Structural and Mechanistic Studies on the Activation and Propagation of a Cationic Allylpalladium Procatalyst in 1,6-Diene Cycloisomerization

[Pd(η3-C3H5)(MeCN)2]OTf acts as an efficient procatalyst for the cycloisomerisation of dimethyl hept-1,6-dienyl-4,4-dicarboxylate (1 a) in CHCl3. The reaction displays a pronounced and variable induction period and gives dimethyl 3-methylene-4-methylcyclopentane-1,1-dicarboxylate (2 a) as the kinetic product. The thermodynamically more favourable tri- and tetra-substituted alkenes dimethyl 3,4-dimethylcylopent-2-ene-1,1-dicarboxylate (3 a) and dimethyl 3,4-dimethylcylopent-3-ene-1,1-dicarboxylate (4 a) are also generated directly (3 a) or by isomerisation (3 a and 4 a) of 2 a. The mechanism of procatalyst activation and the ensuing cycloisomerisation reaction was investigated by NMR spectroscopy (1H, 2H, 13C) and GC analysis of the products arising from isotopically labelled substrates (13C, 2H). Three general mechanisms were considered: hydrometallation, cyclometallation and C−H insertion. These last two were shown to be incompatible with the results. The first, which involves generation and propagation of a palladium hydride species (“Pd−H”), was found to be consistent with both the isotopic distribution and stereochemistry of the reaction product and is supported by the observation of intermolecular transfer of a single 2H label. Due to the high catalytic activity of the palladium hydride and its slow generation, the cycloisomerisation process ultimately yields a mixture of alkene products (2 a, 3 a and 4 a) with incomplete consumption of the procatalyst [Pd(η3-C3H5)(MeCN)2]OTf. The mechanism by which the catalytically active palladium hydride is generated from the procatalyst was studied in detail by NMR spectroscopic analysis of stoichiometric reactions between diene 1 a and [Pd(η3-C3H5)(MeCN)2]OTf. This demonstrated that a carbopalladated complex, namely, [Pd{7,7-(CO2Me)2-(1,2,5,9,10-η5)-dec-1,9-diene)}(OTf)] (15 a), is formed in small quantities by unfavourable displacement of acetonitrile by the diene, followed by a rapid and irreversible β-migratory insertion reaction. Although attempts to isolate 15 a from the reaction mixture were not successful (due to its slow decomposition, low concentration and competing cycloisomerisation), an alternative synthesis in the absence of acetonitrile allowed its isolation and characterisation. However, pure samples of 15 a are completely ineffective as a procatalyst system for cycloisomerisation of 1 a. Further investigation revealed that treatment of 15 a with one equivalent of water results in quantitative β-H elimination to generate triene 16 a (C(1)-allylated 1 a). Thus, addition of catalytic quantities of water to a solution of 1 a in CHCl3 containing 5 mol % 15 a and 10 mol % MeCN results in generation of an active “Pd−H” catalyst for cycloisomerisation. Although procatalyst activation is facilitated by traces of water, no exchange of protons is observed between “Pd−H” and H2O under catalytic turnover. The slow generation of 15 a and the requirement for traces of water for β-H elimination accounts for variability in the induction period when [Pd(η3-C3H5)(MeCN)2]OTf is employed as procatalyst.