Bis-carbenes palladium

Bis(carbene)-palladium pincer complex, on polymer support,... 

In spite of the successful use of NHCs in a number of palladium-catalyzed reactions, no system for hydrogenation was reported until 2005. This can be easily explained as it had been observed that hydridopalladium-carbene species decompose due to attack of the hydride on the carbene, which results in its reductive elimination to yield the corresponding imidazolium salt [ 190]. However, Cavell and co-workers recently showed that the oxidative addition of imidazolium salts to bis-carbenic palladium complexes leads to isolable NHC-hydridopalladium complexes [191]. This elegant work evidenced the remarkable stabilizing effect of NHC ligands in otherwise reactive species and led to the development of the first NHC-palladium catalyst for hydrogenation. 

With the unsymmetric bis-carbene palladium(II) complexes conung from Foley s research group, we are again on firm ground a square planar metal complex and sterically innocent wingtip groups (methyl, benzyl, p-Bu -benzyl) let us suspect the preferred formation of a chelate complex and this is indeed what is observed [316]. 

The Heck reaction was the first catalytic application examined for palla-dium/NHC complexes. A high degree of efficiency was observed for chelating bis-carbene/palladium complexes in this transformation. The reactions involving butyl acrylate with aryl halides were efficiently mediated by such complexes at catalyst loadings as low as 10" mol%. Unactivated chlorides required, however, a catalyst loading up to 1 mol%. The impressive stability of the catalytic system under harsh reaction conditions and its reactivity profile were described as quite impressive [32,56]. 

A bidentate ferrocenyl-A-heterocychc carbene hgand precursor in the form of the bis-imidazolinium salt was deprotonated in situ with KHMDS and reacted with [PdCl2(cod)] to provide a bis(carbene)palladium(n) dichloride complex, which was stmcturally characterized (eq 100). ... 

Caddick and Cloke reported similar exchange reactions as early as 2001. Bis(carbene)-palladium complexes reacted with phosphines, resulting in displacement of the carbene under surprisingly mild conditions. In the case of tris(ort/ o-tolyl)phosphine, an equilibrium between the mixed phosphine-carbene complex and the bis(carbene) complex was established at 60 °C [eqn (2.17)]. ... 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

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. 

Scheme 6.122 Preparation of a pyridyl bis-N-heterocyclic carbene palladium complex.

Bis(butadiene) complexes, with tantalum, 5, 173 Bis(z-butanethiolato) complexes, with bis-Cp Ti(IV), 4, 601 Bis(calixarene) complexes, as organic molecule hosts, 12, 799 Bis(carbene) complexes with gold(I), 2, 287-288 with manganese, 5, 780, 5, 826 with mercury, 2, 429 with palladium, 8, 230 with silver , 2, 206... 

The first such ligand system (14) was developed in Burgess s group and was used in the synthesis of the palladium(II) complex 15, in which the bis-carbene acts as a trans-chelating ligand (Scheme 13) [38]. However, there are no reports of the use of 15 in Pd-catalyzed reactions. 

QCdmJpFJ [QCpmJBr PdCl2 Pd(OAc)2 NaOAc 30 °C. Ligand-free, ultrasound promoted arylation of alkenes and alkynes with aryliodides palladium bis-carbenes and palladium nanoparticles ( 1 nm) are identified after catalysis product extracted with ethyl acetate/petrol ether. [66]... 

Note Depending on stoichiometry, reaction conditions and coligands, monocarbene and bis-carbene as well as dimeric palladium(II) carbene complexes can be synthesised. 

The silver(I) complexes of the furan functionalised bis-carbene ligands were used for the palladium catalysed aryl amination of p-bromotoluene with morpholine as the amine and [Pd(dba)j] as the precatalyst employing an in situ protocol. Performance of the catalyst was poor, but increased somewhat with additional bulk on the wingtip groups (Me < Bu < Mes) and the reaction time. The latter indicates that the catalyst, formed in situ, is stable under catalytic conditions and remains active over prolonged reaction times. 

