Cheng-Xue Zhao

MODIFICATION OF POLYSTYRENE VIA AROMATIC PER(POLY)FLUOROALKYLATION BY PER(POLY) FLUORODIACYL PEROXIDES

Abstract Facile electron-transfer reactions between polystyrene and per(poly)fluorodiacyl peroxides at ~ 20 °C in dichloromethane produced high yields of ring-peer(poly)fluoroalkylated polystyrenes with excellent water- and oil-repellency, chemical resistance and much lowered refractive indices.

Electron paramagnetic resonance spectroscopic monitoring of the reactions of t-butyl perfluoroalkyl nitroxides with alcohols

Abstract Electron paramagnetic resonance spectroscopic monitoring of the reactions of various alcohols with a mixture of perfluoroalkyl nitroxides (R F = CF 3 , n-C 3 F 7 , etc.) and a spin trap (Bu t NO) in F113 (CFCl 2 CF 2 Cl) solutions have been found to provide a novel and informative technique for studying H-abstraction and subsequent radical reactions.

New fluorinated nitroxides. Generation, structure, reaction and application

Abstract A large number of fluorinated nitroxides with a wide variety of structures have been synthesized by mainly making use of the electron-transfer reactions of fluorinated oxidants, such as (R f COO) 2 , R f I and R f SO 2 Br with nitrogen-containing substrates. The nitroxides are spin-adducts of the radical intermediates and both nitroso traps (R f NO or RNO) are formed in these reactions. Such ‘in situ spin trapping reactions’ have served as a useful technique in the mechanistic studies of these radical reactions of N-substrates. The reaction mixtures — blue solutions (called ‘magic blue’) containing very reactive fluorinated nitroxides such as R f N(O·)R f and effective spin traps such as R f NO — have been applied successfully to the study of H-abstraction reactions from very many kinds of substrates and thereby producing a much larger number of fluorinated nitroxides. Much valuable information about the electronegativity of various fluorinated functional groups, substituent effects through spin-delocalization (σ scale), solvation, the temperature dependence of α N and α F values, as well as selective line broadening have been gained from EPR studies of these new fluorinated nitroxides.

ω-hydro-perfluoroalkyl pyrimidinyl nitroxides

Twenty-two polysubstituted 2-aminopyrimidines dissolved in F1 13 (CC1F2-CCl2F) solutions containing ([H(CF2CF2)nCO2]2 n = 1,2) and H(CF2CF2)nNO [↔ H(CF2CF2)n• + NO] have been oxidized into detectable ω-H-perfluoroalkyl pyrimidinyl nitroxides along with neutral products. A mechanism involving Habstraction from the amino groups by H(CF2CF2)nN(O•)(CF2CF2)nH and the subsequent radical steps was suggested for the generation of the nitroxides. Some relationships between aN values and the nature of the ring substituents are discussed.

Modification of linear triblock copolymer SBS via olefinic per(poly)fluoroalkylation by per(poly)fluorodiacyl peroxides

Abstract Facile electron transfer reactions between linear triblock copolymer SBS (polystyrene–polybutadiene–polystyrene) and per(poly)fluorodiacyl peroxides at room temperature (∼20°C) in dichloromethane produced high yields of olefmic per(poly)fluoroalkylated SBS with excellent water and oil repellency, chemical resistance and much lowered refractive indices.

EPR studies on H-abstraction from silicon by “magic blue”-the F, Si-nitroxides

The H-abstraction from benzyl silicanes (3–6), alkoxy silicanes (7–18), phenyl alkoxy silicanes (19–22), and polybenzylsiloxane (23) by “magic blue” solution (in F113, CClF2-CCl2F) containing the H-abstracting agent [H(CF2)4]2NO· (1) and spin trap H(CF2)4 NO (2) was studied by EPR detection of the F, Si-nitroxides (a total of 24 from 21 silicon substrates) generated in the reactions. From the EPR spectra of the nitroxides, we have gained information about the position of H-abstraction, the subsequent radical steps after the H-abstraction, as well as the structures of the transient radical intermediates. This technique may serve as a new method for making multi-and polynitroxides under very mild conditions.

EPR studies on electron transfer reactions between O-benzoyl-N-alkylhydroxylamines and perfluoroacyl peroxides. Generation of n- and sec-alkyl perfluoroacyl nitroxides and sec- and tert-alkyl perfluoroalkyl nitroxides

Both EPR studies and product analysis have disclosed that the oxidation of O-benzoyl-N-alkylhydroxylamines (1, 4, 7) by perfluoroacyl peroxides (2) is an electron transfer-initiated process. Depending on the nature of the alkyl groups in the substrates, the caged radical pair CP1 (RN OCOPh, RfCOO) may react through different radical pathways resulting in different nitroxides along with the neutral products. With primary, benzyl and secondary alkyl groups, a 1,2-acyl shift becomes the predominant pathway resulting in acyl nitroxides (3, 5). With tert-alkyl groups such as tert-butyl, tert-alkyl perfluoroalkyl nitroxides (8) are formed via β-scission of both RN*OCOPh and RfCOO*. A general mechanistic pathway is proposed for the titled reactions generating the quite useful fluorinated nitroxides.

