Palladium complexes cross-couphng

Group 10 TM complexes (110) bearing allyl ligand were synthesized by breaking the corresponding dimer [MC1(R, R -allyl)]2 (109) with two equivalents of free NHC ligand (equation 15). Aryloxy dimer [Ni(allyl)(OAr)]2 (Ar = 2,6-di-fro-propylphenyl) has also been employed for the preparation of NHC-allyl Ni complexes. Since the [(NHC)PdCl(allyl)] complexes exhibited an excellent activity in palladium-catalyzed cross-couphng reactions, this series was extensively studied with special attention to aUyl substituent effects. ... 

In 2009, Ye et al. developed an efficient protocol for the preparation of 3-chloro-and 3-bromo-l-methyleneindenes 256 via a palladium-catalyzed domino reaction from 2-alkenylphenylacetylenes 255 [92] (Scheme 6.68). The product of this reaction can easily be modified to complex structures via palladium-catalyzed cross-couphng... 

The influence of the extent of conversion on enantioselectivity has been studied in the reaction of the Grignard reagent 2b with 4a catalyzed by the nickel complex of (S)-(1 )-BPPFA (14) [12]. A ferrocenylphosphine 16 which is analogous to PPFA but has a tetrahydroindenyl moiety was more enantioselective than PPFA (10a) for the palladium-catalyzed asymmetric cross-couphng of 2a with 4b to give (R)-6 of 79% ee [13] (entry 15). 

A palladium catalyst with a less electron-rich ligand, 2,2-dipyridyl-methylamine-based palladium complexes (4.2), is effective for couphng of aryl iodides or bromides with terminal alkynes in the presence of pyrroMdine and tetrabutylammonium acetate (TBAB) at 100°C in water.However, the reactions were shown to be faster in NMP solvent than in water under the reaction conditions. Palladium-phosphinous acid (POPd) was also reported as an effective catalyst for the Sonogashira cross-couphng reaction of aryl alkynes with aryl iodides, bromides, or chlorides in water (Eq. 4.18). ... 

Palladium complexes of N-heterocychc carbenes as catalysts for cross-couphng reachons 07AG(E)2768. 

Cobalt Oxazoline Palladacycles (COPs) are organocobalt-palladium complexes which catalyse the asymmetric rearrangements of non-chiral allylic trichloroacetamidates with very high enantiomeric selectivity (>90%) to provide chiral allylic amines [it is an aza-Claisen rearrangement, The Overman Rearrangement Overman Carpenter Org React 66 2005, Kirsch, Overman and Watson J Org Chem 69 8101 2004] and in the presence of phenols stereospecific cross-couphng also occurs to provide chiral phenoxyallyl ethers with veiy high (>90%) enantiomeric selectivity [Kirsch, Overman and White Org Lett 9 911 2007, Overman Carpenter Org React 66 2005]. 

Finally, it should be mentioned that two years later, an enantioselective oxidative (boron) Heck-type reaction was reported by Jung and coworkers [32] for a dinuclear NHC-derived CNO-based pincer complex for which for the first time a Pd /Pd" mechanism was suggested to be operative with palladium pincer-type crosscoupling catalysts. Reaction mechanisms without a change of the palladium s oxidation state have never been proposed to be operative for Heck cross-couphng reactions, but have been shown to be operative for xylene-derived selenium-based pincer complexes and related systems in the cross-couphng of vinyl epoxides (and aziridines) with organoboronic acids [24d, 33]. 

Kostas, I.D., Coutsolelos, A.G., Charalambidis, G. and Skondra, A. (2007) The first use of porphyrins as catalysts in cross-couphng reactions a water-soluble palladium complex with a porphyrin ligand as an efficient catalyst precursor for the Suzuki-Miyaura reaction in aqueous media under aerobic conditions. Tetrahedron Lett., 48, 6688-91. 

In a generahzed and simpHfied mechanism, the reaction usually follows the standard catalytic cycle of metal-catalyzed cross-coupling reactions oxidative addition of the C(sp ) -X bond to paUadium(O), followed by coordination of the amine to the resulting palladium complex, occurring with extrusion of HX that is captured by the base. Finally, reductive elimination yields the couphng product, regenerating the catalyticaUy active paUadium(O) species. 

Cross-couphng reactions of alkynylzincs catalyzed by palladium complexes have many similarities with those described earlier with C(sp )-organozincs. Pd(PPh3)4 is the most frequently employed catalyst, but combinations of a palladium source and other common phosphine hgands such as ddpe, dppf, tfp, or DPEPhos (174) are also typically used. 

These observations were rationahzed by the assumption that a pentacoordinate silane is necessary for the cross-coupHng. Both mono- and difluorosilanes are efficient fluoride ion acceptors (from TASF), thereby accessing a pentacoordinate state (Scheme 7.47). The remaining coordination site on sihcon would presumably be occupied by the hahde from the arylpalladium halide complex to allow for a four-centered transmetallation transition state. This last coordination site would not be accessible with a trifluorosilane because it would readily accept two fluoride ions from the promoter, forming an unreactive coordinatively saturated siliconate, and thereby leaving no site for palladium halide complexation. 

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