Photochemical Nickel-Catalyzed C-H Arylation: Synthetic Scope and Mechanistic Investigations.

An iridium photocatalyst and visible light facilitate a room temperature, nickel-catalyzed coupling of (hetero)aryl bromides with activated α-heterosubstituted or benzylic C(sp3)-H bonds. Mechanistic investigations on this unprecedented transformation have uncovered the possibility of an unexpected mechanism hypothesized to involve a Ni-Br homolysis event from an excited-state nickel complex. The resultant bromine radical is thought to abstract weak C(sp3)-H bonds to generate reactive alkyl radicals that can be engaged in Ni-catalyzed arylation. Evidence suggests that the iridium photocatalyst facilitates nickel excitation and bromine radical generation via triplet-triplet energy transfer.

Visible-Light-Mediated Alkenylation, Allylation, and Cyanation of Potassium Alkyltrifluoroborates with Organic Photoredox Catalysts.

Iridium- and ruthenium-free approaches to protected allylic amines and alkyl nitriles under photoredox conditions are reported. An inexpensive organic dye, eosin Y, catalyzes coupling of Boc-protected potassium α-aminomethyltrifluoroborates with a variety of substituted alkenyl sulfones through an α-aminomethyl radical addition-elimination pathway. Allylic and homoallylic amines were formed in moderate yields with high E/Z selectivity. The mechanistic approach was extended using tosyl cyanide as a radical trap, enabling the conversion of alkyltrifluoroborates to nitriles via a Fukuzumi acridinium-catalyzed process.

Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development.

Photoredox catalysis has experienced a revitalized interest from the synthesis community during the past decade. For example, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings by invoking a single-electron transmetalation pathway. This Perspective highlights advances made by our laboratory since the inception of the photoredox/Ni cross-coupling of benzyltrifluoroborates with aryl bromides. In addition to broadening the scope of trifluoroborate coupling partners, research using readily oxidized hypervalent silicates as radical precursors that demonstrate functional group compatibility is highlighted. The pursuit of electrophilic coupling partners beyond (hetero)aryl bromides has also led to the incorporation of several new classes of C(sp2)-hybridized substrates into light-mediated cross-coupling. Advances to expand the radical toolbox by utilizing feedstock chemicals (e.g., aldehydes) to access radicals that were previously inaccessible from trifluoroborates and silicates are also emphasized. Additionally, several organic photocatalysts have been investigated as replacements for their expensive iridium- and ruthenium-based counterparts. Lastly, the net C-H functionalization of the radical partner in an effort to improve atom economy is presented. An underlying theme in all of these studies is the value of generating radicals in a catalytic manner, rather than stoichiometrically.

Rhodium-catalyzed regio- and enantioselective amination of racemic secondary allylic trichloroacetimidates with N-methyl anilines.

We report the chiral diene ligated rhodium-catalyzed dynamic kinetic asymmetric transformation (DYKAT) of racemic secondary allylic trichloroacetimidates with a variety of N-methyl anilines, providing allylic N-methyl arylamines in high yields, regioselectivity, and enantiomeric excess. The rhodium-catalyzed DYKAT method addresses limitations previously associated with this particular class of aromatic nitrogen nucleophiles.