Pyridinium Chlorochromate1

[26299-14-9]  · C5H6ClCrNO3  · Pyridinium Chlorochromate  · (MW 215.57)

(stable versatile oxidizing agent for many functional groups;1a can oxidize activated C-H bonds,2b C-C bonds,2b and C-B bonds;2b can halogenate enol silyl ethers2b)

Alternate Name: PCC.

Physical Data: mp 205 °C (dec).

Solubility: insol dichloromethane, benzene, diethyl ether; sol acetone, acetonitrile, THF.

Form Supplied in: yellow-orange solid; widely available.

Handling, Storage, and Precautions: the dry solid may be stored in contact with air, but in the absence of moisture. Reaction solvents should generally be purified and anhydrous. Reported to be a cancer suspect agent. Should be used in a fume hood.

Oxidation of Primary and Secondary Alcohols.

PCC has the capability to convert primary and secondary alcohols to aldehydes and ketones with great efficiency. The yields are typically equal to or greater than those obtained by the Collins method (see Dipyridine Chromium(VI) Oxide), which customarily requires a fivefold excess of reagent. PCC oxidations are normally carried out in dichloromethane with 1.5 equiv of reagent suspended in the organic solvent at room temperature and are usually complete within 1-2 h (eqs 1-4).2a

More polar solvents, such as acetonitrile or acetone, in which PCC has higher solubility, lead to longer reaction times. Overoxidations are rare, but acids can be directly prepared from aldehydes with stoichiometric Sodium Cyanide and PCC in THF.3 The reagent shows a slightly acidic character. With compounds bearing acid-sensitive groups, the reaction can be buffered with powdered sodium acetate. However, cis-trans isomerization of some allylic alcohols can occur under these conditions (eq 5).2a

PCC appears to be particularly suitable for moderate to large scale preparations, and it has proven to be the reagent of choice for the oxidation of primary alcohols to aldehydes and of secondary alcohols to ketones.2a,b Good yields of ketones and aldehydes are regularly obtained.2a,b Advantages of PCC include the fact that it does not possess the hygroscopic nature of the chromium trioxide-pyridine complex, and it is prepared via a much less hazardous procedure. However, workup and removal of chromium-containing byproducts is often tedious and difficult with the oxidant.2a Several modifications of PCC have been developed to increase both the efficiency and the selectivity of the reagent, and to reduce side reactions.4 The chlorochromate anion has been supported on a polymeric matrix. Poly(vinylpyridinium chlorochromate), easily prepared, is a recyclable and useful reagent in the oxidation of alcohols to carbonyl compounds in 60-100% yield. This reagent has advantages with regard to product isolation, although its use is limited by the scale of the reaction that can be performed conveniently.5 Furthermore, the reactivity of PCC in the oxidation of alcohols can be increased by adding anhydrous Acetic Acid as catalyst to the reaction mixture6 or by using sonochemical conditions.7 The addition of Molecular Sieves in the oxidation of various alcohols results in a dramatic rate enhancement.8a Comparative studies indicate that 3 Å sieves give the best results (3 Å > 4 Å > 10 Å > 5 Å).8b PCC can perform oxidations of slow-reacting alcohols, such as the secondary hydroxyl groups in carbohydrates and nucleosides. These reactions proceed readily in refluxing benzene, but they are very slow in dichloromethane.9 The oxidations are conveniently carried out by adding 3 Å molecular sieves to a suspension of PCC in dichloromethane, providing an efficient synthesis of keto sugars and keto nucleosides (eq 6).8a

The use of molecular sieves in many types of oxidations is now commonplace. 2-Nitro alkanols, available by a nitro-aldol reaction, can be oxidized by PCC and molecular sieves to a-nitro ketones in 60-85% yield. Usually the starting compounds undergo a retro-aldol reaction under acidic conditions (eq 7).8c On the other hand, (Z)-2-butene-1,4-diols are converted by reaction with PCC and molecular sieves into substituted furans in 35-90% yields (eq 8).10

