Silica-entrapped reagents

Fig. 16. One-pot multireactions using opposing reagents entrapped within sol-gel matrices using silica entrapping (a) Nafion and (b) silica anchoring diamine.

The next classical pair of destructive steps that was addressed were oxidations and reductions. Here, the oxidant, pyridinium dichromate, and an H2 reduction catalyst RhCl[P(C6H5)3]3 were entrapped in separate silica sol-gel matrices [24], and with these entrapped reagent and catalyst, several different sequences of one-pot redox reaction pairs were carried out - up to four reactions in one pot - without their mutual destruction and with no need for separation steps one of these sequences is shown in Figure 31.10. More mutual destructive combinations are possible, and the last but not least mentioned here is the two-step reaction with a biocatalyst - an enzyme - and an organometallic catalyst, again... 

The concept is general and one-pot acid/base and enzyme/catalyst enantioselective solid-state syntheses are easily achieved by entrapment of the mutually destructive reagents in two different sol-gel silicas. It is worth pointing out that while acids and bases adsorbed at the surface of polymers are left partly exposed and consequently require acid/base solid-state... 

The microspheres—synthesised via a two-step process (acid-catalysed hydrolysis and condensation of 3-mercaptopropyltrimethoxysilane (MPS) in aqueous solution, followed by condensation catalysed by triethanolamine)—have a narrow size distribution (Figure 5.16) and are considerably more stable than polystyrene divinylbenzene microspheres as shown in phosphoramidite oligonucleotide synthesis by the excellent retention of fluorescence intensity in each of the reagent steps involved in phosphoramidite DNA synthesis (Figure 5.17, in which the organo-silica microsphere free thiol groups are derivatized with ATTO 550 maleimide coupled to the entrapped dye). 

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