Silica-Supported Scavengers

S = Silica Supported Scavenger Figure 3.7 The process of using scavengers. 

Silica-Supported Scavengers 8.3.2.1 Silica-Supported Amines... 

Fig. 7.10 Polymer-supported scavengers polystyrene-bound diamine (top left), silica-based diamine (top right), Stratosphere Plugs (bottom left), and Synphase Lanterns (bottom right).

Fig. 7.11 Scavenging efficiencies of polymer-supported scavengers (a) polystyrene, (b) functionalized silica, (c) plugs, (d) lanterns] at room temperature (left) and under microwave heating (right).

Coupled with the fact that the proportion of trace metal contaminants detected within continuous flow reaction products is inherently low, due to reduced catalyst degradation, the use of a scavenger cartridge at the end of a reaction sequence represents a relatively long-term solution to this problem. Other examples of the use of solid-supported scavengers have been reported by Ley and co-workers [65], where in one example, two scavenger modules, comprising QuadraPure TU (126) and phosphane resin, were used in the synthesis of 1,4-disubstituted-l,2,3-triazoles [66], and by Watts and co-workers [67], where silica-supported copper sulfate was used for the removal of residual dithiol (ppb) in the synthesis of 1,3-dithiolanes and 1,3-dithianes. 

To verify whether the Pd(OH)2/C system was truly heterogeneous or not, a series of three-phase tests were carried out (Scheme 14.2). In the first case, the aryl halide was anchored to a Wang resin via an amide linker. Under standard conditions for 4.5 h, 100% conversion of supported substrate to arylated product was observed. In addition, in the presence of a silica-supported thiol-based scavenger resin (thiols are known to have a very high affinity for palladium), less than 5% of the desired product formed. These results led to the determination of... 

Bis(indolyl)nitroethanes are obtained readily in 7-10 min in high yields (70-86%) on fine TLC-grade silica gel (5-40 pm) by Michael reaction of 3-(2 -nitrovinyl) indole with indoles. The same reaction reported requires 8-14 h for completion at room temperature [77]. Several functionalized resins have been prepared from Merrifield resin via a MW-assisted procedure that utilized mixed solvent system to facilitate the swelling of resins and coupling with microwaves [78], These resins can function as solid supports or polymeric scavengers in solid phase synthesis. 

Figure 3.14 A supported reagent, in a manner similar to a scavenger, consists of a reactive functional group grafted onto insoluble silica. The spent reagent is easily removed by filtration.

There is now an extensive range of supported reactants that use resins, silica, carbons etc, to clean up reactions prior to final purification and is gaining favour in the laboratory. [See section on Scavenger Resins in Chapter 3, at the ends of the sections on Preparation of other adsorbents , FPLC and HPLC .]... 

In toluene at 55° the radical efficiency (/) as determined by means of scavengers is about 75%. On silica the only scavenger that proved to be suitable, of several tested, was BDPA. The radical efficiency on silica, as determined from the fading of the BDPA epr signal, is only 10-20%. This was supported by the ratios of dj, dg, and dj coupling products found in experiments with unsymmetrically deuterated azocumene l-dj. 

Recently, Pattarawarapan and coworker reported the synthesis of a Si-supported DCT for scavenging nucleophiles [27]. 3-Aminopropyltrimethoxysilane was allowed to react with commercial silica gel in toluene. TCT (trichloroazine) was then reacted with the Si-immobilized amine to provide the supported DCT reagent. This was found to sequester both amines (primary and secondary) as well as alcohols with remarkable facility and two libraries of sulfonamides and amides were prepared in parallel format. Filtration was the sole purification step (Scheme 8.18). 

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