[3 + 2] Cycloaddition with photogenerated azomethine ylides in β-cyclodextrin

• Margareta Sohora,
• Leo Mandić and
• Nikola Basarić

Beilstein J. Org. Chem. 2020, 16, 1296–1304, doi:10.3762/bjoc.16.110

• most probably undergoes decarboxylation delivering 1AMY from the S1 state [49]. In CH3CN, 1AMY decays with a rate constant of 2.9 × 106 M−1 s−1, and reacts with methyl acrylate in [3 + 2] cycloaddition with the rate constant 2.7 × 107 M−1 s−1 [49]. Protic solvents such as CH3OH or H2O quench

• and intercepted with AN to yield cycloadducts 7 or 11, respectively. However, the formation of cycloadducts is very inefficient, which may be ascribed to a smaller rate constant for the quenching due to steric hindrance imposed by the bulky cyclohexane or adamantine moiety. Thus, irradiation of 2 gave

Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

• Bernd Strehmel,
• Christian Schmitz,
• Ceren Kütahya,
• Yulian Pang,
• Anke Drewitz and
• Heinz Mustroph

Beilstein J. Org. Chem. 2020, 16, 415–444, doi:10.3762/bjoc.16.40

• possesses an internal activation barrier resulting in a system having a certain energy threshold. Equation 8 shows how temperature affects the rate constant for electron transfer ket The free activation enthalpy [67][72] controls the internal activation barrier, Equation 8 [65]. The free enthalpy of

p-Pyridinyl oxime carbamates: synthesis, DNA binding, DNA photocleaving activity and theoretical photodegradation studies

• Panagiotis S. Gritzapis,
• Panayiotis C. Varras,
• Nikolaos-Panagiotis Andreou,
• Katerina R. Katsani,
• Konstantinos Dafnopoulos,
• George Psomas,
• Zisis V. Peitsinis,
• Alexandros E. Koumbis and
• Konstantina C. Fylaktakidou

Beilstein J. Org. Chem. 2020, 16, 337–350, doi:10.3762/bjoc.16.33

• corresponding activation energy (Equation 1) and free activation energy (Equation 2) were calculated for compound 12 and found 3.14 and 2.95 kcal/mol, respectively. These values were used in Equation 3 in order to calculate the rate constant for the N−O bond dissociation. Accordingly, kr, was found to be 4.27

• chemical reaction below (Scheme 3). The activation free energy for the decarboxylation reaction is only 1.09 kcal/mol and by using Equation 4 (see theoretical calculations section) we find a rate constant kr = 9.87∙1011 s−1, characterizing the reaction as an ultrafast one, with a corresponding life-time of

• energy and free energy of activation are given in Equation 1 and Equation 2, respectively: For the calculation of the rate constant, kr, the Eyring’s classical Equation 3 was used, where in the above equation kB is the Boltzmann’s constant (1.380662∙10−23 J/K), h is the Planck’s constant (6.626176∙10−34

Photoreversible stretching of a BAPTA chelator marshalling Ca²⁺-binding in aqueous media

• Aurélien Ducrot,
• Arnaud Tron,
• Robin Bofinger,
• Ingrid Sanz Beguer,
• Jean-Luc Pozzo and
• Nathan D. McClenaghan

Beilstein J. Org. Chem. 2019, 15, 2801–2811, doi:10.3762/bjoc.15.273

• restoration of the electronic absorption band attributed to the π–π* transition at 30 °C (Figure 5), the rate constant (k) was estimated at 5.4 × 10−5 s−1. Photoisomerization was also recorded in electron-absorption spectroscopy in the presence of calcium (see Figure S2 in Supporting Information File 1). The

A combinatorial approach to improving the performance of azoarene photoswitches

• Joaquin Calbo,
• Aditya R. Thawani,
• Rosina S. L. Gibson,
• Andrew J. P. White and
• Matthew J. Fuchter

Beilstein J. Org. Chem. 2019, 15, 2753–2764, doi:10.3762/bjoc.15.266

• inversely proportional to the rate constant, and this is exponentially dependent on the free-energy barrier according to Eyring theory. Thus, a small variation in the energy barrier of <1 kcal/mol leads to a change of few orders of magnitude in half-life. Vertical electronic transition energies for the

