Hydrazone–palladium catalyzed annulation of 1-cinnamyloxy-2-ethynylbenzene derivatives

Abstract

The annulation of 1-cinnamyloxy-2-ethynylbenzene derivatives using a hydrazone–palladium catalyst system proceeded smoothly and gave the corresponding 2-substituted-3-cinnamylbenzofurans in good-to-excellent yields.

---

Introduction

The benzofuran core structure is found in a wide variety of natural products and pharmacological activities.¹ In particular, 2,3-disubstituted benzofuran derivatives are known as various bioactive compounds.² Therefore, the synthetic methodologies for such a structure have attracted significant attention and subsequent development.³ The annulation of o-ethynylphenol derivatives is known as a simple protocol for the construction of the benzofuran core.⁴ However, this protocol afforded only a 2-substituted benzofuran derivative. Recently, some intermolecular annulations of o-ethynylphenol derivatives with electrophiles for the synthesis of 2,3-disubstituted benzofuran derivatives have been reported.⁵ On the other hand, the Cacchi group⁶ and Monteiro group⁷ independently reported that the palladium-catalyzed annulation of 1-allyloxy-2-ethynylbenzene derivatives afforded 2-substituted-3-allylbenzofuran.

These protocols were attractive for the construction of 2,3-disubstituted benzofuran derivatives. However, they required high temperatures. Furthermore, the Liang group reported the palladium-catalyzed double annulation of 1-allyloxy-2-ethynylbenzene for the construction of 2-substituted-3-allylbenzofuran under relatively mild conditions.⁸ However, they did not attempt the reaction of 1-cinnamyloxy-2-ethynylbenzene. On the other hand, while the Fürstner group reported the platinum-catalyzed annulation of 1-allyloxy-2-ethynylbenzene for the construction of 2-substituted-3-allylbenzofuran, this annulation required a reaction to take place under a poisonous CO atmosphere.⁹

Previously, we demonstrated that easily prepared and air-stable hydrazones (Fig. 1) are effective ligands for palladium-catalyzed C–C bond formation reactions¹⁰ including the Tsuji–Trost type allylic arylation of allylic acetates with arylboronic acids.¹¹ More recently, we also reported the intermolecular¹² and intramolecular¹³ allylic arylation of cinnamyloxybenzene using a hydrazone–palladium catalyst system. Additionally, we demonstrated that a hydrazone–palladium catalyst system was effective for the annulation reaction forming naphthalene derivatives.¹⁴ Herein, we report that the hydrazone–palladium catalyzed annulation of 1-cinnamyloxy-2-(phenylethynyl)benzene was complete in only 1 hour under optimized conditions, and afforded the corresponding 2-substituted-3-cinnamylbenzofuran derivatives in good-to-excellent yields.

Fig. 1 Hydrazones 1 and 2.

