Steric activation in prototropic reactions of pyrazine derivatives. II. The Brønsted relation

1986 ◽  
Vol 64 (6) ◽  
pp. 1090-1092 ◽  
Author(s):  
K. Nagarajan ◽  
T. W. S. Lee ◽  
R. R. Perkins ◽  
Ross Stewart
Keyword(s):  
Transition State ◽  
The Other ◽  
Steric Effects ◽  
Methyl Groups ◽  
Reaction Centre ◽  
Organic Electrolytes ◽  
Methyl Group ◽  
Pyridine Bases ◽  
Tentative Explanation ◽  
High Concentrations

The rate of deprotonation of the 2-methyl group in 1,2,3-trimethylpyrazinium ion by carboxylate, aniline, and pyridine bases has been measured in D2O. Carboxylate ions containing bulky groups near the reaction centre, e.g. ortho benzoates, react faster than predicted by the Brønsted equation that correlates the reactions of unhindered carboxylates. Anilines and pyridines, on the other hand, show conventional steric effects. A tentative explanation for the activation engendered by groups adjacent to the carboxylate centre is based on the known effect that high concentrations of organic electrolytes have on the strengths of carboxylic acids but not of amines. Since ortho carboxylate ions have their relative basicities increased by an alkyl-rich environment, it is argued that the reactive methyl groups of the substrate might provide such an interaction in the transition state.

Absolute stereochemistry of calopiptin

1968 ◽  
Vol 21 (8) ◽  
pp. 2095 ◽  
Author(s):  
JB Mc Alpine ◽  
NV Riggs ◽  
PG Gordon
Keyword(s):  
Mass Spectrometry ◽  
High Resolution ◽  
High Resolution Mass Spectrometry ◽  
The Other ◽  
Methyl Groups ◽  
Lower Field ◽  
Methyl Group ◽  
Low Field ◽  
Absolute Stereochemistry ◽  
Resolution Mass

The α,α?-diaryl-β,β-dimethyltetrahydrofurenoid lignan, calopiptin, from Piptocalyx moorei has been converted by demethylenation and methylation into veraguensin, now also isolated from Trimenia papuana (both species, family Trimeniaceae). Ozonolysis yields (-)-2,3-dimethylsuocinic acid which establishes the absolute trans configuration of the methyl groups. The benzylic proton giving a signal at low field from the other in the p.m.r, spectrum is assigned as trans to the adjacent methyl group by shielding and spin-decoupling arguments. The signal moves to even lower field on nitration of one of the aryl groups, identified as 3,4-dimethoxyphenyl by competitive nitration experiments and high-resolution mass spectrometry. Calopiptin is 2R-(3,4-dimethoxyphenyl)-3S,4S-dimethyl-5S-(3,4-methylenedioxyphenyl)tetrahydrofuran.

Ambident nucleophiles. III. N-bonded vs. C-bonded adduct formation on reaction of imidazolide anions with 1,3,5-trinitrobenzene

1985 ◽  
Vol 63 (4) ◽  
pp. 866-870 ◽  
Author(s):  
M. P. Simonnin ◽  
J. C. Halle ◽  
F. Terrier ◽  
M. J. Pouet
Keyword(s):  
Proximity Effect ◽  
Adduct Formation ◽  
The Other ◽  
Methyl Groups ◽  
Kinetic Control ◽  
Methyl Group ◽  
Low Field ◽  
Substitution Site ◽  
Field Shift ◽  
Subsequent Conversion

Imidazolide anions of imidazole, 2- and 4-methylimidazoles, 4,5-dimethyl and 4,5-diphenylimidazoles react with 1,3,5-trinitrobenzene (TNB) in dimethylsulfoxide (DMSO) to yield, initially, N-bonded σ-adducts under kinetic control. These undergo a subsequent conversion to C-adducts, which are the thermodynamically stable products. The formation of N,C-diadducts from the reversible attack of a second TNB molecule on the imidazolide moiety of the C-adducts is also observed. Provided that the corresponding position of the parent imidazolide anion is free, the formation of C-adducts occurs preferentially at the C-4 or C-5 carbon atom. However, C-2 substitution is observed in 4,5-disubstituted derivatives. Structures and preferred conformations of some adducts and diadducts are deduced from nOe experiments. The mechanisms of the various reactions are discussed. Evidence is presented that adduct formation results in a low-field shift of methyl groups located in a position adjacent to the substitution site, while all the other protons move upfield. This unusual deshielding presumably reflects a proximity effect between a methyl group and the hydrogen atom attached to the sp3 carbon of the adducts.