Reversible switching of arylazopyrazole within a metal–organic cage

• Anton I. Hanopolskyi,
• Soumen De,
• Michał J. Białek,
• Yael Diskin-Posner,
• Liat Avram,
• Moran Feller and
• Rafal Klajn

Beilstein J. Org. Chem. 2019, 15, 2398–2407, doi:10.3762/bjoc.15.232

• linear profile of the recovery suggests that the reaction obeys first-order kinetics, with a rate constant of 0.0975 h−1, corresponding to a thermal half-life of Z-1, τ1/2 ≈ 7.1 hours. This value of τ1/2 is surprisingly small vis-à-vis the previously reported [35] τ1/2 ≈ 10 days in acetonitrile. To

• extent than does aqueous 2. These results suggest that cage 2 can catalyze the thermal back-isomerization of Z-1 to E-1, whose kinetics can be written down as: where the pseudo-first-order rate constant kobs is the product of k and the concentration of the cage, c2, which remains constant over time. To

• increasing concentrations of 2 (A∞ denotes the absorbance at λmax before irradiation; A0 – immediately after exposure to UV light; At – after thermal relaxation for time t). b) Cage concentration dependence of the pseudo-first-order rate constant kobs. c) The proposed mechanism underlying the Pd2

Norbornadiene-functionalized triazatriangulenium and trioxatriangulenium platforms

• Roland Löw,
• Talina Rusch,
• Tobias Moje,
• Fynn Röhricht,
• Olaf M. Magnussen and
• Rainer Herges

Beilstein J. Org. Chem. 2019, 15, 1815–1821, doi:10.3762/bjoc.15.175

• thermal isomerization of QC 1b back to NBD 1a was investigated by 1H NMR measurements (Figure 4). The cycloreversion follows a first order reaction, the rate constant could be determined by logarithmic fitting of the integrals of the CH2 signals next to the nitrogen atoms of the platform as k = 1.06·10−3

• %) after following the cycloreversion within a period of one month are visible in the 1H NMR spectrum. Under nitrogen atmosphere the half-life of the cycloreversion is t1/2 = 732 h (294 K). The rate constant as a function of the temperature follows an Arrhenius-type relationship. The activation energy for

• bridge atom of the TATA platform in QC 1b at time t, [QC]0 corresponding 1H integral at t = 0. A rate constant of k = 0.95·10−3 s−1 was determined from a linear fit of the ΔY/t curve. (a) STM image of self-assembled monolayers of compound 1 on Au(111) (40 × 40 nm2, It = 0.05 nA, Ubias = 0.40 V) and (b

Complexation of a guanidinium-modified calixarene with diverse dyes and investigation of the corresponding photophysical response

• Yu-Ying Wang,
• Yong Kong,
• Zhe Zheng,
• Wen-Chao Geng,
• Zi-Yi Zhao,
• Hongwei Sun and
• Dong-Sheng Guo

Beilstein J. Org. Chem. 2019, 15, 1394–1406, doi:10.3762/bjoc.15.139

• ground-state complex, with drastic changes in both, band shapes and intensities. The calculated rate constant of PET is faster than that of fluorescence and intersystem crossing, which makes it a more favorable deactivation pathway of the first excited singlet state leading to both fluorescence quenching

• non-fluorescent in aqueous solution, but become highly fluorescent in non-polar solvents or when they are bound to proteins and membranes [42][43]. This enhancement is commonly understood in terms of the relocation of the guest into the more hydrophobic environment. The decreased rate constant of

Understanding the unexpected effect of frequency on the kinetics of a covalent reaction under ball-milling conditions

• Ana M. Belenguer,
• Adam A. L. Michalchuk,
• Giulio I. Lampronti and
• Jeremy K. M. Sanders

Beilstein J. Org. Chem. 2019, 15, 1226–1235, doi:10.3762/bjoc.15.120

• ) is the Arrhenius-type rate constant, and f(α) is the functional form of rate, dependent on the specific mechanism of the transformation. Many of the traditional kinetic equations for solid-state transformations (e.g., the Avrami–Erofeyev and Prout–Tompkins models) are derived for single phase solid

