Unraveling the role of prenyl side-chain interactions in stabilizing the secondary carbocation in the biosynthesis of variexenol B

• Moe Nakano,
• Rintaro Gemma and
• Hajime Sato

Beilstein J. Org. Chem. 2023, 19, 1503–1510, doi:10.3762/bjoc.19.107

• group. However, such a large conformational change is unlikely to occur within the enzyme. Therefore, the Curtin–Hammett principle is not applicable to this system. To investigate the details of carbocations and hyperconjugations in the variexenol B biosynthetic pathway, we carried out a bond length

Exploring the role of halogen bonding in iodonium ylides: insights into unexpected reactivity and reaction control

• Carlee A. Montgomery and
• Graham K. Murphy

Beilstein J. Org. Chem. 2023, 19, 1171–1190, doi:10.3762/bjoc.19.86

• these preferential outcomes is not understood, and may derive from rapidly equilibrating species (e.g., 2 and 3) reacting under Curtin–Hammett control, from hard/soft principles dictating the Lewis base’s selectivity with either the stronger (e.g., 3) or weaker (e.g., 2) σ-hole, or possibly from other

Heteroleptic metallosupramolecular aggregates/complexation for supramolecular catalysis

• Prodip Howlader and
• Michael Schmittel

Beilstein J. Org. Chem. 2022, 18, 597–630, doi:10.3762/bjoc.18.62

• geometrical or constitutional situation at the binding sites [92]. In a detailed recent study, the finding of a Hammett correlation in such nanorotors corroborated that a rate-determining dissociation at the rotator–metal binding interaction dictated the rotational speed [93]. Reducing Product Inhibition (RPI

Mechanistic studies of the solvolysis of alkanesulfonyl and arenesulfonyl halides

• Malcolm J. D’Souza and
• Dennis N. Kevill

Beilstein J. Org. Chem. 2022, 18, 120–132, doi:10.3762/bjoc.18.13

• . The solvolyses of these additional types of sulfur(VI) substrates will be the topics of a future review. Keywords: correlation; Grunwald–Winstein equation; Hammett equation; mechanism; solvolysis; sulfonyl halides; Review 1. Introduction to solvolyses of sulfonyl halides In medicinal chemistry

• to synchronous bond-making and bond-breaking, with a small negative charge development on the sulfur at the transition state, for the solvolytic process. Subsequent to this 1927 study, Hammett reported [5][6] in 1937, the development of the Hammett equation, involving the consideration of a scale

• Hammett substituent values obtained can be applied to the solvolyses and other reactions of substrates varying only in the aromatic ring substituent that they carry, under otherwise identical conditions as regards solvent and temperature. Frequently, as in the reactions to be reviewed in this subsection

Breaking paracyclophane: the unexpected formation of non-symmetric disubstituted nitro[2.2]metaparacyclophanes

• Suraj Patel,
• Tyson N. Dais,
• Paul G. Plieger and
• Gareth J. Rowlands

Beilstein J. Org. Chem. 2021, 17, 1518–1526, doi:10.3762/bjoc.17.109

• might better to compare the Hammett acidity function, H0, as this is more suitable for concentrated acids. On this scale triflic acid H0 = −14.1, perchloric acid H0 = −13, and sulfuric acid H0 = −12.0 are more similar and might explain why nitration conditions cause rearrangement [67]. Alternatively

Icilio Guareschi and his amazing “1897 reaction”

• Gian Cesare Tron,
• Alberto Minassi,
• Giovanni Sorba,
• Mara Fausone and
• Giovanni Appendino

Beilstein J. Org. Chem. 2021, 17, 1335–1351, doi:10.3762/bjoc.17.93

• science emphasizes focusing and prizes specialists over generalists. Nowadays, scientists "feed on one plant only", but their ancestors had a more varied diet. Zonation occurred rapidly in chemistry during the first decades of the 20th century, and Hammett, in the introduction of his Physical Organic

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

• accounting for the electron-donating or -withdrawing ability of substituents are of major importance in synthetic organic chemistry. The Hammett constant σ for a variety of substituents [18][19] and improved values, known as σ+, furnished by Brown et al. [20][21] – some of which are listed in Table 1 for

