Sehr starke Organosuperbasen durch Verknüpfung von Imidazol‐ und Guanidinbasen – Synthese, Struktur und Basizität

Vazdar, Katarina; Kunetskiy, Roman A.; Saame, Jaan; Kaupmees, Karl; Leito, Ivo; Jahn, Ullrich

doi:10.1002/ange.201307212

Cited by 15 publications

(3 citation statements)

References 38 publications

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“…[1][2][3][4][5] With high thermal stability, the ease of charge delocalization and coordination properties as well as the possibility to attach up to six different substituents on the guanidine moiety has driven research activities in diverse directions, resulting in their use as superbases, [6,7] ligands for coordination complexes, [8][9][10][11] organocatalysts, [12][13][14][15][16] stimuli-responsive materials, [17] hydrogels, [18] anion exchange polymer electrolytes for fuel cells, [19] and biologically active compounds [20][21][22][23][24][25][26][27][28][29] for drug development. Furthermore, guanidinium salts have also entered the field of ionic liquid crystals (ILCs) [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] as an alternative cationic head group to the imidazolium-derived ILCs, which have dominated this research area so far.…”

Section: Introductionmentioning

confidence: 99%

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

et al. 2014

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A series of phenylguanidinium salts 3 . X, which are linked via an alkoxy spacer either to a 4-decyloxy-or 4-cyanosubstituted biphenyl mesogen, was prepared and the mesomorphism studied. A decyloxybiphenyl core and a spacer of at least C6 chain length were required for mesophase formation. Replacement of the chloride counterion by other anions like bromide or tetrafluoroborate improved the thermal stability of the mesophase. A comparison of substitution pattern (meta v. para) on the phenyl ring revealed decreased melting and clearing points for the bent cationic head group. All guanidinium ionic liquid crystals 3 displayed only smectic A (SmA) phases. A packing model is assumed where the molecules in a bilayer stack over each other in opposite direction with interdigitated terminal decyloxy groups and spacers.bromides 9 giving the tethered compounds p-10a,b and m-10b-e in 78-88 % yield. N-Boc-deprotection to derivatives p-11a,b and m-11b-e was achieved very cleanly with methane sulfonic acid in CHCl 3 . Derivative m-11a was obtained in 94 % yield by acidic hydrolysis [62] of acetamide m-7a. The aniline derivatives 11 were finally treated with chloro-N,N,N 0 ,N 0tetramethylformamidinium chloride [63] and either NEt 3 or NaHCO 3 . Workup strictly required an inert gas atmosphere to provide the neat target guanidinium salts 3 . Cl in 70-98 % yield. To further study the anion effect, decyloxybiphenyl guanidinium chloride m-3a . Cl was submitted to salt metathesis with NaBr, KI, NaOTf, NaBF 4 , KPF 6 , KOAc, and KSCN yielding the corresponding guanidinium ILCs m-3a . X (Scheme 2). Anion Effect in Guanidinium ILCsComparison of the 1 H NMR spectra of phenylguanidinium salts m-3a . X revealed a significant downfield shift of the N-H signal from ,7.5 to 12.0 ppm in the following order: PF 6 , BF 4 , OTf , I , SCN , Br ECl (Fig. 1).This correlation between the anion and the N-H proton shift indicates the presence of contact between the ion pairs that also led to small chemical shifts of the aromatic protons of the phenylguanidinium moiety (Fig. 1, grey). To estimate the effect of concentration on the N-H signal, 1 H NMR measurements of m-3b . Cl with concentrations varying from 0.83 to 2.50 mg mL À1 were carried out. The N-H signal shift of 0.1 ppm turned out to be negligible with respect to the strong dependence of the NH signal on the anion (see Supplementary Material for details). This effect rather might be due to either the interaction of the anion with the anisotropic cone of the aryl ring than due to the conjugation between the guanidinium cation and the aryl ring. Previous DFT calculations of several guanidinium salts revealed no or only very little conjugation, i.e. the guanidinium moiety behaves like an isolated cation. [39] In agreement with these studies, the N-H signal shifted upfield with increasing anion radii [64,65] with an almost linear correlation (Fig. 2).

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Section: Introductionmentioning

confidence: 99%

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

et al. 2014

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“…Although guanidines are generally strong bases, compound 7 was not protonated by water, indicative of a reduced basicity. Reaction of 7 with HOTf in THF gave magenta single crystals of ( 7 +2 H)(OTf) 2 (Figure and Table ).…”

Section: Methodsmentioning

confidence: 99%

“…Although guanidines are generally strongb ases, [31] compound 7 was not protonated by water,i ndicative of ar educed basicity.R eaction of 7 with HOTf in THF gave magenta single crystalso f( 7 + 2H)(OTf) 2 (Figure 4a nd Ta ble 1). As was expected, protonation leads to hypsochromic shift of the Vis bands (to 607 and 431 nm) due to reduction of the donor ability of the guanidinyl groups (Figure 3).…”

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Hetero Diels–Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron‐Rich Pentacene

