bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stereoselective Nucleophilic Additions to the N-Acyliminium Ions: Elucidation of Stereoselectivity By Spectroscopic Conformational Analysis
Authors: Suga Seiji , Yamamoto Junya , Akagi Tomoya , Haisa Masahiro , Mitsudo Koichi 
Published: February 2020

N-Acyliminium ions are versatile cationic intermediates for the synthesis of a variety of alkaloids having biological activities. Enormous numbers of stereoselective reactions and syntheses based on the nucleophilic additions to the N-acyliminium ions as key steps have been developed. Despite extensive efforts to understand these selectivities, however, a strong experimental elucidation has not been established, because it is rather difficult to obtain their conformational information by spectroscopic analysis. N-acyliminium ions are generally generated by an acid promoted reaction from their precursors, and the ions and the precursors are usually in equilibrium. Instability of N-acyliminium ions is also problematic, and makes a direct spectroscopic measurement very difficult.
Recently, we have developed the “cation pool method” and the “indirect cation pool method”, involving generation and accumulation of organic cations without nucleophiles. These methods are advantageous to detect organic cations such as N-acyliminium ions and alkoxycarbenium ions by NMR spectroscopy. For example, using the indirect cation pool method, alkoxycarbenium ions which could be identified by the NMR measurement are rapidly and irreversibly generated from thioacetals by the treatment of electrochemically generated ArS(ArSSAr)+ (Scheme 1).1)

Under the circumstances, we are highly motivated to investigate the conformation of N-acyliminium ions which are practically used for the stereoselective reactions (Scheme 2). N-Acyliminium ions having a piperidine skeleton were chosen because the functionalized piperidines frequently show interesting biologically properties. In addition, the conformations of the 6-membered rings should be relatively more rigid compared with 5-membered ring, and hence it is predicted that the conformational information could easily be obtained. Thus, we attempted to generate a variety of N-acyliminium ions having a substituent from the corresponding precursors by using the “indirect cation pool method”.
In this presentation, we will discuss about stereoselectivities of the nucleophilic additions to the N-acyliminium ions based on the conformational analysis of cationic intermediates.
1) Suga, S.; Matsumoto, K.; Ueoka, K.; Yoshida, J. J. Am. Chem. Soc.
2006, 128, 7710-7711. Matsumoto, K.; Suga, S.; Yoshida, J. Org. Biomol. Chem.
2011, 9, 2586-2596.

Figure 1
Electrocatalytic Diels-Alder Reactions Assisted By Aromatic Redox Tag
Authors: Chiba Kazuhiro , Yamaguchi Yusuke , Ozaki Atsushi , Okada Yohei 
Published: February 2020

Although the Diels-Alder reaction is one of the most classic organic transformations, it is still in the front line of carbon-carbon bond formation toolbox and has been producing modern research topics in synthetic chemistry. Especially, the reaction mechanism has been studied intensively and also expanding to a wide variety of the combination of diene and dienophile. The reaction partners must be electronically matched, namely, electron-rich and electron-deficient substrates are required. In order to overcome this limitation, electron transfer strategy can be used. Typically, the reaction between both an electron-rich diene and dienophile are electronically mismatched, however, one electron oxidation by chemical oxidants or photosensitizers can produce a radical cation of one component, which is then trapped by the other component.
We have been developing electrocatalytic intermolecular carbon-carbon bond formations in lithium perchlorate/nitromethane electrolyte solution. We have designed the reaction based on an aromatic “redox tag” concept, that is, the aromatic ring which provides both as an oxidant and a reductant in an overall electrochemical transformation. Since radicals, ions, and radical ions are all significantly stabilized by an aromatic ring and the redox potentials can be fine-tuned by the number and/or position of substituents, programmed electron transfer events could be achieved. These results in hand, we herein demonstrate electrocatalytic Diels-Alder reactions based on an aromatic redox tag strategy.
Initially, we tried to optimize the reaction conditions by using trans-anethole and isoprene as a model reaction partner. These substrates are both relatively electron-rich, therefore, no reaction occurred without an electron transfer. In this context, we found that only a catalytic amount of electrical input was able to complete the reaction, even in high concentration conditions. We also found that the reaction partners were able to be designed by an expected manner by using an aromatic redox tag concept. In the presentation, the synthesis and electrochemical details will be discussed.
Intramolecular Electron Interaction in Various Oligo-nitro-Thiacalix[4]Arenes and Their Electrochemical Reduction
Authors: Ludvík Jiří , Liška Alan 
Published: February 2020

