Secondary Deuterium Kinetic Isotope Effects in Irreversible Additions of Hydride and Carbon Nucleophiles to Aldehydes:  A Spectrum of Transition States from Complete Bond Formation to Single Electron Transfer

1999 ◽  
Vol 121 (2) ◽  
pp. 326-334 ◽  
Author(s):  
Joseph J. Gajewski ◽  
Wojciech Bocian ◽  
Nathan J. Harris ◽  
Leif P. Olson ◽  
John P. Gajewski
Keyword(s):  
Electron Transfer ◽  
Isotope Effects ◽  
Bond Formation ◽  
Transition States ◽  
Kinetic Isotope Effects ◽  
Single Electron ◽  
Single Electron Transfer ◽  
Carbon Nucleophiles ◽  
Kinetic Isotope

Kinetics and mechanism of oxidation of N,N′-dimethyl-9,9′-biacridanyl by some π acceptors and a one-electron oxidant

1985 ◽  
Vol 63 (2) ◽  
pp. 445-451 ◽  
Author(s):  
Allan K. Colter ◽  
Charles C. Lai ◽  
A. Gregg Parsons ◽  
N. Bruce Ramsey ◽  
Gunzi Saito
Keyword(s):  
Electron Transfer ◽  
Reduction Potential ◽  
Isotope Effects ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Spin Trapping ◽  
Single Electron Transfer ◽  
Kinetic Isotope ◽  
The One ◽  
Rule Out

Oxidation of N,N′-dimethyl-9,9′-biacridanyl (DD) has been investigated as a model for single electron transfer (SET)-initiated oxidation of NADH coenzyme models such as N-methylacridan (DH). Oxidants investigated cover a 1010-fold range of reactivity in acetonitrile and include the π acceptors 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCIBQ), p-chloranil (CA), 2,3-dicyanobenzoquinone (DCBQ), 2,3-dicyano-1,4-naphthoquinone (DCNQ), 2,3-dicyano-5-nitro-1,4-naphthoquinone (DCNNQ), 9-dicyanomethylene-2,4,7-trinitrofluorene (DCMTNF), 9-dicyanomethylene-2,4,5,7-tetranitrofluorene (DCMTENF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE), and the one-electron oxidant tris(2,2′-bipyridyl)cobalt(III), [Formula: see text] The oxidation product is, in every case, N-methylacridinium ion (D+). A mechanism involving a rate-determining electron transfer with simultaneous fragmentation to D+ and N-methyl-9-acridanyl radical (D•) is proposed. This mechanism is supported by the observed dependence of the rate on oxidant reduction potential, by spin-trapping experiments, by kinetic isotope effects in oxidation of 9,9′-dideuterio-DD, and by substituent effects in oxidation of 2,2′- and 3,3′-dimethoxy-DD. The rate of oxidation of DD relative to that of DH is 3.4 × 102 with [Formula: see text] and with the π acceptors varies from ea. 0.3 (BQ) to 8.1 × 104 (DCMTENF). The results rule out a SET-initiated mechanism for oxidation of DH by all of the oxidants studied except TCNQ and DCMTENF.

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.

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.

Competitive oxygen-18 kinetic isotope effects expose O–O bond formation in water oxidation catalysis by monomeric and dimeric ruthenium complexes

2014 ◽  
Vol 5 (3) ◽  
pp. 1141-1152 ◽  
Author(s):  
Alfredo M. Angeles-Boza ◽  
Mehmed Z. Ertem ◽  
Rupam Sarma ◽  
Christian H. Ibañez ◽  
Somnath Maji ◽  
...  
Keyword(s):  
Water Oxidation ◽  
Isotope Effects ◽  
Bond Formation ◽  
Transition States ◽  
Ruthenium Complexes ◽  
Kinetic Isotope Effects ◽  
Oxidation Catalysis ◽  
Kinetic Isotope ◽  
Water Oxidation Catalysis

Competitive 18O KIEs on water oxidation catalysis provide a probe of transition states for O–O bond formation.

Reactive mode composition factor analysis of transition states: the case of coupled electron–proton transfers

2019 ◽  
Vol 21 (45) ◽  
pp. 24912-24918 ◽  
Author(s):  
Mauricio Maldonado-Domínguez ◽  
Daniel Bím ◽  
Radek Fučík ◽  
Roman Čurík ◽  
Martin Srnec
Keyword(s):  
Factor Analysis ◽  
Kinetic Energy ◽  
Isotope Effects ◽  
Transition States ◽  
Kinetic Isotope Effects ◽  
Composition Factor ◽  
Kinetic Isotope ◽  
Proton Transfers ◽  
Relative Kinetic ◽  
Reactive Mode

The kinetic energy distribution in the reactive mode in transition states correlates the asynchronicity of CPET with relative kinetic isotope effects.

scholarly journals Alcohol-Initiated Dediazoniation of Aryldiazonium Ions to Aryl Radicals: A Simple and Efficient Route to Arylboronates

2018 ◽  
Vol 42 (9) ◽  
pp. 481-485
Author(s):  
Xiulian Zhang ◽  
Zhicheng Zhang ◽  
Yongbin Xie ◽  
Yujie Jiang ◽  
Ruibo Xu ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Bond Formation ◽  
Single Electron ◽  
Single Electron Transfer ◽  
Aryl Radical ◽  
Radical Species ◽  
Aryl Radicals ◽  
Efficient Access ◽  
Efficient Route ◽  
Operational Simplicity

A simple and efficient access to arylboronates was achieved with methanol-initiated borylation of aryldiazonium salts. Reduction of aryldiazonium ions by single electron transfer from methanol affords aryl radical species, which undergo a subsequent C–B bond formation with bis(pinacolato)diboron. This highly practical borylation process, which can be carried out on the gram-scale, enjoys operational simplicity as well as mild and catalyst-free conditions.

Mechanism of the benzidine rearrangement. Kinetic isotope effects and transition states. Evidence for concerted rearrangement

1981 ◽  
Vol 103 (4) ◽  
pp. 955-956 ◽  
Author(s):  
Henry J. Shine ◽  
Henryk Zmuda ◽  
Koon Ha Park ◽  
Harold Kwart ◽  
Ann Gaffney Horgan ◽  
...  
Keyword(s):  
Isotope Effects ◽  
Transition States ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope
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