Cross-dehydrogenative alkynylation of sulfonamides and amides with terminal alkynes via Ir(III) catalysis

Guocai Wu , Wensen Ouyang , Qian Chen , Yanping Huo and Xianwei Li *
School of Chemical Engineering and Light Industry, Guangdong University of Technology, No. 100 Waihuan Xi Road Guangzhou Higher Education Mega Center, Guangzhou, 510006, China. E-mail: xwli@gdut.edu.cn

Received 15th October 2018 , Accepted 31st October 2018

First published on 1st November 2018


Primary sulfonamides are widely used structural skeletons in bioactive molecules, however their direct modification via C–H functionalization remains to be developed. Herein, direct cross-dehydrogenative C–H alkynylations of amides and primary sulfonamides with terminal alkynes is reported. This general process features predictable regioselectivity towards multiple functionalized arenes and the stereoselective generation of tetrasubstituted olefins, which might provide insight into the expedient delivery of related bioactive and material molecules.


Introduction

Sulfonamides and amides serve as ubiquitous skeletons occurring in materials science and bioactive molecules, such as celecoxib, brinzolamide, sultiame, piroxicam, brinzolamide, glibenclamide and so on.1 Indeed, great advancement towards their rapid synthesis and late-stage modification has been witnessed, affording various functionalized molecules in a selective fashion. It is worth noting that the direct C–H functionalization2 of amides and sulfonamides serves as an alternative powerful strategy. Nevertheless, to realize a balance of reactivity and regioselectivity, quinolone, pyridine or oxazoline substituted amides are often needed,3 while for sulfonamides, SO2NHAc or SO2NH-fluorinated arenes are often used, which reduces the practicality of the overall process.4 Nevertheless, there remains some limitations in the C–H functionalization of sulfonamides and amides: (1) direct dehydrogenative coupling5 of the C–H/C–H bonds of amides and sulfonamides, which would improve the convenience of the process, is still in its infancy; (2) although they have great bioactivities when used in pharmaceuticals and pesticides, few examples of primary sulfonamide directed C–H functionalization have been shown. Thus, exploration for efficient transformations via the cross-dehydrogenative coupling of primary sulfonamides is highly desirable.

Alkyne functional groups are among the most used key motifs in bioactive and material molecules; they also act as versatile building blocks in organic synthesis, and can be well transformed which leads to a rapid increase in molecular complexity.6 The Sonogashira reaction has been proven to be one of the most powerful strategies for the construction of functionalized alkynes.7 Moreover, direct C–H alkynylation has also been greatly advanced with the assistance of nitrogen heterocycles, functionalized amides with additional chelation systems (Scheme 1, (1)) or specific electron-rich or electron-poor arenes or heterocycles.8,9 In this context, direct oxidative C(sp2)–H and C(sp)–H couplings of arenes with terminal alkynes would be of great synthetic practicality.10 Indeed, Yu10g developed an elegant example of amide-oxazoline directing group promoted oxidative C–H alkynylation; Yi and Li10b utilized –ONHAc as the versatile functionality for the generation of alkyne substituted phenols.


image file: c8qo01105b-s1.tif
Scheme 1 Oxidative C–H alkynylation via metal catalysis.

Driven by our interest in the construction of alkynes,11 we hypothesized that due to the easy Glaser coupling of terminal alkynes, the key issue with realizing the direct C–H alkynylation of sulfonamides is the difficulty of forming an effective concentration of the active organometallic intermediate, probably due to the weak interaction of the sulfonamides with the metal catalyst (Scheme 1, (2)). Thus, the combination of a highly electrophilic metal catalyst and a suitable oxidative system12 might be helpful for facilitating the regioselective C–H alkynylation of sulfonamides. Herein, we report amide and primary sulphonamide assisted dehydrogenative C–H alkynylation, which features great regioselectivity towards multiple functionalized arenes and stereoselectivity for the generation of tetrasubstituted olefins (Scheme 1, (3)).

Results and discussion

We commenced our study with the direct cross-dehydrogenative coupling of 1a with terminal alkyne 2 using various metal catalysts and oxidants (Table 1, entries 2–5). Initial experiments using Pd(II) and Ni(II) catalysts failed to give the desired product 3a. To our delight, [Ru(p-cymene)Cl2]2 and [IrCp*Cl2]2 could catalyse this reaction well with the assistance of AgNTf2, while Rh(III) showed less efficiency. Significantly, acetate salt was beneficial for the formation of the desired product 3a. Further investigation of the reaction conditions revealed that AgNTf2 showed a better performance than AgSbF6, while Ir(III), AgNTf2, NaOAc and Cs2CO3 were crucial parameters for this transformation (entries 7–10). The addition of protonic acid into this catalytic system was proven to be inferior (entry 11). A DCE/HFIP mixed solvent was proven to be optimal for this transformation, while a trace amount of 3a was detected when toluene was used as the solvent (entry 12).
Table 1 Variation of standard conditionsa

