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Oxidation and Reduction Key words: oxidations using chromium and manganese reagents, cleavage of double bonds with ozone and osmium tetroxide, oxidations with aluminum isopropoxide, peracids, Introduction Broader definition of oxidation and reduction respectively refer to the loss and gain of electrons, or an increase in oxidation number (oxidation) Definition and a decrease in oxidation number (reduction). In organic chemistry, the gain of oxygen or loss of hydrogen is often referred to as oxidation. In practice, a series of functional groups have been qualitatively identified in the order of increasing oxidation state. Then, oxidation is referred to as the conversion of one functional group higher in the sequence to another lower in the list. Conversion within a group are neither oxidation nor reduction. It is summarized in following table. This module has been organized based on the reagent that are used for oxidation reactions oxidation Table summarizing O functional groups RH CO arranged according 2 to oxidation state. OH ROH O O RCl CCl4 RNH2 etc. R R R NH2 Cl C Cl C Cl Cl Cl C C etc. Cl Cl C C OH OH etc. reduction I. Oxidation of For oxidation of alcohols to corresponding carbonyl compounds, generally alcohols using Cr(VI) reagents such as K2Cr2O7, Jones reagent, PCC etc., are employed. Cr(VI) reagents Oxidation of alcohols to carbonyl compound occurs via Cr(VI) acid monoester. Mechanism is as follows. OH O CrO3 R1 R2 R1 R2 O HO H O O Cr O O O Cr O + + H2CrO3 R1 C H 1 1 2 O R C H R R R2 R2 Example for chromium oxide based oxidation Oxidation of fused aromatic system is generally carried 4mmol CrO3 out using CrO3 reagent 60 min, rt Juaristi M. et al, Can.J.Chem., O 1984, 62, 2941 O 80% PCC (pyridinium chlorochromate) is other efficient reagent used widely for oxidation of primary and secondary alcohols. E J Corey and W Suggs in 1975 suggested PCC as oxidizing agent. PCC is Reagent can be used in slightly acidic but can be buffered with NaOAc close to stoichiometric O amounts with substrate HN Cl Cr O O O OH H HO H O 1,6-Hexanediol Hexanedial (68%) Corey E J & Suggs W, Tet.Lett., 1975, 16, 2647 OH O Benzhydrol Benzophenone (100%) OH O 4-tertbutylcyclohexanol 4-tertbutylcyclohexanone (97%) Corey and Suggs used H OH following method for O preparation of PCC. R R 100g (1mol) CrO3 is added to conc. HCl, R rapidly with stirring over R 5 min time. Homogenous Presqalene alcohol Presqalene aldehyde (78%) solution obtained is R : (CH ) CCH(CH )CH CH C(CH )=CHCH CH - cooled to 00C. To this, 3 3 3 2 2 3 2 2 1mol of pyridine is added. Yellow-orange OH O solid obtained is filtered H and dried in vacuum. This solid is PCC and is not hygroscopic. It can be stored at room Citronellol Citronellal (82%) temperature. PCC is used particularly for the oxidation of primary alcohol to aldehyde. It does not have any effect on C=C or any other easily oxidizable functional groups. In this reaction, double O PCC, DCM bond is not affected. OH H Geraniol Geranial PCC is used in aprotic solvents, usually, dichloromethane. As no water is present in the reaction mixture, no aldehyde hydrate is formed which is oxidized to carboxylic acid in presence of Cr(VI) PCC, dry CHCl3, Agarwal S; Tiwari H anhy. AcOH, rt, 1h O P; Sharma J P, OH Tetrahedron, 1990, 46, H 1-Hexanol 1-Hexanal (89%) 4417 Another similar oxidant is PDC (pyridiniumdichromate) H O 1.5eq PDC OH DCM, 250 C O O 90% O O NH O Cr O Cr O HN O O pyridiniumdichromate Since PDC is less acidic than PCC it is often used to oxidize alcohols that may be sensitive to acids. In methylene chloride solution, PDC oxidizes primary and secondary alcohols in roughly the same fashion as PCC, but much more slowly. However, in DMF solution saturated primary alcohols are oxidized to carboxylic acids. In both solvents allylic alcohols are oxidized efficiently to conjugated enals and enones respectively. Stanfield C F, et al, J. examples Org. Chem., 1981, 46, 4797 3.5eq PDC 1.5eq PDC COOH DMF, 250 C DCM, 250 C CHO OH 83% 92% 1.5eq PDC, OH DCM, 250C, 24h O O CHO 90% Corey E J & Schmidt COOH G, Tet.Lett., 1979, OH O O 20(5), 399 3.5eq PDC, DMF, 250C, 7-9h H H 85% Collins reagent can be Collins reagent is the mixture of chromium prepared and isolated or trioxide with pyridine in dichloromethane. generated in situ. It is used to selectively oxidize primary alcohols to