Werr

Kaifer

Himmel

2020

Chemistry A European J

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Herein, the first hetero Diels-Alder (DA) reactions with as table, dicationic urea azine derived azo dienophile,s ynthesized by two-electron oxidation of an eutral urea azine are reported. Several charged DA products were synthesized in good yield and fully characterized. The DA adduct of anthracenei si nt hermal equilibrium with the reactants at room temperature, and the reaction enthalpy and entropy were determinedf rom the temperature-dependent equilibrium constant. Furthermore, base additiont os olutionso ft he pentaceneD Ap roduct led to deprotonation, cleavage of the NÀNb ond, and formation of an electron-rich 6,13-bisguanidinyl-substitutedp entacene. The redox and optical properties of this new pentacene derivativew ere studied.F urthermore, the dication resulting from its two-electrono xidation was synthesized and fully characterized. The results disclose an ew elegant route to electron-rich pentacene derivatives. Cycloaddition reactions are among the most elegant coupling reactions in organic chemistry.D iene-ene [4+ +2] cycloadditions, termedD iels-Alder (DA) reactions, lead to new six-membered rings by formation of two new covalent bonds. [1] The large scope of such reactions results from the variety of possible substrates and bonds (e.g.,C ÀC, CÀN) that could be formed. DA reactions are distinguished by their high atom economy, providing access to ring systems with high stereo-and regioselectivity. [2] Hetero DA reactions lead in as ingle step to multifunctionalized compounds,a nd are essential elements of synthetic strategies to build complex aromatics. [3] SomeD Ar eactions are reversible, allowing the liberation of the dienophile and diene components in ar etro Diels-Alder(rDA) reaction initiated thermally, [4] photochemically, [5] or mechanochemically. [6] Withint he timely field of dynamic covalentc hemistry (DCC), [7] the DA reaction is used as one of the prime tools. However, there are only af ew examples of rDA reactions for which the equilibrium could be varied fast near room temperature. [4, 8] The use of an azo compound as ad ienophile has marked the beginning of the DA reaction. In the year 1925, Diels et al. reported the reactionb etween cyclopentadiene and ethyl azodicarboxylate (a)y ieldingasix-membered bridgedh eterocyclic product (Scheme 1). [9] In the meantime, several azo compounds werea pplied as electron-poor dienophiles in DA reactions. [3a, 10] Popular examples are the already mentioneda zodicarboxylic acid esters [11] (a), as well as 4-phenyl-1,2,4-triazoline-3,5-dione(PTAD) [12] (b). The use of the dienophiles azobisformamidine [13] (c)a nd dimethyldiazeniumb romide [14] (d)i sh ampered by their high reactivity and instability,a nd the susceptibility of the DA products to follow-up reactions. [14, 15] The electron-deficient character of the azo group could be increased through inductiono rr esonance effects, leading to higher reactivity in an ormale lectron-demand DA reaction. [16] Thus, the reactivity should decrease in the row d > b > a > c. [3a] However, when the electron d...

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Basicity Limits of Neutral Organic Superbases

Leito

Koppel

et al. 2015

Angewandte Chemie

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The potential limits of superbasicity achievable with different families of neutral bases by expanding the molecular framework are explored using DFT computations.Anumber of different core structures of non-ionic organosuperbases are considered (such as phosphazenes,g uanidinophosphazenes, guanidino phosphorus ylides). As imple model for describing the dependence of basicity on the extent of the molecular framework is proposed, validated, and used for quantitatively predicting the ultimate basicities of different compound families and the rates of substituent effect saturation. Some of the considered bases (guanidino phosphorus carbenes) are expected to reach gas-phase basicity around 370 kcal mol À1 , thus being the most basic neutral bases ever reported. Also,the classical substituted alkylphosphazenes were predicted to reach pK a values of around 50 in acetonitrile,w hich is significantly higher than previously expected.Non-ionic organosuperbases [1] are attracting significant interest because of their practical importance as catalysts [1,2] and auxiliary reagents [3] in synthesis and technology as well as for fundamental challenges. [4,5] Design and synthesis of new superbases has been aflourishing field of research during the last decades. [1,4] Numerous families of superbases (such as phosphazenes, [3] phosphatranes, [6] bisphosphazene proton sponges, [7,8] imidazolidines, [9] imidazolidino-phosphazenes [10] and -guanidines, [11] bis-guanidines [12] )h ave been created and potentially superbasic compound families have been proposed, for example carbenes. [13] More recently different innovative bases have been proposed, such as cyclopropeneimines [14] and silylene bases. [15] Despite av ast range of potential applications, [1] many classes of superbases (guanidino proton sponges, [7] phosphorus ylides [16] )a re still almost unexplored.An early Minireview by Schwesinger [17] summarizes the principal approaches to enhancing the basicity of organosuperbases:1 )the "battery cell" principle, [3] that is,s tepwise expansion of the molecular scaffold by forming alternating formal single and double bonds and thereby enhancing the structure where the positive charge can be delocalized (Scheme 1);2 )stabilization of the protonated form by intramolecular hydrogen bond (chelation);a nd 3) structures that become aromatic on protonation. It was concluded that the most fruitful approach to design of superbases is the battery cell principle.T he best known practical example is the phosphazene family of superbases. [3] Creating as tructure where the protonated form is stabilized by one or more hydrogen bonds is also frequently used [4,7,8] and often remarkable basicity enhancement is seen, especially in the gas phase.H owever,i np ractical usage,n ot only thermodynamic but also kinetic basicity is important and here is the drawback of the chelating bases,a st hey generally are kinetically slow. [7] In this work, we computationally explore (DFT B3LYP 6-311 + G** and DFT BP TZVP) five families of potentially extreme...

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Sehr starke Organosuperbasen durch Verknüpfung von Imidazol‐ und Guanidinbasen – Synthese, Struktur und Basizität

Cited by 15 publications

References 38 publications

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Hetero Diels–Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron‐Rich Pentacene

Basicity Limits of Neutral Organic Superbases

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