Palladacycles dimerization

Jautze S, Seiler P, Peters R (2007) Macrocyclic ferrocenyl-bisimidazoline palladacycle dimers as highly active and enantioselective catalysts for the Aza-Claisen rearrangement of Z-conflgured A-para-methoxyphenyl trifluoroacetimidates. Angew Chem Int Ed 46 ... 

The natural diterpenoid rostratone 16 is synthesized from ethylene ketal as shown in Scheme 17.162 In this synthesis, the Pd-mediated remote acetoxylation is achieved by G-H bond activation by Na2PdCl4 giving palladacycle dimers followed by treatment with pyridine and lead tetraacetate. 

Dinuclear cyclometallated palladium(ii) complexes 81 have been obtained from Schiff base ligands. Cyclopalladated complexes of oximes 82 and 83 and their ligand-exchange reactions have been studied. Ketoximes form palladacycle dimers such as 84. " ... 

A novel racemic palladacyclic dimeric complex 136 was resolved via recrystallization of its diastereomeric (3 )-prolinate derivatives 137 and 138 (Scheme 20). The chemistry and resolution of chiral PMeBuTh was studied using 139 and 140. Complex 139 was also used in the resolution of stilbenediamine. The P-chiral phosphine PBuTh(4-BrC6H4) was resolved using 141. ... 

In 2005, Moyano et al. [60] reported a new type of chiral dimeric ferrocene palladacycle 43 that lacked the element of planar chirality and involved three... 

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

Generally, monophosphine complexes can be generated by decomposition of suitable precursors, among which the most notable are palladacycles (Section 9.6.3.4.7). A spectacular example makes use of spontaneous disproportionation of a dimeric complex of Pd1 with very bulky ligands to give one of the most reactive catalytic systems known so far, which catalyzes the fast crosscoupling of arylboronic acids with aryl chlorides and hindered aryl bromides at room temperature (Equation (28)) 389... 

A key feature of PdCys as precursors of Pd(0) nanoparticles is that reduction of Pd(II) -> Pd(0) involving C-Pd bond cleavage is required. This accounts for both the high temperatures invariably required and the induction period in the absence of reductants. Rosner et al. have developed a detailed kinetic model of a Heck reaction catalyzed by dimeric palladacycles (Rosner et al. 2001 a,b). This model explains the experimental observations and is consistent with an active species... 

Rawal s group developed an intramolecular aryl Heck cyclization method to synthesize benzofurans, indoles, and benzopyrans [83], The rate of cyclization was significantly accelerated in the presence of bases, presumably because the phenolate anion formed under the reaction conditions was much more reactive as a soft nucleophile than phenol. In the presence of a catalytic amount of Herrmann s dimeric palladacyclic catalyst (101) [84], and 3 equivalents of CS2CO3 in DMA, vinyl iodide 100 was transformed into ortho and para benzofuran 102 and 103. In the mechanism proposed by Rawal, oxidative addition of phenolate 104 to Pd(0) is followed by nucleophilic attack of the ambident phenolate anion on o-palladium intermediate 105 to afford aryl-vinyl palladium species 106 after rearomatization of the presumed cyclohexadienone intermediate. Reductive elimination of palladium followed by isomerization of the exocyclic double bond furnishes 102. 

A dimeric 4-hydroxyacetophenone oxime-derived palladacycle was used by Najera et al. as a very efficient precatalyst for selective and microwave-assisted arylations of acrolein diethyl acetal, furnishing 3-arylpropanoate products [45]. In the presence of Cy2NMe and tetrabutylammonium bromide (TBAB), higher reaction rates and slightly improved yields were realized under microwave conditions (Scheme 7). 

Pd(ll) catalysts can be employed as well. Compared to the standard Pd(ll) catalysts, the palladacycle catalyst 7 shows a broader compatibility with functional groups. Terminal alkynes, alkyl halides, and a-halogen ketones are tolerated <1997CB1449>. It is worth mentioning that other catalysts of Pd(ll) preferentially give dimerization products. 

