Redox-Induced Aromatic C–H Bond Functionalization in Metal Complex Catalysis from the Electrochemical Point of View

Budnikova, Yu. G.; Dudkina, Yulia B.; Khrizanforov, Mikhail

doi:10.3390/inorganics5040070

Cited by 32 publications

(23 citation statements)

References 98 publications

(184 reference statements)

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“…This trend appears to be further extended if one were to include the silver complexes, whose oxidation potentials are expected to be much higher [ E 1/2 (Ag 2+ /Ag + ) ≥ 1.6 V vs. Ag/AgCl] than the copper derivatives. In the silver cases, the highly oxidative nature of any hypervalent iodine reagent { E 1/2 [PhI(OAc) 2 /PhI] 2.2 V vs. Ag/AgCl}, putative silver nitrene, or other unidentified Ag (II) intermediate may lead to oxidative decomposition of styrene [ E 1/2 (styrene + /styrene) 2.0 V vs. Ag/AgCl,], or possibly of ligand, at a rate that becomes competitive with nitrene transfer, thereby reducing yields. Thus, as evident from relative M( x L R ) 2+ /M( x L R ) + redox potentials (Table S4), the superiority of the copper(I) complexes over the silver relatives to effect aziridination of styrene might be traced to the enhanced stability of the metal nitrene intermediate brought about by copper's superior ability to back donate electrons into the un‐ or partly‐filled orbitals on the unsaturated nitrogenous fragment (giving a bond with some metal‐iminyl character).…”

Section: Resultssupporting

confidence: 80%

“…For instance, the catalytic activity trends with the electron richness of the copper complexes supported by normal vs. C ‐scorpionates {e.g., E 1/2 (Cu 2+ /Cu + ) = 0.07 V, 0.43 V, 0.63 and 1.0 V vs. Ag/AgCl in CH 3 CN for CuTp iPr2 (CH 3 CN), [Cu(Tpm iPr2 )(CH 3 CN)] + , [Cu( Ts L iPr2 )(CH 3 CN)] + , and [Cu(CH 3 CN) 4 ] + , respectively}. This trend appears to be further extended if one were to include the silver complexes, whose oxidation potentials are expected to be much higher [ E 1/2 (Ag 2+ /Ag + ) ≥ 1.6 V vs. Ag/AgCl] than the copper derivatives. In the silver cases, the highly oxidative nature of any hypervalent iodine reagent { E 1/2 [PhI(OAc) 2 /PhI] 2.2 V vs. Ag/AgCl}, putative silver nitrene, or other unidentified Ag (II) intermediate may lead to oxidative decomposition of styrene [ E 1/2 (styrene + /styrene) 2.0 V vs. Ag/AgCl,], or possibly of ligand, at a rate that becomes competitive with nitrene transfer, thereby reducing yields.…”

Section: Resultsmentioning

confidence: 99%

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Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

et al. 2020

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Two new sterically demanding nitrogen-confused C-scorpionate ligands with a bis(3,5-diisopropylpyrazol-1yl)methyl group bound to the 3-position of a normal pyrazole ( H L iPr2 ) or an N-toluenesulfonyl pyrazole ( Ts L iPr2 ) have been prepared. Reactions between the ligands ( x L iPr2 ) and silver trifluoromethanesulfonate, AgOTf, gave four new compounds of the types [Ag(Ts, 1b; x = H, 2b) depending on the initial metal:ligand ratio. Similarly, the reactions with [Cu(CH 3 CN) 4 ](PF 6 ) produce [a] Dr.to trap and spectroscopically characterize the nitrene intermediates {[(2-py)Bpm R3R5 ]Cu(NTs)} + in CH 2 Cl 2 at -78°C using (2-tBuSO 2 )C 6 H 4 I=NTs [34] as a soluble nitrene source. The spectroscopic and magnetic data were consistent with diamagnetic, singlet species at -78°C. Calculations showed ground state triplet but all spin states [ 1/3 Cu I -nitrene or 1/3 Cu II -N·(iminyl)] and nitrene binding modes (κ 1 N-, κ 2 N,O-) are thermally accessible. These intermediates were capable of stoichiometrically transferring a nitrene unit to a variety of substrates. Then, different {[(2-py)Bpm R3R5 ]Cu(CH 3 CN)}(PF 6 ) complexes were found to be capable catalysts for nitrene transfer to unsubstituted or p-substituted styrenes giving modest yields (> 65 %) of aziridine under mild ocnditions (PhI=NTs/CH 2 Cl 2 /295 K, 24 h).In a previous study, [27] eight complexes of the type [Ag( x L R ) n ](OTf ) (n = 1, 2 and x L R = H L, Ts L, H L*, and Ts L*) were characterized and evaluated for their ability to catalyze aziridination of styrene. Both ESI(+) MS and NMR spectroscopic studies indicated that the analytically pure [Ag( x L R ) n ](OTf ) compounds did not retain their structure in CH 3 CN rather were involved in multiple dynamic equilibria in solution. These dynamic processes remained in the fast exchange regime on the NMR timescale down to the freezing point of the solvent, thereby obfuscating structural information. These silver complexes of nitrogen-confused C-scorpionates showed no or very little capacity to participate in styrene aziridination using PhI= NTs in room temperature CH 2 Cl 2 . However, [Ag( x L R ) n ](OTf ) showed modest catalytic activity at 80°C in CH 3 CN when employing a nitrene generated in-situ from H 2 NTs and PhI(OAc). Interestingly, the bulkiest derivative [Ag( Ts L * ) 2 ](OTf ) was reported to have the highest activity giving 34 % yield of the desired N-tosyl aziridine. This observation prompted the current study to determine if further increasing steric bulk of the ligands would lead to an increase in catalytic activity of silver complexes. Herein we report on the preparation of two new semi-bulky nitrogen confused scorpionates, Ts L iPr2 and H L iPr2 and their silver(I) and copper(I) complexes. The copper(I) complexes were prepared to compare catalytic activity with their silver congeners, and possibly to demonstrate the generality of any trends in ligand sterics on catalytic activity. It was also hoped that the slower ligand exchange rates associated with the smaller copper(I) compared ...

