Cross couphng catalysts

An additional advantage of Freeh s palladium pincer cross-couphng catalyst includes its well-defined fate after catalysis. Treatment of 10 with aqueous hydrochloric acid and thus under work-up conditions led to a rapid and complete catalyst degradation, accompanied by the formation of phosphonate, piperidinium salts, 1,3-diaminobenzene, and other insoluble paUadium-containing products, which were easily separable from the coupling products-an important issue to be considered in particular for pharmaceutical applications. All these advantages make, [ C H3-2,6-(NHP(piperidinyl)2)2 Pd(Cl)] (10) attractive for industrial applications. Indeed, 10 was successfully applied in industry, and nowadays is commercially available. 

CombiPhos-Pd6 is a mixture of POPd, POPdl, POPd2, and other couphng catalysts, which can carry out many cross-coupling reactions. 

When nitroalkenes were used as Michael acceptors, high yields and enantioselectivities of the desired Michael addition products were also obtained (Scheme 5.22). In these reactions, a well-defined chiral Ru amido complex (Figure 5.9) was an efficient catalyst. The mild reaction conditions and high reactivities and stereoselectivities allowed a large-scale reaction in the presence 1 mol% Ru catalyst. By using a chiral Pd(II) catalyst, an asymmetric allylic arylation was reported by Mikami and coworkers to give the cross-couphng product via the activation of both allylic C H and aryl C H bonds in moderate enantioselectivity (Scheme 5.23). ... 

Tandem hydrostaimation/cross-coupling is a powerful tool for C-C bond formation starting from acetylenes. A vinylstaimane derived from butynol by AIBN-mediated hydrostannation underwent the cross-couphng with a dienyl iodide under the influence of palladium catalyst to furnish racemic phthoxazohn A (Scheme 12.106) [208]. In this total synthesis a Z,Z, -triene unit was efficiently assembled by exploiting this hydrostannation/cross-coupling procedure. 

Further investigations into the palladium(ll)-catalysed cross-couphng of olefins with iodopyrimidines have been published. As previously reported,2-iodo-4,6-dimethylpyrimidine and its 4-iodo-2,6-dimethyl isomer do not couple with ethyl acrylate if a reagent mixture of Pd(OAc)2, PhaP, and EtaN is used however, omission of the triphenylphosphine permits the coupling to proceed in reasonable yields. Palladium black can also be used as the catalyst. ... 

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. 

Electrochemical reduction provides a powerful method for maintaining the activity of the Pd° catalyst. Following this method, catalytically inactive Pd species formed in deleterious side reactions with CN are electrochemically intercepted and restored to a catalytically active state. Under these conditions, aryl chlorides undergo cross-couphng with CN at 130 °C in DMF (Scheme 24) Binding to a Cr(CO)3 fragment to form the tt-comple is another way of activating aryl chlorides for the attack of CN in the presence of Pd° catalyst. 

The Inamoto group [44] has reported that the NHC-based pincer complex 30 is an effective catalyst for the cross-coupHng of aryl haUdes and butyl acrylate (Eq. (5.33)). This catalytic system is appHcable to aryl iodides, aryl bromides, and activated aryl chlorides, and compatible with functional groups including nitrile, ketone, and aldehyde. Notably, the reaction of 4-bromobenzonitrile can be retarded by a drop of mercury, suggesting that the catalytic system may be heterogeneous. 

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]. 

Recent years have witnessed an enormous development in catalysts for Suzuki-Miyaura cross-couphng [57], the aim being to achieve higher turnover... 

---