Suzuki-type reactions

To mention a few synthetic appHcations of trialkylsilanols, trimethylsilanol 4 adds readily to 2-chloroacrylonitrile in diethyl ether in the presence of triethylamine as triethylammonium trimethylsilanolate followed by ehmination of triethylamine hydrochloride to give 99 [32] (cf. discussion of the strongly nucleophihc properties of ammonium trimethylsilanolate 155 in Section 4.2.1). The stable potassium trimethylsilanolate 97 has also been used for the saponification of esters (Section 4.7). Dimethylphenylsilanol 100 adds readily to a,y9-unsaturated carbonyl compounds such as methyl vinyl ketone 764 in the presence of Pd(OAc)2 in a Heck-Suzuki-type reaction to give the sihcon-free /9-phenylmethylvinylketone 101 [33]. 

Recently, Suzuki-type reactions in air and water have also been studied, first by Li and co-workers.117 They found that the Suzuki reaction proceeded smoothly in water under an atmosphere of air with either Pd(OAc)2 or Pd/C as catalyst (Eq. 6.36). Interestingly, the presence of phosphine ligands prevented the reaction. Subsequently, Suzuki-type reactions in air and water have been investigated under a variety of systems. These include the use of oxime-derived palladacycles118 and tuned catalysts (TunaCat).119 A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl-modified silica is highly active (>99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water no leaching occurs and the same catalyst sample can be reused eight times without decreased activity.120... 

An efficient aqueous phase Suzuki-Miyaura reaction of activated aryl chlorides with aryl boronic acids has been reported. The method uses a new D-glucosamine-based dicyclohexylarylphosphine ligand for the palladium catalyst and works well with nitro-and cyano-activated chlorides.32 The aryl fluoride bond has been considered inert to palladium-catalysed substitution reactions. However, a computational study, backed up by experiment, shows that the presence of a carboxylate group ortho to fluorine will allow reaction both with phenylboronic acids in a Suzuki-type reaction and with organotin reagents in a Stille-type reaction the presence of the adjacent oxyanion stabilizes the transition state.33... 

Pd(acac)2 efficiently catalyzes the Suzuki-type reaction of potassium alkynyl trifluoroborates 232 with vinylic tellurides (Scheme 127).303 This reaction presents the advantage of using compounds of the type 232, which can be isolated and manipulated in the air. 

Novel 1,2,3-triazolopyridylboronic acids and esters were prepared and were utilized in Suzuki-type reactions <04T4887>. Benzotriazole was regiospecifically substituted at the N- position with alkyl halides or a-halogenated ketones in the absence of base in ionic liquid... 

The rather classic catalyst palladium on activated carbon has been applied by Sun and Sowa et al. [183] and Heidenreich et al. [158] without additional ligands. This simple system was able to convert (mainly activated) aryl chlorides in mixtures of water and an organic solvent (DMA and NMP, respectively). Lysen et al. were able to convert aryl chlorides in pure water and without addition of any ligand [184,185]. Suzuki reactions using Pd/C in an aqueous medium were also reported by Arcadi et al. (in the presence of surfactants) [186] and the group of Leadbeater, who applied microwave techniques [187]. Microwave (as weU as ultrasound) conditions were also employed by Cravotto and Palmisano et al. [188] The substrate scope of Suzuki-type reactions in the presence of Pd/C was extended to halopyri-dines and haloquinoUnes by Tagata and Nishida [189]. 

Details on oxidation and reduction reactions mediated by [(NHC)Pd] complexes can be found in Chapters 12 and 13, respectively. Further reports of interest disclosed in recent years include diboration of alkenes catalysed by a pincer complex, deuteration of C-H bonds with an N,0-functionalised NHC complex, and an intriguing Suzuki-type reaction of [FeI(Cp)(CO)2] with arylboronic acids. 

In biatyl synthesis, although transition metal catalyzed aryl-aryl crosscoupling reactions have been widely exemplified using Suzuki-type reactions, CDC reactions between sp C-H bonds leading to sp C-C bonds are scarce. In 2010, Katsuki reported an enantioselective CDC reaction between two naphthol moieties catalyzed by a chiral iron(salan) complex under aerobic oxidative conditions (Scheme 4.23). Using 4 mol% of the iron(salan) complex 23-A as the catalyst in toluene at 60 °C for 48 h, two different 2-naphthols were coupled and the cross-coupled derivatives were isolated with moderate yields (44-70%) and good ee (87-95%). It must be pointed out that the homocoupling derivatives are also obtained in low yields. 

Alternative sulfonate leaving groups besides triflate have not been reported to be active in Pd catalyzed Suzuki-type reactions. Aryl mesylates, benzenesulfonates and tosylates are much less expensive than triflates and are usually unreactive towards palladium catalysts. However, in the first example of the use of a Ni-catalyst in the Suzuki reaction, aryl mesylates participated in cross-coupling reactions with arylboronic acids in good yields (equation 48) (745). 

Yamada, and Suzuki laid the base for one of the most important and useful transformations for the construction of C-C bonds in the modem day organic chemistry. The Suzuki reaction becomes popular because of the ready availability of a wide range of functionally substituted boron derivatives and the mildness of the coupling reaction itself Suzuki reactions generally employ organic solvents such as tetra-hydrofliran and ethers as well as complex palladium catalysts, which are soluble in these solvents (Miyaura and Suzuki, 1995). The use of microwave heating is a convenient way to facilitate the Suzuki-type reactions in water (Leadbeater and Marco, 2002). 

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