THERMAL (IODIDE) AND PHOTOINDUCED ELECTRON-TRANSFER CATALYSIS IN BIARYL SYNTHESIS VIA AROMATIC ARYLATIONS WITH DIAZONIUM SALTS

The dediazoniative arylation of various aromatic hydrocarbons (Ar'H) w ith diazonium salts (ArN2+) in acetonitrile can be readily effected to biaryls (Ar-Ar') in high yields, simply by the addition of small (cat alytic) amounts of sodium iodide. [In the absence of Ar'H, the competi tive iodination to ArI is nearly quantitative.] Iodide catalysis of bi aryl formation is efficiently mediated by aryl radicals (Ar-.) that pa rticipate in an efficient homolytic chain process in which ArN2+ acts as a 1-electron oxidant. The complex kinetics of such an electron-tran sfer chain or ETC process (Scheme 1) is quantitatively verified by com puter simulation of the Ar'H-dependent (a) competition between arylati on vs iodination and (b) catalytic efficiency of iodide, using the GEA R algorithms. ETC catalysis also pertains to the alternative photochem ical procedure for arylation (in the absence of iodide), in which the deliberate irradiation of the charge-transfer band of the precursor co mplex [ArN2+, Ar'H] initiates the same homolytic chain arylation. The latter underscores the mechanistic generality of the ETC formulation f or various types of catalytic dediazoniations of aromatic diazonium sa lts.