Polymer and small molecule mechanochemistry: closer than ever

• José G. Hernández

Beilstein J. Org. Chem. 2022, 18, 1225–1235, doi:10.3762/bjoc.18.128

• is observed [46]. This difference is believed to be related to the exertion of tensile forces along the glycosidic linkage of the polymer chain during ball milling, which may lower the activation energy for the depolymerization of chitin. Indeed, DFT calculations using the N-acetylglucosamine dimer

• as the model compound showed that the application of pulling forces to selected atoms in the dimer perturb the reaction, making the depolymerization easier to occur [45]. In contrast, no change in the activation energy of the deacetylation step was observed with the introduction of the pulling forces

• . The decrease in the activation energy for the mechanochemical depolymerization of chitin was attributed to force-induced conformational changes in the structure, which destabilize the reactant state upon the introduction of a sufficient pulling force (Figure 3). Evidently, ball milling techniques

Experimental and theoretical studies on the synthesis of 1,4,5-trisubstituted pyrrolidine-2,3-diones

• Nguyen Tran Nguyen,
• Vo Viet Dai,
• Nguyen Ngoc Tri,
• Luc Van Meervelt,
• Nguyen Tien Trung and
• Wim Dehaen

Beilstein J. Org. Chem. 2022, 18, 1140–1153, doi:10.3762/bjoc.18.118

• with an activation energy of about 10.7 kcal·mol−1. On the other hand, if the removal of one H2O molecule occurs from the O–H and N–H groups, IS5 transforms into IS9 through TS14 with a high barrier of 31.8 kcal·mol−1. IS9 is then converted to product 10ab based on the shifting of H atom from the O–H

• bond to N atom (Scheme 4). This process is thermodynamically favorable by a Gibbs free energy (ΔG) of −6.5 kcal·mol−1. In addition, a second way from 4a, through the TS5 transition state, to form the IS3 intermediate requires an activation energy of ca. 33.7 kcal·mol−1. IS3 could release one H2O

• IS8 can be omitted because it is so fast with a small activation energy of 2.1 kcal·mol−1), the formation of product 10ab is superior as compared to its isomer. In terms of the value of the rate constant k, the direction of 10ab formation is in the range of 103–106 times faster than 10ab-v2. The

Comparative study of thermally activated delayed fluorescent properties of donor–acceptor and donor–acceptor–donor architectures based on phenoxazine and dibenzo[a,j]phenazine

• Saika Izumi,
• Prasannamani Govindharaj,
• Anna Drewniak,
• Paola Zimmermann Crocomo,
• Satoshi Minakata,
• Leonardo Evaristo de Sousa,
• Piotr de Silva,
• Przemyslaw Data and
• Youhei Takeda

Beilstein J. Org. Chem. 2022, 18, 459–468, doi:10.3762/bjoc.18.48

• in electron density on the acceptor and the electron-donating power of POZ. Therefore, gradual increase of electron-donating strength brings T1 energy closer to the acceptor T1 energy and leads to a smaller EST gap. But, the activation energy Ea for the DF process, which was calculated from the

• Arrhenius plot obtained from the increase of the DF intensity against temperature, was lower for 1 (Ea = 27 meV) when compared to POZ-DBPHZ (Ea = 47 meV, Table 2) in Zeonex®. The directly determined activation energy of the D–A-type compound is half than that of the D–A–D compound, which is in contradiction

• to the ΔEST value (Table 2). If we support the observation with the DF/PF results that present a stronger TADF property for the mono-substituted derivative 1, the conclusion of misleading ΔEST comparison can be reached. To avoid confusion, a more effective way is to compare only the activation energy

Iridium-catalyzed hydroacylation reactions of C1-substituted oxabenzonorbornadienes with salicylaldehyde: an experimental and computational study

• Angel Ho,
• Austin Pounder,
• Krish Valluru,
• Leanne D. Chen and
• William Tam

Beilstein J. Org. Chem. 2022, 18, 251–261, doi:10.3762/bjoc.18.30

• hypothesized the reductive elimination step was the RDS for rhodium-catalyzed intramolecular hydroacylation reactions [74]. Based on the activation energy of the reverse step of reductive elimination 3TS2 (37.4–41.4 kcal/mol), we predict reductive elimination, and subsequent C–C formation, to be irreversible

