A paradigm shift in rate determining step from single electron transfer between phenylsulfinylacetic acids and iron(III) polypyridyl complexes to nucleophilic attack of water to the produced sulfoxide radical cation: a non-linear Hammett

2016 ◽  
Vol 29 (10) ◽  
pp. 496-504 ◽  
Author(s):  
Perumal Subramaniam ◽  
Jebamoney Janet Sylvia Jaba Rose ◽  
Rajasingh Jeevi Esther Rathinakumari
Keyword(s):  
Electron Transfer ◽  
Radical Cation ◽  
Paradigm Shift ◽  
Single Electron ◽  
Nucleophilic Attack ◽  
Single Electron Transfer ◽  
Polypyridyl Complexes ◽  
Rate Determining Step ◽  
Non Linear

The oxidation of sulfides by chromium(V)

2003 ◽  
Vol 81 (1) ◽  
pp. 75-80 ◽  
Author(s):  
Carmela R Jackson Lepage ◽  
Lynn Mihichuk ◽  
Donald G Lee
Keyword(s):  
Electron Transfer ◽  
Molecular Orbital ◽  
Radical Cation ◽  
Oxygen Transfer ◽  
Single Electron ◽  
Frontier Molecular Orbital ◽  
Molecular Orbital Calculations ◽  
Single Electron Transfer ◽  
Oxidation Of Sulfides

The mechanism for the oxidation of sulfides by [(me4-salen)CrV(O)(pyO)]CF3SO3, where me4-salen is 8,8,8',8'-tetramethylsalen and pyO is pyridine N-oxide, has been investigated. Results from Hammett correlations on the rates of oxidation of substituted thioanisoles, frontier molecular orbital calculations, and product studies are consistent with a mechanism that is initiated by a single electron transfer to give a radical cation intermediate.Key words: oxidation, chromium(V), sulfides, radical cation, oxygen transfer.

Electronically Asynchronous Transition States for C-N Bond Formation by Electrophilic [Coᴵᴵᴵ(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle

2019 ◽  
Author(s):  
Nicolaas P. van Leest ◽  
Martijn A. Tepaske ◽  
Jarl Ivar van der Vlugt ◽  
Bas de Bruin
Keyword(s):  
Electron Transfer ◽  
Bond Formation ◽  
Macrocyclic Ligand ◽  
Transition States ◽  
Lone Pair ◽  
Single Electron ◽  
Nucleophilic Attack ◽  
Single Electron Transfer ◽  
Radical Species ◽  
Nitrene Transfer

The oxidation state of the redox non-innocent TAML (Tetra-Amido Macrocyclic Ligand) scaffold was recently shown to affect the formation of nitrene radical species on cobalt(III) upon reaction with PhI=NNs [J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b11715]. For the neutral [Co<sup>III</sup>(TAMLsq)] complex this leads to the doublet (S = ½) mono-nitrene radical species [Co<sup>III</sup>(TAMLq)(N<sup>•</sup>Ns)], while a triplet (S = 1) bis-nitrene radical species [Co<sup>III</sup>(TAML<sup>q</sup>)(N<sup>•</sup>Ns)<sub>2</sub>]<sup>‒</sup> is generated from the anionic [Co<sup>III</sup>(TAML<sup>red</sup>)]<sup>‒</sup> complex. The one-electron reduced Fischer-type nitrene radicals (N<sup>•</sup>Ns<sup>‒</sup>) are formed through single (mono-nitrene) or double (bis-nitrene) ligand-to-substrate single-electron transfer (SET). In this work we describe the reactivity and mechanisms of these nitrene radical complexes in catalytic aziridination. We report that [Co<sup>III</sup>(TAML<sup>sq</sup>)] and [Co<sup>III</sup>(TAML<sup>red</sup>)]<sup>‒</sup> are both effective catalysts for chemoselective (C=C versus C‒H bonds) and diastereoselective aziridination of styrene derivatives, cyclohexene and 1-hexene under mild and even aerobic (for [Co<sup>III</sup>(TAML<sup>red</sup>)]<sup>‒</sup>) conditions. Experimental (Hammett plots, radical inhibition, catalyst decomposition tests) and computational (DFT, CASSCF) studies reveal that [Co<sup>III</sup>(TAML<sup>q</sup>)(N<sup>•</sup>Ns)], [Co<sup>III</sup>(TAML<sup>q</sup>)(N<sup>•</sup>Ns)<sub>2</sub>]<sup>‒</sup> and [Co<sup>III</sup>(TAML<sup>sq</sup>)(N<sup>•</sup>Ns)]<sup>–</sup> are key electrophilic intermediates in the aziridination reactions. Surprisingly, the electrophilic one-electron reduced Fischer-type nitrene radicals do not react as would be expected for nitrene radicals (i.e. via radical addition and radical rebound). Instead, nitrene transfer proceeds through unusual electronically asynchronous transition states, in which (partial) styrene substrate to TAML ligand (single) electron transfer precedes C-N coupling. The actual C-N bond formation processes are best described as involving a nucleophilic attack of the nitrene (radical) lone pair at the thus (partially) formed styrene radical cation. These processes are coupled to TAML-to-cobalt and cobalt-to-nitrene single-electron transfer, effectively leading to formation of an amido-[gamma]-benzyl radical (Ns–N–CH<sub>2</sub>–<sup>•</sup>CH–Ph) bound to an intermediate spin (S = 1) cobalt(III) center. Hence, the TAML moiety can be regarded to act as a transient electron acceptor, the cobalt center behaves as a spin shuttle and the nitrene radical acts as a nucleophile. Such a mechanism for (cobalt catalyzed) nitrene transfer was hitherto unknown and complements the known concerted and stepwise mechanisms for N-group transfer.

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