Concise Synthesis of Isocoumarins through Rh-Catalyzed Direct Vinylene Annulation: Scope and Mechanistic Insight

Article abstract of DOI:10.1021/acs.orglett.0c02112

Transition-metal-catalyzed activation of inert Ca'H bonds and subsequent Ca'C bond formation have emerged as powerful synthetic tools for the synthesis of elaborate cyclic molecules. In this report, we introduce an efficient synthetic method of 3,4-unsubstituted isocoumarins adopting an electron-deficient CpE Rh complex as the catalyst. The use of vinylene carbonate as a vinylene transfer reagent enables the direct construction of isocoumarins from readily available benzoic acids, without any external oxidants as well as bases. The reaction mechanism is evaluated by computational analysis to find an unprecedented rhodium shift event within the catalytic cycle.

Full text of DOI:10.1021/acs.orglett.0c02112

pubs.acs.org/OrgLett
Letter
Concise Synthesis of Isocoumarins through Rh-Catalyzed Direct
Vinylene Annulation: Scope and Mechanistic Insight
§
§
Gen Mihara, Koushik Ghosh, Yuji Nishii,* and Masahiro Miura*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c02112
Read Online
ACCESS
Metrics & More
Article Recommendations
*
sı Supporting Information
ABSTRACT: Transition-metal-catalyzed activation of inert C−H bonds and subsequent C−C bond formation have emerged as
powerful synthetic tools for the synthesis of elaborate cyclic molecules. In this report, we introduce an efficient synthetic method of
E
3
,4-unsubstituted isocoumarins adopting an electron-deficient Cp Rh complex as the catalyst. The use of vinylene carbonate as a
vinylene transfer reagent enables the direct construction of isocoumarins from readily available benzoic acids, without any external
oxidants as well as bases. The reaction mechanism is evaluated by computational analysis to find an unprecedented “rhodium shift”
event within the catalytic cycle.
he wide prevalence of lactone derivatives in a huge
number of natural products and biologically active
compounds has claimed significant attention in medicinal
isocoumarins, has not been accessible via the catalysis. This is
probably due to the lack of proper reagents which facilitate the
reaction turnover. Indeed, only one example of such a
transformation is found in the literature, albeit with a poor
14% product yield using vinyl acetate in the presence of
T
and synthetic chemistry research. Isocoumarins, also known as
1
H-2-benzopyrans or 3,4-benzopyrones, hold an important
1
6c
position due to their broad pharmacological applications,
which include activities of antimicrobial, antifungal, anticancer,
anti-HIV, anti-inflammatory, and so on. These facts prompted
the synthetic community to establish efficient methods for the
Rh(III) catalyst and stoichiometric Cu(II) oxidant. Ob-
viously, an efficient and comprehensive reaction system for the
isocoumarin core is missing. In addition to this synthetic
difficulty, the formation of stoichiometric metal waste derived
from the oxidant should be avoided.
2
,3
construction of isocoumarin scaffolds. The past two decades
witnessed a considerable advancement in transition-metal-
catalyzed direct annulation of benzoic acids with unsaturated
coupling partners through the activation of their ortho-C−H
To address these issues, we assumed that our catalytic
vinylene transfer strategy would be an effective solution,
employing vinylene carbonate as an oxidizing acetylene
8
3
,4
surrogate (Scheme 1c). This protocol requires no oxidant as
bond. This protocol offers rapid access to the isocoumarins
from readily available starting materials in a step-economical
manner, and is practically beneficial as compared to the
conventional acid- or electrophile-mediated cyclization of
ortho-alkynylated benzoic acid derivatives (Scheme 1a,b).
well as base because the reactant fulfills both functions to give
carbonic acid as a sole side product. In this report, we
introduce a concise synthesis of nonsubstituted isocoumarins,
and the detailed reaction mechanism was investigated by DFT
calculation to find an unprecedented “rhodium shift” event
5
,6
Coupling reactions with internal alkynes
have been
extensively studied since our group reported the first direct
annulation of benzoic acids adopting a Cp*Rh catalyst in
Received: June 26, 2020
5
a,b
2
007.
