The unexpected case of reactions of halogens and interhalogens with halide substituted Pd(ii) σ-butadienyl complexes

Abstract

We have experimentally studied and theoretically interpreted the addition under stoichiometric conditions of halogens or interhalogens to σ-butadienyl palladium complexes bearing the heteroditopic thioquinolines as spectator ligands. The observed reactions do not involve the expected extrusion of the butadienyl fragment but rather the unpredictable substitution of the halide coordinated to palladium and in some cases also of that bound to the terminal butadienyl carbon. We have explained this peculiar reactivity with a mechanistic hypothesis based on a sequence of selective processes of oxidative addition and reductive elimination involving Pd(iv) intermediates.

Reactivity of N-heterocyclic carbene–pyridine palladacyclopentadiene complexes toward halogen addition. The unpredictable course of the reaction

“…9.3 ppm confirms that the opening of the cyclic butadiene occurs trans to the carbene carbon also in this case (ref. 4 and references therein). The four different frequencies of the ester methyl protons and carbons in the 1 H and 13 C NMR spectra, the diastereotopicity of the methylenic -CH 2 -protons, the carbene carbon bound to palladium at 165-170 ppm and the terminal butadienyl carbons bound to iodide at ca.…”

“…The organometallic palladacyclopentadiene fragment Pd(C-COOR) 4 is an important synthetic intermediate that is involved in Pd(0) catalyzed [2 + 2 + 2] alkyne cyclotrimerization and alkyne-alkene, alkyne-diene or alkyne-allene cocyclotrimerization. 1 Its synthesis is based on a well-established protocol 1a-e and the mechanism of its formation was deeply investigated and is well known.…”

Unveiling the promising anticancer activity of palladium(ii)–aryl complexes bearing diphosphine ligands: a structure–activity relationship analysis

“…However, it is important to point out that the halide metathesis carried out with IBr or ICl is selective only in the case of Pd( ii ) complexes bearing N^S ligands. 42 In fact, when P^P ligands are used, the same process leads to mixtures of products, the identities of which are difficult to establish.…”

“…41 Based on studies previously conducted by our group, the presence of an N^S ancillary ligand such as thioquinoline derivatives, enables the replacement of the iodide ligand with a bromide or a chloride by the simple addition of a stoichiometric amount of IBr or ICl, respectively. 42 This reaction is particularly interesting as it avoids the use of silver-based dehalogenating agents, which are light-sensitive and often difficult to remove completely. In addition, the cationic intermediates that form before the entry of the new halide are generally not very stable, leading to very low yields or even preventing the isolation of the product of interest.…”