1,2-Benzothiazines from Sulfoximines and Allyl Methyl Carbonate by

Mar 24, 2017 - 4-Unsubstituted 1,2-benzothiazines are prepared from sulfoximines and allyl methyl carbonate by Rh(III)-catalyzed domino allylation/oxi...
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1,2-Benzothiazines from Sulfoximines and Allyl Methyl Carbonate by Rhodium-Catalyzed Cross-Coupling and Oxidative Cyclization Jian Wen, Deo Prakash Tiwari, and Carsten Bolm* Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany S Supporting Information *

ABSTRACT: 4-Unsubstituted 1,2-benzothiazines are prepared from sulfoximines and allyl methyl carbonate by Rh(III)-catalyzed domino allylation/oxidative cyclization. A wide range of functional groups on the sulfoximine are tolerated. A plausible mechanism is proposed, and chemical modifications of the products have been explored.

S

ulfoximines are natural products1 with important applications in synthetic chemistry2 and biorelated research.3 Among the numerous possible derivatives, 1,2-benzothiazines represent an interesting scaffold consisting of an all-carbon aromatic fragment and an annulated nonaromatic heterocycle with a stereogenic sulfur.4−8 In light of the many applications of structurally related heterocycles,9,10 the use of 1,2-benzothiazines appears underrepresented. For the preparation of 1,2-benzothiazine cores various synthetic approaches have been reported. The first dates back to 1971 when Williams and Cram used a sulfur imination/ cyclization reaction sequence leading to lactam-type product 2 from sulfoxide 1 (Scheme 1).5 Subsequently, Harmata and coworkers demonstrated palladium-catalyzed cross-couplings between ortho-bromo sulfoximine 3 and alkynes 4 providing isomeric heterocycles 5 and 6 (Scheme 1).6 Recently, we applied rhodium-catalyzed C−H-bond functionalizations for the synthesis of 1,2-benzothiazines starting from sulfoximines 7 and internal alkynes 8 or diazo dicarbonyl compounds 10 providing 1,2-benzothiazines 9 and 11, respectively (Scheme 1).7,8 From these results it became apparent that the selective, direct preparation of 4-unsubstituted 1,2-benzothiazines 13 has remained difficult.8a Here, we provide a strategy toward such compounds based on work by Saá and co-workers, who reported rhodium-catalyzed tandem C−H allylations and oxidative cyclizations of anilides leading to indoles.11a In our case, we started from sulfoximes 7 and allyl carbonates 12.11,12 The study was initiated by examining the reaction of sulfoximine 7a with allyl methyl carbonate (12a) in DCE at 100 °C, using a combination of [Cp*RhCl2]2 (4 mol %) and AgSbF6 (16 mol %) as the catalytic system and Cu(OAc)2·H2O (2.1 equiv) as the oxidant. To our delight, the desired product 13a was obtained in 45% yield after 14 h (Table 1, entry 1). At 110 and 90 °C the yield of 13a was slightly lower (Table 1, entries 2 and 3). From a reaction without Cu(OAc)2·H2O a small amount of compound 18a was isolated (for details, see Supporting Information (SI)), suggesting that the transformation involved a rhodium-catalyzed domino allylation/ oxidative cyclization. Screening other solvents revealed DME (dimethoxyethane) as the most effective one providing 13a in a © XXXX American Chemical Society

Scheme 1. Syntheses of 1,2-Benzothiazines

yield of 58% (Table 1, entries 4−7 and SI). Extending or shortening the reaction time (from 14 to 20 and 8 h, respectively) lowered the yield of 13a (Table 1, entries 8 and 9). Replacing the 1:4 mixture of [Cp*RhCl2]2 and AgSbF6 by [Cp*Rh(CH3CN)3][SbF6]2 (4 and 8 mol %) was less efficient (Table 1, entry 10). While substituting allyl carbonate 12a by allyl chloride or allyl alcohol did not lead to 13a, the product was observed in reactions with allyl tert-butyl carbonate (14), allyl benzoate (15), allyl acetate (16), and allyl diethyl Received: February 17, 2017

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DOI: 10.1021/acs.orglett.7b00488 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

Scheme 2. Substrate Scopea

entry

allyl source

solvent

temp (°C)

t (h)

yield of 13a (%)

1 2 3 4 5 6 7 8 9 10b 11 12 13 14 15c,d 16e

12a 12a 12a 12a 12a 12a 12a 12a 12a 12a 14 15 16 17 12a 12a

DCE DCE DCE THF MeOH toluene DME DME DME DME DME DME DME DME DME DME

100 110 90 100 100 100 100 100 100 100 100 100 100 100 100 100

14 14 14 14 14 14 14 20 8 14 14 14 14 14 14 14

45 38 40 31 20 26 58 51 50 51 31 47 50 15 72 46

a

All reactions were performed in sealed tubes on a 0.3 mmol scale (with respect to sulfoximines 7).

a

Reaction conditions: sulfoximine 7a (0.3 mmol), allyl source (0.9 mmol, 3.0 equiv), [Cp*RhCl2]2 (4 mol %), AgSbF6 (16 mol %), Cu(OAc)2·H2O (2.1 equiv), and solvent (1.5 mL) at 100 °C for 14 h in a sealed tube under argon. bUse of [Cp*Rh(CH3CN)3][SbF6]2 (4 mol %) as catalyst; 45% of 13a with 8 mol % of that catalyst. cUse of HOAc (1.0 equiv) as an additive. dScale-up to 1 mmol of 7a: 58% of 13a (see SI for details). eUse of PivOH (1.0 equiv) as an additive.

