Base-catalyzed coupling reaction

Carbamates can be used as protective groups for ammo acids to minimize race-mization in peptide synthesis. Raccmi/ation occurs during the base-catalyzed coupling reaction of an W-protected, catboxyl-uc ivated amino acid, and takes place in the intermediate oxazolone that foro S readily from an N-acyl protected amino... 

To minimize racemization, the use of nonpolar solvents, a minimum of base, low reaction temperatures, and carbamate protective groups (R = O-alkyl or O-aryl) is effective. (A carbamate, R = O-r-Bu, has been reported to form an oxazolone that appears not to racemize during base-catalyzed coupling.) ... 

The Sonogashira reaction is a transition metal-catalyzed coupling reaction which is widely used for the preparation of alkyl-, aryl- and diaryl-substituted acetylenes (Table 4.7) [120]. This reaction is a key step in natural product synthesis and is also applied in optical and electronic applications. Sonogashira reactions involve the use of an organic solvent with a stoichiometric portion of a base for capturing the... 

There are many other transition-metal catalyzed coupling reactions that are based on organic halides in aqueous media. One example is the coupling of terminal alkyne with aryl halides, the Sonogashira coupling, which has been discussed in detail in the chapter on alkynes (Chapter 4). An example is the condensation of 2-propynyl or allyl halides with simple acetylenes in the presence of copper salts. 

Aryl hydrazide-based linker 79 was developed as a traceless handle that released products under mild oxidative conditions (Scheme 42) [91]. Polymeric bound p-iodophenylhydrazide was subjected to a variety of Pd°-catalyzed coupling reactions (Heck, Suzuki, Sonogashira, and Stille). Oxidation with Cu(OAc)2 in MeOH and pyridine released the final products in 50-96% yield. 

Ionic liquids are also alternative reaction media considered as green, mostly because of their lack of vapor pressure and the possibility of reuse [68, 69]. Base-catalyzed Knoevenagel reactions have been described in these media [70-73]. The development of task specific ionic liquids in which the solvent and promoter are coupled is a real breakthrough in the field and some of them have been used in the Knoevenagel condensation [74—81]. 

The 7V-arylation of indoles has been achieved both in palladium and copper catalyzed coupling reactions. In an early example Hartwig succeeded in converting indole to 1-arylindoles in excellent yield, using a palladium -dppf catalyst and caesium carbonate as base (6.71.)."a Buchwald99b and... 

Quite recently, Grigg and co-workers have based several of their ingenious multicomponent reactions on the combination of the above-mentioned palladium-catalyzed coupling reactions of allenes with a subsequent reaction in a one-pot operation. For example, the catalytic re-allylpalladium species generated from the carbopalladation of the allene with iodobenzene was captured by a phenolic nucle-... 

The azetidinium acetates 18 are suitable substrates for the synthesis of 1-arylazetidines 19 <2000EJ01815>. The palladium-catalyzed coupling reaction of azetidines 18 with aryl bromides has led to the synthesis of 1-arylazetidines 19 (Equation 1). This reaction required the presence of a strong base such as /-butoxide. 

Allyl carbonates (Scheme 5) and diene monoxides (Eq. 32) were also employed in the palladium-catalyzed coupling reaction of arylsilanes 136, 37]. The reaction does not require activation by a fluoride ion or an additional base like a hydroxide ion. 

Neutral catalysts or catalyst precursors based on fluorinated ligand systems have been applied in compressed CO2 to a broad range of transformations such as Zn- and Cr-catalyzed copolymerization of epoxides and CO2 [53, 54], Mo-catalyzed olefin metathesis [9], Pd-catalyzed coupling reactions [43, 55, 56] and Pd-catalyzed hydrogen peroxide synthesis [57]. Rhodium complexes with perfluoroalkyl-substituted P ligands proved successful in hydroformylation of terminal alkenes [28, 42, 44, 58], enantioselective hydroformylation [18, 59, 60], hydrogenation [61], hydroboration [62], and polymerization of phenylacetylene... 

Nickel- or palladium-catalyzed coupling reactions of alkyl Grignard or zinc reagents can be applied to heteroaromatic halides and sulfides. The characteristic features are, therefore, based on those described in the preceding sections and hence details are not repeated in this section. Aspects are summarized by the types of heteroaromatic compounds NiCh(DPPP) is used as catalyst, unless stated otherwise hereafter. 

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