Lead(IV) Acetate1


[546-67-8]  · C8H12O8Pb  · Lead(IV) Acetate  · (MW 443.37)

(oxidizing agent for different functional groups;1 oxidation of unsaturated and aromatic hydrocarbons;2 oxidation of monohydroxylic alcohols to cyclic ethers;3 1,2-glycol cleavage;4 acetoxylation of ketones;1 decarboxylation of acids;5 oxidative transformations of nitrogen-containing compounds6)

Alternate Name: lead tetraacetate; LTA.

Physical Data: mp 175-180 °C; d 2.228 g cm-3.

Solubility: sol hot acetic acid, benzene, cyclohexane, chloroform, carbon tetrachloride, methylene chloride; reacts rapidly with water.

Form Supplied in: colorless crystals (moistened with acetic acid and acetic anhydride); widely available, 95-97%.

Analysis of Reagent Purity: iodometrical titration. Drying: in some cases, acetic acid must be completely removed by drying the reagent in a vacuum desiccator over potassium hydroxide and phosphorus pentoxide for several days.

Handling, Storage, and Precautions: the solid reagent is very hygroscopic and must be stored in the absence of moisture. Bottles of lead tetraacetate should be kept tightly sealed and stored under 10 °C in the dark and in the presence of about 5% of glacial acetic acid.

Oxidations of Alkenic and Aromatic Hydrocarbons.

Lead tetraacetate reacts with alkenes in two ways: addition of an oxygen functional group on the double bond and substitution for hydrogen at the allylic position.2 In addition to these two general reactions, depending on the structure of the alkene, other reactions such as skeletal rearrangement, double bond migration, and C-C bond cleavage can occur, leading to complex mixtures of products, and these reactions therefore have little synthetic value (eq 1).1a,b,2,7 Styrenes afford 1,1-diacetoxy derivatives when the LTA reaction is performed in acetic acid (eq 2), while in benzene solution products resulting from the addition of both the methyl and an acetoxy group to the alkenic double bond are formed.7,8 Other nucleophiles, such as azide ion, carbanions, etc. can be introduced onto the alkenic bond in a similar fashion.9 In the LTA oxidation of cyclic alkenes, depending on ring size, structure, solvent, and reaction conditions, several types of products are formed. Thus 1,2-diacetates and 3-acetoxycycloalkenes are obtained from cyclohexene (cyclopentanecarbaldehyde is also formed),10 cycloheptene, and cyclooctene.11 Norbornene reacts with LTA to give rearrangement products in which 2,7-diacetoxynorbornane predominates (eq 3).12 Conjugated dienes undergo 1,2- and 1,4-diacetoxylation,13 while cyclopentadiene in wet acetic acid gives monoacetates of cis-cyclopentene-1,2-diol (eq 4).14

Aromatic hydrocarbons react with LTA in two ways: on the aromatic ring and at the benzylic position of the side chain. Oxidation of the aromatic ring results in substitution of aromatic hydrogens by acetoxy or methyl groups.1c Benzene itself is stable towards LTA at reflux and is frequently used as solvent in LTA reactions. However, mono- and polymethoxybenzene derivatives are oxidized by LTA in acetic acid to give acetoxylation products (eq 5).15 Oxidation of anthracene in benzene gives 9,10-diacetoxy-9,10-dihydroanthracene, whereas in AcOH a mixture of 10-acetoxy-9-oxo-9,10-dihydroanthracene and anthraquinone is obtained.16 The LTA oxidation of furan affords 2,5-diacetoxy-2,5-dihydrofuran (eq 6).17

Aromatic compounds possessing a C-H group at the benzylic position are readily oxidized by LTA to the corresponding benzyl acetates. Benzylic acetoxylation is preferably performed in refluxing acetic acid (eq 7).18 Acetoxylation at the benzylic position can be accompanied by methylation of the aromatic ring, followed sometimes by acetoxylation of the newly introduced methyl group.18

Oxidative Cyclization of Alcohols to Cyclic Ethers.

The LTA oxidation of saturated alcohols, containing at least four carbon atoms in an alkyl chain or an appropriate carbon skeleton, to five-membered cyclic ethers represents a convenient synthetic method for intramolecular introduction of an ether oxygen function at the nonactivated d-carbon atom of a methyl, methylene, or methine group (eq 8).3,19,20 The reactions are carried out in nonpolar solvents, such as benzene, cyclohexane, heptane, and carbon tetrachloride, either at reflux temperature1a,d,3,20,21 or by UV irradiation at rt.22

