[5 + 2] Annulation of N

Sep 7, 2017 - A silver-catalyzed oxidative intermolecular [3 + 2]/[5 + 2] annulation of N-arylpropiolamides with 4-vinyl acids for producing fused 2H-...
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Silver-Catalyzed Intermolecular [3 + 2]/[5 + 2] Annulation of N‑Arylpropiolamides with Vinyl Acids: Facile Synthesis of Fused 2H‑Benzo[b]azepin-2-ones Yang Li,†,‡ Ming Hu,†,‡ and Jin-Heng Li*,†,‡,§ †

State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China § State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China ‡

S Supporting Information *

ABSTRACT: A silver-catalyzed oxidative intermolecular [3 + 2]/[5 + 2] annulation of N-arylpropiolamides with 4-vinyl acids for producing fused 2H-benzo[b]azepin-2-ones is described. This radical-mediated annulation reaction features broad substrate scope and excellent selectivity, and enables the formation of three new C−C bonds through oxidative decarboxylation, [3 + 2]/[5 + 2] annulations, and C(sp2)-H functionalization cascades. Employing this silver-catalyzed oxidative strategy, common terminal alkynes were successfully converted into cyclopentenes via intermolecular [3 + 2] annulation. KEYWORDS: N-arylpropiolamides, intermolecular [3 + 2] annulation, oxidative decarboxylation

A

heterocyclic systems is a continuous need of the synthetic community.3−7 Typical methods for building the core benzo[b]azepin-2-one structure generally rely on the intramolecular annulation processes, including Beckmann rearrangement,3 the ring-closing metathesis (RCM),4 the cross coupling,5 radicalmediated reaction,6 and others,7 but many of which have significant limitations, such as the requirement of harsh conditions and/or tedious multiple reaction steps, and the lack of readily available substrates. The cycloaddition reaction is a powerful synthetic method that allows highly atom- and step-economic preparation of diverse cyclic backbones.8 Despite impressive progress in the field, such a strategy to build the large ring systems, including the seven-membered N-heterocycles, still encounters challenges. In particular, a few available transformations through [6 + 1], [5 + 2], [4 + 3], or [3 + 2 + 2] annulation modalities have been documented,9,10 with the vast majority of such examples concerning the monocyclic azepine synthesis.9 Examples of the intermolecular annulation for accessing diverse benzoazepines are rare10a−h,j and to date only one asymmetric [4 + 3] annulation version toward benzo[b]azepin-2-ones using Nheterocyclic carbene catalysts has been reported by Enders and co-workers (Scheme 1a).10i With this knowledge of the [5 + 2] annulation area, we envisioned that designing readily available

zepines and their derivatives, including benzo[b]azepin-2ones, represent a class of privileged seven-membered Nheterocyclic structural cores in many pharmaceutically active molecules and natural products (Figure 1).1,2 For example,

Figure 1. Examples of important benzo[b]azepin-2-ones.

benzo[b]azepin-2-one 1 is a highly selective CCK-B antagonist and used for the treatment of the ethanol-withdrawal syndrome.2a,b Other benzo[b]azepin-2-ones 2−7 as clinical candidates have also been intensively studied in vitro and/or vivo, which exhibit a wide variety of pharmaceutically activities, such as inhibition of PLK1/VEGF-R2 receptors, inhibition of enzyme, antitumor, opening of BK channel, and/or antithrombosis.2c−j Therefore, development of new efficient synthetic processes for the preparation of such seven-membered N© XXXX American Chemical Society

Received: June 23, 2017 Revised: August 28, 2017

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DOI: 10.1021/acscatal.7b02061 ACS Catal. 2017, 7, 6757−6761

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ACS Catalysis Table 1. Optimization of Reaction Conditionsa