In addition, terminal alkenes and carbene precursors
have also functioned as the coupling partners for the
5s,7
annulation of benzoic acid derivatives.
As a consequence,
it is possible to design various 3-mono- and 3,4-disubstituted
isocoumarins; however, the basic unit, i.e. nonsubstituted
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c02112
A
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Scheme 1. Representative Synthetic Methods for
Isocoumarins
first examined the activity of Cp*Rh (entry 1) and Cp*Ir
(entry 2) complexes; however, the desired product 3a did not
form after attempts even at elevated temperature and with a
higher catalyst loading. We then shifted our attention to an
E
9
electron-deficient Cp ligand, which was found to be effective
for the oxidative annulation of benzoic acids in an elegant
5j
report by Shibata, Tanaka, and co-workers. To our delight,
the catalytic production of 3a was first realized using
E
[
Cp RhCl ] catalyst (entry 3).
2
2
Screening of silver salts suggested the importance of a
noncoordinating anion in this transformation (entries 3−6).
The yield was further improved to 74% after optimization
(
entry 7). Control experiments revealed that both the rhodium
complex and the silver salt were essential for this trans-
formation (entries 8 and 9). We also tested the effect of some
additives to the catalysis. AcOH did not considerably affect the
productivity (entry 10), whereas base additives such as
NaOAc, K CO , and NEt halted the reaction (entries 11−
2
3
3
13). These results implied that acids fulfill an important role in
the reaction mechanism (see below). Other solvents (octane,
PhCF , tert-AmOH, and HFIP) were not effective as compared
3
to DCE (entries 14−17). The product yield was slightly
improved when the reaction was conducted at 120 °C (entry
1
8), and this condition was used for the following study.
Next, we examined the scope of the reaction adopting
various substituted benzoic acids (Scheme 2). In some cases,
,1,2,2-tetrachloroethane was used instead of DCE to suppress
1
the competing esterification of the acids with the solvent
molecules. For the monosubstituted benzoic acids, a series of
functionalities such as alkyl (1b, 1k), aryl (1l), alkoxy (1c),
hydroxy (1d, 1m), chloro (1e), bromo (1f, 1j, 1n),
trifluoromethyl (1g, 1o), aldehyde (1h), and ester (1i) groups
remained untouched during the reaction, giving the corre-
sponding isocoumarins 3 in moderate to high yields. Such
functional group compatibility is highly beneficial for the
synthesis of complex molecules. The meta-substituted acid 1j
was converted into 3j as a major isomer, whereas the minor
isomer formed in negligible amount (<5%).
within the catalytic cycle. Additionally, the developed protocol
was utilized for the synthesis of an indole alkaloid, 2-
bromonorketoyobyrine.
Our initial study focused on the vinylene annulation of
benzoic acid (1a) with vinylene carbonate (2) (Table 1). We
a
Table 1. Optimization Study
Disubstituted as well as benzo-fused acids were also included
in the current protocol. 3,4-Dimethoxy benzoic acid (1p) was
smoothly annulated to give 3p in 73% yield as a major isomer
by activating a C−H bond at the more accessible position. In
contrast, piperonylic acid (1q) preferentially produced a
sterically congested isocoumarin 3q. The minor isomers of
b
entry
additives and conditions
yield
n.d.
n.d.
34%
41%
4%
n.d.
74%
n.d.
c
1
Cp*Rh catalyst, AgNTf (10 mol %)
2
c
2
Cp*Ir catalyst, AgNTf (10 mol %)
2
c
3
AgNTf (10 mol %)
AgSbF (10 mol %)
AgOTf (10 mol %)
AgOAc (10 mol %)
AgSbF (15 mol %)
2
c
4
6
3
p and 3q were observed only in <5% yield. This peculiar
c
5
5e
selectivity was also observed in the literature. 3,5-Dimethoxy
1r), 3,5-dibromo (1s), 4-chloro-2-methyl (1t), and 2,4-
c
6
(
7
8
9
6
dicholoro (1u) benzoic acids proceeded efficiently to give the
corresponding products. Notably, 2-naphthoic acid (1v) was
converted to 3v as a single isomer in 94% yield, whereas the
oxidative annulation of 1v with diphenylacetylene was reported
no Ag salt
no Rh catalyst, AgSbF (15 mol %)
n.d.
