QUANTIFICATION OF THE ELECTROPHILICITIES OF DIAZONIUM IONS

The kinetics of the reactions of arenediazonium ions with arenes, alke nes, allylsilanes, allylstannanes, and silyl ketene acetals have been studied in acetonitrile solution. The reactions follow second-order ki netics, and in several cases rate-determining attack of the diazonium ion has been proven by kinetic isotope effect studies (1a + 2b), by th e independence of the allylsilane reactivities of the rate of desilyla tion (1a + 10a, d) and by the independence of the rate constants of th e diazonium counterion. A decrease of the rate constant with increasin g solvent donor ability (correlation with Gutmann's donor number) was found. The reactions of diazonium ions with pi-nucleophiles roughly fo llow the correlation lgk (20 degrees C) = s (E + N), previously derive d for the reactions of carbocations with nucleophiles. With the E para meters derived for diazonium ions, rate constants for azo couplings wi th aromatic and nonaromatic pi-nucleophiles can be predicted with an a ccuracy of approximate to 10(2). On the basis of E, the electrophilic reactivities of diazonium ions can be compared with those of carbocati ons (Figure 9), and the combination with the nucleophilicity parameter s N (Figure 10) gives a first clue on possible azo coupling reactions. Literature reports are discussed within this scheme.