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

Hydroarylations by cobalt-catalyzed C–H activation

• Rajagopal Santhoshkumar and
• Chien-Hong Cheng

Beilstein J. Org. Chem. 2018, 14, 2266–2288, doi:10.3762/bjoc.14.202

• activation of F2 give intermediate F3. Finally, protonolysis provides the desired product 45 and an active Co(III)Cp*. Due to the low activation energy, the thermodynamically less stable Z diastereomer is preferred in the reaction, which is in contrast to the rhodium(III)-catalyzed reaction [82]. Later, Li

Water-soluble SNS cationic palladium(II) complexes and their Suzuki–Miyaura cross-coupling reactions in aqueous medium

• Alphonse Fiebor,
• Richard Tia,
• Banothile C. E. Makhubela and
• Henok H. Kinfe

Beilstein J. Org. Chem. 2018, 14, 1859–1870, doi:10.3762/bjoc.14.160

• parameterised for transition metal systems and has been found to give geometries, relative energies and activation energy trends in good agreement with high-level density functional theory (DFT) results at a fraction of the computational cost [45][46]. It is therefore adequate for studies in which the prime

• ) (Figure 4). The Pd–Br distance is calculated to be 2.60 Å and is consistent with similar Pd–Br bond lengths found in the literature for single crystal X-ray structures [47][48]. This oxidative addition step involves a low activation energy barrier of −43.9 kcal/mol. This negative energy implies that there

DFT calculations on the mechanism of copper-catalysed tandem arylation–cyclisation reactions of alkynes and diaryliodonium salts

• Tamás Károly Stenczel,
• Ádám Sinai,
• Zoltán Novák and
• András Stirling

Beilstein J. Org. Chem. 2018, 14, 1743–1749, doi:10.3762/bjoc.14.148

• aryl transfer is followed by the O–C bond formation which results in the oxazoline ring. This step requires a moderate 6.5 kcal/mol activation energy (TSrcB) which indicates that this step is very fast under the reaction conditions. After the ring is formed the system is stabilised by releasing a large

Rh(II)-mediated domino [4 + 1]-annulation of α-cyanothioacetamides using diazoesters: A new entry for the synthesis of multisubstituted thiophenes

• Jury J. Medvedev,
• Ilya V. Efimov,
• Yuri M. Shafran,
• Vitaliy V. Suslonov,
• Vasiliy A. Bakulev and
• Valerij A. Nikolaev

Beilstein J. Org. Chem. 2017, 13, 2569–2576, doi:10.3762/bjoc.13.253

• catalyst, proton transfer and, finally, 1,3-shift of alkoxycarbonyl group in the intermediate imine D to produce thiophenes 3 and 5. However, this pathway seems to be less probable, since intramolecular cyclization B → C should have lower activation energy relative to intermolecular interaction of C=S

A mechanochemical approach to access the proline–proline diketopiperazine framework

• Nicolas Pétry,
• Hafid Benakki,
• Eric Clot,
• Pascal Retailleau,
• Farhate Guenoun,
• Fatima Asserar,
• Chakib Sekkat,
• Thomas-Xavier Métro,
• Jean Martinez and
• Frédéric Lamaty

Beilstein J. Org. Chem. 2017, 13, 2169–2178, doi:10.3762/bjoc.13.217

• proton from the amine and to facilitate the departing of the methoxy group by transferring a proton. The activation energy from 16a-solv was computed to be ΔG# = 22.8 kcal mol−1, in good agreement with an easy reaction at room temperature. The reaction was strongly exoergic with ΔG = −17.3 kcal mol−1 and

Are boat transition states likely to occur in Cope rearrangements? A DFT study of the biogenesis of germacranes

• José Enrique Barquera-Lozada and
• Gabriel Cuevas

Beilstein J. Org. Chem. 2017, 13, 1969–1976, doi:10.3762/bjoc.13.192

• [64]. The stability of the wave functions of all the transition states was checked. An unrestricted wave function was used to calculate the activation energy of the cis/trans isomerization of the (Z,E)-germacranolide. All energies were reported with zero-point energy corrections and all TS geometries

• °C (temperature at which the biogenetic transformation was performed) [36]. In case of path N, the activation energy of 4 to reach the Cope TS (TS4-2) is 25.9 kcal/mol, and the relative energy of TS4-2 is 35.0 kcal/mol. The chair TS was, as expected, less energetic than the boat TS. However, before

• difference between TS5-3 and TS6-3 is almost the same than in TS1b-2 and TS4-2, but the energy of boat TS (TS5-3) decreases to 38.0 kcal/mol, which is small enough to be overcome at 200 °C. Thus, hemiacetalization lowers the activation energy of the boat Cope TS which allows the reaction to be completed at a