• . Instead, only the total of all phenomena is considered. In reality, this accumulation and subsequent relaxation is highly complex, involving submolecular (electronic and vibrational) effects, as well as defect generation and temperature development [54][57]. A rate constant, k2, is hence developed as a

Precious metal-free molecular machines for solar thermal energy storage

• Meglena I. Kandinska,
• Snejana M. Kitova,
• Vladimira S. Videva,
• Stanimir S. Stoyanov,
• Stanislava B. Yordanova,
• Stanislav B. Baluschev,
• Silvia E. Angelova and
• Aleksey A. Vasilev

Beilstein J. Org. Chem. 2019, 15, 1096–1106, doi:10.3762/bjoc.15.106

• concentration (1 × 10−4 M) for all dyes. The kinetic data were found to fit well to a single exponential function (Equation 1), giving a rate constant (k) corresponding to a lifetime (1/k) [18]: where A0 and A∞, are the initial and final absorptions, A is the absorption at 500 nm at a time t after termination

Photochemical generation of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical from caged nitroxides by near-infrared two-photon irradiation and its cytocidal effect on lung cancer cells

• Ayato Yamada,
• Manabu Abe,
• Yoshinobu Nishimura,
• Shoji Ishizaka,
• Masashi Namba,
• Taku Nakashima,
• Kiyofumi Shimoji and
• Noboru Hattori

Beilstein J. Org. Chem. 2019, 15, 863–873, doi:10.3762/bjoc.15.84

• 365 nm light (6.02 × 1015 photons s−1) under atmospheric conditions (Figure 1). Clean release of the TEMPO radical was confirmed by measuring the electron paramagnetic resonance (EPR) signals of the typical nitroxide, AN = 15.5 G (g = 2.00232, Figure 1 and Figure 2c). The first-order rate constant for

Cyclopropene derivatives of aminosugars for metabolic glycoengineering

• Jessica Hassenrück and
• Valentin Wittmann

Beilstein J. Org. Chem. 2019, 15, 584–601, doi:10.3762/bjoc.15.54

• cyclopropene reacts two orders of magnitude faster than an amide-linked [28] and that removal of the stabilizing methyl group results in a 9-fold second-order rate constant [27]. However, these studies have been performed with different model compounds and under different reaction conditions and, therefore

• constant. Comparison of the rate constant of ManNCyc with the published one of ManNCyoc (0.99 M−1s−1 [24]), which was determined under the same conditions, shows that the carbamate linkage instead of the amide linkage results in a 33-fold second-order rate constant. Obviously, the presence of the carbamate

• acceptance of the Cp-modified sugar inspired us to develop novel derivatives of glucosamine and galactosamine containing this cyclopropene modification and to explore their behavior in MGE both for membrane-bound and intracellular glycoproteins. Results and Discussion Kinetic studies The second-order rate

Syntheses and chemical properties of β-nicotinamide riboside and its analogues and derivatives

• Mikhail V. Makarov and
• Marie E. Migaud

Beilstein J. Org. Chem. 2019, 15, 401–430, doi:10.3762/bjoc.15.36

• is pH independent at pH below 6.5 and above 12.5. However, in the pH range from 8.5 to 11.0 the hydrolysis reaction shows first order kinetics in OH− (i.e., the hydrolysis rate constant is linearly dependent on pH with a slope of unity). It was also shown that different hydrolysis products were

Catalysis of linear alkene metathesis by Grubbs-type ruthenium alkylidene complexes containing hemilabile α,α-diphenyl-(monosubstituted-pyridin-2-yl)methanolato ligands

• Tegene T. Tole,
• Johan H. L. Jordaan and
• Hermanus C. M. Vosloo

Beilstein J. Org. Chem. 2019, 15, 194–209, doi:10.3762/bjoc.15.19

• ruthenacycle. In addition, the relatively low ruthenium metal positive charge on 9 would cause it to have a high initiation rate constant [19]. On the other hand, the 3-Me group in 6 will strengthen the Ru–N chelation via inductive electron-donation and steric repulsion between the methyl group and the two

Selective ring-opening metathesis polymerization (ROMP) of cyclobutenes. Unsymmetrical ladderphane containing polycyclobutene and polynorbornene strands

