Alkynes functionalization alkynyls

Pyrrole and indole rings can also be constructed by intramolecular addition of nitrogen to a multiple bond activated by metal ion complexation. Thus, 1-aminomethyl-l-alkynyl carbinols (obtained by reduction of cyanohydrins of acetylenic ketones) are cyclized to pyrroles by palladium(II) salts. In this reaction the palladium(II)-complexed alkyne functions as the electrophile with aromatization involving elimination of palladium(II) and water (Scheme 42) (81TL4277). 

Functionalized alkynes from alkynyl(phenyl)iodine(UI) esters. The hyper-valent iodinated acetylenes are formed by reaction of tributylstannylalkynes with PhI(CN)X (X = OTs, OBz, SCN). Elimination of Phi from these derivatives occurs on treatment with AgOTf. The reaction is applicable to diyne substrates. 

In the case of formation of the rhenium(I) complex 69 (Scheme 38), the alkynyl reagent [I(C=CSiMe3)Ph][OTf] has been used to add an additional alkyne functionality to convert a diyne to a triyne. ... 

Rao and Knochel have reported the addition of copper reagents prepared by transmetallation from polyfunctional organozinc iodides to reactive alkynes in a stereochemically well-defined syn-addition. Intramolecular carbocupration of functionalized alkynyl-substituted alkylcopper species such as 74 obtained from the alkyl iodide 75 allows the preparation of highly substituted five-membered car-bocylcles giving stereochemically pure exo-alkylidenecyclopentane derivatives such as 76. The lower reactivity compared to the lithium- or magnesium-copper reagents does not allow the reaction with unactivated terminal alkynes but tolerates ester, nitrile or chloride functions (Scheme 9.23) [34]. 

Despite the great success of the transmetalation process in the enantiose-lective arylation of ketones, its extension to allylation or alkynylation reactions failed, providing the corresponding tertiary alcohols with enantiomeric excesses never higher than 50% ee. On the other hand, more success has been found in the alkenylation of ketones. The process started with the hydrozirconation of terminal alkynes to give the corresponding alkenylzirconium intermediates, which were transmetalated by reaction, in this case, with various ketones in the presence of the HOCSAC ligand. This protocol tolerated the presence of other carbon-carbon multiple bonds on the alkyne, as well as different functionalities and achieved excellent results for alkyl ketones, a,(3-unsaturated ketones and even dialkylketones, as shown in Scheme 4.22. 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

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