering the
high values of indole-1,2-fused seven-membered polycycles in medicinal
chemistry and drug discovery, we were intrigued to develop a
straightforward procedure to access these polycyclic frameworks by using
the N1 and N4\selectlanguage{english}'/C4\selectlanguage{english}' nucleophilicities of indoles via exploiting new
indole precursors (Scheme 1b). Theoretically, the \emph{p} Ka value of
the proton belonging to the N atom of indole can be reduced efficiently
with the aid of electron-withdrawing groups at the C3 position of indole
due to their inductive effects. Thereby, the nucleophilicity of the N
atom can be enhanced significantly to fulfill the following
transformation.\textsuperscript{{[}16{]}} Herein, we report a facile
synthetic protocol targetting indole-1,2-fused 1,4-diazepinones and
azepines via phosphine-catalyzed {[}4+3{]} annulations between
dinucleophilic indoles and MBH carbonates by using the N1 and N4\selectlanguage{english}'/C4\selectlanguage{english}'
nucleophilicities of the newly-derived indoles, in which the indoles act
as novel four atom synthons (Scheme 1b).

\textbf{Scheme 1} The participation of indole unit in the
phosphine-catalyzed annulations.

Results and Discussion

\textbf{Table 1} Optimization of the reaction
conditions\emph{\textsuperscript{a}}\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Entry & Cat. & Solvent & T (\textsuperscript{o}C) &
Yield\emph{\textsuperscript{b}} (\%)\tabularnewline
\midrule
\endhead
1 & PPh\textsubscript{3} & CHCl\textsubscript{3} & 50 &
43\tabularnewline
2 & DABCO & CHCl\textsubscript{3} & 50 & -\tabularnewline
3 & DMAP & CHCl\textsubscript{3} & 50 & -\tabularnewline
4 & (\emph{p}-FC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P &
CHCl\textsubscript{3} & 50 & 47\tabularnewline
5 & (\emph{p}-ClC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CHCl\textsubscript{3} & 50 & 53\tabularnewline
6 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CHCl\textsubscript{3} & 50 & 77\tabularnewline
7 & (\emph{p}-OMeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CHCl\textsubscript{3} & 50 & 72\tabularnewline
8 & PhPEt\textsubscript{2} & CHCl\textsubscript{3} & 50 &
47\tabularnewline
9 & PBu\textsubscript{3} & CHCl\textsubscript{3} & 50 &
55\tabularnewline
10 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CH\textsubscript{2}Cl\textsubscript{2} & 50 & 35\tabularnewline
11 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& THF & 50 & 67\tabularnewline
12 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CH\textsubscript{3}CN & 50 & 83\tabularnewline
13 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& DMF & 50 & 79\tabularnewline
14 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CH\textsubscript{3}CN & 40 & 74\tabularnewline
15 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CH\textsubscript{3}CN & 60 & 86\tabularnewline
16 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CH\textsubscript{3}CN & 65 & 89\tabularnewline
17 & (\emph{p}-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
& CH\textsubscript{3}CN & 70 & 84\tabularnewline
18 & \emph{\textsuperscript{t}}BuOK & CH\textsubscript{3}CN & 65 &
-\tabularnewline
19 & - & CH\textsubscript{3}CN & 65 & -\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{\emph{a}} Reaction conditions: \textbf{1a} (0.10
mmol),\textbf{2a} (0.15 mmol), cat. (20 mol\%) in solvent (1.0 mL) at
the indicated temperature for 6 h. \textsuperscript{\emph{b}} Isolated
yield.

The initial {[}4+3{]} annulation was attempted between
3-formylindole-2-ylamide \textbf{1a} and MBH carbonate \textbf{2a} in
the presences of PPh\textsubscript{3} (20 mol \%) and
CHCl\textsubscript{3} at 50 \selectlanguage{ngerman}°C. Gratifyingly, the desired
indole-1,2-fused 1,4-diazepinone product \textbf{3aa} was obtained in
43\% yield (Table 1, entry 1). The screening of different Lewis base
catalysts indicated that tertiary amines, such as DABCO and DMAP, failed
to support the transformation (Table 1, entries 2 and 3). Subsequently,
some other phosphine catalysts were evaluated (Table 1, entries 4\selectlanguage{english}-9),
and (\emph{p} -MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P
was found to be the most efficient activator to afford \textbf{3aa} in
77\% yield (Table 1, entry 4). Further investigations about the more
compatible solvent showed that the yield of \textbf{3aa} could be
elevated higher up to 83\% in CH\textsubscript{3}CN (Table 1, entries
10\selectlanguage{english}-13). Different reaction temperatures were also examined, and it
revealed that the decreasing of the temperature down to 40 degC led to
inferior outcome (Table 1, entry 14), and the higher temperature as 65
degC yielded the best result (Table 1, entry 16). However, if the
temperature was further raised up to 70 degC, a diminished amount
of\textbf{3aa} was isolated (Table 1, entry 17). Additionally, tertiary
phosphine was proven to be an essential catalyst for this reaction
through the control experiment (Table 1, entries 18 and 19). Therefore,
the optimal reaction was determined to be conducted at 65 degC in
CH\textsubscript{3}CN by using (\emph{p}
-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P as the
catalyst loaded in 20 mol\%.

