Formate derivatives alkyne carbopalladation

The alkyne relay, in particular, has frequently been used for cascade carbopaliadations with ring formation, because a carbon-carbon triple bond is more reactive toward carbopalladation than a carbon-carbon double bond. Thus, methyl -iodocinnamate 70 reacts with diphenylacetylene 71 to yield the methylenediphenylindene derivative 72 (Scheme 22). ... 

The cyclic carbopalladation-cross-coupling tandem reaction has been extensively developed over the past several years. Despite earlier favorable findings with A1 and Zr [84], these metals are still scarcely used. On the other hand, organometals containing Sn and B have been widely used, and favorable results have been obtained for the formation of ffve-membered car-bocycles and heterocycles containing N and O from halodienes [88] (Eqs. 1 and 2 in Scheme 29), haloenynes [89-92] (Eqs. 3-5 in Scheme 29), haloaryl-alkynes [94,95] (Eqs. 6 and 7 in Scheme 29), and allenene derivatives [93,96] (Eqs. 8 and 9 in Scheme 29). 

The palladium-catalyzed reductive cyclization of endiynal-derived imines 72 with formic acid reported by Oh et al. can be considered to be a pyrrole formation through formal 5-exo cyclization between C=N and alkyne moieties (Scheme 19.15) [24]. They proposed a mechanism in which the intermediate 73 is formed by reductive carbopalladation of the internal alkyne by in situ-generated HPdOCOH. The intermediate 73 can then be converted to pyrrole 71 by nucleophilic attack of the imine nitrogen to the palladium nucleus accompanying hydride transfer, followed by reductive elimination. This reaction can be used for furan synthesis without using the amine component (see Scheme 19.34). 

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