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Stereoselective synthesis of 1,3-disubstituted isoindolines via Rh(III)catalyzed tandem oxidative olefination-cyclization of 4-aryl cyclic sulfamidates a,+

DOI: 10.1039/x0xx00000x

Se-Mi Son , Yeon Ji Seo

a,b,+

*a,b

, and Hyeon-Kyu Lee

www.rsc.org/

Rh(III)-catalyzed tandem ortho C-H olefination of cyclic 4-aryl sulfamidates (1) and subsequent intramolecular cyclization is described. This reaction serves as a method for the direct and stereoselective synthesis of 1,3-disubstituted isoindolines (3) starting with enantiomerically enriched 4-aryl cyclic sulfamidates. In the process, the configurational integrity of the stereogenic center in the starting cyclic sulfamidate is completely retained. In addition, the process generates trans-1,3-disubstituted isoindolines exclusively. The isoindoline ring system is an important structural unit in a variety of, pharmaceuticals and bioactive natural and nonnatural products.1 However, only a limited number of synthetic procedures have been developed for the preparation of isoindolines and, in particular, 1,3-disubstituted isoindolines in both racemic as well as enantiomerically enriched forms.1,2 Therefore, new strategies for efficient synthesis of optically active 1,3-disubstituted isoindolines are in high demand. Transition metal catalysed, C-H bond functionalization reactions of arenes bearing pre-installed directing groups have become powerful methods for the direct, and step- and atomeconomic synthesis of structurally diverse substances.3 Oxidative C-H activation and olefination of aromatic compounds bearing directing groups has been achieved by using Pd(II),4 Ru(II),5 and Rh(III)6 complexes as catalysts. More recently, Rh(III) complexes have been explored as catalysts for these reactions. The results show that lower Rh(III) catalyst loadings can be utilized and that C-H activation processes promoted in this manner take place with higher efficiencies and broader functional group compatibilities.6 In addition, a large effort has been devoted to developing methods for site-

selective olefination reactions of arenes that rely on a variety 7 8 of proximal directing groups such as ketones, acids, 4j,6c,9 6b,6i 10 11 esters, amides, anilides, carbamates, 6g,6h,6j 6f 12 13 sulfonamides, triflamides, hydrazines and hydroxyls. These functional groups both increase the activities of the transition metal catalysts and improve the regioselectivity of the reactions. However, despite the fact that significant advances have been made over the last decade, the ability to promote chirality transfer in C-H activation processes remains a great 14 challenge. As a result, the development of asymmetric versions of C-H activation reactions is clearly needed. Diastereoselective transformations induced by C-H bond activation are typically achieved by employing transition metal catalysts that have chiral ligands or by incorporating a chiral auxiliary in the directing group.14a,14b,14e Recently we uncovered a highly efficient method for stereoselective synthesis of 4-aryl cyclic sulfamidates (1) from the corresponding imines (B) which occurs via asymmetric transfer hydrogenation. HCO2H/Et3N serves as the hydrogen source in the new protocol and a well-defined Noyori-type chiral Rh-complex is used as the catalyst. Importantly, the reaction takes place under mild and experimentally convenient conditions15 (rt, 30 min) and efficiently produces the sulfamidate with a high level of enantioselectivity (up to >99% ee, 99% yield) (Scheme 1). Scheme 1. Stereoselective synthesis of 4-aryl cyclic sulfamidates (1) (ref. 15a)

OH NH2 SO2Cl

X



O O N S O HCO2 H/Et3N

O

A

X

B

(S,S)-Rh-cat. EA, rt, 0.5 h

O O HN S O

Ph

Ts N

Ph

N H2

Rh

(R)-1 X up to 99% ee & 99% yield

Cl

(S,S)-Rh-cat

This finding stimulated us to design a new process, in which the cyclic sulfamidate group in the enantiomerically enriched 4-aryl cyclic sulfamidates (1) guides the stereochemistry of regioselective, transition metal catalysed

