Pd-Catalyzed Borylative Cyclization of Allenynes and Enallenes

Pd-catalyzed cyclization of 1,5- and 1,6-allenynes and 1,5-enallenes with bis(pinacolato)diboron affords synthetically useful allylboronates and alkylboronates under smooth conditions in a formal hydroborylative carbocyclization reaction. One C-C and one C-B bond are formed in a single operation. The reaction outcome implies that different mechanisms operate for the reactions of allenynes and enallenes, respectively, the actual pathway depending on the relative reactivity of the alkyne or the alkene versus the allene moiety. The cyclized boronates obtained can be functionalized by oxidation or allylation reaction with aldehydes.