Nielsen et al. have introduced a monoether linked bis-carbene [209] modelled on an amino linked bis-carbene ligand that acts as a C,N,C pincer ligand in a corresponding palladium(II) complex [156]. Synthesis of the ether linked bis-carbene is facile and involves the reaction of the l-co-dichloro-diethylether with 2 equiv. of methylimidazole. Subsequent reaction with silver oxide and carbene transfer to suitable transition metal precursor complexes affords the corresponding complexes (see Figure 3.73). 

Having seen that structural predictions are very difficult, we will now turn to the choice of transition metal. We have already seen the dependence of the coordination mode in square planar complexes on various factors and noticed the preference for polymeric chains with the silver(I) complexes owing to the linearly coordinated silver centre. Chiu et al. [325] reported on a series of arylmethylene and methyl wingtipped bis-carbene complexes of silver(I) (polymeric bridging) and palladium(II) (monomeric chelating). Carbene transfer to palladium was achieved in DMSO since solubility in CHjCfj was very poor. 

Schwarz et al. [326] synthesised a functionalised bis-imidazolium salt with hydroxy end groups on the wingtips [327,328] and used it in the formation of chelating cw-bis-carbene complexes of palladium(ll) applied as catalysts in the Heck reaction. The functional groups were needed to immobilise the catalyst by attachment to a polymeric support [329] (see Figure 3.104). 

Similar cts-bis-carbene chelate complexes of palladium(Il) [327,330,331], but without the hydroxy functional groups on the wingtips, were used by the same research group for the copolymerisation of ethylene and CO. Once again, chelating bisphosphane complexes inspired the synthesis and application of their NHC counterparts [332,333]. The actual, defined catalyst precursors were the cationic complexes formed after haUde abstraction with silver salts in acetonitrile as donor solvent. 

A totally different approach to bis-carbene ligands on a cyclic scaffold comes from Burgess and coworkers [351], They start from AA -dimethyl-l,2-diaminocyclohexane and acetylate this compound with chloroacetic acid chloride. Addition of an N-substituted imidazole yields the chiral bis-imidazolium salt (see Figure 3.110). Reaction with silver(I) oxide and carbene transfer to palladium(II) completes the reaction sequence. 

Figure 3.110 Synthesis of a chiral bis-carbene complex of palladium(ll) using the 1,2-diami-...

The synthesis of the first Janus bis-carbenes (back-to-back carbenes with a symmetric spacer unit - phenylene) was reported by Bielawski and coworkers [376,379,380]. The synthesis of the bis-imidazohum salts starts with a palladium catalysed amination of... 

The concept can be adapted to the introduction of only one (chiral) carboxylic acid wingtip group, even without the introduction of a second one [94]. The adaptation comprises the drop-wise addition of a mixture of ammonia, sodium hydroxide and the a-amino acid in water to a solution of glyoxal and formaldehyde in water at 50°C. Yields are moderate (but excellent compared with the 40-50% achieved by the parent protocol [97]). In the event, Strassner and coworkers [94] used the chiral carboxylic acid functionalised imidazole for the synthesis of the corresponding ester functionalised bis-carbene ligand and their palladium(ll) complexes. 

Despite their apparent steric bulk, both the Fc and bis-ferrocenyl modified carbene ligands could be successfully i Ued in the synthesis of mono- and bis-carbene complexes of mercury(II) [147], mngsten(0) [147], palladium(II) [147,183,192] andsilver(I) [191] (see Hgure 4.57). 

Attachment of the catalyst to a polymeric support can also be facilitated using a bis-carbene as the ligand [253]. The bisimidazoUum salt featuring a hydroxyalkyl sidechain on each of the two imidazolium units was reacted with palladium (II) acetate to form the neutral catalyst complex (see Figure 4.82). Immobilisation was then achieved by reacting the functionalised catalyst with 4-(bromomethyl)phenoxy-methyl polystyrene, known as Wang resin, as the polymeric support. 

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