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

(Z)-Selective Takai olefination of salicylaldehydes

• Stephen M. Geddis,
• Caroline E. Hagerman,
• Warren R. J. D. Galloway,
• Hannah F. Sore,
• Jonathan M. Goodman and
• David R. Spring

Beilstein J. Org. Chem. 2017, 13, 323–328, doi:10.3762/bjoc.13.35

• confidence that these ratios reflect the dominant behaviour of the reaction. Using the appropriate Hammett substituent constant (σm) [12] as a measure of the net electron-withdrawing effect of a given substituent, for the meta-halogenated series, a positive correlation with the relative amount of (Z)-product

• was observed (Figure 1). Whilst Hammett parameters are not tabulated for ortho-substitution, precluding an analogous quantitative analysis, the trend for ortho-halogenated substrates is qualitatively similar, although with even higher amounts of (Z)-product. This implies that electron-withdrawing

Enantioconvergent catalysis

• Justin T. Mohr,
• Jared T. Moore and
• Brian M. Stoltz

Beilstein J. Org. Chem. 2016, 12, 2038–2045, doi:10.3762/bjoc.12.192

• these two complexes was not observed directly for this system, extrapolation from related systems gives a rate more than 105 times greater than the observable rates of C–P reductive elimination. Since the rate of inversion is much greater than the rate of bond formation, Curtin–Hammett/Winstein–Holness

• . Enantioconvergent synthesis of phosphines governed by Curtin–Hammett/Winstein–Holness kinetics (TMS = trimethylsilyl, Is = 2,4,6-triisopropylphenyl). Stoltz’ stereoablative oxindole functionalization. Fu’s type II enantioconvergent Cu-catalyzed photoredox reaction. Stereoablative enantioconvergent allylation and

Potent triazine-based dehydrocondensing reagents substituted by an amido group

• Munetaka Kunishima,
• Daiki Kato,
• Nobu Kimura,
• Masanori Kitamura,
• Kohei Yamada and
• Kazuhito Hioki

Beilstein J. Org. Chem. 2016, 12, 1897–1903, doi:10.3762/bjoc.12.179

• an amido group in place of the methoxy groups of CDMT or DMT-MM. Results and Discussion We decided to use an amido group as a substituent on the triazine ring instead of a methoxy group based on Hammett substituent constants, which indicate that an acetamido group is a more electron-withdrawing group

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

• was observed (86.5:13.5 to 89:11 er). Luo and Cheng also explored the mechanism of the Friedel–Crafts addition of indole to α-substituted vinyl ketones [52][53]. Based on DFT studies, the authors proposed that the stereoselectivity of the reaction was under Curtin–Hammett control (Scheme 28). During

• ketones by enamine catalysis. Curtin–Hammett controlled enantioselective addition of indole. Luo and Cheng’s enantioselective additions to α-branched vinyl ketones. Lou’s reduction–conjugate addition–enantioselective protonation. Luo and Cheng’s primary amine-catalyzed addition of indoles to α-substituted

Conformational equilibrium in supramolecular chemistry: Dibutyltriuret case

• Karina Mroczyńska,
• Małgorzata Kaczorowska,
• Erkki Kolehmainen,
• Ireneusz Grubecki,
• Marek Pietrzak and
• Borys Ośmiałowski

Beilstein J. Org. Chem. 2015, 11, 2105–2116, doi:10.3762/bjoc.11.227

• refer to Supporting Information File 1. Here it is enough to mention that the position of the inflection point is linearly dependent from the substituent (Hammett) constant taken from the publication by Hansch and Taft [53]. To sum up the substituent effect on association it is worth to stress that the

