Synthesis, Radical Reactivity, and Thermochemistry of Monomeric Cu(II) Alkoxide Complexes Relevant to Cu/Radical Alcohol Oxidation Catalysis

Two new monomeric
Cu­(II) alkoxide complexes were prepared and fully characterized as
models for intermediates in copper/radical mediated alcohol oxidation
catalysis: TptBuRCuIIOCH2CF3 with TptBu = hydro-tris­(3-tert-butyl-pyrazol-1-yl)­borate 1 or TptBuMe = hydro-tris­(3-tert-butyl-5-methyl-pyrazol-1-yl)­borate 2. These complexes
were made as models for potential intermediates in enzymatic and synthetic
catalytic cycles for alcohol oxidation. However, the alkoxide ligands
are not readily oxidized by loss of H; instead, these complexes were
found to be hydrogen atom acceptors. They oxidize
the hydroxylamine TEMPOH, 2,4,6-tri-t-butylphenol,
and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical,
and benzene, with formation of HOCH2CF3 (TFE)
and the Cu­(I) complexes TptBuRCuI-MeCN in dichloromethane/1% MeCN or 1/2 [TptBuRCuI]2 in toluene. On the basis of
thermodynamics and kinetics arguments, these reactions likely proceed
through concerted proton–electron transfer mechanisms. Thermochemical
analyses give lower limits for the “effective bond dissociation
free energies (BDFE)” of the O–H bonds in 1/2­[TptBuRCuI]2 + TFE and
upper limits for the free energies associated with alkoxide oxidations
via hydrogen atom transfer (effective alkoxide α-C–H
BDFEs). These values are summations of the free energies of multiple
chemical steps, which include the energetically favorable formation
of 1/2­[TptBuRCuI]2. The effective alkoxide α-C–H bonds
are very weak, BDFE ≤ 38 ± 4 kcal mol–1 for 1 and ≤44 ± 5 kcal mol–1 for 2 (gas-phase estimates), because C–H homolysis
is thermodynamically coupled to one electron transfer to Cu­(II) as
well as the favorable formation of the 1/2­[TptBuRCuI]2 dimer. Treating 1 with the H atom acceptor tBu3ArO• did not result in the expected alkoxide oxidation
to an aldehyde, but rather net 2,2,2-trifluoroethoxyl radical transfer
occurred to generate an unusual 2-substituted dienone–ether
product. Treating 2 with tBu3ArO• gives no reaction, despite evidence
that overall ligand oxidation and formation of 1/2­[TptBuMeCuI]2 is significantly
exoergic. The origin of this lack of reactivity may be due to insufficient
weakening of the alcohol α-C–H bond upon complexation
to copper.