**Contents**


### **About the Editor**

**Victor Borovkov** (Prof. Dr.) received a Ph.D. degree from the Moscow Institute of Fine Chemical Technology (Russia) in 1988. Following a postdoctoral period at Osaka University, he worked at different research organizations in Japan for over twenty years, at the Tallinn University of Technology (Estonia) as a senior research scientist, and at the South-Central University for Nationalities (China) as a full professor. He is a member of the editorial boards of several scientific journals, an author of more than 145 publications, including research papers, reviews, and books, with an h-index of 29 (WOS and Scopus) and 30 (Google Scholar), and 10 patents, and serves as an external peer reviewer of various international journals and scientific foundations. He has headed and participated in different international and industrial collaboration projects and attended numerous international and national conferences as an invited speaker and a member of advisory committees. His current research interests include the chemistry of porphyrins and related macrocycles, supramolecular chemistry, nanotechnology, chirality science, functional chiral materials, and asymmetric catalysis.

### **Preface to "Chiral Auxiliaries and Chirogenesis II"**

Chirality is an inevitable property of our universe, having an enormous impact in different fields ranging from nuclear physics and astronomy to living organisms and human beings. Furthermore, chirality has important applications in various technological processes including pharmaceutical and agrochemical manufacturing. Therefore, in order to highlight the importance of this phenomenon in chemical science, the first Special Issue on "Chiral Auxiliaries and Chirogenesis" was launched in 2017 [1]. This Special Issue has attracted much attention from the scientific community and resulted in a high scientific impact, on the basis of the articles' access statistics and citation indexes (with up to 25 citations as of April 16th 2021 [2]).

This success prompted us to initiate a subsequent Special Issue entitled "Chiral Auxiliaries and Chirogenesis II" on the same subject in 2019, which was completed in 2021. The aim of this second Special Issue, which consisted of one review paper and six research articles, was to address some particular aspects of chiral auxiliary and chirogenesis which were not covered by the first Special Issue.

In particular, Trubitson˜ and Kanger, in their review [3], described new methodologies for the synthesis of chiral N-functionalized indoles. The synthesis of enantioenriched indole derivatives is of grea<sup>t</sup> importance in organic chemistry, especially in light of their potential application in the pharmaceutical industry. The review illustrated efficient applications of organocatalytic and organometallic strategies for the construction of chiral α-N-branched indoles. Both the direct functionalization of the indole core and indirect methods based on asymmetric N-alkylation of indolines, isatins, and 4,7-dihydroindoles were discussed.

Another important class of chiral molecules is macrocycle compounds. Recently, the chirality of a novel group of emerging host molecules, pillar[n]arenes, has attracted increasing attention due to their potential applications for chiral induction, molecular recognition, and asymmetric catalysis. In the research communication [4], Xiao et al. reported the synthesis and successful resolution of planar (P*R*)- and (P*S*)-enantiomeric Boc-protected pillar[4]arene[1]diaminobenzene. Their racemization kinetics were studied. It was shown that hexane and CH2Cl2 can maintain the enantiomeric forms for long periods, because of the complexation of the solvent molecules with the cavity of pillar[4]arene[1]diaminobenzene. The racemization process was accelerated by increasing the temperature or the use of solvents that cannot thread into the cavity of these molecules or can destroy intramolecular hydrogen bonds. This study has provided, for the first time, thermodynamic parameters of the pillararenes in different solvents that will serve as an important guideline in studying the conformational properties of pillar[n]arenes.

A crucial problem of self-induced diastereomeric anisochronism (SIDA) in NMR has been addressed by Baumann et al. in their feature article [5]. This phenomenon may occur when chiral molecules that associate in solution in a dynamic equilibrium that is fast on the NMR timescale have significant condition-dependent NMR chemical shifts. This study was carried out by using alcohol and ester derivatives and highlighted the potential problems that SIDA can cause. Additionally, scalemic samples of both the alcohol and ester compounds were firstly reported to exhibit the self-disproportionation of enantiomers phenomenon by preparative TLC.

