Palladium chemistry carbenes

Hillier, A. C., Grasa, G. A., Viciu, M. S., Lee, H. M., Yang, C., Nolan, S. P. Catalytic cross-coupling reactions mediated by palladium/nucleophilic carbene systems. Journal of Organometallic Chemistry 2002, 653, 69-82. 

A very recent example of carbene/palladium chemistry is reported by Stahl in which (NHC)2Pd add molecular oxygen to form a peroxo complex. Oxidative chemistry involving this type of complex as catalyst is expected to follow. This represents a fundamental departure of NHC ligands from phosphines due to their stability to oxidation Konnick MM, Guzei lA, Stahl SS (2004) J Am Chem Soc 126 10212... 

Cavell KJ, McGuinness DS (2007) Palladium complexes with carbonyl, isocyanide and carbene ligands. In Crabtree RH, Mingos DMP, Canty AJ (eds) Comprehensive organometallic chemistry 111. Elsevier, Amsterdam... 

Dixon, K. R. Dixon, A. C. Palladium Complexes with Carbonyl, Isocyanide and Carbene Ligands, In Comprehensive Organometallic Chemistry II A review of the literature 1982-1994 Puddephatt, R. J. Ed., Elsevier, 1995, Vol. 9, p 193. 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

Switching from palladium to rhodium, we encounter some very interesting chemistry. Zeng et al. [302] reacted the tiidentate PCP phosphino functionalised imidazolium salt with silver(I) oxide and subsequently transferred the carbene to rhodium(I) (see Figure 3.100). Careful selection of the rhodium precursor complex and reaction conditions enables tetrahedral, square bipyramidal and octahedral rhodium(I) and rhodium(III) complexes to be formed. As the authors explained, the activation of the C-Cl bond in methylene chloride in an oxidative addition reaction on rhodium(I) resulting in a rhodium(in) complex requires an electron rich rhodium(I) complex. The presence of a NHC ligand is advantageous in this respect. 

In contrast to the wealth of chemistry reported for catalyzed reactions of diazocarbonyl compounds, there are fewer applications of diazomethane as a carbenoid precursor. Catalytic decomposition of diazomethane, CH2N2, has been reported as a general method for the methylenation of chemical compounds [12]. The efficacy of rhodium catalysts for mediating carbene transfer from diazoalkanes is poor. The preparative use of diazomethane in the synthesis of cyclopropane derivatives from olefins is mostly associated with the employment of palladium cat-... 

As with all such palladium-catalyzed carbon-heteroatom bond-forming chemistry, the ancillary ligand(s) (i.e., L often featuring phosphine or A-heterocyclic carbene donors " employed have a direct influence over the course of the elementary transformations. Electron-rich and sterically demanding ligands promote the formation of low-coordinate compounds of type A that are predisposed to undergo Ar—X... 

Palladium complexes bearing NHC ligands are generally very robust systems with impressive thermal and atmospheric stability. From this point of view, they seem to be ideal candidates for catalytic processes. However, the palladium-carbenic carbon bond can be visualized as a single bond and the usual chemistry of Pd-aryl or Pd-alkyl, such as elimination of C-C, C-H, or C-X, has been both observed and investigated from a theoretical perspective. 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

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