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

Abstract

The reactions of oxidative coupling of dimethylacetylene dicarboxylate (DMA) with palladium (0) olefin complexes bearing a pyridylthioether as ancillary ligand give rise to the corresponding palladacyclopentadienyl complexes as the main product and in some cases to the highly symmetric hexamethyl mellitate molecule, especially when a low alkyne(DMA)/complex ratio is employed. 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. 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. Hexamethyl mellitate is synthesized by means of several catalysts based on different metals with yields ranging from 10% to 65% [2]. In a few cases, palladium complexes were used [3]. The palladium mediated synthesis of the mellitate in fact requires longer time, high temperature and high alkyne excess [4]. Therefore, we decided to investigate exhaustively the reaction leading to the palladacyclopentadienyl species in the awareness that the formation of hexamethyl mellitate, albeit under unusually mild conditions, only represents an interesting side reaction. Thus, we planned an experimental network in which different complexes and different alkyne/complex ratios were analysed.

Mechanistic Studies of Transition-Metal-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

“…Treatment of 151 with dimethyl acetylenedicarboxylate, even at room temperature, led to the formation of the aromatic product. Canovese et al 205 Takahashi et al 211 studied the regioselective synthesis of 2322 zirconacyclopentadienes from the oxidative coupling of two 2323 different alkynes with a low-valent zirconocene precursor. 2324 However, due to the low nucleophilicity of the zirconacyclo-2325 pentadiene, a third unsaturation was not able to insert itself in 2326 the Zr−C bond and, therefore, no benzene derivative was 2327 generated.…”

“…Canovese et al reported the oxidative coupling of dimethyl acetylenedicarboxylate with palladium(0) olefin complexes bearing a pyridylthioether ancillary ligand. Reaction of the alkyne and the palladium complex gave palladacyclopentadiene complex as the main product together with some amounts of the palladium–alkyne complex and hexamethyl mellitate.…”

“…Canovese et al 205 alkyne and the palladium complex gave palladacyclopentadiene complex as the main product together with some amounts of the palladium−alkyne complex and hexamethyl mellitate. The amount of hexamethyl mellitate being formed depended on the alkyne/palladium complex ratio (at high ratios, its formation decreased) and the nature of the complex itself.…”

Synthesis, Structure, and Catalytic Reactivity of Pd(II) Complexes of Proline and Proline Homologs

“…Oxidative palladium(II) catalysis differs from these in that it utilizes molecular oxygen to regenerate the active catalyst in palladium(II) catalyzed coupling reactions. There are two proposed mechanisms for the catalytic cycle [14][15][16][17][18][19][20].…”

Mechanistic and Kinetic Investigation on the Formation of Palladacyclopentadiene Complexes. A Novel Interpretation Involving a Bimolecular Self-Reaction of a Monoalkyne Intermediate

“…As a matter of fact, type 2 complexes, which are produced by the equilibrium displacement of an alkene by an activated alkyne, represent a peculiar class of substrates that can accumulate as unreactive species, which, depending on their electronic and steric characteristics, react with an additional molecule of the alkyne to give palladacyclopentadiene derivatives or react with the palladacyclopentadiene itself to yield mellitate derivatives . The understanding of the initial equilibria in the formation of palladacycles is also of crucial importance for a better understanding of the intimate mechanism governing the first step of other catalytic cycles, such as the formation of functional dienes from coupling reactions between alkynes, organic halides, and organotin reagents. , …”