Activated carbon pyrone groups

As mentioned in Section IV.A, Nelson and Yang [494] have proposed a surface complexation model to describe the effect of pH on adsorption equilibria of chlorophenols on activated carbon. To account for the well known suppression of uptakes as pH increases, they introduce the ionization constants of separate acidic (A) (e.g., carboxyl) and basic (B) (e.g., pyrone) functional groups on the carbon surface ... 

Figure 2.5 Proposed model for stabilization of carbon surfaces upon heat treatment () in N2 (inert gas) and H2. For simplicity and illustrative purposes, only two types of oxygen surface groups (pyrone and lactone) and three other types of active sites are shown. These three types are in order of increasing stability (1) type I isolated unpaired-electron sites Ar-X (where Ar represents the aromatic (graphene layer) (2) type II, Ar-C=CX and (3) type II, divalent in-plane a pair, Ar-Xs (where s represents a localized n electron). Structures C and D are formed in H2. Structure D is much more abundant in H2-treated carbons because of the higher concentration of CH2 radicals. (From ref. 84, with permission. Copyright 1996 American Chemical Society.)...

Carbon-heteroatom double bonds can also participate in this reaction. These include both carbonyl compounds (Scheme 11.37) and imines (Scheme 11.38). Addition to aldehydes is co-catalysed by tin(II) or indium(III) salts. Under these conditions, tetrahydrofiirans are obtained. The presence or absence of the co-catalyst can also switch the reaction from one mode to another (Scheme 11.39). An indium cocatalysed cycloaddition to a 7-pyrone aldehyde 11.117 was used in a synthesis of aureothin 11.122 and A-acetylaureothamine 11.123 (Scheme 11.40). Cross-metathesis of the exo-cyc ic alkene 11.118 allowed a subsequent Suzuki coupling with a gem-dibromide 11.120 that showed the expected selectivity (Section 2.1.4.2). This reaction required the use of thallium ethoxide as the Lewis base to suppress the formation of side products. A Negishi coupling completed the synthesis of aureothin 11.122. Reduction and acylation of the nitro group yielded A-acetylaureothamine 11.123. The latter compound is active digainst Helicobacter pylori, a bacterium behind stomach ulcers. 

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