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

Abstract 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.