Palladium-catalyzed reactions trimethylsilylacetylene

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). 

Palladium-Catalyzed Reactions. Numerous nucleophiles react with (l-bromovinyl)trimethylsilane in the presence of palladium complexes. The bromine has been substituted by phenylthio, vinyl, and aryl groups. This approach gives reasonable yields of the desired products. However, the substitution reactions sometimes lack regiospecificity. For example, a mixture of regioisomers was obtained in eq 6. Two mechanisms have been proposed for the formation of the -substituted product. One involves an elimination step to give trimethylsilylacetylene as an intermediate, which then undergoes catalyzed additions with the nucleophile at either the a- or positions. The other mechanism involves the formation of a pentacoordinated palladium intermediate, leading to the formation of isomeric products. ... 

A useful synthesis of ( )-j -ethoxycarbonylvinylsilanes by palladium-catalyzed regio- and stereospecific hydroesterification (EtOH -I- CO) (or carboethoxylation) of trimethylsilylacetylenes has been reported recently [210]. Alkoxycarbonyl or carbonyl functionalization of vinylsilanes are useful synthetic intermediates [211, 212]. The use of PdCl2(dppf) as a catalyst (with SnCl2 2 H2O as cocatalyst) is found to be superior and gives excellent yields. A key step in the reaction is thought to involve hydropalladation to give 60 or 61. The preference for 60 to 61 is understood... 

Compound 2 was chosen as a direct precursor to cyclo-Ci since it should lose three anthracene molecules in a retro-Diels-Alder reaction under thermal conditions (Scheme 13-1). The synthesis of 2 (Scheme 13-2) started with the Diels-Alder reaction of anthracene and tra 5-dichloroethene [10], followed by dehydrochlorination and subsequently bromination to 10. The latter conversion was best achieved by simply adding elemental bromine to a solution of the vinyl anion formed with n-BuLi [11]. Palladium-catalyzed alkynylation of 10 with trimethylsilylacetylene in n-butylamine followed by deprotection with aqueous KOH in MeOH gave the diethynyl derivative 11 as very unstable crystals, which in one case exploded spontaneously. 

Scheme 15.49. Palladium-catalyzed Sonogashira reaction with trimethylsilylacetylene.

In contrast, palladium(0)-catalyzed coupling of the requisite starting oxazole for the synthesis of hippadine 219 with trimethylsilylacetylene at 80°C did not afford the expected oxazole-alkyne 220 (Fig. 3.64). Instead, they isolated the tricyclic furan 221 derived from a Diels-Alder retro-Diels-Alder reaction in 77% yield. Thermolysis of 221 at 320°C effected an intramolecular Diels-Alder reaction with concomitant desUylation. Subsequent DDQ oxidation of this product (not shown) then provided hippadine. 

Starting from the related o-iodohydroxycoumarin 9 and trimethylsilylacetylene, the disubstituted psoralen 10 was obtained in high yield by Aquila through a palladium-copper-catalyzed Sonogashira/annulation reaction (Scheme 3) [60]. A variety of new psoralen derivatives were subsequently synthesized from 10 by substitution of the reactive trimethylsilyl group. 

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