image file: c8qo01105b-u1.tif

Entry Variation of standard conditions Yieldb (%)
a Reaction conditions: 1a (0.10 mmol), 2 (0.20 mmol) in solvent (1.0 mL), N2, catalyst (4 mol%), 120 °C, 12 h. b Isolated yield. c Addition of tBuOOtBu (1.5 equiv.). d Addition of Cu(OAc)2 (2.0 equiv.).
1 Standard conditions 72
2c Pd(OAc)2 as the catalyst n.r.
3d NiCl2 as the catalyst n.r.
4 [Ru(p-cymene)Cl2]2 as the catalyst 41
5 [RhCp*Cl2]2 as the catalyst 31
6 AgSbF6 instead of AgNTf2 63
7 Without [IrCp*Cl2]2 n.r.
8 Without AgNTf2 27
9 Without NaOAc 15
10 Without Cs2CO3 24
11 Addition of PivOH (0.5 equiv.) 26
12 Toluene as the solvent n.r.


We next studied the scope of this dehydrogenative C–H alkynylation of sulfonamides (Table 2), and various N-substituents were first tested. The results indicated that N-acyl substituent sulfonamide 1b showed lower reactivity than primary sulfonamide 1a in this transformation, which is different from previous reports on sulfonamide (with electron-withdrawing groups on nitrogen atom) directed C–H functionalization and, thus, might provide a complementary strategy for the modification of sulfonamides. Further investigation of N-substitutions revealed that the propionyl group (1c) exhibited similar efficiency to the acyl substituent; while ester (1d) and methyl (1e) groups delivered no desired product. N-Acyl substituent sulfonamides (1f, 1g) exhibited a much lower efficiency than primary sulfonamides (1h–1m). As for primary sulfonamide assisted C–H alkynylation, electron-rich aryl sulfonamides exhibited better performances than those of electron-poor primary sulfonamides. Functional groups on the aryl sulfonamides, such as fluoro (1i), chloro (1j), and bromo (1k), were well compatible; readily transformable functionalities, such as cyan (3l) and nitro (3m), were tolerated. Fused ring systems, such as naphthalene (3n), were also compatible in this transformation. However, to our disappointment, simple aliphatic or aromatic terminal alkynes could hardly participate in this catalytic C–H alkynylation, as only an N–H addition to the C–C triple bond, as well as Glaser coupling of the terminal alkyne products, was observed.

Table 2 C–H alkynylation of sulfonamidesa
a Reaction conditions: 1 (0.10 mmol), 2 (0.20 mmol), Ir(III) (2.0 mol%), AgNTf2 (8 mol%), AgOAc (2.0 equiv.), NaOAc (30 mol%), Cs2CO3 (50 mol%), DCE/HFIP (0.8/0.2 mL) at 90 °C, 12 h. b 2 (0.40 mmol), C–H dialkynylation product was isolated.
image file: c8qo01105b-u2.tif


We further explored the synthetic practicality of this catalytic system in amides (Table 3), which are ubiquitous skeletons in bioactive molecules. N-Substituents, including alkyl groups (5a), morpholine (5b), piperidine (5c), and pyrrolidine (5d) were tolerated. Functional groups on the aromatic amides such as bromide (5e) and benzylic chloride (5g) had good compatibility. For fused rings such as β-amide naphthalene, C–H dialkynylation took place (5f), leading to the dialkynylation product 5f. Double C–H alkynylation products (5h, 5i) could also be obtained with an increase in the amount of alkyne 2 or with bi-directing groups.

Table 3 Dehydrogenative C–H alkynylation of aryl amidesa
a Reaction conditions: 4 (0.10 mmol), 2 (0.20 mmol), Ir(III) (4.0 mol%), AgNTf2 (16 mol%), AgOAc (2.0 equiv.), NaOAc (30 mol%), Cs2CO3 (50 mol%), DCE/HFIP (4[thin space (1/6-em)]:[thin space (1/6-em)]1, 1 mL) at 100 °C, 12 h. b 2 (0.40 mmol). c with AgOAc (1.0 equiv.).
image file: c8qo01105b-u3.tif


Due to their synthetic value in the expedient construction of bioactive molecules, heterocyclic amides were investigated. Furan (5j), thiophene (5k), benzofuran (5l), and benzothiophene (5m) were suitable substrates in this catalytic C–H alkynylation. Notably, pyridines, as key precursors in pharmaceuticals, were often not good coupling partners in common C–H functionalizations, probably due to their strong coordination effect to the metal catalyst. To our delight, nicotinamides (5n, 5o) were suitable substrates in this regioselective C–H alkynylation with moderate efficiency. C4–H alkynylation of an indole derivative (5p) was also observed, which might be attributed to the directing effect.13 Notably, ortho-alkyne aryl primary amide 5q was obtained with NHOMe or NHOPiv directing groups, which provided a complementary methodology for the generation of ortho alkyne substituted aryl primary amides.

