Arylalkynes oxidation

The addition of any one of several dialkyl chlorophosphates to an arylalkyne-derived vinyl zirconocene in the presence of catalytic amounts of CuBr in THF leads to the corresponding vinyl phosphonate in high yields (78—92% see, for example, Scheme 4.38) [25]. Here, alkyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

OL Hydroxy ketones. This hypervalent iodine reagent oxidizes terminal alkyl-or arylalkynes to a-hydroxy ketones (equation I). Of greater interest, it oxidizes ethynylcarbinols to a,a -dihydroxy ketones. ... 

The majority of reported reactions of aryl and heteroaryl substrates with organocopper reagents are examples of Stephens-Castro coupling or the more recent catalytic version of that reaction. The reaction has found recent application in syntheses of C-(6)-substituted pterins and pteridines, substituted pyridines, and the antitumor antibiotic fredericamycin A," to name a few. Aryl iodide can be che-mospecifically displaced in the presence of bromide," and 2,5-dibromopyridine is regioselectively substituted at the 2-position. Substitution of halobenzenes by propargyl alcohol, followed by oxidative cleavage, provides a convenient route to terminal arylalkynes. " Fused heterocycles are formed in reactions of aryl halides bearing nucleophilic ortho substituents. - "... 

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). ... 

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)... 

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. 

A C(sp)-C(sp ) single bond of arylalkynes was cleaved by photoirradiation of a platinum-alkyne complex (Scheme 1.28) [37]. The oxidative addition product was reverted to the starting ii -alkyne complex upon heating. This result indicates the photochemical oxidative addition is an endergonic process. 

Multisubstituted arylalkyne 25 was produced via a Ni-catalyzed reaction from zirconacyclopentadienes. Two equivalents of alkynyl halides are necessary in this reaction. One is as the third alkyne in the cyclization step. The other one acts as an oxidant in further Ni-catalyzed carbon-carbon bond formation (Scheme 11.10) [11]. 

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