• solvolysis of aryl derivatives 13a–i to highlight the importance of the nature of the aromatic substituent on the solvolysis rate (Figure 2) [43]. As anticipated, a faster rate was observed for electron-donating groups, while electron-withdrawing groups slowed the process down. Plotting the Hammett–Brown

• obtained. Similarly, the nature of the aromatic substituent influenced the solvolysis rate, with an observed acceleration for substrates adorned with electron donor substituents and a deceleration for those carrying electron-withdrawing substituents. The Hammett–Brown correlation gave a straight line, with

Exploring the possibility of using fluorine-involved non-conjugated electron-withdrawing groups for thermally activated delayed fluorescence emitters by TD-DFT calculation

• Dongyang Chen and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2021, 17, 210–223, doi:10.3762/bjoc.17.21

• acceptors correlate linearly to the Hammett substituent constant, σp, (Figure 2c) [35]. All of these fluorinated acceptors are much weaker than the more commonly investigated benzonitrile (BN, −1.30 eV), triphenyltriazine (TRZ, −1.72 eV) and benzophenone (BP, −1.58 eV) acceptors. These results indicate that

• , b) Calculated HOMO, LUMO, S1 and T1 energies, as well as HOMO and LUMO topologies of BN, TRZ and BP (isovalue = 0.02). c) Hammett para substituent values (σp) relationship with the calculated LUMO energies for fluorine-containing acceptors PhCF3, PhOCF3, PhOSCF3, and PhSF5. Calculated HOMO, LUMO, S1

Benzothiazolium salts as reagents for the deoxygenative perfluoroalkylthiolation of alcohols

• Armin Ariamajd,
• Nils J. Gerwien,
• Benjamin Schwabe,
• Stefan Dix and
• Matthew N. Hopkinson

Beilstein J. Org. Chem. 2021, 17, 83–88, doi:10.3762/bjoc.17.8

• trifluoromethylthio group (SCF3), for example, is attracting considerable attention due to its strong lipophilicity-enhancing influence (Hansch constant π = 1.44) and electron-withdrawing properties (Hammett constants σp = 0.50 and σm = 0.40) [7][8][9][10][11][12]. On the other hand, longer-chain perfluoroalkylthio

Synthesis of 4-substituted azopyridine-functionalized Ni(II)-porphyrins as molecular spin switches

• Jannis Ludwig,
• Tobias Moje,
• Fynn Röhricht and
• Rainer Herges

Beilstein J. Org. Chem. 2020, 16, 2589–2597, doi:10.3762/bjoc.16.210

• diamagnetic (or the paramagnetic) species in the two photostationary states. The intermolecular association constants of pyridines as axial ligands to Ni-porphyrins as a function of their para substituents usually follow the Hammett relationship [5][12]. Hence, the association constants should also be

• File 1. “Record player” approach for molecular spin switching. a) General principle b) Variation of the substituent R in 4-position of the pyridine unit. Record player molecules synthesized in this work (1f–j) are highlighted. Hammett plot of the investigated pyridine substituents [36]. UV–vis spectra

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

• ester. A common trend between mono- and difluorination was that increasing the electron-releasing ability of a substituent para to the benzylic position decreased the product yield. It is known that polarity matching effects are negligible for benzylic HAT and that BDEs do not correlate with Hammett

Natural dolomitic limestone-catalyzed synthesis of benzimidazoles, dihydropyrimidinones, and highly substituted pyridines under ultrasound irradiation

• Kumar Godugu,
• Venkata Divya Sri Yadala,
• Mohammad Khaja Mohinuddin Pinjari,
• Trivikram Reddy Gundala,
• Lakshmi Reddy Sanapareddy and
• Chinna Gangi Reddy Nallagondu

Beilstein J. Org. Chem. 2020, 16, 1881–1900, doi:10.3762/bjoc.16.156

• sedimentary rocks present in this area. Catalyst characterization The NDL catalyst was ground into a fine powder and then sieved in a 200-mesh sieve. The chemical composition of the catalyst was determined by standard quantitative analysis. The basic strength of the catalyst was analyzed by using Hammett