• Yuan-Zhen Ke,
• Shou-Ling Huang,
• Guoqiao Lai and
• Tien-Yau Luh

Beilstein J. Org. Chem. 2019, 15, 44–51, doi:10.3762/bjoc.15.4

• carried out in THF-d8 at 273 K, the second order rate constant for 4 was 2.1 × 10−3 M−1s−1, whereas norbornene derivative 5 was inert under these conditions. The details are described in the Experimental section and Supporting Information File 1 (Figures S1, S2 and S8–S10). It has been suggested that the

New standards for collecting and fitting steady state kinetic data

• Kenneth A. Johnson

Beilstein J. Org. Chem. 2019, 15, 16–29, doi:10.3762/bjoc.15.2

• additional information, while the ratio of kcat/Km provides a measure of enzyme specificity and is proportional to enzyme efficiency and proficiency. Moreover, kcat/Km provides a lower limit on the second order rate constant for substrate binding. For these reasons it is better to redefine the Michaelis

• to larger errors in the calculated kcat/Km value. On the other hand, the value of kcat/Km is generally well defined from the initial slope of the concentration dependence, as illustrated in Figure 1. Thus, kcat/Km can be understood as the apparent second order rate constant for substrate binding

• . More precisely, kcat/Km is equal to the true second order rate constant for substrate binding to the enzyme multiplied by the probability that the bound substrate will be converted to product and released into solution. This principle can be illustrated using the simplest model: The term, k2/(k−1 + k2

Applications of organocatalysed visible-light photoredox reactions for medicinal chemistry

• Michael K. Bogdos,
• Emmanuel Pinard and
• John A. Murphy

Beilstein J. Org. Chem. 2018, 14, 2035–2064, doi:10.3762/bjoc.14.179

• ). In general, if a molecule is to participate in a reaction in the S1 state, its τf must be greater than 1 ns; N.B. the diffusion rate constant (kdiff) is ≈1–2 × 1010 s−1. The fluorescence quantum yield (Φf) is another key parameter to consider when determining whether the S1 state of a molecule is

• comparable to or larger than the Φf and, more importantly, the rate constant for ISC (kISC) must be similar to the rate constant for fluorescence (kf). The lifetime of the T1 state (τT1) is generally orders of magnitude longer than the timescale of electron transfer (ET), meaning that τT1 does not alter the

Recent advances in phosphorescent platinum complexes for organic light-emitting diodes

• Cristina Cebrián and
• Matteo Mauro

Beilstein J. Org. Chem. 2018, 14, 1459–1481, doi:10.3762/bjoc.14.124

• -state. On the other hand, [Pt(N^C^N)Cl] displayed a metal-perturbed 3π–π* emission as also demonstrated by the relatively high radiative rate constant value. The combination of these two factors explained well the aforementioned good emission efficiencies. As a result, N^C^N-coordinated complexes have

[3 + 2]-Cycloaddition reaction of sydnones with alkynes

• Veronika Hladíková,
• Jiří Váňa and
• Jiří Hanusek

Beilstein J. Org. Chem. 2018, 14, 1317–1348, doi:10.3762/bjoc.14.113

• , for the cycloaddition of the structurally similar ethyl phenylpropiolate in various solvents only a small decrease of the bimolecular rate constant with increasing solvent polarity (in terms of relative permittivity) was observed excluding a transition state having a polarized character. Finally

• corresponding rather to type II or even type I cycloadditions. The most reactive were electron-poor alkynes (acetylene(di)carboxylates, benzoyl phenylacetylene) while electron-rich alkynes (tetradec-1-yne, 1-phenylpropyne) were much less reactive. Unfortunately, the reaction rate constant was not measured for

Phosphodiester models for cleavage of nucleic acids

• Satu Mikkola,
• Tuomas Lönnberg and
• Harri Lönnberg

Beilstein J. Org. Chem. 2018, 14, 803–837, doi:10.3762/bjoc.14.68

• example, the buffer-catalyzed reaction was 103-fold faster than the buffer-independent reaction. The observed rate constant showed both first- and second-order dependence on the buffer concentration, kobs = k1[B] + k2[B][BH+]. The Brönsted β value for the first-order term was 0.77 and this reaction was

