N-Heterocyclic Carbene-Pd Complexes

Pd-N-Heterocyclic Carbene Complexes in Cross-Coupling Applications 135... 

Ionic liquids (ILs) were introduced in catalysis by Yves Chauvin in the 1990s [33a] and have received considerable attention in this field [33b,cj. Yves Chauvin introduced the imidazolium salts that are the most frequently used ILs in catalysis. ILs are valuable media for catalysis with PdNPs because the substituted imidazolium cation is bulky, favoring the electrosteric stabilization of NPs, as do the t-Bu N salts in Fig. 1.1. The size of the cation (that can eventually be tuned by the choice of the N-alkyl substituents) also has an important influence on the stabilization, size and solubUity of the NPs, these factors playing a role in catalysis. ILs are also non-innocent, however, as they readily produce Pd-N-heterocyclic carbene complexes upon deprotonation of the imidazoHum salt at sufficiently high temperature. Thus, these carbene ligands can be bound to the NP surface or give mononuclear mono- or bis-carbene complexes subsequent to leaching of Pd atoms from the PdNP surface (vide infra) [33]. 

Finally, Arnold and Sanford have reported C-Cl bond formation from the Pd N-heterocyclic carbene complex 50 (Eq. 41) [71]. Complex 50 was prepared by oxidation of 49 with PhICl2 at —35°C in CH3CN and was characterized by and C NMR spectroscopy and X-ray crystallography. This complex could... 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Sulfur-functionalized N-heterocyclic carbene complexes of Pd(II) Syntheses, structures and catalytic activities 12MOL2491. 

Bettucci L, Bianchini C, Oberhauser W, Hsiao T-H, Lee HM. Chemoselective aerohic oxidation of unprotected diols catalyzed by Pd-(NHC) (NHC=N-heterocyclic carbene) complexes. J Mo/ Catal A Chem. 2010 322 63-72. 

Budagumpi S, Hague RA, Salman AW. Stereochemical and structural characteristics of sin e- and double-site Pd(II)-N-heterocyclic carbene complexes promising catalysts in organic syntheses ranging from C C coupling to olefin polymerizations. Coord Chem Rep. 2012 256 1787-1830. 

Nielsen DJ, Cavell KJ (2006) Pd-NHC complexes as catalysts in telomerization and aryl amination reactions. In Nolan SP (ed) N-Heterocyclic carbenes in synthesis. WUey-VCH, Weinheim, pp 73-102... 

Utilizing more reactive discrete palladium-N-heterocyclic carbene (NHC) complexes (for example, Pd(carb)2) or in situ generated palladium/imidazolium salt complexes (1 mol% ligand A), Caddick and coworkers were able to extend the rapid amination protocols described above to electron-rich aryl chlorides (Scheme 6.61) [128],... 

On the other hand, the highest reactivity is achieved in the presence of the complex prepared in sim from Pd(acac)2 and N-heterocyclic carbene IMes.HCl (TON = 4.299). With this catalytic system, the dimerization of butadiene is negligible (1%). However, in that case, due to very high activity of the catalytic system, there is no selectivity towards the monotelomer, and a large amount of the ditelo-mer was formed (51 and 41% respectively). 

Herrmann, W. A. Ofele, K. von Preysing, D. Schneider, S. K. Phospha-palladacydes and N-heterocyclic carbene Pd complexes efficient catalysts for CC-coupling reactions. J. Organomet. Chem. 2003, 687, 229-248. 

Tridentate pincer bis-carbene [114] and N-carbamoyl-substituted heterocyclic carbene complexes of Pd(II) [115] have also been used to couple aryl bromides and iodides with aromatic or aliphatic alkynes. Surprisingly, the latter catalytic system requires the use of 1 mol % of PPh3. Its role in the catalytic cycle is still unclear but it might facilitate the initial generation of Pd(0) species. 

Other palladium acetylacetonate complexes have also been developed for cross-coupling reactions. Indeed, an acac paUadacycle complex has recently been observed to effectively catalyze both the Suzuki-Miyaura and Heck reactions. Likewise, the A-heterocyclic carbene complex Pd(acac)Cl(ipr) (96) is an excellent catalyst for the formation of C—N and C—C bonds (equation 27) These catalysts were effective for coupling of ketones... 

Using the bulky N-heterocyclic carbene 130 as a ligand for Pd(0) complexes Schneider and coworkers [104] have recently reported a novel synthetic strategy to five-, six- and seven-membered N-arylated heterocycles 131-135 via sequential Pd-catalyzed intra- and intermolecular arylamination reactions (Scheme 49). 

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