Palladium aryl-alkynyl reactions

R = alkenyl, aryl, allyl, benzyl, propargyl R = alkenyl, aryl, alkynyl, alkyl, benzyl, allyl The Negishi Coupling was the first reaction that allowed the preparation of unsymmetrical biaryls in good yields. The versatile nickel- or palladium-catalyzed coupling of organozinc compounds... 

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). 

Though not proved, alkynyl-ketones could also be involved as reactive intermediates in the regioselective preparation of 3,5-diarylisoxazoles 19 through four-component coupling of terminal alkynes 17, hydroxylamine, carbon monoxide, and aryl iodides 18, in the presence of a palladium catalyst. The reaction proceeds at room temperature and an ambient pressure of CO in an aqueous solvent system <05OL4487>. 

The reaction of aryl(alkynyl)palladium complex with Cul in the presence or absence of PPh3 ligand causes formation of alkynylcopper and aryl(iodo)palladium complexes via transmetalation of the alkynyl ligand from Pd to Cu (Eq. 5.49). 

Exchange reactions of aryl and iodo ligands and of aryl and alkynyl ligands are not observed at all. Analogous alkynyl ligand transfer from aryl(alkynyl)palladium complexes to aryl(iodo)platinum complexes shown in Eq. 5.52 occurs above 35°C to cause complete alkynyl group transfer from Pd to Pt [183]. 

In the former case, the reachon of [PdAr(I)Ln], where L is electron-rich PEt3, was examined (Scheme 6.31). Here, the reachon between the palladium complex, with R = Me, and lequiv. of [Cu(CCPh)(PPh3)]4 led to 74% of the Sonogashira product along with unreacted starting material. When the palladium copper ratio was decreased to 1 2, full conversion to the coupling product was observed. The intermediate of this reachon, an aryl(alkynyl)palladium complex, was formed in 65% as its trans-isomer, when the reaction temperature was lowered to -30 °C. The... 

Similar results have been observed for the arylation of indoUzidine 53, which occurs at C-3 to give 54 (Scheme 11.19) [64]. Attempts to trap a possible Heck intermediate in this reaction using indolizidine 55 as the substrate (via 57 and 58) failed, and led only to a mixture of 56a and 56b. For this, and for the palladium-catalyzed alkynylation of related heterocycles with alkynylbromides [65], an elec-trophihc aromatic substitution was proposed. A Heck process was also excluded in the paUadium-catalyzed arylation of imidazolidones, a reaction for which an intramolecular isotope effect of ku/kj) = 4.5 was found [66]. 

The reaction of sulfinates with alkynyl iodonium salts was successful, as these substrates are less easy to oxidize. Nevertheless, Waser and Chen demonstrated that EBX reagents can also be useful to synthesize alkynyl sulfones, as they allow a new one-pot procedure starting directly from Grignard reagents (Scheme 36) [161]. In this protocol, DABSO (DABCO-S02) is added after formation of the Grignard reagent. Addition of DME and TIPS-EBX 52 gives aryl alkynyl sulfones in 46-85% yield. Eor base sensitive substrates, it was also possible to start from aryl iodides and use a palladium catalyst. 

The palladium-catalyzed reaction of alkenyl-, aryl-, alkynyl- and alkylsiloxanes with aryl, alkyl, and alkenyl halides and triflates in the presence of activators is known as the Hiyama cross-coupling reaction and several reviews have been published. This chapter will present major developments and examples of recent carbon-carbon bond formation methodology and improvements as well as their use in natural products synthesis in the last few years. 

An interesting palladium-catalyzed cascade reaction leading to 3,3-disubstituted oxindoles 190 starting from 2-(alkynyl)aryl isocyanates 189 with benzylic alcohols has been developed by Toyoshima et al. [76] (Scheme 6.53). This reaction integrates a cyclization step and a novel [1,3] rearrangement step. Both benzylic and allylic alcohols could be introduced efficiently to this domino process. Furthermore, the products of this reaction are an important class of heterocycles which are often found in naturally occurring and biologically active molecules. 

Palladium-catalyzed carbon-carbon bond forming reactions like the Suzuki reac-tion as well as the Heck reaction and the Stille reaction, have in recent years gained increased importance in synthetic organic chemistry. In case of the Suzuki reaction, an organoboron compound—usually a boronic acid—is reacted with an aryl (or alkenyl, or alkynyl) halide in the presence of a palladium catalyst. 

Palladium And/Or Copper-Mediated Cross-Coupling Reactions Between 1-Alkynes And Vinyl, Aryl, 1-Alkynyl, 1,2-Propadienyl, Propargyl And Allylic Halides Or Related Compounds. A Review, Rossi. R. Carpita, A. Beilina, F. Org. Prep. Proceed. Int., 1995, 27, 129... 

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