Metal-catalyzed reactions sulfoxidation

A number of metal / -diketonates have been used to catalyze the oxidation of sulfides to sulfoxides, important synthetic intermediates for the construction of various biologically active molecules . For example, an elegant study by Ishii and coworkers demonstrated that VO(acac)2 (35) selectively catalyzed the sulfoxidation of adamantane (41) by SO2/O2 to give 1-adamantane sulfonic acid (42) (equation 10) . Although a number of metal acac complexes were examined as catalysts for this reaction, all but the vanadium compound failed to promote the sulfoxidation. The catalytic oxidation of triarylphosphines using the palladium complex Pd(acac)2 (29) has also been investigated . 

Non-covalent insertion of several modified metal cofactors and synthetic metal complexes into protein cavities such as serum albumin (SA) and Mb has been reported [5, 24, 28, 30, 69], If synthetic metal complexes, whose structures are very different from native cofactors, can be introduced into protein cages, the bioconjugation of metal complexes will be applicable to many proteins and metal complexes. Mn(corrole) and Cn(phthalocyanine) are inserted into SA by non-covalent interactions and the composites catalyze asymmetric sulfoxidation and Diels-Alder reactions with up to 74 and 98% ee, respectively (Fig. 2c) [28, 30], Since the heme is coordinated to Tyrl61 in the albumin cavity, determined by X-ray crystal structure [20], it is expected that both Mn(corrole) and Cu(phtalocyanine) are also bound to albumin with the same coordination. The incorporation of synthetic metal complexes in protein cavities using these methods is a powerful approach for asymmetric catalytic reactions. However, there are still some difficulties in further design of the composites for improving reactivities and understanding reaction mechanisms because detailed structural analyses are not available for most of the composites. 

Complexes of metal + ligand + protein or DNA can also catalyze the Diels Alder cycloaddition or oxidations with hydrogen peroxide. Copper complexes bound to DNA catalyzed the Diels-Alder cycloaddition with up to 99% ee [15, 16], Cu(phthalocyanine) complexed to serum albumin also catalyzed the enantioselective (98% ee) Diels-Alder reaction, but only with very high catalyst loading (10 mol%) and only with pyridine-bearing dienophiles (presumably to complex the copper) [17]. Achiral Cr(III) complexes or Mn(Schiff-base) complexes inserted into the active site of apomyoglobin variants catalyzed the sulfoxidation of thio-anisole with up to 13 and 51% ee, respectively [18, 19]. A copper phenanthroline complex attached to the adipocyte lipid-binding protein catalyzed the enantioselective hydrolysis of esters and amides [20]. 

Having demonstrated the potential of artificial metalloenzymes for the reduction of V-protected dehydroaminoacids, we turned our attention towards organometallic-catalyzed reactions where the enantiodiscrimination step occurs without coordination of one of the reactants to the metal centre. We anticipated that incorporation of the metal complex within a protein enviromnent may steer the enantioselection without requiring transient coordination to the metal. In this context, we selected the palladium-catalyzed asymmetric allylic alkylation, the ruthenium-catalyzed transfer hydrogenation as well as the vanadyl-catalyzed sulfoxidation reaction. Indeed, these reactions are believed to proceed without prior coordination of the soft nucleophile, the prochiral ketone or the prochiral sulfide respectively. Figure 13.5. 

Along with C—N and C—O bond formation reactions, transition-metal-catalyzed C—S bond formation reaction has also been developed. However, the reaction has scarcely been applied to the synthesis of dibenzothiophenes. In this context, Samanta and Antonchick reported the palladium-catalyzed synthesis of 1-formyldibenzothiophene 80 from benzyl phenyl sulfoxides 78 (Scheme 23.30) [41]. 

This compilation embraces a wide variety of subjects, such as solid-phase and microwave stereoselective synthesis asymmetric phase-transfer asymmetric catalysis and application of chiral auxiliaries and microreactor technology stereoselective reduction and oxidation methods stereoselective additions cyclizations metatheses and different types of rearrangements asymmetric transition-metal-catalyzed, organocatalyzed, and biocatalytic reactions methods for the formation of carbon-heteroatom and heteroatom-heteroatom bonds like asymmetric hydroamina-tion and reductive amination, carboamination and alkylative cyclization, cycloadditions with carbon-heteroatom bond formation, and stereoselective halogenations and methods for the formation of carbon-sulfur and carbon-phosphorus bonds, asymmetric sulfoxidation, and so on. 

Asymmetric reactions also occur via oxo metal intermediates 101, 104). Thus, chiral poiphyrin-Fe complexes catalyze oxidation of sulfides with iodosylbenzene in the presence of 1-methylimidazole with high turnover numbers to give optically active sulfoxides in moderate ee (Scheme 45) 105). 

Different metal complexes have shown the ability to catalyze these imination reactions, such as rhodium, copper, and iron.22 In 2005, Bolm found that the disilver(I) complex described in Section 6.2.2 catalyzes the imination of sulfides and sulfoxides... 

As chiral ligands for transition metal complex-catalyzed asymmetric reactions, a variety of novel chiral ferrocenylchalcogen compounds, which possess a planar chirality due to the 1,2-unsymmetrically disubstituted ferrocene structure, have been prepared from chiral ferrocenes (Scheme 1). Thus, chiral diferrocenyl dichalcogenides bearing an optically active dimethylaminoethyl or p-tolyl-sulfoxide moiety 1-10 were prepared by lithiation of the corresponding chiral ferrocenes, highly diastereoselectively, in moderate to high chemical yields. 

Alternative methods include deprotonation of the amine, to enhance its nucleophilicity, by one equivalent of sodium methoxide, sodium amide, metallic sodium,n-butyllithium, sodium hydride in dimethyl sulfoxide or Grignard reagent. Under less drastic basic conditions 2-hydroxypyridine catalyzes the reaction at elevated temperatures (130-170 °C). ... 

Titanium-pillared montmorillonites (Ti-PILC) modified with tartrates were described as heterogeneous Sharpless epoxidation catalysts [33] as well as for the oxidation of aromatic sulfides [34]. Metal oxides modified with histamine showed modest efficiencies for the kinetic resolution of activated amino acid esters (kj /k5 2) [35]. Silica or alumina treated with diethylaluminium chloride and menthol catalyzed the Diels-Alder reaction between cylopentadiene and methacrolein with modest enantioselectivities of up to 31% ee [36]. ZeoHte HY, modified with chiral sulfoxides had remarkable selectivities for the kinetic resolution of 2-butanol (k /kj =39) but unfortunately the catalyst is not very stable... 

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