Ruthenium‐Catalyzed α‐Regioselective Hydroboration of Allenes

Hydroboration of allenes is powerful and atom‐economic approach to the synthesis of organoboranes, such as the highly versatile allylboranes. However, regarding regiocontrol, existing methods uniformly deliver the boron functionality to the less hindered β‐ or γ‐position, but not the α‐position. The latter is particularly challenging for allenes with substantial steric difference between the two terminals and lacking electronic bias (e.g., 1,1‐disubstituted allenes). Herein we report the first highly efficient ruthenium‐catalyzed hydroboration of allenes featuring exclusive α‐regioselectivity, providing access to sterically hindered allyl boranes that are limitedly accessible by conventional methods. DFT studies suggested that the unusual α‐regioselectivity is attributed to the disfavored reductive elimination at the γ‐position due to the high energy cost required to overcome the agnostic interaction and rotation of the key π‐allyl intermediates. This protocol is also applicable to the previously unprecedented α‐hydroalkynylation and underdeveloped α‐hydrosilylation of allenes, thus complementing known catalytic systems and providing convenient access to highly congested yet densely‐functionalized allyl silanes and skipped enynes bearing a fully‐substituted allylic carbon center. It is expected that this ruthenium‐catalyzed system can serve as a new platform for the development of other hydrofunctionalization processes with unorthodox selectivity.