Synthetically useful changes in the properties of pyridinium chlorochromate have been introduced in the oxidation of hydroxy steroids. PCC, in refluxing benzene, is a convenient reagent for the oxidation and concomitant isomerization of steroidal D5-3b-alcohols to the corresponding D4-3-ketones in high yield.11 However, in the presence of anhydrous calcium carbonate, PCC can effect the high yield, selective oxidation of steroidal homoallylic alcohols to the corresponding b,d-unsaturated ketones.12 Remarkable selectivity is obtained in the oxidation of steroid allylic alcohols by adding 2% of Pyridine to the reagent in dichloromethane at 2-3 °C. Quasiequatorial allylic alcohols are oxidized faster than nonallylic axial ones (eq 9).13 Similar properties are found for the combination of PCC and pyrazole (2%),14 PCC and 2,3-dimethylpyrazole (2%),15 and PCC and benzotriazole (2%).16

Oxidation of Unsaturated and Cyclopropyl Tertiary Alcohols.

Tertiary allylic alcohols, obtained by the addition of Vinyllithium or vinylmagnesium chloride to a ketone, undergo oxidative rearrangements to a,b-unsaturated aldehydes in good yields in the presence of PCC. The absence of diene products is remarkable in view of the sensitivity of the starting compounds towards dehydration. The process involves a two-carbon chain-lengthening of the starting ketones, and is synthetically equivalent to a direct crossed-aldol condensation between a ketone and acetaldehyde (eq 10).17

The oxidation of tertiary cyclopropyl alcohols leads to the formation of the corresponding b,g-unsaturated ketones (eq 11). The reaction represents an excellent method for converting ketones to chain-extended b,g-enones, since the starting compounds can be prepared by addition of cyclopropyl organometallic reagents to ketones.18

Cyclic tertiary allylic alcohols, prepared by 1,2-addition of alkyllithium reagents to a,b-enones, are oxidized by PCC to transposed g-alkyl-a,b-unsaturated ketones. Yields are excellent in the case of cyclic alcohols, but only moderate with acyclic alcohols. The overall result of this reaction sequence is an efficient method for alkylative 1,3-carbonyl transposition (eq 12).19a-d

When the tertiary allylic alcohol is suitably disposed near an alkene double bond, PCC is able to perform synthetically useful substituent-directed oxidations, the result of which depends on the relative position of the two groups. Cyclic tertiary g-hydroxy alkenes can undergo a regio- and stereoselective cyclization, giving a single b-hydroxy cyclic ether through an intramolecular C=C oxidation initiated by PCC (eq 13),20a while linear g,d-dihydroxy alkenes are cyclized to cis-2,5-disubstituted tetrahydrofurans (eq 14).20b

On the other hand, the oxidation of tertiary g-hydroxy terminal alkenes brings about both oxidative cyclization and fragmentation of the double bond with loss of one carbon atom, and thus provides an efficient route to g-lactones (eq 15).21

When a tertiary alcohol is homoallylic, reaction with PCC can yield a ketone resulting from oxidative fragmentation. The cyclic tertiary homoallylic alcohol shown in eq 16 furnishes a mixture of two isomeric cyclopentenones in a 1:1.7 ratio in 62% yield.22 Another interesting example of PCC reactivity is the conversion of the acyclic tertiary homoallylic alcohol in eq 17 to a keto lactone, albeit in moderate yield. A large excess of oxidant is necessary.21d

If a homoallylic alcohol is secondary, reaction with PCC leads to the formation of a mixture of g-hydroxy-a,b-unsaturated enones, through a g-oxy functionalization (eq 18).23

Oxidative Cationic Cyclization.

The mild acidic character of PCC has been used to advantage in an essential one step conversion of (-)-citronellol to (-)-pulegone (eq 19).24

The oxidative cationic cyclization is useful for the annulation of linear and cyclic unsaturated alcohols or aldehydes to form cyclohexenones (eqs 20 and 21).25

A limitation is that this reaction cannot be used for formation of cyclopentenones. The reaction is also limited to preparation of b,b-disubstituted a,b-unsaturated cyclohexenones: in fact, cyclization is only observed with substrates capable of affording a tertiary cation as the initial cyclic intermediate. Other substrates (e.g. 1) do not undergo cyclization, even under more forcing conditions. However, the process represents a milder and more efficient alternative to the two-step cationic cyclization for preparing a,b-unsaturated enones.26

Oxidation of Activated C-H Bonds.

