Palladium precatalysts

A domestic oven and open containers were used in the work by Sharifi, who performed couplings between aryl bromides and alkyl amines (yields 32-86%) [79]. The palladium precatalyst Pd[P(o -tolyfbhCk was found to be the most efficent catalyst when toluene was used as solvent and sodium tert-butoxide as base. About the same time, a similar approach to arylating alkyl and aryl amines was reported by Hallberg [80]. Mono-modal microwave... 

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]. 

The palladium-catalyzed cross-coupling reaction of a vinyl or aryl stannane with an arylhalogenide or -triflate is known as a Stille reaction. The mechanism of this Stille reaction is outlined below The palladium precatalyst loses two ligands and forms the catalytic species 36. The catalytic cycle starts with the oxidative addition of the catalytic species 36 into the carbon-triflate bond of 23 forming complex 41, which, however, does not undergo the required transmetallation step with stannane 22. Therefore, the triflate ion is... 

Palladium catalysts have found application in the oxidative kinetic resolution of secondary alcohols such as 1-phenylethanol. (—)-Sparteine, was used to obtain high levels of enantioselection however, it was found that the nature of the palladium source was critical in obtaining a high chemical selectivity factor Pd2(dba)3 proved superior to Pd(OAc)2 but not as effective as Pd(nbd)Cl2-The observed difference in reactivity, for various palladium catalysts, was attributed to subtle differences in the solubility of the palladium-precatalysts in toluene as well as their ability to complex with (—)-sparteine (eq 32). ... 

Table 9. The required amount of the palladium precatalysts with different phosphine ligands in the Suzuki-Miyaura reactions of various aryl halides with arylboronic acids...

Optimal palladium precatalyst loadings (mol%) <0.1, can be as low as 10-5 Optimal range 0.05-0.5 higher and lower values are detrimental 0.5-5 1-20... 

Palladium Precatalysts in Type 1 and Type 2 Mizoroki-Heck Reactions... 

Scenario 4. Nanoparticles are formed during the reaction initiated by soluble palladium precatalysts. Our experience in phosphine-free (mostly aqueous) systems led us at an early stage to hypothesize that these protocols go hand in hand with formation of palladium sols [4, 83]. Since 2000, the observation of palladium sols in Mizoroki-Heck reactions has become ubiquitous. So, if one were to decide to compile a list of references that mention the formation of sols, that list would practically coincide with a list of references which deal with the development of new phosphine-free catalytic systems. In fact, the idea that it is the dispersed palladium metal which catalyses (or, more correctly, serves as precatalyst) the Mizoroki-Heck reaction should be traced back to the seminal pubUcations by Heck in 1972 [2], who clearly attiibuted catalytic activity in a phosphine-free catalytic system to palladium metal formed by reduction of Pd(OAc)2 by the alkene or the amine. This idea fell into oblivion until its rediscovery in the course of a general interest in nanoscale systems in the late 1990s. 

The apparent paradox that, in the systems in which palladium tends to form nanoparticles, TONS increase on decreasing the amount of palladium precatalyst had been noted by us in 1999 [83] and brilliantly and unambiguously established by Reetz and de Vries [36] in a series of studies devoted to the functioning of phosphine-free Mizoroki-Heck reactions. This countervailing trend between the palladium loading and TON certainly has its limitations as soon as the balance between palladium engaged in the cycle and palladium in the reservoir is disturbed, accumulation of palladium nanoparticles will stop and TONS will decrease. 

More recently, a much milder and highly efficient approach has been developed by Komiyama in Japan that applies palladium-catalyzed heteroaryl Heck reaction to access key intermediate 31 from bromoarene 44 and thiazole 45. After intense screening of palladium precatalysts, base, solvent, ligand, co-catalyst Cu(l) salts, and organic acid... 

Palladium-phosphine complexes are the most commonly used catalysts. Palladium-PPhj complexes, either isolated such as Pd(PPhj)4 or formed in situ from typical palladium precatalysts such as Pdjjdbajj or PdjOAc) and PPhj, afford satisfactory results in many cases. For the more demanding situations, other conventional phosphine ligands such as tfp (tris(2-furyl)phosphine), PtBuj, dppf, or DPEPhos (174) can help to achieve superior results. Furthermore, NHC ligands and, in particular, PEPPSl precatalysts also provide very active catalysts. 

By altering the palladium precatalyst to Pd ldbalj and using other inorganic bases and solvents, it was possible to synthesize diarylketones with moderate to excellent yields under air. Since the aryl aldehydes can be easily transformed into several functional groups such as methyl, hydroxymethyl, carboxyl, cyano, and ester (see Scheme 7.29), this methodology appears to be versatile and may provide potential opportunities for the synthesis of complex organic compounds. 

Scheme 8.11 The ablation followed by alkylation of 2-oxindoles in a toluene potassium hydroxide biphasic system catalyzed by a palladium precatalyst and a phase-transfer catalyst TBAB.

The ability of palladium to serve as both nucleophile (Pd(0)) and electrophile (Pd(II)) has led to the development of oxidase-type reactions that exploit the electrophilic nature of Pd(II). One such example is an extension of the above kinetic resolution of secondary alcohols catalyzed by Pd(nbd)Cl2 in the presence of (-)-sparteine (described earlier), for the oxidative cyclization of substituted phenols. This racemic aerobic cyclization utilizes a Pd(II) salt in the presence of pyridine, O2, and SAmolecular sieves. Numerous palladium precatalysts were screened, Pd(TFA)2 was optimal, yielding the desired cyclized product in 87% yield. Pd2(dba)3 also enabled the cyclization but was less effective, providing the desired cyclized product in significantly reduced yield (25%) (eq 48).13 ... 

Figure 9.10 Nickel and palladium precatalysts for living olefin polymerization.

The basic mechanism of the Tsuji-Trost reaction is as follows All palladium precatalysts are converted to the active palladium(0) catalyst 11 in situ, most commonly by phosphine in phosphine assisted catalytic cycles. Following coordination of the allylic reagent 1 to the palladium(0) catalyst 11, oxidative addition occurs to give Jt-allylpalladium(II) complexes 13/14 (this step is also known as ionization). Complexes 13/14 can interconvert via ligand exchange... 

Further studies enriched the field with several palladium precatalyst as [(phen)PdMe(MeCN)]BarF [62], [(n -aUyl) Pd(MeCN)2][OTf] [63], and so on, acting as palladium-hydride species under the reaction conditions. Thus, those are providing good to excellent yields of cyclized products in a high ratio between the major regioisomers and without the need of hydrosilane additives (Scheme 7.45). 

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