Suzuki coupling complexes

The postulated steps that constitute the Suzuki coupling process are shown in Scheme 25. After oxidative addition of the organic halide to the palladium(o) catalyst, it is presumed that a metathetical displacement of the halide substituent in the palladium(ii) complex A by ethoxide ion (or hydroxide ion) takes place to give an alkoxo-palladium(ff) complex B. The latter complex then reacts with the alkenylborane, generating the diorganopalladium complex C. Finally, reductive elimination of C furnishes the cross-coupling product (D) and regenerates the palladium(o) catalyst. 

There is some similarity between the cracking of petroleum and the cracking of biomass. However, biomass is more complex chemically both in terms of structrual types and functional groups. In petrochemistry, hydrocarbons are fractionated and they are then functionalized by oxidation, halogenation, nitration and other chemical processes so as to add value. The commodity chemicals are then built up into more complex molecules using such popular synthetic methods as Friedel Craft reactions, Michael and aldol condensations, and Heck and Suzuki couplings. The speciality products of these reactions are then further elaborated into formulations for use in everyday applications ranging from personal care... 

Suzuki couplings have been used in the synthesis of complex molecules. For example, coupling of two large fragments of the epothilone A structure was accomplished in this way.229... 

Palladium complexes of inexpensive, easily synthesized bis(phosphinite) PCP -pincer ligands show good activity in the Suzuki coupling of deactivated and sterically hindered aryl bromides.256... 

In a more recent study, Wang and coworkers have discussed microwave-assisted Suzuki couplings employing a reusable polymer-supported palladium complex [141]. The supported catalyst was prepared from commercial Merrifield polystyrene resin under ultrasound Bonification. In a typical procedure for biaryl synthesis, 1 mmol of the requisite aryl bromide together with 1.1 equivalents of the phenyl-boronic acid, 2.5 equivalents of potassium carbonate, and 10 mg of the polystyrene-... 

Bedford RB, Cazin CSJ, Hazelwood SL (2002a) Simple mixed tricyclohexyl-phosphane-triarylphosphite complexes as extremely high-activity catalysts for the Suzuki coupling of aryl chlorides. Angew Chem Int Ed 41 4120-4122... 

Three-component assembly of allenes, organic halides and arylboronic acids has been reported in which Suzuki coupling of a Jt-allylpalladium complex with an orga-noboronic acid is utilized (Scheme 16.26) [31], Addition of phosphorus ligands to the reaction mixture greatly decreases either the product yields or E/Z ratios. The decrease in E/Z ratio may be explained based on the fact that donor ligands readily promote anti-syn rearrangement of a Jt-allylpalladium species via a cr-allylpalladium intermediate. 

The reactivity of bromopyrones can be increased through the depletion of their electron density via coordination to an electron withdrawing metal fragment. Fairlamb and co-workers converted 4-bromo-6-methyl-2-pyrone into its irontricarbonyl derivative using Fe2(CO)9 (8.11.), which underwent Suzuki coupling with 4-anisylboronic acid at room temperature to give the coupled complex in 64% isolated yield.19... 

Resin-bound benzylic thioethers can be converted into sulfonium salts by S-alkyla-tion with triethyloxonium tetrafluoroborate. These sulfonium salts react with palladium ) complexes to yield benzylpalladium complexes, which undergo Suzuki coupling with arylboronic acids (Entry 7, Table 3.48),... 

The examples reported to date do not allow a clear ranking of palladium catalysts with regard to their ability to catalyze the Suzuki reaction. It should also be kept in mind that palladium(O) complexes are air-sensitive, and the quality of commercially available catalysts of this type can vary substantially. Some successful Suzuki couplings are listed in Table 5.12. Further examples have been reported [184,186-196],... 

Gordon and Holmes32 used a supported triphenylphosphine-Pd(II) complex as an effective catalyst for Heck and Suzuki couplings in supercritical carbon dioxide (entry 27). After optimization of the amine base for the reaction, the final products have been isolated in good yields and high purity with no traces of metal. 

Bicyclic complexes containing an azetidine ring and a palladium atom 525 have been shown to be effective catalysts in Suzuki coupling reactions with bromobenzenes and with the generally less reactive chlorobenzenes. The catalyst was stable in air for several months without loss of activity and the catalyst loading in the reaction could be lowered to 0.1% without a decrease of yield in the reactions studied <2005JOM2306>. 

The immobilized perfluoro-tagged palladium complex 36 is highly active in Suzuki coupling reactions (> 99% yield), and can easily be recycled by precipitation in a mixture of DME/H2O (10% HC1) 2 1 and reused for three consecutive Suzuki coupling reactions (Table 8). 

---