PCC is of value in the allylic oxidation of compounds containing activated methylenes. 5,6-Dihydropyrans (eq 22)27 and 2,5-dihydrofurans (eqs 23 and 24) are oxidized to the corresponding lactones.28 The reaction is limited to cyclic benzylic and allylic ethers.

PCC is the reagent of choice in the allylic oxidation of D5-steroids to 7-keto derivatives. The reaction can be carried out using PCC in refluxing benzene or PCC in DMSO solution at 100 °C (eq 25).29 These solvents are claimed to be superior to dichloromethane, which must be used in large excess in this reaction.30

The capability of PCC to selectively perform allylic oxidation of methylene groups has frequently been utilized,31 as in the synthesis of a key intermediate related to the taxol ring A (eq 26).31b

In a similar fashion, the smooth oxidation of benzylic hydrocarbons to aryl ketones can be achieved by adding a powdered and homogenized mixture of PCC and Celite to a benzene solution of the substrates (eqs 27 and 28).32 The reaction has been successfully applied to a variety of compounds, such as 1,2,3,4-tetrahydrophenanthrene,33 12-methoxypodocarpa-8,11,13-triene,34 and biscyclophanes.35

The transformation of activated methylene groups to ketones shows particular applicability to the conversion of benzyl alkyl ketones to the corresponding 1,2-diketones in high yields (eq 29).28 Since the starting materials appear to be sensitive to the acidic PCC, anhydrous pyridine must be added to the reaction. This oxidation fails with dialkyl ketones.28

PCC is able to oxidize cyclic 1,4-dienes to dienones (eq 30).36 PCC does not effect oxidation of isolated double bonds, or of more reactive systems such as diphenylmethane or allylbenzene. Complementary selectivity (1:3) is achieved with the Collins reagent.

Phenyloxiranes can undergo a one step C-C bond cleavage to carbonyl compounds by reaction with PCC and molecular sieves (52-75%) (eq 31).37 The presence of a phenyl group seems to be essential for efficient C-C cleavage. Under similar conditions, alkyl-substituted oxiranes are prevalently converted into a-hydroxy ketones (eq 32).

PCC is reported to perform oxidative fragmentation of carbon-carbon bonds of 1,2-diols to give carbonyl compounds in excellent yields (eq 33).38 Hydroxymethyl cyclic ethers also undergo fragmentation; tetrahydrofuranmethanol derivatives lead to g-lactones in 52-95% yield (eqs 34 and 35),39 while 5-hydroxymethyl groups of 1-methoxy-2,3-O-isopropylidene-D-ribose are cleaved to g-lactone derivatives in 50-60% yield (eq 36).40 These transformations usually require more vigorous conditions than those needed for alcohol oxidation.

Oxidation of C=C Activated Double Bonds.

PCC can behave as an oxidizing, weakly electrophilic species, capable of attacking particularly nucleophilic alkenes and of bringing about interesting reactions. PCC oxidizes linear and cyclic enol ethers to esters and lactones in 75-95% yield (eqs 37 and 38).41

According to the reported mechanism, the key step involves a 1,2-hydride shift from the a- to the b-carbon of the ether. When the enol ethers are a,a-disubstituted, there is no possibility for such a hydride shift; reaction with PCC then yields a smooth oxidative cleavage of the nucleophilic alkene to esters or keto lactones in 45-90% yield (eqs 39 and 40).42

The reaction can be applied only to cyclic enol ethers. However, the simplicity of the procedure and the high yields obtained make the methodology synthetically useful for preparative purposes. In fact, the reaction has been extensively applied as a general approach to g-lactones from the corresponding a-alkylidene five-membered enol ethers using as conditions treatment with 4 equiv of PCC and celite at room temperature in dichloromethane (eqs 41 and 42).43 An important feature of this procedure is that, under the reaction conditions, other isolated carbon-carbon double bonds and benzylic groups present in the molecule are not affected.