Calixarene skeleton is a stable supramolecular frame which is electrochemically inactive. Suitable substitution on the upper and lower rim, alternatively structure modification or introduction of heteroatoms lead to unusual and surprising properties of this system.
Thiacalix[4]arenes contain four aromatic units connected by sulphur atoms. This contribution will be focused especially on two series of 1,3-alt-conformers bearing one or more nitro groups as electrochemical probes.
The first series consists of eight para-tetranitro derivatives which resemble each other in their electrochemical response: first two two-electron reversible waves are followed by final step during which a tetrakis(hydroxylamino) derivative is formed. This reduction pattern is analogous to the “standard” methylene-bridged para-tetranitrocalix[4]arenes, where no mutual electron interaction takes place, all nitrogroups are totally indepencent and finally tetraanion tetraradical is formed [1,2].
The second series includes four meta-nitro derivatives. Generally, the electroreductive behaviour of them is more complicated (except the first member which bears only one nitrogroup). Presence of two and three nitro groups in the same molecule in position meta to lower rim (that means in position ortho- and para- towards the thia bridges), exhibit clear intramolecular communication in contrary to the methylene-bridged calixarenes [3]. This phenomenon was confirmed by quantum chemical calculations.

Acknowledgement: This work was supported by the GACR grant No. 13-21704S, by institutional support RVO: 61388955 and by the GAUK project No. 798214. The authors thank prof. Pavel Lhoták for granting the compounds.

References:

[1] Alan Liška, Pavel Vojtíšek, Albert J. Fry, Jiří Ludvík: J.Org.Chem 78 (2013) 10651 – 10656.
[2] Alan Liška, Marco Rosenkranz, Jiří Klíma, Lothar Dunsch, Pavel Lhoták, Jiří Ludvík: Electrochimica Acta 140 (2014) 572 – 578.
[3] Alan Liška, Karolína Flídrová, Pavel Lhoták, Jiří Ludvík: Monatsh. Chem 146 (2015) 857–862.
Short-Step Anodic Synthesis of Color-Tunable Emissive Nucleoside Analogues
Authors: Okada Yohei , Shimada Kouhei , Kitano Yoshikazu , Chiba Kazuhiro 
Published: February 2020

Since natural nucleobases are not emissive, emissive nucleoside analogues have been widely used to monitor fundamental biochemical transformations directly. Nowadays, useful libraries of synthetic emissive nucleoside analogues have been constructed and researchers have a wide variety of choices. A typical approach to prepare these synthetic probes is “pre-formed” manner, namely, a glycosylation is carried out using emissive nucleobases under carefully selected conditions to control both stereoselectivity at the anomeric position and regioselectivity of the aglycon attack. Such a glycosylation usually requires a time-consuming trial-and-error procedure, because the reaction selectivities are affected by the interaction of the protected groups of both the sugar and aglycon. On the other hand, the preparation approach based on “post-forming” manner would be effective since a cumbersome glycosylation could be done in an early synthesis stage.
We have been developing anodic intermolecular cycloadditions in lithium perchlorate/nitromethane electrolyte solution. In this electrolyte solution, unactivated alkenes can function as effective carbon nucleophiles to trap anodically generated cations or radical cations, leading to the formation of corresponding ring systems. Especially, we previously reported that the anodic oxidation of methoxyphenols in the presence of alkene nucleophiles was able to give highly emissive dihydrobenzofuran derivatives through formal [3 + 2] cycloadditions. These results in hand, we herein demonstrate a short-step “post-forming” synthesis of novel emissive nucleoside analogues based on the anodic cycloadditions.
Initially, we tried to attach a methallyl moiety to a ribose scaffold as an alkene nucleophile. After the numerous optimizations, we finally accomplished the stereoselective allylation at the anomeric position to obtain both a- and b-methallyl ribose and we were able to use each corresponding pure form for further transformations. The anomeric stereochemistry was successfully fixed at the first synthesis stage. Methoxyphenols were then anodically oxidized in the presence of the methallyl ribose and we found that emissive dihydrobenzofuran structures were able to be constructed on the sugar moiety in one step. The emission color was significantly affected by the carbonyl substituent, ranging roughly from 400 nm to 500 nm with long stokes shift values. In the presentation, the synthesis details and photophysical properties will be discussed.
Electrochemical Study of Reactivity of Orthophthalaldehyde with Primary Amines
Authors: Donkeng Dazie Joel , Ludvík Jiří 
Published: February 2020