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Section: Resultssupporting

confidence: 80%

Section: Resultsmentioning

confidence: 99%

Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

et al. 2020

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“…Other oxidation agents (lead tetraacetate, phenyliodine(III) diacetate (PIDA) with E° = +1.70 V vs Fc/Fc + in ACN, 50 and silver(I) oxide) for N -alkyl(aryl)-aminocarbonyl-4-aminophenols, 51 were also used in an attempt to obtain the desired oxidation products with a 1,4-benzoquinone imine moiety (see also Scheme S3 , its accompanying explanation, and Figure S2 in the Supporting Information ). The exposure of HL 2 to 1 equiv of PIDA furnished the two-electron oxidized product H L 2c′ and traces of the four-electron oxidized species H L 2c″ .…”

Section: Resultsmentioning

confidence: 99%

Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition

et al. 2021

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Three new thiosemicarbazones (TSCs) HL 1 – HL 3 as triapine analogues bearing a redox-active phenolic moiety at the terminal nitrogen atom were prepared. Reactions of HL 1 – HL 3 with CuCl 2 ·2H 2 O in anoxic methanol afforded three copper(II) complexes, namely, Cu(HL 1 )Cl 2 ( 1 ), [ Cu(L 2 )Cl] ( 2′ ), and Cu(HL 3 )Cl 2 ( 3 ), in good yields. Solution speciation studies revealed that the metal-free ligands are stable as HL 1 – HL 3 at pH 7.4, while being air-sensitive in the basic pH range. In dimethyl sulfoxide they exist as a mixture of E and Z isomers. A mechanism of the E/Z isomerization with an inversion at the nitrogen atom of the Schiff base imine bond is proposed. The monocationic complexes [Cu(L 1 – 3 )] + are the most abundant species in aqueous solutions at pH 7.4. Electrochemical and spectroelectrochemical studies of 1 , 2′ , and 3 confirmed their redox activity in both the cathodic and the anodic region of potentials. The one-electron reduction was identified as metal-centered by electron paramagnetic resonance spectroelectrochemistry. An electrochemical oxidation pointed out the ligand-centered oxidation, while chemical oxidations of HL 1 and HL 2 as well as 1 and 2′ afforded several two-electron and four-electron oxidation products, which were isolated and comprehensively characterized. Complexes 1 and 2′ showed an antiproliferative activity in Colo205 and Colo320 cancer cell lines with half-maximal inhibitory concentration values in the low micromolar concentration range, while 3 with the most closely related ligand to triapine displayed the best selectivity for cancer cells versus normal fibroblast cells (MRC-5). HL 1 and 1 in the presence of 1,4-dithiothreitol are as potent inhibitors of mR2 ribonucleotide reductase as triapine.

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“…Catalysts based on transition metal derivatives capable of taking on different oxidation states (for example, M(0)/M(I)/M(II)/M(III)/M(IV), etc.) are of particular interest [3–17] . By varying the electrode potential, it is possible to control the strength of the oxidizing agent (or reducing agent), passing from one common oxidation state of the catalyst precursor to its active form with a higher (lower) oxidation state, as well as to regulate the potential of electrochemically‐induced transformation of key redox‐active intermediates and the selectivity of the entire reactions.…”

Section: Introductionmentioning

confidence: 99%

Electrochemical Insight into Mechanisms and Metallocyclic Intermediates of C−H Functionalization

Budnikova

2021

The Chemical Record

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Transition metal‐catalyzed C−H activation has emerged as a powerful tool in organic synthesis and electrosynthesis as well as in the development of new methodologies for producing fine chemicals. In order to achieve efficient and selective C−H functionalization, different strategies have been used to accelerate the C−H activation step, including the incorporation of directing groups in the substrate that facilitate coordination to the catalyst. In this review, we try to underscore that the understanding the mechanisms of the catalytic cycle and the reactivity or redox activity of the key metal cyclic intermediates in these reactions is the basis for controlling the selectivity of synthesis and electrosynthesis. Combination of the electrosynthesis and voltammetry with traditional synthetic and physico‐chemical methods allows one to achieve selective transformation of C−H bonds to functionalized C−C or C−X (X=heteroatom or halogen) bonds which may encourage organic chemists to use it in the future more often. The possibilities and the benefits of electrochemical techniques are analyzed and summarized.

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Redox-Induced Aromatic C–H Bond Functionalization in Metal Complex Catalysis from the Electrochemical Point of View

Cited by 32 publications

References 98 publications

Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition

Electrochemical Insight into Mechanisms and Metallocyclic Intermediates of C−H Functionalization

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