PCC has proven to be the reagent of choice for the selective oxidation of a specific class of cyclic enol ethers: 1,4-dioxenyl alcohols. The allylic alcohols can be easily obtained by addition of 2-lithio-5,6-dihydro-1,4-dioxin to ketones and aldehydes. Reaction with PCC thus provides a method for one-carbon homologation of ketones and aldehydes to a-hydroxy acids (39-61%) (eq 43) and a-keto acids (54%) (eq 44), respectively.44 The oxidation occurs regiospecifically at the dioxene site. While the oxidation of the allylic alcohols occurs in a few minutes, the a,b-unsaturated ketones require a longer reaction time, owing to the deactivation of the carbon-carbon double bond.

Aryl-substituted alkenes can be selectively cleaved in the presence of alkyl-substituted alkenes. Treatment of acyclic aryl alkenes with PCC and celite in dichloromethane under reflux results in oxidative opening of the carbon-carbon double bond to the corresponding carbonyl compounds in 72-90% yield (eq 45).45 Alkyl-substituted acyclic alkenes are in general unreactive towards PCC, while cyclic alkenes undergo allylic oxidation, albeit in low yield (eq 46).45

PCC has demonstrated its particular reactivity towards furan derivatives, which are well known for their sensitivity to oxidizing and electrophilic reagents.46 Thus 5-methyl-2-furylcarbinols undergo oxidative ring enlargement to hexenuloses in very high yields (90-94%) (eq 47).47

Another application involves 5-bromo-2-furylcarbinols, which are converted by PCC into g-hydroxybutenolides in 60-75% yield (eq 48).48 Both conversions point out an unusual regiospecific reactivity of pyridinium chlorochromate, since only the oxidation of the furan ring occurs, in spite of the presence of a secondary alcoholic function, which remains untouched. Different behavior is observed when the heteroaromatic nucleus is deactivated by the presence of a nitro group; in this case, PCC oxidizes the alcohol function, leading to alkyl 5-nitro-2-furyl ketones (eq 49).49

A further and important application of PCC involves the conversion of 2,5-dialkylfurans, via oxidative ring fission, to trans-a,b-unsaturated 1,4-dicarbonyl compounds in a very simple manner. The reagent must be utilized in a large excess in dichloromethane at reflux for 24 h. Invariably, the products are obtained with trans configuration. The cis-isomers are formed first and are then isomerized by the acidic reagent (eq 50).50 Lower yields of enedicarbonyl compounds are obtained from 2-alkylfurans (eq 51).50

PCC shows a remarkable reactivity towards more nucleophilic furans, such as 2-alkylthiofurans, which are rapidly converted by action of the reagent in dichloromethane at room temperature for 2 h into S-alkyl 4-oxo-2-alkenethioates in high yields (eq 52).51 (Z)-(E) Isomerization can be observed with longer reaction times, due to the acidic character of PCC. 2-Methoxy-5-methylfuran, which requires only 10 min of reaction, affords methyl (Z)-4-oxo-2-pentenoate, without the (E)-isomer being detectable (eq 53).51

Oxidation of Carbon-Boron Bonds.

PCC is a superior reagent for the oxidation of organoboranes to carbonyl derivatives by a convenient and mild procedure. Trialkylboranes, obtained by treatment of linear and cyclic di- and trisubstituted alkenes with Boron Trifluoride-Lithium Borohydride or Borane-Tetrahydrofuran, afford the corresponding ketones by reaction with PCC in high yields (eqs 54 and 55).52,53 Similar oxidation of dialkylchloroboranes, derived from cyclic alkenes by reaction with Monochloroborane-Dimethyl Sulfide, gives ketones in 70-85% yield (eq 56).54

The success of this method for the conversion of terminal alkenes into aldehydes often depends on the nature of the hydroborating agent. In view of the poor regioselectivity (only 94% primary alkyl groups) and functional group tolerance of borane-THF and borane-dimethyl sulfide,54 Disiamylborane is frequently utilized as a more selective hydroborating agent. Alkyldisiamylboranes are oxidized by PCC to aldehydes in 55-72% yield. The ester groups are compatible with the reaction (eq 57).54,55b The need for a large excess of PCC (6 equiv) for the oxidation of alkyldisiamylboranes can be avoided by using dialkylhaloboranes, which can be prepared with excellent regioselectivity (>99% primary alkyl groups) by hydroboration of terminal alkenes with monochloroborane-dimethyl sulfide complex. Dialkylchloroboranes are hydrolized in the presence of pyridine, and the resulting boronic anhydride is oxidized with PCC (3 equiv) to aldehydes in satisfactory yields (eq 58).54

