Transition-metal Free Visible Light Photoredox-catalyzed Remote C(sp3)-H Borylation Enabled by 1,5-Hydrogen Atom Transfer

Abstract

We describe a transition-metal free, photoredox-catalyzed borylation method of unactivated C(sp3)-H bonds via initiated 1,5-hydrogen atom transfer (HAT). The remote borylation was directed by 1,5-HAT of the amidyl radical, which was generated by photocatalytic reduction of hydroxamic acid derivatives. Substrates bearing primary, secondary and tertiary C(sp3)-H bonds could all be accommodated in this remote borylation protocol, giving moderate to good yields of the desired products (up to 92%), and a series of functional groups were tolerated. Mechanistic studies, including radical clock experiments and DFT calculations, gave detailed insight into the 1,5-HAT borylation process.

Visible-Light-Induced Transition-Metal-Free Redox-Neutral Carboxylation of Remote Benzylic C(sp3)–H Bonds via 1,5-Hydrogen Atom Transfer

“…By substituting the tert-butyl group with either an isopropyl or cyclohexyl group, the substrates can undergo the desired transformation to form carboxylation products with inferior yields, which may be attributed to the replacement of a tertiary N-connected alkyl group with a secondary one, leading to weakened N−H bond and thermodynamic effects on the 1,5-HAT process (2b−2c). 25 α-Oxiamido acids bearing monosubstituted groups at the para-position of the amido bond, such as electron-rich groups (OMe and Me) or electron-poor groups (F, Cl, and Br) on the benzene ring, have been demonstrated to be suitable starting materials, giving the desired products in good yields (2d−2h). When the monosubstituted groups are positioned at the ortho sites of the amide bond, there is a distinct decrease in the yield of the corresponding carboxylation products (2i and 2j), while the substrates bearing monosubstituted groups at the meta-position of the amide bond give moderate to desired carboxylation products (2k, 2i, and 2m).…”