Metal carbonyls epoxides

Transition-metal-catalysed epoxidations work only on allylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isopropoxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

All types of electrophiles have been used with 2-lithio-l,3-dithiane derivatives, including alkyl halides, sulfonates, sulfates, allylic alcohols, arene-metal complexes, epoxides, aziridines, carbonyl compounds, imines, Michael-acceptors, carbon dioxide, acyl chlorides, esters and lactones, amides, nitriles, isocyanates, disulfides and chlorotrialkylsilanes or stannanes. The final deprotection of the dithioacetal moiety can be carried out by means of different types of reagents in order to regenerate the carbonyl group by heavy metal coordination, alkylation and oxidation184 or it can be reduced to a methylene group with Raney-nickel, sodium or LiAIII4. 

Alkenes are converted to epoxides by oxidation with peroxy acids, and thereby they are protected with regard to certain chemical transformations. Alkaline hydrogen peroxide selectively attacks enone double bonds in the presence of other alkenes. The epoxides can be transformed back to alkenes by reduction-dehydration sequences or using triphenylphosphine, chromous salts, zinc, or sodium iodide and acetic acid. A more advantageous and fairly general method consists, however, of the treatment of epoxides with dimethyl diazomalonate in the presence of catalytic amounts of binuclear rhodium(II) car-boxylate salts. This deoxygenation proceeds under neutral conditions and without isomerization or cy-clopropanation of the liberated alkene (Scheme 97). Furthermore, epoxides can be converted to alkenes with the aid of various metal carbonyl complexes. Thus, they may be nucleophilically opened with... 

Complexes of Mo and V as naphthenates [126], carbonyls [169], and acetyla-cetonates [170] have been studied extensively [101,171-173] (Table 1.6). Mo and V are the most active metals for epoxidation with hydroperoxides, and are the basis for one of the industrial production methods of PO. An organomolybdenum compound, T -(C5Bz5)MoO2C1 has been reported with high activity for epoxidation of ds-cyclooctene with TBHP [174]. The authors report a TOF of 0.33 s at 55 °C, but this may have been an initial rate. Using quantities from the paper, an average TOF of 6.9 x 10 s in 4 h is calculated. The catalyst has similar activity for styrene epoxidation, but lower for 1-octene. 

An alternative method for converting a sulfide into an ylide involves the reaction of the sulfide with a carbene or metal carbenoid [10]. We recently reported the successful application of this strategy to carbonyl epoxidation using catalytic quantities of sulfide (Scheme 4) [11,12,13]. 

Rearrangement of epoxides. This relatively inexpensive metal carbonyl can serve as a homogeneous catalyst for rearrangement of epoxides to aldehydes and for isomerization of p,y- to , S-unsaturated aldehydes. The olefin corresponding to the epoxide is a by-product. ... 

In summary of this section, it must noted that, in spite of numerous studies, nowdays we know very little about carbonyl hydrides and other substituted (mixed) carbonyls thermolysis in polymeric systems, as well as in reactive plastics. For example, in some experiments the decomposing metal carbonyls were placed into an epoxide resin heated up to the nanoparticles deposition on the forming polymer surface [121]. It is possible that the highly reactive metal particles in such systems can initiate the epoxy cycle cleavages followed by a three-dimensional space structure formation. Iron carbonyl being decomposed into polybenzimidazole suspension (in transformer oil at 473 K) forms the ferrum nanoparticles (1-11 nm) capable of polymer thermostabization [122]. 

S. S. Balula, A. C. Coelho, S. S. Braga, A. HazeU, A. A. Valente, M. Pillinger, J. D. Seixas, C. C. Romao, I. S. Gongalves, Influence of cyclodextrins on catalytic olefin epoxidation with metal-carbonyl componnds, crystal structure of the TRIMEB complex with CpFefCOl Cl, Organometallics, 2007, 26, 6857-6863. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

As well as for metalated epoxides, the trifluoromethyl moiety also proved an effective organyl-stabilizing group for metalated aziridines. Lithiated aziridine 241 reacted stereoselectively with carbonyl-containing electrophiles, and phenyl disulfide and chlorotrimethylsilane were also trapped in good yield (Scheme 5.60) [70b, 85],... 

The importance of reactions with complex, metal hydrides in carbohydrate chemistry is well documented by a vast number of publications that deal mainly with reduction of carbonyl groups, N- and O-acyl functions, lactones, azides, and epoxides, as well as with reactions of sulfonic esters. With rare exceptions, lithium aluminum hydride and lithium, sodium, or potassium borohydride are the... 

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