Electron paramagnetic resonance studies on the generation and solvation of n-heptafluoropropyl t-butyl nitroxide

Abstract n-Heptafluoropropyl t-butyl nitroxide ( 3 ) has been generated by the use of the electron-transfer reaction of O -benzoyl-N-t-butylhydroxylamine ( 1 ) and heptafluorobutyryl peroxide ( 2 ) in CFCl 2 CF 2 Cl solution. Electron paramagnetic resonance (EPR) measurements of a N and a F β values for 3 in 25 solvents have been carried out at 25±2 °C. The a N values for 3 in 16 aprotic solvents (but not in t-BuOH and acetone) show a linear correlation with the cybotactic solvent parameters E T and Z (Kosower constant) i.e. a N = 3.60 × 10 −2 E T + 10.25 and a N = 1.56 × 10 −2 Z + 10.66. The physical significance for the slope, (slope x E T ) or (slope x Z ), and the extrapolated intercept on the a N axis is linked to the sensitivity of the nitroxide toward solvation, the magnitude of the overall solvation effect on the a N values and the intrinsic a N value of 3 in the ideal gaseous state, respectively. It is believed that the virtually constant value of a N (11.56–11.61 G) for 3 in alcohols and carboxylic acids is caused by H-bond formation between 3 and the solvents.

n-Alkyl perfluoroacyl nitroxides—EPR studies on electron transfer reactions of perfluorodiacyl peroxides with O-benzoyl-n-alkylhydroxylamines

Abstract n-Alkyl perfluoroacyl nitroxides, n-RN(O·)CORF (n-R=n-C4H9, Pr1CH2CH2, PhCH2; RF=CF3, C2F5, n-C3F7, n-C7F15, H(CF2)2, H(CF2)4 and H(CF2)6), have been generated by one-electron oxidation of O-benzoyl-n-alkylhydroxylamines by perfluorodiacyl peroxides. The EPR parameters of 15 useful new fluorinated nitroxides have been determined and the mechanism of their formation discussed.

Perfluorodiacyl peroxides as one-electron oxidants and novel mechanistic probes in the chemistry of aromatic radical cations

Abstract In the reactions of 2,5-di- tert -butyl-1,4-dimethoxybenzene (1) and 2-alkyl-5- tert -butyl-1,4-dimethoxybenzenes (2) with different oxidizing systems, S 2 O 2 8 − Cu 2+ CH 3 CO 2 H, Ce 4+ CH 3 CO 2 H and (φ F CO 2 ) 2 CF 2 CL CFCL 2 , 1 + · and 2 + · are always detectable by EPR. However, the observed reactivity of 1 + · and 2 + · depends greatly on the oxidation systems. In the former two, 1 + · and 2 + · have long lifetimes and do not undergo fragmentation spontaneously; the nucleophile CH 3 CO 2 − in the bulk does not attack 1 + · and 2 + · effectively. In contrast, in (φ F CO 2 ) 2 CF 2 CL CFCL 2 solution, both 1 + · and 2 + · become short-lifed; and large amounts of de- tert -butylation products in the reactions of 1, and de- tert -butylation as well as deprotonation-perfluoro-benzoloxylation products in the reactions of 2, have been isolated. Experimental results imply that the CC bond cleavage involved in de- tert -butylation and CH bond cleavage in deprotonation could be a consequence of an attack by φ F COO · on 1 + · and 2 + · in their original solvent cage. The fact that addition of MeOH to the reaction mixture leads to the formation of t-BuOCH 3 suggests that the tert -butyl group leaves as a carbocation. In the reactions of 2, the deprotonation of 2 + · is further assured by the EPR observation of the corresponding benzyl radicals. The relative importance of deprotonation over de- tert -butylation is greatly dependent on the structure of −CHR 1 R 2 substituents and the solvents. For deprotonation, the reactivity order is found to be cy -PrCH 2 >Me>PhCH 2 >Et⪢ i -Pr. We conclude that the reactions of 1 and 2 with (φ F CO 2 ) 2 are initiated bv electron transfer and followed by fast coupling of 1 + · with φ F CO 2 · in the solvent cage, to form σ-complexes which collapse or react with nucleophile to yield the de- tert - butylation product in the case of 1; and in the cases of 2, fast deprotonation generating benzylic radisals and their fast coupling with φ F COO . in the cage, to form side-chain acyloxylation products. Perfluorodiacyl peroxides are proved to be effective oxidants for generating aromatic radical cations. By taking advantages of the simultaneous generation of the most reactive radical φ F COO · and the most common nucleophile φ F CO − 2 from the extremely fast collapse of their radical anion (φ F CO 2 ) − · 2 in original solvent cage rightafter the electron transfer, we have been able to probe such of the important and complicated mechanistic details of the reactions involving aromatic radical-ions.