· (MW 227.01)
(powerful oxidant, particularly useful for dehydrogenation to form aromatic1a-d and a,b-unsaturated carbonyl compounds;1 oxidizes activated methylene1a-c and hydroxy groups1b to carbonyl compounds; phenols are particularly sensitive1c)
Alternate Name: DDQ.
Physical Data: mp 213-216 °C; E0 &AApprox; 1000 mV.
Solubility: very sol ethyl acetate and THF; moderately sol dichloromethane, benzene, dioxane, and acetic acid; insol H2O.
Form Supplied in: bright yellow solid; widely available.
Analysis of Reagent Purity: UV (l
max [dioxane] 390 nm) and mp.
Purification: recrystallization from a large volume of dichloromethane.
Handling, Storage, and Precautions: indefinitely stable in a dry atmosphere, but decomposes in the presence of water with the evolution of HCN. Store under nitrogen in a sealed container.
Quinones of high oxidation potential are powerful oxidants which perform a large number of useful reactions under relatively mild conditions. Within this class, DDQ represents one of the more versatile reagents since it combines high oxidant ability with relative stability1 (see also Chloranil). Reactions with DDQ may be carried out in inert solvents such as benzene, toluene, dioxane, THF, or AcOH, but dioxane and hydrocarbon solvents are often preferred because of the low solubility of the hydroquinone byproduct. Since DDQ decomposes with the formation of hydrogen cyanide in the presence of water, most reactions with this reagent should be carried out under anhydrous conditions.1a
Dehydrogenation of Hydrocarbons.
The mechanism by which quinones effect dehydrogenation is believed to involve an initial rate-determining transfer of hydride ion from the hydrocarbon followed by a rapid proton transfer leading to hydroquinone formation.1d Dehydrogenation is therefore dependent upon the degree of stabilization of the incipient carbocation and is enhanced by the presence of functionality capable of stabilizing the transition state. As a consequence, unactivated hydrocarbons are stable to the actions of DDQ while the presence of alkenes or aromatic moieties is sufficient to initiate hydrogen transfer.1d,2 The formation of stilbenes from suitably substituted 1,2-diarylethanes3 and the synthesis of chromenes by dehydrogenation of the corresponding chromans (eq 1)4 are particularly facile transformations.
Similar reactions have also found considerable utility for the introduction of additional unsaturation into partially aromatized terpenes and steroids, where the ability to control the degree of unsaturation in the product is a particular feature of quinone dehydrogenations.5 Moreover, the ability to effect exclusive dehydrogenation in the presence of sensitive substituents such as alcohols and phenols (eq 2)5b illustrates the mildness of the method and represents a further advantage.
DDQ is a particularly effective aromatization reagent and is frequently the reagent of choice to effect facile dehydrogenation of both simple (eq 3)6 and complex hydroaromatic carbocyclic compounds.1d,7 Skeletal rearrangements are relatively uncommon features of quinone-mediated dehydrogenation reactions, but 1,1-dimethyltetralin readily undergoes aromatization with a 1,2-methyl shift when subjected to the usual reaction conditions (eq 4).8 Wagner-Meerwein rearrangements have also been observed in the aromatization of steroids (eq 5), although in this instance considerably longer reaction times are required.9 Such reactions provide a unique method for the aromatization of cyclic systems containing quaternary carbon atoms without the loss of carbon.
DDQ is also an effective reagent for the dehydrogenation of hydroaromatic heterocycles, and pyrroles,10 pyrazoles,11 triazoles,12 pyrimidines,13 pyrazines,14 indoles,15 quinolines,16 furans,17 thiophenes,18 and isothiazoles19 are among the many aromatic compounds prepared in this manner. Rearomatization of nitrogen heterocycles following nucleophilic addition across a C=N bond (eq 6)13 is a particularly useful application of DDQ,13,16 and similar addition and reoxidation reactions in acyclic systems have also been reported.20
One particularly important use of DDQ has been in the dehydrogenation of reduced porphyrins, where the degree of aromatization of the product is highly dependent on the relative reagent:substrate stoichiometry.1b Under optimal conditions, excellent yields of partially or fully conjugated products may be isolated.1b,21 The formation of porphyrins from tetrahydro precursors on reaction with 3 equiv of DDQ under very mild conditions (eq 7) typifies one of the more commonly described transformations.21 More recently, DDQ has been used as part of a one-pot sequence for the formation of porphyrins from simple intermediates, although the overall yields in such reactions are generally comparatively low.22
In addition to the formation of neutral aromatic compounds, DDQ is also an effective agent for the preparation of the salts of stable aromatic cations. High yields of tropylium (eq 8) and triphenylcyclopropenyl (eq 9) cations have been isolated in the presence of acids such as perchloric, phosphoric, and picric acid,23 and oxonium,24 thioxonium,23,25 and pyridinium23,26 salts may be prepared in reasonable yields from appropriate starting materials under essentially similar conditions. The formation of the perinaphthyl radical has been reported on oxidation of perinaphthalene with DDQ under neutral conditions,23 although such products are not usually expected.
