Cyclometallation reactions. Part 17. Comparative studies of the manganation and palladation of some substituted azobenzenes

Abstract

Comparative studies of the manganation {with [MnMe(CO)₅]} and palladation (with PdCl₂) of several substituted azobenzenes have demonstrated that these complexes behave as nucleophilic and electrophilic reagents, respectively. Assignment of the isomeric structures of the products is facilitated by ¹⁹F n.m.r. spectroscopy using fluoroazobenzenes. Metallation of the 2- and 4-substituted azobenzenes XC₆H₄N:NPh may give products in which the azobenzene has been metallated either in the Ph ring [isomer (a)] or in the substituted ring [isomer (b)]; 3-substituted azobenzenes may give complexes in which the Ph ring has been metallated [isomer (a)], or isomers in which metallation occurs at either of the carbons ortho to the azo-function in the substituted ring [isomers (b) or (c)]. Using XC₆H₄N:NC₆F₅ the C₆F₅ group is not metallated, and the only complexes formed have structures corresponding to isomer (b)(with 2- or 4-substituted compounds), or to isomers (b) and (c)(in the 3-substituted series). Electron-withdrawing substituents activate the aromatic ring to attack by [MnMe(CO)₅], while electron-donating substituents activate the ring towards electrophilic attack by PdCl₂.