PALLADIUM CATALYST, LIGANDLESS

ACCELERATED SUZUKI COUPLING VIA A LIGANDLESS PALLADIUM CATALYST 4-METHOXY-2 -METHYLBI PHENYL (1,1 -Biphenyl, 4 -methoxy-2-methyl-)... 

ACCELERATED SUZUKI COUPLING VIA A LIGANDLESS PALLADIUM CATALYST 4-METHOXY-2 -METHYLBIPHENYL... 

Accelerated Suzuki Coupling via a Ligandless Palladium Catalyst 4-Methoxy-2 -methylbiphenyl. 

The Suzuki-Miyaura coupling can also be performed with a ligandless palladium catalyst, using a mixture of water and ammonium salt 93 or pyrrolidinium salt 94 (Scheme 38). These BF4 salts have high melting points (125°C and 160 C, respectively) but melt in water to form a biphasic mixture at much lower temperatures ( 50 C and 80°C). The recycling was not efficient, but salts 93 and 94 can be separated and purified, unUke the case with standard ionic Uquids. 

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). 

PALLADIUM CATALYST, LIGANDLESS, 75, 61 20% Palladium hydroxide on carbon, 75, 21 Parr shaker, 75, 24, 226... 

Based on our experience in using A1203 as base in the iodination15 and bromination16 of vinylboronic acids, we hoped that this solid would likewise function as base in the Suzuki reaction. To simplify the reaction, we hoped to use ligandless palladium black [Pd(0)] as catalyst, thus alleviating the need for expensive or custom-made phosphine ligands. 

In the quest for coordinatively unsaturated palladium catalysts, the more radical approach uses ligandless conditions [165, 166] following the work pioneered by Beletskaya [38b, 167]. However, the mechanism of cross-coupling reactions under these conditions is not known [168]. 

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