• reductive elimination transition states, 2bTS3b is the more energetically accessible transition state with an energy barrier of 5.9 kcal/mol, that is 4.9 kcal/mol lower in energy compared to 2aTS3a. This large difference in activation energy (ΔΔG‡) between the two competing transition states offers

• an activation energy of 4.8 to 5.6 kcal/mol, respectively, to produce the aforementioned thermodynamically stable Ir(I) alkoxide intermediates INa3 and INb3. Based on the extremely high energy barrier required for acyl migration over hydride migration, we hypothesize iridium-catalyzed hydroacylation

Iron-catalyzed domino coupling reactions of π-systems

• Austin Pounder and
• William Tam

Beilstein J. Org. Chem. 2021, 17, 2848–2893, doi:10.3762/bjoc.17.196

Fritsch–Buttenberg–Wiechell rearrangement of magnesium alkylidene carbenoids leading to the formation of alkynes

• Tsutomu Kimura,
• Koto Sekiguchi,
• Akane Ando and
• Aki Imafuji

Beilstein J. Org. Chem. 2021, 17, 1352–1359, doi:10.3762/bjoc.17.94

• ), and the C1–Cl bond (C1···Cl: 3.22 Å) was cleaved. The activation energy for this reaction was estimated to be 14.9 kcal/mol. The IRC calculation revealed that the chlorine atom gradually dissociated from the carbenoid carbon atom as the phenyl group approached the carbenoid carbon atom [38][39]. Then

Valorisation of plastic waste via metal-catalysed depolymerisation

• Francesca Liguori,
• Carmen Moreno-Marrodán and
• Pierluigi Barbaro

Beilstein J. Org. Chem. 2021, 17, 589–621, doi:10.3762/bjoc.17.53

• °C provided a hydrocarbon oil in 92% yield, 71.4% of which were attributable to styrene monomer [156]. A decrease of 56 kJ mol−1 for the activation energy of PS depolymerisation was calculated in the presence of the catalysts. More recently, high-porosity montmorillonite (Mt) was used to prepare Mg

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

• analogue (dC–O = 1.438 Å) and strongly inhibits the formation of the α-(trifluoromethyl)bisarylcarbenium ion, as illustrated by the higher activation energy needed for the dehydration (ΔECF3 = 21.0 kcal⋅mol−1 vs ΔECH3 = 14.8 kcal⋅mol−1 at the B3LYP/6-31+G(d,p) level). On the other hand, the first arylation

The preparation and properties of 1,1-difluorocyclopropane derivatives

• Kymbat S. Adekenova,
• Peter B. Wyatt and
• Sergazy M. Adekenov

Beilstein J. Org. Chem. 2021, 17, 245–272, doi:10.3762/bjoc.17.25

• opposite to the CF2 moiety, which was followed by the recyclization of the intermediate diradical (Scheme 42). The activation energy for the rearrangement of 90 was lower by 9.4 kcal/mol than for the parent hydrocarbon system 92. The activation energy of the trans-isomer 91 was greater than that of cis

Pentannulation of N-heterocycles by a tandem gold-catalyzed [3,3]-rearrangement/Nazarov reaction of propargyl ester derivatives: a computational study on the crucial role of the nitrogen atom

• Giovanna Zanella,
• Martina Petrović,
• Dina Scarpi,
• Ernesto G. Occhiato and
• Enrique Gómez-Bengoa

Beilstein J. Org. Chem. 2020, 16, 3059–3068, doi:10.3762/bjoc.16.255

• species to the alkyne 5, as in I (Figure 2). The first step TS1 has a low activation energy (ΔG‡ = 12.2 kcal⋅mol−1) to form the unstable cyclic intermediate II. This short-lived species rapidly reopens through TS2 (ΔΔG‡ = 8.1 kcal⋅mol−1) to give the pentadienyl cation III, which presents a high stability

