The Ligand-free Heck Reaction

In Heck s early publications it was already documented that the Heck reaction on aryl iodides can be carried out using Pd(OAc)2 as catalyst without any added 

One tell-tale sign of the involvement of colloids in catalytic reactions is the dependence of the turnover frequency on the S/C ratio. This is already apparent from Fig. 10.11. A calculation of the turnover frequencies of these reactions reveals the following values after 2h 40 min 309 (0.08mol%), 787 (0.02 mol%), 900 (0.00125 mol%) h . This increase can be easily explained because at higher S/C ratios the size of the nanoparticles is reduced and the ratio between available palladium at the surface vs. total palladium increases [63]. 

Earher, Reetz and coworkers had seen a similar effect when testing the effect of the additive N,N-dimethyIglycine on the Heck reaction between bromobenzene and styrene [lOj. This additive supposedly stabilises the colloids, leading to good yields when between 0.0009 and 1.5 mol% Pd was used. They noted that only at the very low catalyst concentrations is there no need for the additive and rates are the same as without. 

Recently, Leadbeater reported a ligand-free Heck reaction of bromarenes in water in the presence of TBAB using microwaves as the heating source. Here the temperature is around 170 °C, which allowed him to lower the mol% Pd(OAc)2 to 0.0019 obtaining yields of styrenes and cinnamates between 30 and 83% [68]. 

Vries and coworkers, using TEM, EXAFS, EDX and ES-MS. Colloids were clearly visible with TEM, and electrospray-MS showed the presence of anionic palladium species such as [ArPdCh] [58]. The authors also noted the presence of small amounts of butyl chloroacrylate, which suggests that Pd(ii) is reduced to Pd(0) by a Wacker type mechanism. 

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Schmidt, A.R, Al-Halaiqa, A. and Smirnov, V.V. (2006) Effect of macrokinetic factors on the ligand-free Heck reaction with non-activated bromoarenes. J. Mol. Cat. A Chem., 250, 131-7. 

Zhou and coworkers [67] prepared Pd nanoparticles in situ in the H O/TX-lOO/ [bmim][PFg] microemulsion and used this microemulsion system to catalyze the Heck reaction of butyl acrylate with iodobenzene. The reaction time was decreased compared with conventional solvent system. Other Heck reactions relating to the coupling of iodobenzene and methyl acrylate, ethyl acrylate and styrene were also investigated with high yields. All the results indicated that the HjO/TX-100/[bmim] [PF ] microemulsion containing Pd nanoparticles was a very efficient catalyst system for the ligand-free Heck reaction. 

In some of his original work, Heck demonstrated that the reaction of aryl iodides can be carried out using Pd(OAc)2 in the absence of additional ligands. It was subsequently shown by Jeffery that this works particularly well in the presence of tetraalkyl ammonium salts/A detailed mechanism for the ligand-free Heck reaction has been reported by de Vries. 

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