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Methods Discourse | |
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All posts | Subject: THF & BDO Oxidation to GBL with Bromine Reagents | Please login to post | |
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Rhodium (Chief Bee) 09-21-04 22:17 No 532573 
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THF & BDO Oxidation to GBL with Bromine Reagents (Rated as: excellent) |
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Below follows several variations on the theme of produce a solution of elemental bromine (Br2) and hypobromite (HOBr), which is then used to oxidize either tetrahydrofuran (THF) or 1,4-butanediol (BDO) to gamma-butyrolactone (GBL) in varying yields, which in general range between 60-90% of theory. Other reagent combinations may be used to achieve the same goal, and they include the following systems:
Yield Comparison
Oxidation of Diols and Ethers by NaBrO3/NaHSO3 Reagent Satoshi Sakaguchi, Daisuke Kikuchi, and Yasutaka Ishii Bull. Chem. Sec. Jpn., 70, 2561-2566 (1997) (https://www.rhodium.ws/pdf/thf-bd2gbl.nabro3-nahso3.pdf) Abstract NaBrO3 combined with NaHSO3 was found to be an excellent oxidizing reagent of alcohols, diols, and ethers under mild conditions. A variety of aliphatic and cyclic diols were selectively oxidized with satisfactory yields to the corresponding hydroxy ketones and/or diketones, which are difficult to selectively prepare due to a concomitant formation of cleaved products. For example, 2-hydroxycyclohexanone and 1,2-cyclohexanedione were selectively formed by allowing 1,2-cyclohexanediol to react with NaBrO3/NaHSO3 reagent in a selected solvent. On the other hand, an alkyl ether, such as dioctyl ether, reacted with NaBrO3/NaHSO3 in water at room temperature to give octyl octanoate in 82% yield. The same oxidation at higher temperature (60°C) produced the α-brominated ester, octyl 2-bromooctanoate, which is considered to be formed through an alkenyl alkyl ether as the intermediate. The treatment of 1-ethoxy-1-heptene with NaBrO3/NaHSO3 afforded ethyl 2-bromoheptanoate and 2-bromoheptanoic acid as the major products. A preliminary communication by the same authors (only dealing with diol oxidation) has been posted in Post 532536 (Rhodium: "Oxidative Esterification of Butanediol to GBL", Methods Discourse) ____ ___ __ _ Oxidation of Alcohols and Ethers Using Sodium Bromate-Hydrobromic Acid System Shoji Kajigaeshi, Takashi Nakagawa, Noritaka Nagasaki, Hiromichi Yamasaki, and Shizuo Fujisaki Bull. Chem. Soc. Jpn. 59, 747-750 (1986) (https://www.rhodium.ws/pdf/thf-bd2gbl.nabro3-hbr.pdf) Abstract Reaction of primary alcohols or simple ethers, α,ω-diols or cyclic ethers, and secondary alcohols with sodium bromate in the presence of catalytic amount of hydrobromic acid under mild conditions gave dimeric esters, lactones and ketones in fairly good yields, respectively. ____ ___ __ _ Oxidation Using Quaternary Ammonium Polyhalides. III. An Effective Oxidation of Alcohols and Ethers by the Use of Benzyltrimethylammonium Tribromide Shoji Kajigaeshi, Hiroshi Kawamukai, and Shizuo Fujisaki Bull. Chem. Soc. Jpn., 62, 2585-2588 (1989) (https://www.rhodium.ws/pdf/thf-bd2gbl.btma-br3.pdf) Abstract The reaction of primary alcohols or simple ethers and α,ω-diols or cyclic ethers with a stoichiometric amount of benzyltrimethylammonium tribromide (BTMA·Br3) in carbon tetrachloride in the presence of Na2HPO4 aq or in acetic acid in the presence of aq. CH3CO2Na at 60-70°C gave dimeric esters and lactones respectively in good yields. The reaction of secondary alcohols with 1 equiv of BTMA·Br3 in the presence of a buffer at 60°C gave ketones. ____ ___ __ _ Mechanism and synthetic utility of the oxidative cleavage of ethers by aqueous bromine Norman C. Deno, Neil H. Potter J. Am. Chem. Soc. 89, 3550-3554 (1967) (https://www.rhodium.ws/pdf/thf2gbl.bromine-1.pdf) Abstract Aqueous Br2 oxidizes a wide variety of aliphatic ethers at 25°C. Primary alkyl groups are converted to carboxylic acids and secondary alkyl groups to ketones. Bromination products appear when the reaction is conducted under strongly acidic conditions. Their formation can be completely suppressed by conducting the reaction in acetate buffers at pH 5. The reaction is attractive for degradative purposes because of the quantitative yields and the mild conditions. The rates are first order in ether and Br2 and can be depressed by addition of Br-, which converts Br2 to Br3-, or by raising the pH above 5, which converts Br2 to HOBr. Competition experiments indicate that the reaction proceeds by a mechanism involving simultaneous loss of H+ from an α-carbon and an electron pair from the ether oxygen. The mechanism is isoelectronic with the Westheimer mechanism for the oxidation of alcohols by chromic acid. The reaction proceeds in the dark and the selectivity differs from either the light-catalyzed H· abstractions by Br· or the H- abstractions by Ph3C+. This article has been discussed several times in the past, including in Post 308130 (moo: "THF oxidations", Novel Discourse) ____ ___ __ _ Mechanism of oxidation of alcohols by aqueous bromine Norman C. Deno, Neil H. Potter J. Am. Chem. Soc. 89, 3555-3556 (1967) (https://www.rhodium.ws/pdf/thf2gbl.bromine-2.pdf) Abstract The pH-rate profile for the oxidation of 2-propanol by Br2 indicates the presence of two independent mechanisms. At pH 2-6, the rate is invariant with pH and the predominant mechanism is direct attack of Br2 on the 2-propanol. At pH 7-9, the rate follows [HOBr] and appears to involve intermediate formation of ROBr. Both mechanisms are closely related to the oxidative cleavage of ethers by Br2 and involve as a unifying feature the abstraction of an electron pair from the oxygen by bromine and the loss of an α-hydrogen as H+. The Hive - Clandestine Chemists Without Borders |
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