Mn-tmtacn

FIGURE 15. Structures proposed for homogeneous and heterogeneous Mn-tmtacn catalysts... 

Berkessel and Sklorz screened a variety of potential co-ligands for the Mn-tmtacn/H202 catalyzed epoxidation reaction and found that ascorbic acid was the most efficient one. With this activator the authors could oxidize the terminal olefins 1-octene and methyl acrylate with full conversion and yields of 83% and 97%, respectively, employing less than 0.1% of the metal complex (Scheme 86). Furthermore, with E- and Z-l-deuterio-1-octene as substrates, it was shown that the oxygen transfer proceeded stereoselectively with almost complete retention of configuration (94 2%). Besides the epoxidation, also the oxidation of alcohols to carbonyl compounds could be catalyzed by this catalytic system (see also Section in.C). 

SCHEME 86. Mn-tmtacn/H202/Na ascorbate catalyzed epoxidation of terminal olefins... 

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

SCHEME 118. Mn/tmtacn/Na ascorbate catalyzed alcohol oxidation with H2O2... 

W gives ring opening with acid-sensitive epoxides, or the solvent required for good results, e.g. dichloromethane (MTO) and acetonitrile (Mn-tmtacn). Hence, the quest for even better systems continues. 

Mn(tmtacn) Jacobs Terminal olefins Substituted olefins H2O2, CH3CN [167]... 

As an alternative to porphyrin and phthalocyanine catalysts, complexes of Mn and the cyclic triamine l,4,7-trimethyl-l,4,7-tnazacyclononane (tmtacn) clearly deserve more attention [11]. In acetone and at subambient temperature, the activity of Mn-tmtacn matches that of the more active porphyrins, with 1,000 turnovers within a few hours in the styrene epoxidation [12]. Moreover, Mn-tmtacn is colorless after reaction, and because of its relatively moderate price, it has even been commercialized for a short while in laundry powders [13]. A heterogeneous version of Mn-tmtacn would obviously offer even more advantages. We have proposed an immobilization of Mn-tmtacn based on zeolite... 

For grafting the Mn(tmtacn) dimer (L), Si02 and AI2O3 surfaces were functionalized with 2-(carbomethoxy)-ethyl-trimethoxysilane, the surface density of which was controlled at 0.07-0.80nm. The acid treatment that is used to generate... 

Figure 7.4 Homogeneous and heterogeneous Mn(tmtacn) dimers for selective oxidation [43].

In the absence of H2O2, the homogeneous Mn(tmtacn) complex (L) adsorbed on the surface of the carboxylic-acid-functionalized Si02 (N) without any change in its coordination structure as evident from UV/vis spectroscopy. In contrast, the UV/vis peaks characteristic of the Mn(tmtacn) precursor (L) immediately... 

Table 11.5 Oxidation of selected alkenes with Mn-tmtacn [94b].

Hydrogen peroxide decomposition by Mn-tmtacn complexes in CH3CN was shown to be suppressed effectively by addition of oxalate [94d] or ascorbic acid [94a] as coepoxidation activity of the in situ prepared Mn-tmtacn complex [94d]. In general, fidl conversion was reached with less than 1 mol% of catalyst within 1 h. In addition to oxalic acid, several other bi- or polydentate additives, for example, diketones or diacids, in combination with Mn-tmtacn and H2O2 were found to favor alkene epoxidation over oxidant decomposition [94d]. Employing this mixed catalytic system, allylic alkenes (e.g., allyl... 

Scheme 11.10 Selective epoxidation of 1-hexene by Mn-tmtacn using H2O2 in the presence of oxalate [94d].

Table 11.6 Representative examples of epoxidation of terminal and deactivated alkenes with the Mn-tmtacn/oxalate system [94d].

Figure 11.6 Dinuclear Mn-tmtacn complexes 6 and 23, and the proposed structure for the Mn-tmtacn/oxalate oxidation catalyst 22 p(= activated O to be transferred [99].

Although the precise role of the oxalate cocatalytic amount of the bidentate oxalate impedes the formation of the p-peroxo-bridged dimer 23, and as a result the catalase-type decomposition of H2O2, typical of dinuclear complexes, is suppressed [34]. 

Subsequently, additives such as ascorbic add (Scheme 11.11) and squaric add facilitated extension in the epoxidation activity of the Mn-tmtacn complex with H2O2 [94a]. Although only a limited number of substrates were examined, nearly... 

Kilic et al. have reported an alternative approach to controlling the stereochemical outcome of the epoxidation catalyzed by Mn-tmtacn complexes using substrate control via the hydroxyl group of allyhc alcohols and by varying the steric nature of the substituents on the alkene [106]. 

Prompted by the use of oxalate and other additives and hy the observation of ds-diol formation in the hetereogenized system of De Vos and coworkers, a wider search for effective additives both to suppress the catalase type activity of the Mn-tmtacn complexes and to promote ds-dihydroxylation was initiated. An early success was the strongly enhanced ds-dihydroxylation activity observed using [Mn203(tmtacn)2] (Pp6)2 (6) (Figure 11.2) in the combination with aldehydes such as glyoxylic add methylester methyl hemiacetal (gmha (33), Scheme 11.13) [94k]. 

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