THF, oxidation

For internal olefins, the Wacker oxidation is sometimes surprisingly regioselective. By using aqueous dioxane or THF, oxidation of P,y-unsaturated esters can be achieved selectively to generate y-keto-esters (Eq. 3.18).86 Under appropriate conditions, Wacker oxidation can be used very efficiently in transforming an olefin to a carbonyl compound. Thus, olefins become masked ketones. An example is its application in the synthesis of (+)-19-nortestosterone (3.11) carried out by Tsuji (Scheme 3.5).87... 

CH3)3N-S03 (1,2-dichloroethane, Soxhlet apparatus) or with 1,4-ben-zodioxane-S03 (THF). Oxidation of P406 with l,4-dioxane-S03 yields mixtures of P407 and P408 (82). 

Ethers are also attacked by metal atoms e.g., tetrahydrofuran (THF) oxidatively adds to Mo atoms to yield a reactive intermediate that may serve as an olefin hydrogenation catalyst... 

Tetrahydrofuran (THF) oxidation was also observed and documented for the first time for CU2O2 reactivity. The reaction was clean giving mainly... 

Fig. 28. Differences in THF oxidation rates for the [ Cu (R-MePY2) 2(02)] series in CH2CI2.

To form Pt nanopartides, HPS was impregnated in an inert atmosphere with a solution of H2PtCl6 in THF followed by reduction with H2. It is noteworthy that incorporation of the THF solution of platinic acid in microporous HPS results in partial reduction of the Pt(IV) species and formation of Pt(II) complexes where the ligands are the THF oxidation products [89]. The accumulation of Pt(II) complexes, as well as subsequent Pt nanoparticle formation, is effectively restricted by... 

The composition of a mobile phase can change as a result of chemical reactions. For example, THF oxidizes with time. Metals can leach into the mobile phase from metal frits or from brown solvent bottles. But in both cases the amount of reaction product is so small that it scarcely causes a problem. In practice, a change in mobile-phase composition due to chemical reaction is extremely rare. 

To explain the transformation of anilide 14 into tetracyele 15 we should follow the different steps of the mechanism previously discussed. SET from the anilide to the IBX-THF oxidant reagent leads to radical cation 16 that after loss of a proton will be transformed into radical 17. Cyclization into the C=C double bond will lead to cyclic radical 18. At this point, the reaction could end by capture of a hydrogen atom. However, in this ease, vinylcyclopropane radieal 18 is transformed into benzylie radieal 19 by rupture of flie adjacent cyclopropyl ring. Oxidation of this radical in the reaction medium (IBX-THF) leads to cation 20 which cyclizes to 22 (the cyclization is better understood via canonical form 21). Finally, rearomatiza-tion and loss of a proton in 22 yields the observed product 15. The overall reaction is a cascade radical cyclization (Scheme 39.8). 

Note I. A solution of 1-1ithiomethoxyal1ene, prepared from methoxyallene and BuLi-hexane-THF, did not react with ethylene oxide below 20°C. The reaction started at about 30°C, but the reaction mixture became very dark. 

Reducing agent 2 mol Sml2 per oxide oxygen (HMPA/THF)... 

The conversion of primary alcohols and aldehydes into carboxylic acids is generally possible with all strong oxidants. Silver(II) oxide in THF/water is particularly useful as a neutral oxidant (E.J. Corey, 1968 A). The direct conversion of primary alcohols into carboxylic esters is achieved with MnOj in the presence of hydrogen cyanide and alcohols (E.J. Corey, 1968 A,D). The remarkably smooth oxidation of ethers to esters by ruthenium tetroxide has been employed quite often (D.G. Lee, 1973). Dibutyl ether affords butyl butanoate, and tetra-hydrofuran yields butyrolactone almost quantitatively. More complex educts also give acceptable yields (M.E. Wolff, 1963). 