Cathodic hydrodimerization of nitroolefins

• Michael Weßling and
• Hans J. Schäfer

Beilstein J. Org. Chem. 2015, 11, 1163–1174, doi:10.3762/bjoc.11.131

• nitroolefins can be qualitatively seen. Aryl substituents shift the potentials to more negative values according to their Hammett σ values [28]: 10 (4-CF3, σp = 0.53) > 1 (4-H, σp = 0) > 9 (4-CH3O, σp = −0.12). With a Hammett equation for an electrochemical reaction and using the Ep values as E0-values one

• obtains from these three values a Hammett reaction constant ρ = 5.34. This is similar to the reaction constant ρ = 6.37 obtained from the Hammett plot for the one-electron reduction of substituted benzo- and naphthoquinones in DMF [29], which have an electrophore being similar to this of the nitroolefins

• . For the other substituents no σ-values are available to apply the Hammett equation. They are ordered according to decreasing Ep-values in three groups: 11 (2-furyl) > 9 (4-CH3O) > 12 (2-pyrrolyl); 14 (two o-Cl) ≈ 13 (one o-Cl) > 15 (two o-F); another correlation concerns the vinyl substituents at C2

Azobenzene-based inhibitors of human carbonic anhydrase II

• Leander Simon Runtsch,
• David Michael Barber,
• Peter Mayer,
• Michael Groll,
• Dirk Trauner and
• Johannes Broichhagen

Beilstein J. Org. Chem. 2015, 11, 1129–1135, doi:10.3762/bjoc.11.127

• pronounced for electron-donating groups (λmax (1f) = 338 nm, λmax (1a) = 358 nm, λmax (1d) = 404 nm, λmax (1c) = 414 nm, and λmax (1b) = 460 nm). Interestingly, azide 1e with a Hammett constant of σ = 0.08 [12], exhibits a bathochromic shift (λmax (1e) = 356 nm) close to hydroxy azobenzene 1a although it can

• weaker or stronger depending on the functional group in the 4´-position, as they affect the electronic properties of the aromatic system, and this would also be reflected by the Hammett constant. Interestingly, the azobenzene does not adopt a completely planar shape but is distorted with dihedral angles

• of these findings into account, we conclude that the sulfonamide–zinc interaction dominates the binding affinity. The electronic differences of our azobenzene library is expressed by their absorption spectra (as an indicator for the electron richness of the azobenzene) or in their Hammett constants

Microsolvation and sp²-stereoinversion of monomeric α-(2,6-di-tert-butylphenyl)vinyllithium as measured by NMR

• Rudolf Knorr,
• Monika Knittl and
• Eva C. Rossmann

Beilstein J. Org. Chem. 2014, 10, 2521–2530, doi:10.3762/bjoc.10.263

• as 1 or 2. Synthesized with the experimentally most convenient (because least line-broadening [12]) isotope 6Li, almost [9] all species of 1a and 2 were found to have a common sensitivity factor of L = 42.8 Hz in Equation 1, whereas L decreased slightly in a nonlinear fashion with increasing Hammett

• place of 47%) THF solution (−8 °C in Table S2, [15]); however, this weak acceleration might also be due to the increased solvent polarity. The pseudomonomolecular, ionic mechanism in Scheme 3 had previously [11] been deduced through finding a Hammett reaction constant of ρ = +5.2 for the sp2

Derivatives of the triaminoguanidinium ion, 3. Multiple N-functionalization of the triaminoguanidinium ion with isocyanates and isothiocyanates

• Jan Szabo,
• Kerstin Karger,
• Nicolas Bucher and
• Gerhard Maas

Beilstein J. Org. Chem. 2014, 10, 2255–2262, doi:10.3762/bjoc.10.234

• with isocyanates When 1,2,3-tris(benzylamino)guanidinium salts 3 and 5-OTs were combined with three mol equivalents of aryl isocyanates 6a–c, triply carbamoylated salts 7a–f were obtained in good to high yields (Scheme 2). As was expected on the basis of the Hammett σp values [20], p-tolyl isocyanate

