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Electronic Factors (electronic + factor)
Selected AbstractsModulation of Spectrokinetic Properties of o -Quinonoid Reactive Intermediates by Electronic Factors: Time-Resolved Laser Flash and Steady-State Photolysis Investigations of Photochromic 6- and 7-ArylchromenesCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2009Jarugu, Narasimha Moorthy Prof. Abstract A variety of differently substituted 6- and 7-arylchromenes such as that depicted undergo photoinduced CO bond cleavage to yield colored o -quinonoid intermediates. A combined analysis of ,s,ms (laser flash) and real-time kinetic data show that the o -quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating. A ready synthetic accessibility of a series of 6- and 7-arylchromenes via Pd0 -catalyzed Suzuki coupling protocol has permitted a comprehensive investigation of the thermal decay behavior of a broad set of photogenerated o -quinonoid reactive intermediates. It is shown that substantial mesomeric effect between the benzopyran nucleus and the aryl ring at C6 or C7 position of the former renders significant absorption beyond 350,nm such that they are readily photoactivated to yield colored o -quinonoid intermediates. The absorption spectra of the latter are found to be strongly influenced by the substituents on C2-, C6- and C7-aryl rings; indeed the colored absorptions can be conveniently tuned by appropriate choice of substituents. The thermal decay (bleaching phenomenon), which is important from the point of view of their application in ophthalmic lenses, was investigated in each case by ,s,ms as well as real-time absorption spectroscopy. By careful experimentation, we have extracted the decay rate constants for Z,E and E,E o -quinonoid isomers of all 6- and 7-arylchromenes in an attempt to establish a correlation between the electronic attributes with their thermokinetic behavior. From a combined analysis of ,s,ms (laser flash) and real-time kinetic data, it is shown that the colored o -quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. In the same vein, the decay was found to occur slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron-donating. Thus, any substituent on the C2-aryl ring that contributes mesomerically to the development of charge on the quinonoid oxygen, and any substituent on the C6-/C7-aryl ring that exerts ,I effect appear to expedite thermal decay. Furthermore, evidence is obtained for the first time from ,s time-resolved laser-flash spectroscopy for the formation and characterization of the trans,cis (E,Z) o -quinonoid isomer, which has heretofore eluded spectral characterization in the photochromic phenomena of pyrans. [source] Polymers or Supramolecules Generated From a New V-Shaped Bis-monodentate Ligand and the Effect of Steric Hindrance on Coordination Modes of the LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2006Caihua Zhou Abstract A new V-shaped bis-monodentate ligand L (L = 2,3,-dipyridylamine) (1) has been designed and synthesized by alkylation reaction of pyridylamine. An investigation of the charge distributions of the coordination atoms and single-point energy calculations of four conformers of ligand L based on the geometry of conformers optimized by the DFT (density functional theory) method was carried out. The results show that the four conformers of ligand L take on two stable and two less stable configurations. Theory forecasts that two relatively stable configurations present in complexes as probable coordination motifs of the ligand, and that steric hindrance of pyridine nitrogen atoms in isomers will affect its coordination ability together with the electronic factor. This forecast has been demonstrated by the coordination chemistry of ligand L, that is, configuration (a) and (b) of the ligand occur in the following reported complexes, which combines with AgI or CuII through two coordination modes (bidentate bridging or a monodentate mode) resulting in coordination polymers {[Ag (L)2]NO3}n (2), [Cu2(L)2(maa)4]n (maa = methacrylic acid) (3), and the mononuclear molecule [Cu(L)4](ClO4)2·2CH3CH2OH (4). The ligand assumes different coordination modes in the three complexes because of different levels of steric hindrance of the pyridine nitrogen atoms in the conformers. Interestingly, polymers 2 and 3 assume a 1D helical structure and a linear framework, respectively, and 4 has a 2D supramolecular architecture induced from hydrogen bond interactions. In addition, the magnetic properties of 3 have been explored, which shows a strong antiferromagnetic interaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Studies of ethylene,styrene copolymerization with dinuclear constrained geometry complexes with methyl substitution at the five-membered ring in indenyl of [Ti(,5:,1 -C9H5SiMe2NCMe3)]2 [CH2]nJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2004Seok Kyun Noh Abstract The new dinuclear half-sandwich CGC (constrained geometry catalyst) with methyl substitution in indenyl, [Ti(,5:,1 -2-methylindenyl)SiMe2NCMe3]2 [(CH2)n] [n = 6 (10), n = 9 (11), n = 12 (12)], have been synthesized, and structure of these complexes has been characterized by 1H and 13C NMR. The most important feature is that two protons of methylene directly bonded to the indenyl ring become inequivalent to be shown