Results and discussion

Initially, we sought the optimum reaction conditions for the hydrazone–palladium catalyzed annulation of 1-cinnamyloxy-2-(phenylethynyl)benzene (3a) (Table 1). When the reaction of 3a was performed using Pd₂(dba)₃, the pyridine-methyl type bis-hydrazone 1a as a ligand and Et₃N as a base in 1,4-dioxane/H₂O (3/1) at 50 °C for 1 hour under an Ar atmosphere, 3-cinnamyl-2-phenylbenzofuran (4a) was obtained in 96% yield (Table 1, entry 1). On the other hand, only a trace amount of the product was detected using the phenyl-methyl type bishydrazone ligand 1b (entry 2). The reactions using bishydrazone ligands 1c–e bearing 5–7 member rings gave the corresponding product in 15%, 32%, and 15% yields, respectively (entries 3–5). We also tried to use pyridine-type monohydrazone ligands 2a and 2b (entries 6 and 7). However, these ligands were not effective for this annulation. The reaction in the absence of a ligand did not proceed (entry 8). When we tested a PPh₃-palladium catalyst system, which was an effective catalyst system in Cacchi's report, the desired product was obtained in only 13% yield (entry 9). As a result, the pyridine-methyl type bishydrazone ligand 1a was the most suitable for this annulation (entry 1). Next, we checked the reactivity of various palladium sources (entries 1 and 10–13). We found that a reaction using a palladium(II) source, such as Pd(OAc)₂, Pd(tfa)₂, PdCl₂ and PdCl₂(MeCN)₂, gave only a trace amount of the desired product (entries 10–13). Next, the effects of various bases were investigated (entries 1 and 14–18). Although the reaction using Ca(OH)₂ afforded a good yield (entry 18), using Et₃N led to the highest yield in this annulation (entry 1). Various solvents were also tested (entries 1 and 19–24). 1,4-Dioxane was the most suitable solvent for this annulation (entry 1). Additionally, the effect of H₂O was investigated (entries 1 and 25–28). When we reduced the amount of water from 3/1 to 9/1, the yield of 4a was decreased to 33% (entry 25). Furthermore, the reaction without water gave the corresponding product in only a trace amount (entry 26). These results indicate that water is necessary to achieve this annulation. When we alternatively increased the amount of water from 3/1 to 2/1 and 1/1, the yield of 4a was also decreased to 82% and 71%, respectively (entries 27 and 28). Finally, we checked the reaction using 3a under Liang's condition⁸ (entry 29). This annulation was still not complete after 1 hour, despite using 10 mol% of palladium catalyst, and afforded the product 4a in only 48% yield. Therefore, we found that Liang's condition was not suitable for 1-cinnamyloxy-2-ethynylbenzene derivatives.

Table 1 Optimization of the palladium-catalyzed annulation of 1-cinnamyloxy-2-(phenylethynyl)benzene (3a) using the hydrazone ligand^a

Entry	Ligand	Catalyst	Base	Solvent	Yield of 4a (%)
a Reaction conditions: 3a, ligand (5.0 mol%), catalyst (Pd = 5.0 mol%), base (2.0 eq.), solvent (0.25 M) at 50 °C for 1 h under Ar. b Reaction conditions: 3a, PPh3 (20 mol%), Pd2(dba)3·CHCl3 (Pd = 10 mol%), DMF (0.05 M) at 60 °C for 1 h under Ar.
1	1a	Pd 2 (dba) 3	Et 3 N	1,4-Dioxane/H 2 O (3/1)	96
2	1b	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	Trace
3	1c	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	15
4	1d	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	32
5	1e	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	15
6	2a	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	46
7	2b	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	11
8	—	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (3/1)	Trace
9	—	Pd(PPh3)4	Et3N	1,4-Dioxane/H2O (3/1)	13
10	1a	Pd(OAc)2	Et3N	1,4-Dioxane/H2O (3/1)	Trace
11	1a	Pd(tfa)2	Et3N	1,4-Dioxane/H2O (3/1)	Trace
12	1a	PdCl2	Et3N	1,4-Dioxane/H2O (3/1)	Trace
13	1a	PdCl2(MeCN)2	Et3N	1,4-Dioxane/H2O (3/1)	Trace
14	1a	Pd2(dba)3	DIEA	1,4-Dioxane/H2O (3/1)	29
15	1a	Pd2(dba)3	NaOAc	1,4-Dioxane/H2O (3/1)	44
16	1a	Pd2(dba)3	Cs2CO3	1,4-Dioxane/H2O (3/1)	60
17	1a	Pd2(dba)3	K3PO4	1,4-Dioxane/H2O (3/1)	41
18	1a	Pd2(dba)3	Ca(OH)2	1,4-Dioxane/H2O (3/1)	94
19	1a	Pd2(dba)3	Et3N	CPME/H2O (3/1)	9
20	1a	Pd2(dba)3	Et3N	DMA/H2O (3/1)	82
21	1a	Pd2(dba)3	Et3N	DMF/H2O (3/1)	65
22	1a	Pd2(dba)3	Et3N	DMSO/H2O (3/1)	67
23	1a	Pd2(dba)3	Et3N	NMP/H2O (3/1)	62
24	1a	Pd2(dba)3	Et3N	THF/H2O (3/1)	85
25	1a	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (9/1)	33
26	1a	Pd2(dba)3	Et3N	1,4-Dioxane	Trace
27	1a	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (2/1)	82
28	1a	Pd2(dba)3	Et3N	1,4-Dioxane/H2O (1/1)	71
29^b	PPh3	Pd2(dba)3·CHCl3	—	DMF	48

