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Free Rotation (free + rotation)

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Chemistry100%

Selected Abstracts

Oxidation of CH3NH2 and (CH3)2NH by NiIII(cyclam)(H2O)23+ in Aqueous Solutions

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 20 2004
Dror Shamir
Abstract NiII(1,4,8,11-tetraazacyclotetradecane)2+, NiIIL2+, is a good electrocatalyst for the oxidation of CH3NH2 and (CH3)2NH but not of (CH3)3N. The oxidation kinetics of the amines by NiIIIL(H2O)23+ indicate that the amines are good axial ligands to the tervalent nickel complex. The complexes NiIIIL[N(CH3)iH3,i](H2O)3+ are stronger oxidants than the complexes NiIIIL[N(CH3)iH3,i]23+. The oxidation is base-catalyzed and obeys a second-order rate law in NiIIILX2. It is proposed that the key step is NiIII,L(H2O)[N(CH3)iH2,i]2+ + NiIIILX2 , LNiII,N(=CH2)(CH3)i,1H2,i + NiIIL2+ + H3O+ + 2 X. Naturally, N(CH3)3 is not oxidized by this mechanism. Of special interest is the observation that the axial ligands CH3NH2 and (CH3)2NH are oxidized by the central cation, while the cyclam ligand, which has four secondary amine groups bound to the nickel(III) ion, and axially bound pendant primary amine groups, which are covalently linked to the macrocyclic ligand, are relatively stable. This difference in the behavior of axially bound amine groups is attributed to the free rotation of the axially bound N(CH3)iH3,i ligands that is required for the oxidation to proceed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

A New Neighbouring-Group Reaction to Form Pyridopyrrolobenzoxazinediones

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2004
Gerhard Hamprecht
Abstract The azaphthalimide 2a is the first phthalimide oxygen found to undergo a neighboring-group participation reaction with a vicinal N -phenyl carboxylic acid chloride upon nucleophilic addition with alcohols. Owing to the free rotation of the N -phenyl moiety, hetero-anellated benzoxazinedione isomers 3 and 4 are accessible, whereby 3 is preferred to 4 as the pyridine nitrogen in 2a preferentially activates the o -carbonyl group. Yields of up to 92% were obtained when bases such as HCl acceptors were avoided by heating 2a with alcohols. The reaction is restricted to primary and secondary alcohols, as 2a is nonplanar in respect of the heterocyclic and benzene moiety, which prevents tertiary alcohols attacking the pyrrolidinedione carbonyl group. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Non-Dipolar Behavior of Mesoionic Heterocycles: Synthesis and Tautomerism of 2-Alkylthioisomünchnones

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 13 2004
Martín Ávalos
Abstract This paper describes a general preparation of a series of 1,3-thiazolium-4-olates, each bearing an alkyl group at C-2, through reactions between N -arylthiocarboxamides and ,-haloacyl halides. Unlike the 2-aryl-substituted derivatives, such alkylated mesoionic compounds exist in equilibria with their non-dipolar tautomers, the corresponding 2-alkylidene-1,3-thiazolidin-4-ones. The unambiguous characterization of such tautomers and their relative stabilities have now been assessed by spectroscopic and computational studies. The presence of o,o, -disubstituted aryl groups at N-3 of the heterocyclic ring slows down free rotation around the N,Ar bond, thus opening access to a promising class of non-biaryl atropisomers. Finally, treatment of N -arylthioformamides with ,-haloacyl halides gives rise to N -acylthioformamides instead of the corresponding mesoionic species. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Racemization of the gastrointestinal antisecretory chiral drug esomeprazole magnesium via the pyramidal inversion mechanism: A theoretical study

CHIRALITY, Issue 9 2010
Hili Marom
Abstract The pyramidal inversion mechanisms of the 6-methoxy and the 5-methoxy tautomers of (S)-omeprazole were studied, employing ab initio and DFT methods. The conformational space of the model molecule (S)-2-[(3-methyl-2-pyridinyl)methyl]sulfinyl-1H -benzimidazole was calculated, with respect to rotations around single bonds, at the B3LYP/6-311G(d,p) level. All of the resulting conformations were used as starting points for full optimizations of (S)-omeprazole, at B3LYP/6-31G(d), B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-311G(2df,2pd), MP2/6-31G(d), and MP2/6-311G(d,p) levels. Four distinct pathways were found for enantiomerization via the pyramidal inversion mechanism for each of the tautomers of (S)-omeprazole. Each transition state, in which the sulfur, the oxygen and the two carbon atoms connected directly to the sulfur are in one plane, connects two diastereomeric minima. The enantiomerization is completed by free rotation around the sulfur,methylene bond, and around the methylene,pyridine ring bond. The effective Gibbs' free energy barrier for racemization ,G of the two tautomers of (S)-omeprazole are 39.8 kcal/mol (5-methoxy tautomer) and 40.0 kcal/mol (6-methoxy tautomer), indicating that the enantiomers of omeprazole are stable at room temperature (in the gas phase). The 5-methoxy tautomer of (S)-omeprazole was found to be slightly more stable than the 6-methoxy tautomer, in the gas phase. The energy barrier (,G,) for the(S,M) (S,P) diastereomerization of (S)-omeprazole due to the rotation around the pyridine chiral axis was very low, 5.8 kcal/mole at B3LYP/6-311G(d,p). Chirality 2010. © 2010 Wiley-Liss, Inc. [source]