Mechanochemical N-alkylation of imides

• Anamarija Briš,
• Mateja Đud and
• Davor Margetić

Beilstein J. Org. Chem. 2017, 13, 1745–1752, doi:10.3762/bjoc.13.169

• imides and bromides were used to determine the activation energies. It was found that for benzyl and ethyl bromides the activation energy differences are 2–3 kcal mol−1 in favor of the N1 positions in uracil and 7,8-dimethylalloxazine. These calculations are in good accordance with the experimentally

Dimerization reactions of aryl selenophen-2-yl-substituted thiocarbonyl S-methanides as diradical processes: a computational study

• Michael L. McKee,
• Grzegorz Mlostoń,
• Katarzyna Urbaniak and
• Heinz Heimgartner

Beilstein J. Org. Chem. 2017, 13, 410–416, doi:10.3762/bjoc.13.44

• reaction leading to the intermediate delocalized diradical occurs via a transition state with very low activation energy. On the other hand, the calculated spin densities suggest the preference of the formation of the six-membered ring of the 1,4-dithiane, and this reaction course is observed in the case

• of thiobenzophenone S-methanide 1 (Ar1 = Ar2 = Ph). On the other hand, in the hetaryl functionalized thiocarbonyl S-methanide 8 the lowest activation energy was found for the C–C bond formation between C(2) atoms of the selenophen-2-yl ring, leading to the twelve-membered macrocyclic product 9

NMR reaction monitoring in flow synthesis

• M. Victoria Gomez and
• Antonio de la Hoz

Beilstein J. Org. Chem. 2017, 13, 285–300, doi:10.3762/bjoc.13.31

• , temperature gradients, starting concentration and residence times, against a reaction conversion model enables the reaction order, rate constant, Arrhenius parameters, pre-exponential factor, and activation energy values to be determined in a rapid manner from one single flow experiment. The two latter

Direct arylation catalysis with chloro[8-(dimesitylboryl)quinoline-κN]copper(I)

• Sem Raj Tamang and
• James D. Hoefelmeyer

Beilstein J. Org. Chem. 2016, 12, 2757–2762, doi:10.3762/bjoc.12.272

• pass through an intermediate with a formal oxidation state of +3, which may undoubtedly have a large activation energy. Rather, a concerted pathway through a 4-membered transition state will have less localization of charge. A concerted process for coupling of nucleophiles with arylhalides on copper

Synthesis, dynamic NMR characterization and XRD studies of novel N,N’-substituted piperazines for bioorthogonal labeling

• Constantin Mamat,
• Marc Pretze,
• Matthew Gott and
• Martin Köckerling

Beilstein J. Org. Chem. 2016, 12, 2478–2489, doi:10.3762/bjoc.12.242

• observed for all nitro derivatives due to the limited change of the piperazine chair conformation. Therefore, two coalescence points were determined and their resulting activation energy barriers were calculated using 1H NMR. To support this result, single crystals of 1-(4-nitrobenzoyl)piperazine (3a

• the two expected signals at increased temperatures (>67 °C). At the coalescence temperature Tc, the exchange rate is given by the equation kexc = π·Δν/21/2 [25]. As a result, the activation energy (ΔG#exp) to the amide bond rotation can be calculated using the Eyring equation [14][16]. In general

Combined experimental and theoretical studies of regio- and stereoselectivity in reactions of β-isoxazolyl- and β-imidazolyl enamines with nitrile oxides

• Ilya V. Efimov,
• Marsel Z. Shafikov,
• Nikolai A. Beliaev,
• Natalia N. Volkova,
• Tetyana V. Beryozkina,
• Wim Dehaen,
• Zhijin Fan,
• Viktoria V. Grishko,
• Gert Lubec,
• Pavel A. Slepukhin and
• Vasiliy A. Bakulev

Beilstein J. Org. Chem. 2016, 12, 2390–2401, doi:10.3762/bjoc.12.233

• geometry distortion of nitrile oxide 6a has a major contribution to the activation energy barrier, whereas the geometry distortion energy of the enamine is minor. Also, the analysis shows close values of the orbital interaction energy for all the found transition states (Table 1). It should be noted the

• , resulting in smaller geometry distortions. Thus, the lowest electronic activation energy of transition state 1a_1−3a among all calculated transition states is a result of both, an easier attainability of the state governed by longer range orbital interactions, and a smaller value of the total geometry

