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\begin{document}

\title{Ruthenium-Catalysed Asymmetric Intramolecular
Isomerization/Esterification Reaction: Direct Synthesis of Chiral
Dihydrocoumarins}



\author[1]{Lingzi Zhao}%
\author[1]{Xuchao Wang}%
\author[1]{Qing Qiang}%
\author[1]{Xingwei Zhao}%
\author[1]{Feipeng Liu}%
\author[1]{Shenci Lu}%
\author[1]{Zi-Qiang Rong}%
\affil[1]{Northwestern Polytechnical University}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\selectlanguage{english}
\begin{abstract}
An asymmetric isomerization/intramolecular coupling reaction of allylic
alcohols to synthesize chiral dihydrocoumarins was suc-cessfully
accomplished through ruthenium catalysis. This method demonstrates a
wide substrate applicability, excellent tolerance for various functional
groups, and good enantioselectivities (up to 90\% ee). It provides a
convenient pathway to produce a diverse range of structurally distinct
chiral dihydrocoumarins in good efficiency.%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2024} , \emph{42}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202400XXX}

Ruthenium-Catalysed Asymmetric Intramolecular
Isomerization/Esterification Reaction: Direct Synthesis of Chiral
Dihydrocoumarins

Lingzi Zhao,\emph{\textsuperscript{a,++}} Xuchao
Wang,\emph{\textsuperscript{a,++}} Qing
Qiang,\emph{\textsuperscript{a}}Xingwei Zhao,\emph{\textsuperscript{b}}
Feipeng Liu,\emph{\textsuperscript{*,a}} Shenci
Lu,\emph{\textsuperscript{*,a}}and Zi-Qiang Rong
\emph{\textsuperscript{*,a}}

\textsuperscript{a} Frontiers Science Center for Flexible Electronics
(FSCFE), Shaanxi Institute of Flexible Electronics (SIFE) \& Shaanxi
Inst-itute of Biomedical Materials and Engineering (SIBME), Northwestern
Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072,
China. \textsuperscript{b} Honors College, Northwestern Polytechnical
University (NPU), 127 West Youyi Road, Xi'an 710072, China.\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Keywords}\tabularnewline
\midrule
\endhead
Ruthenium catalysis \textbar{} Asymmetric isomerization \textbar{}
Tandem reaction \textbar{} Chiral dihydrocoumarins \textbar{}
Enantioselectivity\tabularnewline
\textbf{Comprehensive Summary}\tabularnewline
An asymmetric isomerization/intramolecular coupling reaction of allylic
alcohols to synthesize chiral dihydrocoumarins was successfully
accomplished through ruthenium catalysis. This method demonstrates a
wide substrate applicability, excellent tolerance for various functional
groups, and good enantioselectivities (up to 90\% ee). It provides a
convenient pathway to produce a diverse range of structurally distinct
chiral dihydrocoumarins in good efficiency.\tabularnewline
\bottomrule
\end{longtable}

\section*{}

{\label{section}}

Background and Originality Content

Transition metal-catalysed isomerization of alkenyl alcohols has emerged
as a powerful synthetic method, gaining widespread recognition and
adoption over the past few decades.\textsuperscript{{[}1{]}} This
efficient process, relying on the isomerization of C=C bonds, offers
several distinct advantages, including atom and step economy, as well as
sustainability. Various catalytic systems have been developed to
facilitate this efficient isomerization towards \selectlanguage{greek}β-\selectlanguage{english}chiral carbonyl
compounds (Scheme 1a).\textsuperscript{{[}2{]}} However, accomplishing
the asymmetric sequential isomerization/transformation tandem reaction
in a one-pot fashion has remained a formidable challenge in the field of
organic synthesis. Only a few successful examples have been documented,
primarily focusing on the asymmetric isomerization/intermolecular
transformation of allylic alcohols (Scheme
1b),\textsuperscript{{[}3{]}}with no previous reports of intramolecular
cases.

Chiral dihydrocoumarin backbones are commonly found in numerous natural
products and pharmaceuticals, making them ubiquitous structural
components.\textsuperscript{{[}4{]}} In recent years, significant
advancements have been made in the construction of chiral
dihydrocoumarins.\textsuperscript{{[}5{]}}Despite these advancements,
considering the vital role played by these heterocyclic compounds in
both natural products and synthetic chemistry, more efficient and direct
methods for the enantioselective synthesis of these compounds are
greatly desirable. Inspired by the progress in the asymmetric
isomerization/transformation of allylic alcohols and motivated by our
ongoing research endeavors,\textsuperscript{{[}6{]}} we set out to
explore the possibility of a new intramolecular tandem transformation.
In our design, the combination of asymmetric isomerization of primary
allylic alcohols and a subsequent intramolecular cross-coupling step
within a single synthetic sequence presented a unique opportunity to
efficiently construct valuable chiral dihydrocoumarin backbones.

