The Role of Structural Effects on the Reactions of Alkoxyl Radicals with Trialkyl and Triaryl Phosphites. A Time-Resolved Kinetic Study

Abstract

A time-resolved kinetic study on the reactions of alkoxyl radicals with trialkyl and triaryl phosphites ((RO)(3)P: R = Me, Et, i-Pr, t-Bu; (ArO)(3)P: Ar = C(6)H(5), 2,4-(t-Bu)(2)C(6)H(3)) has been carried out. In the (RO)(3)P series, the alkoxyl radicals (cumyloxyl (CumO(*)) and benzyloxyl (BnO(*))) undergo addition to the phosphorus center with formation of intermediate tetraalkoxyphosphoranyl radicals (R'OP(*)(OR)(3): R = Me, Et, i-Pr, t-Bu; R' = Bn, Cum). The addition rate constants are influenced by steric effects, decreasing on going from R = Me to R = t-Bu and from BnO(*) to CumO(*). Rate constants for beta-scission of the phosphoranyl radicals R'OP(*)(OR)(3) have also been determined, increasing, for a given alkyl group R, in the order R' = tert-butyl < R' = benzyl < R' = cumyl and, for a given R' group, on going from R = Me to R = i-Pr. This behavior has been explained in terms of the relative stability of the radicals formed after beta-scission, suggesting moreover that steric effects play in this case a minor role. CumO(*) reacts with triaryl phosphites (ArO)(3)P to give phenoxyl radicals, with rate constants that are influenced to a limited extent by substitution of the aromatic rings. The radical scavenging ability of these substrates is briefly discussed.

Kinetic Solvent Effects on Hydrogen Abstraction from Phenol by the Cumyloxyl Radical. Toward an Understanding of the Role of Protic Solvents

“…As an example, the plots of k obs vs [PhOH] for the reactions between CumO • and phenol, carried out in TFE (filled circles) and MeOH (empty circles) solutions are shown in Figure 1. 36 As shown in Table 1, the k H values in alkane and MeCN solution are in excellent agreement with those measured previously by Ingold and co-workers under similar experimental conditions. 11 The lowest k H value was measured when the reaction between CumO • and phenol was studied in MeOH solution (k H = 1.55 × 10 6 M −1 s −1 ).…”

Radical hydroxymethylation of alkyl iodides using formaldehyde as a C1 synthon

“…benzylic) species are generated. 19 In our case this fragmentation would lead to a primary alkyl radical, so we were hopeful that J would be long-lived enough to participate in a following reaction. In particular, we anticipated that J might be able to abstract an I-atom from 1 and therefore establish a phosphoranyl radical-based chain propagating system.…”

Visible Light-Induced Homolytic Cleavage of Perfluoroalkyl Iodides Mediated by Phosphines

“…This should lead to absorption band broadening due to a large ensemble of different conformers. Since a complete analysis of the conformational space and its corresponding excited states of F(CF 2 ) n -I• • • P(OMe) 3 was out of reach, we aimed at the spectroscopic and computational evaluation of different, conformationally locked phosphites, such as the "caged" phosphite 4-methyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane and tri(tert-butyl)phosphite ( t BuO) 3 P [50][51][52]. In the case of the caged phosphite, the oxygen lone pairs point in the same direction as the phosphorus lone pair.…”