Iodine halohydrin reactions

Silver(I) salts are often utilized as catalysts for addition reactions. Kozmin and Sun have recently shown that AgNTf2 is a catalyst of choice for the hydroamination of siloxy alkynes with either secondary amides or carbamates to give silyl ketene am-inals [34]. The addition occurs in a syn selective manner, for instance, the reaction of siloxy alkyne (24) with carbamate (25) produces silyl ketene aminal (26) in 86% yield at room temperature under the influence of 1 mol% of AgNTf2 (Scheme 18.9). A six-membered chelated transition state is proposed to explain the high syn selectivity. Diastereoselective bromohydroxylation and bromomethoxylation reactions of cinnamoyl compounds possessing a chiral auxiliary are also effectively promoted by silver(I) salts such as AgNOs [35]. The asymmetric halohydrin reaction has been successfully applied into stereoselective syntheses of (-)-chloramphenicol and (+)-thiamphenicol. Csp-H iodination [36], hydrosilylation of aldehydes [37], and deprotection of TMS-alkynes [38] are also catalyzed by silver (I) salts. 

Several related procedures for syn hydroxylation of alkenes involve a halohydrin ester (32) as the key intermediate. In Woodward s procedure an alkene in glacial acetic acid is treated with iodine and silver acetate. Acetyl hypoiodite, MeCOjI, formed by reaction of the latter two reagents attacks the alkene, R R 0=CR R in an electrophilic manner, from the less hindered side to give, by overall anti addition. 

NaBr/H20, LiBr on Amberlyst-15 resin, TiCU-LiCl, " SiCL, I2 with a Sml2 catalyst, and Lil on silica gel. Epoxides can be converted directly to 1,2-dichloro compounds by treatment with SOCI2 and pyridine, or with Ph3P and CCl4. These are two-step reactions a halohydrin is formed first and is then converted by the reagents to the dihalide (10-48). As expected, inversion is found at both carbons. Meso epoxides were cleaved enantioselectively with the chiral B-halodiisopinocampheylboranes (see 15-16), where the halogen was Cl, Br, or I. ° Diatomic iodine gives an iodohydrin with a 2,6-bis[2-(o-aminophenoxy) methyl]-4-bromo-l-methoxybenzene catalyst. ... 

In a reaction resembling halohydrin formation (Section 6.17), vicinal haloethers are prepared from alkenes by reaction with an alcohol in the presence of halogens— usually bromine or iodine. This haleotherification proceeds through a cyclic halonium ion, which reacts with the alcohol. 1-Methylcyclohexene undergoes iodoetherification with ethanol in the presence of iodine to give ran -l-ethoxy-2-iodo-l-methylcyclohexane. 

---