Chiral porphyrinoids have important implementations in the fields of chiral sensors, biomimetic functions, asymmetric catalysis, and other applications. Setsune et al. [6] described the synthesis and magnetic properties of linear trinuclear CuII and NiII complexes of porphyrin analogs embedded with chiral binaphthol units. It was found that the observed paramagnetic shifts in the pyrrolic ligand and the binaphthyl ligand could be used to estimate spin delocalization from the terminal metal and the central metal, respectively, and these paramagnetic <sup>1</sup>H NMR data were consistent with the spin densities calculated. Additionally, the strong antiferromagnetic coupling observed for both CuII3 and NiII3 complexes could be ascribed to the unique coordination geometry that was also responsible for the reversible ligation of butylamine only at the central metal ion without decomposition of the trinuclear core. The reported multinuclear complexes of an enantiomerically pure helical porphyrin analog are expected to lead to further exploration of the helical multinuclear complexes.

Further expansion of supramolecular chirogenesis was examined by Osadchuk et al. in [7]. In particular, a comprehensive study on the complexation of ethane-bridged bis(zinc octaethylporphyrin), as a host, with a chiral guanidine derivative, as a guest, was carried out by means of ultraviolet–visible and circular dichroism absorption spectroscopies, single crystal X-ray diffraction, and computational simulation. The formation of a 1:2 host–guest complex was established by X-ray diffraction and spectroscopic titration studies. Such supramolecular organization of the complex results in a screw arrangemen<sup>t</sup> of the two porphyrin subunits, inducing a strong circular dichroism signal in the porphyrin Soret band region. The corresponding computational studies were in a good agreemen<sup>t</sup> with the experimental results. This study was one of the rare examples of comprehensive circular dichroism analysis of chirality induction in bis-porphyrins caused by external chiral ligands, which can be a benchmark approach for the rationalization of supramolecular chirogenesis in bis-porphyrins. Furthermore, the obtained results demonstrate the necessity of careful consideration of all external and internal factors that influence the supramolecular organization of a complex to attain the best match between experimental and simulated circular dichroism spectra.

In a related chirogenic study, <sup>M</sup>ądry et al. reported a new sensitive stereodynamic reporter for primary amines operating on the basis of the point-to-axial chirality mechanism [8]. The throughspace inductor–reporter interactions forced a change in the chromophore conformation toward one of the diastereomeric forms. The structure of the reporter, with the terminal flipping biphenyl groups, led to generating Cotton effects in both lower- and higher-energy regions of the circular dichroism spectrum. The reporter system turned out to be sensitive to the subtle differences in the inductor structure. Despite the size of the chiral substituent, the molecular structure of the inductor– reporter systems in the solid state showed many similarities. The most important one was the tendency of the core part of the molecules to adopt a pseudocentrosymmetric conformation. Supported by a weak dispersion and Van der Waals interactions, the face-to-face and edge-to-face interactions between the π-electron systems present in the molecule were found to be responsible for the molecular arrangemen<sup>t</sup> in the crystal.

The last article in this Special Issue, by Fujiki et al., was devoted to one of the most fundamental questions of chirogenesis [9]. It is known that non-charged semi-flexible and rod-like helical copolymers and π–π molecular stacks reveal sergeants-and-soldiers (Ser-Sol) and majority-rule (Maj) effects in dilute solutions and as a suspension in fluidic liquids. A question remained unanswered as to whether the Ser-Sol and Maj effects between non-charged rod-like helical polysilane copolymers and non-charged, non-helical π-conjugated homopolymers occur when these polysilane copolymers encounter the π-polymer in the co-colloidal systems. Based on different types of chiral polysilane copolymers and detailed analyses of circular dichroism and circularly polarized luminescence results, this paper discussed the origins of noticeable non-mirror-symmetrical Ser-Sol and Maj effects in terms of macroscopic parity violation that differed from rigorous criteria of a molecular parity violation hypothesis. The comprehensive helicity/chirality transfer experiments in the artificial

helical/non-helical polymer co-colloids in the tuned refractive index optofluidic media may lead to possible answers to several unresolved questions in the realms of molecular biology, stereochemistry, supramolecular chemistry, and polymer chemistry: (i) whether mirror symmetry on macroscopic levels is rigorously conserved, (ii) why nature chose L-amino acids and five-membered D-furanose (not six-membered D-pyranose) in DNA/RNA?

While this Special Issue has just been completed, the scientific interest in the published articles is constantly increasing, on the basis of the articles' statistics (with up to four citations as of April 16th 2021 [10]). This may result in a new Special Issue covering additional chirality topics to be launched in *Symmetry* in the near future.