• experimentally observed regioisomer. The reason for the lowest electronic activation energy leading to the experimentally observed product among all calculated transition states, is caused by both relatively better stabilizing orbital interactions and relatively smaller geometry distortion energies. Experimental

Methylenelactide: vinyl polymerization and spatial reactivity effects

• Judita Britner and
• Helmut Ritter

Beilstein J. Org. Chem. 2016, 12, 2378–2389, doi:10.3762/bjoc.12.232

• our postulated radical formation process into account, the dipolar solvent DMF stabilizes the polar educt MLA more than the less polar 1,4-dioxane. Since the formed radical is planar and less polar, the activation energy to this radical formation must be higher in DMF than in 1,4-dioxane [16

Stereo- and regioselectivity of the hetero-Diels–Alder reaction of nitroso derivatives with conjugated dienes

• Lucie Brulíková,
• Aidan Harrison,
• Marvin J. Miller and
• Jan Hlaváč

Beilstein J. Org. Chem. 2016, 12, 1949–1980, doi:10.3762/bjoc.12.184

• distance between C–O to that between C–N was more than one, whereas in the product, this was reversed (Figure 2). Using RB3LYP/6-31G*//RB3LYP/6-31G* theory, for the model reaction between HNO and butadiene, the favored endo-transition state activation energy was found to be 8.6 kcal/mol lower than for the

• exo state. When a number of substituted nitroso compounds were subsequently investigated, the endo-transition state persisted in having a lower activation energy for all compounds tested, relative to the exo. This preference is due to the “exo lone pair effect” resulting from the repulsion between the

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

• Michael Nonnenmacher,
• Dominik M. Buck and
• Doris Kunz

Beilstein J. Org. Chem. 2016, 12, 1884–1896, doi:10.3762/bjoc.12.178

• containing the tert-butyl substituted dipyridocarbene ligand dipiytBu. For iridium complexes [Ir(CO)2Cl(NHC)] (NHC = imidazolidin-2-ylidene) a preferred cis-CO exchange was reported and an activation energy of 12.7–12.9 kcal/mol was determined by NMR spectroscopy for this process [60]. However, an exchange

Cp₂TiCl/D₂O/Mn, a formidable reagent for the deuteration of organic compounds

• Antonio Rosales and
• Ignacio Rodríguez-García

Beilstein J. Org. Chem. 2016, 12, 1585–1589, doi:10.3762/bjoc.12.154

• for both species, which in turn, is dependent upon the zero point for the vibrational energy of both isotopic molecules. As the mass of deuterium is about twice the mass of hydrogen there is a larger activation energy for the C–D bond dissociation than for the C–H bond [4]. The KIE observed allows

Ring-whizzing in polyene-PtL₂ complexes revisited

• Oluwakemi A. Oloba-Whenu,
• Thomas A. Albright and
• Chirine Soubra-Ghaoui

Beilstein J. Org. Chem. 2016, 12, 1410–1420, doi:10.3762/bjoc.12.135

• , therefore, 6 can always interact with it. On the other hand, the LUMO in the π system may not always have the correct symmetry to interact with the b2 orbital on ML2 and it is the evolution of this overlap that has an important impact on the reaction path and activation energy. We will also have an occasion

• halves of the pentalene ligand. The Pt(dpe) group can migrate from one ring to the other via the η3 structure, 42. Again the activation energy associated with the transition state 43 is predicted to be small at 8.6 kcal/mol. We anticipate that pentalene–Pt(PR3)2 will be a highly fluxional molecule. The

• point shortly. One might think that the overlap between b2 ML2 and the LUMO, 49, from the η2 ground state to η4 will be retained and, thus, the activation energy will be small. However, note that at η4 the overlap between the filled b1 fragment orbital, 7, and the au HOMO on C10F8 is turned on and this

Conjugate addition–enantioselective protonation reactions

• James P. Phelan and
• Jonathan A. Ellman

Beilstein J. Org. Chem. 2016, 12, 1203–1228, doi:10.3762/bjoc.12.116

• enantioselectivity is determined by the ratio of Z- to E-enamine, which in turn depends on the activation energy difference in the irreversible C–C bond forming steps. Having developed an enamine catalysis system where the iminium’s conformation in the conjugate addition step determines the enantioselectivity of the

Enantioselective carbenoid insertion into C(sp³)–H bonds

• J. V. Santiago and
• A. H. L. Machado

Beilstein J. Org. Chem. 2016, 12, 882–902, doi:10.3762/bjoc.12.87

• attributed to the stabilization of the partial positive charge on the divalent carbon of the transition state TS-40 provided by the phenyl donor group. The second reason concerns the large difference between the activation energy of these reactions which relies on the development of steric strain through the