The conversion of alcohols to cyclic ethers is a complex reaction involving several steps: (i) reversible alkoxylation of LTA by the substrate; (ii) homolytic cleavage of the RO-Pb bond in the resulting alkoxy-lead(IV) acetate with formation of an alkoxy radical; (iii) intramolecular 1,5-hydrogen abstraction in this oxy radical whereby a d-alkyl radical is generated; (iv) oxidative ring closure to a cyclic ether via the corresponding d-alkyl cation (eq 9).3,20 The crucial step is the formation of the d-alkyl radical by way of 1,5-hydrogen migration. This type of rearrangement is a general reaction of alkoxy radicals, and, independently of the radical precursor, involves a transition state in which the d-CH group must be conformationally suitably oriented with respect to the attacking oxygen radical.1,3,23,24 Regioselective hydrogen abstraction proceeds preferentially from the d-carbon atom, since in that case an energetically favorable quasi-six-membered transition state is involved.3,23,24

The LTA oxidation of primary aliphatic alcohols affords 2-alkyltetrahydrofurans in 45-75% yield. A small amount of tetrahydropyran-type ether is also formed (eq 10).3a,20 The oxidation rate depends on the structural environment of the pro-activated carbon atom, with the rate decreasing in the order: methine > methylene > methyl d-carbon atom.3 When the d-carbon atom is adjacent to an ether oxygen function, the reaction rate and the yield of cyclic ethers increases.25 An ether oxygen attached to the d-carbon atom increases considerably the yield of six-membered cyclic ethers (eq 11). An aromatic ring adjacent to a d-methylene group does not noticeably affect the yield of tetrahydrofuran ethers, but when the phenyl group is attached to an ε-methylene group, the yield of six-membered cyclic ethers are enhanced.26

Secondary aliphatic alcohols containing a d-methylene group afford a cis/trans mixture of 2,5-dialkyltetrahydrofurans in about 33-70% yield (eq 12).20,22 The LTA oxidation of secondary alcohols is much slower than that of primary alcohols and isomeric six-membered cyclic ethers are not formed.20,21 Tertiary aliphatic alcohols, because of unfavorable steric and electronic factors, are less suitable for the preparation of tetrahydrofurans by LTA oxidation.22,27

In the cycloalkanol series, the ease of intramolecular formation of cyclic ether products strongly depends on ring size. Cyclohexanol, upon treatment with LTA, affords only 1% of 1,4-cyclic ether, whereas cycloalkanols with a larger ring, such as cycloheptanol and cyclooctanol, can adopt appropriate conformations necessary for transannular reaction, affording bicyclic ethers in moderate yields (eq 13).28 Large-ring cycloalkanols, such as cyclododecanol, cyclopentadecanol, and cyclohexadecanol, also give the corresponding 1,4-epoxy compounds as major cyclization products.3a,28 However, the special geometry of cyclodecanol is not favorable for the normal reaction and the 1,4-cyclic ether is formed in only 2.5% yield, whereas 1,2-epoxycyclodecane (13%) and the rearranged 8-ethyl-7-oxabicyclo[4.3.0]nonane (13%) are the predominant cyclization products.29

The LTA oxidation of alcohols to cyclic ethers has been successfully applied as a synthetic method for activation of the angular 18- and 19-methyl groups in steroidal alcohols containing a b-oriented hydroxy group at C-2, C-4, C-6, and C-11 (eq 14).3c,30,31 Hydroxy terpenoids with suitable stereochemistry can also undergo transannular cyclic ether formation (eq 15).32

Another possible reaction of alkoxy radical intermediates, formed in the LTA oxidation of alcohols in nonpolar solvents, is the b-fragmentation reaction.3 This process, which competes with intramolecular 1,5-hydrogen abstraction, consists of cleavage of a bond between the carbinol (a) and b-carbon atoms, thus affording a carbonyl-containing fragment and products derived from an alkyl radical fragment (usually acetates and/or alkenes).1a,3,22 Interesting synthetic applications of the LTA b-fragmentation reaction are the formation of 19-norsteroids from their 19-hydroxy precursors and the preparation of 5,10-secosteroids (containing a ten-membered ring) from 5-hydroxy steroids (eq 16).32

In the LTA oxidations of primary and secondary alcohols in nonpolar solvents, the corresponding aldehydes or ketones are usually obtained as minor byproducts (up to 10%).3,20,21 However, in the presence of excess pyridine or in pyridine alone, either with heating or at rt, the cyclization and b-fragmentation processes are suppressed and good preparative yields of aldehydes or ketones are obtained (eq 17).20,21,33 Carbonyl compounds are also obtained when the LTA oxidation of alcohols is carried out in benzene solution in the presence of manganese(II) acetate.33

In addition to cyclic ethers, b-fragmentation products, and carbonyl compounds, acetates of starting alcohols are also usually formed in the LTA oxidation, in yields up to 20%.20

Unsaturated alcohols, possessing an alkenic double bond at the d or more remote positions, react with LTA in nonpolar solvents to give acetoxylated cyclic ethers in good yield (eq 18),34,35 while 5-, 6- and 7-alkenols undergo in great predominance an exo-type cyclization, affording six-, seven- and eight-membered acetoxymethyl cyclic ethers, respectively.35

1,2-Glycol Cleavage.

