Acceptor‐Substituted Ferrocenium Salts as Strong, Single‐Electron Oxidants: Synthesis, Electrochemistry, Theoretical Investigations, and Initial Synthetic Application

Abstract

A series of mono- and 1,1'-diheteroatom-substituted ferrocene derivatives as well as acylated ferrocenes was prepared efficiently by a unified strategy that consists of selective mono- and 1,1'-dilithiation reactions and subsequent coupling with carbon, phosphorus, sulfur and halogen electrophiles. Chemical oxidation of the ferrocene derivatives by benzoquinone, 2,3-dichloro-5,6-dicyanobenzoquinone, AgPF(6), or 2,2,6,6-tetramethyl-1-oxopiperidinium hexafluorophosphate provided the corresponding ferrocenium salts. The redox potentials of the synthesized ferrocenes were determined by cyclic voltammetry, and it was observed that all new ferrocenium salts have stronger oxidizing properties than standard ferrocenium hexafluorophosphate. An initial application of selected derivatives in an oxidative bicyclization revealed that they mediate the transformation under considerably milder conditions than ferrocenium hexafluorophosphate. Quantum chemical calculations of the reduction potentials of the substituted ferrocenium ions were carried out by using a standard thermodynamic cycle that involved the gas-phase energetics and solvation energies of the contributing species. A remarkable agreement between theory and experiment was found: the mean average deviation amounted to only 0.030 V and the maximum deviation to 0.1 V. This enabled the analysis of various physical contributions to the computed reduction potentials of these ferrocene derivatives, thereby providing insight into their electronic structure and physicochemical properties.

Electron‐Transfer Studies of trans‐Platinum Bis(acetylide) Complexes

“…PClnBu 2 , [77] PClnBu(NEt 2 ), [78] BrFc, [79] cis-[PtCl 2 (SEt 2 ) 2 ], [80] trans-[Pt(CϵCFc) 2 -(PnBu 3 ) 2 ], [47] [PtCl 2 (PnBu 3 ) 2 ], [48] and [Cu(CϵCFc)] [81] were prepared according to published procedures.…”

The Unusual Redox Properties of Fluoroferrocenes Revealed through a Comprehensive Study of the Haloferrocenes

“…When FcH had been removed (composition monitored by 1 H NMR spectroscopy between washings), the organic phase was extracted with water until the washings were colorless, dried over MgSO 4 and filtered (50 g silica/n-hexane). 52 A mixture of ferrocene (0.911 g, 4.90 mmol, 1 eq. ), potassium tert-butoxide (0.067 g, 0.60 mmol, 0.12 eq.)…”

Untitled

“…11 Very thorough investigation of the oxidation of ferrocene derivatives by benzoquinone, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, and 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) was performed by Jahn and co-workers. 12 They found that 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is preferred for the oxidation of ferrocene derivatives with strong electron-attracting groups, like Bz, CO 2 i-Pr, POPh 2 , etc. The prepared ferrocenium cations were isolated as the hexafluorophosphate salts and they were stable in acetonitrile or dichloromethane, but slowly reduced to the starting ferrocene derivatives in ethereal solvents, N,N-dimethylformamide, or dimethyl sulfoxide.…”

“…Catalysis by the ferrocenium ion led to much better results (57% yield) than with Brønsted acid (TsOH) catalyzed reactions. Even more importantly, the reaction was more selective, proceeding without the formation of undesired larger (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) calixarenes (Scheme 8).…”