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.

REACTION MECHANISM STUDIES: 3. SOLVENT EFFECTS AND THE NATURE OF THE TRANSITION STATE IN THE DIAXIAL → DIEQUATORIAL REARRANGEMENT

1965 ◽  
Vol 43 (4) ◽  
pp. 847-861 ◽  
Author(s):  
J. F. King ◽  
R. G. Pews
Keyword(s):  
Reaction Mechanism ◽  
Transition State ◽  
Solvent Effects ◽  
Ion Pair ◽  
Polar Solvents ◽  
Concerted Mechanism ◽  
Opening Ratio ◽  
Mechanism Of The Reaction

The rates of the diaxial → diequatorial rearrangement of 2β,3α-dibromocholestane (Ia), and the analogous bromohydrin p-toluenesulfonates (Id and Ie) and anisates (If and Ig) have been measured in various solvents. The change in rate with variation in solvent was found to correlate with the change in the ionizing power of the solvents. The sensitivity of the rate of rearrangement to changes in solvent ionizing power, as measured by the ratio of the rate of rearrangement in nitromethane to that in decalin, was found to be smaller for the dibromide (Ia) than for the esters (Id to Ig). A detailed discussion of the mechanism of the reaction is presented. It is proposed that the following factors (either singly or simultaneously) could lead to the smaller sensitivity to solvent change shown by the dibromide (Ia) as compared with the esters (Id to Ig): (1) a difference in a postulated change of the axial: equatorial opening ratio with change in solvent, and (2) the development of a smaller charge in the transition state for the rearrangement of the dibromide (Ia) as compared with that in the rearrangement of the esters, when the reactions are carried out in the less polar solvents. It is argued that the operation of the latter factor would be most simply interpreted in terms of a merged ion-pair, cyclic-concerted mechanism, as initially suggested by Grob and Winstein.

Reactions of an Aluminium(I) Reagent with 1,2-, 1,3- and 1,5-Dienes: Dearomatisation, Reversibility, and a Pericyclic Mechanism

2019 ◽  
Author(s):  
Clare Bakewell ◽  
Martí Garçon ◽  
Richard Y Kong ◽  
Louisa O'Hare ◽  
Andrew J. P. White ◽  
...  
Keyword(s):  
Reaction Mechanism ◽  
Dft Calculations ◽  
Transition State ◽  
Reaction Pathways ◽  
Aromatic Rings ◽  
Aromatic Character ◽  
Pericyclic Reaction

The reactions of an aluminium(I) reagent with a series of 1,2-, 1,3- and 1,5-dienes are reported. In the case of 1,3-dienes the reaction occurs by a pericyclic reaction mechanism, specifically a cheletropic cycloaddition, to form aluminocyclopentene containing products. This mechanism has been interrogated by stereochemical experiments and DFT calculations. The stereochemical experiments show that the (4+1) cycloaddition follows a suprafacial topology, while calculations support a concerted albeit asynchronous pathway in which the transition state demonstrates aromatic character. Remarkably, the substrate scope of the (4+1) cycloaddition includes dienes that are either in part, or entirely, contained within aromatic rings. In these cases, reactions occur with dearomatisation of the substrate and can be reversible. In the case of 1,2- or 1,5-dienes complementary reactivity is observed; the orthogonal nature of the C=C π-bonds (1,2-diene) and the homoconjugated system (1,5-diene) both disfavour a (4+1) cycloaddition. Rather, reaction pathways are determined by an initial (2+1) cycloaddition to form an aluminocyclopropane intermediate which can in turn undergo insertion of a further C=C π-bond leading to complex organometallic products that incorporate fused hydrocarbon rings.

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.

Determination and use of a transition state for the enzyme estrone sulfatase (ES) from a proposed reaction mechanism

2001 ◽  
Vol 11 (23) ◽  
pp. 3001-3005 ◽  
Author(s):  
Sabbir Ahmed ◽  
Karen James ◽  
Caroline P Owen ◽  
Chirag K Patel ◽  
Mijal B Patel
Keyword(s):  
Reaction Mechanism ◽  
Transition State ◽  
Estrone Sulfatase

SECONDARY KINETIC ISOTOPE EFFECTS IN THE SOLVOLYSES OF 2-(2,4-DIMETIIOXYPHENYL)-1,1-d2-ETHYL AND 2-(3,5-DIMETHOXYPHENYL)-1,1-d2-ETIIYL p-BROMOBENZENESULFONATES

1966 ◽  
Vol 44 (21) ◽  
pp. 2491-2495
Author(s):  
C. C. Lee ◽  
L. Noszkó
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Similar Data ◽  
Kinetic Isotope ◽  
Ion Stability ◽  
Transition State Structures

The secondary α-deuterium kinetic isotope effects in the acetolyses and formolyses of 2-(2,4-dimethoxyphenyl)-ethyl and 2-(3,5-dimethoxyphenyl)-ethyl p-bromobenzenesulfonates (I-OBs and II-OBs, respectively) and their corresponding 1,1-dideuterio analogues (I-OBs-1-d2 and II-OBs-1-d2) were determined. The observed kH/kD values are compared with similar data from the literature for 2-phenylethyl and 2-p-anisylethyl p-toluenesulfonates (III-OTs and IV-OTs, respectively). In the formolyses of III-OTs, IV-OTs, and I-OBs, which proceed either chiefly or exclusively by way of bridged ions as intermediates, the isotope effects appear to increase slightly with increasing bridged-ion stability. For I-OBs and I-OBs-1-d2, acetolyses gave smaller kH/kD values than formolyses because of deuterium scrambling caused by ion-pair returns during the acetolysis. Acetolyses and formolyses of II-OBs and II-OBs-1-d2 gave lower isotope effects than the corresponding reactions with I-OBs and I-OBs-1-d2. The magnitudes of the observed isotope effects in relation to transition-state structures are discussed.

