Arylalkynes, preparation

One drawback to this alkyne annulation chemistry is that it requires either symmetrical alkynes or unsymmetrical alkynes in which the two substitutents on the internal alkyne are sterically quite different or else one obtains mixtures of regioisomers. One way to overcome this problem is to prepare the corresponding arylalkyne through catalytic Pd/Cu chemistry and then effect electrophilic cyclization using organic halides and a Pd catalyst (Scheme 8).9... 

Analogous Pd/Cu-mediated arylalkyne annelation was employed to prepare binaphthyl-based double helical fragments in good yields [111]. Starting from enantiopure dialdehyde 72, both enantiomers of 73 and 74 could be obtained (Fig. 15.27). The structure of racemic 74 was confirmed by X-ray crystallography. 

A relatively inexpensive 2-mcthyl-3-butyn-2-ol is also a useful protecting reagent for stable and volatile arylalkynes. Base-catalyzed retro-Favorsky elimination of acetone from alcohols RC CC(CH3)20H can be successfully applied to prepare terminal acetylenes fEq. (39)] [60]. Treatment with KOH at elevated temperatures is required for elimination of acetone. Many procedures ai e reported. 

The Negishi palladium-catalyzed cross-coupling reaction of alkenyl, aryl, and alkynyl halides with unsaturated organozinc reagents provides a versatile method for preparing stereodefined arylalkenes, arylalkynes, conjugated dienes, and conjugated... 

Terminal arylalkynes can be prepared by oxidation-decarbonylation of 3-arylpropargyl alcohols using manganese dioxide in the presence of alkali. The corresponding arylpiopargyl dcohols are available by palladium-catalysed cross-coupling of aryl halides with commercial propargyl alcohol. The yield of the second step can be improved by the addition of a phase-transfer catalyst (18-crown-6 Scheme 18). ... 

The coupling reaction between an aromatic h ide and TMSA (28) in the presence of a Pd catalyst generated in situ, followed by treatment with potassium carbonate in methanol, provides a simple approach to various ethynylated aromatic derivatives (Scheme 19). By this method even hindered, electron-rich arylalkynes, such as (30), can be prepared from the corresponding aryl iodide by refluxing for 2 d in triethylamine. ... 

A method for the preparation of isoindolo[2,l-a]indoles prescribes treatment of A-(o-iodoaryl)aldimines and arylalkynes with Pd(OAc)2 and a base. The coupling is followed by an annulation event. ... 

Pd-catalyzed coupling reactions of terminal and internal alkynes 1 and 4 with halides are surveyed in this section. Reactions of alkynes with aryl and alkenyl halides are classified in to two types. The first one is the preparation of arylalkynes 2 and alkenylalkynes (1,3-enynes) 3 by the reactions of terminal alkynes 1 with aryl and alkenyl halides in the presence of Pd(0) and Cul as catalysts (section 3.4.2). The second one is insertion of internal alkynes 4 to aryl- and alkenylpalladium bonds formed by oxidative addition of halides, generating alkenylpalladiums 5, which are living species and undergo further transformations before termination. Terminal alkynes also undergo the insertion in the absence of Cul catalyst (section 3.4.3)... 

Sonogashira reaction [4,5], A comprehensive review on Pd-catalyzed alkynylation was published recently by Negishi and Anastasia [6], Facile Pd(0)-CuI-catalyzed coupling of aryl and alkenyl halides with 1-alkynes is used extensively for the preparation of arylalkynes or enynes. 

Preparation of chiral benzylic alcohols has been achieved by asymmetric dihy-drosilylation of terminal arylalkynes, followed by oxidation [24]. The chiral diol 85 with 95 % ee was obtained by direct Pd-catalyzed asymmetric dihydrosilylation of phenylacetylene (82) with HSiCl3 using MOP [(/ )-VI-18], followed by oxidation of 1,2-disilylated product 84, but the yield of 84 was 33 % (the main product was l-trichlorosilyl-2,4-diphenyl-l,3-butadiene). Better results were obtained by hydrosilylation using two catalysts. The first Pt-catalyzed monosilylation to give 83 was followed by the second hydrosilylation catalyzed by the chiral Pd catalyst. The diol 85 with 95 % ee was obtained in 87 % overall yield by this method. 

Instead of the carbonylation of 2-nitrostyrenes, Ragaini and colleagues found that indoles can be prepared from the reaction of nitroarenes with alkynes [42]. By palladium-phenanthroline complexes, 3-arylindoles were efficiently produced from nitroarenes, arylalkynes and CO by an ortho-C-H functionalization of the nitroarene ring (Scheme 9.4). Both electron-withdrawing and electron-donating substituents are tolerated on the nitroarene, except for bromide and activated chloride. Nitroarenes bearing electron-withdrawing substituents react faster, but... 

Terminal alkynes also add to [Tp Rh(CNR)]. Irradiation of the carbodiimide complex 1 in neat 1-alkyne leads to the activation of the sp C-H bond. In cases where other activatable C-H bonds were presented, competitive C-H activation at these positions was observed. For example, t-butylacetylene and trifluoromethyl acetylene give exclusively alkynyl hydride products, whereas 1-octyne and trimethylsilylacetylene also give products resulting from methyl group activation. In both of the latter cases, the sp C-H activation products are unstable and convert to the terminal alkynyl products at room temperature after a few days (Scheme 1). Similarly, the activation of arylalkynes leads to mixtures of sp and sp C-H activation products. The unsaturated fragment [Tp Rh(CNR)] was prepared either... 

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