6
10
11
12
13
14
15
16
17
18
AgSbF (15 mol %), AcOH (30 mol %)
AgSbF (15 mol %), NaOAc (30 mol %)
68%
trace
trace
n.d.
20%
45%
n.d.
6
6
6j
to give a 4:1 mixture of isomers. 1-Napthanoic acid (1w)
AgSbF (15 mol %), K CO (30 mol %)
6
2
3
provided the product 3w in 95% yield, and this reaction could
be conducted in a 1.0 mmol scale. Benzo[b]thiophene
carboxylic acids 1x and 1y were also applicable to afford 3x
and 3y respectively in 47% and 48% yield. Unfortunately, other
heteroaromatic carboxylic acids involving pyridine, pyrrol, and
furan rings were not applicable to the present reaction system
AgSbF (15 mol %), NEt (30 mol %)
6
3
AgSbF (15 mol %), octane solvent
6
AgSbF (15 mol %), PhCF solvent
6
3
t
AgSbF (15 mol %), AmOH solvent
6
AgSbF (15 mol %), HFIP solvent
AgSbF (15 mol %), at 120 °C
55%
77% (74%)
6
6
(
not shown).
a
E
Reactivity of the nonsubstituted vinylene moiety of
Reaction conditions: 1a (0.2 mmol), 2 (0.2 mmol), [Cp RhCl ]
2.5 mol % metal), DCE (0.5 mL), 80 °C, 15 h. Determined by
NMR analysis. Isolated yield is in parentheses. n.d. = not detected.
2
2
b
isocoumarins has not been explored much possibly due to
their poor availability. Thus, we conducted some derivatization
of the coupling products to evaluate synthetic utility (Scheme
(
c
2
.0 equiv (0.4 mmol) of 2 were used.
B
https://dx.doi.org/10.1021/acs.orglett.0c02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Scheme 2. Substrate Scope of the Isocoumarin Synthesis
The bromination product 6 was converted to 7 upon basic
10
treatment through the selective HBr extraction, and the
structure of 7 was unambiguously determined by an X-ray
crystallographic analysis (CCDC 2008750). The bromo-
function in 7 may be a useful synthetic handle for further
derivatization of the molecule. Additionally, the reaction of 3w
with PhMgBr provided the corresponding keto-aldehyde 8 in
7
3% yield.
To highlight the synthetic utility, we conducted the synthesis
of an indole alkaloid (Scheme 4). A coupling product 3f was
Scheme 4. Synthesis of 2-Bromonorketoyobyrine (10)
treated with tryptamine under microwave irradiation to afford
the corresponding isoquinolinone 9. Sequential Lewis acid
mediated cyclization and oxidation by CeCl3 delivered a
pentacyclic product, 2-bromonorketoyobyrine (10), in 42%
11
yield over two steps. Installation of functional groups onto
the inherently less reactive ring over the pyrrol moiety is
12
challenging for the manipulation of indole alkaloids, and
cumbersome multistep synthesis is usually inevitable. As shown
in this particular example, the present method may provide
synthetically useful building blocks for various multicyclic
compounds.
a
E
A proposed reaction mechanism for the annulation of
benzoic acid (1a) with vinylene carbonate (2) is shown in
Scheme 5. The reaction initiates with the coordination of 1a to
Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol), [Cp RhCl ] (2.5
2
2
mol %), AgSbF (15 mol %), DCE (0.5 mL), 120 °C, 15 h.
6
b
E
c
[
Cp RhCl ] (4.0 mol %), AgSbF (30 mol %). Tetrachloroethane
2 2 6
was used instead of DCE.
