Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Named Reactions/Baeyer-Villiger Rearrangement

The Baeyer-Villiger oxidation is an organic reaction in which a ketone is oxidized to an ester by treatment with peroxy acids or hydrogen peroxide. Adolf von Baeyer Baeyer A. Villiger, V. Einwirkung des Caro'schen Reagens auf Ketone Ber. 1899 32 3 3625–3633 10.1002/cber.189903203151 http://www3.interscience.wiley.com/cgi-bin/abstract/112379267/ABSTRACT abstract Adolf von Baeyer Baeyer A. Villiger, V. Ueber die Einwirkung des Caro'schen Reagens auf Ketone Ber. 1900 33 1 858–864 10.1002/cber.190003301153 http://www3.interscience.wiley.com/cgi-bin/abstract/112334541/ABSTRACT abstract Key features of the Baeyer-Villiger oxidation are its stereospecificity and predictable regiochemistry. Crudden, C. M.; Chen, A. C.; Calhoun, L. A. A Demonstration of the Primary Stereoelectronic Effect in the Baeyer-Villiger Oxidation of α-Fluorocyclohexanones Angew. Chem. Int. Ed. 2000 39 16 2851–2855 10.1002/1521-3773(20000818)39:16<2851::AID-ANIE2851>3.0.CO;2-Y It is named after the German chemist Johann Friedrich Wilhelm Adolf von Baeyer (1835-1917) and the Swiss chemist Victor Villiger (1868-1934). 400px|center|The Baeyer-Villiger oxidation Reagents typically used to carry out this rearrangement are meta-chloroperoxybenzoic acid (mCPBA), peroxyacetic acid, or peroxytrifluoroacetic acid. Burton, J.W.; Clark, J.S.; Derrer, S.; Stork, T.C.; Bendall, J.G.; Holmes, A.B. Synthesis of Medium Ring Ethers. 5. The Synthesis of (+)-Laurencin J. Am. Chem. Soc. 1997 119 32 7483–7498 10.1021/ja9709132 http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1997/119/i32/abs/ja9709132.html Abstract Reactive or strained ketones (cyclobutanones, norbornanones) react with hydrogen peroxide or hydroperoxides to form lactones. The original reagent in the 1899 publication is Caro's acid discovered just a year earlier. Michael Renz, Bernard Meunier 100 Years of Baeyer-Villiger Oxidations European Journal of Organic Chemistry 1999 4 737–750 1999 10.1002/(SICI)1099-0690(199904)1999:4<737::AID-EJOC737>3.0.CO;2-B Disodium phosphate or sodium bicarbonate is often added as a buffering agent to prevent transesterification or hydrolysis. Mechanism The reaction mechanism of this oxidative cleavage involves first addition of the peroxy acid to the carbonyl forming a tetrahedral intermediate also called the Criegee intermediate for its similarity with rearrangement of that name. The transition state for this step is envisioned as a hydrogen relay involving three peroxy acid molecules with linear O-H-O interactions.The Role of Hydrogen Bonds in Baeyer-Villiger Reactions Shinichi Yamabe and Shoko Yamazaki J. Org. Chem.; 2007; 72(8) pp 3031 - 3041; (Article) doi:10.1021/jo0626562 Next is a concerted migration of one of the adjacent carbons to oxygen with loss of a carboxylic acid. If the migrating carbon is chiral, the stereochemistry is retained. 500px|center|Mechanism of the Baeyer-Villiger oxidation Migratory aptitude: H > tertiary alkyl > cyclohexyl > secondary alkyl, aryl > primary alkyl > methyl In the transition state for this migration step the R-C-O-O dihedral angle should be 180° in order to maximise the interaction between the filled R-C sigma bond and the antibonding O-O sigma bond. This step is also (at least in silico) assisted by two or three peroxyacid units enabling the hydroxyl proton to shuttle to its new position. For unsymmetrical ketones, the migrating group is usually the one that can best stabilize positive charge. Thus, cyclic ketones produce lactones and aldehydes usually produce carboxylic acids, although formates can also be formed if the migrating group is tertiary or an electron rich vinyl group or aromatic ring (Dakin reaction). Sometimes the alcohol is formed when the formate is hydrolytically unstable. Biocatalytic BV oxidation The Baeyer-Villiger oxidation can also be performed by biocatalysis with a so-called Baeyer-Villiger monooxygenase or BVMO. Though largely an experimental technique it shows the promise of enantioselectivity and green chemistry for this reaction type. Current stumbling blocks are confinement to water as the reaction medium, substrate specificity, dependence on the stoichiometric use and costs of cofactors such as NADPH and the costs associated with BVMO's themselves because lengthy purification steps are required. In vivo oxidations with metabolically active microbial cells introduce complications on their own. In one study Towards practical biocatalytic Baeyer-Villiger reactions: applying a thermostable enzyme in the gram-scale synthesis of optically active lactones in a two-liquid-phase system Frank Schulz; François Leca; Frank Hollmann; Manfred T Reetz Beilstein Journal of Organic Chemistry 2005 1 10 http://bjoc.beilstein-journals.org/content/pdf/1860-5397-1-10.pdf PDF 10.1186/1860-5397-1-10 10 16542025 the enzyme purification issue is addressed and a special thermally stable monooxygenase is isolated from a specific E. coli strain. This enzyme converts racemic 2-phenylcyclohexanone with oxygen to the corresponding (R)-lactone with 50% chemical yield and 94% enantiomeric excess with in a biphasic system of water and hexane. The NADPH cofactor is regenerated in each catalytic cycle by action of a second dehydrogenase enzyme which consumes isopropanol as a sacrificial catalyst. The solubility of the organic reactant and product is low in the aqueous phase thus averting inhibition. On the other hand the catalytic turnover number for this reaction is much larger than can be obtained with classical organic asymmetric catalysts. center|400px|Biocatalytic BV oxidation References