scholarly journals Spectroscopic and computational evidence for the concerted mechanism of the Wagner‐Jauregg reaction

2020 ◽  
Author(s):  
Samuel S. Tartakoff ◽  
Abigail A. Enders ◽  
Wenyao Zhang ◽  
Adam D. Hill
Keyword(s):  
Concerted Mechanism

Kinetic Studies of [4n+2]π-Thermal Cyclodimerization of 1-(3-Pyridazinyl)-3-oxidopyridinium Betaines

1992 ◽  
Vol 57 (9) ◽  
pp. 1951-1959 ◽  
Author(s):  
Madlene L. Iskander ◽  
Samia A. El-Abbady ◽  
Alyaa A. Shalaby ◽  
Ahmed H. Moustafa
Keyword(s):  
Energy Gap ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
First Order ◽  
Thermodynamic Activation Parameters ◽  
Cyclic Transition State ◽  
Complete Dissociation ◽  
Homo Lumo

The reactivity of the base induced cyclodimerization of 1-(6-arylpyridazin-3-yl)-3-oxidopyridinium chlorides in a pericyclic process have been investigated kinetically at λ 380 nm. The reaction was found to be second order with respect to the liberated betaine and zero order with respect to the base. On the other hand dedimerization (monomer formation) was found to be first order. It was shown that dimerization is favoured at low temperature, whereas dedimerization process is favoured at relatively high temperature (ca 70 °C). Solvent effects on the reaction rate have been found to follow the order ethanol > chloroform ≈ 1,2-dichloroethane. Complete dissociation was accomplished only in 1,2-dichloroethane at ca 70 °C. The thermodynamic activation parameters have been calculated by a standard method. Thus, ∆G# has been found to be independent on substituents and solvents. The high negative values of ∆S# supports the cyclic transition state which is in favour with the concerted mechanism. MO calculations using SCF-PPP approximation method indicated low HOMO-LUMO energy gap of the investigated betaines.

Reaction mechanism studies. 5. The mechanism of the diaxial → diequatorial rearrangement of β-chlorothioethers

1968 ◽  
Vol 46 (1) ◽  
pp. 9-13 ◽  
Author(s):  
J. F. King ◽  
K. Abikar
Keyword(s):  
Reaction Mechanism ◽  
Transition State ◽  
Nitro Group ◽  
Ion Pair ◽  
Concerted Mechanism

p-Methoxy- and p-nitro substituted analogues (1b and 1c) of the diaxial β-chlorothioether 2β-chloro-3α-(phenylthio)-5α-cholestane (1a), have been prepared and found to undergo the diaxial → diequatorial rearrangement. The rates of rearrangement of these compounds show the sequence p-methoxy > H > p-nitro. It is concluded that the transition state for the rearrangement is polarized in the sense of a sulfonium chloride (3). The rearrangement of 1a is 1600 times faster in butanol than in decalin (at 110°). There is thus no inherent insensitivity to solvent change in a rearrangement in which there may be a "four-atom arrangement" in the transition state, a conclusion relevant to previous work on the diaxial → diequatorial rearrangement of 1,2-dibromides (1). It was further found that the nitro group slowed the rearrangement (at 110°) more in butanol than in decalin, an observation regarded as consistent with, but not requiring, the incursion of a merged ion-pair, cyclic concerted mechanism.

scholarly journals Fast and Highly Chemoselective Alkynylation of Thiols with Hypervalent Iodine Reagents Enabled through a Low Energy Barrier Concerted Mechanism

2014 ◽  
Vol 136 (47) ◽  
pp. 16563-16573 ◽  
Author(s):  
Reto Frei ◽  
Matthew D. Wodrich ◽  
Durga Prasad Hari ◽  
Pierre-Antoine Borin ◽  
Clément Chauvier ◽  
...  
Keyword(s):  
Energy Barrier ◽  
Low Energy ◽  
Hypervalent Iodine ◽  
Concerted Mechanism

Kinetics and Mechanism of the Aminolysis of S-Aryl O-Ethyl Dithiocarbonates in Acetonitrile

2004 ◽  
Vol 69 (12) ◽  
pp. 2174-2182 ◽  
Author(s):  
Hyuck Keun Oh ◽  
Ji Young Oh ◽  
Dae Dong Sung ◽  
Ikchoon Lee
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Interaction Constant ◽  
Kinetics And Mechanism ◽  
Cyclic Structure ◽  
Concerted Mechanism ◽  
Tetrahedral Intermediate ◽  
Kinetic Isotope ◽  
Hydrogen Bonded

