The oxidation of halohydrins

The mechanism and stereochemistry of halogenation, physical methods / 273 Dihalogenation / 276 The use of A-halo compounds / 280 The use of other reagents / 282 The oxidation of halohydrins / 283 Vinylogous a-halo ketones / 284... 

Peroxidases, in particular haloperoxidases, catalyze the oxidation of olefins in the presence or absence of halide ions. In the latter case, epoxides are produced directly (Fig. 6, route A), while in the former process halohydrins are formed (Fig. 6, route B), which may be transformed to epoxides either enzymatically by halohydrin epoxidase [128] or chemically by base treatment. 

Cleavage of epoxides. As with simpler haloboranes, these reagents cleave epoxides of cycloalkenes to give, after nonoxidative workup, halohydrins in 65-90% yield. When carried out at -78 to -100°, the cleavage can show high en-antioselectivity. Thus the halodiisopinocampheylboranes derived from (+ )-pinene react with the oxide of cyclohexene or of cyclopentene to furnish (1R,2R) halo-... 

Reactions of Oxirans with Organometallic Compounds. - A general method for the preparation of halohydrin esters via the reaction of organic halides with CO (20 atm) in the presence of PhPd(PPh3)2l has been described.In the particular case of benzyl chloride and cyclohexene oxide, (228) (57.5%) was obtained. A possible mechanism for this reaction is shown in Scheme 14. 

Halohydrins may be used as intermediates in protection of olefins as epoxides and there are some instances of the use of halohydrin acetates to protect double bonds. Overton [49] protected an olefin against both oxidative and reductive conditions by use of the halohydrin acetate and Levine and Wall [50] noted that formation of a halohydrin acetate of the A -olefin in (7) caused the Cj i methyl group to be protected in some way against bromination. Bromhydrin acetates can be... 

Oxides are usually formed in the laboratory by the peroxidation of olefins with H Oj, peracids, or by the dehydrohalogenation of halohydrins. 

On the other hand, the halohydrin (chloro and bromo) 908 is converted into a ketone via oxidative addition and //-elimination in boiling benzene with catalysis by Pd(OAc)2 and tri(o-tolyl)phosphine in the presence of K2C03[765,766],... 

Electrochemical Process. Applying an electrical current to a brine solution containing propylene results in oxidation of propylene to propylene oxide. The chemistry is essentially the same as for the halohydrin process. AH of the chemistry takes place in one reactor. Most of the reported work uses sodium or potassium bromide as the electrolyte. Bromine, generated from bromide ions at the anode, reacts with propylene and water to form propylene bromohydrin. Hydroxide generated at the cathode then reacts with the bromohydrin to yield propylene oxide (217—219). The net reaction involves transfer of two electrons ... 

Propylene oxide has found use in the preparation of polyether polyols from recycled poly(ethylene terephthalate) (264), haUde removal from amine salts via halohydrin formation (265), preparation of flame retardants (266), alkoxylation of amines (267,268), modification of catalysts (269), and preparation of cellulose ethers (270,271). 

Uses. /-Butyl hypochlorite has been found useful in upgrading vegetable oils (273) and in the preparation of a-substituted acryflc acid esters (274) and esters of isoprene halohydrins (275). Numerous patents describe its use in cross-linking of polymers (qv) (276), in surface treatment of mbber (qv) (277), and in odor control of polymer latexes (278). It is used in the preparation of propylene oxide (qv) in high yield with Httle or no by-products (269,279). Fluoroalkyl hypochlorites are useful as insecticides, initiators for polymerizations, and bleaching and chlorinating agents (280). 

With Acyl Halides, Hydrogen Halides, and Metallic Halides. Ethylene oxide reacts with acetyl chloride at slightly elevated temperatures in the presence of hydrogen chloride to give the acetate of ethylene chlorohydrin (70). Hydrogen haUdes react to form the corresponding halohydrins (71). Aqueous solutions of ethylene oxide and a metallic haUde can result in the precipitation of the metal hydroxide (72,73). The haUdes of aluminum, chromium, iron, thorium, and zinc in dilute solution react with ethylene oxide to form sols or gels of the metal oxide hydrates and ethylene halohydrin (74). 

Urea hydrogen peroxide adduct (UHP) was employed in metal-catalysed asymmetric epoxidation [98] and Baeyer-Villiger oxidation [99,100]. Since the presence of urea does not change the course of the reaction, this will not be described here. Conversion of epoxides to halohydrins with elemental... 

Ethylene oxide is used in the manufacture of raw materials and can be used to sterilize the surface of finished products and containers. Unfortunately, ethylene oxide is a genotoxic carcinogen and its use is not accepted without justification. In any event, tight controls are required on residues of ethylene oxide and its halohydrin-related substances. For raw materials the amount of these residues is limited to 1 and 50 pig/g, respectively for finished products 1 and 50 pg/g, respectively (with any affected ingredients subject to the control limits for raw materials) and for containers, based on simulated use, 1 and 50 pg/mL container volume, respectively. 

Hydratropaldehyde has been prepared by hydrolysis of phenylmethylglycidic ester,2 3 4 by chromyl chloride oxidation of cumene,5 by the elimination of halogen acid or water from halohydrins or glycols,5 8 and by the distillation at ordinary pressure of methylphenylethylene oxide.9,10... 

Silica gel-based catalytic systems have been described as efficient promoters for a number of organic reactions.28 Illustrative examples include the oxidative cleavage of double bonds catalyzed by silica-supported KM11O4,29 reaction of epoxides with lithium halides to give /i-halohydrins performed on silica gel,30 selective deprotection of terf-butyldimethylsilyl ethers catalyzed by silica gel-supported phosphomolybdic acid (PMA),31 and synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts.32... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

The reactions of anthranilic acids and esters (65CB983, 75BSF(2)283) with halohydrins give the 4,l-benzoxazepin-5-one system (357). o-Methylaminobenzamide reacts with ethylene oxide to give (358) which can be cleaved to (359) by treatment with aqueous ammonia (66JOC4268). 

Alkenes are converted to halohydrins by the treatment of halides and water. When halohydrins are treated with a strong base (NaOH), an intramolecular cyclization occurs and epoxides are formed. For example, 1-butene can be converted to butylene oxide via butylene chlorohydrin. 

The indirect electrochemical generation of propylene oxide via propylene chloro- or bromohydrin using anodically generated hypochlorite or hypobromite has been studied very intensively. The reason is the lack of a technically useful process for the synthesis of propylene oxide by way of heterogeneous catalysis. The propylene halohydrins are saponified using the cathodically generated sodium hydroxide (Eqs. (42)-(47)) (Table 4. No. 12-15)... 

The primary product of hypohalite photolysis is a 1,5-halohydrin, the stability of which increases on passing from iodo- to bromo- to chloro-hydrins. If spontaneous elimination of hydrogen halide does not occur, the halohydrins can be converted into tetrahydrofurans by base treatment. In several instances69, 75, 88)115 it has been possible to isolate the intermediate halohydrin. Oxidation of this intermediate followed by solvolysis leads to the corresponding hydroxy ketones or in the presence of base to cyclopropyl ketones.5,57... 

---