Regio‐Divergent and Stereo‐Controlled Alder‐Ene Reaction: Harnessing the Unique Electronic and Steric Properties of B(MIDA) Moiety

1,4-Dienes are pivotal structural motifs in bioactive natural products and serve as valuable intermediates in synthetic chemistry. Among the various methods for their synthesis, the ruthenium-catalyzed Alder-ene reaction stands out due to its redox-neutral, atom- and step-efficient nature. However, controlling regioselectivity with unsymmetrical alkynes remains a persistent challenge. In this study, we demonstrate that propargyl B(MIDA)-substituted alkynes are effective substrates for regio-divergent Alder-ene reactions. By selecting between [CpRu] and [Cp*Ru] catalysts, we can selectively obtain either linear or branched 1,4-dienes via γ- or β-allylation. Mechanistic investigations reveal that σ(C─B)→σ(Ru─C)* hyperconjugation plays a critical role for γ-selectivity, while steric repulsion between the Cp* ligand and the bulky B(MIDA) group governs the β-selectivity. This approach provides a straightforward and efficient means to access two distinct allylic boron-functionalized 1,4-dienes, each featuring differentiated double bonds, which are highly amenable to further chemical elaboration.