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reactions of arenes with olefins taking place through sequential ortho C-H bond activation and oxidative C-C bond formation (Scheme 2). Moreover, we anticipated that the configuration of the stereogenic center in the 4-aryl cyclic sulfamidate (1) substrates would be retained in the process and control the relative configuration of the newly generated chiral center in the coupling product. Studies exploring this proposal have led to the development of the first Rh-catalyzed tandem oxidative ortho C-H bond activation and oxidative C-C bond forming reactions of chiral cyclic-sufamidate substituted arenes with olefins that generate the 1,3-disubstitute isoindolines (3) in a highly stereoselective manner (Scheme 3).

Table 1. 4-Aryl cyclic sufamidates 1 scope of the reaction 2a

a

Scheme 2. Directing group assisted, oxidative C-H functionalization

Scheme 3. Oxidative C-H olefination-cyclization of 4-aryl cyclic sulfamidate 1

(R)-4-Phenylsulfamidate ((R)-1a) and methyl acrylate (2a) were chosen as substrates to explore features of the new tandem process. The results of an extensive study, screening various transition metal catalysts and oxidants (see SI, Table S2 & S3), showed that 4 mol% of [RhCp*Cl2]2 and 200 mol% of Cu(OAc)2 promote reaction of (R)-1a with 2a (t-amyl alcohol, o 110 C, 5 h) to form the 1,3-disubstituted isoindoline (1R,3S)3a in 92% isolated yield. The structure and stereochemistery of (1R,3S)-3a were unambiguously determined by using X-ray crystallographic analysis (CCDC-1435134). The pathway for production of (1R,3S)-3a likely involves initial Rh-catalyzed oxidative C-C bond formation between the two ortho C-H sites in (R)-1a with methyl acrylate followed by cyclization via aza-Michael addition of the sulfamide nitrogen to the β-carbon of one acrylate moiety (see SI-1, Scheme S1). Importantly, an uncyclized, bis-olefinated intermediate 4 is not detected under the reaction conditions (Scheme 3). Moreover, the conversion of (R)-1a (99.8% ee) to (1R,3S)-3a (99.5% ee) takes place with complete retention of enantiomeric purity and the configuration of the newly generated stereogenic C3center in 3a is (3S) corresponding to exclusive formation of trans-1,3-di-substituted isoindoline (1R,3S)-3a.

a Reaction conditions: 1 (0.3 mmol), 2a (0.9 mmol), [RhCp*Cl2]2 (4 mol%), Cu(OAc)2 (200 mol%), t-AmOH (3 mL), 110 oC for 5-12 h in sealed tubes. bee was determined using chiral HPLC. cIsolated yields after silica gel chromatographic purification. dThe structures and stereochemistry of (1R,3S)-3a (CCDC-1435134) and (1R,3S)-3c (CCDC-1400887) were determined by using X-ray crystallographic analysis. eThe structures of (1R,3S)-3d and (1R,3S)-3e were determined by using 2D-NOSEY spectroscopy analysis (see SI-2). fAgOAc was used as the oxidant, instead of Cu(OAc)2, in toluene solvent.

The results emerging from the initial phase of this investigation show that the [RhCp*Cl2]2 promoted reaction between (R)-1a and 2a takes place in a single step with formation of two C-C and one C-N bond with complete control of the stereochemistry at the newly generated stereogenic center in (1R,3S)-3a. With optimized reaction conditions in hand, the scope and limitations of the coupling reaction were explored using a variety of 4-aryl cyclic sulfamidates 1 with methyl acrylate (2a). Observations made in this effort (Table 1) demonstrate that various 4-aryl cyclic sulfamidates (1) bearing a variety of electron-donating and -withdrawing aryl substituents react with 2a under the optimized conditions to form the corresponding 1,3-disubstituted isoindolines (3) in high yields and with excellent levels of stereoselectivity. In addition, the substrate 1b, bearing a substituent at the orthoposition also participate in this process to produce the monoolefinated and cyclized product, 3b, in excellent yield (96%). meta-Substituted 4-aryl cyclic sulfamidates, 1d and 1e, also react to form the corresponding isoindolines, 3d and 3e, efficiently. Interestingly, in these processes cyclization occurs only at the sterically more hindered position (determined by

