Chemically Triggered C-ON Bond Homolysis of Alkoxyamines. 5. Cybotactic Effect

In a recent work (Org. Lett. 2012, 14, 358), we showed that the rate constant k(d) of the chemically activated C-ON bond homolysis of alkoxyamines was subject to solvent effects. However, we showed that solvent effects were weak for the nonactivated alkoxyamine 1 (diethyl (1-(tert-butyl(1-(pyridin-4-yl)ethoxy)amino)-2,2-dimethylpropyl)phosphonate) and its N+-O- oxide activated version 3. On the other hand, the activated N-methylated version 2 of 1 experienced a strong solvent effect for a radical reaction, i.e., a 24-fold increase in kd from tert-butylbenzene (tBuPh) to 2,2,2-trifluoroethanol (TFE). Good correlations were observed with the normalized Reichardt solvent polarity constant E-T(N) and the nitrogen hyperfine coupling constant of the released nitroxide a(N,SGI), meaning that the stabilization of the nitroxide played an important role in C-ON bond homolysis in alkoxyamines. The Kalmet-Abboud-Taft relationships described successfully the solvent effect for each diastereoismer of 1 and 2, as for example with the minor diastereoisomer of 1 log(k(d)'/s(-1)) = -4.84 + 0.37 pi* + 0.21 alpha and that of 2 log(k(d)'/s(-1)) = -3.11 + 0.37 pi* + 0.47 alpha, with pi* being the polarity/polarizability and a the hydrogen bond donor (HBD) ability of the solvent. Surprisingly, the HBD effect is larger for 2 than for 1, whereas no extra lone pair is available in 2. This amazing effect was ascribed to the solvation of the counteranion, which is expected to be better solvated in a HBD solvent.