The Substitution Proper of C-Coupling Reactions

For a long time azo coupling was considered to belong to the group of electrophilic aromatic substitutions. 

3 Higher pH (pH 9) is, however, not to be recommended because of the competing homolytic dediazoniation (i.e. decomposition) of diazonium ions 

The discussions in the 50 s on the mechanism of these substitutions concentrated mainly on the problem as to whether they were one-step reactions in which the release of hydrogen is associated with the rate-determining part of the reaction (the so-called Sg3 mechanism), or if they were two-step reactions (Sg2) in which the initial attack of the electrophile on the aromatic substrate is followed by the release of the hydrogen ion. 

Unequivocal evidence for the Sg2 mechanism came in 1955 in three papers by Zollinger i i ), in which general base catalysis and primary kinetic hydrogen isotope effects of azo coupling reactions of various types were treats. In particular, such reactions with no isotope effects (k,j/kp s 1.0) and no general base catalysis, others with large isotope effects (k /kp = 6.5) and (practically) lin r base catalysis, and intermediate cases with isotope effects around 3.0 and less-than-linear base catalysis were found. 

All these reactions fit the steady-state equation (52) for the 8 2 mechanism (50,51). 

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Applications of MO calculations to studies of reaction mechanisms are of limited value because of the considerable demands placed on the computational method to guarantee reliable results. It is not always easy to formulate the task properly <89CHE1321>. Nevertheless, efforts to elucidate the diazo coupling reaction of imidazole have been assisted by MNDO calculations of the heats of formation of the possible Wheland intermediates <89JCS(P2)2055> (see Section 3.02.5.3.7), and an FMO analysis of the effects of methyl- and benzo-substitution on the facility of proton transfer from C-2 of imidazole, benzimidazole, and their 1-methyl derivatives has thrown light on the mechanism <86CJC1240>. 

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