• indicators. The catalyst was characterized by XRD, IR, Raman, SEM, and EDAX analysis. The chemical composition of the NDL was determined by adopting a standard quantitative analysis [75]. The obtained results are summarized in Table 1. The basic strength of the NDL catalyst (H_) was measured using Hammett

• indicators, namely bromothymol blue (H_ = 7.2), phenolphthalein (H_ = 9.8), 2,4-dinitroaniline (H_ = 15.0), and nitroaniline (H_ = 18.4) as Hammett indicators. In each case, 5 mL of a methanolic solution of the Hammett indicator was added to 50 mg of the catalyst, shaken well, and then allowed to equilibrate

Models of necessity

• Timothy Clark and
• Martin G. Hicks

Beilstein J. Org. Chem. 2020, 16, 1649–1661, doi:10.3762/bjoc.16.137

• for calculating partial charges. Such methods, although seemingly outdated, are uniquely fast because of the limits in computer power when they were developed. Chematica [58] uses among other parameters the Hammett substituent constant [59] developed in the 1930s. These uses of group contribution

• ], developing a ML-suitable model for intermolecular interactions is feasible. As noted above, old models such as Gasteiger–Marsili [38][39] charges or Hammett constants [59] have been used in recent applications. This development ignores the immense improvements in hard- and software of the last four decades

Fluorohydration of alkynes via I(I)/I(III) catalysis

• Jessica Neufeld,
• Constantin G. Daniliuc and
• Ryan Gilmour

Beilstein J. Org. Chem. 2020, 16, 1627–1635, doi:10.3762/bjoc.16.135

• substrate, a Hammett analysis was conducted. To that end, log(kR/kH) was plotted against the Hammett σ−p parameter for the para-substituted pentynyl benzoates with R = OMe, H, Cl, and NO2 (Figure 4A). However, the resulting slope of the Hammett plot was essentially zero (ρ ≈ 0). In contrast, a second

• Hammett analysis in which the substitution of the iodoarene catalyst was explored (R = Me, H, Cl, and Br) revealed a build-up of positive charge (ρ < 0) during the rate-determining step (Figure 4B). From a methodological perspective, this study provides a conceptual platform to unmask the valuable α

• Hammett analyses, have identified key determinants that are essential for reaction efficiency. X-ray crystal structure analysis of ketone 2 reveals a dihedral angle of φO=C-C-F = −3.7°. Although the origin of this specificity requires clarification, this fluorohydration reaction constitutes a rare case of

NHC-catalyzed enantioselective synthesis of β-trifluoromethyl-β-hydroxyamides

• Alyn T. Davies,
• Mark D. Greenhalgh,
• Alexandra M. Z. Slawin and
• Andrew D. Smith

Beilstein J. Org. Chem. 2020, 16, 1572–1578, doi:10.3762/bjoc.16.129

• nature of the aromatic substituent within the trifluoroacetophenone component, followed by a ring opening with allylamine, showed that the introduction of electron-withdrawing groups (positive Hammett sigma constants) [56], such as p-bromo, p-fluoro, and p-trifluoromethyl groups, respectively, were well

Cascade trifluoromethylthiolation and cyclization of N-[(3-aryl)propioloyl]indoles

• Ming-Xi Bi,
• Shuai Liu,
• Yangen Huang,
• Xiu-Hua Xu and
• Feng-Ling Qing

Beilstein J. Org. Chem. 2020, 16, 657–662, doi:10.3762/bjoc.16.62

• SCF3, 0.88 for CF3, and 0.61 for SMe) [1] that helps permeation across biological membranes. Furthermore, the strong electron-withdrawing properties of the SCF3 group (Hammett constants: σp = 0.50, σm = 0.40) [2] with respect to metabolic stability have attracted considerable interest in pharmaceutical

Palladium-catalyzed Sonogashira coupling reactions in γ-valerolactone-based ionic liquids

