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Metal-free Borylation

Mechanism + Description

A frustrated Lewis Pair (FLP) is a compound or mixture containing a Lewis acidic atom and a Lewis base atom that cannot combine to form a classical donor-acceptor adduct due to steric hindrance. The catalytic activity of FLP follows the addition of the electron rich π-system followed by loss of H2. σ-bond metathesis with the borane generates the product and regenerates the catalyst.

General comments

A number of routes exist to the borylation of arenes/heteroarenes/π-bonds without using metal catalysts.These involve the transformation of Ar-N, Ar-Halogen and Ar-H bonds into Ar-B compounds. Some involve the use of highly reactive intermediates like diazonium salts, or the use of organocatalysis. A number of ambiphilic or FLP molecules are known to activate boranes and facilitate carbon borylation – usually with fairly electron-rich π-systems.

Key references

Fontaine, F.-G.; Rochette, E. Ambiphilic Molecules: From Organometallic Curiosity to Metal-Free Catalysts. Acc. Chem. Res. 2018, 51, 454-464.

Chen, K.; Wang, L.; Meng, G.; Li, P. Recent Advances in Transition-Metal-Free Aryl C-B Bond Formation. Synthesis 2017, 49, 4719-4730

Farre, A.; Briggs, R.; Pubill-Ulldemolins, C.; Bonet, A. Developing a Bench-Scale Green Diboration Reaction toward Industrial Application. Synthesis 2017, 49, 4775-4782.

Yang, K.; Song, Q. Transition-Metal-Free Regioselective Synthesis of Alkylboronates from Arylacetylenes and Vinyl Arenes. Green Chem. 2016, 18, 932-936.

Qi, X.; Li, H.-P.; Peng, J.-B.; Wu, X.-F. Borylation of Aryldiazonium Salts at Room Temperature in an Aqueous Solution Under Catalyst-Free Conditions.Tetrahedron Lett. 2017, 58, 3851-3853.

Légaré, M.-A.; Rochette, E.; Lavergne, J. L.; Bouchard, N.; Fontaine, F.-G. Bench-Stable Frustrated Lewis Pair Chemistry: Fluoroborate Salts as Precatalysts for the C–H Borylation of Heteroarenes. Chem. Commun. 2016, 52, 5387-5390.

Lavergne, J. L.; Jayaraman, A.; Castro, L. C. M.; Rochette, E.; Fontaine, F.-G. Metal-Free Borylation of Heteroarenes Using AmbiphilicAminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C–H Bond Activation J. Am. Chem. Soc. 2017, 139, 14714-14723.

Relevant Scale-up Example – Metal-free Borylation

Jayaraman, A.; Castro, L. C. M.; Fontaine, F.-G. Org. Process Res. Dev2018, 22, 1489–1499.