Reactions of Alcohols and Halides

Preparation of Alkenes by Elimination Reactions of Alcohols and Alkyl Halides... 

Several examples have been reported of the use of palladium-mediated oxidation reactions of alcohols and alkyl halides. Palladium(II) acetate in the presence of iodobenzene converts primary and secondary alcohols into carbonyl compounds under solid-liquid two-phase conditions [20], However, other than there being no further oxidation to carboxylic acids, the procedure has little to commend it over other methods. It is relatively slow with reaction times in the order of 2 days needed to achieve yields of 55-100%. 

The reaction of alcohols, alkyl halides or alkenes with alkenes in the presence of anhydrous hydrogen fluoride should lead to conjugated alkylation of the alkenes with simultaneous introduction of an alkyl group and a fluorine atom to give products 1. 

Reaction of Alcohol and Hydrobromic Acid.—The third reaction which proves the presence of the hydroxyl group in alcohol is with the halogen binary acids. When hydrochloric, or better hydrobromic or hydriodic acid, acts upon hot alochol a partial decomposition of the alcohol takes place and the ethyl halide and water are formed... 

In a variation of this reaction, alkyl halides can be converted to carbamates, by treatment with a secondary amine and K2CO3 under phase-transfer conditionsThe reaction of alcohols and alkyl halides can lead to carbonates. 

The classical permanent protecting group of carbohydrate hydroxyl functions is probably the benzyl ether. It is very stable and can be readily removed under essentially neutral conditions. For this reason, numerous benzylation and 0-debenzylation procedures have been described. Benzyl ether formation is usually achieved by the reaction of alcohols and benzyl halides in the presence of a base such as sodium hydride in anhydrous DMF (O Scheme 2) [11], or a mild base (Ag20) in THF using a phase-transfer catalyst [12]. Benzylation can also be accomplished by the use of an acidic catalyst with benzyltrichloroacetimidate as the reagent [13]. A method using the reductive etherification of TMS ethers under non-basic conditions has also been reported [14]. 

Reactions of Alcohols and Ethers with Hydrogen Halides Reaction of the C-0 Bond by Nucleophilic Substitution... 

Predicting the Reactions of Alcohols, Alkyl Halides, and Amines... 

Although the reaction of alcohols and mineral acids is usually quite efficient, there are times when the use of mineral acids in chemical reactions must be avoided. Therefore, the availability of alternative methods for the preparation of halides from alcohols is essential. To prepare an alkyl chloride from an alcohol, any reagent must contain chlorine atoms, just as a brominating agent must... 

In the reaction of aryl and alkenyl halides with 1,3-pentadiene (248), amine and alcohol capture the 7r-allylpalladium intermediate to form 249. In the reactions of o-iodoaniline (250) and o-iodobenzyl alcohol (253) with 1,3-dienes, the amine and benzyl alcohol capture the Tr-allylpalladium intermediates 251 and 254 to give 252 and 255[173-175]. The reaction of o-iodoaniline (250) with 1,4-pen tadiene (256) affords the cyclized product 260 via arylpalladiuni formation, addition to the diene 256 to form 257. palladium migration (elimination of Pd—H and readdition to give 258) to form the Tr-allylpalladium 259, and intramolecular displacement of Tr-allylpalladium with the amine to form 260[176], o-Iodophenol reacts similarly. 

Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

One important experimental fact is that the rate of reaction of alcohols with hydro gen halides increases m the order methyl < primary < secondary < tertiary This reac tivity order parallels the carbocation stability order and is readily accommodated by the mechanism we have outlined... 

The SnI mechanism is generally accepted to be correct for the reaction of tertiary and secondary alcohols with hydrogen halides It is almost certainly not correct for methyl alcohol and primary alcohols because methyl and primary carbocations are believed to be much too unstable and the activation energies for their formation much too high for them to be reasonably involved The next section describes how methyl and primary alcohols are converted to their corresponding halides by a mechanism related to but different from S l... 

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