Heck colloidal palladium systems

Beller, M. et al., First palladium-catalyzed heck reactions with efficient colloidal catalyst systems, J. Organomet. Chem., 520, 257, 1996. 

This universally known [66] reaction was discovered independently by Heck and Mizoroki about 30 years ago. Basically it consists of the arylation or vinylation of alkenes and is generally catalyzed in solution by palladium species (Scheme 5.7). One of the major problems of the early homogeneous systems was the precipitation of palladium black. Addition of phosphanes improves the stabiUty however oxidation of this Ugand is a drawback for easy purification of the products. Consequently, development of heterogeneous catalysis [67] through supported palladium or stabilized colloidal palladium catalysts is an area of great interest... 

The same catalyst was compared with another palladacycle (23) at 140 °C and longer reaction times (14-48 h) (Table 15.1, entry 2) [66]. The catalyst was very active in coupling iodobenzene with styrene or methyl acrylate (TON (1.3-1.5) x 10 ), whereas lower conversions and yields were observed due to catalyst deactivation for the less reactive bromoacetophenone for the coupling with methyl acrylate (TON (2.7-3.0) x 10 ). The catalytic system could be recycled after the addition of CgFnBr to give a biphasic mixture. The data obtained by transmission electron microscopy indicates that colloidal palladium nanoparticles were formed as active species for the Mizoroki-Heck reaction. 

In this chapter, we discuss selected literature on palladium catalysts (or catalyst precursors) for the Heck, Suzuki and Sonogashira reactions. The review covers simple homogeneous palladium complexes, Ugand-free palladium catalytic systems, stable palladium colloids and particles and supported palladium catalysts. It focuses on the role of palladium nanoparticles (as catalyst precursors or formed in situ during the course of the reaction) from a mechanistic point of view. 

The development of Pd colloids as catalyst for C—C coupling reactions is rather recent [5]. The first example was reported by BeUer et al. in 1996 they used preformed Pd coUoids stabihsed by tetra-octylammonium bromide prepared following the Bonneman procedure in the Heck arylation [6]. The colloidal system was effective for the Heck arylation of styrene or butyl acrylate by activated aryl bromides, but showed only moderate to little activity for deactivated aryl bromides and aryl chlorides. To obtain these results, the authors found that the colloidal pre-catalysts must be added slowly to the reaction mixture to avoid the formation of inactive palladium black at the beginning of the reaction. 

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