Palladium imidazolylidene

Another strategy was successfully implemented by synthetic deprotonation of the acidic C2 group of the imidazolium cation by basic ligands of metal complexes, forming carbenes (Scheme 12). When Pd(OAc)2 was heated in the presence of [BMIM]Br, a mixture of palladium imidazolylidene complexes formed. The palladium carbene complexes have been shown to be active and stable catalysts for the... 

Although such catalyst systems are known to have rather high productivities for the reaction, the addition of several equivalents of 1,3-dialkylimidazolium salts per equivalent of palladium leads to complete deactivation of the catalyst, which was attributed to the formation of highly stable palladium imidazolylidene complexes (Scheme 18). 

Scheme 6.11 Proposed mechanism for the formation of palladium-imidazolylidene species under the conditions employed in Suzuki reactions...

Marsh, K.N., Boxall, J.A. lichtenthaler, R. (2004). Room Temperature Ionic Liquids and Their Mixtures, a Review, Fluid Phase EquUtb., pp. 219, 93-98 Mathews, C.J., Smith, P.J. Welton, T. (2000). Palladium Catalysed Suzuki Cross-Coupling Reactions in Ambient Temperature Ionic Liquids, Chem. Commun., 14, pp. 1249-1250 McNulty, J., Capretta, A., Wilson, J., Dyck, J., Adjabeng, G. Robertson, A. (2002). Suzuki Cross-Coupling Reactions of Aryl Halides in Phosphonium Salt Ionic Liquid imder Mild Conditions, Chem, Commun., 17, pp. 1986-1987 McLachlan, F., Mathews, C.J., Smith, P.J. Welton, T. (2003). Palladium-Catalyzed Suzuki Cross-Coupling Reactions in Ambient Temperature Ionic Liquids Evidence for the Importance of Palladium Imidazolylidene Complexes, Organometallics, 22, pp. 5350-5357... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

With the enthusiasm currently being generated by the (so-called) stable carbenes (imidazolylidenes) [7], it is surprising that there are few reports of imidazolium-based ionic liquids being used to prepare metal imidazolylidene complexes. Xiao et al. have prepared bis(imida2olylidene)palladium(II) dibromide in [BMIMJBr [8]. All four possible conformers are formed, as shown in Scheme 6.1-3. 

A series of palladium(O) mono-imidazolylidene complexes have also been prepared and used for the Heck reaction of aryl chlorides in [NBu4]Br.29 The authors report that two of their catalysts, (1,3-dimesi-tylimidazolylidene)(naphthoquinone)palladium(0) and (1,3-dimesitylimida-zolylidene)(benzoquinone)palladium(0) (Fig. 5) remain stable throughout the reaction and will couple even non-activated aryl chlorides in good yields. Clearly, the authors believe that the imidazolylidene complex is responsible for this activity. 

Fig. 6. The phosphine-imidazolylidene palladium complex [(bmimy)(PPh3)2PdX]+.

The process is carried out in two steps the first step involves the dissolution of Pd(PPh3)4 in thoroughly degassed and dried [bmim][BF4] at 110°C under N2 in the presence of 4-bromotoluene and NaCl, until a yellow-orange solution results. In the second step the solution is cooled to room temperature, and a solution of NivCO in water and phenylboronic acid are added. The cross coupling was achieved by stirring at 110°C under N2. The in situ formation of mixed phosphine/imidazolylidene palladium complexes in the catalytically active solutions has been demonstrated." ... 

The main advantage of these coordinating anions is that they stabilize the active species. This is particularly obvious in the case of palladium complexes, whose tendency to decompose into black metal is well documented. Imidazolium-based ionic liquids can generate in situ formation of metal-imidazolylidene carbene complexes by a deprotonation of the imidazolium cation. The ease of deprotonation depends on the nucleophilicity of the anions. In this case, NAILs may act as both solvents and catalyst ligand precursors [13],... 

Couplings. Both bis(imidazolylidene) and mixed imidazolylidene-phsophine complexes of palladium (1,2) are good catalysts for promoting Suzuki and Stille coupling reactions. 

Fig. 5.3-15 Phosphine-imidazolylidene palladium ion [(BMimy) (PPh3)2PdX]+ as identified from the reaction mixture of a Suzuki reaction in [BMIM][BF4j.

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

As observed in Heck reactions, in the reactions performed in 1,3-dialkylimidazolium ILs, the formation of carbenes that may be involved in the stabilization of catalytically active species is not uncommon. The in situ formation of a mixed phosphine-imidazolylidene palladium complex, in [G4GiIm]BF4, has been observed under conditions employed in many palladium-catalyzed coupling reactions (Scheme 40). ... 

Very recently, Albrecht and coworkers synthesized air- and moisture-stable abnormal imidazolylidene-palladium(II) complexes by the oxidative addition... 

Similar trends were observed in a complementary study using the palladium complexes 14 and 15 comprising sterically identical normal and abnormal bis(imidazolylidene) ligands (Scheme 5.4). ° In the presence of chlorine, complex 14 was stable and did not react, whereas the abnormal carbene complex decomposed to [PdCU] " and a doubly chlorinated bisimidazolium dication 16. This outcome was explained by oxidative CI2 addition to complex 15, followed by reductive Ccarbene Cl bond formation. Obviously, this process was unfavourable with normally bound imidazolylidenes. It is worth noting that an analogue of complex 15 with no alkyl substituents at the C5 and C5 positions induced reductive Ccarbene-Ccarbene bond formation. The higher propensity of abnormal carbenes to be reductively cleaved was rationalised by the enhanced electron donor properties of the non-classical carbenes, which made them more susceptible towards elimination processes. Evidently, steric factors could be ruled out in these systems. 

The increased donor ability of abnormally bound imidazolylidenes increased the nucleophilicity of the metal centre. Abnormal NHC-palladium complexes were thus shown reactive towards Lewis acids. When the abnormal NHC complex 15 was treated with AgBp4, the adduct 18 was formed while normal carbene complexes underwent the expected halide abstraction to form 17 (Scheme 5.5). Crystallographic analysis revealed short Ag- -Pd distances of 2.8701 A, suggesting a strong metal-metal interaction. Theoretical calculations indicated that the palladium centre acted as a Lewis base in this adduet, despite its formal dipositive charge. No such adduct formation was observed with analogous normal NHC-palladium complexes. 

Ruiz-Botella S, Peris E. Phenylene and biphenylene-bridged bis-imidazolylidenes of palladium. Influence of the presence of pyrene tag s on the catalytic activity of the complexes. Organometallics. 2014 33 5509-5516. 

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