The above results drove us to further develop the stereoselective synthesis of enynes from acrylamides, since there were still some limitations in the precedent work:14 (1) stereospecific alkenyl halides or related organometallic reagents are needed, which are often not easy to access; (2) highly substituted alkenes show a lower efficiency. Inspired by the chelation effect used in the stereoselective synthesis of functionalized alkenes, developed by Loh, Chang and others,15 we assumed that the stereospecific generation of highly substituted stereospecific enynyl amides might be operational with the assistance of amides. To our delight, various acrylamides were suitable substrates in this catalytic C–H alkynylations, delivering tri- and tetra-substituted olefins with great stereoselectivity (Table 4). Moreover, N-substituents of cinnamamides, such as morpholine (7b, 7j), piperidine (7c, 7i) were well compatible; functional groups, such as bromo (7e, 7h), methoxyl (7f) and trifluoromethyl (7g) were tolerated in this transformation.

Table 4 Stereoselective C–H alkynylation of enamidesa
a Reaction conditions: 6 (0.10 mmol), 2 (0.20 mmol), Ir(III) (4.0 mol%), AgNTf2 (16 mol%), AgOAc (2.0 equiv.), NaOAc (30 mol%), Cs2CO3 (50 mol%), DCE/HFIP (4[thin space (1/6-em)]:[thin space (1/6-em)]1, 1 mL) at 90 °C, 12 h.
image file: c8qo01105b-u4.tif


The development of regioselective C–H functionalization might provide valuable insight into the late-stage modification of bioactive molecules that contain multiple reactive C–H bonds.16 Herein, with this dehydrogenative C–H alkynylation reaction, great regioselectivity towards 1,4-disubstituted arenes was observed (Table 5): 3° amide outcompetes 3° sulfonamide or primary sulfonamide, affording C–H alkynylation that took place at the C–H position ortho to aromatic amides; NHAc outcompetes SO2NH2 (9b), while SO2NH2 is stronger than CO2Et (9e) in this transformation, and tertiary amide outcompetes tertiary sulfonamide, affording Probenecid derivatives (9d, 9e); alkynylation at the C–H bond ortho to aryl amide that contained primary sulfonamide led to the 9f product with great regioselectivity. Notably, regioselective C–H alkynylation of Celebrex that contained primary sulfonamide and nitrogen heterocycles was also observed. These results demonstrated that the directing priority of these functionalities is as follows: R1CONHR2, R1CONR2R3, NHCOR > SO2NH2 > CO2R; N-heterocycle > SO2NH2; CONR1R2 > SO2NR1R2. This might provide alternative strategies for the late-stage modification of bioactive molecules, due to the versatile transformation of C–C triple bonds.

Table 5 Regioselective C–H alkynylation of multiple functionalized arenesa
a Reaction conditions: 8 (0.10 mmol), 2 (0.20 mmol), Ir(III) (4.0 mol%), AgNTf2 (16 mol%), AgOAc (2.0 equiv.), Cs2CO3 (30 mol%), DCE/HFIP (4[thin space (1/6-em)]:[thin space (1/6-em)]1, 1 mL) at 100 °C, 12 h. b 2 (0.30 mmol).
image file: c8qo01105b-u5.tif


Primary synthetic applications were further performed in Scheme 2: (1) terminal alkynes could be readily accessed with the treatment of the obtained products with TBAF, e.g., ortho-ethynyl benzamide 5a′ and enynyl amides 7a′ were obtained in quantitative yield; (2) a sequential process that involved C–H alkynylation/desilylation/Click reaction proceeded well to afford multiple functionalized benzofurans 5l′′; (3) a programmed Sonogashira reaction and the directed C–H alkynylation of bromo substituted amides 3e performed well, affording multiple alkyne substituted aryl amides 5e′′.


image file: c8qo01105b-s2.tif
Scheme 2 Synthetic transformation and preliminary mechanistic studies.

As the detailed mechanism remains to be ascertained, preliminary mechanistic studies were performed. KIE values were observed from (kH/kD = 3.2) isotope effect studies of 4d (eqn (4), Scheme 2), which indicated that C–H bond cleavage might be involved in the rate-determining step of this transformation. Moreover, we also synthesized silver acetylide17 to investigate its reaction with 4d under the standard conditions, and the desired product 5d was detected in 58% yield (eqn (5)). These results indicated that the process involved the IrIII-catalyzed C–H activation of amides, delivering a cyclometalated IrIII complex, while the generation of alkynyl-Ag may undergo transmetalation to give an IrIII-alkynyl intermediate, and subsequent oxidant promoted reductive elimination might take place to afford the desired product (ESI).

Conclusion

In summary, we have developed a general methodology for the dehydrogenative C–H alkynylation of amides and primary sulfonamides with terminal alkynes. Stereoselective construction of tetra-substituted olefins was demonstrated through a potential application in materials science. Predictable regioselectivity was observed towards multiple functionalized arenes, which provides insight into the regio-divergent modification of pharmaceuticals, such as Probenecid and Celebrex. Further investigation of the mechanistic details and synthetic application of this transformation is under way.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support by the National Natural Science Foundation of China (21602032, 61671162) and the One Hundred Young Talent program of Guangdong University of Technology.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qo01105b

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