Scheme 5. A Proposed Reaction Mechanism
3
). The double bond of isocoumarin 3w could be hydro-
genated cleanly with Pd/C to give 4 in 96% yield. Epoxidation
Scheme 3. Derivatization of the Coupling Product 3w
E
2+
the catalytically active [Cp Rh] species formed in situ, and
the subsequent C−H bond activation is effected with the
assistance of the carboxyl directing group to deliver a five-
membered rhodacycle intermediate. Coordination and mi-
gratory insertion of vinylene carbonate into the Rh−C bond
produce a seven-membered intermediate, in which the β-
hydrogen elimination is configurationally restricted. Thereafter,
a formal “rhodium shift” to the carbonate moiety takes place.
was effected using a dioxirane (TFDO) to provide 5 in 71%
yield. These results suggested that the double bond retains
considerable olefinic character. In accordance with this idea,
the double bond preferentially reacted with bromine in the
presence of FeBr catalyst keeping the aromatic unit unreacted.
3
C
https://dx.doi.org/10.1021/acs.orglett.0c02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Figure 1. Gibbs free energy profile of the reaction. Values in parentheses are relative enthalpies. The calculations were conducted at the ωB97X-D/
-311+G(d,p)&SDD/PCM(DCE)//ωB97X-D/6-31G+(d,p)&LanL2DZ/PCM(DCE) level.
6
We assume this occurs in a concerted manner, so that the
overall process would be redox-neutral. The product was
liberated via the β-oxygen elimination, closing the catalytic
cycle.
In order to shed light on the reaction mechanism in detail,
we conducted a computational study with respect to the
E
reaction of 1a with 2. Here the −CO Et groups on the Cp
2
ligand were replaced with −CO Me to simplify the calculation.
2
Figure 2. (a) An optimized molecular geometry of the transition state
tsEF: carbon (gray), hydrogen (white), oxygen (red), and rhodium
(green). (b) The atom labeling and selected bond lengths. Cp ligand
Additionally, acetic acid was employed as a supporting ligand
instead of benzoic acid and carbonic acid, which would
participate in the actual catalytic system. Figure 1 displays an
energy profile of the reaction relative to a benzoate complex
E
was omitted for the sake of clarity.
1
3,14
preA.
The C−H activation was calculated as a CMD
(
concerted metalation−deprotonation) process adopting the
E
acetate anion as an internal base to give a rhodacycle
intermediate intA. After the ligand exchange (intB−intC),
two possible configurations for the insertion of vinylene
carbonate to the Rh−C bond were considered (for details, see
the Supporting Information). A transition state in which the
carbonate moiety is located on the “opposite” side of the CpE
ligand requires 3.9 kcal/mol higher energy than that of the
other, thereby the path intC−tsCD was kinetically more
favorable. The direct C−O bond formation through reductive
elimination from intD or its AcOH adduct intE was found to
be not feasible. Alternatively, a concerted rhodium shift was
calculated to have a rather acceptable activation barrier of 34.0
acids through the Cp Rh-catalyzed vinylene annulation upon
adopting vinylene carbonate as an oxidizing acetylene
surrogate. This protocol exhibited broad functional group
compatibility and was utilized to the synthesis of a new indole
alkaloid molecule. The first mechanistic study for the vinylene
transfer reaction was conducted by the DFT calculations to
find an unprecedented intramolecular S 2-type rhodium shift
N
event, ensuring the redox-neutral catalytic process.
ASSOCIATED CONTENT
■
*
sı Supporting Information
(
32.9) kcal/mol (tsEF). In this transition state, the carbon
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02112.
atom directly bonded to the Rh atom had a trigonal
bipyramidal geometry where the two oxygen atoms occupied
its axial positions (Figure 2). The sum of the bond angles
within the equatorial plane was 359.3°, and the central carbon
atom exhibited a positive APT charge of +1.461. According to
these aspects, this process can be seen as an intramolecular
SN2-type reaction, and it is noteworthy that the coordinated
acid enhances the leaving ability of the carbonate fragment
through hydrogen bonding. This may be one of the reasons
why the reaction was inhibited by base additives (see Table 1).
The sequence traverses another transition state for the β-
oxygen elimination (tsFG) to produce the coupling product
Experimental procedures, kinetic experiments, computa-
1
13
tional data, and copy of H and C NMR spectra
PDF)
(
Atomic coordinates of all calculated molecules (XYZ)
Accession Codes
CCDC 2008750 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(intG).
