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Ligand-Free Iron-Catalyzed Regiodivergent Hydroboration of Unactivated Terminal Alkenes
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submitted on 02.01.2020 and posted on 07.01.2020by Wei Su, Ruixiao Qiao, Xiaoli Zheng, Xia Tian, Jianrong Han, Shiming Fan, Qiushi Cheng, Shouxin Liu
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.