\textbf{Scheme 1} Isomerization of alkenyl alcohols.

The key to realizing this process lay in finding a suitable transition
metal catalyst capable of achieving asymmetric redox isomerization to
generate a chiral \selectlanguage{greek}β-\selectlanguage{english}substituted aldehyde intermediate in situ, while
also promoting the subsequent ester formation reaction to yield the
final target product. Ruthenium, a promising transition metal, has
demonstrated high activity and stereoselectivity in catalysing
asymmetric isomerization
reactions.\textsuperscript{{[}7{]}}Additionally, it is well-suited for a
variety of cross-coupling reactions, significantly expanding the
accessible chemical structures. In this communication, we present a
ruthenium-catalysed asymmetric isomerization/intramolecular coupling
reaction of allylic alcohols, resulting in the efficient synthesis of
various chiral dihydrocoumarins (Scheme 1c).

Results and Discussion

We initiated an exploration of the intramolecular tandem reaction using
2-(3-hydroxy-1-phenylprop-1-en-1-yl)-5-methoxyphenol \textbf{1a} as the
model substrate. In the presence of Ru(cod)Cl\textsubscript{2} (5
mol\%), 2,2,6,6-tetramethylpiperidine (TMP) (2.0 equiv.) as the base,
and chiral diphosphine \textbf{L1} (6 mol\%) as the ligand, the
isomerization/intramolecular esterification reaction can be achieved,
and the desired lactone product \textbf{2a} could be ultimately obtained
in an impressive 83\% yield, albeit with a low enantiomeric excess (ee)
of 14\% (Table 1, entry 1). Encouraged by this promising outcome, we
expanded our investigation to encompass a range of commercially
available chiral phosphine ligands, including phosphoramidites and
bidentate phosphorus ligands. Gratifyingly, the application of the
chiral phosphoramidite ligand \textbf{L2} facilitated the smooth
formation of \textbf{2a} , affording a 58\% yield with commendable
enantioselectivity

\textbf{Table 1} Summary of selected optimization of reaction conditions

\emph{\textsuperscript{a}}\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
entry & ligand & base & solvent & yield (\%) & ee (\%)\tabularnewline
\midrule
\endhead
1 & L1 & TMP & toluene & 83 & 14\tabularnewline
2 & L2 & TMP & toluene & 58 & 78\tabularnewline
3 & L3 & TMP & toluene & 67 & 68\tabularnewline
4 & L4 & TMP & toluene & 70 & 13\tabularnewline
5 & L5 & TMP & toluene & 26 & 13\tabularnewline
6 & L6 & TMP & toluene & 7 & 3\tabularnewline
7 & L7 & TMP & toluene & 52 & 2\tabularnewline
8 & L8 & TMP & toluene & 72 & 21\tabularnewline
9 & L9 & TMP & toluene & 62 & 69\tabularnewline
10 & L2 & K\textsubscript{2}CO\textsubscript{3} & toluene & 75 &
83\tabularnewline
11 & L2 & DBU & toluene & 77 & 39\tabularnewline
12 & L2 & K\textsubscript{2}CO\textsubscript{3} & DCE & 78 &
89\tabularnewline
13 & L2 & K\textsubscript{2}CO\textsubscript{3} & Ph-CF\textsubscript{3}
& 72 & 87\tabularnewline
\bottomrule
\end{longtable}

\emph{\textsuperscript{a}} All the reactions were run with 0.1 mmol of
1a in 2 ml DCE at 130 \textsuperscript{o}C for 20
h.\emph{\textsuperscript{b}} Determined by\textsuperscript{1}H NMR using
1,1,2,2-tetrachloroethane as the internal standard. Abbreviations: DCE,
Dichloroethane. DBU, 1,8-diazabicyclo{[}5.4.0{]}undec-7-ene. TMP,
2,2,6,6-Tetramethylpiperidine

(78\% ee) (Table 1, entry 2). Similarly, other chiral phosphoramidite
ligands, such as \textbf{L3} and \textbf{L9} , also yielded favorable
results (Table 1, entries 3 and 9). Building upon these initial
findings, we directed our attention towards optimizing the choice of
base and solvent parameters. It was observed that the use of the
inorganic base K\textsubscript{2}CO\textsubscript{3} resulted in the
production of \textbf{2a} with good enantioselectivity (83\% ee) and
outstanding NMR yield (75\%). Furthermore, 1,2-Dichloroethane (DCE) has
been emerged as the most advantageous solvent for this reaction.
Subsequent efforts to refine the experimental conditions did not yield
significant improvements (see more details in the Supporting
Information).

