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Ligand-Free Iron-Catalyzed Regiodivergent Hydroboration of Unactivated Terminal Alkenes
preprintsubmitted on 02.01.2020, 14:29 and posted on 07.01.2020, 13:16 by Wei Su, Ruixiao Qiao, Xiaoli Zheng, Xia Tian, Jianrong Han, Shiming Fan, Qiushi Cheng, Shouxin Liu
The control of regioselectivities has been recognized as the elementary issue for alkene hydroboration. Despite considerable progress, the specificity of alkene substrates or the adjustment of ligands were necessary for specific regioselectivities, which restrict the universality and practicability. Herein, we report a ligand-free iron-catalyzed regiodivergent hydroboration of unactivated terminal alkenes that obtains both Markovnikov and anti-Markovnikov hydroboration products in excellent regioselectivities. Notably, solvents and bases were shown to be crucial factors influencing the regioselectivities and further studies suggested the iron-boron alkoxide ate complex is the key intermediate that determines the unusual Markovnikov regioselectivity. Terminal alkenes with diverse structures (mono-substituted and 1,1-disubstituted, open-chain and exocyclic) underwent the transformation smoothly. The reaction does not require the addition of auxiliary ligands and it can be performed on a gram scale, thus providing an efficient and sustainable method for the synthesis of primary, secondary, and tertiary alkyl borates.