Substitution effect and effect of axle’s flexibility at (pseudo-)rotaxanes

• Friedrich Malberg,
• Jan Gerit Brandenburg,
• Werner Reckien,
• Oldamur Hollóczki,
• Stefan Grimme and
• Barbara Kirchner

Beilstein J. Org. Chem. 2014, 10, 1299–1307, doi:10.3762/bjoc.10.131

• (standard values were chosen here) was set to 1.3 Å. The Hammett parameters are taken from [43]. Computational details for periodic calculation The periodic calculations were carried out with the Vienna ab-initio simulation package VASP 5.3 [44][45]. We utilized the GGA functional PBE [46] in combination

• also witnessed for diketopiperazine-based amide rotaxanes [57]. In Table 2, also the Hammett σ parameters are given. These substituent parameters [43] are the difference of the pKa values of substituted and non-substituted benzoic acids, they can be correlated with the interaction energies resulting in

• the Hammett σ parameters. The values are given in Table 1. Black curves: DB and red curves: SB. The solid lines are fits to all energies of the para-substitution only, the dotted lines are linear regressions to all interaction energies. Comparison of the X-ray structure of the rotaxane with the

Oxidative 3,3,3-trifluoropropylation of arylaldehydes

• Akari Ikeda,
• Masaaki Omote,
• Shiho Nomura,
• Miyuu Tanaka,
• Atsushi Tarui,
• Kazuyuki Sato and
• Akira Ando

Beilstein J. Org. Chem. 2013, 9, 2417–2421, doi:10.3762/bjoc.9.279

• substantially. p-Methyl and p-methoxybenzaldehydes gave 3h and 3i in 26% and 17% yield, respectively (Table 2, entries 7 and 8). meta-Substitution of benzaldehyde with a methoxy group gave 3j in 58% yield (Table 2, entry 9). These results are in accordance with the Hammett equation. ortho-Substitution of

Sequential Diels–Alder/[3,3]-sigmatropic rearrangement reactions of β-nitrostyrene with 3-methyl-1,3-pentadiene

• Peter A. Wade,
• Alma Pipic,
• Matthias Zeller and
• Panagiota Tsetsakos

Beilstein J. Org. Chem. 2013, 9, 2137–2146, doi:10.3762/bjoc.9.251

• . Nitronic ester 26c with a sulfone group (Hammett σp 0.68) [20] at the C(I) position rearranges at 80 °C to 28b whereas analogous nitronic ester 26d with phenacyl group, a less powerful W-group (Hammett σp 0.43) [20], requires 120 °C for rearrangement to 28c [2]. The [3,3]-sigmatropic rearrangement of 26a

Kinetics and mechanism of the anilinolysis of aryl phenyl isothiocyanophosphates in acetonitrile

• Hasi Rani Barai and
• Hai Whang Lee

Beilstein J. Org. Chem. 2013, 9, 615–620, doi:10.3762/bjoc.9.68

• Table 1 summarizes the second-order rate constants, kH and kD, with X-anilines and deuterated X-anilines, respectively. Table 2 and Table 3 summarize the Hammett (ρX(H and D)) and Brönsted (βX(H and D)) coefficients with X, and Hammett coefficients (ρY(H)) with Y, respectively. The second-order rate

• substituent effects of X on the reaction rates are not compatible with a typical nucleophilic substitution reaction. The Hammett and Brönsted plots with X are biphasic concave upwards with a break region between X = H and 4-Cl (break point of σX ≈ 0.13), giving unusual positive ρX and negative βX values with

• and substrates, respectively, is described in Equation 2 and Equation 3. The two ρXY values are obtained because the Hammett plots with X are biphasic. Figure 2 shows the plots of ρX versus σY and ρY versus σX to determine the ρXY values, according to Equation 3. The signs of ρXY are positive with

Equilibrium constants and protonation site for N-methylbenzenesulfonamides

• José A. Moreira,
• Ana M. Rosa da Costa,
• Luis García-Río and
• Márcia Pessêgo