Dehydrogenation of Carbonyl Compounds.
DDQ and other high oxidation potential quinones are versatile reagents for the synthesis of a,b-unsaturated carbonyl compounds,1e a reaction that has found extensive application in the chemistry of 3-keto steroids.1b The regiochemical course of this dehydrogenation is highly dependent on the initial steroidal geometry; thus the 5a- and the 5b-series usually furnish D1- and D4-3-keto steroids, respectively (eq 10).1b The selection of one isomer over the other is likely to reflect the relative steric crowding of the C-4 hydrogen atom in the two series, but other factors may play a role in those instances where the anticipated product is not formed.1b
A rather more unusual situation exists during the dehydrogenation of D4-3-keto steroids where the product formed is dependent on the oxidizing quinone. Thus whereas DDQ gives the D4,6 ketone, chloranil and a number of other quinones yield only the D1,4 isomer (eq 11), a result that has been rationalized on the basis of DDQ proceeding via the kinetic enolate while less reactive quinones proceed via the thermodynamic enolate.27
While DDQ is an effective reagent for the formation of a,b-unsaturated steroidal ketones, the dehydrogenation of cyclohexanones to the corresponding enone only proceeds well when the further reaction is blocked by gem-dialkyl substitution.1b,28 Tropone, by contrast, has been prepared from 2,4-cycloheptadienone (eq 12), although the yield was somewhat low.29 Heterocyclic enones such as flavones30 and chromones31 may be efficiently prepared from flavanones and chromanones, respectively, under similar conditions to those used for the dehydrogenation of steroids, and the dehydrogenation of larger ring heterocyclic ketones has been described.32 Ketone enol ethers have also been shown to undergo facile dehydrogenation
to a,b-unsaturated ketones with DDQ, although the nature of the product formed may be dependent on the presence or absence of moisture.33 Prior formation of the silyl enol ether is a potentially more versatile procedure that has been shown to overcome the problems generally associated with the dehydrogenation of unblocked cyclohexanones (eq 13),34 particularly when the acidic hydroquinone formed during the reaction is neutralized by the addition of N,O-Bis(trimethylsilyl)acetamide (BSA)34 or a hindered base.35 Preparation of the enone derived from either the kinetic or the thermodynamic enolate is possible in this manner.34,35a
Quinone dehydrogenation reactions of carbonyl compounds are mostly limited to the more readily enolized ketones, and analogous reactions on esters36 and amides37 require stronger conditions and are far less common unless stabilization of the incipient carbonium ion is possible. Oxidation in the presence of the silylating agent bis(trimethylsilyl)trifluoroacetamide (BSTFA) considerably improves the dehydrogenation of steroidal lactams (eq 14) by facilitating the breakdown of the intermediate quinone-lactam complex.38 Similar dehydrogenations of carboxylic acids are rare, but reaction of the a-anion of carboxylate salts generated in the presence of HMPA has given modest yields of a number of a,b-unsaturated fatty acids.39
Oxidation of Alcohols.
Saturated alcohols are relatively stable to the action of DDQ in the absence of light, although some hindered secondary alcohols have been oxidized in reasonable yield on heating under reflux in toluene for extended periods of time (eq 15).40 It has been suggested that oxidation proceeds in this instance as a result of relief of steric strain.40 Allylic and benzylic alcohols, on the other hand, are readily oxidized to the corresponding carbonyl compounds,1b,41 and procedures have been developed which utilize catalytic amounts of the reagent in the presence of a stoichiometric amount of a second oxidant.42 Since the rate of oxidation of allylic alcohols is greater than that for many other reactions,43 the use of DDQ provides a selective method for the synthesis of allylic and benzylic carbonyl compounds in the presence of other oxidizable groups.