Unless otherwise noted, all reactions were carried out with 0.2 mmol
of\textbf{1} in 4 mL DCE at 130 \textsuperscript{o}C for 20 h.

\textbf{Figure 1} The scope of substrate \textbf{1}

With the optimal conditions in hand, the substrate scope of this
intramolecular enantioselective tandem reaction was examined as
illustrated in Figure 1. A variety of substituted
2-(3-hydroxy-1-phenylprop-1-en-1-yl)phenols with diverse aryl groups
demonstrated a smooth progression through the tandem process, affording
the desired lactones with moderate yields and a good level of
enantioselectivity. However, the 2-(\emph{tert} -butyl) or
2,4-di-\emph{tert} -butyl-6-(3-hydroxy-1-phenylprop-1-en-1-yl)phenol was
subjected into the reaction conditions, delivering reduced
enantioselectivity with 68\% and 49\% ee respectively (\textbf{2g}
and\textbf{2h} ). The introduction of various substituents on another
aryl group was well-tolerated, yielding the corresponding lactones in
good yields and enantioselectivities (\textbf{2i} -\textbf{2q} ).
Notably, product \textbf{2q} holds significant promise as it can undergo
further transformations, ultimately leading to the formation of a chiral
\selectlanguage{greek}β,β-\selectlanguage{english}diaryl carboxylic acid that serves as a potent agonist for GPR40,
rendering it a prospective drug candidate for diabetes treatment.
Encouragingly, even a heterocyclic substituent, such as the thiophene
group, proved compatible with the reaction conditions, providing the
desired product \textbf{2r} in a 60\% isolated yield, albeit with 77\%
ee. Furthermore, the reaction consistently demonstrated its efficiency,
delivering the desired dihydrocoumarins with various substituents on the
two aryl groups (\textbf{2s} -\textbf{2x} ). Remarkably, when the aryl
substituents at the \selectlanguage{greek}β-\selectlanguage{english}position of the primary allylic alcohol were
replaced with alkyl substituents, the reaction still proceeded
efficiently, yielding the desired dihydrocoumarins in good efficiency
(\textbf{4a} -\textbf{4h} ). We were delighted to confirm that
electron-neutral, electron-rich, and electron-deficient substituents on
the aromatic ring were well tolerated, resulting in the corresponding
products with excellent yields. The presence of tert-butyl group
substituents in the aromatic ring also led to good yield and
enantioselectivity (\textbf{4e} ). Unfortunately, when the
R\textsubscript{4} group was replaced with the bulkier ethyl group
(\textbf{4h} ) instead of the methyl group (\textbf{4a} ), it led to
lower enantioselectivity.

Unless otherwise noted, all reactions were carried out with 0.2 mmol
of\textbf{1} in 4 mL DCE at 130 \textsuperscript{o}C for 20 h.

\textbf{Figure 2} The scope of substrate \textbf{3} .

To gain deeper insights into the mechanics of this reaction,
particularly concerning the isomerization of primary allylic alcohols
and the formation of the new C-O bond, isotopic labeling studies were
conducted, as depicted in Figure 3. Deuterium labeling experiments were
executed using \textbf{1x} -\emph{d}\textsubscript{\textbf{2}} to
elucidate on the plausible mechanism. The analysis
by\textsuperscript{1}H NMR revealed that deuterium in
dihydrocoumarin\textbf{2x} -d \textsubscript{2} was integrated at both
C1 (0.51D) and C2 (0.14D), providing evidence that the hydrogen transfer
during the reaction followed a chain-walking process involving iterative
\selectlanguage{greek}β-\selectlanguage{english}H elimination and reinsertion steps. Furthermore, a hydrogen signal
was detected \emph{via} gas chromatography during the reaction
of\textbf{1y} , indicating that a dehydrogenative process was
incorporated into this transformation process.

Based on the experimental findings and previous research on Ru-catalyzed
alkene double bond isomerization,\textsuperscript{{[}8{]}} we propose a
specific reaction mechanism. The plausible catalytic cycle is
illustrated in Figure 3c, encompassing the isomerization of alkenyl
alcohol and the subsequent coupling processes for lactone formation. The
reaction commences with the activation of the reactant through a
deprotonation step in the presence of the base, leading to the formation
of alkoxide \textbf{I} . Subsequently, a \selectlanguage{greek}β-\selectlanguage{english}hydride elimination event
occurs, resulting in the generation of aldehyde intermediate
\textbf{II}into complex \textbf{III} . Tautomerization then leads to the
formation of intermediate \textbf{IV} . Swift protonation and subsequent
esterification processes yield \textbf{VI} , which can further release
H\textsubscript{2} gas to produce the desired chiral
3,4-dihydrocoumarins while regenerating the ruthenium catalyst for
subsequent reactions.

