Chemically triggered C-ON bond homolysis of alkoxyamines. Part 4: solvent effect
Résumé
In a recent work (Org. Lett., 2012, 14, 358), we showed that the rate constants k(d) for the C-ON bond homolysis of chemically activated alkoxyamines were subject to solvent effects. We then investigated solvent effects on the non-activated alkoxyamine 1 ((diethyl(1-(tert-butyl(1-(pyridin-4-yl)ethoxy) amino)-2,2-dimethylpropyl) phosphonate) and its N+-O- oxide activated version 2, using 14 solvents exhibiting different solvent parameters - dipolar moments mu, dielectrical constants epsilon, cohesive pressures c, Reichardt solvent polarity constants E-T, viscosity eta, hydrogen bond donor and hydrogen bond acceptor constants alpha and beta, respectively, and nitrogen hyperfine coupling constants a(N). Weak solvent effects were observed both for 1 (4-5-fold from n-octane to 2,2,2-trifluoroethanol TFE) and for 2 (2-fold from n-octane to water) although k(d) increased 27-fold in n-octane and 19-fold in TFE from 1 to 2. It was shown that the C-ON bond homolysis rate constant k(d) increased with the a(N) values, meaning that the stabilization of the nitroxide was the main factor involved in the solvent effect. Approaches relying on the Koppel-Palm and Kalmet-Abboud-Taft relationships failed to describe the solvent effect for all diastereoisomers of 1 and 2. Nevertheless, the solvent polarity/polarizability (pi*) and hydrogen bond donor (alpha) properties are the main effects involved in the solvent effects at TS and on products.