Straightforward synthesis of benzosiloles was achieved by the invention of Pd/acid-catalyzed intramolecular anti-hydroarylation of aryloxyethynyl(aryl)silanes via ortho-C-H bond activation. The aryloxy group bound to the ethynyl carbon is the key factor for this transformation.

The title reaction of silylethynyl 2,6-(tBu) 2 phenyl ethers took place in the presence of a palladium catalyst and pivalic acid to give 2-silylmethylenechromanes. The initial products were converted by treatment with acetic acid to thermodynamically stable 4H-chromenes, which were further reacted with activated aldehyde and ketone. Addition of a carbonhydrogen bond to a carboncarbon unsaturated bond is an ideal transformation in green chemistry, as by-products are not generated. Efforts so far have been focused mainly on sp 2 CH bond activation followed by insertion of unsaturated bonds. On the other hand, the corresponding sp 3 CH activation is almost limited to benzylic and allylic positions or to the sites next to a heteroatom such as oxygen and nitrogen and to carbonyl and imino groups.1,2 Typical examples are the nickelcatalyzed additions of unactivated sp 3 CH bonds to alkynes or alkenes using 8-aminoquinoline as the directing group.3 Additional examples involve sp 3 CH bond activation followed by addition to alkenes or alkynes to achieve annulation reaction.4,5 Thus, the hydroalkylation of carboncarbon unsaturated bonds via unactivated sp 3 CH bond activation has not yet been challenged. We have found that alkynoxy groups (OC¸C) work as a directing group for the activation of adjacent CH bond and as an addition partner. 68 In the course of this study, we disclosed that palladium-catalyzed hydrobenzylation reaction of alkynyl ortho-tolyl ethers occurs via benzylic sp 3 CH activation. 6a We envisaged that this alkynoxy group strategy might be applicable to hydroalkylation via aliphatic sp 3 CH activation. Herein, we report the intramolecular hydroalkylation reaction of alkynyl ethers via aliphatic sp 3 CH activation in the presence of palladium(0)/pivalic acid dual catalysis.We first exposed tert-butyldimethylsilylethynyl 2,6-(tBu) 2 -4-Me-phenyl ether (1a) to 5 mol % of Pd(OAc) 2 and PCy 3 , the standard hydrobenzylation conditions, 6a at 120°C, but no reaction occurred. We assumed that bulky phosphines are not suitable for steric reason at the target reaction center. Accordingly, we switched to less bulky trialkylphosphines and were pleased to find that PBu 3 was effective for the desired hydroalkylation. As a result, syn-adduct 2a was obtained via CH bond cleavage in a tBu group, albeit with a low yield (Table 1, Entry 1). Replacing Pd(OAc) 2 by Pd(dba) 2 was totally futile (Entry 2), suggesting that zero-valent palladium complexes do not show any catalytic activity. Then, considering the catalytic conditions for previous hydrobenzylation reactions, a catalytic amount of pivalic acid was added to the reaction in Run 2. This results in the formation of 2a in 77% yield as a Z/E mixture (95:5) along with an endo-isomer 3a (14%) (Entry 3), a thermodynamically more stable isomer of 2a. On the other hand, use of 10 mol % of PBu 3 suppressed the reaction (Entry 4). Since Pd(OAc) 2 can be reduced by triorganophosphine, 9 we examined catalytic conditions using Pd(OAc) 2 and 2 equiv of PBu 3 in the presence of ...

Understanding the formation of benzosiloles by the intramolecular palladium-catalyzed annulation of alkynyl­(aryl)­silanes is crucial for achieving synthetic diversity toward the enhancement of the chemistry of siloles. By a combination of density functional theory calculations and experiments, we describe not only the whole mechanism of reaction but also the drawbacks that block this type of reaction. We also unravel the role of the phosphine ligand, without which the reactions could not go forward. Moreover, in silico predictive catalysis is presented here since the substitution of the phosphine ligand by an N-heterocyclic carbene (NHC) promises milder experimental conditions. A screening of substrates with different electronic properties was carried out to further understand the two fundamental steps of the reaction: stereoisomerization and concerted metalation–deprotonation.

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