CD Spectroelectrochemistry, a Method for the Characterization of Chiral Electron Transfer Reagents and Chiral Reaction Intermediates

Redox-active tetraaryldibenzoquinodimethanes

Quinodimethanes (QDs) are a class of non-aromatic π-conjugated compounds that are well-known to be interconvertible scaffolds in many response systems. While parent ortho- and para-QDs (o-QD and p-QD) can be easily converted to benzocyclobutenes or oligomers/polymers by the formation of C-C bonds at α-positions, the attachment of four phenyl groups to these reactive sites makes o-Ph₄QD and p-Ph₄QD long-lived. We have demonstrated that further dibenzo-annulation of such tetraaryl QD units also drastically increases their stability, and many tetraarylated dibenzoquinodimethane derivatives have been developed. This Feature Article shows our milestones in creating functional redox systems, where drastic changes in structure occur upon electron transfer (dynamic redox "dyrex" behaviour).

Electrochemistry of Quinones with Respect to their Role in Biomedical Chemistry

Quinones are ubiquitous in nature and form one of the largest class of antitumor agents approved for clinical use. They are known to be efficient in inhibiting cancer cells growth. Under physiological conditions they can undergo non-enzymatic one-electron reduction to give the moderately toxic species of semiquinone radical-anion. Thus, electrochemical study of quinones might provide a basic knowledge on semi-quinone radicals formation in both in vivo and in vitro under different media. Several processes are outlined briefly and discussed in the present article. Previously we investigated the electrochemical and spectral properties of ω-N-quinonyl amino acids. Such quinone-bearing peptides are known to be cytotoxic and of potential clinical significance. We were able to prove that the ω-amino quinonyl compounds are very effective in producing stable semiquinone radicals. Moreover, a direct relation was found between the first reduction potentials of the quinonyl moiety and their reactivity towards the ω-amino acids. In order to increase our knowledge of such amino quinonyl compounds and enlarge the arsenal of such cytotoxic compounds, a series of N,N-diquinonyl amines (1-6) bearing an internal proton (stems from the NH moiety) were synthesized. Their electron-transfer capabilities were probed by cyclic voltammetry measurements, in dichloromethane. It was found that the acidic NH group linking the two quinonyl moieties undergoes an initial electrochemical reduction step and generates a nitride anion. This step is followed by further reductions to yield quasi-stable semiquinone radicals and polyanions, Since these acidic diquinones (1-6) serve also as a source of internal proton donors even in non-polar medium, they might cause protonation of basic radical-anions and polyanion intermediates during the various electrochemical stages. The processes are demonstrated and discussed by analyzing different mechanistic schemes. The successful generation of relatively stable semiquinone radicals is a prerequisite for the manifestation of site directed antitumor activity by these bis-quinonyl amino derivatives. Based on the values of their redox potentials some of them could be promising candidates for clinical development.

ECD spectroelectrochemistry: A review

The electronic circular dichroism (ECD) spectroscopy is probably the most important chiraloptical method, and the role of chirality in contemporary chemistry, pharmacy, and material science constantly increases. On the other hand, the electrochemical methods are also very sensitive tools for studying multivarious redox processes. Nevertheless, the first ECD spectroelectrochemical (SEC) study was only published by Daub, Salbeck and Aurbach in 1988, and since then, the ECD SEC method has been mentioned in only thirty papers. By the summer of 2020, the ECD SEC studies were mainly focused around molecular systems for organic, and marginally, inorganic chiroptical switching studies of biochemical redox reactions. The review provides more details about the ECD SEC studies carried out so far. At the end, we suggest some future applications for the ECD spectroelectrochemistry.

Assembly of an Axially Chiral Dynamic Redox System with a Perfluorobiphenyl Skeleton into Dumbbell- or Tripod-type Electron Donors

The incorporation of F atoms endows a diethenylbiphenyl-based electron donor with configurational stability and SN Ar reactivity. The former enables the dynamic redox pair of (Rax)-1/(Rax ,R,R)-1(2+) to exhibit drastic UV/Vis and CD spectral changes upon electrolysis, whereas the latter makes it possible for (Rax)-1 to serve as a useful chiral synthon for the production of larger assemblies [(Rax ,Rax)-2 d,p,m and (Rax ,Rax ,Rax)-3] containing two or three dyrex units. These dyads and triad also exhibit a clean electrochiroptical response with isosbestic points owing to one-wave multi-electron transfer.

Multi-input–Multi-output Molecular Response System Based on Dynamic Redox Behavior of Hexaphenylethane-type Electron Donors with the Tetrahydrophenanthrazepine Skeleton: Strong Chiroptical Signals through the Transmission of Point Chirality to Helicity

The title heterocyclic donors undergo reversible C-C bond formation/cleavage upon electron transfer (dynamic redox behavior). The helical sense in both neutral and cationic states is interconvertible by facile ring flipping. The pi-type asymmetric center on the azepine nitrogen atom induces a significant degree of diasteromeric preference, thus endowing strong CD activity based on exciton coupling. Chiroptical properties could be modified not only by redox reactions but also by heat and protonation. The present redox pairs can serve as unprecedented three-way-input (e, H+, delta) and two-way-output (UV/Vis, CD) response systems.

Chemistry of porphyrin-appended cellulose strands with a helical structure: spectroscopy, electrochemistry, and in situ circular dichroism spectroelectrochemistry

Around 100 porphyrin units have been selectively linked at C(6)-O to a cellulose (Avicel). The properties of the metal-free and zincated porphyrin-celluloses 2 and Zn-2 have been determined by optical and electrochemical methods. Circular dichroism indicates a helical arrangement of the porphyrin units and reveals intra-chain coupling reminiscent, in the broadest sense, of strands of nucleic acids. Cyclic voltammetry and spectroelectrochemistry have been used to characterize the radical ions and dianions. The electrochromism of the oxidation of cellulose 2 to porphyrin radical cations of 2 has been employed for both molecular switching and the transduction of an electrochemical input into chiroptical signal expression.

Thermodynamic calculations for reactions involving hydrogen halide polymers, ions, and Lewis acid adducts. 4. The temperature dependence of some reactions of HF and HCl

Ab initio MP2/6-311++G(d,p) structure optimizations and frequency calculations have been carried out on 10 polymers, (HX)n; on eight polyhalohydrogenate(1-) anions, HnXn+1-, and on four polyhalohydrogen(I) cations, Hn+1Xn+, (X = F and Cl) at a number of temperatures from 188 to 673 K, and at various total pressures. From the results, ΔG° and K values have been calculated for some reactions involving these species, and the relative amounts determined under a variety of conditions:

nHX –> (HX)n (n = 2-6) HX + HnXn+1- àHn+1Xn+2- (n = 0-3) HX + Hn+1Xn+ àHn+2Xn+1+ (n = 0, 1) There are some marked differences between the fluorine and chlorine series, but generally, the relative amounts of the large molecules and ions decrease as the temperature increases. For all reactions, ln K can be expressed as function to the fourth power in 1/T indicating that ΔH° is more or less temperature dependent over a range of 400-500°.Key words: ab initio, thermodynamics, HCl, HF.