Similarity perruthenate

For example, whereas the solid oxidation catalyst MCM-41-entrapped perruthenate can be used for the conversion of benzyl alcohols only, a similarly perruthenated-doped amorphous ORMOSIL is equally well suited for a variety of different alcohol substrates.35 On the other hand, a uniform pore structure ensures access to the active centres, while in an amorphous material made of non-regular porosity hindered or even blocked sites can well exist (Figure 1.16), rendering the choice of the polycondensation conditions of paramount importance. 

Just as Ru(VIII) chemistry is dominated by RuO, Ru(VII) chemistry is dominated by salts of the perruthenate ion, [RuO ] , particularly TPAP, ( Pr N)[RuO ]. Another similarity between RuO and [RuO ]" in solution is their reluctance to expand their coordination spheres beyond tetrahedral. The principal salts are listed first. There are few Ru(VII) complexes apart from [RuO ] . 

Similarly to oxidative olefin cleavage with periodate, the first intermediate formed is ester 35, here a perruthenate at the oxidation level +VI1 A -elimination releases ketone 6 and the mthenium(V) acid 36 /V-Methylmorpholine-N-oxide (NMO) serves in this case to regenerate the perruthenate(VII) speck s, and must therefore be introduced in stoichiometric quantity. 

This has several advantages over the other methods mentioned above chief among which is the much lower cost of reagents and the fact that the use of heavy metals is limited to catalytic amounts. In a similar approach, hypochlorite was used in the presence of perruthenate catalysts to oxidize acetals of ascorbic and iso-ascorbic acid to optically pure acetals of glyceric acid (9). 

As mentioned above, treatment of the aldol adducts 150 a/b with NMO produced the phenol 152. The interesting oxidation properties of NMO had previously been investigated by Sulikowski et al. on the model compound 157 [85] (Scheme 40). They observed the formation of the hemiacetal 159 in 60% yield and assumed attack of the nucleophilic N-oxide on the quinonemethide tautomer 158 (or on the anion of 158). A related reaction was observed in our group in which the diol 94 was methoxylated at C-6 to 95 by treatment with methoxide ions [82] (Scheme 27). An internal redox step is postulated to account for the reductive 0-N-bond cleavage with concomitant oxidation of the hydroquinone back to the quinone. Without the presence of perruthenate, aromatization with formation of a C-5 phenolic hydroxy group was observed, a reaction later exploited in the synthesis of angucycline 104-2 [87] (see Scheme 49). Thus, based on similar mechanistic principles, the chemical results of the NMO oxidations were quite different compound 147 gave the C-6 phenol 152 [86] whereas 157/158 were converted to the C-5 phenol 160 [85]. 

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