Quantification of the Electrophilicities of Diazonium Ions

Abstract

AbstractThe kinetics of the reactions of arenediazonium ions with arenes, alkenes, allylsilanes, allylstannanes, and silyl ketene acetals have been studied in acetonitrile solution. The reactions follow second‐order kinetics, and in several cases rate‐determining attack of the diazonium ion has been proven by kinetic isotope effect studies (1a + 2b), by the independence of the allylsilane reactivities of the rate of desilylation (1a + 10a, d) and by the independence of the rate constants of the diazonium counterion. A decrease of the rate constant with increasing solvent donor ability (correlation with Gutmann's donor number) was found. The reactions of diazonium ions with π‐nucleophiles roughly follow the correlation lgk (20°C) = s (E + N), previously derived for the reactions of carbocations with nucleophiles. With the E parameters derived for diazonium ions, rate constants for azo couplings with aromatic and nonaromatic π‐nucleophiles can be predicted with an accuracy of ≈102. On the basis of E, the electrophilic reactivities of diazonium ions can be compared with those of carbocations (Figure 9), and the combination with the nucleophilicity parameters N (Figure 10) gives a first clue on possible azo coupling reactions. Literature reports are discussed within this scheme.