Reactions of aliphatic free radicals with copper cations in aqueous solutions. Part 3.—Reactions with cuprous ions: a pulse radiolysis study

Abstract

Helium-saturated solutions containing copper sulphate and an aliphatic alcohol (methanol, ethanol or propan-2-01) were pulse irradiated. The kinetics of the reactions thus initiated were followed by the spectrophotometric and conductometric techniques. The results indicate that all three radicals, -CR1R20H, react with Cuii to yield Cu&+ CRlR20+ H,O+. =CH20H radicals react with Cu&, according to CuAq+4M20H CuIl-CH,OH+, K = 5 x 10 dm" rnol-'.? The corresponding equilibrium constants for CulI-CH(CH3)OH+ and CUI~-C(CH~)~OH+ are too low to be measured and the corresponding intermediates are not observed. Cuk, reacts with the p radicals, =CH2CH20H and -CH2CH(CH3jOH, according to Cu&The latter intermediates decompose via acid catalysed reactions to yield C u j i + H2C=CR1R2 + H20.Copper salts were shown to affect a variety of reactions involving aliphatic free radicals including many catalytic processes in aprotic and protic media.Ib The results indicate that aliphatic free radicals react with cuprous, as well as with cupric ions via a mechanism involving the formation of " short-lived " intermediates having a copper-carbon bond(2) Recently while studying the kinetics of formation and decomposition of several CUI~~--R intermediates in aqueous solutions, we found that when Cu,', ions are t In writing CuII--R + or CuIII---R*+ we follow the accepted nomenclature for organometallic compounds. This notation does not imply that the nature of the chemical bond is ionic. Furthermore, the results, see discussion, indicate that the Cu-C bond has at least partially a covalent nature.* Clearly one cannot prepare solutions which contain Cug ions but no Cu&+ ions.

Reductive Termination of Cyanoisopropyl Radicals by Copper(I) Complexes and Proton Donors: Organometallic Intermediates or Coupled Proton–Electron Transfer?

“…A possible contribution of IS-PCET to RRT, however, cannot be excluded for proton donors able to coordinate to the metal center. 31 Overall, while this study is consistent with the proposition, made in a previous contribution, 28 that the saturated PMA-H made by reductive termination of PMA • chains in the presence of methanol in toluene solution may originate from the protonolysis of a PMA-Cu II Br/Me6TREN intermediate, the contribution (or dominance) of alternative PCET pathways cannot yet be totally excluded. In addition, we have shown in our study that, at least for the termination of the cyanoisopopyl radical, the C6D6 solvent does not contain sufficient amounts of water impurities to yield a significant impact of RRT, even for the most active Cu I Br/Me6TREN system.…”

“…The involvement of metal complexes to mediate PCET is well established 30 and an IS-PCET process was found kinetically competent in a recent mechanistic study of Fe(acac)3-catalyzed radical alkene cross-coupling. 31 Scheme 6. Possible pathways for reductive radical termination.…”

Photoreduction of IrCl62? complex in alcohol solutions and its reaction with hydroxyalkyl radicals

“…6 room temperature are shown in Table I. For deoxygenated solutions, after a laser pulse, we observe not only the instant bleaching in ab-…”

The fate of copper catalysts in atom transfer radical chemistry

“…The rate constants for radical addition to copper(I) (kd,OMRP) are similar for both initiators (10 6 M -1 s -1 ) and correlate well with the earlier work of Meyerstein et al who reported rate constants using pulse radiolysis for a series of reactions of alkyl and allyl radicals binding to copper(I) polyamine complexes as 10 6 -10 8 M -1 s -1 . [59][60][61] By contrast, the rate constant for the reverse reaction (namely, homolysis of the Cu II -C bond -ka,OMRP) is three orders of magnitude larger for [Cu II (Me6tren)(CHMeCOOEt)] + than the corresponding [Cu II (Me6tren)(CH2COOEt)] + complex. This indicates that the additional methyl group on the propionate ligand plays a key role in accelerating dissociation of the Cu II -C bond.…”