Computational study of the cyclopalladation mechanism of azobenzene with PdCl2 in N,N-dimethylformamide

Mechanism of Mechanochemical C−H Bond Activation in an Azobenzene Substrate by PdII Catalysts

“…Taking into account the clear in situ observation of the adducts as well as the acceleration effect of DMF or H 2 O that both enable the C−H bond cleavage by facilitating proton elimination, whereas MeCN addition retards the reaction, breaking of the C−H bond is most likely the slow step in the palladation by PdCl 2 under milling conditions. This agrees with the studies in solution and calculations (Scheme ) . Observed rates for the C−H bond cleavage could be related to the basicity of the employed additives which according to Gutmann's scale increases in order: MeCN<H 2 O<DMF.…”

“…Studies of the palladation of azobenzenes by PdCl 2 and PdCl 2 (DMF) 2 suggest that mechanism of double palladation in solution consists of four successive steps (Scheme ): i) formation of the mononuclear adduct A in which one azo‐nitrogen is bound to the palladium; ii) intramolecular C−H bond activation resulting in the formation of a monopalladated intermediate M ; iii) formation of a monopalladated adduct MA in which the free azo‐nitrogen coordinates the second Pd precursor; and iv) the second intramolecular C−H bond activation producing a dipalladated product D . Taking this into account, we have tried to obtain possible intermediates, that is, the AZB adduct species, the monopalladated complex and its adduct (Scheme ).…”

Dicyclopalladated Complexes of Asymmetrically Substituted Azobenzenes: Synthesis, Kinetics and Mechanisms

“…The kinetic results obtained herein for the monocyclopalladated complexes ( C1 ) are consistent with a reaction mechanism in which azobenzene ( L ) coordinates the Pd atom through the azo-nitrogen in the first fast pre-equilibrium step producing the mononuclear adduct LPdCl 2 (DMF) (α- R ), from which the intramolecular C–H bond activation starts in the second rate-determining step, as shown in Scheme . 1 H NMR and computational studies of analogous reactions indicated that the adducts with two ligands were not present, , although such species were found as the first intermediates in the cyclopalladation of amines …”

“…The monocyclopalladation mechanism for the nonsubstituted azobenzene was studied by Babićet al 8 The study ruledout the coordinated chlorides and the solvent molecules from the bulk as the proton-accepting species and the dimeric adduct species as the reactants entering the cyclopalladation. The proton transfer was found to be the rate-determining step.…”

“…Although a number of synthetic studies of monocyclopalladated azobenzenes have been reported, experimental support for the cyclopalladation mechanism of azobenzenes is still lacking. ,,, The generally accepted cyclopalladation mechanism is the electrophilic substitution in which the hydrogen dissociation is often nucleophilically assisted by the coordinated or free bases, which accept the leaving proton. , In addition, the experimental data related to monocyclopalladation suggest that the phenyl ring with an electron-donating substituent is more favorable for ortho -cyclopalladation . Furthermore, adducts in which the Pd atom is bound to azobenzenes through coordinative bonds to the azo-nitrogen are generally assumed to be the first intermediates in the cyclopalladation process .…”