Beilstein J. Org. Chem. 2011, 7, 1732–1738, doi:10.3762/bjoc.7.203

• , respectively). The solvation parameter m* is always higher than 0.5 and points to a decrease in the importance of solvation on the cation stabilization as the electron-donor character of the substituent increases. Hammett plots of the equilibrium constants showed a better correlation with the σ+ substituent

• the pH scale, defined for dilute aqueous solutions. With this purpose, Hammett defined the so-called “Hammett acidity function”, H0, which is no more than a measure of the deviation, relative to ideality, provoqued by the changes in the medium as the acid concentration increases. Time has proved that

Anion–π interactions influence pK_a values

• Christopher J. Cadman and
• Anna K. Croft

Beilstein J. Org. Chem. 2011, 7, 320–328, doi:10.3762/bjoc.7.42

• measured by potentiometric titration in 50:50 mixtures of acetonitrile/water with tetrabutylammonium hydroxide (TBAH) as the base. As an example, the plot obtained for compound 1 with TBAH is shown in Figure 3. The derived pKa' values, along with the Hammett σp-constant for each R-substituent are presented

• and the actual pKa. It is worth remembering, however, that the resulting pKa' value of every derivative is relative to the others and therefore any effects of the R-substituents are likely to be preserved. The pKa' values correlate linearly with the corresponding Hammett σp-constants for the R

• -substituents, affording a correlation constant (R2) of 0.916. The linear dependence is illustrated in Figure 5. The slope of the Hammett plot confirms that the intermediate or product is stabilised by electron deficient groups, consistent with deprotonation. The magnitude of the slope is relatively small

Heavy atom effects in the Paternò–Büchi reaction of pyrimidine derivatives with 4,4’-disubstituted benzophenones

• Feng-Feng Kong,
• Jian-Bo Wang and
• Qin-Hua Song

Beilstein J. Org. Chem. 2011, 7, 113–118, doi:10.3762/bjoc.7.16

• the ISC processes in the triplet 1,4-diradicals, k2I/k2I’ versus k3I/k3I’ and (iv) the relative rate constants of the bond-forming and bond-breaking step from the singlet 1,4-diradical C5-1DR1 and C6-1DR1, k2b/k2c versus k3b/k3c. According to the Curtin–Hammett principle [20], the ratios of the

Backbone tuning in indenylidene–ruthenium complexes bearing an unsaturated N-heterocyclic carbene

• César A. Urbina-Blanco,
• Xavier Bantreil,
• Hervé Clavier,
• Alexandra M. Z. Slawin and
• Steven P. Nolan

Beilstein J. Org. Chem. 2010, 6, 1120–1126, doi:10.3762/bjoc.6.128

• stretching frequencies (νCOav) were used to provide the TEP (Table 1). As expected, the backbone substitution pattern has a profound effect on the electronic donor capacity of the NHC, and a linear correlation between the electronegativity of the backbone substituent (measured as the Hammett parameter, σp

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate

• Michael North and
• Marta Omedes-Pujol

Beilstein J. Org. Chem. 2010, 6, 1043–1055, doi:10.3762/bjoc.6.119

• from 51V NMR spectroscopy suggested that propylene carbonate coordinates to VO(salen)NCS, blocking the free coordination site, thus inhibiting its Lewis acidity and accounting for the reduction in catalytic activity. This explanation was further supported by a Hammett analysis study, which indicated

• that Lewis base catalysis made a much greater contribution to the overall catalytic activity of VO(salen)NCS in propylene carbonate than in dichloromethane. Keywords: cyanohydrin; Hammett; kinetics; propylene carbonate; vanadium; Introduction The last 15 years have witnessed an explosion of activity

• dichloromethane is also consistent with a less tightly organized transition state, again consistent with only one of the reaction components interacting with the catalyst. To investigate this further, a Hammett analysis was undertaken using a range of substituted benzaldehydes. Hammett analysis It is well