Suzuki phosphine free systems

A different example of triphasic catalysis for the Heck, Stille and Suzuki reactions relied on a three-phase microemulsion/sol-gel transport system. Gelation of an z-octyl(triethoxy)silane, tetramethoxysilane and Pd(OAc)2 mixture in a H2O/CH2CI2 system led to a hydrophobicitized sol-gel matrix that entrapped a phosphine-free Pd(ii) precatalyst. The immobilized precatalyst was added to a preformed microemulsion obtained by mixing the hydrophobic components of a cross coupling reaction with water, sodium dodecyl sulfate and a co-surfactant, typically zz-propanol or butanol. This immobilized palladium catalyst was leach proof and easily recyclable. 

Another useful variation of the phosphine-free Suzuki reaction uses a heterogeneous system with neat water as solvent and tetrabutylammonium chloride as promoter and phase-transfer catalyst (Scheme 42). Aryl bromides were shown to give higher yields than aryl iodides, because of inhibition of phase-transfer by the hberated iodide ion. ... 

A potentially interesting development is the microwave-assisted transition-metal-free Sonogashira-type coupling reaction (Eq. 4.10). The reactions were performed in water without the use of copper(I) or a transition metal-phosphine complex. A variety of different aryl and hetero-aryl halides were reactive in water.25a The amount of palladium or copper present in the reaction system was determined to be less than 1 ppm by AAS-MS technique. However, in view of the recent reassessment of a similarly claimed transition-metal-free Suzuki-type coupling reaction, the possibility of a sub-ppm level of palladium contaminants found in commercially available sodium carbonate needs to be ruled out by a more sensitive analytical method.25 ... 

Alkylboronic acids, which are often less reactive in Suzuki reactions than aryl-boronic acids, are suitable substrates for this catalyst, with turnover numbers (TONs) as high as 9700 being achieved with this system. In addition, Fu was able to demonstrate that the air-stable and crystalline tetrafluoroborate salt, P(t-Bu)3HBp4 was equally efficient as the low-melting and air-sensitive free phosphine P(t-Bu)3 [29],... 

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