Rhodium(III)-Catalyzed Redox-Neutral Cascade [3 + 2] Annulation of N-Phenoxyacetamides with Propiolates via C-H Functionalization/Isomerization/Lactonization

Article abstract of DOI:10.1021/acs.orglett.8b03082

A Rh(III)-catalyzed cascade [3 + 2] annulation of N-phenoxyacetamides with propiolates under mild conditions using the internal oxidative O-N bond as the directing group has been achieved. This catalytic system provides a regio- and stereoselective access to benzofuran-2(3H)-ones bearing exocyclic enamino motifs with exclusive Z configuration selectivity, acceptable to good yields and good functional group compatibility. Mechanistic investigations by experimental and density functional theory studies suggest that a consecutive process of C-H functionalization/isomerization/lactonization is likely to be involved in the reaction.

Full text of DOI:10.1021/acs.orglett.8b03082

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium(III)-Catalyzed Redox-Neutral Cascade [3 + 2] Annulation of
N‑Phenoxyacetamides with Propiolates via C−H Functionalization/
Isomerization/Lactonization
Jin-Long Pan,†,⊥ Peipei Xie,§,⊥ Chao Chen,† Yu Hao,† Chang Liu,† He-Yuan Bai,† Jun Ding,†
Li-Ren Wang,¶ Yuanzhi Xia,*,§ and Shu-Yu Zhang*,†,‡
‡
†Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs & School of Chemistry and Chemical Engineering, Key
¶
Laboratory for Thin Film and Microfabrication of Ministry of Education, and Zhiyuan College, Shanghai Jiao Tong University,
Shanghai 200240, P. R. China
§College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China
S
* Supporting Information
ABSTRACT: A Rh(III)-catalyzed cascade [3 + 2] annulation of N-
phenoxyacetamides with propiolates under mild conditions using
the internal oxidative O−N bond as the directing group has been
achieved. This catalytic system provides a regio- and stereoselective
access to benzofuran-2(3H)-ones bearing exocyclic enamino motifs
with exclusive Z configuration selectivity, acceptable to good yields
and good functional group compatibility. Mechanistic investigations
by experimental and density functional theory studies suggest that a
consecutive process of C−H functionalization/isomerization/lactonization is likely to be involved in the reaction.
Scheme 1. Transition-Metal-Catalyzed C−H Activation and
Annulation with Alkynes
ransition-metal-catalyzed C−H bond functionalization/
Tannulation cascade reactions continue to capture the
attention of chemists and provide a powerful and convenient
alternative to synthesize heteroatom-containing compounds in
an ecofriendly and step-economical manner.1
In the past decade, transition-metal-catalyzed C−H function-
alization/[n + 2] annulation of aromatic substrates bearing
directing groups (DG) with internal alkynes, acting as two-
carbon synthons, to synthesize heterocycles has been well
documented.2 In addition to these methods, increasing attention
has been devoted to transition-metal-catalyzed C−H function-
alization reactions using N-phenoxyacetamides as the privileged
substrates. They offer opportunities and possibilities to develop
new transformations by utilizing the unique reactivity of O−
NHAc acting as a redox-active DG, which avoids the use of
stoichiometric amounts of external oxidants.3−10 In 2013, Liu
and Lu pioneered an efficient synthesis of substituted
benzofurans and ortho-hydroxyaryl enamides via a Cp*Rh(III)-
catalyzed coupling between N-phenoxyacetamides and alkynes
with tunable selectivity under mild reaction conditions (Scheme
1, eq 1).3a Followed by this pioneering work, the research groups
of Liu,3b−e Wang,4 Zhao,5 Yi,6 You,7 Glorius,8 Zhou,9 and
others10 have made great advances in transition-metal-catalyzed
C−H functionalization reactions of N-phenoxyacetamides. In
contrast, few examples were reported involving internal alkynes,
serving as one-carbon coupling partners, in transition-metal-
catalyzed C−H activation/[n + 1] annulations.11 In 2015, Chang
disclosed the first example of Cp*Rh(III)-catalyzed redox-
neutral [4 + 1] annulation of arylnitrones with internal alkynes to
afford indolines using the nitrone group as the internal oxidative
directing group accompanied by oxygen atom transfer (Scheme
