Highly active thermomorphic fluorous palladacycle catalyst precursors for the Heck reaction; evidence for a palladium nanoparticle pathway

Quadrol-Pd(II) complexes: phosphine-free precatalysts for the room-temperature Suzuki-Miyaura synthesis of nucleoside analogues in aqueous media

Commercially available Quadrol, N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine (THPEN), has been used for the first time as a N^N-donor neutral hydrophilic ligand in the synthesis and characterization of new water soluble palladium(II) complexes containing chloride, phthalimidate or saccharinate as co-ligands. [PdCl2(THPEN)] (1) [Pd(phthal)2(THPEN)] (2), [Pd(sacc)2(THPEN)] (3) and the analogous complex with the closely related N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (THEEN) [Pd(sacc)2(THEEN)] (4) were efficiently prepared in a one-pot reaction from [PdCl2(CH3CN)2] or Pd(OAc)2. Structural characterization of 1 and 3 by single crystal X-ray diffraction produced the first structures reported to date of palladium complexes with Quadrol. The resultant palladium complexes are highly soluble in water and were found to be effective as phosphine-free catalysts for the synthesis of functionalized nucleoside analogues under room-temperature Suzuki-Miyaura cross-coupling conditions between 5-iodo-2'-deoxyuridine (& 5-iodo-2'-deoxycytidine) with different aryl boronic acids in neat water. This is the first report of the coupling process performed on nucleosides in water at room temperature.

Perfluorinated phosphine and hybrid P–O ligands for Pd catalysed C–C bond forming reactions in solution and on Teflon supports

The synthesis of two types of phosphine ligands that feature perfluorinated ponytails is reported. A bidentate (RfCH₂CH₂)₂PCH₂CH₂P(CH₂CH₂Rf)₂ (Rf = CF₃(CF₂)_n; n = 5, 7) and an alkoxyphosphine made by ring opening a fluorous epoxide, RfCH₂CH(OH)CH₂PR₂ (Rf = CF₃(CF₂)₇), have been prepared and spectroscopically characterised. The electronic effects of the fluorous chains have been elucidated from either the ¹J_Pt–P or ¹J_P–Se coupling constants in Pt(ii) or phosphine selenide compounds. Whilst the bidentate phosphines do not give stable or active Pd catalysts, the hybrid ligand does allow Susuki, Heck and Sonogashira catalysis to be demonstrated with low catalyst loadings and good turnovers. Whilst a fluorous extraction methodology does not give good performance, the ligand can be adsorbed onto Teflon tape and for the Suzuki cross coupling reaction the catalytic system can be run 6 times before activity drops and this has been traced to oxidation of the ligand. Additionally the crystal structure of the hybrid phosphine oxide is reported and the non-covalent interactions discussed.

Phosphine ligands containing a perfluorous ponytail can be sorbed onto Teflon tape and used as ligands for C–C cross coupling reactions with little leaching.

Synthesis of a new series of Ni(ii), Cu(ii), Co(ii) and Pd(ii) complexes with an ONS donor Schiff base: crystal structure, DFT study and catalytic investigation of palladium and nickel complexes towards deacylative sulfenylation of active methylenes and regioselective 3-sulfenylation of indoles via thiouronium salt formation

A series of Ni(ii), Cu(ii), Co(ii), and Pd(ii) complexes have been synthesized with a chelating Schiff base ligand coordinated to a metal center with ONS donor atoms. The ligand and complexes are characterized by elemental analysis and spectroscopic techniques like FT-IR, 1H-NMR, and UV-Visible spectroscopy. The single crystal structure of the Pd(ii) complex is obtained by X-ray diffraction analysis and exhibits slightly distorted square planar geometry. The structure is optimized by DFT, TD-DFT calculation to elaborate the electronic structure and NBO for the charge distribution analysis of the Pd(ii) complex. The synthesized Pd(ii) and Ni(ii) complexes as catalysts have been investigated in the C-S cross-coupling of indoles and active methylenes. The metal propelled regioselective transformation afforded 3-sulfenylated indoles while β-diketones favored deacylated monosulfenyl ketones in an excellent yield via thiouronium salt formation. The Pd(ii) complex displays slightly better reactivity whereas the Ni(ii) complex is cost-efficient. The method is fast, easy to handle and cost effective in terms of high reactivity of catalysts, use of non-toxic solvents, and cheaper aryl halides and thiourea replace conventional sulfur sources, providing a practical access to organic transformations.