\textbf{Figure 3} Mechanism exploration.

Conclusions

In summary, we have successfully developed a ruthenium catalytic system
for the enantioselective isomerization of alkenyl alcohols, followed by
a tandem coupling reaction that enables the synthesis of diverse chiral
dihydrocoumarins. This protocol demonstrates a wide substrate
applicability, excellent tolerance for various functional groups, and
good enantioselectivities, and presents its potential for the
construction of GPR40 agonists. Ongoing efforts are dedicated to further
elucidating the underlying reaction mechanism.

Experimental

\textbf{General procedure for the synthesis of 2}

A flame--dried 15 mL cylindrical pressure vessel was charged
with\textbf{1} (0.2 mmol, 1.0 equiv.). The cylindrical pressure vessel
was directly transferred into a nitrogen-filled glovebox without caps.
Then, RuCl\textsubscript{2}(cod) (2.8 mg, 0.01 mmol, 5 mol\%),
\textbf{L2}(5.1 mg, 0.012 mmol, 6 mol\%),
K\textsubscript{2}CO\textsubscript{3}(55.3 mg, 0.4 mmol, 2.0 equiv.) and
4.0 mL dry DCE were added. Then the cylindrical pressure vessel was
tightly sealed, transferred out of the glovebox and stirred at 130 \selectlanguage{ngerman}°C
for 20 h. After the completion of the reaction, the solvent was removed
in vacuo and the residue was purified by flash column chromatography on
silica gel to give the desired 4-aryldihydrocoumarin compounds.

\textbf{General procedure for the synthesis of 4}

A flame--dried 15 mL cylindrical pressure vessel was charged
with\textbf{3} (0.3 mmol, 1.0 equiv.). The cylindrical pressure vessel
was directly transferred into a nitrogen-filled glovebox without caps.
Then, RuCl\textsubscript{2}(cod) (4.2 mg, 0.015 mmol, 5 mol\%),
\textbf{L3}(7.8 mg, 0.018 mmol, 6 mol\%),
K\textsubscript{2}HPO\textsubscript{4}(104.5 mg, 0.6 mmol, 2.0 equiv.)
and 4.0 mL dry benzene were added. Then the cylindrical pressure vessel
was tightly sealed, transferred out of the glovebox and stirred at 130
°C for 20 h. After the completion of the reaction, the solvent was
removed in vacuo and the residue was purified by flash column
chromatography on silica gel to give the desired
4-methyldihydrocoumarins.

Supporting Information

The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2024xxxxx.

Acknowledgement (optional)

This project was supported by National Natural Science Foundation of
China (21801206), Shaanxi Fundamental Science Research Project for
Chemistry \& Biology (22JHQ002), the Program for Young Talents of
Shaanxi Province (5113190023), and the Innovation Foundation for Doctor
Dissertation of Northwestern Polytechnical University (CX2023104).

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\end{enumerate}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
Manuscript received: XXXX, 2024 Manuscript revised: XXXX, 2024
Manuscript accepted: XXXX, 2024 Accepted manuscript online: XXXX, 2024
Version of record online: XXXX, 2024\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{Ruthenium-Catalysed Asymmetric Intramolecular
Isomerization/Esterification Reaction: Direct Synthesis of Chiral
Dihydrocoumarins} Lingzi Zhao,\emph{\textsuperscript{a,++}} Xuchao
Wang,\emph{\textsuperscript{a,++}} Qing Qiang,\emph{\textsuperscript{a}}
Xingwei Zhao,\emph{\textsuperscript{b}} Feipeng
Liu,\emph{\textsuperscript{*,a}} Shenci Lu,\emph{\textsuperscript{*,a}}
and Zi-Qiang Rong \emph{\textsuperscript{*,a}} \emph{Chin. J. Chem.}
\textbf{2024}, \emph{42}, XXX---XXX. \textbf{DOI:
10.1002/cjoc.202300XXX}\tabularnewline
Herein, an asymmetric isomerization/intramolecular coupling reaction of
allylic alcohols to synthesize chiral dihydrocoumarins was successfully
accomplished through ruthenium catalysis. This method demonstrates a
wide substrate applicability, excellent tolerance for various functional
groups, and good enantioselectivities (up to 90\% ee). It provides a
convenient pathway to produce a diverse range of structurally distinct
chiral dihydrocoumarins in good efficiency.\tabularnewline
\bottomrule
\end{longtable}

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