Additional Considerations

The Fensterbank group reported on the metal-free photooxidation of alkylsilicates to generate alkyl radicals, which could then be used in the metal-catalyzed cross-coupling with suitable acceptors. This protocol makes use of the organic dye 1,2,3,5-tetrakis(carbozol-9-yl)4,6-dicyano-benzene (4CzIPN), which allows for the facile generation of alkyl radicals from the alkylsilicate. In this work the potassium silicate was used (Scheme 8).⁸

The Molander group with their interest in both organoboron and organosilicon chemistry have worked on the Csp³-Csp² cross-coupling of alkylsilicates with various borylated aryl bromides. The alkylsilicate cross-coupling occurs without affecting the arylboronate, which is then available for Suzuki-Miyaura cross-coupling. Whereas the pinacol, neopentyl and the N-methyliminodiacetoxy boronates reacted well the 1,8-diaminonaphthalene-derivatives did not. A total of 25 examples were reported (Scheme 9).⁹

The chemoselective Ni/photoredox dual catalyzed sp³-sp² cross coupling as a function of the aryl halide showed a definite enhanced reactivity of the iodide substrates over the bromides. This represents the first investigation of the chemoselective sp³-sp² cross-coupling. An extensive investigation of a variety of bromo(iodo)arenes and alkylsilicates revealed excellent functional group tolerance although a benzylic alcohol gave no yield (Scheme 10).¹⁰

The alkylsilicate Csp³-Csp² cross-coupling with aryl triflates gave good yields of the alkylated arene with the exception of the reaction with a strong electron-donating group substituted aryl triflate. The chemoselectivity of the cross-coupling alkylation favored the reaction with an aryl bromide over that with an aryl triflate (Scheme 11).¹¹