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Physical Data: the following data are for a 90% aqueous solution: flash point 35 °C; d 0.901 g cm-3; n20D 1.3960. The density of a 70% aqueous solution is 0.937 g cm-3.
Solubility: sol alcohol, ether, chloroform; slightly sol H2O, DMSO.
Handling, Storage, and Precautions: eye protection and rubber gloves should be worn when handling this material; avoid skin contact; this reagent should be handled only in a fume hood. Eye and skin irritant; immediately flush with water if contact is made with the eyes. Flammable liquid; oxidizer; sensitive to shocks and sparks. May react explosively with reducing agents. Store in an explosion-proof container, and keep away from reducing materials and strong acids and bases. Avoid using high strength solutions; do not distill. The use of molecular sieves for drying is not recommended.
The title reagent is used in oxidations of various substrates to give epoxides, ketones, aldehydes, carboxylic acid esters, and nitro or azoxy compounds. The reagent and its metal complexes have been extensively reviewed.1-3 This article describes representative applications to problems in organic synthesis.
Chlorohydroxylation of nonfunctionalized alkenes can be accomplished (eq 3) through the reaction of TBHP with Titanium(IV) Chloride.8 Chlorohydroxylation can also be done asymmetrically to the alkenes of allylic alcohols using TBHP with Dichlorotitanium Diisopropoxide and an asymmetric tartrate catalyst, and the stereochemistry can be controlled by the ratio of titanium to tartrate (eq 4) varying (see below).
Alkenes with an allylic hydrogen can be selectively oxidized to the allylic alcohol by TBHP in the presence of Selenium(IV) Oxide (eq 5).9 This is preferable to the oxidation using stoichiometric SeO2 by itself, which leads to reduced forms of selenium and can make isolation and purification of the product difficult. Less substituted alkenes require 0.5 equivalents of SeO2 while for more substituted alkenes it may be present in catalytic amounts. The regioselectivity of this reaction favors the more substituted site being oxidized. The addition of small amounts of carboxylic acids also aids this reaction with certain alkenes (eq 5). See also Selenium(IV) Oxide-t-Butyl Hydroperoxide.
Alkenes can also be oxidized to give rearranged allylic alcohols using TBHP with phenylselenenic acid and Diphenyl Diselenide (eq 6).10 The reaction proceeds through a b-hydroxyl phenylselenide adduct of the alkene, which then eliminates the selenide to give the allylic alcohol and a phenylselenenic acid byproduct. This method is preferable to the use of phenylselenenic acid with hydrogen peroxide, since the latter can lead to epoxidation of the alkene of the product to give the epoxy alcohol. This method also does not oxidatively remove the selenium from the phenylselenenic acid byproduct, as the H2O2 method does, allowing the phenylselenenic acid to be recovered and easily reduced back to diphenyl diselenide.
Propargylic carbons can also be oxidized, using TBHP and SeO2 (eq 8).14 Unlike allylic systems, propargylic systems show a great tendency towards oxygenation on both sides of the triple bond, and are generally more reactive towards a oxygenation. A mixture of the propargylic alcohol, ketone, diol, and ketol will generally result from this reaction. If there are two sites possible for oxygenation, methine and methylene groups have about the same reactivity towards these conditions, while methyl groups show a lesser preference for oxidation. In symmetrical alkynes the diol is prevalent, and in some cases where the alkyne is in conjugation with other p systems the ketone is an important product, whereas in most other cases the ketone and ketol are minor products. If the alkyne has one methine and one methylene substituent, the enynone can be an important product.
If a chromium(VI) catalyst is used in the presence of TBHP, propargylic carbons will be oxidized to the alkynic ketone (eq 9).15 The more highly substituted alkyl substituent on the alkyne is preferentially oxidized, and symmetrical alkynes give the monoketone accompanied by the diketone.
Oxidation of p-allylpalladium complexes can also occur with TBHP using a molybdenum(IV) catalyst to give the allylic alcohol (eq 10).16 Hydroxyl attack will occur axially, syn to the complexed palladium. This conversion can also be carried out with peroxy acids or singlet oxygen, but these methods are not as selective.
TBHP is widely used as an epoxidizing agent, both synthetically and industrially.18 TBHP has been used to effect regiospecific, stereospecific, and asymmetric epoxidations. In general, the rates of epoxidations using TBHP are slowed by polar solvents, and increased with higher alkyl substitution of the alkene. TBHP is considered superior to hydrogen peroxide for epoxidations, because it is soluble in hydrocarbon solvents, while hydrogen peroxide can readily transform the epoxide to the vic-glycol.
Epoxidations of alkenes in compounds containing other functional groups can also be accomplished using TBHP with a molybdenum catalyst (eq 12).19,20 For nonconjugated dienes, more highly substituted alkenes can be selectively epoxidized over less substituted alkenes. Conjugated dienes are less susceptible to epoxidation than isolated alkenes, but the regioselectivity for the different double bonds of a conjugated system follows the same pattern as that for isolated alkenes (eq 13).