• . The second milestone on the way to guanidine-based cleaving agents was the finding that tris[2-(benzimidazol-2-ylamino)ethyl]amine (5) could rather rapidly degrade RNA [77]. The first-order rate constant for the cleavage of an individual phosphodiester linkage of a 30-mer RNA sequence was 3.3∙10−6 s−1

• second-order rate constant for the ethylenediamine-catalyzed cleavage of ApA has been reported to be 1.2∙10−6 L mol−1 s−1 at pH 8 and 50 °C [83]. Cyclic polyamines are somewhat better catalysts (Figure 9). The tetracation of 1,4,16,19-tetraoxa-7,10,13,22,25,28-hexaazacyclotriacontane (9) cleaves ApA

D–A–D-type orange-light emitting thermally activated delayed fluorescence (TADF) materials based on a fluorenone unit: simulation, photoluminescence and electroluminescence studies

• Lin Gan,
• Xianglong Li,
• Xinyi Cai,
• Kunkun Liu,
• Wei Li and
• Shi-Jian Su

Beilstein J. Org. Chem. 2018, 14, 672–681, doi:10.3762/bjoc.14.55

• generally results in a low radiative rate constant (kr) to compete with a large nonradiative rate constant (knr) [12]. The increasing nonradiative transition processes and large knr play a role in competition with RISC and radiative transition processes and seriously restrict the development of orange and

• lifetime of the prompt and delayed components of the time-resolved transient PL decay curves at room temperature, the PLQY of their respective components and rate constant of different kinetic processes were calculated, as shown in Table 4. The rate constants were calculated following Equations 1–4 below

• [5][7][16]. where kr, knr, kisc, and krisc represent the rate constant of radiative, nonradiative, intersystem crossing and reverse intersystem crossing, respectively; Φ, ΦPF, ΦTADF, τPF and τTADF represent the photoluminescence quantum yield, quantum yield of the prompt component, quantum yield of

Enzyme-free genetic copying of DNA and RNA sequences

• Marilyne Sosson and
• Clemens Richert

Beilstein J. Org. Chem. 2018, 14, 603–617, doi:10.3762/bjoc.14.47

• reactivity exists. When we determined the rates for each of the different templating triplets, we found that the rate constant for extension on the poorest templating sequence (CAG) and on the best templating sequence (TCT) differed by less than two orders of magnitude, with rate constants k'CAG = 100 h−1 M

• -limiting step are found, so that the modeling requires no more than a single rate constant for the covalent step (kcov). Determining the rate constant experimentally requires knowledge of Kd, so that a defined concentration of the kinetically relevant species can be entered in the rate equation for what is

• with monomers that were not carefully purified. The high reactivity was traced to a species with a chemical shift of −10.8 ppm in the 31P NMR spectrum that was identified as the imidazolium bisphosphate (Figure 12 and Figure 13) [34]. We calculated a second order rate constant for the reaction of the

Recent advances on organic blue thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs)

• Thanh-Tuân Bui,
• Fabrice Goubard,
• Malika Ibrahim-Ouali,
• Didier Gigmes and
• Frédéric Dumur

Beilstein J. Org. Chem. 2018, 14, 282–308, doi:10.3762/bjoc.14.18

Synthesis of naturally-derived macromolecules through simplified electrochemically mediated ATRP

• Paweł Chmielarz,
• Tomasz Pacześniak,
• Katarzyna Rydel-Ciszek,
• Izabela Zaborniak,
• Paulina Biedka and
• Andrzej Sobkowiak

Beilstein J. Org. Chem. 2017, 13, 2466–2472, doi:10.3762/bjoc.13.243

• S9 and S10, Supporting Information File 1). The dependence was linear (R = 0.997). The rate constant of the chemical reaction between the CuI complex and QC-Br5, i.e., the C’ reaction of the catalytic process (EC’), using the equations from the classic works of Savéant, Vianello [62] and Nicolson

Grip on complexity in chemical reaction networks

• Albert S. Y. Wong and
• Wilhelm T. S. Huck

Beilstein J. Org. Chem. 2017, 13, 1486–1497, doi:10.3762/bjoc.13.147

• were determined from kinetic studies in isolated individual reactions, allowing accurate simulations to test specific details of the experiments. We used the model to vary the rate constant that is induced by the changes to the molecular structure. First we show in Figure 7b that the tuning of R1