• László Orha,
• József M. Tukacs,
• László Kollár and
• László T. Mika

Beilstein J. Org. Chem. 2019, 15, 2907–2913, doi:10.3762/bjoc.15.284

• the 4-substituents, i.e., no Hammett-plot can be obtained for the above reaction, it can be stated that the rate determining step of the Sonogashira coupling is not related to the formation of Pd-aryl species. Subsequently, a series of different acetylenes, which can readily be dissolved in [TBP

A diastereoselective approach to axially chiral biaryls via electrochemically enabled cyclization cascade

• Hong Yan,
• Zhong-Yi Mao,
• Zhong-Wei Hou,
• Jinshuai Song and
• Hai-Chao Xu

Beilstein J. Org. Chem. 2019, 15, 795–800, doi:10.3762/bjoc.15.76

• –Hammett scenario (Scheme 4) [42]. Specifically, the equilibrium of the conformations IV and IV′ is much faster than their respectively cyclizations to give V and V′. Since TS2 is relatively lower in energy than TS3, V is formed as the major product. Conclusion In summary, we have developed a

Sigmatropic rearrangements of cyclopropenylcarbinol derivatives. Access to diversely substituted alkylidenecyclopropanes

• Guillaume Ernouf,
• Jean-Louis Brayer,
• Christophe Meyer and
• Janine Cossy

Beilstein J. Org. Chem. 2019, 15, 333–350, doi:10.3762/bjoc.15.29

• -position of the aromatic ring could contribute to the stabilization of TS1, in which the Ar–C3–C2–P dihedral angle is close to 0°, by considering the mesomeric form TS2. The observed dependence of the diastereoselectivity on the σ+ Hammett constant of the para substituents further supported the proposed

Bioinspired cobalt cubanes with tunable redox potentials for photocatalytic water oxidation and CO₂ reduction

• Zhishan Luo,
• Yidong Hou,
• Jinshui Zhang,
• Sibo Wang and
• Xinchen Wang

Beilstein J. Org. Chem. 2018, 14, 2331–2339, doi:10.3762/bjoc.14.208

• ., that the variation of the ligands has a profound effect on the observed redox potentials. Figure 3 displays the plots of the Hammett σp parameters for the ligands versus the half-wave potentials (E1/2) for 1-R complexes, and the potentials increase linearly as a function of σp, giving an indicator of

• the electronic influence of the substituents on E1/2. The E1/2 values for the complexes increase in the following order: 1-R, R = OMe < Me < H < Br < CN. The Hammett analysis provides a positive slope value, indicating that the E1/2 value is favored by electron-withdrawing ligands [48][55]. As the

• ligand becomes more electron withdrawing, the reduced electron density at the metal center makes the Co center in the complex easier to reduce and more difficult to oxidize [55]. Most surprisingly, the potentials could be predicted simply by considering the Hammett σp values. Therefore, the observed

A conformationally adaptive macrocycle: conformational complexity and host–guest chemistry of zorb[4]arene

• Liu-Pan Yang,
• Song-Bo Lu,
• Arto Valkonen,
• Fangfang Pan,
• Kari Rissanen and
• Wei Jiang

Beilstein J. Org. Chem. 2018, 14, 1570–1577, doi:10.3762/bjoc.14.134

• determined by ITC titrations (Figures S15–S26 in Supporting Information File 1), and the data are shown in Table 2. The logarithm of the corresponding association constants of 11+–21+ over 3+ were parabolic as the function of Hammett parameter (σp) [39] (Figure 2). For substituents C(Me)3, OMe, Me, SMe, F

• the substituents to leverage the primary cation···π interactions and thus the final binding affinities. This may explain the parabolic distribution of binding affinities over the Hammett parameters (σp) of the substituents as shown in Figure 2. The conformational adaptivity or flexibility allows ZB4

• . Hydrogen atoms of the host are removed and butyl groups are shortened to methyl groups for viewing clarity. The X-ray single crystal structures of 2+@ZB4-IV (b) and 3+@ZB4-III (c) have been reported previously (see [37]). Parabolic free-energy relationship between log(KR/KH) and Hammett parameter σp. KR