The aminolysis of S-aryl O-ethyl dithiocarbonates with benzylamines are studied in acetonitrile at -25.0 °C. The βX (βnuc) values are in the range 0.67-0.77 with a negative cross-interaction constant, ρXZ = -0.24, which are interpreted to indicate a concerted mechanism. The kinetic isotope effects involving deuterated benzylamine nucleophiles (XC6H4CH2ND2) are large, kH/kD = 1.41-1.97, suggesting that the N-H(D) bond is partially broken in the transition state by forming a hydrogen-bonded four-center cyclic structure. The concerted mechanism is enforced by the strong push provided by the EtO group which enhances the nucleofugalities of both benzylamine and arenethiolate from the putative zwitterionic tetrahedral intermediate.

Azorhizobium caulinodans chemotaxis is controlled by an unusual phosphorelay network

2021 ◽  
Author(s):  
Emily N. Kennedy ◽  
Sarah A. Barr ◽  
Xiaolin Liu ◽  
Luke R. Vass ◽  
Yanan Liu ◽  
...  
Keyword(s):  
Competitive Advantage ◽  
Bacterial Colonization ◽  
Root Nodules ◽  
Bacterial Species ◽  
Root Surface ◽  
Phosphoryl Group ◽  
Flagellar Motor ◽  
Azorhizobium Caulinodans ◽  
Concerted Mechanism ◽  
Phosphoryl Groups

Azorhizobium caulinodans is a nitrogen-fixing bacterium that forms root nodules on its host legume, Sesbania rostrata . This agriculturally significant symbiotic relationship is important in lowland rice cultivation, and allows for nitrogen fixation under flood conditions. Chemotaxis plays an important role in bacterial colonization of the rhizosphere. Plant roots release chemical compounds that are sensed by bacteria, triggering chemotaxis along a concentration gradient toward the roots. This gives motile bacteria a significant competitive advantage during root surface colonization. Although plant-associated bacterial genomes often encode multiple chemotaxis systems, A. caulinodans appears to encode only one. The che cluster on the A. caulinodans genome contains cheA , cheW , cheY2 , cheB , and cheR . Two other chemotaxis genes, cheY1 and cheZ , are located independently from the che operon. Both CheY1 and CheY2 are involved in chemotaxis, with CheY1 being the predominant signaling protein. A. caulinodans CheA contains an unusual set of C-terminal domains: a CheW-like/Receiver pair (termed W2-Rec), follows the more common single CheW-like domain. W2-Rec impacts both chemotaxis and CheA function. We found a preference for transfer of phosphoryl groups from CheA to CheY2, rather than to W2-Rec or CheY1, which appears to be involved in flagellar motor binding. Furthermore, we observed increased phosphoryl group stabilities on CheY1 compared to CheY2 or W2-Rec. Finally, CheZ enhanced dephosphorylation of CheY2 substantially more than CheY1, but had no effect on the dephosphorylation rate of W2-Rec. This network of phosphotransfer reactions highlights a previously uncharacterized scheme for regulation of chemotactic responses. IMPORTANCE Chemotaxis allows bacteria to move towards nutrients and away from toxins in their environment. Chemotactic movement provides a competitive advantage over non-specific motion. CheY is an essential mediator of the chemotactic response with phosphorylated and unphosphorylated forms of CheY differentially interacting with the flagellar motor to change swimming behavior. Previously established schemes of CheY dephosphorylation include action of a phosphatase and/or transfer of the phosphoryl group to another receiver domain that acts as a sink. Here, we propose A. caulinodans uses a concerted mechanism in which the Hpt domain of CheA, CheY2, and CheZ function together as a dual sink system to rapidly reset chemotactic signaling. To the best of our knowledge, this mechanism is unlike any that have previously been evaluated. Chemotaxis systems that utilize both receiver and Hpt domains as phosphate sinks likely occur in other bacterial species.

1,3-Dipolar cycloaddition of nitrones to oxa(aza)bicyclic alkenes

2019 ◽  
Vol 6 (19) ◽  
pp. 3360-3364 ◽  
Author(s):  
Yongqi Yao ◽  
Wen Yang ◽  
Qifu Lin ◽  
Weitao Yang ◽  
Huanyong Li ◽  
...  
Keyword(s):  
Dft Calculations ◽  
Reaction Pathways ◽  
Dipolar Cycloaddition ◽  
Concerted Mechanism ◽  
Mild Conditions

A new 1,3-dipolar cycloaddition of oxa(aza)bicyclic alkenes with nitrones has been developed without any catalyst and additive under mild conditions. The proposed concerted mechanism is investigated by DFT calculations of the reaction pathways.

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