Palladium‐Catalyzed Aerobic Intermolecular Cyclization of Acrylic Acid with 1‐Octene to Afford α‐Methylene‐γ‐butyrolactones: The Remarkable Effect of Continuous Water Removal from the Reaction Mixture and Analysis of the Reaction by Kinetic, ESI‐MS, and XAFS Measurements

Abstract

Further study of our aerobic intermolecular cyclization of acrylic acid with 1-octene to afford α-methylene-γ-butyrolactones, catalyzed by the Pd(OCOCF(3))(2)/Cu(OAc)(2)⋅H(2)O system, has clarified that the accumulation of water generated from oxygen during the reaction causes deactivation of the Cu cocatalyst. This prevents regeneration of the active Pd catalyst and, thus, has a harmful influence on the progress of the cyclization. As a result, both the substrate conversion and product yield are efficiently improved by continuous removal of water from the reaction mixture. Detailed analysis of the kinetic and spectroscopic measurements performed under the condition of continuous water removal demonstrates that the cyclization proceeds in four steps: 1) equilibrium coordination of 1-octene to the Pd acrylate species, 2) Markovnikov-type acryloxy palladation of 1-octene (1,2-addition), 3) intramolecular carbopalladation, and 4) β-hydride elimination. Byproduct 2-acryloxy-1-octene is formed by β-hydride elimination after step 2). These cyclization steps fit the Michaelis-Menten equation well and β-hydride elimination is considered to be a rate-limiting step in the formation of the products. Spectroscopic data agree sufficiently with the existence of the intermediates bearing acrylate (Pd-O bond), η(3)-C(8)H(15) (Pd-C bond), or C(11)H(19)O(2) (Pd-C bond) moieties on the Pd center as the resting-state compounds. Furthermore, not only Cu(II), but also Cu(I), species are observed during the reaction time of 2-8 h when the reaction proceeds efficiently. This result suggests that the Cu(II) species is partially reduced to the Cu(I) species when the active Pd catalytic species are regenerated.

Mechanistic Insights on Pd/Cu-Catalyzed Dehydrogenative Coupling of Dimethyl Phthalate

“…To the best of our knowledge, Stults, Friedman, and co-workers were the first to use synchrotron XAFS in a study of homogeneous catalysts using a Rh complex . Following this pioneering study, homogeneous catalysts involving Ti, V, Cr, Mo, Mn, Fe, Ru, Ni, Pd, Au, and Cu were studied by XAFS. Note that one of the authors succeeded in elucidating the mechanism of paramagnetic Fe-catalyzed Kumada–Tamao–Corriu coupling by a solution-phase XAFS analysis .…”

Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques

“…Authors founded that, starting from Cr(III) in the Cr(acac) 3 precursor, a (PNP)Cr(II)-(CH 3 ) 2 complex (PNP = bidentate diphosphinoamine) is formed upon adding bidentate diphosphinoamine and modified methylaluminoxane (MMAO), which is the active species that converts ethylene to 1-octene by passing a reversible redox cycle, while reduction to Cr(I) leads to deactivation. Miyoshi et al[831] deeply investigated the aerobic intermolecular cyclization of acrylic acid with 1-octene to afford -methylene--butyrolactones, catalysed by the Pd(OCOCF 3 ) 2 /Cu(OAc) 2 H 2 O system. They found that the accumulation of water generated from oxygen during the reaction causes deactivation of the Cu co-catalyst.…”

“…Furthermore, Cu K-edge XANES data showed that not only Cu(II), but also Cu(I), species are present when the reaction proceeds efficiently. Authors concluded that the Cu(II) species is partially reduced to the Cu(I) species when the active Pd catalytic species are regenerated[831].…”

Palladium-Catalyzed Oxidative Annulation of Acrylic Acid and Amide with Alkynes: A Practical Route to Synthesize α-Pyrones and Pyridones

“…On the basis of the above results, a plausible mechanism for the Pd-catalyzed oxidative annulations is illustrated in Scheme . We proposed that the initial step involved the coordination and ligand exchange of the acrylic derivative 2 with Pd II to provide X–Pd intermediate I . , Then, exo coordination of 1a and insertion of the diarylethyne molecule into I gave the vinyl–palladium complex II . Subsequently, the intramolecular Heck-type process afforded the alkyl-palladium species III , which would undergo β-hydride elimination to release product 3 and Pd 0 .…”