\textbf{Scheme 2} Substrate scope for MBH carbonates \textbf{2} and
indole-2-ylamides \textbf{1} .\emph{\textsuperscript{a, b}}

\textsuperscript{\emph{a}} Reaction conditions: \textbf{1} (0.10
mmol),\textbf{2} (0.15 mmol), (\emph{p}
-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P (20 mol\%) in
CH\textsubscript{3}CN (1.0 mL) at 65 \textsuperscript{o}C for 6 h.
\textsuperscript{\emph{b}} Isolated yield.

Afterwards, we moved our attention to the substrate scopes (Scheme 2).
Firstly, it turned out that the MBH carbonates bearing various ester
groups can give the indole-1,2-fused 1,4-diazepinones \textbf{3} in good
to high yield (\textbf{3aa} \selectlanguage{english}-\textbf{3ad} ), and more bulky esters, such
as the one embodying the \emph{tert} -butyl group, provided
\textbf{3ac}with relatively lower yield. On the other hand, it is
exciting to observe both the electron-donating (\textbf{3ba}
\selectlanguage{english}-\textbf{3da} ) and electron-withdrawing (\textbf{3ea} , \textbf{3fa} )
substituents at the 5-position of the indole ring were well-tolerated,
and the related products (\textbf{3ba} \selectlanguage{english}-\textbf{3fa} ) can all be
isolated in good yields (65\selectlanguage{english}-75\%). Furthermore, the reaction can be
undergoing smoothly as well for the indole substrates \textbf{1}
containing some other\emph{N} -alkoxy groups, e.g. \emph{N} -OEt,
\emph{N} -OBn and\emph{N} -O\emph{\textsuperscript{t}} Bu, which can
afford the cyclized products \textbf{3ga} \selectlanguage{english}-\textbf{3ia} in 66\selectlanguage{english}-81\%
yields. In addition,\emph{N} -phenyl amide could also be compatible in
the system, leading to the product \textbf{3ja} in 52\% yield. As
expected, the formyl group (R\textsuperscript{3}) substituted on the C3
position of the indole ring could be replaced by different
electron-withdrawing groups, such as the 3-chloro analogue that produced
\textbf{3ka} in 61\% yield. As a comparison, no reaction was detected
under the optimized condition when indole-2-ylamides \textbf{1l} was
utilized as the raw material. This phenomenon indicated the importance
of the electron-withdrawing group R\textsuperscript{3} and its electron
inductive effect as mentioned in ahead. The structure of \textbf{3ia}
was confirmed by single-crystal X-ray diffraction analysis (CCDC
2092646).\textsuperscript{{[}17{]}}

\textbf{Scheme 3} Substrate scope for 2-((1H-indol-2-yl)
methyl)malonates \textbf{4} .\emph{\textsuperscript{a, b}}

Meanwhile, the achievements of indole-1,2-fused azepines by means of the
similar {[}4+3{]} annulation reactions of MBH carbonate \textbf{2a} with
2-((1H-indol-2-yl) methyl)malonates \textbf{4} were also realized
(Scheme 3). Based on the transformation outcomes, the steric hindrance
of the ester group in the precursors \textbf{4} did not show significant
effects to hamper the reaction, and the oriented compounds\textbf{5aa}
\selectlanguage{english}-\textbf{5fa} with different ester moieties or electron-withdrawing
groups were all isolated successfully in moderate to good yields. It
could be noted that the yield of \textbf{5ea} was somewhat lower than
those of other analogues. Since the electron-withdrawing ability owing
by the imine group is weaker, it leads to the worse result as described
above.

\textbf{Scheme 4} Gram synthesis and transformations of the
product\textbf{3aa} .

To showcase the scalability of the strategy aforementioned, a 5.0 mmol
scale reaction of \textbf{1a}was carried out, and the product
\textbf{3aa} could be obtained in 85\% yield. Subsequently, the
resulting \textbf{3aa} was subjected to some further derivatizations to
verify its synthetic utilities (Scheme 4). Firstly, the formyl group of
\textbf{3aa} was transferred with
(acetylylmethylene)triphenylphosphorane via Wittig reaction to give the
corresponding alkene \textbf{6aa} in 83\% yield with 1:1 E/Z ratio, and
the same C=O double bond can be selectively reduced by the treatment of
NaBH\textsubscript{4} to afford alcohol \textbf{7aa} in 78\% isolation
yield. Furthermore, the methoxy group embodied by the N4\selectlanguage{english}' atom
in\textbf{3aa} can be cleaved readily with SmI\textsubscript{2} to
provide the secondary amide \textbf{8aa} in 58\% yield.