1, eq 2).11a More recently, Gooßen demonstrated an example of
Ru(II)-catalyzed formal [3 + 3] annulation of benzoic acids with
propargylic alcohols through oxidative C−H hydroarylation and
subsequent intramolecular lactonization to provide γ-alkylidene-
δ-lactones bearing exocyclic C−C double bonds, in which only
one of two alkyne carbon atoms was installed in the lactone ring
Received: September 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03082
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Letter
(Scheme 1, eq 3).12 Inspired by the above elegant work, we
envisioned that a combination of C−H activation and intra-
molecular lactonization using N-phenoxyacteamides and
propiolates would be highly interesting and promising. In line
with our interest in transition-metal-catalyzed C−H activa-
tions,13 herein, we present our new development of rhodium-
(III)-catalyzed redox-neutral cascade [3 + 2] annulation of
readily available N-phenoxyacetamides and propiolates to
furnish a straightforward approach to benzofuran-2-(3H)-one
skeletons bearing exocyclic enamino motifs, by means of O−
NHAc serving as an internal oxidative directing group (Scheme
1, eq 4). Interestingly, benzofuran-2(3H)-ones represent an
important class of valuable synthons and privileged structural
cores in numerous natural products, drug candidates, and
biologically active compounds.14
reaction was sluggish when 1,4-dioxane was employed as the
solvent (entries 2−4). Interestingly, 3-phenylpropiolates with
ethyl and benzyl substituents bonded on oxygen afforded 3aa in
similar yields (entries 5 and 6). However, lower yields were
obtained with phenyl, iso-propyl, and tert-butyl groups (entries
7−9). This may due to the steric effect, which is not favorable to
the lactonization step. Next, a survey of additives indicated that
KOAc was more practical and provided better results (entries
10−12). Finally, we were pleased to find that the desired product
3aa could be isolated in 70% yield by slightly increasing catalyst
loading (entries 13 and 14). Control experiments showed that
Cp*Rh(III) was crucial toward the success of this reaction as its
omission led to no formation of 3aa. Other representative Cp*M
catalysts (M = Ir, Ru, or Co) did not show any catalytic activity.
Increasing the amount of 1a did not give a higher yield.
Moreover, N-phenoxypivalamide (PhONHPiv) did not afford
the corresponding product. Gratifyingly, the reaction was not
sensitive to the air atmosphere and moisture. Thus, we obtained
the optimal catalytic system consisting of 1a (0.2 mmol), 2a (1.0
equiv), [Cp*RhCl2]2 (3.5 mol %), and KOAc (0.25 equiv) in
MeCN at room temperature for 24 h.
We evaluated our hypothesis with N-phenoxyacetamide 1a
and 3-phenylpropiolate 2 (Table 1). To our delight, upon the
a b
,
Table 1. Optimization of Reaction Conditions
After establishing the optimized reaction conditions, we
sought to examine the scope and limitations of N-phenox-
yacetamides in the present Cp*Rh(III)-catalyzed redox-neutral
C−H functionalization/isomerization/lactonization cascade
reactions. As shown in Scheme 2, para-substituted N-
phenoxyacetamides containing alkyl and phenyl groups led to
the construction of the desired benzofuran-2-(3H)-one skel-
etons bearing exocyclic enamino motifs in reasonable yields
(3ba−3fa). Moreover, 4-vinyl and 4-halo groups were
compatible, the corresponding products could be generated
entry
solvent
MeOH
DCE
additive
2
yield (%)
1
2
3
4
5
6
7
8
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOPiv
KOAc
2a
2a
2a
2a
2a-2
2a-3
2a-4
2a-5
2a-6
2a
58
61
29
66
66
64
47
49
13
68
69
58
1,4-dioxane
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
a b
,
Scheme 2. Substrate Scope
9
10
11
12
2a
2a
2a
2a
NaOAc
KOAc
KOAc
c
d
13
e
76 (70)
14
65
a
Reaction conditions: 1a (0.20 mmol), 2 (0.20 mmol), [Cp*RhCl2]2
(2.5 mol %), and additive (0.25 equiv) in solvent (2.0 mL) at room
temperature for 24 h. DCE = 1,2-dichloroethane. Yields were
determined by H NMR analysis with 1,3,5-trimethoxybenzene as an
internal standard. [Cp*RhCl2]2 (3.5 mol %) was used. Isolated
yield was shown in parentheses. [Cp*RhCl2]2 (2.0 mol %) was used.
