Non-redox metal ions can promote Wacker-type oxidations even better than copper(ii): a new opportunity in catalyst design

Abstract

In Wacker oxidation and inspired Pd(ii)/Cu(ii)-catalyzed C-H activations, copper(ii) is believed to serve in re-oxidizing of Pd(0) in the catalytic cycle. Herein we report that non-redox metal ions like Sc(iii) can promote Wacker-type oxidations even better than Cu(ii); both Sc(iii) and Cu(ii) can greatly promote Pd(ii)-catalyzed olefin isomerization in which the redox properties of Cu(ii) are not essential, indicating that the Lewis acid properties of Cu(ii) can play a significant role in Pd(ii)-catalyzed C-H activations in addition to its redox properties. Characterization of catalysts using UV-Vis and NMR indicated that adding Sc(OTf)3 to the acetonitrile solution of Pd(OAc)2 generates a new Pd(ii)/Sc(iii) bimetallic complex having a diacetate bridge which serves as the key active species for Wacker-type oxidation and olefin isomerization. Linkage of trivalent Sc(iii) to the Pd(ii) species makes it more electron-deficient, thus facilitating the coordination of olefin to the Pd(ii) cation. Due to the improved electron transfer from olefin to the Pd(ii) cation, it benefits the nucleophilic attack of water on the olefinic double bond, leading to efficient olefin oxidation. The presence of excess Sc(iii) prevents the palladium(0) black formation, which has been rationalized by the formation of the Sc(iii)H-Pd(ii) intermediate. This intermediate inhibits the reductive elimination of the H-Pd(ii) bond, and facilitates the oxygen insertion to form the HOO-Pd(ii) intermediate, and thus avoids the formation of the inactive palladium(0) black. The Lewis acid promoted Wacker-type oxidation and olefin isomerization demonstrated here may open up a new opportunity in catalyst design for versatile C-H activations.

Transformation of Methyl Linoleate to its Conjugated Derivatives with Simple Pd(OAc)2/Lewis Acid Catalyst

“…However, when the ratio continuously increased from 1 to 2, the efficiency improvement is relatively minor. This phenomenon is similar to those in allylbenzene isomerization, but distinctly different from that in Wacker-type oxidations catalyzed by Pd(OAc) 2 /Lewis acid [33][34][35]. In Wacker-type oxidation, the Sc(III)/Pd(II) ratio of 4 is much more efficient than the ratio of 1.…”

“…In previous studies, it has been observed by UV-Vis spectra that adding non-redox metal salts like Sc(OTf) 3 or Al(OTf) 3 to Pd(OAc) 2 in acetonitrile would generate the new Pd(II) species, and it was further evidenced by NMR studies [33][34][35]. Taken together with the facts that heterobimetallic Pd(II) dimers having an acetate bridge with bivalent or trivalent metal ions, including Ba 2+ , Sr 2+ , Ca 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cd 2+ , Zn 2+ Ce 3+ , Nd 3+ , Eu 3+ and Ce 4+ , have been widely reported [45][46][47][48], we proposed the formation of the heterobimetallic Pd(II)/Sc(III) or Pd(II)/Al(III) dimer having an acetate bridge as the key species for Wacker-type oxidation, olefin isomerization and oxidative coupling reactions [33][34][35]. In the present studies, the UV-Vis kinetics of Pd(OAc) 2 /Sc(OTf) 3 in toluene/t-BuOH solvent also indicated the formation of the new Pd(II) species.…”

“…Electronic supplementary material The online version of this article (doi:10.1007/s11746-017-3052-5) contains supplementary material, which is available to authorized users. roles in olefin oxidations in addition to its redox properties [33]. Inspired by these findings, we explored non-redox metal ions as Lewis acid-promoted olefin isomerization and its oxidative coupling with indoles by a Pd(OAc) 2 catalyst.…”

A General Strategy for Open‐Flask Alkene Isomerization by Ruthenium Hydride Complexes with Non‐Redox Metal Salts

“…This phenomenon shed light on the mechanism because the manipulation of the catalytic properties always originated from the interaction of non‐redox metals with transition‐metal complexes through metal−oxo−metal or metal−H−metal bonds. This interaction has been observed in our previous studies– and in the work of others . Thus, the presence of Ru−H in the RuH 2 (CO)(PPh 3 ) 3 /Mg 2+ system was essential to promote the isomerization from which it was speculated that the regulation of the non‐redox metals on the Ru center may originate from metal−H−metal bonds.…”

“…The role of Mg 2+ was mainly: 1) to twist the first coordination sphere of the Ru center. This twisting, together with steric hindrance from OTf − anions, induced the dissociation of the PPh 3 ligand and the formation of an unoccupied coordination site for alkene and 2) to interact with the Ru center through hydride, because of which the strain associated with the Ru−H bond could be relieved by invoking a Mg‐bridged species that benefitted the isomerization of the alkene . This acceleration was similar to the case of the redox isomerization of propargyl alcohols, in which the interaction of In 3+ with Ru species through Ru−Cl−In 3+ also benefitted the isomerization process .…”