Sonogashira reaction systems

Cationic phosphine ligands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. They were shown to form active catalytic systems in Pd-mediated C-C coupling reactions between aryl iodides and alkynes (Castro-Stephens-Sonogashira reaction) [72, 74] and Rh-catalyzed hydroformylation of olefins in aqueous two-phase systems [75]. 

Pyrazinone bearing a phenylalkynyl substituent at position C-3, was prepared in 47% yield via Sonogashira reaction with 7 equiv of phenylacetylene in a mixture of toluene/triethylamine (2 1) using Pd[P(Ph3)2]Cl2 and Cul as the catalyst system (Scheme 37). 

The performance of the Sonogashira reaction is claimed to be the first example of a homogeneously metal-catalyzed reaction conducted in a micro reactor [120], Since the reaction involves multi-phase postprocessing which is needed for the separation of products and catalysts, continuous recycling technology is of interest for an efficient production process. Micro flow systems with micro mixers are one way to realize such processing. 

Figure 4.69 Flow scheme of a Sonogashira reaction performed in a micro flow system [120].

A potentially interesting development is the microwave-assisted transition-metal-free Sonogashira-type coupling reaction (Eq. 4.10). The reactions were performed in water without the use of copper(I) or a transition metal-phosphine complex. A variety of different aryl and hetero-aryl halides were reactive in water.25a The amount of palladium or copper present in the reaction system was determined to be less than 1 ppm by AAS-MS technique. However, in view of the recent reassessment of a similarly claimed transition-metal-free Suzuki-type coupling reaction, the possibility of a sub-ppm level of palladium contaminants found in commercially available sodium carbonate needs to be ruled out by a more sensitive analytical method.25 ... 

Sonogashira reaction. The first system consisted in the use of the oxime palladacycles 7a-f at elevated temperatures, without the aid of Cul or an amine base, for the coupling of aryl iodides and bromides. They also reported on the use of complex 48b in aqueous media for the coupling of aryl iodides and bromides and terminal acetylenes in excellent yields. ... 

Doucet H, Hierso JC (2007) Palladium-based catalytic systems for the synthesis of conjugated enynes by sonogashira reactions and related alkynylations. Angew Chem Int Ed 46 834-871... 

Highly elaborate architectures, such as 346 could be prepared from 344 and 345 using the Sonogashira reaction. These systems could be used as hgands for intramolecular electron-transfer quenching systems [121]. 

The Sonogashira coupling reaction of terminal alkynes with aryl or vinyl halides is a useful tool for carbon—carbon bond formation, and has found wide employment in areas such as natural product synthesis, the pharmaceutical industry, and material sciences. Novel recyclable Pd catalysts with fluorous ponytails in the ligand 2,2 -bipyridine were reported in a copper-free Pd-catalyzed Sonogashira reaction in a fluorous biphasic system (FBS) (Equation 4.19). The catalysts are only soluble in perfluorinated solvents at room temperature [41],... 

Through a survey of a number of Pd/ligand combinations, Pd/NHC-based systems were found to possess the desired activity. Interestingly, organic bases such as NEt3, which are commonly used in Sonogashira reactions of aryl electrophiles, were ineffective under the conditions described in Eq. 14. 

In this chapter, we discuss selected literature on palladium catalysts (or catalyst precursors) for the Heck, Suzuki and Sonogashira reactions. The review covers simple homogeneous palladium complexes, Ugand-free palladium catalytic systems, stable palladium colloids and particles and supported palladium catalysts. It focuses on the role of palladium nanoparticles (as catalyst precursors or formed in situ during the course of the reaction) from a mechanistic point of view. 

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