Baeyer–Villiger oxidation
| Baeyer-Villiger oxidation | |
|---|---|
| Named after | Adolf von Baeyer Victor Villiger |
| Reaction type | Organic redox reaction |
| Identifiers | |
| Organic Chemistry Portal | baeyer-villiger-oxidation |
| RSC ontology ID | RXNO:0000031 |
The Baeyer-Villiger oxidation (also called Baeyer-Villiger rearrangement) is an organic reaction that forms an ester from a ketone or a lactone from a cyclic ketone.[1] Peroxyacids or peroxides are used as the oxidant.[1] The reaction is named after Adolf Baeyer and Victor Villiger who first reported the reaction in 1899.[1]
Contents
Reaction mechanism
In the first step of the reaction mechanism, the peroxyacid protonates the oxygen of the carbonyl group.[1] This makes the carbonyl group more susceptible to attack by the peroxyacid.[1] In the next step of the reaction mechanism, the peroxyacid attacks the carbon of the carbonyl group forming what is known as the Criegee intermediate.[1] Through a concerted mechanism, one of the substituents on the ketone migrates to the oxygen of the peroxide group while a carboxylic acid leaves.[1] This migration step is thought to be the rate determining step.[2] Finally, deprotonation of the oxygen of the carbonyl group produces the ester.[1]
The products of the Baeyer-Villiger oxidation are believed to be controlled through both primary and secondary stereoelectronic effects.[3] The primary stereoelectronic effect in the Baeyer-Villiger oxidation refers to the necessity of the oxygen-oxygen bond in the peroxide group to be antiperiplanar to the group that migrates.[3] This orientation facilitates optimum overlap of the 𝛔 orbital of the migrating group to the 𝛔* orbital of the peroxide group.[1] The secondary stereoelectronic effect refers to the necessity of the lone pair on the oxygen of the hydroxyl group to be antiperiplanar to the migrating group.[3] This allows for optimum overlap of the oxygen nonbonding orbital with the 𝛔* orbital of the migrating group.[4] This migration step is also (at least in silico) assisted by two or three peroxyacid units enabling the hydroxyl proton to shuttle to its new position.[5]
The migratory ability is ranked tertiary ≻ secondary ≻ phenyl ≻ primary.[6] Allylic groups also migrate better than primary groups but not as well as secondary groups.[4] If there is an electron withdrawing group on the substituent, then it decreases the rate of migration.[7] There are two explanations for this trend in migration ability.[8] One explanation relies on the carbocation resonance structure of the Criegee intermediate.[8] Keeping this structure in mind, it makes sense that the substituent that can maintain positive charge the best would be most likely to migrate.[8] Tertiary groups are more stable carbocations than secondary groups, and secondary groups are more stable than primary.[9] Therefore, the tertiary ≻ secondary ≻ primary trend is observed.
Another explanation uses stereoelectronic effects and steric bulk to explain the trend.[10] As mentioned, the substituent that is antiperiplanar to the peroxide group in the transition state will be the group that migrates.[3] This transition state has a gauche interaction between the peroxyacid and the non-migrating substituent.[10] If the bulkier group is placed antiperiplanar to the peroxide group, the gauche interaction between the substituent on the forming ester and the carbonyl group of the peroxyacid will be reduced.[10] Thus, it is the bulkier group that ends up antiperiplanar to the peroxide group making it the group that migrates.[10] This explains the trend of tertiary ≻ secondary ≻ primary because tertiary groups are generally bulkier than secondary and primary groups.
Historical background
In 1899, Adolf Baeyer and Victor Villiger first published a demonstration of the reaction that we now know as the Baeyer-Villiger oxidation.[11][12] They used peroxymonosulfuric acid to make the corresponding lactones from camphor, menthone, and tetrahydrocarvone.[12][13]
There were three suggested reaction mechanisms of the Baeyer-Villiger oxidation that seemed to fit with observed reaction outcomes.[14] These three reaction mechanisms can really be split into two pathways of peroxyacid attack.[15] The first pathway has the peroxyacid attack the oxygen of the carbonyl group.[15] The second pathway has the peroxyacid attack the carbon of the carbonyl group.[15] The first pathway could lead to two possible intermediates: Baeyer and Villiger suggested a dioxirane intermediate, while Georg Wittig and Gustav Pieper suggested a peroxide intermediate with no dioxirane formation.[15] A second pathway was suggested by Rudolf Criegee.[15] In this pathway, the peracid attacks the carbonyl carbon producing what is now known as the Criegee intermediate.[15]
In 1953, William von Eggers Doering and Edwin Dorfman elucidated the correct pathway for the reaction mechanism of the Baeyer-Villiger oxidation by using oxygen-18 to label benzophenone.[14] The three different mechanisms each lead to a different distribution of labelled products. The Criegee intermediate leads to a product that is only labelled on the oxygen of the carbonyl group.[14] The product of the Wittig and Pieper intermediate is only labeled on the oxygen of the ester.[14] The Baeyer and Villiger intermediate leads to a 1:1 distribution of both of the above products.[14] The outcome of the labelling experiment supported the Criegee intermediate.[14] It is now believed that the mechanism follows the Criegee intermediate.[1]
Stereochemistry
The migration does not change the stereochemistry of the group that transfers.[16][17] Therefore, if it is a chiral group that migrates, the chirality of that group will not be changed.
