Kinetic studies for Lewis acid catalyzed hydrogen atom donation from water

Very recently, water was discovered to act as a hydrogen atom trapping reagent in radical reactions when it was coordinated with a Lewis acid (Et 3B or Cp2TIIICl). Since no kinetic data was available for reaction of water as a hydrogen atom source, our goal was to take advantage of suitable radical clocks to obtain kinetics of these water trapping reactions for synthetic benefit and better mechanism understanding.;Kinetics of ring opening of the 1-cyclobutyldodecyl radical were studied by indirect kinetic methods using a PTOC ester as a radical precursor and Bu3SnH trapping. An Arrhenius function over the temperature range -20 °C to 47 °C was log kR = (13.2)--(13.5)/2.3 RT, and kR = 1.5 x 103 s-1 at 20 °C. This radical ring opening can now be used for calibration of slow radical reactions using "radical clock" indirect kinetic methods.;In the Barton-McCombie deoxygenation reactions, water and methanol were H-atom sources when they were complexed with Et3B. In addition, the alpha-hydrogens of triethylborane were also reactive towards radicals. Rate constants for reactions of secondary alkyl radicals with triethylborane and its complexes with water, deuterium oxide, methanol, and THE at ambient temperature were determined by radical clock methods. Cyclization of the 1-undecyl-5-hexyl radical and ring opening of the 1-cyclobutyldodecyl radical were evaluated as clock reactions. The resulting Arrhenius function for reaction of the Et 3B-CH3OH complex with a secondary alkyl radical was log kH = (7.9)--(4.7)/2.3RT. The Arrhenius function for reaction of the Et3B-OH2 complex with a secondary alkyl radical was log kH = (6.9)--(3.2)/2.3 RT.;Radical reactions were conducted with titanium-catalyzed water as a trapping reagent. The cyclobutyldodecyl radical clock was utilized for the indirect kinetic study of this trapping process. A secondary alkyl radical abstracted hydrogen atom from Cp2TiCl-H2O with a rate constant of 9.5 x 104 M-1 s-1 at 23 °C. The resulting Arrhenius function for reaction of the Cp 2TiCl-H2O complex with a secondary alkyl radical was log kH = (9.1)--(5.5)/2.3RT.