The selective formation of aldehydes from various types of double bonds in dienes can be conveniently carried out via PCC oxidation of the resulting organoboranes, prepared by utilizing either disiamylborane (eq 59) or Chloro(thexyl)borane (eq 60) for the regioselective monohydroboration of the diene.54

Other organoboranes, such as trialkyl borates and trialkyl boroxins, can also be oxidized with PCC. Trialkyl borates are rapidly prepared either by esterification of boric acids or by reaction of alcohols with Borane-Dimethyl Sulfide. These intermediates, on reaction with PCC in boiling dichloromethane, are oxidized to give good yields of aldehydes and ketones (eq 61).56 The method does not seem to have significant advantages over the direct oxidation of the alcohols, but it may prove useful for the one-pot conversion of carboxylic acids into the corresponding aldehydes. Both aliphatic and aromatic carboxylic acids are easily reduced by borane-dimethyl sulfide to trialkoxyboroxins, which are smoothly oxidized to aldehydes with PCC (eq 62).57

Oxidation of Silicon-Containing Molecules.

PCC is able to effect the deprotection-oxidation of p-hydroquinone silyl ethers to p-quinones, except where there are electron-withdrawing substituents on the aromatic ring. The reaction proceeds both with bis(trimethylsilyl) and bis(t-butyldimethylsilyl) ethers of p-hydroquinones in 60-99% yield. The bis(trimethylsilyl) ethers are slightly more reactive than the bis(t-butyldimethylsilyl) ethers (eq 63).58

The deprotection-oxidation method can be applied to the preparation of ketones from trimethylsilyl-protected secondary alcohols.59 However, the Pyridinium Dichromate-Chlorotrimethylsilane combination seems to be more efficient for the transformation. A related application of PCC involves its capability to selectively convert 1-trialkylsilyl-1,2-diols into aldehydes, presumably by electrofugal loss of the silicon moiety (eq 64).60

Both cyclic and linear enol silyl ethers can be converted to a-iodocarbonyl compounds by treatment with the pyridinium chlorochromate-Iodine system in dichloromethane in 76-100% yield (eq 65). The yields of a-iodocarbonyl compounds from enol methyl ethers and dihydropyran are lower (19-53%) (eq 66). The method fails with enamines.61

Oxidation of Oximes.

PCC can be conveniently employed for oxidative cleavage of oximes to carbonyl compounds. Aldehydes and ketones can be recovered from the oxime derivatives by treatment with PCC in dichloromethane at room temperature for 12-24 h in 30-85% yield.62 PCC-Hydrogen Peroxide is more effective as a deblocking agent: cleavage of oximes occurs within 10 min at 0-10 °C when 30% hydrogen peroxide is added to an acetone solution of PCC. Ketoximes are converted to ketones in 55-88% yield. For aldoximes, over-oxidation to acids has been observed.63 On the other hand, aryl hydroxylamines can be oxidized to aryl C-nitroso compounds by treatment with PCC in tetrahydrofuran in 50-90% yield.64

Oxidation of Sulfur-Containing Molecules.

PCC dimerizes aromatic, but not aliphatic, thiols to their corresponding disulfides in good yields.65 Sulfides may undergo oxidation to give sulfones.4

Changes in the properties and reactivity of pyridinium chlorochromate have been brought about by altering the amine ligand associated with the chlorochromate anion. It has been found that heterocyclic chlorochromates show a trend where their strength as oxidant is inversely proportional to the donor strength of the heterocyclic ligand. Several aromatic amines have been examined, which give rise to the related chlorochromate-derived reagents. Enhanced reactivity is found for complexes containing a bidentate ligand.4

Related Reagents.

Pyridinium Chlorochromate-Alumina.

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Giovanni Piancatelli

University of Rome La Sapienza and CNR, Rome, Italy

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