Heterometallic CoIIIZnII Schiff Base Catalyst for Mild Hydroxylation of C(sp3)–H Bonds of Unactivated Alkanes: Evidence for Dual Mechanism Controlled by the Promoter

Oksana Nesterova,Olga Vassilyeva,Dmytro Nesterov,Armando Pombeiro,Katerina Kasyanova,Brian Skelton,Elena Buvaylo

doi:10.3390/catal9030209

Abstract

The novel Schiff base complex [CoIIIZnIIL3Cl2]·CH3OH (1) was synthesized by interaction of zinc powder, cobalt(II) chloride and methanol solution of the pre-formed HL in air (HL is the product of condensation of o-vanillin and methylamine) and characterized by IR, UV-Vis and NMR spectroscopy, ESI-MS and single crystal X-ray diffraction analysis. In the heterometallic core of 1 the two metal centers are bridged by deprotonated phenoxy groups of the L− ligands with the cobalt-zinc separation of 3.123 Å. Catalytic investigations demonstrated a pronounced activity of 1 towards mild alkane oxidation with m-chloroperbenzoic acid (m-CPBA) as an oxidant and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) as the model substrate. The influence of the nature of different promoting agents of various acidities (from HOTf to pyridine) on the catalytic process was studied in detail and a pronounced activity of 1 in the presence of nitric acid promoter was found, also showing a high retention of stereoconfiguration of the substrate (>99% for cis-1,2-DMCH). The best achieved yield of tertiary cis-alcohol based on the oxidant was 61%, with a turnover number (TON) of 198 for nitric acid as promoter. The 18O-incorporations into the alcohols when the reactions were performed under 18O2 atmosphere using acetic and nitric acid promoters, suggest that the cis-1,2-DMCH hydroxylation proceeds by two distinct pathways, a non-stereoselective and a stereoselective one (with and without involvement of a long-lived free carbon radical, respectively). The former dominates in the case of acetic acid promoter and the latter is realized in the case of HNO3 promoter.

Highlights

Selective transformation of inactive aliphatic C−H bonds into suitable functional groups is a principal goal of modern chemistry [1,2,3,4,5,6,7,8]
The Schiff base HL was synthesized in situ by condensation of o-vanillin and CH3 NH2 ·HCl in HL was in situ by condensation o-vanillin
For an for an acid-free system oxidizing with m-chloroperbenzoic acid (m-CPBA) we have shown that cis-1,4-DMCH affords almost twice lower yields than its 1,2-isomer [26]

Summary

Introduction

Selective transformation of inactive aliphatic C−H bonds into suitable functional groups is a principal goal of modern chemistry [1,2,3,4,5,6,7,8]. It is known metalloenzymes, such as cytochrome P450 or methane monooxygenase (MMO), can functionalize C–H bonds of a range of substrates, including alkanes, under very mild conditions (MMO), can functionalize. C–H bonds of a range of substrates, including alkanes, inspired under very mild and using dioxygen or peroxides as terminal oxidants [11,12,13,14]. These observations chemists conditions and using dioxygen or peroxides as terminal oxidants [11,12,13,14]. Nowadays these families are iron represented mainly by which could mimic the reaction mechanisms of enzymatic oxidations [15]

Methods

Results

Conclusion