Synthesis, base strength, and prototropic behaviour of a number of alkyllumazines

1985 ◽  
Vol 63 (12) ◽  
pp. 3290-3293 ◽  
Author(s):  
Ross Stewart ◽  
S. J. Gumbley
Keyword(s):  
Sulfuric Acid ◽  
Isotopic Exchange ◽  
The Other ◽  
Methyl Groups ◽  
Hydrogen Atoms ◽  
Methyl Group ◽  
Neutral Compounds ◽  
Protonated Forms ◽  
Base Strength ◽  
Acid Catalyzed

A number of lumazines and 5-deazalumazines containing a methyl group at C-7 have been prepared, their pKBH+ values determined, and measurements made of the rates at which the hydrogen atoms of their 7-methyl groups undergo isotopic exchange in aqueous sulfuric acid. The presence of an alkyl group at N-8 in the protonated forms of these compounds activates the neighbouring methyl group at C-7; the effect is considerably larger than that previously observed for a methyl group at C-6, which is the other neighbouring position. The comparison of methyl and hydrogen at N-8 can be made only for the acid-catalyzed reaction because the structures of the neutral compounds, which take part in the base-catalyzed reaction, are not analogous.

Pyrolysis of aryl azides. III. Steric and electronic effects upon reaction rate

1975 ◽  
Vol 28 (10) ◽  
pp. 2147 ◽  
Author(s):  
LK Dyall
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Reaction Rate ◽  
Steric Effects ◽  
Electronic Effects ◽  
Aryl Azides ◽  
Phenyl Azide ◽  
Methyl Group ◽  
First Order ◽  
Ortho Substituent

First-order rate constants have been measured for the pyrolysis of 15 phenyl azides in decalin solution. The rate for phenyl azide is increased only slightly by all para and many ortho substituents; in these cases Eact and ΔSact values are related linearly. ��� The very large rate increases when the ortho substituent is phenylazo, nitro, acetyl or benzoyl cannot be from steric or normal electronic effects and therefore identify a specific involvement of these groups in the transition state. This rate enhancement is reduced to scarcely significant levels by a 6-chloro or 6-methyl group in 2- nitrophenyl azide, but not by a 6-nitro group. These results raise doubts about recent claims1 to establish mechanism by measuring polar effects on rates of pyrolysis of azides in which steric effects might also operate.

scholarly journals Benzazole derivatives: V. The reactivity of some 2-p-tolyl-and 2-p-methylstyryl-benzimidazolium salts

2004 ◽  
Vol 58 (12) ◽  
pp. 548-551
Author(s):  
Corina Cernatescu ◽  
Eugenia Comanita
Keyword(s):  
Aromatic Aldehydes ◽  
The Other ◽  
Steric Effects ◽  
Methyl Group ◽  
Condensation Reactions ◽  
Other Hand ◽  
Benzimidazolium Salts

The reactivity of the p-substituted methyl group in 1-methyl-2-p-tolylbenzimidazolium and 1-methyl-2-p-methylstyrylbenzimidazolium iodides with p-nitroso-dimethylaniline (p-NDMA) and aromatic aldehydes was studied. According to literature data on the steric effects produced by the bulky N-l methyl group, the 2-p-tolyl substituted salt does not react with p-NDMA. On the other hand, 2-p-methylstyryl substituted salts give condensation reactions, similarly to 1,2,3-trimethylbenzimidazolium iodides. The compounds obtained from the reaction with aromatic aldehydes can be used as dyeing substances, photosensitizers or as dienic monomers, due to the conjugated chain from C-2.

scholarly journals Characterization of Glycine SarcosineN-Methyltransferase and Sarcosine DimethylglycineN-Methyltransferase

