Rhodium-Catalyzed Sequential Cycloisomerization/Aldol Addition of Cyclopropene Carboxylic Acids with Isatins

of Cyclopropene Carboxylic Acids with Isatins

Letter

Rhodium-Catalyzed Sequential Cycloisomerization/Aldol Addition

†

Dan Zhang,* Xin Wang, Mengchu Zhang, and Wenhao Hu*

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sı Supporting Information

ABSTRACT: A unique, rhodium-catalyzed reaction of cyclopropene carboxylic acids with isatins has been developed, which takes

place through a trapping process of transient cyclic carboxylic oxonium ylides. This reaction tolerates a wide variety of cyclopropene

carboxylic acids and N-protected or unprotected isatins and represents a new, direct strategy to access valuable γ-substituted γ-

butenolide architectures containing an oxindole moiety in very high yields under mild reaction conditions.

ransient ylides are reactive intermediates in many carbene

transfer processes. For example, protic ammonium and

Based on this chemistry, a broad array of novel multi-

component reactions and cyclizations are developed, providing

eﬃcient approaches to access α-amino or α-hydroxy acid

derivatives and heterocycles. However, the previous studies are

severely conﬁned to acceptor and donor/acceptor carbenes,

which are convenient to generate from the stable and easily

oxonium ylides, derived from metal carbenes and the protic

amino groups (including amines, carbamates, and amides, etc.)

or hydroxyl groups (e.g., alcohols, water, phenols, and

carboxylic acids), respectively, are considered as the key

intermediates in N−H and O−H insertion reactions. In recent

years, these high energy intermediates from α-carbonyl diazo

compounds have been demonstrated to be trapped by

manipulated α-diazocarbonyl compounds. The donor car-

benes, such as vinyl carbenes, are rarely involved in the

trapping process, as they are diﬃcult to access from

corresponding vinyl diazo precursors that are unstable,

explosive, and toxic. Thus, the development of green access

to these carbenes would provide an opportunity to bring them

to the ylide-trapping chemistry.

electrophiles (Scheme 1a) by our group and the Che,

Gong, Szabo,

Schneider, Moody, and Bull groups.

Scheme 1. Electrophilic Trapping of Transient Protic Ylides

Recent development of cyclopropenes as nondiazo pre-

cursors of vinyl carbenes by Gevorgyan, Lee, Doyle, Cossy,

Wang, Shi, and Vicente et al. has led to many interesting

transformations. The readily accessible cyclopropene pos-

sesses a highly strained three-menbered ring and could be

converted into donor-type vinyl metal carbenes via transition-

metal-induced ring opening under safe and practical conditions

with 100% atom economy. Beneﬁting from these progress, we

recently uncovered a trapping process of alcoholic oxonium

ylide using cyclopropene as the vinyl carbene source. In the

process, a dirhodium complex induces isomerization of

cyclopropene alcohols to generate functional vinyl carbenes,

which further transform into oxonium ylides and are

Received: June 10, 2020

XXXX American Chemical Society

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

subsequently trapped by isatins at the vinylogous site (Scheme

to 3aa in 38% yield with 44:56 dr, and JohnPhosAu(MeCN)-

b). Inspired by the successful use of donor carbenes in the

SbF enhanced the yield to 72% with a low dr value. We then

5b,14

trapping chemisry, we turn to the capture of carboxylic

oxonium ylides, which is more challenging because the higher

acidity of carboxylic acids would promote the proton transfer

turned to rhodium catalysts.

and resulted in the desired product in 62% yield with a higher

dr of 83:17 (entry 4). Delightedly, when Rh (esp) was

Rh (OAc) was also eﬀective

2 4

and compromise the stability of the ylide intermediates. In

employed, the yield of 3aa was improved to 95% and the

order to facilitate the generation of carboxylic oxonium ylides,

cyclopropene carboxylic acid, a substrate incorporating the

source of vinyl carbene and a carboxyl group (COOH), was

utilized as an ylide precursor (Scheme 1c). We envisioned that

the activation of cyclopropene carboxylic acids by transition

metals would lead to an electrophilic vinyl carbene that could

be intercepted rapidly by the tethered carboxyl group to form

diastereoselectivity increased to 84:16, in a shorter reaction

time (12 h; entry 5). For other catalysts, AgOTf decreased

both the yield and dr value, and AgSbPF could maintain a

^hⁱ^g^h^yⁱ^e^l^d^b^u^t^dⁱ^mⁱⁿⁱ^s^h^t^h^e^dⁱ^a^s^t^e^r^e^o^s^e^l^e^c^tⁱ^vⁱ^t^y^,^w^hⁱ^l^e^C^u^O^T^f2

