Alkenyl triflates terminal alkyne alkenylation

The formation of disubstituted alkynes by coupling of terminal alkynes, followed by intramolecular attack of an alcohol or amine, is used for the preparation of benzofurans and indoles. The benzo[il)]furan 356 can be prepared easily by the reaction of o-iodophenol with a terminal alkyne[262]. The 2-substituted indole 358 is prepared by the coupling of 2-ethynylaniline (357) with aryl and alkenyl halides or triflates, followed by Pd(ll)-catalyzed cycliza-tion[263]. 

The palladium-catalyzed arylation and alkenylation of terminal alkynes with aryl or alkenyl hahdes in presence of a copper(l) co-catalyst is called Sonogashira reaction. In the same way as in the other cross-coupling reactions described before, it is possible to immobihze the alkyne or the aromatic bromides, iodides or triflates on sohd supports (Scheme 3.15). 

Chlorobenzenes activated by coordination of Cr(CO)3 react with terminal alkynes[253]. The 1-bromo-l,2-alkadiene 346 reacts with a terminal alkyne to afford the alka-l,2-dien-4-yne 347(254]. Enol triflates are used for the coupling with terminal alkynes. Formation of 348 in the syntheses of ginkgolide[255] and of vitamin D are examples[256]. Aryl and alkenyl fluorides are inert. Only bromide or iodide is attacked when the fluoroiodoalkene 349 or fluoroiodoar-ene is subjected to the Pd-catalyzed coupling with alkynes[257-259]. 

The alkenyl iodide or triflate 369 reacts in the absence of Cul with two moles of terminal alkyne 370 to form the substituted fulvene 371. The reaction can be explained by the intermolecular insertion of the alkyne twice, followed by the intramolecular insertion of the alkene, and / -elimination to form the fulvene 371(268]. 

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. 

The carbonylation of aryl iodide in the presence of terminal alkynes affords acyl alkynes. Bidentate ligands such as DPPF give good results [241]. When PI13P is used, phenylacetylene is mainly converted to diphenylacetylene. The alkynyl ketones 488 are prepared by the reaction of the alkenyl triflate 487 with phenylacetylene and CO [242],... 

Terminal alkynes can be alkenylated by alkenyl triflates (bromides, iodides) in the presence of catalytic amounts of a palladium(O) complex (or a precursor thereof) and usually an additional substoichiometric amount of copper(I) iodide (Cul), and they can be arylated by aryl triflates (bromides, iodides). These reactions are called Cacchi coupling reactions if triflate reagents are employed, and Sonogashira-Hagihara coupling reactions if halides are used. 

Schaus and Panek also employed oxazole triflates as coupling partners in their palladium-catalyzed synthesis of vinyloxazoles for application to the C(26)-C(31) subunit of phorboxazole. They reported an improved procedure of preparing 2-phenyl-4-oxazole triflate 984 from 2-phenyl-4(57/)oxazolone 983 (Scheme 1.263). With 984 in hand, they developed a one-pot Cp2ZrCl2 catalyzed carboalumination of a terminal alkyne to produce an intermediate vinyl alane (not shown), which was then coupled with 984 to generate a 4-( )-alkenyl-2-phenyloxazole, e.g., 985 or 987, respectively. 

Several types of functionalized alkenyl(aryl)iodonium salts have been prepared by the addition of hyperva-lent iodine reagents to alkynes. Reactions of terminal alkynes with iodosylbenzene and triflic acid proceed as a highly stereoselective anti-addition to afford ( )-(P-trifluoromethanesulfonyloxyaIkenyl)phenyliodonium triflates 310 in high yield (Scheme 2.90) [446-448], Similar products were obtained Ifom the reactions of internal alkynes and parent acetylene [447],... 

Similarly functionalized alkenyl(aryl)iodonium triflates 312 were prepared by the addition of (aryl)fluoroiodonium triflates 311 to terminal alkynes (Scheme 2.91) [45,48]. 

Oxidative addition of alkenyl halides, triflates, and other esters to zerovalent palladium compounds has been long known as a viable route to palladium(ii)-alkenyl complexes. Stereospecific coupling reactions involving mono- and (E)- or (Z)-dihalo-alkenes with palladium-copper catalysis under modified Sonogashira conditions are quite versatile and useful. These proceed through oxidative addition of the alkenyl halide via palladium(ii) alkenyl complexes, followed by coupling with a nucleophile (usually an alkynylcopper reagent obtained in situ with co-catalytic copper(i) from terminal alkynes in the presence of, in this case, a base like piperidine instead of diethylamine). ... 

Vinyl triflates undergo carbonylative coupling with terminal alkynes to yield alkenyl alkynyl ketones in a reaction catalyzed by palladium acetate and dppp in the presence of triethylamine. When applied to 2-hydroxyaryl iodides (eq 35), subsequent attack by the hydroxyl group on the alkyne yielded flavones and aurones. The cyclization result depends on the reaction conditions. l,8-diazabicyclo[5.4.0]undec-7-ene as base in DMF yields mainly the six-membered ring flavone, whereas the only product observed when ert iloying potassium acetate in anisole was the flve-membered ring aurone. ... 

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