Palladium-catalyzed Annulation of 2-Substituted Silylethynyloxybiaryls through δ-C–H Activation

Abstract

The intramolecular hydroarylation of 2-(silylethynyloxy)-biphenyls takes place with the aid of a palladium catalyst to give 6-methylene-6H-dibenzo [b,d]pyrans. The product can be transformed into 6,6-disubstituted dibenzopyrans via protodesilylation, followed by 1,2-addition. 1,4-Bis[2-(triisopropylsilylethynyloxy)phenyl]benzene is shown to undergo a dual intramolecular insertion reaction leading to the formation of pentacyclic condensed aromatics.Heteroatom-bridged condensed aromatics and heteroaromatics play key roles as biologically active agents and functional materials.1 Typical examples are dibenzofurans, 1b -pyrroles, and -siloles. 1d Moreover, two atom-bridged biaryls are also attractive conjugated π-electron building blocks. For the synthesis and functionalization of such compounds, an approach involving CH bond activation is ideal in view of efficient use of natural resources and step economy.2,3 For example, 6H-benzo[c]chromenes 4,5 can be prepared by the metal-catalyzed intramolecular arylaryl bond-forming reaction using CH bond-cleaving intramolecular cross-coupling.2c2h,2j An economical approach for the synthesis of such cycles is an intramolecular insertion of unsaturated bonds into CH bonds with the aid of transition-metal catalysts.3 Along this line we have disclosed that an O-alkynyl group is effective for site selective CH activation by palladium(0) complexes, which allows the synthesis of various oxygen-containing heterocycles from alkynyl aryl ethers. 68 Herein we report that an intramolecular insertion of a silylethynyl group into a δ-aryl-CH bond in 2-(silylethynyloxy)biaryls proceeds in the presence of a palladium catalyst to give 3-membered cyclic structures through novel annulation. This system can be easily applied to the synthesis of various oxygen-containing condensed cycles.We first examined the intramolecular hydroarylation of 2-triisopropylsilylethynyloxybiphenyl (1a) in the presence of Pd(OAc) 2 (5 mol %) and tricyclohexylphosphine (PCy 3 , 5 mol %) in toluene (0.1 M of 1a) at 90°C for 6 h and obtained9 Apparently, the δ-aryl-CH bond is selectively cleaved to allow the ethyne moiety to insert between the CH bond. Other bulky silyl substituents on alkynyl carbon were potent for the hydroarylation. tert-Butyldiphenylsilyl (1b) and tert-butyldimethylsilyl (1c) groups were effective for this transformation to give the corresponding dibenzopyran 2b and 2c in 89% and 37% 10 yields, respectively. However, less bulky silyl groups such as triethylsilyl and dimethylphenylsilyl or carbonaceous substituents like C(OMe)(CH 2 ) 5 gave the heterocycles in much lower or null yields. Substrates like 2-PhC 6 H 4 CH(OMe)C¸CTIPS and 2-PhC 6 H 4 C(O)C¸CTIPS were futile to give cyclization products, thus showing that the presence of bulky silyl substituents and the ethereal oxygen is essential for this hydroarylation. Of note, the intramolecular hydroarylation of 1a proceeded on a gram-scale to give 2a in 90% yield (1.28 g from 4.0 mmol of 1a).Pd(OAc) 2 (5 mol%) PCy 3 (5 mol%)We next examined the sco...

Transition‐Metal‐Catalyzed π‐Bond‐Assisted CH Bond Functionalization: An Emerging Trend in Organic Synthesis

“…Recently, Hiyama and co‐workers demonstrated the intramolecular CH activation of 2‐(silylethynyloxy)biphenyls 17 in the presence of 5 mol % Pd(OAc) 2 and 20 mol % PCy 3 (Cy=cyclohexyl) in toluene to give 6‐methylene‐6 H‐ dibenzo[ b,d ]pyrans 18 27. Under similar reaction conditions, bis(silylethynyloxy)terphenyl 19 underwent dual CH activation and carbocyclization to give pentacycle product 20 in excellent yield.…”

Collaborative Interaction of Carbon–Carbon Unsaturated Bond Groups with Transition-metal Catalysts for C–H Bond Functionalization

“…18 We showed that 2-silylethynyloxy biaryls underwent intramolecular hydroarylation via C2¤H bond activation by a combination of Pd(OAc) 2 /PCy 3 catalyst to give dibenzopyran derivatives (Scheme 12). 19 In the case of the reaction using 2,2¤-bis(alkynoxy)biphenyl, the monoannulation product is only provided without the generation of the double annulation product (Scheme 13). Based on these results, this reaction is considered to be triggered by the formation of both the alkynoxy-and twisted aryl-ligated complexes, followed by C2¤ H bond activation, leading to final products via intramolecular insertions.…”

Synthetic Transformations through Alkynoxy–Palladium Interactions and C–H Activation

“…We examined the intramolecular hydroarylation of 2-triisopropylsilylethynyloxybiphenyl ( 1i ) using various palladium catalysts and found that the conditions with Pd­(OAc) 2 (5 mol %) and PCy 3 (5 mol %) in toluene (0.1 M) at 90 °C were optimal for the hydroarylation; 6-( Z )-triisopropylsilylmethylidene-6 H -dibenzo­[ b , d ]­pyran ( 24a ) was obtained in 88% yield (Scheme ). The use of Pd­(dba) 2 or PdCl 2 instead of Pd­(OAc) 2 was ineffective. Various biaryl substrates were used to form the corresponding cycles.…”

“…33 We examined the intramolecular hydroarylation of 2-triisopropylsilylethynyloxybiphenyl (1i) using various palladium catalysts and found that the conditions with Pd(OAc) 2 (5 mol %) and PCy 3 (5 mol %) in toluene (0.1 M) at 90 °C were optimal for the hydroarylation; 6-(Z)triisopropylsilylmethylidene-6H-dibenzo [b,d]pyran (24a) was obtained in 88% yield (Scheme 21). 34 The use of Pd(dba The hydroarylation products could be converted into 6,6disubstituted dibenzopyrans (Scheme 23). Protodesilylation of 23a using hydrogen bromide in acetic acid gave 6methyldibenzo [b,d]pyrylium bromide (27), which was nucleophilically attacked at C6 by the cuprate reagent BuCu(CN)Li prepared from BuLi and CuCN to give 6-butylated 6methyldibenzopyran 28 (two steps).…”