Donor–Acceptor Fluorophores for Visible-Light-Promoted Organic Synthesis: Photoredox/Ni Dual Catalytic C(sp³)–C(sp²) Cross-Coupling

Abstract

We describe carbazolyl dicyanobenzene (CDCB)-based donor–acceptor (D–A) fluorophores as a class of cheap, easily accessible, and efficient metal-free photoredox catalysts for organic synthesis. By changing the number and position of carbazolyl and cyano groups on the center benzene ring, CDCBs with a wide range of photoredox potentials are obtained to effectively drive the energetically demanding C­(sp3)–C­(sp2) cross-coupling of carboxylic acids and alkyltrifluoroborates with aryl halides via a photoredox/Ni dual catalysis mechanism. This work validates the utility of D–A fluorophores in guiding the rational design of metal-free photoredox catalysts for visible-light-promoted organic synthesis.

Photoredox Catalysis in Nickel-Catalyzed Cross-Coupling

“…This is not of particular concern because cyclopropyltrifluoroborates couple extremely well under normal Pd-catalyzed conditions [19,20], but this result does serve to showcase one of the limitations of the process and also points to the complementary nature of the photoredox/Ni dual catalytic method relative to that of two-electron cross-coupling methods (Scheme 7). The efficacy of the photoredox/Ni dual catalysis cross-coupling of benzyltrifluoroborates was tested using inexpensive, donor-acceptor fluorophores instead of the expensive Ir catalyst for the photoredox cycle (Scheme 8) [21]. After optimization, the alternative 4CzIPN photoredox catalyst was found efficient, affording the desired coupled products in yields comparable to those obtained using iridium photoredox catalysts.…”

“…As in the case of the alkyltrifluoroborates (Scheme 9), the use of donor-acceptor fluorophore catalysts was also investigated for the photoredox/Ni cross-coupling of carboxylic acid partners [21]. The alternative 4CzIPN photoredox catalyst was found efficient to afford the desired coupled products in yields comparable to those obtained using iridium complexes.…”

Recent advances in dual transition metal–visible light photoredox catalysis

“…They developed new carbazolyl dicyanobenzene-based fluorophores, and successfully applied as transition metal free photoredox catalyst along with nickel coupling catalyst for the decarboxylative coupling of Boc protected proline and aryl iodides. 25 …”

“…40 The photogenerated aryl radical adds to the Au(I) catalyst to form Au(II), then one electron oxidation results an Au(III) highly active species. By π-bond coordination to the Au(III) catalyst, a Meyer-Schuster rearrangement takes place and the propargylic alcohols turn into α,β-arylated unsaturated carbonyl compounds (25). The arylation reaction underwent in methanol, under argon atmosphere by irradiation of blue light.…”

Photoredox Generation of Carbon‐Centered Radicals Enables the Construction of 1,1‐Difluoroalkene Carbonyl Mimics

“…Pleasingly, both the inexpensive organometallic photocatalyst, Ru(bpy) 3 (PF 6 ) 2 , and the organic photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [13] facilitated the desired transformation without the need for additives (Scheme 2). Control studies verified that each component of the reaction mixture was necessary.…”