---

With the optimized reaction conditions obtained (Table 1, entry 1), we investigated the scope and limitation of this annulation using various substituted 1-allyloxy-2-ethynylbenzene derivatives 3 (Table 2). The reaction of 2-cinnamyloxy-1-(phenylethynyl)naphthalene (3b) as a starting material also proceeded and gave 1-cinnamyl-2-phenylnaphthofuran (4b) in good yield (entry 2). Although 10 mol% of palladium and the ligand were required in some cases, the reactions of substrates 3c–e bearing an electron-donating or withdrawing group on the aromatic ring were also tolerated (entries 3–5). On the other hand, the yield of the product was low when we employed methylbenzoate 3f as a starting material (entry 6). Next, we investigated the electronic effect of the cinnamyloxy group. We found that the annulations of 4-substituted cinnamyloxy benzenes 3g–j were also tolerated (entries 7–10). In particular, the corresponding benzofuran derivative 4h was obtained in a quantitative yield when we employed cinnamyloxybenzene 3h to this annulation (entry 8). Furthermore, we succeeded in synthesizing 2-alkyl-3-cinnamylbenzofuran 3k in 73% yield (entry 11). Allyloxybenzene derivatives 3l and 3m were allowed to react and 2-phenyl-3-allylbenzofurans 4l and 4m were obtained in 97% and 96% yields, respectively (entries 12 and 13). The reaction of crotyloxybenzene 3n afforded the mixture of 2-phenyl-3-crotylbenzofuran (4n) (E/Z = 91/9) and 3-(but-3-en-2-yl)-2-phenylbenzofuran (4n′) (4n/4n′ = 89/11) in 92% yield (entry 14). However, prenyloxybenzene 3o was not tolerated in this reaction because the oxidative addition was interrupted by two methyl groups at the allylic position in terms of steric hindrance and the electronic effect (entry 15). Finally, we attempted to use 4-substituted phenylethynylbenzene derivatives in this annulation. The reaction of substrates 3p–s afforded the corresponding products 4p–s in moderate-to-good yields (entries 16–19). All of the reactions, except that using 3o, proceeded for only 1 hour and gave the corresponding benzofuran derivatives in good-to-excellent yields. Next, in order to confirm whether this reaction is an intermolecular or intramolecular reaction, crossover reactions in the presence of 5.0 mol% of Pd₂(dba)₃ and 10 mol% of 1a were carried out using 3c and 3l as starting materials (Scheme 1). As a result, the corresponding products 4c and 4l were isolated. Furthermore, two crossover products 4m and 4a were also isolated. From this result, we concluded that this reaction occurred in an intermolecular manner.

Scheme 1 Crossover reaction.

Table 2 Scope and limitation of the annulation of 1-cinnamyloxy-2-ethynylbenzene 3 using the hydrazone ligand^a