• derivative 1 (Figure 3), confirming the differences that the 2- and 3-substitution, respectively, exert in the reaction outcome. Starting with V, the acetate rearrangement is rate determining (ΔΔG‡ = 14.2 kcal⋅mol−1), and more importantly, the activation energy for the cyclization in TS6 is very low (ΔΔG

• conjugation. However, according to the energy profile, this observation does not have a reflection in the deprotonation step, which seems to be affected partially by the steric hindrance around the two hydrogen atoms, being clearly higher in Ha (a 2.2 kcal⋅mol−1 higher activation energy of TS10 than for TS8

Dirhamnolipid ester – formation of reverse wormlike micelles in a binary (primerless) system

• David Liese,
• Hans Henning Wenk,
• Xin Lu,
• Jochen Kleinen and
• Gebhard Haberhauer

Beilstein J. Org. Chem. 2020, 16, 2820–2830, doi:10.3762/bjoc.16.232

• contour length decreases with increasing temperature according to the following Arrhenius equations, Equation 9 and Equation 10 [32]: where Ea is the activation energy, R is the gas constant, and A is the preexponential factor. The semilogarithmic plots of η0 and τR versus 1/T (Figure 7b and c) indicate

• an Arrhenius plot like behavior, and the activation energy calculated from the slopes of the two plots equals 119 kJ/mol, close to those found in other wormlike micelles [48][49][50][51][52]. According to Equation 10, G0 is independent of the temperature, which is not true in our case. This might be

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• photocatalysts (PCs) generate excited state substrates that can then undergo reactions that would be impossible in the ground state [4]. A challenge for enantioselective catalysis is stifling the racemic background reaction, which is generally achieved through a lower activation energy for the catalysed process

Photosensitized direct C–H fluorination and trifluoromethylation in organic synthesis

• Shahboz Yakubov and
• Joshua P. Barham

Beilstein J. Org. Chem. 2020, 16, 2151–2192, doi:10.3762/bjoc.16.183

• reactions. Keywords: C–H activation; energy transfer; fluorination; photocatalysis; photosensitization; visible light; Review 1 Introduction 1.1 Importance of direct C–H fluorination/trifluoromethylation and photosensitization in organic synthesis 1.1.1 Importance of fluorine atoms in organic molecules

• electrophilic fluorinating source. Thereafter, the AQN–Selectfluor® exciplex abstracts a hydrogen atom from the C3 position of pentane to form a secondary radical, a Selectfluor® N-radical cation, HF and AQN. The activation energy barrier relative to RC2 was only 9.9 kcal⋅mol−1 (Scheme 17). The Selectfluor® N

• -radical cation can abstract a hydrogen atom from the C3 2° C–H bond of another pentane molecule in an overall exergonic process with an activation energy barrier of 2.0 kcal⋅mol−1 from Int4 to afford Int5 and ultimately Int6 (Scheme 18). The authors did not mention the possibility of a mechanism whereby

One-pot synthesis of oxazolidinones and five-membered cyclic carbonates from epoxides and chlorosulfonyl isocyanate: theoretical evidence for an asynchronous concerted pathway

• Esra Demir,
• Ozlem Sari,
• Yasin Çetinkaya,
• Ufuk Atmaca,
• Safiye Sağ Erdem and
• Murat Çelik

Beilstein J. Org. Chem. 2020, 16, 1805–1819, doi:10.3762/bjoc.16.148

• the transition state and so lowering the activation energy barrier (Figure 3a). On the other hand, stabilization of the benzylic cation is not possible along the IRC path for TS1′ (Figure 3b), since the bond distance C2–C(Ph) is found as around 1.50 Å showing a single bond character. This can be the

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

• characterized accordingly as stationary points (minima or maxima) on the corresponding potential energy surfaces (PESs). Equations 1–5 were used for the calculations of the rates and the physicochemical data of the N−O bond dissociation of the most active compound 12 in radicals. The corresponding activation

• 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

The reaction of arylmethyl isocyanides and arylmethylamines with xanthate esters: a facile and unexpected synthesis of carbamothioates