The first purine-8-ylidene palladium complex was synthesised shortly afterwards by the Herrmann group [104]. Cleavage of a dimeric palladacycle with the azolium salt in the presence of sodium acetate (to preserve the acetate ligand on palladium) resulted in the respective monomeric palladium(II) carbene adducts (see Figure 6.45). 

NHC-palladacycle complexes (125) have been obtained, as depicted in equation (17), from the corresponding dimer (124) and two equivalents of NHC ligand. Conveniently, this route was found compatible with diverse functions and so aminopalladacycles and phosphapalladacycles have been prepared. 

A variety of palladium(II) and palladium(O) complexes serve as effective precatalysts or precursors to the active palladium(O) catalyst. The most commonly used precatalysts in Heck chemistry are Pd(OAc)2, Pd2(dba)3, and PdCl2(PPh3)2. Typical catalyst loading is in the range of 5-10mol-%. The discovery of the unique catalytic activity of a dimeric palladacycle (Pd2(P(o-Tol)3)( Li-OAc)2) by Herrmann and Belle/ has set a milestone in palladium catalysis as it allows the use of even unreactive chloroaryl substrates intransformations. 

Many types of palladacyclic complexes have been used as precursors to catalysts for a variety of coupling processes146,147. Mixtures of dimeric palladacycles containing bridging halide, acetate or trifluoroacetate ions and a phosphine or carbene ligand have been studied as catalysts for the animation of aryl halides. The isolated phosphine adducts can also be applied in catalysis. 

Reactions of this type are postulated as equilibria in synthetically important insertion reactions of alkenes, alkynes and other unsaturated molecules into the Pd-C bonds of dimeric palladacycles.271,272 Further, this reaction can form the basis for... 

The first example of an o/t/20-alkylation/Mizoroki-IIcck coupling was reported by Catellani [4] in 1997. Using the PNP dimer as a catalyst in the presence of an aryl halide, norbomene, an alkyl iodide, a terminal olefin and a base at room temperature, 1,2,3-trisubstituted benzenes (Scheme 16), were synthesized through alkylation of a palladacycle of type 35, followed by Mizoroki-Heck coupling with an arylpalladium(II) species of type 36. Although the synthetic scope of the reaction was limited, the importance of the report reveals an unprecedented catalytic transformation where two aryl C-H bonds are converted to sp2-sp3 C-C bonds followed by a standard Mizoroki-Heck coupling. The 1,2,3-trisubstitution pattern generated in the products would be very difficult to obtain via conventional methods. 

In the mechanistic studies of the reactivity of (arylnorbomyl) palladacycles [39], Catellani observed the ort/zo-arylation of the PNP dimer to generate unsymmetrical homobiaryl products. This o/t/io-arylation was combined with the Mizoroki-Heck... 

Immobilized dicyclohexylphosphine ligand, 310, has been used as the starting point for the preparation of supported palladacycles 313 and 314. Simply stirring 310 and dimeric palladium complexes 311 and 312 for 1 h in dichloromethane gave 313 and 314, respectively (Scheme 104). They are active Suzuki coupling catalysts on the first use but cannot be recycled. 

This pointed to the intermediacy of a palladacycle which was actually isolated from the reaction of phenylnorbornylpalladium chloride dimer with sodium phenoxide at room temperature by trapping it with phenanthroline (62% yield) (Eq.lO) [20]. 

It seems highly likely that all palladium-catalysed reactions that commence with an oxidative addition as the first step of the catalytic cycle proceed though a Pd(0) / Pd(ll) mechanism. Thus one needs to conclude that all palladacycles and pincers are converted to some form of Pd(0) in these reactions. In many cases this was shown to be in the form of palladium nanoparticles. However, with the more reactive iodoarenes it is possible that most of the catalyst is in the form of an anionic or neutral monomeric or dimeric palladium species. 

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