\textbf{Scheme 5} Proposed reaction mechanism.

A plausible mechanism for this {[}4+3{]} annulation process was proposed
based on the results obtained above and some published reports relied on
the phosphine-triggered allylic alkylation/intramolecular Michael
cyclization of MBH carbonates (Scheme 5).\textsuperscript{{[}18{]}} In
the beginning, the nucleophilic addition of the phosphine catalyst on
the MBH carbonate \textbf{2a} would take place with a simultaneous
decarboxylation to yield intermediate \textbf{I} , and the\emph{tert}
-butoxide anion generated at the same time serves as a base to abstract
the proton of the N1 atom belonging to the indole ring to induce the ion
pair of phosphonium-nucleophile anion \textbf{II.}Subsequently, Ion pair
\textbf{II} undergoes S\textsubscript{N}2\selectlanguage{english}' substitution to afford the
allylic alkylation intermediate \textbf{III}and reborns the phosphine
catalyst. Then, the N4\selectlanguage{english}' atom in the amide moiety of \textbf{III} is
believed to be deprotonated due to its acidic property to produce
intermediate \textbf{IV} under the influence of the in situ generated
\emph{tert} -butoxide anion. Finally, the intermediate\textbf{IV}
undergoes an intramolecular Michael cyclization to furnish\textbf{3aa}
as the final cyclic product.

Conclusions

In summary, a novel phosphine-catalyzed {[}4+3{]} annulation reaction of
indole derivatives with MBH carbonates by exploiting the N1 and N4\selectlanguage{english}'/C4\selectlanguage{english}'
nucleophilic sites of indole has been explored. A variety of
indole-1,2-fused 1,4-diazepinones and azepines were facilely obtained in
good to high yields in one step. This annulation protocol features
metal-free conditions, good functional group tolerance, easy scalability
and versatile transformations of the products, highlighting its
potential synthetic utility in drug discovery. Further development of
other new reagents and related annulation reactions for the construction
of medium-ring systems are ongoing in our laboratory.

Experimental

\textbf{General procedures for the synthesis of products 3 and 5}

Indole derivatives \textbf{1} (0.10 mmol), (\emph{p}
-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P (20 mol\%)
and CH\textsubscript{3}CN (1.0 mL) were added to a dry 15 mL flask at
room temperature. Then MBH carbonates \textbf{2} (1.5 equiv., 0.15 mmol)
was added. This solution was stirred at 65\textsuperscript{o}C until the
complete consumption of the starting materials monitored by TLC. After
the removal of the solvent, the residue was purified by flash column
chromatography (petroleum ether: EtOAc = 2:3) on silica gel to afford
indole-1,2-fused 1,4-diazepinones\textbf{3} .

Indole derivatives \textbf{4} (0.10 mmol), (\emph{p}
-MeC\textsubscript{6}H\textsubscript{4})\textsubscript{3}P (20 mol\%)
and CH\textsubscript{3}CN (1.0 mL) were added to a dry 15 mL flask at
room temperature. Then MBH carbonate \textbf{2a} (1.5 equiv., 0.15 mmol)
was added. This solution was stirred at 65\textsuperscript{o}C until the
complete consumption of the starting materials monitored by TLC. After
the removal of the solvent, the residue was purified by flash column
chromatography (petroleum ether: EtOAc = 2:3) on silica gel to afford
indole-1,2-fused azepines\textbf{5} .

Supporting Information

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

Acknowledgement

We are grateful for the financial support from the funding for
school-level research projects of Yancheng Institute of Technology (No.
XJR2022019, XJ201719).

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\end{enumerate}\selectlanguage{english}
\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{Phosphine-catalyzed {[}4+3{]} annulation reaction of indole
deriva-tives with MBH carbonates: A facile access to indole-1,2-fused
1,4-diazepinones and azepines} Yannan Zhu,\textsuperscript{a} Haoran
Jiang,\textsuperscript{a} and Yi-Ning Wang*\textsuperscript{a}
\emph{Chin. J. Chem.} \textbf{2023}, \emph{41}, XXX---XXX. \textbf{DOI:
10.1002/cjoc.202300XXX}\tabularnewline
Phosphine-catalyzed {[}4+3{]} annulations between indoles and MBH
carbonates was developed, leading to the direct construction of
indole-1,2-fused 1,4-diazepinones and azepines in good to excellent
yields.\tabularnewline
\bottomrule
\end{longtable}

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