Pd-carbene complexes

The (l-ethynyl)-2-propenyl acetate derivative 111 undergoes an interesting PdCl2(PhCN)2-catalyzed cyclization to form the 2-cyclopentenone 112[47], A Pd-carbene complex is assumed to be an intermediate of the formation of 112. 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

Scheme 5.2-3 Formation of a Pd-carbene complex by deprotonation of the imidazolium cation.

The use of imidazolium-based ionic liquids in Pd-catalyzed Heck reactions always carries with it the possibility of in situ formation of Pd-carbene complexes (for more details see Section 5.2.2.3). The formation of these under the conditions of the Heck reaction was confirmed by investigations by Xiao et al. [30], who described a significantly enhanced reactivity of the Heck reaction in [BMIM]Br in relation to the same reaction in [BMIM][Bp4] and explained this difference by the fact that formation of Pd-carbene complexes was observed only in the bromide melt. 

A bromoallene was demonstrated to act as an allyl dication equivalent. When treated with Pd(0) in an alcoholic solvent, an ei-hydroxybromoallene provides a mediumsized heterocycle (Scheme 16.101) [106]. The oxidative addition of a bromoallene to Pd(0) generates an allenylpalladium species, which is successively transformed into a Jt-allylpalladium complex through the attack of the hydroxyl group on the sp carbon followed by the protonation of the resulting Pd-carbene complex. Finally, the products are provided as a mixture of regioisomers by the nucleophilic attack of the external methanol. 

Apart from palladium(O) and palladium(II) salts, such as Pd2(dba)3, PdCl2 and Pd(OAc)2, several well-defined Pd-carbene complexes have been employed in ionic liquids, as have some palladacycle complexes. However, ligands used are in most cases neutral and have not been included to facilitate catalyst retention in the ionic liquid. 

Cerium salts turned out to be the most active (Ce Cu > Mn > Co > Cr > Mo Fe) in DPC synthesis in CH2CI2 solution using PdCl2, TBAB, HQ, and an ICC [50]. Though the activity of Ce and Cu acetates was comparable (yield 76%, Pd TON 250), cerium cocatalysts delivered better selectivity [50]. In the catalytic system including a Pd carbene complex, a Ce salt, NBu Br, and HQ in CH2CI2, the activity of cerium cocatalysts increased in the order Ce(OAc)g < Ce(trop) <... 

Scheme 1.7 Formation of Pd-carbene complexes by reaction between palladium acetate and imidazolium salts followed by decomplexation at high temperature and formation of catalytically active PdNPs for Heck-type reactions [33e,i,jj.

In conclusion, IL are favorable media for the electrostatic stabilization of preformed NPs at room temperature and subsequent catalysis, whereas they give for instance Pd-carbene complexes upon deprotonation of the imidazolium cation, yielding, at high temperature, PdNP catalysts whose mechanism of action is discussed in the ligand-free catalysis section. 

At the same time as new applications of ionic liquids are discovered on almost a daily basis, limitations of these reaction media are also uncovered. While studying the Morita-Baylis-Hillman reaction in ionic liquids, Aggarwal observed that [bmim][Cl] was deprotonated by the weak base present in the reaction mixture, leading, after reaction with benzaldehyde, to salt 111 (Scheme 49). Deprotonation of imidazolium salts with strong bases (KCyBu or NaH) is well known, providing, for example, an easy route to Pd-carbene complexes (Section 2.3.5.1). However, this observation limits the use of imidazolium-based ionic liquids even in weakly basic conditions, where they can react with electrophiles. It also explains previous works reporting low yields for reactions performed in these conditions, such as the Horner-Wadsworth-Emmons reaction in [emimlCPFe] or [emim][BF4]. ... 

Another good ligand is carbene XVI-6, and its derivatives. Synthesis of these carbene ligands is rather simple. Pd-carbene complexes prepared in situ are active catalysts. For example, reaction of chlorotoluene with A-methylaniline was carried out at 100 °C using the imidazol-2-ylidene complex XVI-2 to give 23 in... 

IL-modified sihca gel supports have also been widely used for the deposition of MNPs. As an example, a silica gel modified by N-3-(3-triethoxysilylpropyl)-3-methyhmidazolium-based ILs ((TESP)MI.X X = Q , NOj", Bp4 , and PFg ) was employed as a support for the deposition of Pd MNPs [56]. Initially, Pd(OAc)2 was supported on the IL-modified silica and then reduced to Pd MNPs using ethanol. In the case of the IL containing X = C1 , it was related to the possible formation of a Pd carbene complex. However, for the other IL supports, Pd MNPs of 2-10 nm were attained. These supported Pd catalysts were successfully applied to the heterogeneous hydrogenation ofcirmamaldehyde, which, in all cases, affords hydrodnnamaldehyde as the exclusive product (entries 14-16 in Table 13.2). Indeed, except in the case where a Pd carbene complex was formed, excellent... 

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