syntheses of compounds 13n−q, which were obtained in yields ranging from 60% to 71%. Presumably due to steric reasons, ortho-chloro-substituted sulfoximine 7r did not react well, providing only a trace of 1,2-benzothiazine 13r. Under standard conditions, attempts to use substituted allyl carbonates 12b and 12c as a coupling partner for sulfoximine 7a remained unsuccessful. However, performing the catalyses in the absence of Cu(OAc)2·H2O proceeded well leading to the corresponding ortho-substituted products 18a−c in yields ranging from 45% to 50% (Scheme 3). In each case, the Zisomer of the product was formed with high preference (as determined by NMR spectroscopy).15

phosphate (17). In all cases, however, the yield of 13a was lower than that when using 12a (Table 1, entries 11−14). Finally, the addition of acetic acid (1 equiv) proved beneficial, providing 13a in 72% yield (Table 1, entry 15). Switching from acetic acid to pivalic acid as an additive lowered the yield of the product to 46% (Table 1, entry 16). With the optimized conditions in hand, the substrate scope was evaluated. First, various sulfoximines were subjected to the reaction with allyl methyl carbonate (12a). In general, the yields of the resulting 1,2-benzothiazines 13 were moderate to good (Scheme 2). In the series of S-aryl S-methyl sulfoximines with para-substituted arenes a slight electronic effect was observed. Thus, those substrates with electron-donating substituents led to slightly higher product yields than those having electron-withdrawing groups (e.g., 13b and 13c versus 13g−i). While exclusive site selectivity was observed in the reaction with meta-bromo-substituted S-aryl S-methyl sulfoximine 7j providing 13j as a single regioisomeric product in 54% yield, meta-methoxy-substituted 7k provided 36% of a 2:1 mixture of (unseparated) regioisomers 13k and 13k′. In addition, two S-2-naphthyl-substituted sulfoximines showed exquisite site selectivity leading to 13l and 13m in 63% and 58% yield, respectively. Noteworthy, the latter product distinguished itself from the previously discussed ones by having an S-n-propyl substituent instead of an S-methyl group. S-Phenyl sulfoximines with other S-alkyl moieties led to the corresponding 1,2-benzothiazines as well, as exemplified by the

Scheme 3. Ortho-Olefinations with Allyl Carbonatesa,b

a

All reactions were performed in sealed tubes on a 0.3 mmol scale (with respect to sulfoximines 7a). bThe Z/E ratios were determined by 1 H NMR spectroscopy.

Two mechanistic scenarios explain the results (Scheme 4). As shown for sulfoximine 7a, both start with the formation of five-membered rhodacycle A. Experimental evidence for its involvement was reported earlier.7a Starting from A, two pathways can be distinguished. The first is an oxidative Mizoroki−Heck/β-elimination type pathway,13 which is initiated by migratory insertion of the allylic double bond of B

DOI: 10.1021/acs.orglett.7b00488 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

13a with NBS/AIBN in carbon tetrachloride gave dibromobenzothiazine 19 in 70% yield.16 The reaction of 19 with deprotonated pyrazole in tetrahydrofuran at room temperature afforded substitution product 20 in 51% yield.17 Analogously, dibenzylamine and methyl phenyl amine gave 21 and 22 in 80% and 89% yield, respectively.18 Finally, 19 was treated with sodium azide in DMF leading to the corresponding azide, which was reacted with phenyl acetylene in toluene at room temperature under catalysis with copper(I)-thiophene-2carboxylate providing “click product” 23 in 71% yield (over two steps).19 In summary, we developed a protocol for the preparation of 4-unsubstituted 1,2-benzothiazines by rhodium(III)-catalyzed domino allylation/oxidative cyclization of NH-sulfoximines with allyl methyl carbonate. It allows access to a variety of synthetically valuable products in a straightforward manner. Their chemical potential has been demonstrated by initial functionalization reactions.

Scheme 4. Suggested Reaction Pathways

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00488. General experimental procedure and characterization details (PDF)

12a into the Rh−C bond11,14 affording seven-membered oxygen-chelated rhodacycle B. Upon β-oxygen elimination, B is converted into olefin-coordinated Rh(III) complex D. Alternatively, D can be formed via Rh(III) species C, which is generated by coordination of the carbonate oxygen to the metal center. The rhodium/oxygen interaction facilitates an intramolecular nucleophilic substitution leading to complex D by fragmentation. From D, product 13a is formed by sequential metal-mediated ring closure and β-hydride elimination followed by double bond isomerization. In this process a Rh(I) species is expelled and reoxidized to Rh(III) by the Cu(II) salt. If the latter is absent, complex D is protonated leading olefin 18a presumably through a β-hydride elimination/isomerization sequence involving a [Rh−H] complex generated in situ. The synthetic potential of the products was demonstrated by modifying 1,2-benzothiazine 13a, which was selected as representative starting material (Scheme 5). Treatment of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carsten Bolm: 0000-0001-9415-9917 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.W. is grateful to the China Scholarship Council for predoctoral stipends. D.P.T. acknowledges support by Alexander von Humboldt Foundation.

Scheme 5. Selected Transformations of 13a

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DOI: 10.1021/acs.orglett.7b00488 Org. Lett. XXXX, XXX, XXX−XXX