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Module I Oxidation Reactions

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NPTEL – Chemistry – Reagents and Organic reactions Module I Oxidation Reactions Lecture 1 1.1 Osmium Oxidants 1.1.1 Introduction Osmium is the densest (density 22.59 gcm-3) transition metal naturally available. It has seven naturally occurring isotopes, six of which are stable: 184Os, 187Os, 188Os, 189Os, 190Os, and 192Os. It forms compounds with oxidation states ranging from -2 to +8, among them, the most common oxidation states are +2, +3, +4 and +8. Some important osmium catalyzed organic oxidation reactions follow: 1.1.2 Dihydroxylation of Alkenes Cis-1,2-dihydroxylation of alkenes is a versatile process, because cis-1,2-diols are present in many important natural products and biologically active molecules. There are several methods available for cis-1,2-dihydroxylation of alkenes, among them, the OsO4-catalzyed reactions are more valuable (Scheme 1). OsO4 vapours are poisonous and result in damage to the respiratory tract and temporary damage to the eyes. Use OsO4 powder only in a well-ventilated hood with extreme caution. Y. Gao, Encylcopedia of Reagents for Organic Synthesis, John Wiley and Sons, Inc., L. A. Paquette, New York, 1995, 6, 380. OH OsO4 Na2SO3 Et2O, 24 h OH OH OsO4 OH Na2SO3 Et O, 24 h 2 Scheme 1 Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 122 NPTEL – Chemistry – Reagents and Organic reactions The use of tertiary amine such as triethyl amine or pyridine enhances the rate of reaction (Scheme 2). OH OsO4, Pyridine K2CO3, KOH OH Et O, 30 min 2 Scheme 2 Catalytic amount of OsO4 can be used along with an oxidizing agent, which oxidizes the reduced osmium(VI) into osmium(VIII) to regenerate the catalyst. A variety of oxidizing agents, such as hydrogen peroxide, metal chlorates, tert-butyl hydroperoxide, N-methylmorpholine-N-oxide, molecular oxygen, sodium periodate and sodium hypochlorite, have been found to be effective (Scheme 3- 7). HO OH OsO4, H2O2 HO C CO H 2 2 tert-BuOH, 12 h, rt HO2C CO2H Me C8H17 Me C8H17 Me OsO4, H2O2 Me tert-BuOH, 24 h, rt O O OH HO Scheme 3 OH OsO , t-BuOOH Me 4 Me Me Me t-BuOH, Et4NOH OH Me OsO4, t-BuOOH Me t-BuOH, Et NOH Ph 4 OH Ph OH Scheme 4 Joint initiative of IITs and IISc – Funded by MHRD Page 2 of 122 NPTEL – Chemistry – Reagents and Organic reactions OH OsO , NaClO OH 4 3 HO OH H2O, 50 °C OH OsO4, AgClO3 Me CO2H Me CO2H H2O, 0 °C OH HO H H OAc HO OAc OsO4, Ba(ClO3)2 H O, 5 h O Me 2 H O Me CO Me H 2 CO2Me CO2Me CO2Me Me HO Me OsO4, KClO3 HO CO2H H2O, 50 °C CO2H CO2H CO H 2 Scheme 5 MeO C 2 MeO2C Me Me OsO4, NMO HO CO H HO 2 aqueous acetone CO2H CO H 2 t-BuOH CO2H AcO AcO OH Me Me OH OsO4, NMO Me aqueous acetone Me t-BuOH O O Scheme 6 Joint initiative of IITs and IISc – Funded by MHRD Page 3 of 122 NPTEL – Chemistry – Reagents and Organic reactions O O OsO4, NaIO4 aqueous acetone AcO AcO O Me Me O O O Me Me OsO , NaIO Me 4 4 Me O dioxane-water H O Me H O Me Me O Me Scheme 7 In the latter case, the resultant diols undergo oxidative cleavage to give aldehydes or ketones. This reaction is known as Lemieux-Johnson Oxidation. NaIO4 oxidizes the reduced osmium(VI) to osmium(VIII) along with the oxidative cleavage of the diols. Mechanism The reaction involves the formation of cyclic osmate ester, which undergoes oxidative cleavage with NaIO4 to give the dicarbonyl compounds (Scheme 9). OsO4 O O NaIO OsO + 4 CHO 4 Os OHC addition to O O oxidative double bond cleavage osmate ester Scheme 9 1.1.3 Sharpless Asymmetric Dihydroxylation Although osmylation of alkenes is an attractive process for the conversion of alkenes to 1,2-diols, the reaction produces racemic products. Sharpless group attempted to solve this problem by adding chiral substrate to the osmylation reagents, with the goal of producing a chiral osmate intermediate (Scheme 10). The most effective chiral additives were found to be the cinchona alkaloids, especially esters of dihydorquinidines such as DHQ and DHQD. The % ee of the diol product is good to excellent with a wide range of alkenes. Joint initiative of IITs and IISc – Funded by MHRD Page 4 of 122 NPTEL – Chemistry – Reagents and Organic reactions O O O OH O O Os Unligated Pathway Os + R OH O O R Racemic Product O R O O OH O O R Ligand Accelerated Pathway O O O Os L* OH Os + L* Os O O O O R Optically Active Product O L* R Scheme 10 Et Et N N O R R O H H R = acyl, aryl MeO OMe N N Dihydroquinoline