Orthophthalaldehyde (OPA) is an aromatic dialdehyde with aldehydic groups in positions 1 and 2. The two formyl groups exhibit inductive, resonance and steric effects. Due to the inductive and resonance effects, one aldehydic group is activated by the other one and, as a result, the molecule is strongly reactive towards nucleophiles. In addition to this, the mutual ortho-position of the carbonyls enables intramolecular ring formation.
The reactivity of OPA with amines is widely used since many years in the determination of amino acids [1] and in application of OPA as a biocide [2]. In the literature related to these applications the mechanism of reaction of OPA with amines is not completely understood. The present study was made in order to describe the detailed mechanism of OPA with primary amines using DC- and DP-polarography and UV/Vis spectrophotometry. The experiments were performed in aqueous buffered solutions of different pH, in non-aqueous acetonitrile and in mixed acetonitrile/H2O media with aqueous or non-aqueous stock solution of OPA. It has been observed that at pH between 7.5 and 10.5, OPA is reduced in three waves of reduction instead of two as mostly mentioned in the literature [3,4]. Its reduction is strongly influenced by pH-dependent hydration. It was found that there is a significant difference in reaction pattern, when using hydrated or non-hydrated form of OPA, respectively. Although the dialdehydic form was always considered as the species reacting with amines [5,6], the experiments showed that the reactivity of OPA with amines is directly related to the hydrated form of OPA, while the non-hydrated OPA undergoes hydration reaction prior the reaction with amines. It was also found that the reaction rate of OPA with primary amines is strongly influenced by the number of H-atoms at the α–carbon atom which may reflect the steric hindrance.

Acknowledgement: The material support from the grant GACR No. 13-21704S and the institutional support RVO 61388955 are acknowledged.

References

(1) Norton, D. R.; Howell, N. Analytical Chemistry
1954, 26, 1116-1119.
(2) Simoes, M.; Simoes, L. Journal of Basic Microbiology
2007, 47, 230-242.
(3) Norton, D. R.; Mann, C. K. Analytical Chemistry
1954, 26, 1111-1115.
(4) Zuman, P.; Salem, N.; Kulla, E. Electroanalysis
2009, 21, 645-649.
(5) Kulla, E.; Zuman, P. Organic & Biomolecular Chemistry
2008, 6, 3771-3780.
Redox-Responsive 4 H-Bond Arrays for Supramolecular Applications
Authors: Smith Diane K. , Cedano Mario , Clare Laurie A. , Darzi Ghazwan M 
Published: February 2020

Due to the ease of synthesis and strong H-bonding, Meijer’s ureidopyrimidone (UPy) 4 H-bonding array has been extensively investigated for use in a variety of functional supramolecular assemblies and polymers. However, somewhat surprisingly, there are relatively few examples of electroactive UPy’s, particularly ones in which electron transfer has a significant effect on the dimerization of the self-complementary AADD or ADAD arrays. Recently we have described an electroactive UPy that contains a phenylenediamine redox couple formed by attachment of a p-dimethylaminophenyl group to R1. In this system the AADD tautomer is favored. Two electron oxidation of the phenylenediamine leads to a sufficient increase in the acidity of one of the urea H donor sites such that proton transfer occurs, resulting in breakup of the dimer. In more recent work we have been investigating the effect of redox couples attached at the R2 position. In one example, based on initial studies by Alexander et al. (Chem Comm 2007, 22, 2246), a ferrocene is attached to this position. CV studies indicate that like the phenylenediamine system, oxidation of the ferrocene leads to break up of the dimer. In another example, an alkyl pyridinium group is attached to the R2 position to create a “monoquat” (N-alkyl-4,4'-bipyridinium) type redox couple. In this case, the weaker binding DADA array is favored in the starting oxidation state. Reduction of the pyridinium should increase the negative charge on the pyrimidinol leading to stronger rather than weaker H-bonding as observed in the other cases. Initial CV studies suggest that this is happening, although the voltammetry is much more complicated than that observed in the other examples. More detailed investigations, currently in progress, will help to ascertain whether the binding strength is indeed increasing upon reduction.

