Acetic Anhydride1

[108-24-7]  · C4H6O3  · Acetic Anhydride  · (MW 102.09)

(useful for the acetylation of alcohols,2 amines,3 and thiols,4 oxidation of alcohols,5 dehydration,6 Pummerer7 reaction, Perkin8 reaction, Polonovski9 reaction, N-oxide reaction,10 Thiele11 reaction, ether cleavage,12 enol acetate formation,13 gem-diacetate formation14)

Physical Data: bp 138-140 °C; mp -73 °C; d 1.082 g cm-3.

Solubility: sol most organic solvents. Reacts with water rapidly and alcohol solvents slowly.

Form Supplied in: commercially available in 98% and 99+% purities. Acetic anhydride-d6 is also commercially available.

Analysis of Reagent Purity: IR, NMR.15

Preparative Methods: acetic anhydride is prepared industrially by the acylation of Acetic Acid with Ketene.1b A laboratory preparation of acetic anhydride involves the reaction of sodium acetate and Acetyl Chloride followed by fractional distillation.16

Purification: adequate purification is readily achieved by fractional distillation. Acetic acid, if present, can be removed by refluxing with CaC2 or with coarse magnesium filings at 80-90 °C for 5 days. Drying and acid removal can be achieved by azeotropic distillation with toluene.17

Handling, Storage, and Precautions: acetic anhydride is corrosive and a lachrymator and should be handled in a fume hood.

Acetylation.

The most notable use of acetic anhydride is for the acetylation reaction of alcohols,2 amines,3 and thiols.4 Acids, Lewis acids, and bases have been reported to catalyze the reactions.

Alcohols.

The most common method for acetate introduction is the reaction of an alcohol with acetic anhydride in the presence of pyridine.2 Often, Pyridine is used as the solvent and reactions proceed nearly quantitatively (eq 1).

If the reaction is run at temperatures lower than 20 °C, primary alcohols can be acetylated over secondary alcohols selectively.18 Under these conditions, tertiary alcohols are not acylated. Most alcohols, including tertiary alcohols, can be acylated by the addition of DMAP (4-Dimethylaminopyridine) and Acetyl Chloride to the reaction containing acetic anhydride and pyridine. In general, the addition of DMAP increases the rate of acylation by 104 (eq 2).19

Recently, Vedejs found that a mixture of Tri-n-butylphosphine and acetic anhydride acylates alcohols faster than acetic anhydride with DMAP.20 However, the combination of acetic anhydride with DMAP and Triethylamine proved superior. It is believed that the Et3N prevents HOAc from destroying the DMAP catalyst.

Tertiary alcohols have been esterified in good yield using acetic anhydride with calcium hydride or calcium carbide.21 t-Butanol can be esterified to t-butyl acetate in 80% yield under these conditions. High pressure (15 kbar) has been used to introduce the acetate group using acetic anhydride in methylene chloride.22 Yields range from 79-98%. Chemoselectivity is achieved using acetic anhydride and Boron Trifluoride Etherate in THF at 0 °C. Under these conditions, primary or secondary alcohols are acetylated in the presence of phenols.23

a-D-Glucose is peracetylated readily using acetic anhydride in the presence of Zinc Chloride to give a-D-glucopyranose pentaacetate in 63-72% yield (eq 3).24

Under basic conditions, a-D-glucose can be converted into b-D-glucopyranose pentaacetate in 56% yield (eq 4).

In the food and drug industry, high-purity acetic anhydride is used in the manufacture of aspirin by the acetylation of salicylic acid (eq 5).25

Amines.

The acetylation of amines has been known since 1853 when Gerhardt reported the acetylation of aniline.3 Acetamides are typically prepared by the reaction of the amine with acetic anhydride (eq 6).

A unique method for selective acylation of secondary amines in the presence of primary amines involves the use of 18-Crown-6 with acetic anhydride and triethylamine.26 It is believed that the 18-crown-6 complexes primary alkylammonium salts more tightly than the secondary salts, allowing selective acetylation.