as two separated resonances at 2.9 and 3.0 ppm, probably due to the formation of planar chirality caused by a titanium complex formation. It has been found that the dinuclear CGCs with methyl substitution at an indenyl ring were very active catalysts for ethylene and styrene copolymerization. The activity increases in the order of 10 < 11 < 12, which indicates that the presence of a longer bridge between two active sites contributes to facilitate the polymerization activity of the dinuclear CGC more effectively. This result might be understood by the implication that the steric factor rather than the electronic factor may play a major role to direct the polymerization behavior of the dinuclear CGC. It is found that the dinuclear catalysts are very efficient to incorporate styrene in the polyethylene backbone. The styrene contents in the formed copolymers ranged from 5 to 40% according to the polymerization conditions. One can observe strong signals at 29.7 ppm of the polyethylene sequences, and, in addition, peaks at 27.5, 36.9, and 46. 2ppm (S,,, S,,, and T,,, respectively) of sequences of EESEE. Weak peak at 25.3 ppm are attributed to S,,, which represents SES sequence. The absence of a signal for T,, at 41.3 ppm and for S,, at 43.6 ppm shows there is no styrene,styrene sequences in copolymers. This result indicates that the dinuclear CGC are very effective to generate well-distributed poly(ethylene- co -styrene)s. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1712,1723, 2004 [source] Molecular Library Obtained by Allene Insertion into the Pd,C Bond of Cyclopalladated Complexes: Biological and Pharmacological EvaluationEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2004Claude Sirlin Abstract A minilibrary of cationic N-heterocycles has been prepared and evaluated. The potential for the preparation was a result of the high versatility of palladium-mediated chemistry. The synthesis of the novel molecules was based on intramolecular quaternization of tertiary amine attached allylpalladium complexes. The steric and electronic factors of the reaction are discussed. The structures of the synthesized molecules made them candidates for precise biological and pharmacological evaluations. Of the various N-heterocyclic compounds, 2,2-dimethyl-3-methylenenaphtho[def]quinolizinium showed antibacterial activity at micromolar concentrations. This compound also proved to be a nanomolar competitive antagonist for the channel site of the nicotinic acetylcholine receptor. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Kinetics of elimination of several heterocyclic carbamates in the gas phase,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2002Yannely Brusco Abstract The kinetics of the gas-phase elimination of several heterocyclic carbamates were determined in a static system over the temperature range 190.0,409.7,°C and the pressure range 26.5,125,Torr (1,Torr,=,133.3,Pa). The reactions in seasoned vessels, with the free radical inhibitor cyclohexene and/or toluene always present, are homogeneous and unimolecular and obey a first-order rate law. The observed rate coefficients are represented by the following Arrhenius equations: for tert -butyl-1-pyrrolidine carboxylate, log k1 (s,1),=,(11.36,±,0.31) ,(145.4,±,3.1)kJ,mol,1 (2.303RT),1; for 1-(tert -butoxycarbonyl)-2-pyrrolidinone, log k1 (s,1),=,(11.54,±,0.29) ,(140.8,±,2.8)kJ,mol,1 (2.303RT),1; for tert -butyl-1-pyrrole carboxylate, log k1 (s,1),=,(12.12,±,0.05) ,(145.2,±,1.0),kJ,mol,1 (2.303RT),1; and for 1-ethylpiperazine carboxylate, log k1 (s,1),=,(12.05,±,0.19) ,(188.2,±,4.6),kJ,mol,1 (2.303RT),1. The saturated heterocyclic carbamates show a decrease in rates of elimination due to electronic factors. Heterocyclic carbamates with a nitrogen atom able to delocalize its electrons with ,-bonds present in the ring were found to enhance the rates due to resonance interactions. Copyright © 2002 John Wiley & Sons, Ltd. [source] Theoretical studies on the S,N interaction in sulfinamidesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2002Prasad V. Bharatam Abstract The potential energy surface of sulfinamide H(O)S,NH2 (1) was searched, using ab initio and density functional methods, to study the conformational preferences. High-accuracy G2MP2 calculations showed that the S,N rotational barrier in 1 is 7.0,kcal,mol,1. The inversion around N in 1 goes through a very low energy barrier. Charge analysis using the NPA method was performed to elucidate the electronic factors responsible for the observed trends in the S,N interactions. The strength of negative hyperconjugation in 1 was estimated using NBO analysis and by studying the substituent effect. The repulsions between the lone pairs on oxygen and nitrogen and the nN , ,*S,O negative hyperconjugation play an important role in the conformations. Copyright © 2002 John Wiley & Sons, Ltd. [source] Computational Investigations on the General Reaction Profile and Diastereoselectivity in Sulfur Ylide Promoted AziridinationCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2007Deepa Janardanan Abstract Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me2S+CH,R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHCNCO2Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3- trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches. [source] | ||||||||||