The oxidation of benzylic alkyl groups proceeds rapidly in those instances in which stabilization of the incipient carbonium ion is possible44,45 and a number of polycyclic aromatic compounds have been oxidized in good yield to the corresponding benzylic ketones on brief treatment with DDQ in aqueous acetic acid at rt.44 The reaction is postulated to proceed via an intermediate benzylic acetate which is hydrolyzed and further oxidized under the reaction conditions.44 It is interesting that 1-alkylazulenes, which are cleaved by many of the more common oxidants, are cleanly oxidized following a short exposure to DDQ in aqueous acetone (eq 16), while under the same conditions no oxidation of C-2 alkyl substituents takes place.46 As expected, the oxidation was shown to be disfavored by the presence of strongly electron-withdrawing substituents.46
The stabilization of benzylic carbonium ions is also a feature of arenes containing electron-donating substituents, especially those having 4-alkoxy or 4-hydroxy groups, and such compounds are particularly effective substrates for oxidation by DDQ. Thus 6-methoxytetralone has been prepared in 70% yield from 6-methoxytetralin on treatment with DDQ in methanol,47 although it is possible to isolate intermediate benzylic acetates if the oxidation is carried out in acetic acid (eq 17).48 An interesting variant of the oxidation in inert solvents in the presence of either Cyanotrimethylsilane49 or Azidotrimethylsilane50 results in the isolation of good to excellent yields of benzyl cyanides and azides, respectively.
Benzylic oxidation of alkoxybenzyl ethers is particularly facile, and since some of the more activated derivatives are cleaved under conditions which leave benzyl, various ester, and formyl groups unaffected, they have found application in the protection of primary and secondary alcohols.51 Deprotection with DDQ in dichloromethane/water follows the order: 3,4-dimethoxy > 4-methoxy > 3,5-dimethoxy > benzyl and secondary > primary, thus allowing the selective removal of one function in the presence of another.51 2,6-Dimethoxybenzyl esters are readily cleaved to the corresponding acids on treatment with DDQ in wet dichloromethane at rt, whereas 4-methoxybenzyl esters are stable under these conditions.52 Oxidative cleavage of N-linked 3,4-dimethoxybenzyl derivatives with DDQ has also been
DDQ is a powerful oxidizing agent for phenols, and carbonium ion stabilization via the quinone methide makes benzylic oxidation of 4-alkylphenols a highly favored process.47,54 With methanol as the solvent it is possible to isolate a-methoxybenzyl derivatives in reasonable yield.55
Phenolic Cyclization and Coupling Reactions.
The oxidation of phenolic compounds which either do not possess benzylic hydrogen atoms, or which have an alternative reaction pathway, can result in a variety of interesting products. Cyclodehydrogenation reactions leading to oxygen heterocycles represent a particular application of phenolic oxidation by DDQ, and is common when intramolecular quenching of the intermediate phenoxyl radical is possible (eqs 18-20).56-59 These reactions necessarily take place in nonpolar solvents and have given such products as coumarins,56 chromenes (eq 18),57 benzofurans (eq 19),58 and spiro derivatives (eq 20).59
Phenols and enolizable ketones that cannot undergo a,b-dehydrogenation may afford intermolecular products arising from either C-C or C-O coupling on treatment with DDQ in methanol.60 2,6-Dimethoxyphenol, for example, results predominantly in oxidative dimerization (eq 21), while the hindered 2,4,6-tri-t-butylphenol generates the product of quinone coupling (eq 22).60 Various other unusual products have been observed on DDQ oxidation of phenols and enolic compounds, their structure being dependent on that of the parent compound.41,60,61
In addition to the key reactions above, DDQ has been used for the oxidative removal of chromium,62 iron,63 and manganese64 from their complexes with arenes and for the oxidative formation of imidazoles and thiadiazoles from acyclic precursors.65 Catalytic amounts of DDQ also offer a mild method for the oxidative regeneration of carbonyl compounds from acetals,66 which contrasts with their formation from diazo compounds on treatment with DDQ and methanol in nonpolar solvents.67 DDQ also provides effective catalysis for the tetrahydropyranylation of alcohols.68 Furthermore, the oxidation of chiral esters or amides of arylacetic acid by DDQ in acetic acid provides a mild procedure for the synthesis of chiral a-acetoxy derivatives, although the diastereoselectivity achieved so far is only
While quinones in general are well known dienophiles in Diels-Alder reactions, DDQ itself only rarely forms such adducts.70 It has, however, been shown to form 1:1 adducts with electron-rich heterocycles such as benzofurans and indoles where it forms C-O and C-C adducts, respectively.71
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Derek R. Buckle
SmithKline Beecham Pharmaceuticals, Epsom, UK
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