Figure 1
Limits of Acetonitrile Dryness Via Molecular Sieve Method
Authors: Staley Patrick Andrew , Lopez Eric M , Metzger Joy , Smith Diane K. 
Published: February 2020

Acetonitrile (MeCN) is a common solvent in synthetic chemistry and electrochemistry where it is often used when a polar, aprotic solvent is needed.1,2 In the past few years, the importance of controlling water content of aprotic solvents has been pointed out several times2,3 and the suggestion of allowing the solution to sit for a time over 3A molecular sieves has been proposed.4 There should be no surprise that there is a definite limit to the utility of this method, as complexes with closely spaced Lewis acids and bases have been shown to have catalytic effects in the past5 and molecular sieves often consist of main group metal oxides and halides.
We report here a definite limit to solvent dryness where MeCN left over 3A molecular sieves for more than a few days will decay into one or more products that include acidic goups, aromatic groups, and one or more functional groups that will adhere to an electrode surface after a reduction/oxidation cycle. We present CV experiments involving duroquinone (DQ) as a probe molecule to show how surface coverage of the electrode differs between three common electrode materials—Pt, Au, and glassy carbon—changes over time, and also how CV characteristics typical of protonation show up after just allowing the MeCN to sit over the molecular sieves for a little while. Finally, we use NMR spectroscopy, Raman spectroscopy, and mass spectrometry to gain some understanding of the identity of the one or more decay products.
1. D. Bradley, G. Williams, and Michelle Lawton. “Drying fo Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants”. J. Org. Chem. (2010) Vol. 75, iss. 24, pp. 8351-8354.
2.Yanlan Hui, Elaine Lay Khim Chng, Cheryl Yi Lin Chng, Hwee Ling Poh, and Richard D. Webster. “Hydrogen-Bonding Interactions between Water and the One- and Two- Electron-Reduced Forms of Vitamin K1: Applying Quinone Electrochemistry To Determine the Moisture Content of Non-Aqueous Solvents”. J. Am. Chem. Soc. (2009) Vol. 131, iss. 4, pp. 1523-1534.
3. Patrick A. Staley, Eric M. Lopez, Laurie A. Clare, and Diane K. Smith. “Kinetic Stabilization of Quinone Dianions via Hydrogen Bonding by Water in Aprotic Solvents”. J. Phys. Chem. C. (2015) Vol. 119, iss. 35, pp. 20319-20327.
4. Malcolm E. Tessensohn, Hajime Hirao, and Richard D. Webster. “Electrochemical Properties of Phenols and Quinones in Organic Solvents are Strongly Influenced by Hydrogen-Bonding with Water”. J. Phys. Chem. C. (2013) Vol. 117, iss. 2, pp. 1081-1090.
5. Deng Zhaoxiang, Qui Wenfeng, Li Weijia, and Li Yadong. “Cyclotrimerization of nitriles catalyzed by Li3N”. Chinese Science Bulletin. (2004) Vol. 49, No. 2, pp. 127-130.
Kinetics of Fast Electrochemical Processes Studied By Impedance Spectroscopy. Frequency Dispersion at High Frequency Range
Authors: Kulboka Pawel , Polczynski Piotr , Jurczakowski Rafal Robert 
Published: February 2020

The difficulty of the kinetics determination for very fast electrochemical reaction by using impedance spectroscopy (EIS) is related to the fact that the charge transfer resistance for this process is much smaller than the solution resistance. Moreover, for very fast reactions only a low frequency part of the spectra can be observed when potentiostat is coupled with frequency response analyzer in a classical three electrode arrangement. In such case, the high frequency part of the spectra is very often entirely covered by artifacts related to a relatively low time constant of the potentiostat. A special experimental setup is needed for such experiments, in which impedance is measured in stand-alone mode of the frequency response analyzer. The solution resistance can be minimized by using twin electrode arrangement, in which two identical electrodes are facing each other [1]. However, at high frequency part of the impedance spectra a distinct frequency dispersion occurs due to an unequal and frequency dependent current distribution. Therefore a special attention has to be devoted to control the geometry of the electrode arrangement. Unequal current distribution can be eliminated by using recessed electrodes [2]. However, even for recessed electrodes, a distinct frequency dispersion can be observed at kHz region. This effect is more pronounced in the presence of specific adsorption [3]. To date its origin has not been fully understood.
In this study high frequency impedance measurements (up to 1 MHz) have been carried out in a twin electrode and hanging meniscus configurations. Recessed electrodes have been used for the first time to study the kinetics of hydrogen electrosorption at Pt(111) at high frequency range. Hydrogen absorption in palladium thin layers (50 nm) covered by platinum monolayers (1ML/Pd) have also been investigated as model systems involving diffusion processes related to hydrogen absorption. In the 1ML/Pd system, the hydrogen insertion is particularly fast [4], the resistance of the charge transfer, related to this process, is in the order of 0.02 Ωcm2. For these model electrochemical processes the frequency dispersion at high frequency rage will be discussed.
Literature:
[1] B. Losiewicz, R. Jurczakowski, A. Lasia, Electrochim. Acta, 80 (2012) 292-301
[2] Frateur, I.; Huang, V. M. W.; Orazem, M. E.; Pebere, N.; Tribolletd, B.; Vivier, V., Electrochim. Acta 53 (2008) 7386-7395.
[3] T. Pajkossy, Z. Phys. Chem.,226 (2012) 935-943
[3] P. Połczyński, R. Jurczakowski, J. Power Sources, 2016 in press
Investigation of a Redox-Responsive 4 H-Bond Array Capable of Strong Self-Dimerization
Authors: Darzi Ghazwan M , Smith Diane K. 
Published: February 2020