b
1
c
d
e
treatment of 1a with 2a in the presence of dimeric [Cp*RhCl2]2
as catalyst precursor and CsOAc as an additive in MeOH at room
temperaturefor24h, theunexpectedannulationproduct3aawas
obtained with a NMR yield of 58% (entry 1). The unexpected Z-
configuration of the newly formed exocyclic double bond of 3aa
was unambiguously confirmed by X-ray diffraction analysis, and
it was speculated that the Z-geometry may be stabilized by the
N−H···O type intramolecular interaction (hydrogen bond)
(δH(NH) ≈ 11 ppm, and the distance between the hydrogen
atom of amide and the oxygen atom of the ester carbonyl group
was nearly 2.0 Å).15 Intrigued by the formation of 3aa, we then
attempted to further extensively optimize the reaction
parameters (see Table S3 in Supporting Information (SI) for
detailed optimization studies). First, a screening of solvents
revealed that MeCN was the optimal reaction medium, while the
a
Reaction conditions: 1 (0.20 mmol), 2 (0.20 mmol), [Cp*RhCl2]2
(3.5 mol %), and KOAc (0.25 equiv) in MeCN (2.0 mL) at rt for 24
h. Isolated yield. Performed with [Cp*RhCl2]2 (5.0 mol %) at 50
°C.
b
c
B
DOI: 10.1021/acs.orglett.8b03082
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Letter
though higher reaction temperatures and increased catalyst
loadings were required (3ga−3ia). For meta-substituted
substrates, the reaction underwent smoothly and showed
excellent regioselectivity occurring at the less sterically hindered
position (3ja−3ma). Substrates bearing an alkyl group or a
chlorine atom at the ortho-position or two methyl groups at the
para-and meta-positionsonthearylringsgave thecorresponding
products in acceptable yields (3na−3qa) under the enhanced
conditions. Substrates containing electron-withdrawing groups
(such as CF3 and CO2Me) showed poor reactivity.
To further illustrate the substrate scope, we subsequently
explored a wide range of propiolates under standard conditions.
To our satisfaction, various electron-donating and -withdrawing
functionalities (such as alkyl, phenyl, formyl, acetyl, hydrox-
ymethyl, trifluoromethyl, nitro, dimethylamino, ethers, and
halides) on the para- or meta-positions of the aromatic rings
were well-tolerated, giving the corresponding benzofuran-2-
(3H)-one derivatives in yields ranging from 60% to 77% (3ab−
3aq and 3at−3av). Propiolates with a substituent on the 2-
position of the aryl group showed low reactivity (3ar and 3as).
Additionally, the reaction could be successfully extended to 3-
thiophen-2-yl and 3-thiophen-3-yl propiolates, delivering the
desired products in 78% and 69% yields, respectively (3aw and
3ax). Disappointingly, 3-alkyl and 3-alkenyl propiolates gave
unsatisfactory results, suggesting that the 3-(hetero)aryl groups
were essential under our catalytic system.