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using 2D-NOSEY spectroscopy, see SI-2). Cyclic sulfamidate derivatives of thiophene or furan also produce oxidativeolefination products under the reaction conditions. However, in these cases, only the respective uncyclized indolines 3l and 3m are generated. It should be noted that 4-furan substituted cyclic sulfamidate 1m is unstable under the reaction conditions and, as a result, 3m is formed in a low yield (31%). Interestingly, 2-naphthyl-sulfamidate 1n gave no products under the standard reaction conditions but, when Rh(III)catalyst loading was increase (4 mol% → 8 mol %), the isoindoline 3n was obtained in 36% yield. Moreover, when AgOAc is employed as the oxidant instead of Cu(OAc)2, the isolated yield was improved to 72%. In this process, cyclization occurs only at the sterically more hindered position (determined by using 2D-NOSEY spectroscopy, see SI-2). Recently, Chang and coworkers descibed a rhodiumcatalyzed ortho-olefination of ethyl benzoate with methyl acrylate, which demonstrates that an ester moiety can also 6c serve as an effective chelating/activating group. To compare the chelating ability of ester and cyclic sufamidate groups, 4(4-methoxycarbonyl-phenyl) cyclic sulfamidate (1k) was subjected to the reaction conditions. The process generates 3k as the sole product (94%), showing that the cyclic sufamidate moiety has a stronger chelating ability than does the ester group (Scheme 4).

phenyl vinyl sulfone (2e), and acrylamide (2f) also provided the corresponding products 3bd, 3be and 3bf. Interestingly, oxidative coupling reaction of 1a with excess (4 eq.) acrylonitrile (2d) leads to formation of the monoolefinationcyclization product 3ad (CCDC-1432488) mainly. In contrast, methyl methacrylate (2h) does not undergo oxidative olefination with 1a under the optimized conditions. Also, reaction of 4-chlorostyrene (2g) with 1a does not occur under the standard conditions but di-olefinated derivative 3ag is generated in 30% yield when AgOAc is employed as the 6g oxidant instead of Cu(OAc)2. Finally, reaction of methyl vinyl ketone (2i) with 1b afforded the olefination-cyclization product 3bi, albeit low yield (37%). Table 2. Olefins scope of the reaction

a

O O HN S O

H

Cu(OAc)2 (2 eq) t-Amyl-OH, 110oC

H R 3(% ee,b % yieldc)

2a-2i

CO2But

CO2Et

SO2Ph

CN

H

EtO2C S

2a

2b

CON(Me)2

H

EtO2C R

2d Me

2g

2f

Scheme 4. Comparison of chelating ability of ester and cyclic sulfamidate groups

2c Ph-4-Cl

O O S O

N

H

t

BuO2C

H

COMe

2h

CO2Et (R)-1a(99.8%ee) -> (1R,3S)-3ab(99.5%ee), 86%

2i

O O S O

N

O O S O N R

2e

CO2Me

O O S O

N

[RhCp*Cl 2]2 (4 mol%)

R (3 eq)

+ X 1a: X=H 1b: X=Me

CO2Me

H

R

H

NC

O O S O N

NC

H

O O S O

N

S

H

H

CO2Et (S)-1a (99.8%ee) -> (1S,3R)-3abd,e(>99%ee), 84%

CO2tBu 1a(99.8%ee) -> 3ac(99.5%ee), 79%

e

PhO2S

N-Methyl- and N-Boc-4-phenyl cyclic sulfamidates, 1o and 1p, were prepared in order to explore the influence of Nsubstituents on the chelation-assisted oxidative C-H olefination reaction. Studies with 1o and 1p show that both do not participate in reactions with methyl acrylate to generate the corresponding olefinated products, 4o and 4p, under the optimized conditions (Scheme 5).

