Preparation of 1,2-Diols

Diols are almost always given snbstitntive lUPAC names. As the name of the product in the example indicates, the snbstitntive nomenclatnre of diols is similar to that of alcohols. The snflix -diol replaces -ol, and two locants, one for each hydroxyl group, are required. Note that the final -e of the alkane basis name is retained when the suffix begins with a consonant (-diol), bnt dropped when the snflix begins with a vowel (-0I). 

Vicinal diols are diols that have their hydroxyl groups on adjacent carbons. Two commonly encountered vicinal diols are 1,2-ethanediol and 1,2-propanediol. 

Ethylene glycol and propylene glycol are common names for these two diols and are acceptable lUPAC names. Aside from these two compounds, the lUPAC system does not use the word glycol for naming diols. 

In the laboratory, vicinal diols are normally prepared from alkenes using the reagent osmium tetraoxide (OsOa). Osmium tenaoxide reacts rapidly with alkenes to give cyclic osmate esters. 

Osmate esters are fairly stable but are readily cleaved in the presence of an oxidizing agent such as tert-butyl hydroperoxide. 

Diols are compounds with two hydroxyl groups, and the following additional rules are used to name them  

The positions of both hydroxyl groups are identified with numbers placed before the parent. 

Notice that an e appears in between the parent and the suflfix. In a regular alcohol, the e is dropped (i.e., propanol or hexanol). A few simple diols have common names that are accepted by lUPAC nomenclature. 

The term glycol indicates the presence of two hydroxyl groups. Diols can be prepared from diketones via reduction using any of the reducing agents that we have seen. 

Alternatively, diols can be made via dihydroxylation of an alkene. In Chapter 9, we explored reagents for achieving either syn or anti dihydroxylation. 

Sample Solution (a) Reaction with ethylene oxide results in the addition of a —CH2CH2OH unit to the Grignard reagent. The Grignard reagent derived from o-bromotoluene (or o-chlorotoluene or 

This kind of chemical reactivity of epoxides is rather general. Nucleophiles other than Grignard reagents react with epoxides, and epoxides more elaborate than ethylene oxide may be used. These features of epoxide chemistry will be discussed in Sections 16.11-16.13. 

Write an equation showing how 3-methylpentane-l,5-diol could be prepared from a dicarbo) lic acid. 

The 2004 lUPAC name for this diol is 3-methylpentane-l,5-diol. 

Write equations showing how 3-methyl-l,5-pentanediol could be prepared from a dicarboxylic acid. 

Ethylene glycol and propylene glycol are prepared industrially from the corresponding alkenes by way of their epoxides. Some applications were given in the box in Section 6.21. 

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How to Use the Book to Locate Examples of the Preparation of Difunctional Compounds. The difunctional index on p. xi gives the section and page corresponding to each difunctional product. Thus Section 327 (Alcohol, Thiol-Ester) contains examples of the preparation of hydroxyesters Section 323 (Alcohol, Thiol-Alcohol, Thiol) contains examples of the preparation of diols. 

Many literature preparations of difunctional compounds are extensions of the methods applicable to monofunctional compounds. As an example, the reaction RCI — ROH might be used for the preparation of diols from an appropriate dichloro compund. Such methods are difficult to categorize and may be found in either the monofunctional or difunctional sections, depending on the focus of the original paper. 

Addition of water to carbonyl compounds acid-catalysed hydration. Preparation of diols... 

Preparation of diols Acid-catalysed epoxides are easily cleaved by water. Water reacts as the nucleophile, and this is referred to as a hydrolysis. For example, hydrolysis of ethylene oxide in the presence of acid-catalyst produces 1,2-ethanediol (ethylene glycol). 

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

For the preparation of diols from olefins see Section 323 (Alcohol - Alcohol). 

Heating stilbene with benzoyl peroxide and iodine in carbon tetrachloride at 80 °C for 48 h gives an 83% yield of the dibenzoyl ester of hydrobenzoin (l,2-dibenzoxy-l,2-diphenylethane) [230]. Esters of vicinal diols are intermediates in the preparation of diols by the Prevost and the Woodward methods (equation 78). 

As in the asymmetric epoxidation reaction, the development of such a powerful tool for the enantioselective preparation of diols spurred its application to a very wide variety of synthetic problems and the invention of new methods of manipulating the diol products [64], A few examples confer some of the flavor of this work. 

Preparation of Alcohols via Reduction Preparation of Diols Preparation of Alcohols via Grignard Reagents... 

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