AROMATIC SUBSTITUTION: PART V. THE MECHANISM OF THE REACTION OF PHENYLLITHIUM WITH PYRIDINES. DEUTERIUM ISOTOPE EFFECT STUDIES

1963 ◽  
Vol 41 (12) ◽  
pp. 3127-3131 ◽  
Author(s):  
R. A. Abramovitch ◽  
Choo-Seng Giam
Keyword(s):  
Isotope Effect ◽  
Experimental Error ◽  
Aromatic Substitution ◽  
Deuterium Isotope Effect ◽  
Hydride Ion ◽  
Nucleophilic Aromatic Substitutions ◽  
Mechanism Of The Reaction

The ratios of isomers formed in the reaction of 3-alkylpyridines with phenyllithium under various conditions appear to be invariant within the limits of experimental error. A number of experiments, including the reaction of phenyllithium with 3-picoline-2d and with an excess of a mixture of pyridine and pyridine-2d, are described which establish that the hydride ion elimination stage in these nucleophilic aromatic substitutions is not important in determining the observed orientation and that the addition stage is either virtually irreversible or that the equilibrium lies far on the product side and is not rapidly reversible. A convenient new procedure is described for working up the reaction mixtures.

The effect of steric crowding on an E2 transition state

1976 ◽  
Vol 54 (14) ◽  
pp. 2339-2341 ◽  
Author(s):  
George Stanley Dyson ◽  
Peter James Smith
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Nitrogen Isotope ◽  
Bond Rupture ◽  
Deuterium Isotope Effect ◽  
Steric Crowding ◽  
Reaction Proceeds ◽  
Hydrogen Deuterium ◽  
Nitrogen Bond ◽  
Mechanism Of The Reaction

The mechanism of the reaction of 9-(4-substituted benzyl)fluorene-9-trimethylammonium ions with ethoxide is a normal E2 process. The magnitude of the primary hydrogen–deuterium isotope effect at 60 °C increased with increasing electron-donating ability of the 4-substituent, i.e., 4.15, 5.10, 5.34, 5.65, 5.75, and 5.91 for the 4-CF3, 4-Br, 4-Cl, 4-H, 4-CH3, and 4-OCH3 substituents, respectively. The magnitude of the nitrogen isotope effect at 70 °C decreased with increased electron-donating power of the 4-substituent, i.e., [(k14/k15)–1]100 = 1.24, 0.95, 0.92, 0.91, and 0.80 for the 4-CF3, 4-F, 4-H, 4-CH3, and 4-OCH3 substituents, respectively. A small Hammett ρ value of +1.33 was observed for the reaction. It is concluded that the reaction proceeds via a transition state where the proton is more than one-half transferred to base. It is further concluded that for a reaction in which the 4-substituents decrease the rate, both carbon–hydrogen and carbon–nitrogen bond rupture is more advanced in the transition state. This variance with Hammond's postulate is discussed in the light of steric crowding at the transition state.

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