Entry	3	R^1	R^2	R^3	R^4	Yield of 4 (%)
a Reaction conditions: 1-cinnamyloxy-2-ethynylbenzene 3 (0.25 mmol), Pd2(dba)3 (2.5 mol%), 1a (5 mol%), Et3N (2.0 eq.), 1,4-dioxane/H2O (3/1) (0.25 M) at 50 °C for 1 h under Ar. b Pd2(dba)3 (5.0 mol%) and 1a (10 mol%) were used. c E/Z mixture of 3n (83/17) was used. d The E/Z isomer ratio was determined as 91/9 by ^1H NMR spectroscopy. e The 4n/4n′ ratio was determined as 89/11 by ^1H NMR spectroscopy.
1	3a	H	Ph	Ph	H	96 (4a)
2	3b	–(CH=CH)2 (3,4)	Ph	Ph	H	91 (4b)
3^b	3c	4-OMe	Ph	Ph	H	88 (4c)
4^b	3d	5-Cl	Ph	Ph	H	95 (4d)
5	3e	5-CF3	Ph	Ph	H	94 (4e)
6^b	3f	5-COOMe	Ph	Ph	H	39 (4f)
7^b	3g	H	p-MeC6H4	Ph	H	81 (4g)
8^b	3h	H	p-MeOC6H4	Ph	H	99 (4h)
9^b	3i	H	p-CNC6H4	Ph	H	76 (4i)
10^b	3j	H	p-FC6H4	Ph	H	95 (4j)
11^b	3k	H	^nBu	Ph	H	73 (4k)
12	3l	H	Ph	H	H	97 (4l)
13	3m	4-OMe	Ph	H	H	96 (4m)
14^b	3n ^c	H	Ph	Me	H	92 (4n + 4n′)^d^,^e
15	3o	H	Ph	Me	Me	N.R.
16^b	3p	H	Ph	p-MeC6H4	H	57 (4p)
17^b	3q	H	Ph	p-MeOC6H4	H	77 (4q)
18^b	3r	H	Ph	p-ClC6H4	H	70 (4r)
19	3s	H	Ph	p-CF3C6H4	H	67 (4s)

---

Therefore, we tried to carry out an intermolecular reaction using o-(phenylethynyl)phenol and cinnamyloxybenzene under optimized reaction conditions (Scheme 2). Unfortunately, the intermolecular reaction proceeded to afford only 2-phenylbenzofuran in 67% yield without producing 3a. Consequently, the use of 1-cinnamyloxy-2-ethynylbenzene derivatives as the starting material was found to be essential for constructing 2-phenyl-3-cinnamylbenzofuran.

Scheme 2 Intermolecular reaction.

From these results, a plausible mechanism for the annulation of 1-cinnamyloxy-2-ethynylbenzene derivatives is illustrated in Scheme 3. First, the hydrazone–palladium(0) complex coordinates itself with the olefin of the starting material. An oxidative addition to the hydrazone–palladium(0) complex at the allylic position of the starting material takes place to generate an ion pair of a phenoxy anion and a π-allyl palladium(II) cation. Next, hydrolysis of the ion pair by water generates an o-(phenylethynyl)phenol¹⁵ and π-allyl-hydroxy palladium(II) complex. This complex coordinates with the triple bond of o-ethynylphenol. Then, the annulation of o-ethynylphenol occurs by the deprotonation of the hydroxyl group to generate a π-allyl palladium(II) benzofuran intermediate. Finally, reductive elimination occurs to produce a 2-substituted-3-allylbenzofuran product and to regenerate the hydrazone–palladium(0) complex. At this point, the catalytic cycle was complete. We assumed that these palladium complexes were stabilized by the hydrazone ligand, and this annulation proceeded smoothly in spite of using a cinnamyloxy substrate as a starting material.

Scheme 3 Plausible reaction mechanism.

Conclusions

In summary, we found that the palladium-catalyzed annulation of 1-cinnamyloxy-2-ethynylbenzene derivatives 3 using hydrazone 1a as a ligand, even at low temperature proceeded smoothly and gave the corresponding 2-substituted-3-cinnamylbenzofurans 4 in good-to-excellent yields in a very short reaction time.

Acknowledgements

This work was supported by the COE Program of Chiba University.