In some cases, tertiary amines undergo a displacement reaction with acetic anhydride. A simple example involves the reaction of benzyldimethylamine with acetic anhydride to give dimethylacetamide and benzylacetate (eq 7).27

Allylic tertiary amines can be displaced by the reaction of acetic anhydride and sodium acetate.28 The allylic acetate is the major product, as shown in eq 8.

Cyclic benzylic amines may undergo ring opening upon heating with acetic anhydride (eq 9).29

a-Amino acids react with acetic anhydride in the presence of a base to give 2-acetamido ketones.30 This reaction is known as the Dakin-West reaction (eq 10) and is believed to go through a oxazolone mechanism. The amine base of choice is 4-dimethylaminopyridine. Under these conditions, the reaction can be carried out at room temperature in 30 min.

Cyclic b-amino acids rearrange to a-methylene lactams upon treatment with acetic anhydride, as shown in eq 11.31

Thiols.

S-Acetyl derivatives can be prepared by the reaction of acetic anhydride and a thiol in the presence of potassium bicarbonate.4 Several disadvantages to the S-acetyl group in peptide synthesis include b-elimination upon base-catalyzed hydrolysis. Also, sulfur to nitrogen acyl migration may be problematic.

Oxidation.

The oxidation of primary and secondary alcohols to the corresponding carbonyl compounds can be achieved using Dimethyl Sulfoxide-Acetic Anhydride.5 The reaction proceeds through an acyloxysulfonium salt as the oxidizing agent (eq 12).

The oxidations often proceed at room temperature, although long reaction times (18-24 h) are sometimes required. A side product is formation of the thiomethyl ethers obtained from the Pummerer rearrangement.

If the alcohol is unhindered, a mixture of enol acetate (from ketone) and acetate results (eq 13).32

The oxidation of carbohydrates can be achieved by this method, as Hanessian showed (eq 14).33

Aromatic a-diketones can be prepared from the acyloin compounds; however, aliphatic diketones cannot be prepared by this method.34 The reaction proceeds well in complex systems without epimerization of adjacent stereocenters, as in the yohimbine example (eq 15).35 This method compares favorably with that of Dimethyl Sulfoxide-Dicyclohexylcarbodiimide.

Dehydration.

Many functionalities are readily dehydrated upon reaction with acetic anhydride, the most notable of which is the oxime.6 Also, dibasic acids give cyclic anhydrides or ketones, depending on ring size.36

An aldoxime is readily converted to the nitrile as shown in eq 16.37

When oximes of a-tetralones are heated in acetic anhydride in the presence of anhydrous Phosphoric Acid, aromatization occurs as shown in eq 17.38

Oximes of aliphatic ketones lead to enamides upon treatment with acetic anhydride-pyridine, as shown in the steroid example in eq 18.39

Upon heating with acetic anhydride, dibasic carboxylic acids lead to cyclic anhydrides of ring size 6 or smaller. Diacids longer than glutaric acid lead to cyclic ketones (eq 19).36

N-Acylanthranilic acids also cyclize when heated with acetic anhydride (eq 20). The reaction proceeds in 81% yield with slow distillation of the acetic acid formed.40

Pummerer Reaction.

In 1910, Pummerer7 reported that sulfoxides react with acetic anhydride to give 2-acetoxy sulfides (eq 21). The sulfoxide must have one a-hydrogen. Alternative reaction conditions include using Trifluoroacetic Anhydride and acetic anhydride.

b-Hydroxy sulfoxides undergo the Pummerer reaction upon addition of sodium acetate and acetic anhydride to give a,b-diacetoxy sulfides. These compounds are easily converted to a-hydroxy aldehydes (eq 22).41

2-Phenylsulfonyl ketones rearrange in the presence of acetic anhydride-sodium acetate in toluene at reflux to give S-aryl thioesters (eq 23).42 Upon hydrolysis, an a-hydroxy acid is obtained.

