Attack of Substituted Alkynes on Olefin Palladium(0) Derivatives of Pyridylthioethers. The First Kinetic Study on the Mechanism of Formation of Palladacyclopentadiene Complexes

Abstract

The formation of metallacyclopentadienyl derivatives was studied under controlled conditions, and the kinetics and mechanism of reactions between pyridylthioether olefin Pd(0) substrates and substituted alkynes of the type ZCtCZ (Z ) COOMe, COOEt, COOt-Bu) leading to the corresponding palladacyclopentadienyl species were systematically investigated. In the case of less hindered ancillary ligands the attack of the alkyne forming a reactive monoalkyne intermediate was found to be the rate-determining step. In this respect the rates of reaction were discussed in terms of the substituent-induced basicity of sulfur and nitrogen of the ancillary ligands. The associative nature of the attack was unequivocally established, and the formation of a transition state bearing a monodentate ancillary ligand was proposed. In the case of more hindered ancillary ligands a partially stabilized monoalkyne intermediate is formed irrespective of the olefin in the starting complex, and this species strongly influences the reaction progress. Formation of hexamethylmellitate under mild conditions is also observed.

Oxidative coupling of activated alkynes with palladium(0) olefin complexes: Side production of the highly symmetric hexamethyl mellitate species under mild conditions at low alkyne/complex molar ratios

“…It is apparent that: At high alkyne/complex molar ratios the formation of hexamethyl mellitate decreases. This result must be considered general and conclusive and explains why no mellitate is observed under UV/Vis kinetic conditions when the formation of the palladacyclopentadienyl derivative is studied [1]. The highest mellitate production was obtained at different alkyne/complex ratios as a function of the ancillary ligand.…”

“…If we consider the mechanism of formation of palladacyclopentadiene complexes proposed elsewhere [1] and reported in Scheme 2, of which formation of mellitate can be thought of as a side reaction, we can rationalize these findings by supposing that the mellitate arises from slow reaction of the cyclopalladate main product with the monoalkyne palladium (0) Scheme 3. In this context, the reactivity of the monoalkyne derivatives 2 undergoing two competitive reactions is probably crucial. As a matter of fact the Pd (0) hindered ancillary ligand MeN-St-Bu gives rise to the formation of the largest quantity of monoalkyne species among all the studied pyridylthioether compounds [1].…”

“…A plausible mechanism based on several pieces of experimental evidence is proposed, and the most effective complexes promoting the hexamethyl mellitate synthesis were identified. Ó 2006 Elsevier B.V. All rights reserved.Keywords: Hexamethyl mellitate; Oxidative coupling; Pd(0) and Pd(II) pyridylthioether complexes; Activated alkynesWe have recently studied the mechanism of the attack of substituted alkynes on pyridylthioether olefin palladium(0) complexes leading to the formation of pyridylthioether palladacyclopentadiene derivatives; the reaction studied and the species involved are reported in Scheme 1 [1].It was soon apparent that the majority of the reactions studied under spectrophotometric conditions ([alkyne]/ [complex] P 50) proceeded smoothly to the formation of the palladacyclopentadienyl complexes. In the case of the complex bearing the hindered ancillary ligand MeN-StBu however, the reaction path was markedly complicated by the accumulation of a scarcely reactive intermediate which was identified as the monoalkyne Pd(0) derivative.…”

“…We have recently studied the mechanism of the attack of substituted alkynes on pyridylthioether olefin palladium(0) complexes leading to the formation of pyridylthioether palladacyclopentadiene derivatives; the reaction studied and the species involved are reported in Scheme 1 [1].…”

“…In the case of the complex bearing the hindered ancillary ligand MeN-StBu however, the reaction path was markedly complicated by the accumulation of a scarcely reactive intermediate which was identified as the monoalkyne Pd(0) derivative. The analysis of the kinetic problem by means of the NMR technique, while allowing a solution to the first puzzle, raised a further chemical question [1]. As a matter of fact, these complexes, when reacting with dimethylacetylene dicarboxylate (DMA) under NMR experimental conditions (T = 298 K, [complex] % 1 · 10 À2 mol dm À3 , 0.25 < [DMA]/ [complex] < 60) beside the main product cyclopalladate gave rise to the formation of the highly symmetric hexamethyl mellitate molecule.…”