N-amination of 2,6-di-tert-butylpyridine

Summary Despite its low nucleophilicity 2,6-di-tert-butylpyridine (DTBP) easily undergoes N-amination. Other hidered pyridines react similarly. Comparison of the NMR carbon chemical shifts of 2,6-disubstituted 1-aminopyridinium perchlorates and those of the respective 1-methylpyridinium salts shows that the changes are parallel. 1-Amino-2,6-di-tert-butylpyridinium perchlorate does not react withp-dimethylaminobenzaldehyde. However, other hindered 1-(4-dimethylaminobenzylideneamino)pyridinium salts were obtained by a standard procedure.

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N-Aminierung von 2,6-di-tert-Butylpyridin

Zusammenfassung Trotz seiner geringen Nucleophilie ist an 2,6-Di-tert-butylpyridin (DTBP) leicht eine N-Aminierung durchzuführen. Andere gehinderte Pyridine reagieren ähnlich. Ein Vergleich der NMR-Kohlenstoffverschiebungen von 2,6-disubstituiertem 1-Aminopyridiniumperchlorat mit denjenigen der entsprechenden 1-Methylpyridiniumsalze zeigt, daß die Änderungen parallel verlaufen. 1-Amino-2,6-di-tert-butylpyridiniumperchlorat reagiert nicht mitp-Dimethylaminobenzaldehyd, hingegen wurden andere gehinderte 1-(4-Dimethlylaminobenzylidenamino)pyridiniumsalze über Standardmethoden erhalten.

     

Molecular structure of [(tBu)2Al(μ-OPh)]2

The molecular structure of [(^tBu)₂Al(-OPh)]₂ has been determined. The intramolecular steric interaction between the phenyl groups and thetert-butyl ligands results in the geometry about aluminum being significantly distorted from tetrahedral, with the AlC₂ planes are pitched 62° with respect to the Al₂O₂ plane. The greater distortion from tetrahedral about aluminum, and the orientation of the phenoxide ring more nearly perpendicular to the M₂O₂ core as compared to that in [(^tBu)₂Ga(-OPh)]₂ are all consistent with increased^tBu...Ph steric interaction as a consequence of the smaller M₂O₂ core for [(^tBu)₂Al(-OPh)]₂. Crystal data: tetragonal, I4₁/acd,a=16.44(1),c=21.41(1) Å,V=5788(7) ų,Z=8,R=0.047,R _w=0.045.     

Scope and limitations of HClO4-SiO2 as an extremely efficient, inexpensive, and reusable catalyst for chemoselective carbon-sulfur bond formation

The scope and limitations of perchloric acid adsorbed on silica gel (HClO₄-SiO₂) as a highly efficient, inexpensive, and reusable catalyst for chemoselective carbon-sulfur bond formation by conjugate addition of thiols to α,β-unsaturated ketones under solvent-free conditions and at rt are reported. In the case of 1,3-diphenylpropenone, the reactions were best carried out either at 80 °C under solvent-free conditions or at rt in MeOH. The reaction of aryl, arylalkyl, alkyl thiols, and alkane dithiols with cyclic and acyclic α,β-unsaturated ketones afforded excellent yields of the corresponding β-sulfidocarbonyls after 2 min to 2 h. In the case of dithiols, the bis-thia-Michael adducts were formed. The rate of the reaction was found to be dependent on the electronic and steric factors of the α,β-unsaturated ketones and the thiols. A substituent at the β-carbon of the α,β-unsaturated ketone offered steric hindrance for conjugate addition and such substrates required longer times. In case of aromatic thiols, the presence of the nitro group reduced the nucleophilicity of the sulfhydryl sulfur atom resulting in slower rate of reaction for 4-nitrothiophenol compared to that of thiophenol and 4-methylthiophenol. For alkane thiol, the reaction rate was influenced by the steric crowding of the alkyl group attached to the sulfhydryl moiety. The rate of reaction for alkane thiols was sluggish compared to that of aryl thiols. The influence of the β-substituent on the rate of thia-Michael addition was utilized for selective reaction during inter-molecular competitions of cyclohexen-2-one versus 3-methyl-2-cyclohexenone, cyclohexen-2-one versus 4-methyl-3-penten-2-one, and 4-phenyl-3-buten-2-one versus 4-methyl-3-penten-2-one with thiophenol with 100:0, 91:9, and 70:30 selectivities, respectively. The difference in the rate of reaction of thiophenol and 4-nitrothiophenol was also reflected during the inter-molecular competition for the reaction with cyclohexen-2-one with an excellent selectivity of 100:0. The influence of the steric factor of the alkyl group in alkane thiol resulted in 62:38 selectivity during the inter-molecular competition of ethane thiol and tert-butanethiol with cyclohexen-2-one. During inter- and intra-molecular competitions of thia- versus aza- and thia- versus oxa-Michael addition reactions, chemoselective thia-Michael addition took place. The chemoselective thia-Michael addition over aza-Michael addition during intra-molecular competion reactions with 2-aminothiophenol was utilized for an efficient one-pot synthesis of benzothiazepines.     

