Ligand-free Heck reactions

Because aryl phosphines are not only costly but can also act as aryl sources themselves, giving rise to unwanted byproducts, there has been steady interest in extending ligand-free Heck reactions to aryl bromides and aryl chlorides. Reetz and de Vries recently found that these can be performed with high efficiency using stabilized colloidal Pd catalysts [21]. If the palladium is kept at a low concentration between 0.01 and 0.1 mol%, precipitation of the Pd(0) is avoided and the colloids serve as a reservoir for the catalytically active species (Scheme 5). This economically attractive method has been successfully applied on an industrial scale by DSM [22]. 

Ligand-free Heck Reactions using Low Pd-Loading... 

Scheme 10.5 Recycle of palladium in ligand-free Heck reaction by reoxidation of precipitated Pd black.

Scheme 10.6 Catalytic cycle of ligand-free Heck reaction between aryl iodides and acrylate ester (Species in grey detected by ES-MS or EXAFS).

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]. 

Several reports exist on the use of ionic Uquids as solvent for ligand-free Heck reactions. Zhang found that 2-n-butyl-l,l,3,3-tetramethyIguanidinium acetate was an exceUent solvent for a Ugand-free Heck reaction with 0.16mol% of PdCl2. Remarkably, this reaction needs no extra base, presumably since the solvent can act as a base [73]. EarUer studies found slower reactions using imidazolium type ionic Uquids as solvent when compared to conventional media [33, 74]. Presumably, this is due to formation of palladium carbine-type complexes [75]. 

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. 

Reetz, M.T. and de Vries, J.G. (2004) Ligand-free Heck reactions using low Pd-loading. Chem. Commun., 1559-63. 

Scheme 7.15 Ligand-free Heck reactions on halobenzenes.

Precatalyst Pd(OAc) in Ligand-Free Heck Reactions Heck reactions Ifom aryl iodides are catalyzed by Pd(OAc)2 without any phosphine hgand [4b]. Their efficiency is greatly improved by additives such as ammonium salts R N+X (X=Br, Cl) (Jeffery [6a]). The thermolytic decomposition of Pd(OAc)2 generates Pd° nanoparticles stabilized by R N+X (Scheme 19.3a, Reetz [6b]). 

De Vries has developed ligand-free Heck reactions involving ArBr in the presence of low loading of Pd(OAc)2 (0.01-0.lmol%) and NaOAc as a base [6c]. Soluble Pd" clusters stabilized by Na X" (X"=AcO", Br") are formed that inhibit the reversible formation of inactive palladium black and slowly deliver the active ligand-free Pd (Scheme 19.3b) [6d]. Lower the Pd(OAc)2 loading, higher the TOR Aryl chlorides are unreactive. 

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. 

The Heck reaction (see Section 7.4) is one of the first reported Pd-catalyzed cross-coupling reactions. Reaction 3.2.5.2 is an example of a ligand-free Heck reaction and has been studied by EXAFS and its variants. 

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