In summary, we have introduced a direct synthesis of 3,4-
unsubstituted isocoumarins from readily available benzoic
D
https://dx.doi.org/10.1021/acs.orglett.0c02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Miyauchi, Y.; Fukui, M.; Sugiyama, H.; Uekusa, H.; Satoh, T.; Miura,
M.; Tanaka, K. Chem. - Eur. J. 2016, 22, 14190−14194. (k) Yedage, S.
L.; Bhanage, B. M. Green Chem. 2016, 18, 5635−5642. (l) Singh, K.
S.; Sawant, S. G.; Dixneuf, P. H. ChemCatChem 2016, 8, 1046−1050.
(m) Jiang, G.; Li, J. X.; Zhu, C.; Wu, W.; Jiang, H. Org. Lett. 2017, 19,
4440−4443. (n) Mandal, R.; Sundararaju, B. Org. Lett. 2017, 19,
AUTHOR INFORMATION
Corresponding Authors
■
Masahiro Miura − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0
871, Japan; orcid.org/0000-0001-8288-6439;
2544−2547. (o) Song, L.; Xiao, J.; Dong, W.; Peng, Z.; An, D. Eur. J.
Email: miura@chem.eng.osaka-u.ac.jp
Org. Chem. 2017, 2017, 341−349. (p) Ruiz, S.; Carrera, C.;
Villuendas, P.; Urriolabeitia, E. P. Org. Biomol. Chem. 2017, 15,
Yuji Nishii − Frontier Research Base for Global Young
Researchers, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan; orcid.org/0000-0002-
8
904−8913. (q) Singh, K. S.; Sawant, S. G.; Kaminsky, W. J. Chem.
Sci. 2018, 130, 120. (r) Qiu, Y.; Tian, C.; Massignan, L.; Rogge, T.;
Ackermann, L. Angew. Chem., Int. Ed. 2018, 57, 5818−5822.
6
824-0639; Email: y_nishii@chem.eng.osaka-u.ac.jp
Authors
Gen Mihara − Department of Applied Chemistry, Graduate
(s) Nguyen, T. T.; Grigorjeva, L.; Daugulis, O. Angew. Chem., Int.
Ed. 2018, 57, 1688−1691. (t) Liu, G.; Kuang, G.; Zhang, X.; Lu, N.;
Fu, Y.; Peng, Y.; Zhou, Y. Org. Lett. 2019, 21, 3043−3047. (u) Sihag,
P.; Jeganmohan, M. J. Org. Chem. 2019, 84, 2699−2712. (v) Lu, Q.;
Mondal, S.; Cembellín, S.; Greßies, S.; Glorius, F. Chem. Sci. 2019, 10,
6560−6564. (w) Yugandar, S.; Nakamura, H. Chem. Commun. 2019,
55, 8382−8385.
School of Engineering, Osaka University, Suita, Osaka 565-
871, Japan
0
Koushik Ghosh − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
(6) Selected examples: (a) Kajita, Y.; Kurahashi, T.; Matsubara, S. J.
0
871, Japan
Am. Chem. Soc. 2008, 130, 17226−17227. (b) Xie, H.; Sun, Q.; Ren,
G.; Cao, Z. J. Org. Chem. 2014, 79, 11911−11921. (c) Li, X. G.; Liu,
K.; Zou, G.; Liu, P. N. Adv. Synth. Catal. 2014, 356, 1496−1500.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02112
(
d) Mo, J.; Wang, L.; Cui, X. Org. Lett. 2015, 17, 4960−4963.
(e) Banerjee, A.; Santra, S. K.; Mohanta, P. R.; Patel, B. K. Org. Lett.
015, 17, 5678−5681. (f) Tan, H.; Li, H.; Wang, J.; Wang, L. Chem. -
Author Contributions
§G.M. and K.G. contributed equally to this work.
Notes
2
Eur. J. 2015, 21, 1904−1907. (g) Prakash, R.; Shekarrao, K.; Gogoi,
S.; Boruah, R. C. Chem. Commun. 2015, 51, 9972−9974. (h) Kaishap,
P. P.; Sarma, B.; Gogoi, S. Chem. Commun. 2016, 52, 9809−9812.
The authors declare no competing financial interest.