Reagents
Although many different peroxyacids are used for the Baeyer-Villiger oxidation, some of the more common oxidants include meta-chloroperbenzoic acid (mCPBA) and trifluoroperacetic acid (TFPAA).[2] The reactivity differs depending on the choice of the peroxyacid.[4] The general trend of reactivity correlates to the strength of the corresponding acid (or alcohol in the case of the peroxides).[4] The stronger the acid, the more reactive will the corresponding peroxyacid be in performing the Baeyer-Villiger oxidation.[4] The trend of reactivity of some reagents is TFPAA ≻ 4-nitroperbenzoic acid ≻ mCPBA and performic acid ≻ peracetic acid ≻ hydrogen peroxide ≻ tert-butyl hydroperoxide.[4] The peroxides are much less reactive than the peroxyacids.[2] In fact, hydrogen peroxide requires a catalyst in order to be used as an oxidant in the Baeyer-Villiger oxidation.[6][18]
Limitations
The use of peroxyacids and peroxides when performing the Baeyer-Villiger oxidation can cause the undesirable oxidation of other functional groups.[19] Alkenes and amines are a few of the groups that can be oxidized.[19] However, methods have been developed that will allow for the tolerance of these functional groups.[19] For instance, if there is an alkene present in the ketone, the alkene could potentially undergo oxidation to the epoxide.[19] In general, electron-poor alkenes will prefer the Baeyer-Villiger oxidation, while electron-rich will prefer the epoxidation.[20] However, it may depend on the reagents that are used.[20] For example, there are methods that will selectively choose the formation of the epoxide or the ester.[21] In 1962, G. B. Payne reported that the use of hydrogen peroxide in the presence of a selenium catalyst will produce the epoxide, while use of peroxyacetic acid will form the ester.[21]
Modifications
Catalytic Baeyer-Villiger oxidation
There has been interest in making the Baeyer-Villiger oxidation work with hydrogen peroxide as an oxidant in the presence of a catalyst.[6] Using hydrogen peroxide as an oxidant makes the reaction more environmentally friendly as the waste produced would just be water.[6] The use of benzeneseleninic acid derivatives as a catalyst has been reported to give high selectivity with hydrogen peroxide as the oxidant.[22]
Baeyer-Villiger monooxygenases
Another way to create a catalytic Baeyer-Villiger oxidation is by using enzymes as the catalyst.[6] Baeyer-Villiger monooxygenases (BVMOs) use dioxygen to perform the Baeyer-Villiger oxidation.[6] These enzymes are capable of enantioselective oxidations of prochiral substrates.[6]
Asymmetric Baeyer-Villiger oxidation
There have been attempts to use organometallic catalysts to perform an enantioselective Baeyer-Villiger oxidation. [6] The first reported instance of an asymmetric Baeyer-Villiger oxidation on a prochiral ketone used dioxygen as the oxidant and a copper catalyst.[20] Other catalysts followed such as platinum and aluminum catalysts.[20]
Applications
Zoapatanol
Zoapatanol is a biologically active molecule that occurs naturally in the zeopatle plant.[23] The zeopatle plant has been used in Mexico to make a tea that can induce menstruation and labor.[23] In 1981, Vinayak Kane and Donald Doyle reported a synthesis of zoapatanol.[24][25] They used the Baeyer-Villiger oxidation to make a lactone that served as a crucial building block that ultimately led to the synthesis of zoapatanol.[24][25]
Steroids
Steroids are an important class of molecules for use in therapeutics.[26] For instance, testololactone has been identified as an anticancer agent.[26] In 2013, Alina Świzdor reported the transformation of dehydroepiandrosterone to testololactone by use of a fungus that produces Baeyer-Villiger monooxygenases.[26] The fungus formed testololactone from dehydroepiandrosterone via a Baeyer-Villiger oxidation.[26]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ The Role of Hydrogen Bonds in Baeyer-Villiger Reactions Shinichi Yamabe and Shoko Yamazaki J. Org. Chem.; 2007; 72(8) pp 3031–41; (Article) doi:10.1021/jo0626562
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 10.0 10.1 10.2 10.3 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 15.0 15.1 15.2 15.3 15.4 15.5 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 19.0 19.1 19.2 19.3 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 20.0 20.1 20.2 20.3 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 26.0 26.1 26.2 26.3 Lua error in package.lua at line 80: module 'strict' not found.
External links
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