Palladium complexes binuclear species

A versatile route to 3-benzoheteropines has been reported starting from o-phthalaldehyde, including the first preparations of 3-benzarsepines and the parent 3-benzothiepin and 3-benzoselenepins <96CC2183>. l,7-Dihydro-l//-dibenzo[c,c]tellurepin has been prepared from 2,2 -bis(bromomethyl)biphenyl and potassium tellurocyanate and its complexes with palladium and ruthenium species have been studied, a number of mono- and binuclear complexes are formed <96RTC427>. 

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]. 

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. 

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]. 

With the aim to prevent the formation of the inactive binuclear complex 1, which is very fast using 1,10-phenanthroline, a possible way is the addition to the reaction media of a competitor of carbon monoxide in binding the reduced palladium species. 

Pd2(dba)3 - CHCl. This is the most frequently employed precursor to a variety of chiral Pd(0) complexes. A chloroform molecule is cocrystallizing with the binuclear palladium moiety par unit. A number of variants with different cocrystaUized solvents are known and show similar reactivity with the chloroform adduct, which includes dba (dibenzalacetone) cocrystals [i.e., Pd2(dba)3-(dba)]. This species is often referred as Pd2(dba)4 or Pd(dba)2. In the presence of stronger ligands, such as phosphines, dba ligands in the complex are easily replaced to give new palladium species. In many cases, the released dibenzalacetone is difficult to remove and remains in the reaction mixture. Sometimes, the remaining dibenzalacetone works as an inhibitor thus, special attention must be paid to the dba. 

In 2009, this possibility was realized by Ritter and coworkers. The two-electron oxidation of dipalladium(II) compound 148 at low temperature (-30 °C) afforded the dipalladium(lll) compound 149 with significant Pd-Pd distance contraction from 2.84A in 148 to 2.57A in 149 (Entry 1, Table 10.9) (Scheme 10.68) [108]. The existence of a Pd(III)-Pd(III) bond was further proven by the diamagnetism of 149, which was the result of spin pairing of two d Pd(III) centers. Warming 149 to ambient temperature led to bimetallic reductive elimination to form a C-Cl bond, along with unidentified Pd(II) species. This was the first clearly defined example of carbon-heteroatom reductive elimination from a binuclear transition metal complex, and created a new horizon of palladium organometallic chemistry based on synergetic Pd(III)-Pd(III) bond [113]. 

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