scholarly journals Transition between [R]- and [S]-stereoisomers without bond breaking

2020 ◽  
Vol 22 (26) ◽  
pp. 14983-14991 ◽  
Author(s):  
Shampa Raghunathan ◽  
Komal Yadav ◽  
V. C. Rojisha ◽  
Tanashree Jaganade ◽  
V. Prathyusha ◽  
...  
Keyword(s):  
Transition State ◽  
Bond Breaking ◽  
Intermediate Structure ◽  
Planar Transition

First examples of racemization of tetrahedral tetracoordinated centers via a planar transition state or an intermediate structure.

scholarly journals Transition between [R]- and [S]-Stereoisomers without Bond Breaking

2020 ◽  
Author(s):  
Shampa Raghunathan ◽  
Komal Yadav ◽  
V. C. Rojisha ◽  
Tanashree Jaganade ◽  
V. Prathyusha ◽  
...  
Keyword(s):  
Transition State ◽  
Theoretical Calculations ◽  
Bond Breaking ◽  
Kinetic Properties ◽  
Intermediate Structure ◽  
Structure Dynamic ◽  
Planar Transition ◽  
First Time

We for the first time shown that transition between (R) and (S) stereoisomers via a planar transition state or an intermediate structure without having to break a bond is possible. Rigorous theoretical calculations have been used to study this novel phenomenon and to characterize the energetic, structure, dynamic and kinetic properties.

Transition between [R]- and [S]-Stereoisomers without Bond Breaking

2020 ◽  
Author(s):  
Shampa Raghunathan ◽  
Komal Yadav ◽  
V. C. Rojisha ◽  
Tanashree Jaganade ◽  
V. Prathyusha ◽  
...  
Keyword(s):  
Transition State ◽  
Theoretical Calculations ◽  
Bond Breaking ◽  
Kinetic Properties ◽  
Intermediate Structure ◽  
Structure Dynamic ◽  
Planar Transition ◽  
First Time

We for the first time shown that transition between (R) and (S) stereoisomers via a planar transition state or an intermediate structure without having to break a bond is possible. Rigorous theoretical calculations have been used to study this novel phenomenon and to characterize the energetic, structure, dynamic and kinetic properties.

Classical Racemization of Tetrahedral Tetracoordinate Centers via a Planar Transition State or an Intermediate Structure

2019 ◽  
Author(s):  
Shampa Raghunathan ◽  
Komal Yadav ◽  
V. C. Rojisha ◽  
Tanashree Jaganade ◽  
V. Prathyusha ◽  
...  
Keyword(s):  
Transition State ◽  
Theoretical Calculations ◽  
Kinetic Properties ◽  
Intermediate Structure ◽  
Structure Dynamic ◽  
Planar Transition ◽  
First Time

We for the first time shown that transition between (R) and (S) stereoisomers via a planar transition state or an intermediate structure without having to break a bond is possible. Rigorous theoretical calculations have been used to study this novel phenomenon and to characterize the energetic, structure, dynamic and kinetic properties.

Berechnung kinetischer Isotopeneffekte für die Reaktion von Methyljodid mit Cyanid-Ion

1966 ◽  
Vol 21 (9) ◽  
pp. 1377-1384
Author(s):  
A. V. Willi
Keyword(s):  
Transition State ◽  
Secular Equation ◽  
Isotope Effects ◽  
Force Constants ◽  
Bond Breaking ◽  
Bending Force ◽  
A Value ◽  
Order Of Magnitude ◽  
Deuterium Isotope Effects ◽  
The Difference

Kinetic carbon-13 and deuterium isotope effects are calculated for the SN2 reaction of CH3I with CN-. The normal vibrational frequencies of CH3I, the transition state I · · · CH3 · · · CN, and the corresponding isotope substituted reactants and transition states are evaluated from the force constants by solving the secular equation on an IBM 7094 computer.Values for 7 force constants of the planar CH3 moiety in the transition state (with an sp2 C atom) are obtained by comparison with suitable stable molecules. The stretching force constants related to the bonds being broken or newly formed (fCC, fCC and the interaction between these two stretches, /12) are chosen in such a way that either a zero or imaginary value for νʟ≠ will result. Agreement between calculated and experimental methyl-C13 isotope effects (k12/ k13) can be obtained only in sample calculations with sufficiently large values of f12 which lead to imaginary νʟ≠ values. Furthermore, the difference between fCI and fCC must be small (in the order of 1 mdyn/Å). The bending force constants, fHCI and fHCC, exert relatively little influence on k12/k13. They are important for the D isotope effect, however. As soon as experimental data on kH/kD are available it will be possible to derive a value for fHCC in the transition state if fHCI is kept constant at 0.205 mdynA, and if fCI, fCC and f12 are held in a reasonable order of magnitude. There is no agreement between experimental and calculated cyanide-C13 isotope effects. Possible explanations are discussed. — Since fCI and fCC cannot differ much it must be concluded that the transition state is relatively “symmetric”, with approximately equal amounts of bond making and bond breaking.

