Structural and mechanistic studies on the activation and propagation of a cationic allylpalladium procatalyst in 1,6-diene cycloisomerization

[Pd(eta (3)-C3H5)(MeCN)(2)]OTf acts as an efficient procatalyst for the cyc loisomerisation of dimethyl hept-1,6-dienyl-4,4-dicarboxylate (1a) in CHCl3 . The reaction displays a pronounced and variable induction period and give s dimethyl 3-methylene-4-methylcyclopentane- 1,1 -dicarboxylate (2a) as the kinetic product. The thermodynamically more favourable tri- and tetra-subs tituted alkenes dimethyl 3,4-dimethylcylopent-2-ene-1,1-dicarboxylate (3a) and dimethyl 3,4-dimethylcylopent-3-ene-1,1-dicarboxylate (4a) are also gen erated directly (3a) or by isomerisation (3a and 4a) of 2a. The mechanism o f procatalyst activation and the ensuing cycloisomerisation reaction was in vestigated by NMR spectroscopy (H-1, H-2., C-13) and GC analysis of the pro ducts arising from isotopically labelled substrates (C-13, H-2). Three gene ral mechanisms were considered: hydrometallation, cyclometallation and C-H insertion. These last two were shown to be incompatible with the results. T he first, which involves generation and propagation of a palladium hydride species ("Pd-H"), was found to be consistent with both the isotopic distrib ution and stereochemistry of the reaction product and is supported by the o bservation of intermolecular transfer of a single H-2 label. Due to the hig h catalytic activity of the palladium hydride and its slow generation, the cycloisomerisation process ultimately yields a mixture of alkene products ( 2a. 3a and 4a) with incomplete consumption of the procatalyst [Pd(eta (3)-C 3H5)(MeCN)(2)]OTf. The mechanism by which the catalytically active palladiu m hydride is generated from the procatalyst was studied in detail by NMR sp ectroscopic analysis of stoichiometric reactions between diene 1a and [Pd(e ta (3)-C3H5)(MeCN)(2)]OTf. This demonstrated that a carbopalladated complex , namely. [Pd{7,7-(CO2Me)(2)-(1,2,5,9,10-eta (5))-dec- 1,9-diene)}(OTf)] (1 5a), is formed in small quantities by unfavourable displacement of acetonit rile by the diene, followed by a rapid and irreversible beta -migratory ins ertion reaction. Although attempts to isolate 15a from the reaction mixture were not successful (clue to its slow decomposition. low concentration and competing cycloisomerisation). an alternative synthesis in the absence of acetonitrile allowed its isolation and characterisation. However, pure samp les of 15 a are completely ineffective as a procatalyst system for cycloiso merisation of 1a. Further investigation revealed that treatment of 15a with one equivalent of water results in quantitative beta -H elimination to gen erate triene 16a (C(1)-allylated 1 a). Thus, addition of catalytic quantiti es of water to a solution of 1a in CHCl3 containing 5 mol% 15 a and 10 mol % MeCN results in generation of an active "Pd-H" catalyst for cycloisomeris ation. Although procatalyst activation is facilitated by traces of water, n o exchange of protons is observed between "Pd-H" and H2O under catalytic tu rnover. The slow generation of 15 a and the requirement for traces of water for beta -H elimination accounts for variability in the induction period w hen [Pd(eta (3)-C3H5)(MeCN)(2)]OTf is employed as procatalyst.