Mechanism of Borane‐Catalyzed Oligomerization of Arylallenes and Arylacetylenes.

Recently, it was shown that Piers' borane [HB(C6F5)2]2 can induce rapid oligomerization reactions of arylacetylenes and arylallenes. Using phenylallene PhCH=C=CH2 and phenylacetylene PhCCH as typical substrates, the detailed mechanisms of borane‐induced oligomerization are disclosed by extensive state‐of‐the‐art DFT calculations. Following initial regioselective hydroboration of unsaturated C=C or C≡C bonds, further oligomerization and structural rearrangement reactions may occur through unique boryl‐enabled conversions such as 1,3‐boryl‐shift, allylboration, 1,2‐carboboration, and 1,5‐electrocyclization. Together with stepwise transborylation with hydroboranes such as pinacolborane HBpin, such fundamental reactions may pave a catalytic way to borane‐induced oligomerization of arylacetylenes and arylallenes. [ABSTRACT FROM AUTHOR]