Palladium Colloids in the Heck Reaction

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. 

Reetz and coworkers pointed out the role of Pd colloids in phosphine-free Pd-catalysed Heck reactions, as typically encountered under Jeffery s conditions (Pd(OAc)2, Bu4NBr, K2CO3). Studying the Heck reaction of iodobenzene with ethyl 

Further elucidation of the reactivity of Pd colloids was reported by Bradley and Blackmond [13]. These authors prepared a series of well defined homopolymer-stabilised Pd colloids, varying the particle size, and used them in the Heck reaction between 4-bromobenzaldehyde and butyl acrylate. Interestingly, the authors found a correlation between initial rate and particle size the initial rate increases with decreasing particle size. No correlation was reported regarding the palladium specific surface area. Whereas the precise morphology of the Pd particles could not be defined, the authors found that they were quite equidimensional, allowing a representation as regular polyhedra to apply surface defect site statistics to char- 

Prakash and Thompson used monodisperse poly(4- and poly(2-vinylpyridine) nanospheres as stabilisers to support 1-4 nm Pd nanoparticles in a study concerning the Heck, Suzuki and Stille couphng reactions [14]. The material was found to be very active. However, the reactions were Umited to the couphng of the highly reactive 4-bromo-nitrobenzene with, respectively, butyl acrylate, phenylboronic acid and phenyl-trimethylstannane. No noticeable influence of the nature of the stabilisers was reported in this study. The authors argue that the material was stable under the reaction conditions, but this conclusion is only based on macroscopic TEM analyses. In view of the low Pd concentration which was required to perform effective Heck, Suzuki or Stille coupling reactions, it is not surprising that no major variation of the Pd particle size was observed. 

Gin and coworkers used Pd nanoparticles stabilised by a cross-hnked lyotrophic liquid crystal (LLC) in the Heck reaction of aryl bromides with styrene or butyl acrylate. The material was prepared, after preparing a LLC structured polymer (Fig. 

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The immobilized, colloidal palladium catalyst, Si02-(C3H6SH)nPd is reported to induce the Heck reaction [14a] between ethyl iodide and ethyl acrylate. XPS data showed the presence of Pd(Il) on the surface of the colloidal Pd particles, owing to air oxidation this explains the different behavior of this and the Pd/C catalyst. Addition of BujN.HI and iodine greatly reduced the induction period. The catalytic activity of propylene carbonate-stabilized palladium colloids in the Heck reaction has been investigated [14b]. 

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