equivalent 2,2,6,6-tetramethylpiperidine-N-oxyl(TEMPO), 1,1-
diphenylethylene, or butylated hydroxytoluene (BHT) as a
radical scavenger, the desired product 3aa could still be isolated
in maintained yields, implying that the reaction is most likely not
basedonaradicalpathway(seetheSIformoredetails). Toprobe
the active intermediate, astable five-memberedrhodacycle B was
prepared.5d The Cp*Rh(III) complex B reacted with equal
equivalent2atogive3aainaNMRyieldof43%inthepresenceof
2 equiv of HOAc, concomitantly generated in the catalytic cycle,
which is essential for the formation of product. Moreover, the
Cp*Rh(III) complex B was proven to be a robust catalyst for the
cascade reaction of 1a and 2a to give 3aa without or with catalytic
amount of KOAc in 43% and 62% yields, respectively. The above
resultsdemonstratedthattherhodacycleBismostlikelyinvolved
inthecatalyticcycle(Scheme4a). Underthestandardconditions
Scheme 4. Experimental Mechanistic Investigations
Wethenturnedourattentiontoproveboththepracticalityand
effectiveness of this method in organic synthesis. Notably, the
cascade [3 + 2] annulation of 1a with 2a was scaled up to 6 mmol
under the standard conditions to give access to 3aa in 66% yield.
The resultant enaminoate moiety can be used to prepare
heterocycles, and readily be converted to chiral β-amino acids by
metal-catalyzedasymmetrichydrogenations.16 Theobtained3aa
could be conveniently transformed to enamine 4, which could be
further converted to the corresponding 2H-azirine 5 via
intramolecular cyclization mediated by PhI(OAc)2 (Scheme
3a).17 Importantly, this method can be applied to late-stage
Scheme 3. Synthetic Applicability
in the presence of D2O, 1a was recovered and 42% deuterium
incorporationwasobserved. Incontrast, thereactionof1aand2a
in the presence of D2O gave rise to the desired product 3aa in a
maintained yield, and deuterium was not incorporated in the
product (<5% D). These experiments illustrated that C−H bond
activation was largely reversible, and that the C−C bond
formation was not reversible (Scheme 4b). The KIE value for
competitive and parallel reactions were determined to be 2.85
and 1.05, respectively, which suggested that C−H bond cleavage
might not to be involved in the rate-determining step (Scheme
4c, see the SI for more details).
modification of bioactive product derivatives, as exemplified by
the transformation of dopamine derivative 1r to give the
corresponding product 3ra with moderate yield and good
functional group tolerance (Scheme 3b).
To get a better understanding of the reaction mechanism, a
series of experiments were then carried out. In the presence of
Based on the aforementioned experimental results, together
with precedent literature,8c,e,9a,18 we have proposed a plausible
C
DOI: 10.1021/acs.orglett.8b03082
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Figure 1. DFT-computed potential energy surface for Rh(III)-catalyzed [3 + 2] annulation of 1a with 2a.
mechanism for the Cp*Rh(III)-catalyzed cascade [3 + 2]
annulation (see the SI for more details).
to carbonyl group via TS8 to form the addition intermediate Q.
Finally, the elimination of OMe from complex R is realized via
TS9, leading to product 3aa and Cp*Rh(OAc)(OMe), from
which the regeneration of the active Cp*Rh(OAc)2 is quite
favorable. If no Rh(III) is contained in the lactonization process,
much higher activation barriers are required.19 The overall
activation energy required for transforming I into 3aa is 32.4
kcal/mol. Thisisthe highestbarrierinthe wholereaction(Figure
1), suggesting the lactonization controls the rate of the reaction.
This is in good agreement with the observation of 6 as an
intermediate in reaction of 1a with 2a-6.20
Insummary, wehave developedanunexpected cascade[3+ 2]
annulation of readily available N-phenoxyacetamides and
propiolates via rhodium(III)-catalyzed redox-neutral C−H
functionalization/isomerization/lactonization using the O−
NHAc as an internal oxidative directing group. The reaction
provides an access to benzofuran-2(3H)-ones containing
exocyclic enamino motifs featuring exclusive Z-configuration
selectivity, moderate to good yields, broad substrate scope,
excellent functional group compatibility, and mild reaction
conditions. Experimental and computational mechanistic
investigationsrevealthatthecascadereactionispossiblyinvolved
in a C−H functionalization/isomerization/lactonization se-
quence. Further applications of this strategy are in progress in
our laboratory.