Scheme 1. Annulation toward Fused 2H-Benzo[b]azepin-2ones

reactants with regard to both the five-atom units and the unsaturated hydrocarbons through new annulation strategies would furnish the 2H-benzo[b]azepine architectures. Herein, we report an oxidative [3 + 2]/[5 + 2] annulation of Narylpropiolamides with 4-vinyl acids by means of AgNO3 catalyst and K2S2O8 oxidant for preparing fused 2H-benzo[b]azepin-2-ones (Scheme 1b), wherein N-arylpropiolamides serve as a five-atom unit and a two-atom units through transformations of a C−C triple bond and an arene C(sp2)-H bond. The reaction is initiated by decarboxylation of vinyl acids,11 which allows the formation of the three new C−C bonds in one pot. Importantly, this silver-catalyzed oxidative strategy was applicable to [3 + 2] annulation of common terminal alkynes with 4-vinyl acids for the preparation of cyclopentenes. We initiated our efforts toward screening reaction conditions for the [3 + 2]/[5 + 2] annulation of N-methyl-N-phenylpropiolamide (1a) and 2,2-dimethylpent-4-enoic acid (2a) (Table 1). Treatment of substrate 1a with acid 2a, 20 mol % AgNO3, and 3.5 equiv of K2S2O8 in H2O/MeCN (1:2) at 40 °C for 16 h afforded the desired product 3aa in 75% yield (entry 1). While the absence of AgNO3 resulted in no conversion (entry 2), a lower amount of AgNO3 (10 mol %) only decreased the yield (entry 3), and a higher amount of AgNO3 (30 mol %) had no effect by comparison with the results at 20 mol % AgNO3 (entry 4). Other silver salts, including AgOAc, Ag2CO3, and Ag2O, were examined: each of which exhibited high catalytic activity but was less efficient than AgNO3 (entries 5−7). Among the amount of K2S2O8 examined, the preferred loading turned out to be 3.5 equiv, and without K2S2O8 the reaction could not provide 3aa (entries 1 and 8−10). Other oxidants were tested: oxone displayed a rather lower reactivity, but both TBHP and DTBP were inert (entries 11−12). A screen of the solvent effect reveals a mixture of H2O/MeCN with the 1:2 v/v ratio as the best option, and the amount of water affected the reaction by controlling the solubility of the K2S2O8 in reaction media (entries 1 and 13−16). The reaction could be successfully performed, albeit giving a slightly diminishing yield (entry 17). A lower temperature (30 °C) reduced the yield sharply (entry 18), and a higher temperature (50 °C) gave identical yield to that of 40 °C (entry 19). However, three ligands, including 1,2bis(diphenylphosphino)ethane (dppe), 1,10-phenanthroline (1,10-Phen), and 2,2′-bipyridine, were proven to have a negative effect on the reaction (entries 20−22). Finally, the amount of acid 2a was examined (entries 1 and 23−25). Using 1 equiv of acid 2a resulted in 59% yield of 2a after prolonging the reaction time (entry 23). Delightfully, satisfactory results, which are identical to those of 2 equiv of 2a, were achieved following addition of 1.2 equiv or 1.5 equiv of acid 2a for 32 h (entries 24 and 25). These results showed that the amount of

entry

variation from the optimal conditions

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17b 18 19 20c 21d 22e 23f 24g 25h

none without AgNO3 AgNO3 (10 mol %) AgNO3 (30 mol %) AgOAc instead of AgNO3 Ag2CO3 instead of AgNO3 AgO instead of AgNO3 without K2S2O8 K2S2O8 (3 equiv) K2S2O8 (4 equiv) oxone instead of K2S2O8 TBHP or DTBP instead of K2S2O8 H2O/MeCN (1:1) H2O/MeCN (1:4) H2O/MeCN instead of H2O/acetone H2O/MeCN instead of H2O/CH2ClCH2Cl none at 30 °C at 50 °C none none none none none none

75 0 37 76 60 54 62 0 60 75 31 trace 62 46 41 19 69 36 73 58 48 trace 59 73 75

a

Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), AgNO3 (20 mol %), K2S2O8 (3.5 equiv), H2O/MeCN (1:2; 3 mL), argon, 40 °C, and 16 h. bUnder O2 (1 atm) atmosphere. cDppe (20 mol %) was added. d 1,10-Phen (20 mol %) was added. e2,2′-Bipyridine (20 mol %) was added. f2a (0.2 mmol) for 32 h. g2a (0.24 mmol) for 32 h. h2a (0.3 mmol) for 32 h.

