Normal, Leveled, and Enhanced Steric Effects in Alkoxyamines Carrying a β-Phosphorylated Nitroxyl Fragment

The design of new R1R2NOR3 alkoxyamines for various applications relies on the accurate prediction of two kinetic parameters, the C-ON bond homolysis rate constant (k(d)) and its re-formation rate constant (k(c)). Relationships to describe the steric and polar effects of the R1R2NO fragment ruling k(d) have been developed. For all cyclic nitroxyl fragments, the steric effect is described as the sum of the bulkiness of the R-1 and R-2 groups (i.e., normal steric effect), while for the noncyclic nitroxyl fragment (except for one case), a leveled steric effect is assumed. In this work, we show that the normal steric effect also applies to noncyclic nitroxyl fragments and that for one case an enhanced steric effect is also observed, i.e., experimental k(d) > 5-fold larger than the predicted value.