scholarly journals Theoretical study on pegylation reaction mechanisms of IFN-α-2a, IFN-α-2b and IFN-β-1a

2017 ◽  
Vol 82 (7-8) ◽  
pp. 841-850
Author(s):  
Mohammad Taqavian ◽  
Daryoush Abedi ◽  
Fatemeh Zigheimat ◽  
Leila Zeidabadinejad
Keyword(s):  
Polyethylene Glycol ◽  
Reaction Mechanism ◽  
Dft Calculations ◽  
Transition State ◽  
Reaction Mechanisms ◽  
Theoretical Study ◽  
Computational Study ◽  
Single Transition ◽  
Experimental Works ◽  
Ab Initio And Dft

Ab initio and DFT calculations have been carried out to study the reaction mechanism between interferons (IFNs) ?-2a, ?-2b and ?-1a and polyethylene glycol (PEG) group. The calculations show that the mechanisms are concerted, in agreement with the results of experimental works. However, although it appears that there is one single transition state, the characteristics of its structure reveal a very synchronous reaction mechanism. The reactions are clearly exothermic and as well have feasible activation energies. Our computational study shows that the lowest transition state energies are related to Lys 134, His 34 and Met 1 of IFN-?-2a, IFN-?-2b and IFN-?-1a, respectively.

scholarly journals Mechanistic Molecular Motion of Transition-Metal Mediated Beta-Hydride Transfer: Quasiclassical Trajectories Reveal Dynamically Ballistic, Dynamically Unrelaxed, Two Step, and Concerted Mechanisms

2020 ◽  
Author(s):  
Josh Wheeler ◽  
Ryan Carlsen ◽  
Daniel Ess
Keyword(s):  
Transition State ◽  
Density Functional ◽  
Hydrogen Transfer ◽  
Transition States ◽  
Surface Shape ◽  
Concerted Mechanism ◽  
Hydrogen Elimination ◽  
Migratory Insertion ◽  
One Step ◽  
Quasiclassical Trajectories

<div>The transfer of a -hydrogen from a metal-alkyl group to ethylene is a fundamental</div><div>organometallic transformation. Previously proposed mechanisms for this transformation involve either a</div><div>two-step -hydrogen elimination and migratory insertion sequence with a metal hydride intermediate</div><div>or a one-step concerted pathway. Here, we report density functional theory (DFT) quasiclassical direct</div><div>dynamics trajectories that reveal new dynamical mechanisms for the -hydrogen transfer of</div><div>[Cp*RhIII(Et)(ethylene)]</div><div>Despite the DFT energy landscape showing a two-step mechanism with a Rh-H</div><div>intermediate, quasiclassical trajectories commencing from the -hydrogen elimination transition state</div><div>revealed complete dynamical skipping of this intermediate. The skipping occurred either extremely fast</div><div>(typically <100 femtoseconds (fs)) through a dynamically ballistic mechanism or slower through a</div><div>dynamically unrelaxed mechanism. Consistent with trajectories begun at the transition state, all</div><div>trajectories initiated at the Rh-H intermediate show continuation along the reaction coordinate. All of</div><div>these trajectory outcomes are consistent with the Rh-H intermediate <1 kcal/mol stabilized relative to</div><div>the -hydrogen elimination and migratory insertion transition states. For Co, which on the energy</div><div>landscape is a one-step concerted mechanism, trajectories showed extremely fast traversing of the</div><div>transition-state zone (<50 fs), and this concerted mechanism is dynamically different than the Rh</div><div>ballistic mechanism. In contrast to Rh, for Ir, in addition to dynamically ballistic and unrelaxed</div><div>mechanisms, trajectories also stopped at the Ir-H intermediate. This is consistent with an Ir-H</div><div>intermediate that is stabilized by ~3 kcal/mol relative to the -hydrogen elimination and migratory</div><div>insertion transition states. Overall, comparison of Rh to Co and Ir provides understanding of the</div><div>relationship between the energy surface shape and resulting dynamical mechanisms of an</div><div>organometallic transformation.</div>

Eliminations promoted by weak bases. IX. Stereochemistry of olefin formation from trans-2-Phenylcyclopentyl p-bromobenzenesulfonate promoted by lithium chloride in acetone

1983 ◽  
Vol 36 (9) ◽  
pp. 1821 ◽  
Author(s):  
DJ McLennan ◽  
C Lim
Keyword(s):  
Transition State ◽  
Kinetic Analysis ◽  
Lithium Chloride ◽  
Ion Pairs ◽  
Chloride Ion ◽  
Ion Pair ◽  
Current Idea ◽  
Alternative Models ◽  
Weak Bases ◽  
Rate Limiting

Parker, Winstein, and their coworkers have previously established that in the E2C elimination of trans-2-phenylcyclopentyl p-bromobenzenesulfonate induced by Bu4NCl in acetone some 9% of the olefinic product is produced by a syn-elimination. In view of the current idea that syn-eliminations in solution are assisted by association of the base with its counterion, the stereochemistry of the reaction induced by lithium chloride in acetone has been studied. There is no increase in the amount of syn-elimination, and kinetic analysis reveals that lithium chloride ion pairs are completely unreactive. 1-Phenylcyclopentene is not produced by rate-limiting attack of chloride ion on a preformed symmetrical phenonium ion pair. These results do not serve to distinguish between two alternative models of the E2C transition state.

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