Notes and references

1. (a) C. D. Poon and B. M. Fung, Synthesis, 1991, 2061; (b) Z. Tomaszewski, M. P. Johnson, X. Huang and D. E. Nichols, J. Med. Chem., 1992, 35, 2061; (c) M. Sauter and W. Adam, Acc. Chem. Res., 1995, 28, 289; (d) O. Als, E. Sozialer and U. R. Institutionellen, Arbeitsmedizin, 1999, 14, 1700; (e) P. Máximo, A. Lourenço, S. S. Feio and J. C. Roseiro, J. Nat. Prod., 2002, 65, 175; (f) T. Pacher, C. Seger, D. Engelmeier, S. Vajrodaya, O. Hofer and H. Greger, J. Nat. Prod., 2002, 65, 820; (g) K. S. Song and I. Raskin, J. Nat. Prod., 2002, 65, 76; (h) B. L. Flynn, E. Hamel and M. K. Jung, J. Med. Chem., 2002, 45, 2670; (i) Y. L. Lin, Y. Y. Chang, Y. H. Kuo and M. S. Shiao, J. Nat. Prod., 2002, 65, 745; (j) E. Tsuji, K. Ando, J. Kunitomo, M. Yamashita, S. Ohta, S. Kohno and Y. Ohishi, Org. Biomol. Chem., 2003, 1, 3139; (k) C. J. Hamilton, A. Saravanamuthu, A. H. Fairlamb and I. M. Eggleston, Bioorg. Med. Chem., 2003, 11, 3683; (l) A. Katona, M. I. Toşa, C. Paizs and J. Rétey, Chem. Biodiversity, 2006, 3, 502; (m) S. M. Bakunova, S. A. Bakunov, T. Wenzler, T. Barszcz, K. A. Werbovetz, R. Brun, J. E. Hall and R. R. Tidwell, J. Med. Chem., 2007, 50, 5807; (n) K. Ando, Y. Kawamura, Y. Akai, J.-I. Kunitomo, T. Yokomizo, M. Yamashita, S. Ohta, T. Ohishi and Y. Ohishi, Org. Biomol. Chem., 2008, 6, 296; (o) K. Asoh, M. Kohchi, I. Hyoudoh, T. Ohtsuka, M. Masubuchi, K. Kawasaki, H. Ebiike, Y. Shiratori, T. A. Fukami, O. Kondoh, T. Tsukaguchi, N. Ishii, Y. Aoki, N. Shimma and M. Sakaitani, Bioorg. Med. Chem. Lett., 2009, 19, 1753; (p) R. K. Thalji, N. Aiyar, E. A. Davenport, J. A. Erhardt, L. A. Kallal, D. M. Morrow, S. Senadhi, C. L. Burns-Kurtis and J. P. Marino, Bioorg. Med. Chem. Lett., 2010, 20, 4104; (q) A. M. Venkatesan, O. Dos Santos, J. Ellingboe, D. A. Evrard, B. L. Harrison, D. L. Smith, R. Scerni, G. A. Hornby, L. E. Schechter and T. H. Andree, Bioorg. Med. Chem. Lett., 2010, 20, 824; (r) C. Antczak, D. Shum, B. Bassit, M. G. Frattini, Y. Li, E. Stanchina, D. A. Scheinberg and H. Djaballah, Bioorg. Med. Chem. Lett., 2011, 21, 4528; (s) J. Rangaswamy, H. V. Kumar, S. T. Harini and N. Naik, Bioorg. Med. Chem. Lett., 2012, 22, 4773; (t) T. T. B. Nguyen, T. Lomberget, N. C. Tran, E. Colomb, L. Nachtergaele, S. Thoret, J. Dubois, J. Guillaume, R. Abdayem, M. Haftek and R. Barret, Bioorg. Med. Chem. Lett., 2012, 22, 7227; (u) W. Eccles, J. M. Blevitt, J. N. Booker, C. C. Chrovian, S. Crawford, A. R. de Leon, X. Deng, A. M. Fourie, C. A. Grice, K. Herman, L. Karlsson, A. M. Kearney, A. Lee-Dutra, J. Liang, R. Luna, D. Pippel, N. Rao, J. P. Riley, A. Santillán, B. Savall, V. M. Tanis, X. Xue and A. L. Young, Bioorg. Med. Chem. Lett., 2013, 23, 811; (v) X.