resulted in a low yield of 3a (entries 6−8). Subsequently, using

Rh (esp) as catalyst, a series of solvents were screened to

further improved the diastereoselectivity (entries 9−15), but

no better results were achieved. Among these investigations,

ethyl acetate as the solvent could slightly increase the yield

cyclic carboxylic oxonium ylide. The conjugated CC

functionality in this architecture would stabilize the inter-

mediate and facilitate the trapping by an electrophile (isatin) at

the carbenic site. Compared to the rearrangement of

cyclopropene carboxylic acids or derivatives to produce furan

(

98%) and maintain dr value (84:16). When elevating the

reaction temperature (77 °C), the reaction completed in 1.5 h

but caused a decrease of diastereoselectivity (entry 16). We

also tried some chiral rhodium catalysts, but no induction of

enantioselectivities or improvement of diastereoselectivies was

achieved (see the Supporting Information ), suggesting that

rhodium may not involve the step of aldol reaction. Therefore,

the reaction conditions shown in entry 11 were chosen as the

optimized ones.

or butenolide derivatives, this trapping process of cyclic

carboxylic oxonium ylides could deliver valuable γ-butenolide

derivatives containing an oxindole moiety under mild

1,12

conditions.

Furthermore, this is the ﬁrst aldol-type reaction

of carboxylic oxonium ylides.

We commenced the present study by conducting the

reaction of N-benzyl isatin (1a) and 3-phenyl-cyclopropenyl-

With the optimal conditions identiﬁed, we next evaluated

the scope of this unique transformation (Scheme 2). With

respect to the N-benzyl isatins, all substituents tested on the

aromatic ring, including halides (F, Cl, Br), strong electron-

withdrawing groups (CF , NO ), and electron-donating groups

-acid (2a) under the promotion of transition metals in DCM

at 20 °C. First, three widely used gold complexes in

cyclopropene chemistry were tested (Table1, entries 1−3),

in which (PPh )AuNTf was not eﬀective, (PPh )AuOTf led

(

Me, MeO), regardless of the positions, are well tolerated, and

the desired products (3aa−3al) were obtained in almost

quantitative yields with moderate to good diastereoselectivity

Table 1. Optimizations of Reaction Conditions

(

up to 86:14 dr). Notably, the reaction could be conducted in

gram scale for 3ak, which was obtained in almost quantitative

yield (99%, 1.06 g) with 82:18 dr. Furthermore, other N-

substituents, such as acetyl (Ac), tert-butyloxycarboryl (Boc),

and methyl (Me), had no deleterious eﬀect on the yields and

diastereoselectivities (3am−3ao). Moreover, when it came to

the unprotected isatins, the reaction also worked well for all

assessed substrates with various substituents on diﬀerent

positions of the aryl ring and led to 3ap−3aw in high yields,

in which the temperature was adjusted to 75 °C to improve the

reactivity.

We then surveyed the substituent eﬀect of the cyclopropene

carboxylic acids (Scheme 3). Generally, the reaction proceeded

well and delivered corresponding γ-butenolide products in very

high yields with moderate to good dr values. For example, the

substrate with an ortho-Br was compatible with reaction

conditions (3ba), and the meta-substituted substrate aﬀorded

the desired products (3bb−3bc) in 99% and 96% yield,

respectively. Para-substituted cyclopropene carboxylic acids

were also nicely tolerated in this reaction and provided 3bd−

3bh with good results (95−99% yield). In addition, for the

reactants possessing di- and trimethoxy phenyl groups, we also

observed high yields of the desired products (3bi−3bj).

Furthermore, the bicyclic and tricyclic substrates could be used

without a decrease in yield (3bk−3bl), and particularly, the

cyclopropene carboxylic acid derived from isoxepac was also

eﬀective to the reaction, resulting in the polycyclic heterocyclic

product 3bl in 98% yield.

Entry

ML_n

solvent

CH Cl

T/°C yield/%

(PPh )AuNTf

N.R.

−

(PPh )AuOTf

JohnPhos Au(MeCN)

CH Cl

44:56

38:62

CH Cl

SbF

^R^h⁽^O^A^c⁾4

CH Cl

83:17

84:16

33:67

30:70

81:19

83:17

67:33

82:18

^R^h⁽^e^s^p⁾2

CH Cl

AgOTf

CH Cl

^A^g^S^b^F6

CH Cl

^C^u^O^T^f2

CH Cl

(CH Cl)

^R^h⁽^e^s^p⁾2

^C^H^C^l3

^R^h⁽^e^s^p⁾2

MTBE

^R^h⁽^e^s^p⁾2

toluene

EtOAc

acetone

PhCl

^R^h⁽^e^s^p⁾2

99 (98 ) 84:16

^R^h⁽^e^s^p⁾2

72:28

81:19

79:21

^R^h⁽^e^s^p⁾2

Rh (esp)₂

EtOAc

Reaction conditions: 1a (0.10 mmol), 2a (0.15 mmol), metal

catalyst (gold, silver or copper) (5.0 mol %) or Rh L (2.0 mol %),

solvent (1.5 mL), 20 °C, 24 h. Determined by H NMR analysis of

the crude products based on internal standard (1,3,5-trimethox-

To prove the synthetic utility of the reaction, the major

diastereoisomer of products were used as the reactant for

further transformations (Scheme 4). For example, hydro-

ybenzene). Determined by H NMR analysis of the crude products.

