Oxidation TEMPO catalysis

The oxidation of different cellulosic materials by TEMPO catalysis has highlighted the importance of the substrate morphology [36]. Crystalline native celluloses were unaffected even after long reaction times, whereas amorphous celluloses were promptly oxidized at Cg. 

Other important successes have been achieved in developing clean, green, methods to oxidize alcohols, for example, the Ru/TEMPO (tetramethylpiperidiny-loxyl radical) catalysis, shown in Figure 9.4, for the aerobic oxidation of alcohols. ... 

The most convenient and commonly employed method for promoting the generation of TEMPO-oxoammonium is instead the so-called Anelh procedure It resorts to NaClO as the regenerating oxidant in 1 1 molar ratio with the substrate, under co-catalysis by EUlr. In a two-phase CH2Cl2-water system at 0°C, the oxidation of a primary alkanol does take place, to give the aldehyde selectively and in good yields (entry 3) after a few minutes. In contrast, if the reaction is run in aqueous solution with 2 molar equivalents of... 

Our group have developed 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO)-functionalized PEG for biomimetic oxidation of alcohols together with CuCl in compressed C02, through a so-called mono-phase reaction, two-phase separation process to recover the catalyst, thus leading to conducting a homogeneous catalysis in a continuous mode [62]. 

Catalysis by TEMPO has the advantage of being general for oxidation of both benzylic or non benzylic alcohols to aldehydes, whereas catalysis by PINO, although limited to the synthesis of aromatic aldehydes, has the advantage that the radical is generated in situ from the less expensive N-hydroxyphthalimide, which can be more easily recovered and recycled. 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

Bragd, P.L., van Bekkum, H., and Besemer, A.C., TEMPO-mediated oxidation of polysaccharides survey of methods and applications, Tropics Catalysis, 27, 49-66, 2004. 

Oxidations Metal-catalyzed aerobic oxidation of organic compounds has been reviewed. Aerial oxidation of primary and secondary alcohols is mediated by TEMPO in the presence of HCl and NaN02- Secondary benzylic alcohols undergo aerial oxidation (or with r-BuOOH) based on catalysis by AuCl - neocuproine," but another report describes the oxidation of both primary and secondary alcohols (to acids and ketones, respectively) using nanoclusters of gold that are stabilized by poly(A-vinyl-2-pyrtolidone). ... 

In principle, homogeneous catalysis can overcome this latter problem. This has been demonstrated by further research reporting that the homogeneous catalyst TEMPO could be used for the chemoselective oxidation of the primary alcohols in polysaccharides to give the corresponding polyuronic acids. As expected, this method is selective for primary alcohols [35]. 

Highly efficient rhodium-catalyzed direct arylations were accomplished through the use of 2,2, 6,6 -tetramethylpiperidine-N-oxyl (TEMPO) as terminal oxidant [17]. Thereby, a variety of pyridine-substituted arenes was regioselectively functionalized with aromatic boronic acids (Scheme 9.5). However, in order for efficient catalysis to proceed, 4equiv. of TEMPO were required. The use of molecular oxygen as terminal oxidant yielded, unfortunately, only unsatisfactory results under otherwise identical reaction conditions. However, a variety of easily available boronic acids could be employed as arylating reagents. 

A Ni(II)/Ni(III) catalytic cycle is proposed (Scheme 29). Catalysis is initiated by the coordination of 46 to Ni(II) followed by a ligand exchange and the cleavage of C-H bonds gives the cyclometalated Ni(II) complex 50, which is oxidized to the Ni (III) species 65 by TEMPO. Reductive elimination gives the desired product 60 with the generation of a Ni(I) species that is oxidized to Ni(II) species by TEMPO. 

Cooperative catalysis using chiral imidazolidinones and metal catalysts is not restricted to carbon electrophiles. In 2012, the MacMillan group [50] disclosed a general approach to undertake enantioselective -oxidation of aldehydes with TEMPO by combining organocatalysis with copper catalysis (Scheme 26.5b). 

Tetra-O-benzyl-a-D-glucopyranosyl azide was obtained by nucleophilic displacement reaction of the corresponding a-glucosyl phosphate with azide ion under phase-transfer catalysis conditions. Oxidation of unprotected D-glycopyranosyl azides (with NaOQ, NaHCOj, catalytic TEMPO, H2O) afforded the corresponding D-glycopyraniuronosyl azides, which were isolated as the O-acetylated methyl ester derivatives in good yields. ... 

---