To gain further details into these unexpected cascade
annulation reactions, density functional theory (DFT) calcu-
lations were performed (Figure 1).19 Upon the activation of 1a
with Cp*Rh(III), the first formation of B is quite facile with an
activation barrier around 20 kcal/mol via sequential N−H
deprotonation via TS1 and C−H cleavage via TS2. Although this
process is slightly exergonic, the reverse process is possible if no
further reaction occurs, explaining the observation of deuterium
incorporation in Scheme 4b. From B, π complex K could be
formed by coordination with 2a, and the following migratory
insertion via TS3 requires an overall barrier of 23.3 kcal/mol.
This step is highly regioselective as the insertion via the
alternative orientation would be unfavorable both kinetically
and thermodynamically.19 Upon the favorable generation of C,
the nitrenoid D could be formed by N−O cleavage via TS4 with
an activation barrier of 25.1 kcal/mol. This intermediate is
unstableandundergoesC−Nformationby1,2-alkenylmigration
via TS5, leading to cyclic enamine intermediate E highly
exergonically.
As isomerization of the enamino moiety was observed in
products, details of this intriguing result were next unveiled. It
was found that the protonation of E occurs first via TS6 to give a
freephenol moietyand thattheRh(III) isbondedtotheenamine
moiety in L. The enamine−imine isomerism could be the reason
for C−C double bond isomerization since the C-bonded
intermediates M and N are close in energies to the N-bonded
intermediates L and O, respectively (Figure 1). We found that
this mechanism is the most favorable as other possibilities were
calculated to be higher in energies.19 By incorporation of one
HOAc, protonation of the amino moiety in O delivers
intermediate I via TS7. The N−H···O interaction in I should
bethedrivingforceforthisisomerizationprocess, asitis5.4kcal/
mol lower in energy than the trans-isomer I′.19
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03082.
Experimental procedures, characterization data of all new
compounds, DFT data, andcopies of NMRspectra (PDF)
Accession Codes
To form the lactone moiety in final product 3aa, the
intramolecular substitution process was studied theoretically. It
is suggested that this process could be realized by an addition−
elimination mechanism in which the Rh(III) is involved. In this
mechanism, a first deprotonation of the phenol with Cp*Rh-
(OAc)2 to form P is required. After that, the O−Rh moiety adds
CCDC 1845127, 1845991, and 1850276 contain the supple-
mentary 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 Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
D
DOI: 10.1021/acs.orglett.8b03082
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Letter
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xyz@wzu.edu.cn.
*E-mail: zhangsy16@sjtu.edu.cn.
ORCID
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Jin-Long Pan: 0000-0002-1085-4566
Yuanzhi Xia: 0000-0003-2459-3296
Shu-Yu Zhang: 0000-0002-1811-4159
Author Contributions
⊥These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the “Thousand Youth TalentsPlan”,
NSFC (21672145 and 21572163), the Shuguang program
(16SG10) from Shanghai Education Development Foundation
and Shanghai Municipal Education Commission, and the
Science and Technology Commission of Shanghai Municipality
(17JC1403700). We thank Prof. Changkun Li (School of
Chemistry and Chemical Engineering, Shanghai Jiao Tong
University) for helpful suggestions and comments on this
manuscript.
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DOI: 10.1021/acs.orglett.8b03082
Org. Lett. XXXX, XXX, XXX−XXX

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(19) All DFT calculations were done at the (SMD)M06/6-311+
+G(d,p)/SDD//6-311+G(d,p)/LANL2DZleveloftheory, andrelative
salvation free energies (ΔGsol in kcal/mol) are given. See the SI for more
details.
(20) When 1a was treated with tert-butyl 3-phenylpropiolate 2a-6
instead of 2a, the unlactonized 3-aminoacrylate ester 6 was generated in
42% yield, the structure of which was unambiguously supported by the
X-ray diffraction analysis.
F
DOI: 10.1021/acs.orglett.8b03082
Org. Lett. XXXX, XXX, XXX−XXX