LTA is one of the most frequently used reagents for the cleavage of 1,2-glycols and the preparation of the resulting carbonyl compounds (eq 19).1,4 The reactions are performed either in aprotic solvents (benzene, nitrobenzene, 1,2-dichloroethane) or in protic solvents such as acetic acid.36,37 The rate of LTA glycol cleavage is highly dependent on the structure and stereochemistry of the substrate. In general, there is correlation between the oxidation rate and the spatial proximity of the hydroxy groups.36 1,2-Diols having a geometry favoring the formation of cyclic intermediates are much more reactive than 1,2-diols whose structure does not permit such intermediates to be formed (eq 20).38,39 The oxidation rates often provide a reliable means for the determination of the stereochemical relationship of the hydroxy groups.39,40

1,2-Glycol cleavage by LTA has been widely applied for the oxidation of carbohydrates and sugars (eq 21).4,37 Because of structural and stereochemical differences, the reactivity of individual glycol units in sugar molecules is often different, thus rendering the LTA reaction a valuable tool for structural determination and for degradation studies in carbohydrate chemistry.41

a-Acetoxylation of Ketones.

The reaction of enolizable ketones with LTA is a standard method for a-acetoxylation (eq 22).1,3,42 The reactions are usually carried out in hot acetic acid or in benzene solution at reflux. The reaction proceeds via an enol-lead(IV) acetate intermediate, which undergoes rearrangement to give the a-acetoxylated ketone. Acetoxylation of ketones is catalyzed by Boron Trifluoride.43 Enol ethers, enol esters, enamines,1 b-dicarbonyl compounds, b-keto esters, and malonic esters are also acetoxylated by LTA.42

Decarboxylation of Acids.

Oxidative decarboxylation of carboxylic acids by LTA depends on the reaction conditions, coreagents, and structure of acids, and hence a variety of products such as acetate esters, alkanes, alkenes, and alkyl halides can be obtained.1,5 The reactions are performed in nonpolar solvents (benzene, carbon tetrachloride) or polar solvents (acetic acid, pyridine, HMPA).5 Mixed lead(IV) carboxylates are involved as intermediates, and by their thermal or photolytic decomposition decarboxylation occurs and alkyl radicals are formed (eq 23).5,44,45

Oxidation of alkyl radicals by lead(IV) species give carbocations and, depending on the reaction conditions and structure of the substrate acids, various products derived from the intermediate alkyl radicals and corresponding carbocations (dimerization, hydrogen transfer, elimination, substitution, rearrangement, etc.) are obtained.5 Decarboxylation of primary and secondary acids usually affords acetate esters as major products (eq 24).44 When a mixture of acetates and alkenes is formed, it is recommended (in order to improve the yields of acetate esters) to run the reaction in the presence of potassium acetate (eq 25).5 The LTA decarboxylation of tertiary carboxylic acids gives a mixture of alkenes and acetate esters.46 For the preparative oxidative decarboxylation of acids to alkenes, see Lead(IV) Acetate-Copper(II) Acetate.

A useful modification of the LTA reaction with carboxylic acids is the oxidation in the presence of halide ions, whereby the corresponding alkyl halides are obtained (eqs 26 and 27).47 Halodecarboxylations of acids are performed by addition of a molar equivalent of the metal halide (lithium, sodium, potassium chloride) to a carboxylic acid and LTA, the reactions being performed in boiling benzene solution.5,47 For the iododecarboxylation of acids, see Lead(IV) Acetate-Iodine.

Bis-decarboxylation of 1,2-dicarboxylic acids by LTA is a useful method for the introduction of alkenic bonds (eq 28).48 The reactions are performed in boiling benzene in the presence of pyridine or in DMSO. In some cases, LTA bis-decarboxylation can be effected by using acid anhydrides (eq 29).49 Bis-decarboxylation of 1,1-dicarboxylic acids yields the corresponding ketones (eq 30).50

Oxidative Transformations of Nitrogen-Containing Compounds.

The LTA oxidation of aliphatic primary amines containing an a-methylene group results in dehydrogenation to alkyl cyanides (eq 31).51 However, aromatic primary amines give symmetrical azo compounds in varying yield (eq 32).52

Primary amides react with LTA in the presence of alcohols to give the corresponding carbamates (eq 33), but in the absence of alcohol, isocyanates are formed.53

Aliphatic ketoximes, upon treatment with LTA in an inert solvent, undergo acetoxylation at the a-carbon producing 1-nitroso-1-acetoxyalkanes (eq 34),54 whereas hydrazones afford azoacetates (eq 35) or, when the reactions are performed in alcohol solvent, azo ethers.55 Arylhydrazines, N,N-disubstituted hydrazines,56 and N-amino compounds57 are oxidized by LTA to different products.

Other Applications.

By LTA oxidation of phenols, acetoxycyclohexadienones, quinones, and dimerization products can be formed.58 Alkyl sulfides,59 alkyl hydroperoxides,60 and organometallic compounds61 are also oxidized by LTA.

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Mihailo Lj. Mihailović & &ZZbreve;ivorad &CCbreve;eković

University of Belgrade, Yugoslavia

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