54765-14-9

The kinetics of oxidation of a large series of aliphatic amines by the "high-potential" oxidant (batho)2CuII (batho = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonate) was studied under pseudo-first-order conditions (excess amine) in water or in 30% aqueous methanol (v/v) at 25 °C over the pH range 7-11. The oxidations follow bell-shaped pH-rate profiles, with the low-pH leg reflecting the fact that only the free amine base is subject to oxidation and the high-pH leg representing conversion of (batho)2CuII to an ineffective oxidant at high pH. The latter is thought to be (batho)CuII(OH2)OH on the basis of the observed effect of [batho] on rate at high pH, and curve fitting of the rate data yielded estimates of the unitless Keq values governing this conversion. The variation in rate with degree of N-substitution and other structure-reactivity trends (such as the effect of ring size and the non-rate-retarding effect of 2,4,6-trimethyl substitution on PhCH2NR2) support a mechanism involving initial outer-sphere one-electron transfer, followed by proton transfer to the solvent, and then a rapid second one-electron oxidation to give imine/iminium product. Inner-sphere coordination of chelating amines shuts down the redox reaction, presumably as a consequence of displacement of the batho ligand(s) needed for high oxidant strength. Deuterium kinetic isotope effect (DKIE) measurements (i) comparing PhCD2N(CD3)2 vs PhCH2N(CH3)2 (intermolecular DKIE) and (ii) determining N-dealkylation preference in the case of PhCH2N(CH3)CD2Ph (intramolecular DKIE) suggest that the initial electron transfer is mainly rate-limiting. A rate comparison between erythro and threo diastereomers of 1,2-diphenyl-2-piperidinoethanol indicates a stereoelectronic preference for one-electron oxidation at nitrogen when held antiperiplanar to a β-hydroxyl. Stoichiometry studies using an excess of the Cu(II) oxidant reveal regioselective and chemoselective factors governing the overall amine-to-iminium oxidations.