Ortho‐alkynyl biaryls react in the presence of catalytic amount of Brønsted acids to give phenanthrenes in high yields under mild conditions. The activity and selectivity of this transformation are governed by the substitution pattern of the diarylalkyne moiety. Selectivity shifts are observed between the carbophilic Lewis and Brønsted acid‐catalyzed cycloisomerization involving alkyne activation.magnified image
The first example of an enantioselective carbocyclization of an alkyne-containing substrate catalyzed by chiral Brønsted acids was achieved. The use of the 2-hydroxynaphthyl substituent on the alkyne as ad irecting group constituted the key parameter enabling both efficient regioselective protonation of the carbon-carbon triple bond and chiral induction. The keyc ationici ntermediate could be depicted either as ac ationic vinylidene ortho-quinonem ethide or as tabilized vinyl cation. Atropoisomeric phenanthrenes derivatives were produced in high yields and good enantioselectivities under mild, metal-free reactionc onditions in the presence of chiral Ntriflylphosphoramide catalysts. The carbenic nature of the cationic intermediate was also exploitedt odescribe an example of alkyne/alkane cycloisomerization.
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