Active Palladium Colloids via Palladacycle Degradation as Efficient Catalysts for Oxidative Homocoupling and Cross-Coupling of Aryl Boronic Acids

Active palladium colloids formed upon degradation of a palladacyclic complex (Herrmann-Beller 1) have been isolated for the first time and thoroughly characterized with techniques such as transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure spectroscopy. The synthesized palladium colloids have been utilized as efficient catalysts for the oxidative homocoupling of aryl boronic acids. Cross-coupling of two different aryl boronic acids has also been made possible using these active palladium colloids. This is the first report of this kind of coupling between aryl boronic acids.

Sonogashira (Cu and amine free) and Suzuki coupling in air catalyzed via nanoparticles formed in situ from Pd(ii) complexes of chalcogenated Schiff bases of 1-naphthaldehyde and their reduced forms

The activation of the coupling reactions with the Pd(ii)-complexes (0.05–0.01 mol% loading) is significant in 1–2 h under mild conditions.

Convenient protocols for Mizoroki-Heck reactions of aromatic bromides and polybromides with fluorous alkenes of the formula H2C[double bond, length as m-dash]CH(CF2)n-1CF3 (n = 8, 10)

The fluorous alkenes H₂C[double bond, length as m-dash]CHR_fn (R_fn = (CF₂)_n-1CF₃; n = 8, 10) undergo the Mizoroki-Heck reaction with a variety of aromatic monobromides and polybromides such as 1,3- and 1,4-C₆H₄Br₂, 1,3,5-C₆H₃Br₃, 1,3,5-C₆H₃Br₂Cl, 1,4-XC₆H₄Br (X = CF₃, R_f8, COCH₃, CN, 1,4-OC₆H₄Br), 1,2-O₂NC₆H₄Br, 5-bromoisoquinoline, 5-bromopyrimidine, 3-bromo-5-methoxypyridine, and 3,5-dibromopyridine (sixteen examples, 78% average isolated yield). Typically, 1.2-2.4 equiv. of alkene are employed per Ar-Br bond, together with Pd(OAc)₂ catalyst (4-5 mol%/Ar-Br bond), n-Bu₄N⁺ Br^- (0.8-1.0 equiv./Ar-Br bond), NaOAc (1.2-2.4 equiv./Ar-Br bond), and 3 : 1 w/w DMF/THF as solvent (120 °C). No effort is necessary to exclude air or moisture, and reactions may be conducted on >10 g scales. Only E isomers of the products Ar(CH[double bond, length as m-dash]CHR_fn)_m are detected. Thirteen representative examples are hydrogenated (Pd/C, balloon pressure H₂), giving Ar(CH₂CH₂R_fn)_m (92% average isolated yield).

Bis(N-substituted oxamate)palladate(ii) complexes as effective catalysts for sustainable Heck carbon–carbon coupling reactions in n-Bu4NBr as the solvent

Bis(oxamato)palladate(ii) complexes are structurally well-defined precatalysts for the Heck vinylation of aryl halide derivatives inn-Bu₄NBr as benign ionic liquid.

Oxime-derived palladacycles as source of palladium nanoparticles

Oxime-derived palladacycles are very efficient and versatile pre-catalysts for a wide range of carbon-carbon bond coupling reactions in air, under very low loading conditions, and employing reagent-grade chemicals. This tutorial review presents the main achievements, advantages and limitations of oxime palladacycles as a source of highly active palladium nanoparticles for high-turnover catalyzed Heck, as well as other homo- and cross-coupling reactions usually carried out employing organic or aqueous solvents. Comparison with other ligandless Pd(II) catalysts is also presented. Recent advances to develop supported oxime-derived palladacycles in order to facilitate precatalyst recovery and reuse in cross-coupling reactions, especially under aqueous reaction conditions, are also discussed.