2a mainly affected the reaction rate, suggesting that competing premature oxidation of key tertiary radicals is not the major factor. With the optimal reaction conditions in hand, the scope of this intermolecular [3 + 2]/[5 + 2] annulation was next investigated (Tables 2 and 3). As shown in Table 2, a wide range of N-arylpropiolamides 1b−r were subjected to this [3 + 2]/[5 + 2] annulation with acid 2a, AgNO3 and K2S2O8 (3bara). Initially, the substitution effect on the nitrogen atom of substrates 1 was examined: substrates 1b−d with a N-Et group a N-Bn group or a N-allyl group were converted to 3ba-ca with useful yields,12 but substrate 1e−f with a free N−H group or a N−Ac group were inert (3ea−fa). Gratifyingly, several substituents, including Me, MeO, Br, CF3, Cl, and Ph groups, on the aryl ring of the N-aryl moiety were well tolerated, and their electron and position nature had an fundamental influence on the reaction (3ga−ra). While both substrate 1g bearing a weak electron-donating p-Me group and substrate 1i with a weak electron-withdrawing p-Br group delivered high yields of 3ga and 3ia, respectively, substrates 1h and 1j having a strong electron effect group (p-MeO or p-CF3) afforded 3ha and 3ja with the slightly diminishing yields. Using m-Me-substituted substrate 1k gave 3ka in 86% yield, but o-Me-substituted substrate 1m furnished 3ma in only 56% yield. Substrates 1n-o 6758

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ACS Catalysis Table 2. Variation of the N-Arylpropiolamides (1)a

regioselectivity toward the ortho position to the chloride group (3la/3la′, 3pa/3pa′ and 3qa/3qa′) compared with the m-Mesubstituted substrate 1k (3ka/3ka′). For N-(naphthalen-2-yl)substituted substrate 1r, the reaction was smoothly performed and afforded 3ra in 57% yield. We found that the [3 + 2]/[5 + 2] annulation protocol was applicable to an array of vinyl acids 2 (Table 3) In the presence of substrate 1a, AgNO3 and K2S2O8, 2,2-diethylpent-4-enoic acid (2b) efficiently furnished 3ab in good yield. Interestingly, acids 2c−e, containing a cyclopentyl group, a cyclohexyl group or a cycloheptyl group on the 2 position, were transformed into spirocycle-fused 2H-benzo[b]azepin-2-ones 3ac−ae in 59%− 67% yields. Using 2-ethyl-2-methylpent-4-enoic acid (2f) or 2methyl-2-phenylpent-4-enoic acid (2g) successfully delivered 3af−ag. Notably, the protocol could be expanded to annulation with secondary and primary acids 2h−i, providing 3ah−ai in high yields. Substituents at the terminal or internal alkene were consistent with the optimal conditions: 2,2-diethylhex-4-enoic acid (2j) gave 3aj in 55% yield, and 2,2-diethyl-4-methylpent-4enoic acid (2k) assembled 3ak in 42% yield. Pleasingly, 2,2diethylhex-5-enoic acid (2l) successfully underwent the annulation to build six-membered ring-fused 2H-benzo[b]azepin-2-one 3al. As shown in Scheme 2, benzyl propiolate (1s) underwent [3 + 2] annulation with acid 2a, AgNO3 and K2S2O8, affording Scheme 2. [3 + 2] Annulation of Other Alkynes (1)

a

Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), AgNO3 (20 mol %), K2S2O8 (3.5 equiv), H2O/MeCN (1:2; 3 mL), argon, 40 °C, and 16 h. The regioselective ratio is given in the parentheses as determined by GC-MS analysis of the crude product.

