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Stereochemistry of the σ‐Complex Intermediate in Sterically Hindered Electrophilic Aromatic Substitutions. (20th communication on diazo coupling reactions)
Author(s) -
Snyckers F.,
Zollinger H.
Publication year - 1970
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19700530606
Subject(s) - chemistry , steric effects , diazo , reaction rate constant , substituent , electrophile , photochemistry , kinetic isotope effect , intramolecular force , medicinal chemistry , stereochemistry , catalysis , kinetics , deuterium , organic chemistry , physics , quantum mechanics
The kinetics of the diazo coupling reactions of diazotized sulfanilic acid with 9 derivatives of 2‐naphthol, each containing a substituent in the 8‐position, have been measured. The reactions proceed by general base catalysis. The rate constants ( k 1 ) for the formation of the steady‐state intermediate in the presence of varying concentrations of a base (pyridine) have been evaluated from rate measurements. Similarly the ratios k 2 / k 1 (where k 2 is the rate constant for proton transfer from the steady‐state intermediate to the base, and k −1 is the rate constant for the dissociation of the intermediate) have been determined. The dependence of log k 1 on σ + m and of the ratio k 2 / k −1 on a steric parameter R f (defined in this paper) is interpreted as evidence for the steady‐state intermediate existing as a benzodienone σ‐complex with the sp 3 ‐bound hydrogen in a pseudo‐equatorial position and the electrophile pseudo‐axial. A sterically caused destabilisation of the intermediate can be excluded. The steric influence on the ratio k 2 / k −1 is therefore due to the steric influence on k 2 only. Diazo coupling of 8‐(2′‐pyridyl)‐2‐naphthol is subject to intramolecular base catalysis, as demonstrated by kinetic hydrogen isotope effects.