• Narasimhamurthy Rajeev,
• Toreshettahally R. Swaroop,
• Ahmad I. Alrawashdeh,
• Shofiur Rahman,
• Abdullah Alodhayb,
• Seegehalli M. Anil,
• Kuppalli R. Kiran,
• Chandra,
• Paris E. Georghiou,
• Kanchugarakoppal S. Rangappa and
• Maralinganadoddi P. Sadashiva

Beilstein J. Org. Chem. 2020, 16, 159–167, doi:10.3762/bjoc.16.18

• been reported for other carbene hydrolyses [37][38][39]. As can be seen from Figure 5, the highest activation energy barrier is 42.2 kJ/mol. We had previously considered an alternative mechanism in which a benzylic proton is instead removed by the base [40]. For the previous mechanism, which we have

• now recalculated at the B3LYP/6-311++G(d,p) level of theory in DMF (using a PCM, see Scheme S1 and Figures S15 and S16, Supporting Information File 1), an activation energy barrier of 20.6 kJ/mol was obtained for the formation of the resulting benzylic α-carbanion and H2. This benzylic α-carbanion

• anion, either by a stepwise or by a concerted five-membered ring transition state, followed by several subsequent steps could lead to the observed products. However, the mechanism involving all of those steps required an improbably higher overall activation energy of 173.9 kJ/mol (Figure S16, Supporting

[1,3]/[1,4]-Sulfur atom migration in β-hydroxyalkylphosphine sulfides

• Katarzyna Włodarczyk,
• Piotr Borowski and
• Marek Stankevič

Beilstein J. Org. Chem. 2020, 16, 88–105, doi:10.3762/bjoc.16.11

• , leading to intermediate I, and the overall transformation proceeded with remarkable stabilization. In the next step, dissociation of the C−O bond occurred through transition state II, finally leading to alkene III. The activation energy for the C−O bond cleavage in 20, a tertiary alcohol, was similar to

• activation energy (31.0 kcal/mol), which was followed by slight stabilization of the formed γ-carbanion (−6.1 kcal/mol). The latter should then undergo intramolecular C–S bond formation, followed by hydrolysis, finally leading to the rearranged product. It was then decided to investigate the

Understanding the role of active site residues in CotB2 catalysis using a cluster model

• Keren Raz,
• Ronja Driller,
• Thomas Brück,
• Bernhard Loll and
• Dan T. Major

Beilstein J. Org. Chem. 2020, 16, 50–59, doi:10.3762/bjoc.16.7

• . Here, the energy gain was likely due to the fact that the carbocation in intermediate C was located 4.21 Å away from the pyrophosphate group, which stabilized it (Table 1 and Figure 2). Moreover, π–cation interactions with F107 contributed to the stabilization as well. The activation energy for the

• site model. The required activation energy to form D was 4.1 kcal/mol lower in the active site model than the gas phase, likely due to π–cation interactions with F107 and F149 and greater proximity to the negatively charged pyrophosphate group. Another conformational difference between the gas phase

• formed interactions with N103, W186, and especially with I181. The relative energy difference between H and I was also similar in the gas phase and in the enzyme model. However, the activation energy was higher by 3.0 kcal/mol in the active site model, possibly due to steric effects. I was stabilized via

Microwave-assisted synthesis of 2-substituted 4,5,6,7-tetrahydro-1,3-thiazepines from 4-aminobutanol

• María C. Mollo,
• Natalia B. Kilimciler,
• Juan A. Bisceglia and
• Liliana R. Orelli

Beilstein J. Org. Chem. 2020, 16, 32–38, doi:10.3762/bjoc.16.5

• alcohols. In both cases, PPSE would activate the OH group for nucleophilic attack, and the plausible reaction mechanism would involve an intramolecular SN2-type displacement. However, the lower reactivity of the carboxamide oxygen (as an O-nucleophile), together with the comparatively high activation

• energy associated to the formation of a seven-membered heterocycle would favour the competitive ring closure leading to the five-membered ring (Scheme 2, reaction a), which would involve attack of the carboxamide nitrogen to the ω-carbon. On the other hand, the higher nucleophilicity of the sulfur in the

Click chemistry towards thermally reversible photochromic 4,5-bisthiazolyl-1,2,3-triazoles

