Palladium complexes binuclear

However, some binuclear -jr-allyl palladium complexes have been obtained by facile decarboxylations from palladium chloride and some... 

In 1998, a new type of Pd(II) binuclear complex was reported which was effective for Mannich reactions of an imine derived from glyoxylate and anisidine with silicon enolates [38,39]. In these reactions, use of solvents including a small amount of water was essential. It was shown that water played an important role in this system water not only activated the Pd(II) complex to generate a cation complex, but also cleaved the N-Pd bond of the intermediate to regenerate the chiral catalyst. This reaction reportedly proceeded via an optically active palladium enolate on the basis of NMR and ESIMS analyses. A unique binuclear palladium-sandwiched enolate was obtained in the reaction of the p-hydroxo palladium complex with the silyl enol ether [(Eq. (9)]. 

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

Multinuclear metal complexes that may act as active catalysts or off-cycle species can also be easily identified and studied via ESl-MS. For example, analysis of a simple Pd-catalyzed allylic substitution reaction lead to the discovery of two reversibly formed binuclear bridged palladium complexes (Fig. 6) that act as a reservoir for the active mononuclear catalyst [21], The observation of dimers when using ESl-MS is common and it is crucial to confirm that they truly exist in solution and are not just formed during the ESI process, in this case the detection was supported by P and H NMR studies of stoichiometric reaction mixtures and in situ XAFS experiments [49]. 

In an ESI-MS monitoring study of the Suzuki-Miyaura reaction using a dichloro-bis(aminophosphine) palladium precatalyst, binuclear Pd complexes were detected after the reaction went to completion, indicating a catalyst sink or a resting state. Addition of starting reagents resumes the reaction, suggesting the active role of the binuclear complex as a reservoir of mononuclear active catalyst. Other interpretations propose the involvement of Pd nanoparticles in which binuclear Pd complexes act as a precursor or perhaps even the active catalyst, but the last possibility seems unlikely. A mechanism for this transformation was proposed based on the intercepted species (Scheme 10) [62]. 

Scheme 10. Proposed mechanistic cycle for Suzuki-Miyaura cross-coupling based on tbe observation of binuclear palladium complexes. L=l.l -(cyclohexylpbospbanediyl)dipiperidine.

The binuclear palladium complex [(tBu2PH)PdPtBu2]2 with oxygen gave a very efficient catalyst for the hydrogenation of a,/3-unsaturated carbonyls. 0 Glueck and co-workers have undertaken studies on aspects of platinum-catalyzed hydrophosphinations of activated olefins.71,72... 

Reaction of the isonitrile-palladium complex with a spiro trisilane 21 affords binuclear palladium complex 22 in high yield (Eq. 6) [21]. 

The reaction of 3-halo-l,2-bis(dialkylamino)cyclopropenylium halides 5 with palladium black or platinum black in refluxing acetonitrile afforded binuclear complexes of the type 6. When the palladium complexes 6 (M = Pd) were treated with tributylphosphane in dichloromethane at — 60 "C and the mixture warmed to room temperature, a nucleophilic cleavage occurred to give the mononuclear complexes 7. The structure of complex 7 (X = Cl, R = Me) was determined by X-ray crystallography. ... 

Sodeoka et al. have found fhat binuclear /z-hydroxo palladium complex 87 is an efficient catalyst of fhe asymmetric Mannich-type reaction of a-N-arylimino esters with SEE (Scheme 10.80) [226]. Mechanistic studies using NMR and electrospray ionization mass spectrometry suggest that a unique binuclear palladium-sandwiched enolate 88 is involved in fhe palladium-catalyzed reaction. 

In particular, the palladium complex with 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, was selected as the most efficient catalyst. The presence of the methyl substitution in 2,9-positions was necessary to avoid the formation of binuclear complexes like 1 (detected using 1,10-phenanthroline as palladium ligand), which are stable in reaction conditions and are responsible of the fast irreversible deactivation of the catalyst. 

In 1971, an interesting application of the chlorobridged Pd(II) complexes with orthometallated chiral amines was demonstrated by Otsuka and co-workers resolution of racemic chiral phosphincs. The binuclear species reacts with tertiary phosphines or arsines to form two equivalents of mononuclear complexes (Scheme 3). If both the phosphines and the orthometallated palladium complexes were chiral, the mononuclear products could be a mixture of diastereomers. With appropriate combinations of the chiral racemic phosphines and the enantiomerically pure orthometallated palladium species, one of the two enantiomers of the phosphines reacts with the palladium complex selectively to give a specific diastereomer of the mononuclear palladium complexes, leaving the other enantiomer of the phosphine unreacted. 

Both binuclear and trinuclear oxalamidinate palladium complexes were employed by Rau, Walther and coworkers [79] in copper-free Sonogashira reactions. The peculiarity of the system was seen to reside in the presence of chemical bridges that allowed electronic communication between metal centers (Scheme... 

Ricci and coworkers [64] studied oxazoline moiety fused with a cyclopenta[P]thio-phene as ligands on the copper-catalyzed enantioselective addition of Et2Zn to chalcone. The structure of the active Cu species was determined by ESI-MS. Evans and coworkers [65] studied C2-symmetric copper(II) complexes as chiral Lewis acids. The catalyst-substrate species were probed using electrospray ionization mass spectrometry. Comelles and coworkers studied Cu(II)-catalyzed Michael additions of P-dicarbonyl compounds to 2-butenone in neutral media [66]. ESI-MS studies suggested that copper enolates of the a-dicarbonyl formed in situ are the active nucleophilic species. Schwarz and coworkers investigated by ESI-MS iron enolates formed in solutions of iron(III) salts and [3-ketoesters [67]. Studying the mechanism of palladium complex-catalyzed enantioselective Mannich-type reactions, Fujii and coworkers characterized a novel binuclear palladium enolate complex as intermediate by ESI-MS [68]. 

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