Radical Arene Addition vs Radical Reduction: Why Organometal Hydride Chain Reactions Stop and How To Make Them Go

摘要

Nonideal kinetic chain analysis was used to examine the kinetic limitations of free-radical synthesis. Homolytic aromatic substitution (HAS: ArH + R center dot - ArR + H center dot) occurs in a chain-terminating side reaction to the tributyltin hydride (SnH) reduction chain (RX + SnH + (i center dot)(cat). - RH + SnX). Kinetic modeling of premixed and slow reagent addition reactions have clarified the mechanisms of SM HAS, with the azo initiator (iNNi) acting not only as radical source but also (as an H center dot acceptor) as the redox catalyst for aromatization, and/or as a postaddition oxidant. Refractory halides and other hitherto baffling anomalies may arise from the build up of ipso (rather than ortho)-cycloadduct radicals in the steady-state radical population. The implications of these findings for "tin-free" radical chains (and emerging photoredox methods) are considered via historical and recent examples of the effects of chain-degrading radical transfer (to substrate, product, solvent, initiator, and/or reagent ligands) on the reagent's chain.