For cyclic systems, vanadium- and molybdenum-catalyzed reactions give predominantly the cis product (eq 16).23 There are several factors that can affect the selectivity of this reaction for cyclic allylic alcohols. With increasing ring size, the selectivity decreases slightly for vanadium-catalyzed reactions, and more dramatically for molybdenum-catalyzed reactions. The selectivity was also observed to be better for cyclic allylic alcohols where the hydroxyl is in a quasi-axial position.
Epoxidation of vinyl allenes by TBHP and Vanadyl Bis(acetylacetonate) catalyst leads to the formation of cyclopentenones (eq 23).35 The intermediate in this reaction is an epoxide of the allene. The stereochemistry of the double bond can be retained. The stereoselectivity is kinetic in nature, and can be lost due to epimerization of the kinetic product if the reaction is continued for long periods of time.
Double bonds of silyl enol ethers can be oxidatively cleaved to the corresponding ketones or carboxylic acids using TBHP with MoO2(acac)2 as a catalyst (eq 24).36 The ease with which enols can be generated regiospecifically makes this a very powerful method in organic synthesis. This reaction selectively cleaves the double bond of silyl enol ethers in the presence of other double bonds within the molecule.
Oxidation of a,b-unsaturated esters and ketones with palladium-catalyzed TBHP gives b-keto esters or 1,3-diketones (eq 27).39 Hydrogen peroxide can also be used as the oxidant for this reaction.
Reactions with Other Functional Groups.
In the presence of catalytic amounts of diphenyl diselenide, TBHP oxidizes benzylic and allylic alcohols to the corresponding ketones (eq 29).41 Saturated alcohols can be oxidized to the corresponding carbonyl compounds as well if bis(2,4,6-trimethylphenyl) diselenide is used as the catalyst and a small amount of a secondary or tertiary amine is present. These conditions do not affect other double bonds present in the substrate. This system can also be used for the oxidation of a-hydroxy selenides and thiols, selectively oxidizing the hydroxy function to the carbonyl.
Oxidation of alcohols can also be done using TBHP and a chromium(VI) catalyst (eq 31).44 This system works best for allylic, benzylic, and propargylic alcohols, and will selectively oxidize these in the presence of other alcohols.
Oxidation of Selenides and Selenoxides.
Alkenes can be produced oxidatively from selenides, through the selenoxides and elimination. This is done by stirring TBHP with basic alumina and the appropriate selenide (eq 37).51 This transformation can also be accomplished by treatment of the selenide with Hydrogen Peroxide, Ozone followed by Triethylamine, periodate, or peroxy acids.
Reactions of TBHP with compounds containing nitrogen have been used to effect a variety of oxidations, both of the nitrogen atom itself and of adjacent carbon atoms. Tertiary amines react with TBHP in the presence of vanadium and molybdenum catalysts to give amine oxides (eq 38).52 This transformation can also be done with cumene and pentene hydroperoxides.
Secondary amines are oxidized to imines by TBHP in the presence of ruthenium(II) catalysts (eq 39).53 Tertiary amines are oxidized by TBHP in the presence of ruthenium catalyst to give a-(t-butyldioxy)alkylamines, which decompose to iminium ion intermediates when treated with acid (eq 40).54 N-Methyl groups are selectively oxidized when other N-alkyl or -alkenyl groups are present.
Using catalytic amounts of copper, cobalt, or manganese salts, TBHP reacts with molecules that contain a slightly activated carbon-hydrogen bond, replacing the activated hydrogen with a peroxy group. This transformation can also be accomplished with other hydroperoxides. Carbon-hydrogen bonds a to an alkene (eqs 44 and 45),59,60,61 phenyl groups,60 carbonyls,61 nitriles,62 oxygen,60,61 or nitrogen (eq 46)63 atoms are activated towards this reaction. The primary function of the metal salts in these reactions is to initiate decomposition of the hydroperoxide.
Peroxy t-butyl organosilanes can be prepared by reacting TBHP with the appropriate silyl chloride and pyridine, ammonia, or triethylamine (eq 49).68 Peroxides of a number of other heteroatoms in organic compounds, such as germanium,69 boron,70 cadmium,71 tin,72 aluminum,73 and mercury,74 can also be synthesized using TBHP.
Aldehydes react with TBHP in the presence of catalytic amounts of copper, cobalt, or manganese salts to give the t-butyl ester (eq 51).76 In the absence of a metal catalyst, benzaldehyde will react with TBHP to give a mixture of the meso and racemic forms of benzopinacol dibenzoate.
Conversion of Halides to Alcohols.
Grignard reagents react with TBHP to give the appropriate alcohol or phenol (eq 56).81 This provides an alternative method for the conversion of halides to alcohols or phenols. Because the hydrogen of the peroxide is activated, either two equiv of Grignard reagent must be used or the magnesium salt of the hydroperoxide, prepared from the hydroperoxide and ethylmagnesium bromide.
Conversion of Alcohols to Halides.
In cases where traditional methods fail, alcohols can be converted to halides by a radical chain reaction.82 This is accomplished by transforming the alcohol into a chloroglyoxylate, reacting it with TBHP, and warming this in the presence of a halogen donor such as CCl4 or BrCCl3, to initiate a radical reaction where first a t-butoxyl radical is eliminated, then CO2 is eliminated twice in succession, leaving an alkyl radical which then reacts with the halogen donor to give the halide (eq 57).
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