Suppression of racemization in the carbonylation of amino acid-derived aryl triflates

The carbonylation of enantiopure phenylglycine-derived aryl triflates was achieved to afford 4-carboxyphenylglycine analogs with high enantiomeric excesses (88 to >99% ee). Amide analogs of phenylglycine were well-tolerated in the hydroxy- and methoxycarbonylation processes, providing efficient access to benzoic acid and ester building blocks. The % ee of the product was dependent on the relative steric bulk of both the amino acid substrate and the requisite amine base, with ⁱPr₂NEt proving optimal in minimizing product racemization.     

Syntheses of extreme sterically hindered 4-methoxyboronic acids

4-Iodoanisoles 3a,b, 3d and 4-bromoanisoles 4a-d were readily obtained. An extreme steric hindrance precluded obtaining 3c. Catalytic borylation of 3a,b, 3d followed by hydrolysis of boronic ester 26a,b, 26d easily provided the boronic acids 5a,b, 5d. Compounds 5a and 5d were also synthesised, starting from 4a and 4d, by halogen/metal exchange. Because of a too important steric hindrance, this last reaction failed with 4c and 4b led to the unexpected but stable boronic ester 6. The final obtaining of 5b required a strongly basic hydrolysis with heating.     

结构受阻对取代苯乙烯基吡嗪光物理行为的影响

有关藻类化合物衍生物的荧光发光及其顺反异构化是长期来一直受到广泛注意的问题［’，2］，近年来特别在发现了街的两侧苯环联有推、拉电子能力基团、构成了分子内共轭的电行转移化合物后，其荧光发射有了较大幅度的增长［3，4］，使人们对这一问题的研究更加关切．由于荧光的增长会导致异构化反应的减少，促使人们提出所谓的激发态多衰变通道的观点［5，6］以激发的英分子为例；已认识到其中的双键扭转应为异构化（无辐射衰变的一种）的主要通道，而蔑分子的平面态或双键二侧单键的扭转态则是荧光发射（辐射衰变）的主要途径·所有这些看…     

Structure and chromotropic properties of mono- and binuclear mixed-ligand nickel(II) complexes containing 1,3-diketonate and polyamine ligands

Abstract  Mixed-ligand Ni^II complexes have been synthesized with 1,3-diketonate and bulky tri- or tetra-amine ligands. Their structures were determined by X-ray crystallography, and solvatochromic behavior was examined by UV–vis–NIR spectroscopy. Steric effects of the bulky ligands on the coordination distances and spectral properties are discussed. Graphical Abstract  

Visualization and quantification of anisotropic effects on the H NMR spectra of 1,3-oxazino[4,3-a]isoquinolines—indirect estimates of steric compression

The anisotropic effects of the phenyl, α- and β-naphthyl moieties in four series of 1,3-oxazino[4,3-a]isoquinolines on the ¹H chemical shifts of the isoquinoline protons were calculated by employing the Nucleus Independent Chemical Shift (NICS) concept and visualized as anisotropic cones by a through-space NMR shielding grid. The signs and extents of these spatial effects on the ¹H chemical shifts of the isoquinoline protons were compared with the experimental ¹H NMR spectra. The differences between the experimental δ (¹H)/ppm values and the calculated anisotropic effects of the aromatic moieties are discussed in terms of the steric compression that occurs in the compounds studied.     

Effect of high pressure on Biginelli reactions. Steric hindrance and mechanistic considerations

The effect of high pressure is examined in 3-CC Biginelli reactions. This effect is small when moderately hindered aldehydes or ureas are involved. However, particularly in the case of bulky aldehydes, the sensitivity of the reaction to pressure increases with increasing steric congestion in line with earlier studies described in the reference list. The results also provide insights into the mechanism. Such a result highlights the synthetic utility of high pressure activation for the preparation of hindered Biginelli products.