(
i) Yedage, S. L.; Bhanage, B. M. J. Org. Chem. 2017, 82, 5769−5781.
(j) Youn, S. W.; Yoo, H. J. Adv. Synth. Catal. 2017, 359, 2176−2183.
k) Mayakrishnan, S.; Arun, Y.; Maheswari, N. U.; Perumal, P. T.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant No. JP
9K15586 (Grant-in-Aid for Young Scientists) to Y.N. and JP
7H06092 (Grant-in-Aid for Specially Promoted Research) to
M.M. The authors thank Dr. Hiroaki Iwamoto (Osaka
University) for his assistance with DFT calculations.
■
1
1
(
Chem. Commun. 2018, 54, 11889−11892. (l) Tao, L.-M.; Li, C.-H.;
Chen, J.; Liu, H. J. Org. Chem. 2019, 84, 6807−6812. (m) Luo, M.;
Zhang, T.; Cai, F.; Li, J.; He, D. Chem. Commun. 2019, 55, 7251−
7254.
(7) Selected examples: (a) Miura, M.; Tsuda, T.; Satoh, T.; Pivsa-
Art, S.; Nomura, M. J. Org. Chem. 1998, 63, 5211−5215. (b) Nandi,
D.; Ghosh, D.; Chen, S. J.; Kuo, B. C.; Wang, N. M.; Lee, H. M. J.
Org. Chem. 2013, 78, 3445−3451. (c) Zhang, M.; Zhang, H.-J.; Han,
T.; Ruan, W.; Wen, T.-B. J. Org. Chem. 2015, 80, 620−627. (d) Liang,
Y.-F.; Yang, L.; Torben, R.; Ackermann, L. Chem. - Eur. J. 2018, 24,
REFERENCES
■
(
1) For selected reviews, see: (a) Saeed, A. Eur. J. Med. Chem. 2016,
1
16, 290−317. (b) Saddiqa, A.; Usman, M.; Cakmak, O. Turk. J.
Chem. 2017, 41, 153−178.
1
6548−16552. (e) Sun, R.; Yang, X.; Li, Q.; Xu, K.; Tang, J.; Zheng,
X.; Yuan, M.; Fu, H.; Li, R.; Chen, H. Org. Lett. 2019, 21, 9425−9429.
f) Huang, Y.; Lyu, X.; Song, H.; Wang, Q. Adv. Synth. Catal. 2019,
61, 5272−5276. (g) Liu, Y.; Wu, J.; Qian, B.; Shang, Y. Org. Biomol.
Chem. 2019, 17, 8768−8777.
8) (a) Ghosh, K.; Nishii, Y.; Miura, M. ACS Catal. 2019, 9, 11455−
1460. (b) Ghosh, K.; Nishii, Y.; Miura, M. Org. Lett. 2020, 22,
3547−3550.
(
2) For selected reviews, see: (a) Pal, S.; Chatare, V.; Pal, M. Curr.
Org. Chem. 2011, 15, 782−800. (b) Saikia, P.; Gogoi, S. Adv. Synth.
(
3
Catal. 2018, 360, 2063−2075.
(
3) For a recent review, see: Kuang, G.; Liu, G.; Zhang, X.; Lu, N.;
Peng, Y.; Xiao, Q.; Zhou, Y. Synthesis 2020, 52, 993−1006.
4) For selected reviews, see: (a) Satoh, T.; Miura, M. Chem. - Eur. J.
(
1
(
2
010, 16, 11212−11222. (b) Colby, D. A.; Bergman, R. G.; Ellman, J.
A. Chem. Rev. 2010, 110, 624−655. (c) Boyarskiy, V. P.; Ryabukhin,
E
D. S.; Bokach, N. A.; Vasilyev, A. V. Chem. Rev. 2016, 116, 5894−
(9) For other reactions using Cp ligand, see: (a) Shibata, Y.;
Tanaka, K. Angew. Chem., Int. Ed. 2011, 50, 10917−10921. (b) Fukui,
M.; Hoshino, Y.; Satoh, T.; Miura, M.; Tanaka, K. Adv. Synth. Catal.
