Synthesis of Thieno[3,2-b]benzofurans by Palladium-catalyzed Intramolecular C–H/C–H Coupling

Abstract

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 ...

D-π-D chromophores based on dithieno[3,2-b:2',3'-d]thiophene (DTT): Potential application in the fabrication of solar cell

“…The structures of these novel planar π-conjugated compounds 2a-d were fully characterized on the basis of 1 H-NMR, 13 C-NMR spectra (see Figure S1-8 in the Supporting Information). [36][37] Simultaneously, the structures of 2a-d were further established on the basis of the base peak molecular ion at m/z [M + ] 369.00 for 2a, 508.11 for 2b, 456.04 for 2c and 482.13 for 2d in their mass spectra. All results were consistent with the proposed structures.…”

Palladium(II)-Catalyzed Direct C–H Alkenylation of Thienothiophene and Related Fused Heteroarenes

“…7 Meanwhile, we recently reported that thieno[3,2-b]-benzofuran frameworks can be readily constructed by the palladium-catalyzed dehydrogenative cyclization of 3-phenoxythiophenes. 8 A series of thienobenzofurans 1c−e prepared by our method could be employed as the substrates in the present alkenylation. Thus, the corresponding 2-styrylthienobenzofurans 3ca−ea were obtained under the standard room-temperature conditions (Table 2, entries 7−9).…”

Synthesis and circularly polarized luminescence properties of BINOL-derived bisbenzofuro[2,3-b:3’,2’-e]pyridines (BBZFPys)

“…This method is straightforward and highly step-economical, enabling us to produce condensed (hetero)acenes from rather simple polyarenes, in which several aromatic units are connected with each other through appropriate linker units [2][3][4][5][6][7][8][9][10][11]. Recently, we reported the synthesis and optical properties of a series of furan-fused aromatics via the formal dehydrogenative coupling adopting oxygen atom as the linker [12][13][14][15][16][17]. In particular, bisbenzofuro [2,3- were found to exhibit intense photoluminescence with relatively high quantum efficiency (Φ flu up to 0.70), indicating that the BBFZPy scaffold may serve as a key fluorophore unit in certain light-emitting functional materials (Scheme 1) [14].…”