2001 ◽  
Vol 67 (5) ◽  
pp. 2044-2050 ◽  
Author(s):  
Antti Nyyssölä ◽  
Tapani Reinikainen ◽  
Matti Leisola
Keyword(s):  
Escherichia Coli ◽  
Osmotic Pressure ◽  
Glycine Betaine ◽  
The Other ◽  
Substrate Specificities ◽  
Methyl Group ◽  
High Substrate ◽  
Ectothiorhodospira Halochloris ◽  
High Concentrations

ABSTRACT Glycine betaine is accumulated in cells living in high salt concentrations to balance the osmotic pressure. Glycine sarcosineN-methyltransferase (GSMT) and sarcosine dimethylglycineN-methyltransferase (SDMT) of Ectothiorhodospira halochloris catalyze the threefold methylation of glycine to betaine, with S-adenosylmethionine acting as the methyl group donor. These methyltransferases were expressed inEscherichia coli and purified, and some of their enzymatic properties were characterized. Both enzymes had high substrate specificities and pH optima near the physiological pH. No evidence of cofactors was found. The enzymes showed Michaelis-Menten kinetics for their substrates. The apparent Km andV max values were determined for all substrates when the other substrate was present in saturating concentrations. Both enzymes were strongly inhibited by the reaction productS-adenosylhomocysteine. Betaine inhibited the methylation reactions only at high concentrations.

Temperature dependence of the crystal structure of 3,5-di-tert-butylpyrazole from 10 K

2004 ◽  
Vol 60 (5) ◽  
pp. 621-625 ◽  
Author(s):  
Alexandre N. Sobolev ◽  
Allan H. White
Keyword(s):  
Pyrazole Ring ◽  
The Other ◽  
Methyl Groups ◽  
Asymmetric Unit ◽  
Temperature Dependent ◽  
Methyl Group ◽  
Butyl Group ◽  
Structure Determinations ◽  
History Of ◽  
The Subject

A single crystal of 3,5-di-tert-butylpyrazole has been the subject of structure determinations at 10, 100, 120 and 299 K. Above ∼ 110 K, the structure is Pbca, a ≃ 11.3, b ≃ 20.6, c ≃ 10 Å, Z = 8. Below 110 K, the c axis is doubled and the space group becomes Pb21 a, Z = 16. Here the asymmetric unit is comprised of four molecules, arranged as a pair of overlapping hydrogen-bonded dimers, all components being ordered. In each case one methyl group of each t-butyl group lies coplanar with its parent pyrazole ring and, within each quasi-inversion-related dimer, one molecule has both of these methyl groups directed away from the N2 component `cis', while in the other, one methyl group is toward (`trans'). Above 110 K the four molecules collapse into one, with one t-butyl group (corresponding to an overlaid `cis/trans' pair of the low-temperature form) modelled as disordered. The disorder has been previously described as `temperature dependent' (Deacon et al., 2001), but the present studies suggest it to be dependent on the history of the particular specimen as well.

scholarly journals Metals in Calluna vulgaris, Empetrum nigrum, Festuca vivipara and Thymus praecox ssp. arcticus in the geothermal areas of Iceland

2021 ◽  
Author(s):  
Adam Rajsz ◽  
Bronisław Wojtuń ◽  
Aleksandra Samecka-Cymerman ◽  
Paweł Wąsowicz ◽  
Lucyna Mróz ◽  
...  
Keyword(s):  
Active Sites ◽  
Calluna Vulgaris ◽  
Bioaccumulation Factor ◽  
The Other ◽  
Extreme Situation ◽  
Empetrum Nigrum ◽  
Geothermal Activity ◽  
High Concentrations ◽  
And Control ◽  
Festuca Vivipara