In the absence of sodium acetate, 2-phenylsulfonyl ketones give the typical Pummerer product. Since b-keto sulfoxides are available by the reaction of esters with the dimsyl anion, this overall process leads to one-carbon homologated a-hydroxy acids from esters (eq 24).42

Also, 2-phenylsulfonyl ketones can be converted to a-phenylthio-a,b-unsaturated ketones via the Pummerer reaction using acetic anhydride and a catalytic amount of Methanesulfonic Acid (eq 25).43

The Pummerer reaction has been used many times in heterocyclic synthesis as shown in eq 26.

The Pummerer rearrangement of 4-phenylsulfinylbutyric acid with acetic anhydride in the presence of p-Toluenesulfonic Acid leads to butanolide formation (eq 27).44 Oxidation with m-Chloroperbenzoic Acid followed by thermolysis then leads to an unsaturated compound.

An unusual Pummerer reaction takes place with penicillin sulfoxide, leading to a ring expansion product as shown in eq 28.45

This led to discovery of the conversion of penicillin V and G to cephalexin,46 a broad spectrum orally active antibiotic (eq 29).

A Pummerer-type reaction was carried out on a dithiane protecting group to liberate the corresponding ketone (eq 30).47 These were the only reaction conditions which provided any of the desired ketone.

Perkin Reaction.

The Perkin reaction,8 developed by Perkin in 1868, involves the condensation of an anhydride and an aldehyde in the presence of a weak base to give an unsaturated acid (eq 31).

The reaction is often used for the preparation of cinnamic acids in 74-77% yield (eq 32).

Aliphatic aldehydes give low or no yields of acid. Coumarin can be prepared by a Perkin reaction of salicylaldehyde and acetic anhydride in the presence of triethylamine (eq 33).

Polonovski Reaction.

In the Polonovski reaction,9 tertiary amine oxides react with acetic anhydride to give the acetamide of the corresponding secondary amine (eq 34).

In nonaromatic cases, the Polonovski reaction gives the N-acylated secondary amine as the major product and the deaminated ketone as a minor product (eq 35).48

This reaction has been extended to a synthesis of 2-acetoxybenzodiazepine via an N-oxide rearrangement (eq 36).

An application of the Polonovski reaction forms b-carbonylenamines. N-Methylpiperidone is reacted with m-CPBA followed by acetic anhydride and triethylamine to give the b-carbonyl enamine (eq 37).49

Reaction with N-Oxides.

Pyridine 1-oxide reacts with acetic anhydride to produce 2-acetoxypyridine, which can be hydrolyzed to 2-pyridone (eq 38).10

Open chain N-oxides, in particular nitrones, rearrange to amides (almost quantitatively) under acetic anhydride conditions (eq 39).50

Heteroaromatic N-oxides with a side chain react with acetic anhydride to give side-chain acyloxylation (eq 40).51

This reaction has been used in synthetic chemistry as the method of choice to form heterocyclic carbinols or aldehydes.

Thiele Reaction.

The Thiele reaction converts 1,4-benzoquinone to 1,2,4-triacetoxybenzene using acetic anhydride and a catalytic amount of Sulfuric Acid.11 Zinc chloride has been used without advantage. In this reaction, 1,4-addition to the quinone is followed by enolization and acetylation to give the substituted benzene (eq 41).

With unhindered quinones, BF3 etherate is a more satisfactory catalyst but hindered quinones require the more active sulfuric acid catalyst. 1,2-Naphthoquinones undergo the Thiele reaction with acetic anhydride and sulfuric acid or boron trifluoride etherate (eq 42).

Ether Cleavage.

Dialkyl ethers can be cleaved with acetic anhydride in the presence of pyridine hydrochloride or anhydrous Iron(III) Chloride. In both cases, acetate products are produced. As shown in eq 43, the tricyclic ether is cleaved by acetic anhydride and pyridine hydrochloride to give the diacetate in 93% yield.12

Simple dialkyl ethers react with iron(III) chloride and acetic anhydride to produce compounds where both R groups are converted to acetates (eq 44).52

Cleavage of allylic ethers can occur using acetic anhydride in the presence of iron(III) chloride (eq 45). The reaction takes place without isomerization of a double bond, but optically active ethers are cleaved with substantial racemization.53

Cleavage of aliphatic ethers occurs with the reaction of acetic anhydride, boron trifluoride etherate, and Lithium Bromide (eq 46). The ethers are cleaved to the corresponding acetoxy compounds contaminated with a small amount of unsaturated product.54 In some cases, the lithium halide may not be necessary.

