The palladium-catalyzed intramolecular oxidative C-H/C-H coupling of 2-aryloxypyridines as challenging substrates is investigated to construct an important class of N,O-mixed heteroacenes, i.e. benzofuropyridines. It is found that 2,6-diaryloxypyridines efficiently undergo double cyclization to provide bisbenzofuro[2,3-b:3',2'-e]pyridines of interest in materials chemistry.

Treatment of diaryloxy-and triaryloxybenzenes in the presence of a Pd(TFA) 2 catalyst and an AgOAc oxidant produces the corresponding benzobis-and benzotrisbenzofurans in good yields through double and triple intramolecular CH/CH coupling reactions, respectively.

The intramolecular CH/CH coupling of 3-aryloxythiophenes and 3-aryloxybenzo [b]thiophenes has been found to occur effectively in the presence of Pd(TFA) 2 and AgOAc as catalyst and oxidant, respectively, to afford the corresponding cyclized products, thieno [3,2-b] 2 The syntheses of TBF 3 and BTBF 4 were carried out only by some conventional multistep methods. For instance, the mother TBF and BTBF were obtained from the combinations of 3-chloro-2-formylbenzo[b]furan/2-mercaptoacetate ester and methyl thiosalicylate/ 2-acetyloxybenzyl bromide, respectively. Thus, development of new, effective methods leading to TBF and BTBF as well as TBT and BTBT is desired. Among the possible approaches to the target structures, a metal-mediated direct CH coupling method appears to be attractive, as it may provide a shortcut and atom-economic process. 5,6 Recently, Takimiya and co-workers reported a palladium-catalyzed intramolecular direct arylation that leads to BTBT series of compounds (Scheme 1, X = S, Y = Br). 7,8 It may be conceived that a more direct method, which is intramolecular CH/CH coupling (Y = H), is also highly useful (Scheme 1).The formal dehydrogenative coupling may be regarded as a potential variant of the oxidative cyclization of diphenyl ether or sulfide to dibenzofuran or dibenzothiophene, for which some palladium-catalyzed protocols have been reported. 9,10 Consequently, we attempted to develop a new protocol for the transformation and found an effective set of conditions for the synthesis of TBF and BTBF, although that for TBT and BTBT still remains a challenge. 11 The results are described herein. We recently reported that 3-arylthiophenes efficiently undergo dehydrogenative direct annulation upon treatment with alkynes under Cp*Rh(III) catalysis (eq 1).12 Based on this work, it was expected that 3-aryloxythiophenes could afford thieno[3,2-b]benzofurans through intramolecular CH/CH coupling under similar conditions, without the addition of alkynes. Indeed, it was found that 3-phenoxythiophene (1a) can be converted to thieno[3,2-b]benzofuran (2a) by rhodium catalysis, as depicted in eq 2. However, the product yield was less than 30% under various conditions examined. Then, we turned our attention to palladium catalysis. Fagnou and co-workers described that the dehydrogenative cyclization of diphenyl ether, leading to dibenzofuran, occurs when using Pd(OAc) 2 as the catalyst in the presence of K 2 CO 3 under air in pivalic acid.9e However, the attempted cyclization of 1a under the reported conditions was not successful, with no product 2a being detected. In a subsequent experiment, it was observed that the reaction takes place when using AgOAc as the oxidant in propionic acid at 110°C to give 2a in 37% yield (Entry 1 in Table 1). The use of Pd(TFA) 2 (TFA: trifluoroacetate) in place of Pd(OAc) 2 increased the yield to 52% (Entry 2). Acetic acid and pivalic acid could be used, but the yields were lower than that in propionic acid (Entries 3 and 4). Eliminating K 2 CO 3 somewhat increased the yield (Entry ...

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