Table 3. Variation of the 4-Vinyl Alkyl Acids (2)a

cyclopentene 3sa in 68% yield. A number of common terminal alkynes, either arylalkynes 3t−aa or alkylalkyne 1ab, were suitable substrates for the [3 + 2] annulation reaction (products 3ta−aab). For example, phenylacetylene (1t) was converted into 3ta with 81% yields. Alkynes 1u−y with a Me, a MeO, or a CN group succeeded to furnish the corresponding cyclopentenes 3ua−ya. For heteroarylalkynes 1z−aa, cyclopentenes 3za−aa were prepared in good yields. Gratifyingly, aliphatic alkyne 1ab was viable for accessing 3aba in 48% yield. Unfortunately, internal alkynes, including diphenylacetylene (1ac) and diethyl acetylenedicarboxylate (1ad), were not suitable substrates for the reaction (3aca−ada). Using secondary and primary acids 2d or 2l reacted with phenylacetylene (1t) succeeded to form the corresponding products 3ta and 3tl in good yields. Notably, the annulation reaction of 1a with 2a was completely suppressed by TEMPO and afforded product 4 which is determined by GC-MS analysis (eq 1; Scheme 3). We

a Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), AgNO3 (20 mol %), K2S2O8 (3.5 equiv), H2O/MeCN (1:2; 3 mL), argon, 40 °C, and 16 h. The d.r. value is given in the parentheses as determined by 1H NMR spectroscopic analysis of the crude product.

with an o-Ph group or an o-Cl group were also highly reactive, offering 3na−oa in moderate yields. Disubstituted substrates 1p−q were viable for efficiently producing 3pa−qa. Notably, both the weak electron-withdrawing effect and the lone electron pair nature of the chloride group would be stabilized the ortho-carbon-centered radicals, thus mainly making the 6759

DOI: 10.1021/acscatal.7b02061 ACS Catal. 2017, 7, 6757−6761

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ACS Catalysis ORCID

Scheme 3. Control Experiment and Possible Mechanism

Jin-Heng Li: 0000-0001-7215-7152 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (Nos. 21625203 and 21472039) for financial support.

also found that the annulation reaction of 1a with 2a was also inhibited when using the other radical scavenger (e.g., 2,6ditert-butyl-4-methylphenol and hydroquinone). The results support the reaction via a free-radical process. To understand the current [3 + 2]/[5 + 2] annulation reaction, the possible mechanism outlined in Scheme 3 was proposed.11 Initially, oxidative decarboxylation of vinyl acid 2a by AgNO3 combination with K2S2O8 oxidant affords the alkyl radical intermediate A.11 Addition of the alkyl radical intermediate A across the C−C triple bond of substrate 1a gives the vinyl radical intermediate B, which would sequentially react with the C−C double bond to produce the alkyl radical intermediate C. The second annulation of the alkyl radical intermediate C occurs and delivers the cyclohexadienyl radical intermediate D. Finally, oxidation of D through hydrogen atom transfer to persulfate radical or single electron oxidation by Ag(II), and subsequent deprotonation provides product 3aa.11 In summary, we have developed the first silver-catalyzed oxidative intermolecular [3 + 2]/[5 + 2] annulation of Narylpropiolamides with 4-vinyl acids for the synthesis of fused 2H-benzo[b]azepin-2-ones. This reaction is compatible with a wide range of N-arylpropiolamides and 4-vinyl acids using a catalytic amount of AgNO3 combined with a cheap inorganic oxidant, which should be valuable in building N-heterocycle motifs. Moreover, this silver-catalyzed oxidative strategy was compatible with a wide range of common alkynes, including propiolate, arylalkynes, herteroarylalkyne, and alkylalkyne, which underwent the intermolecular [3 + 2] annulation with 4-vinyl acids leading to cyclopentenes.

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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/acscatal.7b02061. X-ray data of 3ba (CIF) Experimental procedures and characterization of all compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 6760

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Letter

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DOI: 10.1021/acscatal.7b02061 ACS Catal. 2017, 7, 6757−6761