• Chenxia Zhang,
• Kaori Morinaka,
• Mahmut Kose,
• Takashi Ubukata and
• Yasushi Yokoyama

Beilstein J. Org. Chem. 2019, 15, 2161–2169, doi:10.3762/bjoc.15.213

• visible region was observed at three different temperatures. The first order reaction rate constants of the thermal back reactions at different temperatures were then determined. Arrhenius plots of ln k against 1/T gave pre-exponential factors (A) and Arrhenius activation energy Ea of these thermally

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

• %) 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

• the cycloreversion was determined by linear fit of ln (k) as a function of 1/T. The cycloreversion of QC 1b to NBD 1a has an activation energy of 111 kJ mol−1 (degassed deuterated benzene). The switching efficiency of NBD 2a to QC 2b is quantitative (≈100%) after irradiation with 385 nm, whereas the

Water inside β-cyclodextrin cavity: amount, stability and mechanism of binding

• Stiliyana Pereva,
• Valya Nikolova,
• Silvia Angelova,
• Tony Spassov and
• Todor Dudev

Beilstein J. Org. Chem. 2019, 15, 1592–1600, doi:10.3762/bjoc.15.163

• -CD (5.3 ± 0.9 and 7.2 ± 0.6 kcal mol−1 H2O for the first and second water release, respectively) [19]. The variation of the endothermic peak maximum with the rate of heating in DTA permits an activation energy of β-CD dehydration about 60 kJ mol−1 (14.3 kcal mol−1) to be also estimated, applying the

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

• modification from that originally proposed by Butyagin [29], in which the rate is proportional to the frequency of collision (A), and the initial activation energy (E0) for the chemical reaction. For systems in which the reaction is limited by the distribution of mechanical energy, it has been suggested that

• the relative activation energy of mechanochemical reactions or facilitates the energy accumulation when compared to experiments run under ball mill NG conditions. These two explanations are not mutually exclusive. While the activation may be facilitated by partial dissolution of components, it is

• the accumulation of energy and mixing effects. The induction time is significantly shorter under LAG conditions. This can be explained by either a lower activation energy under LAG conditions, or aggregation playing a more important role under NG conditions. The crystal breaking process is likely to

Silanediol versus chlorosilanol: hydrolyses and hydrogen-bonding catalyses with fenchole-based silanes

• Falco Fox,
• Jörg M. Neudörfl and
• Bernd Goldfuss

Beilstein J. Org. Chem. 2019, 15, 167–186, doi:10.3762/bjoc.15.17

• the PCM model is used [69][70]. The free Gibbs energies of the respective structures are discussed. The activation energy (Ea) is the difference between the educt and the TS and the reaction energy (Er) is the difference between the educts and products of the respective steps. The mechanism of

• ) 8ax (Figure 5). For the TS of the front1 attack mechanism, the lowest activation energy (Ea = 32.6 kcal mol−1, Table 3, entry 1, Figure 5 and Figure 6) is found, closely followed by the front2 attack mechanism (Ea = 33.2 kcal mol−1, Table 3, entry 2, Figure 5 and Figure 7). The side attack mechanism

• hydrolysis to BIFOXSi(OH)2 (9) and comparison with glycoxydichlorosilane. The activation energy (Ea) is the difference of the free Gibbs energy of the educt and the TS and the reaction energy (Er) is the difference the free Gibbs energies of the educts and products of the respective steps side and front1

Some mechanistic aspects regarding the Suzuki–Miyaura reaction between selected ortho-substituted phenylboronic acids and 3,4,5-tribromo-2,6-dimethylpyridine

• Piotr Pomarański,
• Piotr Roszkowski,
• Jan K. Maurin,
• Armand Budzianowski and
• Zbigniew Czarnocki

Beilstein J. Org. Chem. 2018, 14, 2384–2393, doi:10.3762/bjoc.14.214

• then the activation energy of rotation (ΔG#) using the Eyring equation [40]. Therefore, we estimated the respective energy barriers to be as ΔG#7-8 = 21.7 kcal/mol and ΔG#9-10 = 23.4 kcal/mol (Table 3). In order to verify our observations that may result from additional O-chelation of palladium we used