The design of stimuli-responsive systems in which supramolecular structure changes in response to external signals, such as the change in voltage of an electrode, is important for many applications, for example, self-healing polymers and gels, triggered release of entrapped molecules for drug delivery, and “smart” materials. In this study, a four H-bond ureidopyrimidinone (UPy) array with an alkyl-pyridinium, (RP), redox center has been synthesized, UPy(RP), shown below. This array prefers the tautomer that presents an ADAD H-bond motif in the starting oxidation state. Due to electrostatic repulsions and unfavorable secondary H bond interactions, this motif would form a dimer with relatively weak H-bonding. Upon 2e- reduction, where 1e- is gained per R-pyridinium redox center, the H-bond strength should increase due to the loss of the repulsive charges, making the nitrogen a stronger hydrogen acceptor. Because the nitrogen is now a better hydrogen acceptor, there could be a possibility of an intermolecular proton transfer. This would encourage the tautomer to have an AADD motif that will make the H-bonding stronger by increasing the favorable secondary H-bond interactions.
UPy dimers can exist in two tautomeric forms called pyrimidinol and pyrimidinone. In order to more efficiently study the electrochemistry of the two forms, two compounds, 4-acetylpyridinium (AcP) and N-methyl-4,4’-bipyridinium (MeV+), were used as model compounds. AcP resembles the pyrimidinone tautomer and MeV+ resembles the pyrimidinol tautomer. The cyclic voltammetry (CV) scans for the two model compounds in CH2Cl2 and CH3CN show two widely spaced, reversible redox waves. In contrast, CV’s of UPy(RP) in these solvents show two closely spaced reductions. The first occurs at a potential very similar to that seen with simple model compounds. The second is considerably positive of that observed for the model compounds, consistent with strong stabilization of the doubly reduced form by H-bonding. However, the voltammetry is complex. The first reduction is partially reversible if the scan direction is switched immediately after the peak, but irreversible if the scan direction is switched after the second reduction peak. Going through the second reduction also leads to the appearance of new oxidation waves at more positive potentials. This behavior suggests, not surprisingly, that the second reduction induces proton transfer and tautomerization. It is possible that the new oxidation peaks are due to oxidation of a more strongly H-bonded dimer. Further studies to help elucidate what is actually happening in this system will include concentration dependent CV’s and analysis of binding strength using NMR.

Figure 1
Orientational Control of Electroactive Photosystem I on Chemically Modified Electrodes
Authors: Cliffel David , Gizzie Evan Alexander 
Published: February 2020

The ability to control the orientation of proteins on chemically modified electrodes and within films of conducting or redox polymers would allow for the efficient electron transfer. Unfortunately, extraction of proteins and protein complexes like photosystem I from biological membranes often scrambles the precise orientational control found in native plants. In this talk, we will present alternative methods and chemical reactions that can help maintain the memory of the correct protein orientation as originally in the membrane after the protein complex of photosystem I is extracted from spinach. The resulting voltammetry and photochronoamperometry of these modified electrodes will be compared to non-modified protein extractions.
An Electroenzymatic CO2 Conversion to Formate on Cu Electrode Using NADH Co-Enzyme
Authors: Jung Kwang-Deog , Kim Seung-Han , Yoon Sungho 
Published: February 2020

Mimicking photosynthesis using has been attempted to convet CO2. The reaction scheme consists of electrochemical NADH regeneration and enzymatic CO2 reduction. Carbon and metal electrodes were not suitable for electrochemical NADH regenreation because the inactive NADH formation by dimerization of NADH was not avoidable. Electron mediators were required to get nearly 100% active NADH by the electrochemical method. Rh carbene