Difunctionalization with similar or different nucleophiles has wide synthetic applications. The oxidative diacetoxylation of butadiene with Pd(OAc)i affords 1,4-diacetoxy-2-butene (344) and l,2-diacetoxy-3-butene (345). The latter can be isomerized to the former. An industrial process has been developed based on this reaction. The commercial process for l,4-diacetoxy-2-butene (344) has been developed using the supported Pd catalyst containing Te in AcOH. 1,4-Butanedioi and THF are produced commercially from 1,4-diacetoxy-2-butene (344)[302]. 

As an application of maleate formation, the carbonylation of silylated 3-butyn-l-ol affords the 7-butyrolactone 539[482], Oxidative carbonylation is possible via mercuration of alkynes and subsequent Lransmetallation with Pd(II) under a CO atmosphere. For example, chloromercuration of propargyl alcohol and treatment with PdCF (1 equiv.) under 1 atm of CO in THF produced the /3-chlorobutenolide 540 in 96% yield[483]. Dimethyl phenylinale-ate is obtained by the reaction of phenylacetylene, CO, PdCU, and HgCl2 in MeOH[484,485]. 

Butane-Based Transport-Bed Process Technology. Du Pont aimounced the commercialization of a moving-bed recycle-based technology for the oxidation of butane to maleic anhydride (109,149). Athough maleic anhydride is produced in the reaction section of the process and could be recovered, it is not a direct product of the process. Maleic anhydride is recovered as aqueous maleic acid for hydrogenation to tetrahydrofuran [109-99-9] (THF). 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

Other THF polymerization processes that have been disclosed in papers and patents, but which do not appear to be in commercial use in the 1990s, include catalysis by boron trifluoride complexes in combination with other cocatalysts (241—245), modified montmorrillonite clay (246—248) or modified metal oxide composites (249), rare-earth catalysts (250), triflate salts (164), and sulfuric acid or Aiming sulfuric acid with cocatalysts (237,251—255). 

PTMEG is a polymeric ether susceptible to both thermal and oxidative degradation. It usually contains 300—1000 ppm of an antioxidant such as 2,6-di-/ f2 -butyl-4-hydroxytoluene (BHT) to prevent oxidation under normal storage and handling conditions. Thermal decomposition in an inert atmosphere starts at 210—220°C (410—430°E) with the formation of highly flammable THE. In the presence of acidic impurities, the decomposition temperature can be significantly reduced contact with acids should therefore be avoided, and storage temperatures have to be controlled to prevent decomposition to THF (261). 

Hydrogenation. Gas-phase catalytic hydrogenation of succinic anhydride yields y-butyrolactone [96-48-0] (GBL), tetrahydrofiiran [109-99-9] (THF), 1,4-butanediol (BDO), or a mixture of these products, depending on the experimental conditions. Catalysts mentioned in the Hterature include copper chromites with various additives (72), copper—zinc oxides with promoters (73—75), and mthenium (76). The same products are obtained by hquid-phase hydrogenation catalysts used include Pd with various modifiers on various carriers (77—80), Ru on C (81) or Ru complexes (82,83), Rh on C (79), Cu—Co—Mn oxides (84), Co—Ni—Re oxides (85), Cu—Ti oxides (86), Ca—Mo—Ni on diatomaceous earth (87), and Mo—Ba—Re oxides (88). Chemical reduction of succinic anhydride to GBL or THF can be performed with 2-propanol in the presence of Zr02 catalyst (89,90). 

The nitrogen of these aminocarboranes can be alkylated to give, eg, 7-[N(CH3)3]-7-CB2qH22 [31117-16-5]. These compounds give closo-2-(Z. . ]Y, [38102-45-0] upon treatment with Na in tetrahydrofuran (THF) followed by iodine oxidation (eq. 63) (126). 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

Birch reduction of indole with lithium metal in THF in the presence of trimethylsilyl chloride followed by oxidation with p-benzoquinone gave l,4-bis(trimethylsilyl)indoIe (106). This is readily converted in two steps into l-acetyl-4-trimethylsilylindole. Friedel-Crafts acylation of the latter compound in the presence of aluminum chloride yields the corresponding 4-acylindole (107) (82CC636). 

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