1,2-Carboboration of Arylallenes by In Situ Generated Alkenylboranes for the Synthesis of 1,4-Dienes

We herein report a novel method for the coupling of unactivated alkynes and arylallenes, which relies on an unprecedented and regioselective 1,2-carboboration of the allene by an alkenylborane. The alkenylborane is conveniently prepared in situ by hydroboration of an alkyne with Piers' borane, i. e., HB(C6F5)(2). The boryl-substituted 1,4-dienes that are formed by this carboboration are well-suited for a subsequent Suzuki-Miyaura coupling with aryl iodides. This allowed us to develop a three-step, one-pot protocol for the synthesis of aryl-substituted 1,4-dienes. The generality of the reaction was demonstrated by the synthesis of twenty dienes with modular variations of all three reaction partners. The mechanism of the new 1,2-carboboration was investigated using dispersion corrected double-hybrid DFT computations that allowed us to rationalize the chemo- and regioselectivity of this key step.