Trimethylsilylacetylene, aryl coupling

Scheme 14 Aryl coupling of trimethylsilylacetylene via a palladium-catalyzed Sonogashira protocol...

The series of wide-bite-angle, bulky ligands derived from a cyclobutene scaffold gave Pd complexes (117) showing appreciable activity in the cross-coupling of reactive aryl bromides with trimethylsilylacetylene. A considerable shift of electron density to the phosphorus atoms, probably arising from alternative aromatic canonical structures, may account for the ligand properties.422... 

Although the Sonogashira reaction is normally performed with a copper cocatalyst, a copper-free, one-pot procedure for direct coupling with l-aryl-2-trimethylsilylacetylenes has been developed <2005T2697>. The procedure uses a mixture of palladium acetate and tri(o-tolyl)phosphine as catalyst in the presence of tetra- -butylammonium chloride... 

TMS group is used for protection of terminal alkynes. However, alkynylsilanes themselves can be used for the coupling with aryl and alkenyl triflates using Pd-CuCl as a catalyst [74], Thus the internal alkyne 160 is prepared by stepwise reactions of two different triflates 157 and 159 with trimethylsilylacetylene (134) via 158. 

A catalytic amount of CuCl was found to activate alkynyl(trimethyl)silanes in the palladium-catalyzed coupling reaction with aryl triflates (Eq. 7) [12]. The catalytic cycle is considered to involve the transfer of an alkynyl group from an alkynylsilane to Cu(I) and then to palladium(II). A sequential palladium-catalyzed reaction of trimethylsilylacetylene gives unsymmetrical diarylacet-ylenes (Eq. 8). 

The catalyst system for the coupling reaction was a Pd(II)-tri-phenylphosphine complex, usually prepared in situ, with excess triphenyl-phospUne and either cuprous iodide or cupric acetate as a co-catalyst. Alternatively, a preformed catalyst mixture prepared from these reagents may be utilized (see Experimental Section). When 2-methyl-3-butyn-2-ol was used as the protected acetylene, the intermediates 5a-d were converted to the corresponding aryl acetylenes 6a-d by a retro-Favorskii-Babayan (8) reaction utilizing potassium r-butoxide in toluene under conditions of slow distillation. In the case of p-iododimethylaniline (3e), trimethylsilylacetylene was used as the ethynyl source. The intermediate (5e) was treated with hydroxide to generate the free aryl acetylene 6e. The syntheses of 6d and 6e are described in the Experimental section below. 

The sp -sp coupling reaction can be extended to the synthesis of terminal alkynes by use of protected alkynes such as trimethylsilylacetylene (28 TMSA) or 2-methyl-3-butyn-2-ol (29), followed by subsequent removal of the protecting group (Schemes 17, 18 and 20). - - Thus, commercially available TMSA (28) reacts with aryl bromides or iodides in the presence of a palladium complex and copper(I) iodide, followed by treatment with dilute aqueous potassium hydroxide in methanol or a P source, such... 

The most commonly used protecting group for acetylenes is the trimethylsilyl (TMS) group.f Thus, commercially available trimethylsilylacetylene provides an excellent starting material for the synthesis of aryl and alkenyl acetylenes. Triisopropylsilyl groups are also useful for the step wise deprotection with TMS groups (Scheme 37). Cross-coupling of aryl or alkenyl halides with trimethylsilylacetylene proceeds in the presence of a Pd catalyst and Cul, followed by treatment with dilute aqueous or in... 

Trimethylsilylacetylene couples smoothly with aryl iodides in the presence of catalytic amounts of bis(triphenylphosphine)palladium dichloride and copper(l) iodide to give, following deprotection, ethynylarenes (e.g. Scheme 110). Aryl bromides react only when they are suitably activated by electron-withdrawing... 

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