Suzuki reactions fluorous

Scheme 7.83 Suzuki reactions utilizing fluorous-tagged acid-labile protecting groups.

Palladium nanoparticles coated with a fluorous-derivatized surfactant (or stabilizer) have been used to catalyse Heck and Suzuki reactions in a C8F17Br-benzene biphase [11], The reaction between phenylboronic acid and cinnamyl bromide was investigated and the reaction is illustrated in Scheme 10.10. 

The palladium nanoparticle is prepared from the reaction of the stabilizer, 4,4 -bis(perfluorooctyl)dibenzylideneacetone with palladium(II) chloride. The average size of the nanoparticle varied according the ratio of PdCF to the stabilizer, but was typically around 4 or 5 nm. The initial yield observed in the Suzuki coupling reaction was 90%, but decreased to 78% after five consecutive runs. Fluorous boronates (alternative precursors in Suzuki reactions), have also been developed for use in fluorous biphasic processes [12], A generic structure of a fluorous boronate is shown in Figure 10.2. 

Figure 10.2 General structure of fluorous boronates used in fluorous biphase Suzuki reactions...

These critical aspects of the classical fluorous biphasic catalysis led in recent works to the development of protocols for the conversions with modified catalyst systems in non-fluorinated hydrocarbons as solvents. As part of the BMBE lighthouse project, Gladyzs and coworkers appHed this concept to C - C coupHng reactions (Suzuki reaction) and other metal-catalyzed addition reactions (hydrosilylation, selective alcoholysis of alkynes), which have direct relevance for the synthesis of fine chemicals and specialties [74]. 

Fluorous phase modifications of the Stille reaction were shown by Curran et al. to be accelerated by microwave irradiation.10 Similarly, Hallberg et al. demonstrated that such irradiation gives remarkably fast solid-phase Suzuki reactions, in the generation of biaryl units.9 Their reaction involved the coupling of a tethered (Rink amide TentaGel) aryl iodide or bromide with several boronic acids under 45 W of irradiation at 2450 MHz in sealed... 

A general method for fluorous Suzuki reaction of perfluorooctanesulfonates has been developed using [Pd(dppf)Cl2 ] (dppf = l,l/-bis(diphenyl-... 

Using conditions similar to those developed for fluorous Suzuki reactions, aryl sulfides were synthesized by reaction of fluorous sulfonates with thi-oles (Scheme 5) [32]. The reaction mixtures were in this case also purified by F-SPE. 

Scheme 17 illustrates another fluorous sulfonate-based synthesis of library scaffolds. The tagged substrates were taken through aldol condensation and cycloaddition reactions to form the pyrimidine ring 18. The intermediates were then reacted with boronic acids for Suzuki reactions to form biaryl compounds 19 [31], reacted with HCO2H to give traceless detagged products 20 [34], or reacted with amine to form products 21 [33]. 

This review is divided into four main sections, covering the Heck, Stille, and Suzuki reactions, with miscellaneous reactions being included at the end. Processes featuring alkynes in copper co-catalyzed Sonogashira-type couplings have been included in the section on Heck reactions. This review does not cover carbon-carbon bond formation processes using immobilized catalysts. Similarly, fluorous-phase syntheses " and those on polyethylene glycol " are excluded. 

Fluorous DEAD was prepared and used in a Mitsunobu reaction.Fluorous triaryl phosphine was used for a Wittig reaction.Fluorous boronates were prepared and used in functional transformations and a Suzuki-Miyaura coupling reaction. 

Further examples of microwave-assisted Suzuki cross-couplings involving supported substrates/catalysts or fluorous-phase reaction conditions are described in Chapter 7. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

The time consuming chromatographical purification of heterocycles 28 and 29 slowed down the rate of library production. A phase separation using fluorous chemistry was employed by Zhang and Lu to address the workup and purification of fused 3-aminoimidazo[ l,2-a]pyridines (such as 30) [54]. Thus, attachment of a perfluorooctanesulfonyl tag to aldehydes and subsequent Ugi three-component microwave-assisted condensations with 2-aminopyridines and isocyanides furnished the desired heterocycles 30, which were conveniently isolated by fluorous solid-phase extraction. The fluorous tag could be subsequently used as an activating group in the post-condensation modifications, such as Suzuki-Miyaura cross-coupling reactions. 

The same group reported on a library synthesis of 3-aminoimidazo[l,2-a]-pyridines/pyrazines by fluorous multicomponent reactions. Here the overall yields, as well as the yields for the separate Suzuki-Miyaura reactions that were a part of the synthesis, were relatively low due to competing reactions and the poor reactivities of the substrates, but the speed of the microwave-mediated syntheses and ease of separation underlined the usefulness of fluorous reagents [140]. A recent paper further illustrated the use of Suzuki-Miyaura couplings of aryl perfluorooctylsulfonates in the decoration of products derived from 1,3-dipolar cycloadditions [141]. 

Parallel synthesis of a bicyclic proline library 29 has been developed by a two-step synthesis involving 1,3-dipolar cycloaddition of fluorous benzalde-hydes followed by Pd-catalyzed Suzuki coupling reactions with boronic acids (Scheme 20) [43]. Both reactions were conducted under microwave irradiation and the reaction mixtures were purified by F-SPE without the need of performing a following chromatography. 

Ladlow and coworkers recently developed an acid-labile fluorous benz-aldehyde protecting group 43 to facilitate the parallel synthesis of sulfonamides 44 (Scheme 25) [56]. The Suzuki coupling reaction was conducted under microwave irradiation. All the intermediates and the final products were purified by F-SPE. 

The successful demonstration of the fluorous biphasic concept for performing organometallic catalysis sparked extensive interest in the methodology and it has subsequently been applied to a wide variety of catalytic reactions, including hydrogenation [59], Heck and Suzuki couplings [60, 61] and polymerizations [62]. The publication of a special Symposium in print devoted to the subject [63] attests to the broad interest in this area. 

Zhang, W. and Chen, C.H.-T. 2005. Fluorous synthesis of biaryl-substituted prolines by 1,3-dipolar cycloaddition and Suzuki coupling reactions. Tetrahedron Letters, 46 1807-10. 

Other biphasic C—C bonding reactions were carried out with fluorous solvents, for instance Suzuki- and Sonogashira-couplings [124] or ethene or propene oligomerizations [125, 126], Further new solvent systems use ionic liquids for the linear dimerisation of 1-butene to octenes [127] or the hydrovinylation of styrene with a combination ionic liquid/supercritical carbon dioxide [128] (cf. Section 7.4). 

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