EPR study of the long-range effect of a β-carbon chiral center on the magnetic equivalence of β-methylene protons

Abstract

PETER S M~T Hand W~L L~A M HARLEY DONOVAN. Can. J. Chem. 66, 2304 (1988.Using aqueous TiC1,-based continuous-flow, radical-generating methods at 25°C we have carried out a comprehensive EPR study o f . hydroxyl-and aminyl-radical addition to eight alkene substrates that yielded radicals of general formula XCH~-CY-C*HR'R~, where C* is a chiral center, to investigate the magnetic inequivalence of p-CH2 protons arising from a nonadjacent p-C chiral center. We attribute the observed nonequivalence to the long-range influence of the chiral center in these radicals, and note that our results are consistent with the reasonable expectation that the presence of bulky groups in the radicals is likely to favor particular conformations, leading to more easily observed inequivalence of the p-CH2 proton splittings. Operant a25"C et utilisant des mCthodes de production de radicaux basCes sur des Ccoulements continus de solutions aqueuses de TiCl,, on a rCalisC une ttude rpe dCtaillCe d'additions des radicaux hydroxyle et aminyle sur huit alcenes conduisant des radicaux de formule gCnCrale X C H~--C Y -C * H R ' R '~~~S lesquels C* est un centre chiral; ces expkriences ont CtC rCalisCes afin d'ivaluer si les protons CH2 en p , qui sont sournis a l'influence d'un centre chiral qui n'est pas adjacent, sont equivalents. On a observt qu'ils ne sont pas Cquivalents et on attribue ce resultat a l'influence longue distance du centre chiral present dans ces radicaux; de plus, on note que nos rCsultats sont en accord avec les attentes qu'on peut raisonnablement avoir si on adrnet que la prCsence de groupements volumineux dans les radicaux devrait favoriser des conformations particulikres et ainsi conduire a une inequivalence plus facilernent observable des couplages des protons CH2 en p.[Traduit par la revue] IntroductionRecently (I), we presented the results of an EPR study of three XCH~-CH-CH(OH)CH~X radicals, with -X = -CN, -COOH, and -OH, formed from symmetrical 1,2-disubstituted alkenes in aqueous solution at ca. 25°C with use of the TiC13-H202, continuous-flow, radical-generating method (2). Two of these radicals showed magnetic inequivalence of their p-CH2 protons due to the presence of a nonadjacent P-chiral carbon, an EPR phenomenon that has received little attention (3-7) and, in the case of acyclic C-centered radicals, no systematic study (5-7). Similar long-range effects of molecular asymmetry are numerous in the NMR literature (8-12), and the adjacent chiral effect, namely, magnetic inequivalence of p-CH2 protons adjacent to a y-C chiral center or of y-CH2 protons adjacent to a p-C chiral center, is common in EPR spectroscopy (1 3-23).Previously (I), we stressed the importance to spectral interpretation of recognizing the possibility of there-being inequivalent p-CH2 protons in radicals with a nonadjacent chiral center. The present investigation successfully extends our previous study ofthe nonadjacent chiral effect; specifically, to determine how often inequivalence of p-CH2 protons can occur in radicals of general structu...

Electron paramagnetic resonance study of radicals derived from 2- and 3-fluoropropene

“…The experimental arrangements and procedures were essentially as before (14) except that all measurements were taken using a Varian E-231 multipurpose single cavity. The field and g-value standard was potassium peroxylamine disulfonate (Alpha, 95%) (15,16) in an aqueous solution saturated at room temperature with potassium carbonate (Fisher, anhydrous, ACS reagent), the temper-'~u t h o r to whom correspondence may be addressed.…”

“…All radicals were observed using either a two-or a three-stream, rapid-mixing, continuous-flow epr system (14,20,21). The epr sample flow cells employed were of the aqueous, mix-and-flow type and were either of the Borg design (22), as before (14), or as supplied by Wilmad Glass Company, Inc. (Cat.…”

“…The epr sample flow cells employed were of the aqueous, mix-and-flow type and were either of the Borg design (22), as before (14), or as supplied by Wilmad Glass Company, Inc. (Cat. no.…”

“…I. stream 0.10 M in H202, and both streams 0.10 M in HISO, and of equal concentration, within the range 0.015-0.020 M, in 1 or 2, unless stated otherwise. When using the TiC1,-NH20H method (14), the reaction conditions were as for the TiC1,-H,0, work except that the H,O, in the oxidizing stream was replaced with 1.0 M NH,OH*HCl. When employing the TiC1,-Na,S20x system, the reaction conditions employed first were as for the TiC1,-H?02 work, i.e., with the reaction mixture at pH ca.…”