2014, 356, 1638−1644. (c) Hoshino, Y.; Shibata, Y.; Tanaka, K. Adv.
Synth. Catal. 2014, 356, 1577−1585. (d) Takahama, Y.; Shibata, Y.;
Tanaka, K. Chem. - Eur. J. 2015, 21, 9053−9056. (e) Takahama, Y.;
Shibata, Y.; Tanaka, K. Org. Lett. 2016, 18, 2934−2937. (f) Fukui, M.;
Shibata, Y.; Hoshino, Y.; Sugiyama, H.; Teraoka, K.; Uekusa, H.;
Noguchi, K.; Tanaka, K. Chem. - Asian J. 2016, 11, 2260−2264.
(g) Yoshimura, R.; Shibata, Y.; Tanaka, K. J. Org. Chem. 2019, 84,
13164−13171.
5
986. (d) Sambiagio, C.; Schonbauer, D.; Blieck, R.; Dao-Huy, T.;
̈
Pototschnig, G.; Schaaf, P.; Wiesinger, T.; Zia, M. F.; Wencel-Delord,
J.; Besset, T.; Maes, B. U. W.; Schnurch, M. Chem. Soc. Rev. 2018, 47,
603−6743. (e) Santhoshkumar, R.; Cheng, C.-H. Chem. - Eur. J.
019, 25, 9366−9384.
5) Selected examples: (a) Ueura, K.; Satoh, T.; Miura, M. J. Org.
̈
6
2
(
Chem. 2007, 72, 5362−5367. (b) Ueura, K.; Satoh, T.; Miura, M. Org.
Lett. 2007, 9, 1407−1409. (c) Shimizu, M.; Hirano, K.; Satoh, T.;
Miura, M. J. Org. Chem. 2009, 74, 3478−3483. (d) Ackermann, L.;
Pospech, J.; Graczyk, K.; Rauch, K. Org. Lett. 2012, 14, 930−933.
(
2
e) Chinnagolla, R. K.; Jeganmohan, M. Chem. Commun. 2012, 48,
(10) For a recent example of indium-mediated synthesis of 4-
substitued isocoumarins, see: Kita, Y.; Yata, T.; Nishimoto, Y.; Chiba,
K.; Yasuda, M. Chem. Sci. 2018, 9, 6041−6052.
030−2032. (f) Frasco, D. A.; Lilly, C. P.; Boyle, P. D.; Ison, E. A.
ACS Catal. 2013, 3, 2421−2429. (g) Deponti, M.; Kozhushkov, S. I.;
Yufit, D. S.; Ackermann, L. Org. Biomol. Chem. 2013, 11, 142−148.
(11) Bai, L.-G.; Zhou, Y.; Zhuang, X.; Zhang, L.; Xue, J.; Lin, X.-L.;
Cai, T.; Luo, Q.-L. Green Chem. 2020, 22, 197−203.
(
4
h) Unoh, Y.; Hirano, K.; Satoh, T.; Miura, M. Tetrahedron 2013, 69,
454−4458. (i) Warratz, S.; Kornhaaß, C.; Cajaraville, A.; Niepo
B.; Stalke, D.; Ackermann, L. Angew. Chem., Int. Ed. 2015, 54, 5513−
517. (j) Kudo, E.; Shibata, Y.; Yamazaki, M.; Masutomi, K.;
̧
tter,
(12) Recent examples for the synthesis of related alkaloids: (a) Pan,
X.; Bannister, T. D. Org. Lett. 2014, 16, 6124−6127. (b) Li, K.; Ou, J.;
Gao, S. Angew. Chem., Int. Ed. 2016, 55, 14778−14783.
5
E
https://dx.doi.org/10.1021/acs.orglett.0c02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
(
(
13) For preliminary calculations, see the Supporting Information.
14) We have calculated the energies of Cp*Rh complexes
corresponding to some key intermediates and transition states;
however, no significant energy change was obtained to rationalize the
E
superior activity of Cp Rh catalyst in the present reaction.
F
https://dx.doi.org/10.1021/acs.orglett.0c02112
Org. Lett. XXXX, XXX, XXX−XXX