Suzuki solid supported catalysts

An alternative to solid-supported catalysts are catalysts that are insoluble themselves [136]. A pyridine-aldoxime ligand was presented and evaluated in the Suzuki-Miyaura reaction using water as a solvent. Using an Irori kan to contain the polymeric catalyst, the reaction could be repeated 14 times without any noticeable reduction in efficiency. The optimized reaction conditions were then used to create a small library of approximately 30 biaryl compounds using aryl iodides, bromides, triflates as well as an activated chloride (Scheme 56) [136]. 

The use of solid supported, recyclable catalysts, is a well-assessed technique in classic organic chemistry, and many exhaustive reviews dealing with this subject are available [105, 115]. The use of solid supported catalysts for library synthesis in solution has also been reported. Among others, Kobayashi et al. presented the use of a new supported scandium catalyst for 3CC reactions leading to solution libraries of amino ketones, esters, and nitriles (24-member model discrete library) [116], or to quinolines (15-member model discrete library) [117], and Jang [118] presented a polymer bound Pd-catalyzed Suzuki coupling of organoboron compounds with halides and triflates. This area was also briefly reviewed recently [119]. 

Applications of Suzuki and Heck cross-coupling reactions were adopted by Gallop and coworkers [131] to prepare diverse biaryl- and styryl-substituted (3-lactams on solid support. The catalyst system, [PdCl2(dppf)]-TEA, was found to be efficient to promote C-C bond formation around a (3-lactam template (Schemes 26 and 27). 

Since the renaissance of solid-phase organic chemistry in 1992, carbon-carbon bond formation reactions on solid support have probably been the best studied reactions. Many different facets of the Suzuki, Heck and Stille reactions have been evaluated. The influence of linkers, catalyst, solvents, microwave, polymer-bound aryl halides or polymer-bound arylboronic acids (or stannanes) have been studied in detail. 

Tliere are some excellenl reviews on the use of solid bases in Fine C hemistry 9. 14, 16, 17, 175, 184]. In this section we will comment on some of those base-catalyzed processes. For simplicity purposes, we will not refer to bifunclional catalysis (i.e, metal phase solid base) but only to catalysis by bases. Tlicrefore, for instance, we will not mention on reactions as important as Heck-, Suzuki-, Sonogashira- and Slille-typc ones (248, 249). Similarly, when talking on selective hydrogenation of cilral, wx will not comment on the use of metal-supported catalysts 1250-252], but only on solid bases. 

An early report by Konig deals with rapid parallel Suzuki reactions in water with phase-transfer catalysts. The solid support used in these reactions was PEG, and a variety of aryl palladium precursors were evaluated aryl halides as well as aryl tri-flates and nonaflates (Scheme 15.8). The inclusion of PEG is appealing, because it not only helps to solubilize the reagents but is also suggested to stabilize the palladium catalyst in the absence of phosphine ligands. Both the polymers and the esters were reported to withstand 10 min of 900 W multimode MW irradiation whereas the thermal conditions induced substantial ester cleavage (up to 45%). Nonaflates were found to be associated with lower yields and diminished product purity [32]. 

Another solid-support Suzuki coupling is the hydroboration of vinyl polystyrene with 9-BBN in THF to provide [29] polymer supported borane. This borane is coupled to give different aryl, vinyl, or alkyl iodides. Pd(OAc)2 is employed as the catalyst for aryl iodides and PdCl2(dppf) in other cases to yield the coupled adduct in 55-85% yield (Scheme 31.7) on the basis of the mass of the product cleaved or by comparison with IR integration to standards. 

Metal nanoparticles (MNPs) are excellent catalysts working under mild conditions either homogeneously or heterogenized on a solid support (zeolite, charcoal, nanotubes). For instance, Haruta discovered that AuNPs on Ti02 catalyze CO oxidation by O2 to CO2 under subambient conditions, with many applications. Alkene and arenes hydrogenation under ambient conditions and cross C-C bond formation (Suzuki, Sonogashira, Heck, Stille, Corriu-Kumada) with aryl halides are catalyzed by noble metal NPs (Ru, Rh, Pd or Pt). 

Since the initial report by Suzuki in 1979, the optimization of reaction parameters, such as base and solvent, has been explored by many different groups.Replacement of the base with KF or CsF has been demonstrated to improve functional group tolerance in some systems." Use of microwave heating, water or ionic liquids as solvents, solid supports, or other non-traditional conditions can also provide advantages. However, the research that has arguably led to the broadest expansion of scope has been focused on catalyst development. 

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