Studies of the borderline between stepwise and concerted mechanisms of elimination: leaving group effects in elimination of (fluoren-9-ylmethyl)-ammonium ions

1983 ◽  
Vol 36 (9) ◽  
pp. 1831 ◽  
Author(s):  
F Larkin ◽  
RAM O'Ferrall
Keyword(s):  
Proton Transfer ◽  
Sodium Methoxide ◽  
Reaction Path ◽  
Intramolecular Proton Transfer ◽  
Ammonium Ions ◽  
Enhancing Effect ◽  
Leaving Group ◽  
Substituent Constants ◽  
Leaving Groups ◽  
Intermediate Rate

By comparison with related substrates β-elimination of (fluoren-9-ylmethyl)trimethylammonium ion to form dibenzofulvene in methanolic sodium methoxide is unexpectedly rapid. The reaction has been investigated as a possible example of an E2 mechanism at the borderline between concerted and stepwise reaction paths. Evidence from buffer saturation in dimethylaminoacetonitrile buffers and comparisons with other amine leaving groups suggests, however, that the mechanism is probably ElcB, with ionization to a carbanion intermediate rate-determining. As measured by σ* for the trimethylammonium group the rate of elimination is 105 times faster than predicted from a Taft correlation of rates of carbanion formation. Probably this reflects the sensitivity of substituent constants for monopoles to solvent and reaction, and a rate-enhancing effect from N-methylation of (fluoren-9-ylmethy1)amine. Comparison of the trimethylammonium leaving group with eliminations of neutral amines also confirms an earlier conclusion that intramolecular proton transfer is not an important reaction path in eliminations of amines activated by a β-fluorenyl group.

scholarly journals Theoretical Study of the H-+CH3Cl SN2 Reaction

1979 ◽  
Vol 32 (10) ◽  
pp. 2289 ◽  
Author(s):  
ER Talaty ◽  
JJ Woods ◽  
G Simons
Keyword(s):  
Ab Initio ◽  
Theoretical Study ◽  
Reaction Path ◽  
Planar Structure ◽  
Sn2 Reaction ◽  
Leaving Group ◽  
Leaving Groups

A theoretical study of the C3v reaction path of the H- + CH3Cl → CH4+Cl- SN2 reaction with the use of the GAUSSIAN 70 program reveals an earlier departure of the leaving group than in other ab initio studies of similar reactions involving poorer leaving groups, and predicts an intermediate planar structure that is highly ionic and which should be represented as a tight ion triplet, H-[CH3+]Cl-.

scholarly journals Dependence of tautomerism on substituent type in o-hydroxy Schiff bases

2019 ◽  
Vol 38 (1) ◽  
pp. 85 ◽  
Author(s):  
Çiğdem Albayrak Kaştaş ◽  
Gökhan Kaştaş
Keyword(s):  
Proton Transfer ◽  
Schiff Bases ◽  
Substituent Effect ◽  
Density Functional ◽  
Energy Surface ◽  
Intramolecular Proton Transfer ◽  
Functional Theory ◽  
Phenolic Oxygen ◽  
Substituent Constants ◽  
Transfer Potential

Quantum computational methods were used to elucidate the structures of the o-hydroxy Schiff bases with different substituents. It is possible for a Schiff base to have different tautomeric structures depending on intramolecular proton transfer from the phenolic oxygen atom to the nitrogen atom. Proton transfer results in two tautomeric structures known as the phenol-imine and keto-amine forms. To explain the substituent effect on the proton transfer process in five o-hydroxy-Schiff bases, possible geometric structures in gas phase were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. To describe tautomerism including intramolecular proton transfer, potential energy surface (PES) scans were performed starting from the optimized geometry of the phenol-imine form. HOMA indices were calculated in order to estimate p-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.

The effect of steric size of leaving group on rates of the competing syn- and anti-pathways in biomolecular elimination

1980 ◽  
Vol 45 (8) ◽  
pp. 2171-2178
Author(s):  
Jiří Závada ◽  
Magdalena Pánková
Keyword(s):  
Comparative Analysis ◽  
Homologous Series ◽  
Leaving Group ◽  
Leaving Groups

Approximate rates of the competing syn- and anti-pathways have been determined in t-C4H9OK-t-C4H9OH promoted elimination from two homologous series of tosylates: I-OTs trans-III (R = H, CH3, C2H5, n-C3H7, i-C3H7, t-C4H9) and II-OTs trans-IV (R = CH3, C2H5, n-C3H7, i-C3H7, t-C4H9). A comparison has been made with rates of the same processes in the (+) elimination of the corresponding trimethylammonium salts I-N(CH3)3 trans-III and (+) II-N(CH3)3 trans-IV. The title effect is demonstrated by a comparative analysis of the rate patterns obtained for the two leaving groups.

scholarly journals An enhanced sampling QM/AMOEBA approach: The case of the excited state intramolecular proton transfer in solvated 3-hydroxyflavone

2021 ◽  
Vol 154 (18) ◽  
pp. 184107
Author(s):  
Michele Nottoli ◽  
Mattia Bondanza ◽  
Filippo Lipparini ◽  
Benedetta Mennucci
Keyword(s):  
Excited State ◽  
Proton Transfer ◽  
Intramolecular Proton Transfer ◽  
Enhanced Sampling
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