Cyclic ethers are cleaved to o-bromoacetates using Magnesium Bromide and acetic anhydride in acetonitrile (eq 47).55

The reaction occurs with inversion of configuration, as shown in eq 48.

Trimethylsilyl ethers are converted to acetates directly by the action of acetic anhydride-pyridine in the presence of 48% HF or boron trifluoride etherate (eq 49).56

Miscellaneous Reactions.

Primary allylic alcohols can be prepared readily by the action of p-toluenesulfonic acid in acetic anhydride-acetic acid on the corresponding tertiary vinyl carbinol, followed by hydrolysis of the resulting acetate.57 The vinyl carbinol is readily available from the reaction of a ketone with a vinyl Grignard reagent. Overall yields of allylic alcohols are very good (eq 50).

Enol lactonization occurs readily on an a-keto acid using acetic anhydride at elevated temperatures.58 The reaction shown in eq 51 proceeds in 89% yield. In general, acetic anhydride is superior to acetyl chloride in this reaction.59

Aliphatic aldehydes are easily converted to the enol acetate using acetic anhydride and potassium acetate (eq 52).13 This reaction only works for aldehydes and is the principal reason for the failure of aldehydes to succeed in the Perkin reaction. Triethylamine and DMAP may also catalyze the reaction.

A cyclopropyl ketone is subject to homoconjugate addition using acetic anhydride/boron trifluoride etherate. Upon acetate addition, the enol is trapped as its enol acetate (eq 53).60

When an aldehyde is treated with acetic anhydride/anhydrous iron(III) chloride, geminal diacetates are formed in good to excellent yields.14 Aliphatic and unsaturated aldehydes can be used in this reaction as shown in eqs 54-56. Interestingly, if an a-hydrogen is present in an unsaturated aldehyde, elimination of the geminal diacetate product gives a 1-acetoxybutadiene.

If an aldehyde is treated with acetic anhydride in the presence of a catalytic amount of Cobalt(II) Chloride, a diketone is formed (eq 57).61

However, if 1.5 equiv of cobalt(II) chloride is added, the geminal diacetate is formed (eq 58).62

Apparently, the reaction in eq 58 occurs only when the starting material is polyaromatic or with compounds whose carbonyl IR frequency is less than 1685 cm-1.

Acetic anhydride participates in several cyclization reactions. For example, enamines undergo a ring closure when treated with acetic anhydride (eq 59).63

o-Diamine compounds also cyclize when treated with acetic anhydride (eq 60).64

Twistane derivatives were obtained by the reaction of a decalindione with acetic anhydride, acetic acid, and boron trifluoride etherate (eq 61).65

A condensation/cyclization reaction between an alkynyl ketone and a carboxylic acid in the presence of acetic anhydride/triethylamine gives a butenolide (eq 62).66

A few rearrangement reactions take place with acetic anhydride. A Claisen rearrangement is involved in the formation of the aromatic acetate in eq 63.67 The reaction proceeds in 44% yield even after 21 h at 200 °C.

Complex rearrangements have occured using acetic anhydride under basic conditions, as shown in eq 64.68

Aromatization occurs readily using acetic anhydride. Aromatization of a-cyclohexanones occurs under acidic conditions to lead to good yields of phenols (eq 65).69 However, in totally unsubstituted ketones, aldol products are formed.

Aromatization of 1,4- and 1,2-cyclohexanediones leads to cresol products (eq 66) in over 90% yield.70

Alkynes and allenes are formed by the acylation of nitrimines using acetic anhydride/pyridine with DMAP as catalyst (eqs 67 and 68).71 Nitrimines are prepared by nitration of ketoximes with nitrous acid.

Lastly, acetic anhydride participates in the Friedel-Crafts reaction.72 Polyphosphoric Acid is both reagent and solvent in these reactions (eq 69).


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Regina Zibuck

Wayne State University, Detroit, MI, USA



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