aldehyde, and will Isolation of reagent often tolerate many other functional groups in the molecule. leads to improved yields. CrO3-2Py R H R DCM OH O It can be used as an alternative to Jones reagent and PCC in oxidation of secondary alcohols. Moreover, Collins reagent is especially useful for oxidations of acid sensitive compounds. This complex is both difficult and dangerous to prepare, as it is very hygroscopic and can inflame during its preparation. It is required to be used in a sixfold excess in order to complete the reaction. examples O one of the steps in the O synthesis of O O prostaglandin F2α CrO3-2Py DCM, 00C employs Collin’s reagent as oxidant H OH AcO Corey E J, JACS, 1969, AcO O 91, 5675 one of the steps in OH synthesis of longifolene O employs Collin’s reagent H CrO -2Py, DCM, 3 H as oxidant rt, 15min McMurry J E, JACS, OH 1972, 94, 7132 H O 100% one of the steps in synthesis of triquinacene employs Collin’s reagent as oxidant CrO3-2Py, DCM Woodward RB, JACS, H O 1964, 86, 3162 OH 74% Jones described for the Jones reagent is used for the oxidation of primary and first time a convenient secondary alcohols to carboxylic acids and ketones, respectively, that do and safe procedure for a not contain acid sensitive group. chromium (VI) based oxidants, that paved way for some further developments such as Collins Reaction and pyridinium dichromate. It is chromium oxide, sulfuric acid and acetone. A mixture of potassium or sodium dichromate and dilute sulfuric acid can also be used. CrO3, aq.H2SO4 O acetone R OH R OH OH CrO3, aq.H2SO4 O acetone R R' R R' O Mechanism : O H2SO4 H2O HO Cr OH Chromium trioxide or Cr O O sodium dichromate with O Chromic acid dilute H2SO4 in situ forms chromic acid . O O H2SO4 H2O Chromic acid and HO Cr O Cr OH Na2Cr2O7 alcohol then through O O chromate ester gives carbonyl compound in Dichromic acid .. presence of base (water H2O O OH O H in this case). VI VI R' HO Cr OH + HO Cr O The intramolecular R R' reaction occurs by way O H O R . .. of β-elimination through O O . cyclic transition state. + H O ..Cr IV 3 R R' HO O O VI OH Aldehydes can form O HO Cr OH H2O hydrates in presence of O R OH water and further R H H oxidized to carboxylic . .. O . acid in presence of H .. O OH O VI H2O Cr(VI) reagents. HO Cr O + .. IVCr -H3O R OH HO O O R The Jones reagent is The oxidation of primary allylic and benzylic alcohols gives aldehydes. prepared by adding Some alcohols such as benzylic and allylic alcohols give aldehydes that chromium trioxide to do not form hydrates in significant amounts; these can therefore be dilute sulfuric acid in selectively oxidized with unmodified Jones Reagent to yield aldehydes. acetone and is added to the alcohol at 0-25oC. K2Cr2O7, H2SO4 CHO OH 0 H2O, acetone, 0-25 C The excess Cr(VI), if any is remained, is destroyed CrO , H SO , H O, in the reaction workup 3 2 4 2 O acetone, 0-250C by adding isopropyl alcohol. OH H For the synthesis of aldehydes, the Collins Reaction or use of more modern although more expensive chromium (VI) reagents such as PCC and PDC can be an appropriate choice. Tertiary alcohols cannot be oxidized by this reagent. It is a powerful oxidizing reagent and exhibits only poor chemoselectivity. OH CrO3, aq.H2SO4 O 0 oxidation of secondary H2O, acetone, 0-25 C alcohol gives ketone whereas primary alcohol is oxidized to aldehyde OH CHO COOH first and then to CrO3, aq.H2SO4 0 (O) carboxylic acid. H2O, acetone, 0-25 C H H Jones reagent, acetone, Panda J; Ghosh S & 00C - rt, 1h Ghosh S, ARKIVOC, O O 2001(viii), 146 H H H OH O MnO2 is used widely as oxidant in organic synthesis. II. Oxidation using It oxidizes allylic alcohols to corresponding aldehydes or ketones. Mn reagents O MnO2 R OH (a) Mn(IV) reagent R H The configuration of double bond is preserved in the reaction. Also, acetylenic alcohols and benzylic alcohols are oxidized under similar conditions. Applications of MnO2 are numerous. These include many kinds of reactions such as amine oxidation, aromatization, oxidative coupling, and thiol oxidation. Activity of MnO2 S S depends upon method of MnO2, DCE preparation and choice 2 2 1 R R1 R of solvent R N N 1,2-Diols are cleaved by MnO2 to dialdehydes or diketones. OH MnO2, DCM, O Ph rt, 24h Ph 2 Ph H OH MnO , rt, 5d CH OH 2 examples 2 Pet. ether vit.A Taylor R J K; et al, Acc. Chem. Res., 2005, CHO 38, 851.
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