Reaction time: 36 h. Reaction time: 12 h. Isolated yield. Reaction

time: 1.5 h.

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Scheme 2. Scope of Isatins

Scheme 4. Transformations of Products

genation of 3ak led to γ-butyrolactone 4 in 99% yield with

excellent diastereoselectivity. Furthermore, 1,3-dipolar cyclo-

addition of 3ak or 3be with N-benzyl-N-methoxymethyl-N-

trimethylsilyl) methylamine (5) as the precursor of the

(

azomethine ylide aﬀorded bicyclic heterocycle 6a or 6b in 98%

yield and 96% yield, respectively, with a good dr value. The

relative conﬁguration of 6b was conﬁrmed by X-ray single

crystal analysis, which indicated that the 1,3-dipolar cyclo-

addition proceeded on the less hindered face of the alkene, i.e.

trans to the oxindolyl moiety and cis to the H at the γ-carbon.

Finally, a reaction mechanism was proposed in Scheme 5.

First, the rhodium catalyst binds to the CC bond of

Scheme 5. Proposed Reaction Mechanism

For 3ap−3aw, temperature = 75 °C, reaction time = 1−3 h. Isolated

yields are provided.

Scheme 3. Scope of Cyclopropene Carboxylic Acids

cyclopropene 2 and induces the opening of the strained three-

membered ring to produce vinyl carbene A. Intramolecular

interaction of the carbene center with the carbonyl oxygen of

the tethered carboxylic acid moiety results in carbonyl ylide B.

Disassociation of the rhodium catalyst will give the nonmetal

associated ylide C and its more stable resonance structure, 3-

aryl-2-hydroxyfuran (D), which is nucleophilic at the C5-

position and trapped by isatins 1 via aldol-type addition to

form desired products 3. A competing reaction versus the

trapping process is the proton transfer, leading to O−H

insertion product 7.

To further elucidate the reaction pathway, a control

experiment using the performed cycloisomerization product

(

7) of cyclopropene carboxylic acid was carried out (Scheme

). First, the rhodium-catalyzed transformation of 3-(4-

bromophenyl)-cyclopropenyl-3-carboxylic acid in the absence

of isatin 1a could provide γ-butenolide 7 in 56% yield. When 7

was treated with isatin 1a under standard conditions, no

reaction took place and 3bf was not detected. This control

experiment indicated that the proposed reactive protic

oxonium ylides (B or C) could not be generated from

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Scheme 6. Control Reaction Using Preformed O−H

Insertion Product 7 as the Nucleophile

Author Contributions

†

D.Z. and X.W. contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Financial support by National Natural Science Foundation of

China (No. 21901259), Guangdong Basic and Applied Basic

Research Foundation (No. 2020A1515011116), and the

Program for Guangdong Introducing Innovative and En-

trepreneurial Teams (No. 2016ZT06Y337) are greatly

acknowledged.

butenolide 7, and the pathway involving trapping of reactive

intermediates is more likely.

In conclusion, we had developed a unique reaction of readily

accessible cyclopropene carboxylic acids and isatins under

rhodium catalysis. This reaction proceeds through an electro-

philic trapping of highly reactive carbonyl ylides generated

from nondiazo precursors, providing a direct protocol to γ-

butenolides possessing an oxindole moiety in excellent yields

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ASSOCIATED CONTENT

sı Supporting Information

■

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.orglett.0c01957.

Synthetic procedures, characterizations and copies of

NMR spectra (PDF)

Accession Codes

CCDC 1956386 and 2008050 contain 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

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

(3) (a) Zhou, C.-Y.; Wang, J.-C.; Wei, J.; Xu, Z.-J.; Guo, Z.; Low, K.-

AUTHOR INFORMATION

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Dan Zhang − Guangdong Key Laboratory of Chiral Molecule

and Drug Discovery, School of Pharmaceutical Sciences, Sun

Yat-Sen University, Guangzhou 510006, China; orcid.org/

000-0003-0200-8527 ; Email: zhangdan5@mail.sysu.edu.cn

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Wenhao Hu − Guangdong Key Laboratory of Chiral Molecule

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Yat-Sen University, Guangzhou 510006, China;

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Xin Wang − Guangdong Key Laboratory of Chiral Molecule and

Drug Discovery, School of Pharmaceutical Sciences, Sun Yat-Sen

University, Guangzhou 510006, China

Mengchu Zhang − Guangdong Key Laboratory of Chiral

Molecule and Drug Discovery, School of Pharmaceutical

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