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.

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.

Beiträge zur Chemie des Siliciums und Germaniums, XXXIX [1]. Darstellung, Eigenschaften und 29Si-NMR-Spektren von höheren Kaliumsilanylen KSinH2n+1 / Contributions to the Chemistry of Silicon and Germanium, XXXIX [1]. Preparation, Properties and 29Si-NMR Spectra of Higher Potassium Silanyls

1985 ◽  
Vol 40 (10) ◽  
pp. 1301-1305 ◽  
Author(s):  
Franz Fehér ◽  
Michael Krancher
Keyword(s):  
High Field ◽  
Nmr Spectra ◽  
The Other ◽  
29Si Nmr ◽  
Reaction Conditions ◽  
Low Field ◽  
Field Shift ◽  
29Si Nmr Spectra ◽  
Silicon And Germanium

Diluted solutions of potassium disilanyl, 2-potassium trisilanyl and 2-potassium isotetrasilanyl in m onoglym e have been prepared from potassium silyl and di- or trisilane under suitable reaction conditions. The purity of the silanyls varies between 95 and 99%. Their stability increases from potassium di- to 2-potassium isotetrasilanyl. The 29Si NMR spectra exhibit a strong high field shift for the metal substituted silicon atoms and a m oderate low field shift for the other silicon atoms compared with the signals of the corresponding silanes.

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.

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.

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 Ground state and stability of the fractional plateau phase in metallic Shastry–Sutherland system TmB4

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Matúš Orendáč ◽  
Slavomír Gabáni ◽  
Pavol Farkašovský ◽  
Emil Gažo ◽  
Jozef Kačmarčík ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Phase Diagram ◽  
Ground State ◽  
Constant Magnetic Field ◽  
Paramagnetic Phase ◽  
The Other ◽  
Zero Field ◽  
Plateau Phase ◽  
Low Field ◽  
Zero Field Cooling

AbstractWe present a study of the ground state and stability of the fractional plateau phase (FPP) with M/Msat = 1/8 in the metallic Shastry–Sutherland system TmB4. Magnetization (M) measurements show that the FPP states are thermodynamically stable when the sample is cooled in constant magnetic field from the paramagnetic phase to the ordered one at 2 K. On the other hand, after zero-field cooling and subsequent magnetization these states appear to be of dynamic origin. In this case the FPP states are closely associated with the half plateau phase (HPP, M/Msat = ½), mediate the HPP to the low-field antiferromagnetic (AF) phase and depend on the thermodynamic history. Thus, in the same place of the phase diagram both, the stable and the metastable (dynamic) fractional plateau (FP) states, can be observed, depending on the way they are reached. In case of metastable FP states thermodynamic paths are identified that lead to very flat fractional plateaus in the FPP. Moreover, with a further decrease of magnetic field also the low-field AF phase becomes influenced and exhibits a plateau of the order of 1/1000 Msat.

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.

Proximity effects in 29Si, 13C, and 1H NMR spectra of ortho substituted phenoxytrimethylsilanes

1990 ◽  
Vol 55 (8) ◽  
pp. 2019-2026 ◽  
Author(s):  
Jan Schraml ◽  
Václav Chvalovský ◽  
Harald Jancke ◽  
Peter Koehler ◽  
Mikhail F. Larin ◽  
...  
Keyword(s):  
Oxygen Atom ◽  
1H Nmr ◽  
Proximity Effect ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
Proximity Effects ◽  
1H Nmr Spectra ◽  
Methyl Groups ◽  
Conformational Preferences

NMR Spectra of eight ortho substituted phenoxytrimethylsilanes, 2-X-C6H4-OSi(CH3)3 (X = Cl, Br, OCH3, NH2, NO2, OSi(CH3)3, CH3, and H), are reported. In contrast to analogous ortho substituted methoxybenzenes the 13C chemical shifts of C-2 and C-6 aromatic carbons do not exhibit consistent trends indicating different conformational preferences in the trimethylsiloxybenzenes. Under the influence of the ortho substituents the nuclei of OSi(CH3)3 group (29Si, 13C, and 1H) are deshielded; compounds with X = CH3 (and H) appear anomalous in this respect. It is argued that this proximity effect is not due to an interaction involving terminal methyl groups but involves the oxygen atom of the OSi(CH3)3 group; it is most likely due to an interaction with unshared electrons of the ortho substituent.

The Rates and Products of Addition of 4-Chlorobenzenesulfenyl Chloride to a Series of Side Chain Methyl Substituted Styrenes

1974 ◽  
Vol 52 (9) ◽  
pp. 1807-1812 ◽  
Author(s):  
George H. Schmid ◽  
Dennis G. Garratt
Keyword(s):  
Steric Hindrance ◽  
Side Chain ◽  
Methyl Groups ◽  
Methyl Group

The rates of addition and the product compositions have been determined for the addition of 4-chlorobenzenesulfenyl chloride to a series of seven side chain methyl substituted styrenes in 1,1,2,2-tetrachloroethane at 25°. Unlike the addition to the corresponding series of methylated ethylenes, the effect of the methyl groups is not cumulative. The effect of the methyl groups depends upon whether or not the β-methyl group is cis to the phenyl. When it is cis, the rate of addition is decreased compared to styrene and substitution of additional methyl groups has only a small effect on the rate of addition. In compounds lacking a cis-β-methyl group the rate of addition more closely resembles that for addition to the methylated ethylenes. Steric hindrance between the cis-methyl and phenyl groups is believed to be the cause of this difference in behavior between the ethylene and styrene series.

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