O (Me)2N

H

O O S O

N

H

PhO2S

O O S O

H

H Me

H

O O S O

N

H Me 1b(92%ee) -> 3bf(-%ee), 54%

O Me

O O S O

N

1a(99.8%ee) -> 3add(-%ee), 54% O O HN S O

1b(92%ee) -> 3be(93%ee), 58% H

Me

Ph-4-Cl

N

SO2Ph 1a(99.8%ee) -> 3ae(-%ee), 24%

H

H

Ph-4-Cl 1a(99.8%ee) -> 3agf(-%ee), 30% Me O

MeO2C

H Me 1b(92%ee) -> 3big(-%ee), 37%

Ph-4-Cl

O O HN S O

Me 1b(92%ee) -> 3bgf(-%ee), 43%

O S

N

1b(89.2%ee) -> 3bd(90%ee), 51%

O Me CO2Me

1a(99.8%ee) -> 3ah(-%ee), 0%

a

Reaction conditions: see Table 1. bee was determined using chiral HPLC. Isolated yields after silica gel chromatographic purification. dThe absolute structures of (1S,3R)-3ab (CCDC-1400877) and 3ad (CCDC-1432488) were determined by using X-ray crystallographic analysis. e(S)-1a was used instead of (R)-1a. fReaction conditions: [RhCp*Cl2]2 (5 mol %), AgOAc (4 eq), t-AmOH, 120 oC, 48 h. g1.1 equiv. of 2i was employed. c

Scheme 5. Influence of N-substituents of 4-aryl cyclic sulfamidates

The olefin scope of the new process was probed using reactions of 2a-2i with 1a or 1b. The results (Table 2) show that not only methyl but also ethyl and t-butyl acrylates undergo efficient coupling reactions with 4-phenyl cyclic sulfamidate (1a) to form the corresponding adducts 3ab, and 3ac. Reaction of 1b with other olefins such as acrylonitrile (2d),

The isoindolines generated in the reactions described above contain cyclic sulfamidate groups, which participate in reaction with various nucleophiles. As a result, this functionality can serve as a versatile handle to introduce other 15,16 groups into the isoindoline ring system. For example, treatment of 5, produced by NiCl2-mediated NaBH4 reduction 6g o of 3c, with NaN3 (DMF, 60 C, 4 h, 92%) leads to generation of the 1-(azido-methyl)-3-(methoxycarbonyl methyl) substituted isoindoline 6. In addition, treatment of 5 with pcresol (NaH, DMF, rt, 12 h, 88%) produces the 1-[(4-

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methylphenoxy)-methyl]-3-(methoxycarbonyl isoindoline 7.

Journal Name methyl)-

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Scheme 6. Synthetic Applications

5

6 In summary, a new Rh(III)-catalyzed tandem ortho C-H olefination and cyclization reaction of 4-aryl cyclic sulfamidates (1) has been developed. The process serves as a direct and stereoselective method for synthesis of 1,3-disubstituted isoindolines 3 from enantiomerically enriched 4-aryl cyclic sulfamidates (1). The isoindolines generated in this process have trans-1,3 relative configurations and the enantiomeric purity of the cyclic sulfamidate is completely retained in the product. The resulting 1,3-disubstituted isoindoline 3c, which contain the nucleophile reactive cyclic sulfamate moiety, can be further transformed to azido- (6) or phenoxide-substituted (7) analogs in high yields. This research was financially supported by grants from the National Research Foundation of Korea (2008-2004732) and Korea Research Institute of Chemical Technology (SI1512).

7 8 9

Notes and references

10

1

11

2

3

4

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Stereoselective synthesis of 1,3-disubstituted isoindolines via Rh(III)-catalyzed tandem oxidative olefination-cyclization of 4-aryl cyclic sulfamidates.

Rh(III)-catalyzed tandem ortho C-H olefination of cyclic 4-aryl sulfamidates (1) and subsequent intramolecular cyclization are described. This reactio...
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