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...

Palladium(0)-catalyzed insertion/annulation sequence between aryl silylethynyl ethers and internal alkynes was found to proceed through activation of ortho-C–H bonds assisted by alkynoxy groups and gave stereoselectively (Z)-2-silylmethylenechromenes. These products could be easily converted into 2,2-alkylated 2H-chromene derivatives, an important structural motif in medicinal chemistry and materials science. Various aryl silylethynyl ethers and alkynes can be transformed under the reaction conditions, and a wide range of chromenes is thus accessible. When unsymmetric alkynes are employed, a regioselective annulation takes place, especially those containing aryl and/or bulky substituents. Catalytic systems based on palladium(0), such as Pd(OAc)2/PCy3/Zn, [Pd(dba)2]/PCy3, or Pd(PCy3)2 exhibit excellent catalytic activity, and the best performance is observed for Pd(PCy3)2 in combination with Zn(OAc)2 as an additive. Substituents on the aryl group in the alkynyl aryl ethers rarely affect the reaction rate. Deuterium-labeling experiments suggest that the ortho-hydrogen atom migrates to the 2-methylene position in the chromene products. The cleavage of the C–H bond is considered to be the rate-determining step in these reactions.

The treatment of silylethynyloxyarenes with allylic pivalates in the presence of a palladium catalyst led to efficient C-H bond cleavage in both substrates and a novel annulation reaction to give 2,3-bismethylenechromanes. When ortho-allylated silylethynyloxybenzenes were used as the substrates, the same products were obtained. This result shows that site-selective intramolecular hydrovinylation is involved in the annulation reaction. The synthetic utility of the products was demonstrated by the construction of condensed polycycles.

The treatment of silylethynyloxyarenes with allylic pivalates in the presence of apalladium catalyst led to efficient CÀHbond cleavage in both substrates and anovel annulation reaction to give 2,3-bismethylenechromanes.W hen orthoallylated silylethynyloxybenzenes were used as the substrates, the same products were obtained. This result shows that siteselective intramolecular hydrovinylation is involved in the annulation reaction. The synthetic utility of the products was demonstrated by the construction of condensed polycycles.

Palladium-Catalyzed Annulation of 2-Substituted SilylethynyloxybiarylsThrough -C-H Activation. -The intramolecular hydroarylation of 2-(silylethynyloxy)-biaryls affords the corresponding cyclized products with a Z-configurated silylethynyl group. Analogously, compound (XI) leads to (XII) by dual annulation. Dibenzopyran (IIa) can be easily transformed to 6,6-disubstituted dibenzopyran derivatives (V) via desilylated intermediate (III) which can also be isolated with 89% yield. -(MINAMI, Y.; ANAMI, T.; HIYAMA*, T.; Chem. Lett. 43 (2014) 11, 1791-1793, http://dx.doi.org/10.1246/cl.140725 ; Res. Dev. Initiative, Chuo Univ., Bunkyo, Tokyo 112, Japan; Eng.) -M. Bohle 21-209