AbstractThis investigation was conducted to identify the content of metals in Calluna vulgaris (family Ericaceae), Empetrum nigrum (family Ericaceae), Festuca vivipara (family Poaceae) and Thymus praecox subsp. arcticus (family Lamiaceae), as well as in the soils where they were growing in eight geothermal heathlands in Iceland. Investigation into the vegetation of geothermal areas is crucial and may contribute to their proper protection in the future and bring more understanding under what conditions the plants respond to an ecologically more extreme situation. Plants from geothermally active sites were enriched with metals as compared to the same species from non-geothermal control sites (at an average from about 150 m from geothermal activity). The enriched metals consisted of Cd, Co, Cu, Fe and Ni in C. vulgaris; Cd, Mn and Ti in E. nigrum; Hg and Pb in F. vivipara; and Cd, Fe and Hg in T. praecox. Notably, C. vulgaris, E. nigrum, F. vivipara and T. praecox had remarkably high concentrations of Ti at levels typical of toxicity thresholds. Cd and Pb (except for C. vulgaris and F. vivipara) were not accumulated in the shoots of geothermal plants. C. vulgaris from geothermal and control sites was characterised by the highest bioaccumulation factor (BF) of Ti and Mn; E. nigrum and F. vivipara by the highest BF of Ti and Cr; and T. praecox by the highest BF of Ti and Zn compared to the other elements. In comparison with the other examined species, F. vivipara from geothermal sites had the highest concentration of Ti in above-ground parts at any concentration of plant-available Ti in soil.

Removal of trace contaminants using molecularly imprinted polymers

2006 ◽  
Vol 53 (11) ◽  
pp. 205-212 ◽  
Author(s):  
M. Le Noir ◽  
B. Guieysse ◽  
B. Mattiasson
Keyword(s):  
Activated Carbon ◽  
Molecularly Imprinted Polymers ◽  
The Other ◽  
Molecular Imprinted Polymers ◽  
Molecularly Imprinted ◽  
Imprinted Polymers ◽  
Trace Contaminants ◽  
High Concentrations ◽  
Trace Concentrations ◽  
Granulated Activated Carbon

This work was conducted to study the potential of molecularly imprinted polymers (MIPs) for the removal of oestradiol at trace concentrations (1 ppm–1 ppb). An MIP synthesised with 17β-oestradiol as template was compared to non-imprinted polymers (NIP) synthesised under the same conditions but without template, a commercial C18 extraction phase and granulated activated carbon. At 1 ppb oestradiol was recovered by 98±2% when using the MIP, compared to 90±1, 79±1, and 84±2% when using the NIP, a C18 phase, or granulated activated carbon, respectively. According to these levels, the MIP was capable of producing an effluent with a quality 5–10 times higher than the other materials. The same levels of oestradiol recovery were achieved with the MIP when supplying 17β-oestradiol at 0.1 ppm. Phenolic compounds added as interferences bound less to the MIP than to the NIP, confirming the selectivity of the MIP. Oestradiol biodegradation was also demonstrated at high concentrations (50 ppm), showing the pollutants can be safely destructed after being enriched by molecular extraction. This study demonstrates the potential of molecular imprinted polymers as a highly efficient specific adsorbent for the removal of trace contaminants.

Net charges and valence AO's in the methylamines; the hydrogen basis set and the properties of nitrogen

1985 ◽  
Vol 63 (7) ◽  
pp. 1487-1491 ◽  
Author(s):  
Giuseppe Del Re ◽  
Sándor Fliszár ◽  
Michel Comeau ◽  
Claude Mijoule
Keyword(s):  
Basis Set ◽  
Basis Sets ◽  
Methyl Groups ◽  
Extended Form ◽  
Amine Nitrogen ◽  
Methyl Group ◽  
Net Charges ◽  
Extended Basis

Net charges and valence AO's for ammonia, methylamine, dimethylamine, and trimethylamine were calculated using extended basis sets. Superposition effects, evaluated by replacing Pople's standard 6-31G* basis by an extended form in which the basis of the ammonia H atoms and of the methyl groups of trimethylamine are retained in the treatment of each molecule, indicate that the quality of the treatment of amine nitrogen atoms is strongly dependent on the number of methyl groups. A new, augmented basis is proposed for the hydrogens, which appears to be reasonably well balanced: comparison with familiar (e.g., 6-31G*) calculations illustrates in what manner the treatment of nitrogen is worsened when even just one methyl group is replaced by hydrogen unless the impoverishment of the basis is suitably taken care of.

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