-Y. Li, B.-F. He, H.-J. Luo, N.-Y. Huang and W.-Q. Deng, Bioorg. Med. Chem. Lett., 2013, 23, 4617; (w) H. Watanabe, M. Ono, S. Iikuni, M. Yoshimura, K. Matsumura, H. Kimura and H. Saji, Bioorg. Med. Chem. Lett., 2014, 24, 4834; (x) R. Naik, D. S. Harmalkar, X. Xu, K. Jang and K. Lee, Eur. J. Med. Chem., 2015, 90, 379; (y) R. J. Nevagi, S. N. Dighe and S. N. Dighe, Eur. J. Med. Chem., 2015, 6, 561.
2. (a) C. C. Felder, K. E. Joyce, E. M. Briley, M. Glass, K. P. Mackie, K. J. Fahey, G. J. Cullinan, D. C. Hunden, D. W. Johnson, M. O. Chaney, G. A. Koppel and M. Brownstein, J. Pharmacol. Exp. Ther., 1998, 284, 291; (b) M. Halabalaki, N. Aligiannis, Z. Papoutsi, S. Mitakou, P. Moutsatsou, C. Sekeris and A. L. Skaltsounis, J. Nat. Prod., 2000, 63, 1672; (c) B. Carlsson, B. N. Singh, M. Temciuc, S. Nilsson, Y. L. Li, C. Mellin and J. Malm, J. Med. Chem., 2002, 45, 623; (d) V. Wallez, S. Durieux-Poissonnier, P. Chavatte, J. A. Boutin, V. Audinot, J.-P. Nicolas, C. Bennejean, P. Delagrange, P. Renard and D. Lesieur, J. Med. Chem., 2002, 45, 2788; (e) I. Hayakawa, R. Shioya, T. Agatsuma, H. Furukawa, S. Naruto and Y. Sugano, Bioorg. Med. Chem. Lett., 2004, 14, 4383; (f) M. Ohno, M. Miyamoto, K. Hoshi, T. Takeda, N. Yamada and A. Ohtake, J. Med. Chem., 2005, 48, 5279; (g) B. L. Flynn, G. S. Gill, D. W. Grobelny, J. H. Chaplin, D. Paul, A. F. Leske, T. C. Lavranos, D. K. Chalmers, S. A. Charman, E. Kostewicz, D. M. Shackleford, J. Morizzi, E. Hamel, M. K. Jung and G. Kremmidiotis, J. Med. Chem., 2011, 54, 6014; (h) Y. He, J. Xu, Z.-H. Yu, A. M. Gunawan, L. Wu, L. Wang and Z.-Y. Zhang, J. Med. Chem., 2013, 56, 832; (i) S. He, P. Jain, B. Lin, M. Ferrer, Z. Hu, N. Southall, X. Hu, W. Zheng, B. Neuenswander, C.-H. Cho, Y. Chen, S. A. Worlikar, J. Aubé, R. C. Larock, F. J. Schoenen, J. J. Marugan, T. J. Liang and K. J. Frankowski, ACS Comb. Sci., 2015, 17, 641.
3. (a) A. S. K. Hashmi, T. M. Frost and J. W. Bats, Org. Lett., 2001, 3, 3769; (b) M. C. Willis, D. Taylor and A. T. Gillmore, Org. Lett., 2004, 6, 4755; (c) C. Chen and P. G. Dormer, J. Org. Chem, 2005, 70, 6964; (d) M. Carril, R. SanMartin, I. Tellitu and E. Domínguez, Org. Lett., 2006, 8, 1467; (e) B. Zhao and X. Lu, Org. Lett., 2006, 8, 5987; (f) B. L. Zhan, F. D. Wang and J. M. Yue, Synlett, 2006, 567; (g) B. Lu, B. Wang, Y. Zhang and Y. D. Ma, J. Org. Chem., 2007, 72, 5337; (h) J. Oppenheimer, W. L. Johnson, M. R. Tracey, R. P. Hsung, P.