Dihydroquinidine DHQ DHQD DHQD Attack DHQD OHOH R s RM RL Rs RM OsO4 t-BuOH:H O, oxidant RL 2 Rs RM DHQ RL Attack OHOH DHQ RL = larger substituent Rs = smaller substituent RM = medium substituent Scheme 11 If the alkene is oriented as shown in Scheme 11, the natural dihydroquinidine (DHQD) ester forces delivery of the hydroxyls from the top face ( -attack). Conversely, dihydorquinine (DHQ) esters deliver hydroxyls from the bottom face ( -attack). Joint initiative of IITs and IISc – Funded by MHRD Page 5 of 122 NPTEL – Chemistry – Reagents and Organic reactions The reactions are generally carried out in a mixture of tert-butyl alcohol and water at ambient temperature (Scheme 12). AD-mix O O O O t-BuOH-H2O HO OH Me Me AD-mix H H Me Me t-BuOH-H2O OH Me Me OH Me Me AD-mix H H Me Me t-BuOH-H O 2 OH Me Me OH Scheme 12 Features: The reaction is stereospecific leading to 1,2-cis-addition of two OH groups to the alkenes It typically proceeds with high chemoselectivity and enantioselectivity The reaction conditions are simple and the reaction can be easily scaled up The product is always a diol derived from cis-addition. It generally exhibits a high catalytic turnover number It has broad substrate scope without affecting the functional groups Joint initiative of IITs and IISc – Funded by MHRD Page 6 of 122 NPTEL – Chemistry – Reagents and Organic reactions 1.1.4 Aminohydroxylation Similar to cis-1,2-dihydroxylation, cis-1,2-aminohydroxylation of alkenes has been developed by reaction with chloroamine in the presence of catalytic amount of OsO4. In this process, alkene reacts with chloroamine in the presence of OsO4 to give sulfonamides that is readily converted into the cis-1,2-hydroxyamines by cleavage with sodium in liquid ammonia (Scheme 13). This process provides a direct cis-aminohydroxylation of alkenes, but the major problem is the poor regioselectivity for unsymmetrical alkenes. TsHN R' Na/liq. NH K2OsO2(OH)4 3 H2N R' R R' R Ts-N(Na)Cl, H2O-t-BuOH OH R OH OH OH Na/liq. NH3 K2OsO2(OH)4 Ts-N(Na)Cl, H2O-t-BuOH NH2 NHTs Scheme 13 Mechanism The catalytically active species in the reaction most likely is an imidotrioxo osmium(VIII) complexes, which is formed in situ from the osmium reagent and the stoichiometric nitrogen source, i.e. chloroamine (Scheme 14). Experiments under stoichiometric conditions have been shown that imidotrioxo osmium(VIII) complexes transfer the nitrogen atom and one of the oxygen atoms into the substrate. The major regioisomer normally has the nitrogen placed distal to the most electron withdrawing group of the substrate. R R R O R O O R H2O Os + O Os O HO R O R NX O N R O Os NX XHN X O Scheme 14 Joint initiative of IITs and IISc – Funded by MHRD Page 7 of 122 NPTEL – Chemistry – Reagents and Organic reactions 1.1.5 Asymmetric Aminohydroxylation The asymmetric cis-1,2-aminohydroxylation of alkenes with chloroamine has been explored using the chiral osmium catalyst derived from OsO4 and cinchona alkaloids, dihydroquinidine ligands (DHQD)2-PHAL and dihydroquinine ligands (DHQ)2-PHAL. Et Et N N N N R O O R *Ak-O O-Ak* H H R = MeO OMe N N Dihydroquinoline Dihydroquinidine Phtholazine (PHAL) DHQ DHQD The face selectivity for the aminohydroxylation can too be reliably predicted (Scheme 15). (DHQD)2-PHAL HNX OH OHNHX Top ( ) Attacl (DHQD)2-PHAL Rs + Rs RM RM RL RL N-Source: XNClNa Rs RM RL O-source: H2O R R Catalyst: OsO s R s RM 4 RL M + RL (DHQ) -PHAL Bottom ( ) Attack 2 HNX OH OHNHX R = larger substituent (DHQ)2-PHAL L Rs = smaller substituent X = SO2R, ROCO RM = medium substituent Scheme 15 An alkene with these constraints receives the OH and NHX groups from above, -face, in the case of DHQD derived ligand and from the bottom, i.e. -face, in the case of DHQ derivative. For example, the asymmetric aminohydroxylation of methyl cinnamate gives the following face selectivity based on the chiral ligand (Scheme 16). Joint initiative of IITs and IISc – Funded by MHRD Page 8 of 122 NPTEL – Chemistry – Reagents and Organic reactions NHX O OH O DHQD Ph OMe + Ph OMe O OH NHX p-TolSO2NClNa X = p-TolSO Ph OMe 2 NHX O OsO4, H2O OH O Ph OMe + Ph OMe DHQ OH NHX Scheme 16 With respect to the yield, regio- and enantioselectivity, reaction depend on number of parameters, e.g. the nature of starting material, the ligand, the solvent, the type of nitrogen source (sulfonamides), carbamates and carboxamides as well as the size of its substituent.
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