Ligand-Free Iron-Catalyzed Regioselectivity-Controlled Hydroboration of Aliphatic Terminal Alkenes

Abstract

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 was necessary for specific regioselectivities, which restrict the universality and practicability. Herein, we report a ligand-free iron-catalyzed regiodivergent hydroboration of aliphatic terminal alkenes that obtains both Markovnikov and anti-Markovnikov hydroboration products in high regioselectivities. Notably, solvents and bases were shown to be crucial factors influencing the regioselectivities and further studies suggested that the iron–boron alkoxide ate complex is the key intermediate that determines the unusual Markovnikov regioselectivity. Terminal alkenes with diverse structures (monosubstituted 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.

Rhodium-Catalyzed Regiodivergent Synthesis of Alkylboronates via Deoxygenative Hydroboration of Aryl Ketones: Mechanism and Origin of Selectivities

“…The control of the regioselectivity has been the core issue in this reaction. Generally, the catalyzed and uncatalyzed hydroboration of alkenes gives anti-Markovnikov products, − ,− although the metal-catalyzed hydroboration of vinylarenes usually leads to Markovnikov regioselecitivity. − However, ligand-, borane-, and/or catalyst-controlled regiodivergent hydroboration of alkenes is rare (only six examples, Figure A). − These existing protocols for divergently controlling regioselectivity in hydroboration were realized by varying the catalyst, borane, and/or the ligand, and provide access to divergent anti-Markovnikov and Markovnikov products. Of particular note is that the regiodivergent synthesis of linear and branched alkylboronates from readily available starting materials except alkenes is yet to be reported to the best of our knowledge.…”

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

“…[a] See Scheme Chemistry-A European Journal complexes of Co and Fe were developed (entry 11; [233] see also entry 12 [234] ). Commercially available iron and cobalt tetrafluoroborates were used as the pre-catalysts.…”

“…Very recently, Liu and co-workers reported ligand-free ironcatalyzed regiodivergent hydroboration of aliphatic terminal alkenes that can lead to either "Markovnikov" or "anti-Markovnikov" hydroboration products with high regioselectivity. [234] Interestingly, the solvent and the counterion were the crucial factors affecting the reaction outcome. In the case of Fe(OTs) 3 -tBuOLi-NMP system (Table 10, entry 4), the "Markovnikov" regioselectivity was observed, while the FeBr 3 -tBuOLi-tBuOMe system (Table 9, entry 12) gave "anti-Markovnikov" products.…”

Recent Advances in Nonprecious Metal Catalysis

“…Liu and Su reported a ligand-free Fe-catalyzed hydroboration of aliphatic terminal alkenes. Interestingly, good control over Markovnikov or anti-Markovnikov hydroboration was obtained by selecting the iron catalyst source, base, and solvent, resulting in high regioselectivities (49:1 vs 1:49) . While regioselective anti-Markovnikov hydroboration is obtained with FeBr 3 in MeOtBu, the Markovnikov hydroboration product is observed in amide solvents (e.g., FeCl 2 or Fe­(OTs) 3 , NMP) (Scheme ).…”