Long‐range polar and steric effects in propionate‐SG1‐type alkoxyamines (SG1‐CHMeCOOX): a multiparameter analysis

Abstract

The effects of the substituent X on the homolysis rate constants (k d ) of SG1-propionate type alkoxyamines (SG1-CHMeCOOX) are analyzed by a multiparametric equation with y, the steric constant and s I , the polar inductive/field Hammett constant of X. An influence of long-range polar and steric effects on k d was observed, that is, decrease in k d with increasing size of the X group and increase in k d with increasing polarity of the X group.

Effect of the Carboxylate Salt on the CON Bond Homolysis of SG1‐Based Alkoxyamines

“…Furthermore, no differences in positive or negative charge distribution are noted between 1 b, 1 e, 1 g, 2, 2', and 5 ( Table 4). Because of the steric hindrance and electrostatic interactions, the carboxylic groups for the RS/SR diastereoisomers of 1 b, 1 e, 1 g, 2, and 2' are turned toward the phosphoryl group, whereas for the other diastereisomers they are in a position opposite both to the tBu and the phosphoryl groups (see Figures 5 and 6, and hC 7 C 6 O 5 N 4 i in Table 3) A recent study [22,31] showed that the difference in k d for the diastereoisomers of 5 is due to an anchimeric interaction which depends on the torsion angle hO 5 C 6 C 7 O 10 i (see Figure 7). Calculations for 2 and 1 b showed that the difference between these angle values and those of 5 is small.…”

“…31 P NMR studies of 1 a in C 6 D 6 revealed only one peak, whereas two peaks, corresponding to the two diastereoisomers, were observed in [D 6 ]DMSO (DMSO = dimethyl sulfoxide). For the alkoxyamines 1 a-f, the 31 P NMR shifts are clearly different from those of the starting material 2 and characteristic for each salt (see Table 2 and Experimental Section).…”

“…Recently, [29][30][31] we showed that k d decreased with increasing bulkiness of the alkyl ester group. This trend is similar for both diastereoisomers.…”

“…For the alkoxyamines 1 a-f, the 31 P NMR shifts are clearly different from those of the starting material 2 and characteristic for each salt (see Table 2 and Experimental Section). Based on our experience (obtained from X-ray data [22] and a collection of 31 P NMR shifts [22, for the diastereoisomers of SG1-CMeHCOOR type alkoxyamines), we assume that the isomer with the slowest k d always exhibits the highest 31 P shift associated with the RR/SS configurations.…”

“…The calculations provided geometric data, such as bond lengths l, interatomic distances d, valence angles a, and torsion angles q (see Table 3), which are in general related to the effects ruling the CÀON bond homolysis in alkoxyamines. [22,31] They also provided the conformations of each molecule (Figure 2). For comparison, the geometrical parameters of 5 are also reported in Table 3.…”