-Y. Yao, R. Liu and K. Zhao, Org. Lett., 2007, 9, 2361; (i) A. S. K. Hashmi and M. Wölfle, Tetrahedron, 2009, 65, 9021; (j) J. Hu, L.-Y. Wu, X.-C. Wang, Y.-Y. Hu, Y.-N. Niu, X.-Y. Liu, S. Yang and Y.-M. Liang, Adv. Synth. Catal., 2010, 352, 351; (k) R. N. Nair, P. J. Lee, A. L. Rheingold and D. B. Grotjahn, Chem. – Eur. J., 2010, 16, 7992; (l) Y. Liu and S. Ma, Org. Lett., 2012, 14, 720; (m) B. Yin, C. Cai, G. Zeng, R. Zhang, X. Li and H. Jiang, Org. Lett., 2012, 14, 1098; (n) H. Yuan, K.-J. Bi, B. Li, R.-C. Yue, J. Ye, Y.-H. Shen, L. Shan, H.-Z. Jin, Q.-Y. Sun and Q.-Y. W.-D. Zhang, Org. Lett., 2013, 15, 4742–4745.
4. (a) C. Koradin, W. Dohle, A. L. Rodriguez, B. Schmid and P. Knochel, Tetrahedron, 2003, 59, 1571; (b) X. Li, A. R. Chianese, T. Vogel and R. H. Crabtree, Org. Lett., 2005, 7, 5437; (c) K. Hiroya, S. Itoh and T. Sakamoto, Tetrahedron, 2005, 61, 10958; (d) M. Yoshida, Y. Morishita, M. Fujita and M. Ihara, Tetrahedron, 2005, 61, 4381; (e) Y. Zhang, Z.-J. Xin, J.-J. Xue and Y. Li, Chin. J. Chem., 2008, 26, 1461; (f) A. Varela-Fernández, C. González-Rodríguez, J. A. Varela, L. Castedo and C. Saá, Org. Lett., 2009, 11, 5350; (g) W. Huang, J. H.-C. Liu, P. Alayoglu, Y. Li, C. A. Witham, C.-K. Tsung, F. D. Toste and G. A. Somorjai, J. Am. Chem. Soc., 2010, 132, 16771; (h) A. S. K. Hashmi, T. D. Ramamurthi and F. Rominger, Adv. Synth. Catal., 2010, 352, 971; (i) R. N. Nair, P. J. Lee and S. B. Grotjahn, Top. Catal., 2010, 53, 1045; (j) I. R. Siddiqui, M. A. Waseem, S. Shamim, Shireen, A. Srivastava and A. Srivastava, Tetrahedron Lett., 2013, 54, 4154; (k) M. Kim, S. Lee, K. Kim, D. Shin, H. Kim and H. Song, Chem. Commun., 2014, 50, 14938; (l) O. S. Morozov, A. V. Lunchev, A. A. Bush, A. A. Tukov, A. F. Asachenko, V. N. Khrustalev, S. S. Zalesskiy, V. P. Ananikov and M. S. Nechaev, Chem. – Eur. J., 2014, 20, 6162; (m) S.-X. Sun, J.-J. Wang, Z.-J. Xu, L.-Y. Cao, Z.-F. Shi and H.-L. Zhang, Tetrahedron, 2014, 70, 3798; (n) J. Liu, Z. Liu, P. Liao and X. Bi, Org. Lett., 2014, 4, 2; (o) A. Chakraborty, K. Jyothi and S. Sinha, Tetrahedron Lett., 2014, 55, 6795.
