TMSCF 3 -Mediated Conversion of Salicylates into α,α-Difluoro-3-coumaranones: Chain Kinetics, Anion-Speciation, and Mechanism.

As reported by Zhao, the TBAT ([Ph 3 SiF 2 ] - [Bu 4 N] + )-initiated reaction of ethyl salicylate with TMSCF 3 in THF generates α,α-difluoro-3-coumaranones via the corresponding O -silylated ethoxy ketals. The mechanism has been investigated by in situ 19 F and 29 Si NMR spectroscopy, CF 2 -trapping, competition, titration, and comparison of the kinetics with the 3-, 4-, 5-, and 6-fluoro ethyl salicylate analogues and their O -silylated derivatives. The process evolves in five distinct stages, each arising from a discrete array of anion speciations that modulate a sequence of silyl-transfer chain reactions. The deconvolution of coupled equilibria between salicylate, [CF 3 ] - , and siliconate [Me 3 Si(CF 3 ) 2 ] - anions allowed the development of a kinetic model that accounts for the first three stages. The model provides valuable practical insights. For example, it explains how the initial concentrations of the TMSCF 3 and salicylate and the location of electron-withdrawing salicylate ring substituents profoundly impact the overall viability of the process, how stoichiometric CF 3 H generation can be bypassed by using the O -silylated salicylate, and how the very slow liberation of the α,α-difluoro-3-coumaranone can be rapidly accelerated by evaporative or aqueous workup.