[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

• , substitution effects in the 3-(4-substituted phenyl) group of sydnones were studied and a relatively low Hammett reaction constant ρ ≈ +0.8 was estimated from four derivatives (MeO, Me, H and Cl). An even smaller dependence of the rate constants on the solvent polarity and substituent effect sensitivity (ρ

• )sydnones with DMAD for which positive Hammett ρ-values were observed [54][55]. On the other hand Huisgen and Gotthardt [54] measured bimolecular rate constants for the above-mentioned reaction of 4-methyl-3-phenylsydnone and various acetylenes in p-cymene at 140 °C (Table 2) and found a reactivity sequence

• sydnones with strained cycloalkynes such as bicyclo[6.1.0]non-4-yne-9-methanol (BCN) or 3,3,6,6-tetramethylthiacyclohept-4-yne (TMTH). It was found that the reaction of BCN with 3-(4-substituted phenyl)sydnones roughly obeys a Hammett correlation with ρ ≈ +1.35 ± 0.25 [43] thus indicating a type III

Polysubstituted ferrocenes as tunable redox mediators

• Sven D. Waniek,
• Jan Klett,
• Christoph Förster and
• Katja Heinze

Beilstein J. Org. Chem. 2018, 14, 1004–1015, doi:10.3762/bjoc.14.86

• correlated with the sum of Hammett constants. The 1/1+–4/4+ redox couples are chemically stable under the conditions of electrolysis as demonstrated by IR and UV–vis spectroelectrochemical methods. The energies of the C=O stretching vibrations of the ester moieties and the energies of the UV–vis absorptions

• characteristic relationship is supported by correlating the electrochemical data with the Hammett substituent constants [37][39][74][75]. Typically, the E1/2 data of substituted ferrocenes correlate linearly with the sum ∑σp of the Hammett values σp of para-substituents [37][39][74]. For esters 1–4, the

• electrochemical potentials E1/2 (vs FcH/FcH+) versus sum of Hammett values ∑σp did not give a satisfactory linear relation. Within this approach, the relative positions of ester groups and hence their different electronic influence to the electrochemical potential is not considered. The influence of a

Syntheses, structures, and stabilities of aliphatic and aromatic fluorous iodine(I) and iodine(III) compounds: the role of iodine Lewis basicity

• Tathagata Mukherjee,
• Soumik Biswas,
• Andreas Ehnbom,
• Subrata K. Ghosh,
• Ibrahim El-Zoghbi,
• Nattamai Bhuvanesh,
• Hassan S. Bazzi and
• John A. Gladysz

Beilstein J. Org. Chem. 2017, 13, 2486–2501, doi:10.3762/bjoc.13.246

• isolated in analytically pure form from the reaction of the corresponding aryl iodide and Cl2, even though the nitro groups render the iodine atom less Lewis basic and thermodynamically less prone to oxidation. The Hammett σ values associated with CF2CF3 and CF2CF2CF3 substituents (σp 0.52; σm 0.47–0.52

One-pot syntheses of blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones by T3P^® activation of 3-arylpropiolic acids

• Melanie Denißen,
• Alexander Kraus,
• Guido J. Reiss and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2017, 13, 2340–2351, doi:10.3762/bjoc.13.231

• anhydrides as reactive intermediates blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones are formed by consecutive pseudo three-component syntheses in a one-pot fashion. The Stokes shifts correlate excellently with the Hammett–Taft σR parameter indicating an extended degree of resonance stabilization

• λmax,em are found in a range from 408.5 to 512.5 nm with Stokes shifts lying between 3900 and 2500 cm−1 (Table 6, entries 1, 2 and 4–6, Figure 6). This electronic substituent effect of R1 on the Stokes shifts was further corroborated by linear structure–property relationships based upon Hammett–Taft

• correlations with the consanguineous series 4a,b,d–f. Correlation studies of the longest wavelength absorption maxima λmax,abs, the shortest wavelength emission maxima λmax,em, and the Stokes shifts Δ with the Hammett–Taft parameters σp, σR, σp+, and σp− [55] disclose an interesting insight on electronic