Sonogashira solvent-free

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)... 

Some early examples involving microwave-assisted solvent-free Sonogashira couplings using palladium powder doped on alumina/potassium fluoride as catalyst were described by Kabalka and coworkers (Scheme 4.4) [150], In addition, this novel catalytic system has been used in microwave-assisted solvent-free Sonogashira coupling-cyclization of ortho-iodophenol with terminal alkynes, and similarly of ortho-ethynylphenols with aromatic iodides, to generate 2-substituted benzo[b]furans... 

Scheme 7.126 Solvent-free Sonogashira coupling-cyclization.

Scheme 15 Nickel-catalyzed solvent-free Sonogashira coupling...

Type of reaction C-C bond formation Reaction condition solvent-free Keywords Sonogashira cross-coupling, alkyne... 

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]. 

Scheme 15.50. A solvent-free Sonogashira coupling employing nickel catalysis.

The microwave-assisted solvent-free Sonogashira coupling-cyclization of 0-iodophenols 4 with terminal alkynes 5 and that of o-ethynylphenols 6 with organic iodides 7 to generate 2-substituted benzo[h]furans (8 and 9) has been... 

Fulmer DA, Shearouse WC, Medonza ST, Mack J. Solvent-free Sonogashira coupling reaction via high speed ball milling. Green Chem 2009 11 1821-5. 

Reproduced from Thorwirth R, Stolle A, Ondruschka B. Fast copper-, ligand- and solvent-free Sonogashira coupling In a ball mill. Green Chem 2010 12 985-91. With permission from the Royal Society of Chemistry. 

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