Manganese complexes alcohols

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

Manganese complexes, metal-complexed alcohol reduction, 25 Metal catalysts, organosilicon hydride mixtures, 5, 11... 

The synthesis of the Y zeolite-encapsulated manganese complex of the salen ligand has been reported recently [51]. It was found to have catalytic activity in the oxidation of cyclohexene, styrene, and stilbene with PhlO. Typically, 1 Mn(salen) is present per 15 supercages, resulting in catalytic turn-overs in the order of 60. The reactions investigated with the respective product yields are given in Scheme 5. Typical oxidation products are epoxides, alcohols and aldehydes. In comparison to the homogeneous case encapsulation seems to lower the reaction rate. From cyclohexene the expected oxidation product cyclohexene oxide is present in excess and is formed on the Mn(salen) site. 2-cyclohexene-l-ol is probably formed on residual Mn cations via a radical mechanism. 

Metal salts and complexes have also often been used as redox catalysts for the indirect electrochemical oxidation of alcohols. Particularly, the transformation of benzylic alcohols to benzaldehydes has been studies. For this purpose oxoruthe-nium(IV) and oxoruthenium(V) complexes have been applied as redox catalysts. In a similar way, certain benzyl ethers can be cleaved to yield benzaldehydes and the corresponding alcohols using a di-oxo-bridged binuclear manganese complex Electrogenerated 02(804)3 was used to generated 1-naphthaldehyde from 1-naphthylmethanol... 

The mechanism of the reaction of the alcohol (or water) with the acyl complex to produce ester (or acid) and regenerate the cobalt hydride complex is not known. Because the reaction of the analogous manganese complex with alcohols is known to proceed through a hemiacetal-like complex, this mechanism has been written for the carboxylation reaction (equation 42). 

Catalytic, asymmetric epoxidations are one of the most important asymmetric processes. In 1980 Katsuki and Sharpless reported a stoichiometric asymmetric epoxidation of allylic alcohols, a method that was later improved to become a catalytic process.9 Moreover, catalytic asymmetric epoxidations of unfunctionalized olefins using salen-manganese complexes have been reported independently by several groups.10-12 In striking contrast to these successful achievements, an efficient catalytic asymmetric epoxidation of enones with broad generality has not been developed.13-22... 

Oxidative addition of the 16-electron manganese center on the silyl ether leads to silyl-manganese complex III. Attack on III by an incoming alcohol (ROH) leads to complex TV. Pseudorotation gives all four possible structures of IV. Complex IV, having a free proton,... 

The applicability of the Sharpless asymmetric epoxidation is however limited to functionalized alcohols, i.e. allylic alcohols (see Table 4.11). The best method for non-functionalized olefins is the Jacobsen-Kaksuki method. Only a few years after the key publication of Kochi and coworkers on salen-manganese complexes as catalysts for epoxidations, Jacobsen and Kaksuki independently described, in 1990, the use of chiral salen manganese (111) catalysts for the synthesis of optically active epoxides [276, 277] (Fig. 4.99). Epoxidations can be carried out using commercial bleach (NaOCl) or iodosylbenzene as terminal oxidants and as little as 0.5 mol% of catalyst. The active oxidant is an oxomanganese(V) species. 

Ordinary alkenes (without an allylic OH group) do not give optically active alcohols by the Sharpless protocol because binding to the catalyst is necessary for enantioselectivity. Simples alkenes can be epoxidized enantioselectively with sodium hypochlorite (NaOCl, commercial bleach) and an optically active manganese-complex catalyst. An important variation of this oxidation uses a manganese-salen complex with various oxidizing agents, in what is called... 

Manganese salen complex catalyzes C—H oxidation of organic molecules with NaOCl or PhIO, giving alcohols . Larrow and Jacobsen observed kinetic resolution in the benzylic hydroxylation . Katsuki and coworkers used the axis chiral salen manganese complexes for the benzyl hydroxylation and ether hydroxylation, and attained higher ee with the ligand possessing (/f,/f)-diamine and (R)-axis chirality (equation 84). ... 

Since the distribution coefficients of PAN and its manganese complex are high, it is sufficient to extract the aqueous solution with only one portion of chloroform. Carbon tetrachloride, benzene, or isoamyl alcohol can be used as extractants. Only slight changes are observed in the value of A,max for the complex in the different solvents. 

A wide variety of N-alkyl hydrazinedicarboxylic esters may be obtained in excellent yields by the hydrohydrazination reaction depicted in Eq. 49.215 Use of cobalt complexes results in more highly regioselective reactions at the cost of lower reaction rates as compared to additions where manganese complexes are employed. Di(fert-butyl) azodicarboxylate is the preferred azo ester reduction of the N=N double bond becomes more prominent when less hindered azo esters are used. Alcoholic solvents are essential the reaction fails when methylene chloride or THF is used. 

The main drawback in Sharpless epoxidation is that the substrate must bear a functional group to achieve the precoordination required for high enantioselec-tivity (as in the case of allyl alcohol). This restriction is not applicable to the epoxidation of alkyl- and aryl-substituted olefins with manganese complexes of chiral Schiffs bases as catalysts. Very high enantioselectivities can be obtained in these reactions (Jacobsen, 1993). The most widely used catalysts that give high enantioselectivity are those derived from the Schiff bases of chiral diamines such as [SiS] and [RR] 1,2-diphenylethylenediamine and [SS] and [RR] cyclohexyl-1,2-diamine. An example is the synthesis of cromakalim. 

The active species of the polymerization has been considered to be a manganese alcoholate, that is formed by the insertion of the epoxide into the Mn-OAc bond of the initiator. In the presence of methanol or acetic acid, the polymerization of 11 (R = Me) has been found to proceed with immortal character, to give a narrow MWD polyether with the number of the molecule exceeding that of the initiator molecule 6 (X = OAc). An example is shown by the polymerization at a feed mole ratio of 11 (R = Me) to 6 (X = OAc) of 400 at 30 C (Figure 9), where the molecular weight of the polymer produced at 100% monomer conversion can be controlled by the mole ratio of methanol to 6 (X = OAc), while the MWD is almost constant and close to unity. Therefore, the manganese complex 6... 

Mn catalysts that show activity in alkene or alcohol oxidation with H2O2 are potentially active in the oxidation of sulfides also. The Mn-tmtacn catalysts and a number of tn-sifu-formed complexes employing ligands such as 39 are examples of such catalysts (see above). These complexes were found to be highly active in the oxidation of sulfides to sulfoxides. For example, the dinudear manganese complex based on tmtacn (6) performs efficiently in the oxidation of aryl alkyl sulfides and generally results in fiJl conversion within 1 h. Unfortunately, as is often the case, in... 

Anionic Additions to Aldehydes. The addition of l,3-bis(silyl)-propenes to aldehydes and ketones to yield the vinyl silyl alcohol was explored. This was done using TBAF and good to excellent yields were achieved (eq 11). A further extension of this work was the addition of the (l,3-bis(silyl)allyl)lithium to ketones and aldehydes (eq 12). In this reaction, the substituted silyl diene was isolated in moderate to good yields. These substrates were then explored as ligands for both iron and manganese complexes. 

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