Mapping the conformational itinerary of β-glycosidases by X-ray crystallography

2003 ◽  
Vol 31 (3) ◽  
pp. 523-527 ◽  
Author(s):  
G.J. Davies ◽  
V.M.-A. Ducros ◽  
A. Varrot ◽  
D.L. Zechel
Keyword(s):  
Transition State ◽  
Reaction Pathway ◽  
Tight Binding ◽  
Three Dimensional ◽  
Intermediate Structure ◽  
Active Centre ◽  
X Ray ◽  
Solution Structures ◽  
X Ray Crystallography ◽  
Covalent Intermediate

The conformational agenda harnessed by different glycosidases along the reaction pathway has been mapped by X-ray crystallography. The transition state(s) formed during the enzymic hydrolysis of glycosides features strong oxocarbenium-ion-like character involving delocalization across the C-1–O-5 bond. This demands planarity of C-5, O-5, C-1 and C-2 at or near the transition state. It is widely, but incorrectly, assumed that the transition state must be 4H3 (half-chair). The transition-state geometry is equally well supported, for pyranosides, by both the 4H3 and 3H4 half-chair and 2,5B and B2,5 boat conformations. A number of retaining β-glycosidases acting on gluco-configured substrates have been trapped in Michaelis and covalent intermediate complexes in 1S3 (skew-boat) and 4C1 (chair) conformations, respectively, pointing to a 4H3-conformed transition state. Such a 4H3 conformation is consistent with the tight binding of 4E- (envelope) and 4H3-conformed transition-state mimics to these enzymes and with the solution structures of compounds bearing an sp2 hybridized anomeric centre. Recent work reveals a 1S5 Michaelis complex for β-mannanases which, together with the 0S2 covalent intermediate, strongly implicates a B2,5 transition state for β-mannanases, again consistent with the solution structures of manno-configured compounds bearing an sp2 anomeric centre. Other enzymes may use different strategies. Xylanases in family GH-11 reveal a covalent intermediate structure in a 2,5B conformation which would also suggest a similarly shaped transition state, while 2S0-conformed substrate mimics spanning the active centre of inverting cellulases from family GH-6 may also be indicative of a 2,5B transition-state conformation. Work in other laboratories on both retaining and inverting α-mannosidases also suggests non-4H3 transition states for these medically important enzymes. Three-dimensional structures of enzyme complexes should now be able to drive the design of transition-state mimics that are specific for given enzymes, as opposed to being generic or merely fortuitous.

Pyrazolines IX. Chemistry of some 3-cyano-3-carbomethoxy-1-pyrazolines

1969 ◽  
Vol 47 (21) ◽  
pp. 3965-3973 ◽  
Author(s):  
Donald E. McGreer ◽  
Y. Y. Wigfield
Keyword(s):  
Transition State ◽  
Kinetic Studies ◽  
Product Distribution ◽  
Bond Breaking ◽  
State Structure ◽  
Transition State Structure ◽  
Aryl Group

Product and kinetic studies have been made on the pyrolysis of a series of 3-cyano-3-carbomethoxy-1-pyrazolines substituted at C-4 with a methyl and an aryl group (phenyl, p-methoxyphenyl, and p-nitrophenyl). Evidence is presented supporting a transition state structure in which little progress to bond breaking of the C-5 to N bond has taken place at the transition state. The product distribution is largely dependent on the stereochemistry of the initial pyrazoline and to a lesser degree on the nature of the aryl group at C-4. The cis pyrazolines gave products resulting primarily from aryl migration while the trans pyrazolines gave products predominantly from methyl migration.

scholarly journals Ab initio Study of Nucleophilic Aromatic Substitution of Polyfluorobenzene

1999 ◽  
Vol 23 (9) ◽  
pp. 528-529
Author(s):  
Kiyoshi Tanaka ◽  
Makoto Deguchi ◽  
Satoru Iwata
Keyword(s):  
Transition State ◽  
Ab Initio ◽  
Transition States ◽  
Nucleophilic Aromatic Substitution ◽  
Bond Breaking ◽  
Aromatic Substitution ◽  
Ab Initio Study ◽  
Rate Determining Step ◽  
Elimination Mechanism ◽  
The Stability

Calculations at ab initio levels of theory of the nucleophilic aromatic substitution of pentafluoronitrobenzene with amines demonstrate an addition–elimination mechanism (SNAr), with the rate-determining step at the second transition state involving C–F bond breaking, and support the ortho-selectivity of the reactions based on the stability of the second transition states.

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