Suzuki solvent-free

Solvent-free microwave-assisted Suzuki reaction on AI2O3 as soHd support using KF as base has also been described in the hterature by Villemin [38] (Scheme 20). Ligand-free Pd(OAc)2 was used as a precatalyst. In a reaction... 

A recent publication by the group of Barbarella has disclosed the rapid preparation of poorly soluble unsubstituted and modified a-quinque- and sexithiophenes by the extensive use of bromination/iodination steps and microwave-assisted Suzuki and Sonogashira cross-couplings (Scheme 6.16) [42]. Suzuki reactions were either carried out under solvent-free conditions on a strongly basic potassium fluoride/ alumina support for the synthesis of soluble oligothiophenes, or in solution phase for the preparation of the rather insoluble a-quinque- and sexithiophenes. In both cases, 5 mol% of [l,l -bis(diphenylphosphino)ferrocene]dichloropalladium(II)... 

A similar reaction was reported by Kabalka et al. where ligandless and solvent-free Suzuki couplings were performed with potassium fluoride on alumina. This reaction is very interesting as the catalyst used was palladium powder, the least expensive form of palladium available32. The authors demonstrated the simplicity of the procedure by efficient isolation of the biaryl products via a simple filtration. This could be done as the palladium catalyst remains adsorbed on the alumina surface. A small amount of water in the matrix was beneficial for the outcome of the reactions. Recycling of the catalyst was possible by adding fresh potassium fluoride to the palladium/alumina surface and the catalytic system remained effective at least through six reaction cycles (Scheme 2.6). 

Scheme 2.6 Solvent-free and ligandless Suzuki coupling on potassium fluoride impregnated alumina.

Recently, microwave-assisted Suzuki couplings under solvent-free conditions resulting in thiophene oligomers have also been reported41. 

Melucci, M., Barbarella, G. and Sotgiu, G., Solvent-free, microwave-assisted ynthesis of thiophene oligomers via Suzuki coupling, /. Org. Chem., 2002,67, 8877-8884. 

Toda, F., Suzuki, T., Higa, S. Solvent-free Dieckmann condensation reactions of diethyl adipate and pimelate. J. Chem. Soc., Perkin Trans. 11998, 3521-3522. 

Heck and Stille coupling reactions, also allowing reactions to be conducted under solvent-free conditions. The synergic effect of polymer 96 and TSIL not only reflected an increased stability during storage but also an enhanced activity during catalysis. Scheme 1.62 shows a typical Suzuki reaction occurring in an aqueous biphasic medium. 

T pe of reaction C-C bond formation Reaction condition solvent-free Keywords Suzuki coupling, boronic acid... 

A rapid MW-assisted palladium-catalyzed coupling of heteroaryl and aryl boronic acids with iodo and bromo-substituted benzoic acids, anchored on TentaGel has been achieved [222]. An environmentally friendly Suzuki cross-coupling reaction has been developed that uses polyethylene glycol (PEG) as the reaction medium and palladium chloride as a catalyst [223]. A solvent-free Suzuki coupling has also been reported on palladium-doped alumina in the presence of potassium fluoride as base [224]. This approach has been extended to the Sonogashira coupling reaction wherein terminal alkynes couple readily with aryl or alkenyl iodides on palladium-doped alumina in the presence of triphenylphosphine and cuprous iodide (Scheme 8.89) [225]. 

A few interesting papers have appeared with ligandless and solvent-free Suzuki-Miyaura reactions using cheap palladium powder and potassium fluoride on alumina. The catalysts have been recycled and used through several reaction cycles and the products were collected by a simple filtration, adding to the preparative ease of the method [35]. Potassium fluoride on alumina has also been used in the solvent-free synthesis of unsymmetrical ketones, with good results [36, 37]. 

Solvent-free Suzuki reactions have also been reported under monomode mi- ... 

Suzuki and Wada explored the Knoevenagel condensation of malononitrile and methyl cyanoacetate under solvent-free conditions (Scheme 2.45) [40]. Different alkaline earth carbonates and fluospar were used as mild catalysts for the condensation of 2 equiv. of malononitrile and 1 equiv. of aromatic aldehyde to obtain products in high yields, after short milling (30 min. Table 2.42). When methyl cyanoacetate was ball milled with aldehydes, the reaction occurred with the stereoselective formation of (fl-cyanocinnamic esters 133. However, significant oxidative loss of aldehydes was encountered. The addition of a small amount of hydroquinone as antioxidant was found to solve the problem and drastically increase yields. 

Reproduced from Wada S, Suzuki H. Calcite and fluorite as catalyst for the Knoevenagel condensation of malononitrile and methyl cyanoacetate under solvent-free conditions. Tetrahedron Lett 2003 44 399-401. Copyright (2003), with permission from Elsevier. 

Reproduced from Wada S, Hayashi N, Suzuki H. Noticeable facilitation of the bismuth-mediated Barbier-type allylation of aromatic carbonyl compounds under solvent-free conditions. Org Bio-mo Chem 2003 1 2160-3. With permission from the Royal Society of Chemistry. 

Nielsen SF, Peters D, Axelsson O. The Suzuki reaction under solvent-free conditions. Synth Commun 2000 30 3501-9. 

Bernhardt F, Trotzki R, Szuppa T, Stolle A, Ondruschka B. Solvent-free and time-efficient Suzuki-Miyaura reaction in a ball mill the solid reagent system KF-AI2O3 under inspection. Beilstein J Org Chem 2010 6. http //dx.doi.Org/10.3762/bjoc.66.7. 

Adapted with permission Horn Wada S, Urano M, Suzuki H. The newborn surface of dull metals in organic synthesis bismuth-mediated solvent-free one-step conversion of nitroarenes to azoxy- and azoarenes. J Org Chem 2002 67 8254-7, Copyright 2002, American Chemical Society. 

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