Boron chemistry in a new light

Abstract

Photocatalysis has recently opened up new avenues for the generation of radical species under visible light irradiation conditions. A particularly fascinating class of photocatalyzed transformations relies on the activation of stable boron species with visible-light since it allows the creation of boryl and/or carbon radicals through single electron transfer or energy transfer without the need for specific and costly equipment. This new paradigm has found numerous applications in synthetic organic chemistry, catalysis, and macromolecular chemistry. In this minireview, the concepts underlying photoactivation of boron-species as well as applications to the creation of C-H, C-C, C-O, B-C and B-S bond are discussed.

Fig. 1. Selected examples of catalysts used in visible-light photoredox chemistry and their excited-state potentials.

Scheme 1. Conjugate addition of B-alkylcatecholboranes to olefins (EWG = electron withdrawing group).

Scheme 2. Photobleaching of cyanine borate.

Scheme 3. Photocatalytic C–O bond formation from borates and TEMPO.

Scheme 4. Postulated reaction mechanisms for the C–O and C–C bond formations.

Scheme 5. Photocatalytic C–C bond formation from borates and electron-deficient alkenes.

Scheme 6. Redox-neutral α-heteroatom methylations of olefins (PMP: para-methoxyphenyl).

Scheme 7. Methyl-acridinium-mediated photocatalytic C–C bond formation from potassium trifluoroborates and electron-deficient alkenes.

Scheme 8. Visible-light-mediated deboronative alkynylation reaction.

Scheme 9. Mechanistic pathway of the deboronative alkynylation.

Scheme 10. Deboronative/decarboxylative alkenylation reaction.

Scheme 11. Mechanistic pathway accounting for the deboronative/decarboxylative alkenylation reaction.

Scheme 12. Hypothetical mechanism of the photoredox/nickel cross-coupling.

Scheme 13. Photoredox cross-coupling of benzylic trifluoroborates and aryl bromides.

Scheme 14. Photoredox cross-coupling of secondary alkyl trifluoroborates and aryl bromides.

Scheme 15. Barton-type decarboxylative experiment with dipp-Imd-BH₃ as a hydrogen donor.

Scheme 16. Visible-light-mediated formation of 3,5-dimethylpyrazole-boryl radical.

Scheme 17. (a) Visible-light-driven polymerization of trimethylolpropane triacrylate; (b) visible-light-mediated polymerization in water of hydroxymethyl- and methyl-acrylates; (c) plausible mechanism of the NHC-boryl-assisted photopolymerization.

Scheme 18. First synthesis of NHC-boryl sulfides and proposed mechanistic pathway.

Scheme 19. di-Me-Imd-BH₃-mediated reduction of organohalides.

Scheme 20. Ruthenium-catalyzed photoredox conversion of arylboronic acids into phenols.

Scheme 21. Methylene-blue- and iron-oxide-catalyzed photoredox conversion of arylboronic acids into phenols.

Scheme 22. BODIPY photocatalyzed oxidation of thioanisole under visible light and proposed mechanism.

Scheme 23. Aza-BODIPY photocatalyzed oxidation of 1-naphthol under sunlight.

Scheme 24. BODIPY-photocatalyzed preparation of pyrrolo[2,1-a]-isoquinolines from N-substituted maleimides and proposed mechanism.

Scheme 25. BODIPY-photocatalyzed arylation of furan and thiophene and proposed mechanistic pathway.

Scheme 26. BODIPY-mediated photopolymerization of divinylether monomer and proposed mechanism.

Scheme 27. Uncaging reaction of BODIPY-caged histamine derivative.

Scheme 28. Uncaging reaction of model biogenic molecules.

Scheme 29. Cu/Ru-catalyzed trifluoromethylation of boronic acids and plausible mechanism.

Scheme 30. Cu/Ir-catalyzed Chan–Lam coupling reaction of anilines and boronic acids and plausible mechanism. ^aWith Cu(OAc)₂ (5 mol%) and myristic acid (10 mol%).

Scheme 31. Visible-light-induced borylation of aryldiazonium salts.