5. (a) Y. Kondo, F. Shiga, N. Murata, T. Sakamoto and H. Yamanaka, Tetrahedron, 1994, 50, 11803; (b) A. Arcadi, S. Cacchi, M. Del Rosario, G. Fabrizi and F. Marinelli, J. Org. Chem., 1996, 61, 9280; (c) M. Nakamura, L. Ilies, S. Otsubo and E. Nakamura, Org. Lett., 2006, 8, 2803; (d) M. Nakamura, L. Ilies, S. Otsubo and E. Nakamura, Angew. Chem., Int. Ed., 2006, 45, 944; (e) C. Martínez, R. Álvarez and J. M. Aurrecoechea, Org. Lett., 2009, 11, 1083; (f) A. Boyer, N. Isono, S. Lackner and M. Lautens, Tetrahedron, 2010, 66, 6468; (g) S. Cacchi, G. Fabrizi and A. Goggiamani, Org. Biomol. Chem., 2011, 9, 641; (h) Z. Han, L. Zhang, Z. Li and R. Fan, Angew. Chem., Int. Ed., 2014, 53, 6805; (i) J. Li, Z. Zhu, S. Yang, Z. Zhang, W. Wu and H. Jiang, J. Org. Chem., 2015, 80, 3870; (j) X. Zhao, L. Zhang, X. Lu, T. Li and K. Lu, J. Org. Chem., 2015, 80, 2918.
6. S. Cacchi, G. Fabrizi and L. Moro, Synlett, 1998, 741.
7. N. Monteiro and G. Balme, Synlett, 1998, 746.
8. Z. Liang, S. Ma, J. Yu and R. Xu, J. Org. Chem., 2007, 72, 9219.
9. A. Fürstner and P. W. Davies, J. Am. Chem. Soc., 2005, 127, 15024.
10. (a) T. Mino, Y. Shirae, M. Sakamoto and T. Fujita, Synlett, 2003, 882; (b) T. Mino, Y. Shirae, M. Sakamoto and T. Fujita, J. Org. Chem., 2005, 70, 2191; (c) T. Mino, Y. Shirae, Y. Sasai, M. Sakamoto and T. Fujita, J. Org. Chem., 2006, 71, 6834; (d) T. Mino, Y. Shirae, T. Saito, M. Sakamoto and T. Fujita, J. Org. Chem., 2006, 71, 9499; (e) T. Mino, H. Shindo, T. Kaneda, T. Koizumi, Y. Kasashima, M. Sakamoto and T. Fujita, Tetrahedron Lett., 2009, 50, 5358; (f) T. Mino, S. Suzuki, K. Hirai, M. Sakamoto and T. Fujita, Synlett, 2011, 1277; (g) T. Mino, M. Shibuya, S. Suzuki, K. Hirai, M. Sakamoto and T. Fujita, Tetrahedron, 2012, 68, 429; (h) T. Mino, K. Watanabe, T. Abe, T. Kogure and M. Sakamoto, Heterocycles, 2013, 87, 2015.
11. (a) T. Mino, K. Kajiwara, Y. Shirae, M. Sakamoto and T. Fujita, Synlett, 2008, 2711; (b) T. Mino, T. Koizumi, S. Suzuki, S. K. Hirai, K. Kajiwara, M. Sakamoto and T. Fujita, Eur. J. Org. Chem., 2012, 678.
12. T. Mino, T. Kogure, T. Abe, T. Koizumi, T. Fujita and M. Sakamoto, Eur. J. Org. Chem., 2013, 1501.
13. K. Watanabe, T. Mino, T. Abe, T. Kogure and M. Sakamoto, J. Org. Chem., 2014, 79, 6695.
14. K. Watanabe, T. Mino, C. Hatta, S. Ito and M. Sakamoto, Org. Biomol. Chem., 2015, 13, 11645.
15. In the case of using NaOAc (Table 1, entry 15), a trace amount of 2-phenylbenzofuran¹⁶ was also obtained via the annulation of o-ethynylphenol.
16. G. W. Kabalka, L. Wang and R. M. Pagni, Tetrahedron, 2001, 57, 8017.

---

Footnotes

† Dedicated to Professor Barry M. Trost on the occasion of his 75th birthday.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c6qo00112b

---