Phospha-palladacycles and N-heterocyclic carbene palladium complexes: efficient catalysts for CC-coupling reactions

A simple procedure for the polymer-supported N-heterocyclic carbene-rhodium complex via click chemistry: a recyclable catalyst for the addition of arylboronic acids to aldehydes

A novel polymer-supported carbene-rhodium complex was prepared using a simple procedure via click chemistry. This polymer-supported N-heterocyclic carbene-rhodium complex was characterized and used as a catalyst for the addition of arylboronic acids to aldehydes in good to excellent yields.

Amino Oxazolines as Easily Accessible Water Stable Ligands for Palladium Catalysed Aqueous Heck Reaction

A series of amino oxazolinyl ligands were screened for palladium catalysed Heck reaction. These ligands work well as phosphine free system in aqueous, micellar medium, can be effectively recovered and reused for subsequent cycles.

Synthesis of Highly Stable 1,3-Diaryl-1H-1,2,3-triazol-5-ylidenes and their Applications in Ruthenium-Catalyzed Olefin Metathesis

The formal cycloaddition between 1,3-diaza-2-azoniaallene salts and alkynes or alkyne equivalents provides an efficient synthesis of 1,3-diaryl-1H-1,2,3-triazolium salts, the direct precursors of 1,2,3-triazol-5-ylidenes. These N,N-diarylated mesoionic carbenes (MICs) exhibit enhanced stability in comparison to their alkylated counterparts. Experimental and computational results confirm that these MICs act as strongly electron-donating ligands. Their increased stability allows for the preparation of ruthenium olefin metathesis catalysts that are efficient in both ring-opening and ring-closing reactions.

Charged tags as probes for analyzing organometallic intermediates and monitoring cross-coupling reactions by electrospray-ionization mass spectrometry

Organic iodides bearing a cationic quaternary ammonium group at a remote position react with zerovalent Pd complexes, Zn, or In leading to a C-I bond insertion. The resulting charge-tagged organometallics can be detected by electrospray-ionization mass spectrometry, which provides detailed information on their stoichiometry, oxidation state, and coordination sphere. The properties of the observed organopalladium and -zinc intermediates largely agree with previous findings, whereas the organoindium species show a surprisingly high tendency to form ate complexes. Magnesium also undergoes insertion into the C-I bond of the charge-tagged organic iodides, but instead of the expected organomagnesium intermediates only the corresponding hydrolysis products could be detected in the diluted solutions. Electrospray-ionization mass spectrometry can also be used to study the reactivity of the charge-tagged species, as was demonstrated for a Pd-catalyzed Negishi cross-coupling reaction. The presented approach permits a straightforward identification of the rate-limiting step and the determination of the corresponding second-order rate constant.

Bis-N-heterocyclic carbene palladium(IV) tetrachloride complexes: synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates

This Article describes the preparation and isolation of novel octahedral CH(2)-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPd(IV)Cl(4) [L = (NHC)CH(2)(NHC)] from LPd(II)Cl(2) and Cl(2). In intermolecular, nonchelation-controlled transformations LPd(IV)Cl(4) reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C-H bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE Pd(IV) complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a Pd(IV)-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl(+)-transfer from the pentacoordinated Pd(IV)-intermediate LPd(IV)Cl(3)(+) to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl(2) from a reductive elimination process. Instead, a ligand-mediated direct Cl(+)-transfer from LPd(IV)Cl(3)(+) to the π-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPd(IV)Cl(3)(+). The presence of a large excess of added Cl(-) slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both Pd(IV)-Cl ionization and Cl(+)-transfer from LPd(IV)Cl(3)(+). (1)H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl(-)···H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.

Palladium motion in cyclomeric compounds: a theoretical study

Density functional theory (DFT)/B3LYP calculations have been carried out to study successive intramolecular 1,n palladium shifts (n = 3-5) in palladium complexes of organic cyclomers. Such shifts of the PdBr(phosphine) moiety, which is bound to the cyclomer and which exchanges concomitantly with a hydrogen atom distant by n carbon atoms from palladium, might lead to an endless motion around the cyclomer. The cyclomers that have been analyzed are either the [1,1,1,1]paracyclophane 1, a THF-based 16-crown-4 structure 2, or the THF-ethylene cyclic dimer 3 (THF = tetrahydrofuran). We show that the [1,1,1,1]paracyclophane 1 is not a good candidate for a circular motion of the metallic moiety. This is due to the very high barrier of the 1,2 Pd/H exchange within the phenyl ring. Hence only a pendulum movement of the metallic moiety between two adjacent phenyl rings can easily take place. For the THF-based systems 2 and 3, the processes along the exo face are found to involve high energy barriers. Processes along the endo face are more accessible, especially for 2 where an endless motion made of successive 1,2 and 1,5 shifts is characterized by barriers that are somewhat less than 30 kcal mol(-1).

An efficient and reusable polymer-supported palladium catalyst for the Suzuki cross-coupling reactions of aryl halides

A reusable, air-stable polymer-anchored palladium (II) Schiff base complex catalyst, P-[{(NCH)C6H4}Pd(OAc)]2 was prepared and was found to be highly active in Suzuki cross-coupling reactions of aryl halides with phenyl boronic acid in an aqueous medium to give biaryl products in high yields.

Synthesis and structure of a bis-N-heterocyclic carbene complex of uranium tetrachloride exhibiting short ClC(carbene) contacts

Treatment of UCl(4) with one or two equivalents of IPr [IPr = {C(NArCH)(2)}, Ar = 2,6-diisopropylphenyl] gives [UCl(4)(IPr)(2)] as the sole isolable uranium-containing containing product; an X-ray diffraction study showed close Cl...C(carbene) contacts, but DFT analysis suggests the contacts are the result of aryl chloride steric repulsions as the Cl...C orbital interaction is negligible.