Dehydration of tertiary alcohols

The dehydration of alcohols is mostly an acid-catalyzed reaction and much work has been done by Taft and co-workers to elucidate the mechanism (7 5-7 7). These investigators proved that the intermediate in the dehydration of tertiary alcohols or hydration of branched olefins in dilute acid solutions resembles the conjugate acid of the olefin and it is... 

It can be assumed that the dehydration of tertiary alcohols proceeds through the participation of Bninsted acid sites of the aluminas, A H+. The reaction may be presented as follows ... 

The dehydration of tertiary alcohols over aluminas can be interpreted by a carbonium ion mechanism. 

Monocyclic tetrahydrophosphorin derivatives can be prepared, for example by dehydration of tertiary alcohols which are available by the action of lithium alkyls on 4-oxophos-phorinane derivatives (equation (16)). P- Oxidation and dehydrogenation of the ketone with Se02 is also possible (equation (17)) (66AG(E)588). Electrophilic substitution in position 2 (6) of phosphorinane 1-oxides and 1-sulfides can be achieved by a Wittig-Horner a-deprotonation, e.g. equation (18) (72BSF(2)402l, 79CJC723). 

Methylpropene can be removed from the reaction mixture by distillation and easily is made the principal product by appropriate adjustment of the reaction conditions. If the 2-methylpropene is not removed as it is formed, polymer and oxidation products become important. Sulfuric acid often is an unduly strenuous reagent for dehydration of tertiary alcohols. Potassium hydrogen sulfate, copper sulfate, iodine, phosphoric acid, or phosphorus pentoxide may give better results by causing less polymerization and less oxidative degradation which, with sulfuric acid, results in the formation of sulfur dioxide. 

The Martin sulfurane is an excellent reagent for the rapid dehydration of tertiary alcohols.12 Mechanistically, the dehydration involves ligand exchange around sulfur, diphenylsulfonium ion formation, and El elimination of diphenylsulfoxide to give the alkene (Scheme 8.11). 

Dehydration. Although early studies of dehydration of tertiary alcohols were shown to involve an anti-elimination, little use of this stereoselectivity has been applied to preparation of (E)- or (Z)-alkenes. This reaction is useful for preparation of (E)- and (Z)- A22- or A23-sterols, and in combination with svn-hydroboration of alkenes for isomerization of trisubstituted steroidal alkenes.1... 

Slerically hindered olefins can be obtained by dehydration of tertiary alcohols with HMFl at elevated temperatures. Thus (2) can be obtained from (1) in this way in 77%... 

From your knowledge of the dehydration of tertiary alcohols, which olefin should predominate in the product of the dehydration of 2-methyl-2-butanol and why ... 

These points may be illustrated by two examples acid-catalyzed dehydration of tertiary alcohols (Scheme 3), and solvolytic dehydrohalogenation of secondary alkyl halides (Scheme 4). ... 

The mechanism by which proton acids catalyze the dehydration of primary and secondary alcohols in water is not perfectly well understood (1). There is universal agreement that the dehydration of tertiary alcohols can be explained by an El mechanism (1,2) involving either a II complex ( ) or a symmetrically solvated carbonium ion (4) as the key reaction intermediate. Although an occasional text ( ) also describes the dehydration of primary alcohols by an El mechanism, authoritative reviews (1/4) conclude that a concerted E2 type mechanism is more probable. The dehydration behavior of secondary alcohols is presumed to be similar to primary alcohols (4). Discussions of the gas phase dehydration of alcohols by heterogeneous Lewis acid catalysts admit more possibilities. In their authoritative review Kut, et al. (1) consider E1-, E2-, and ElcB-like mechanisms, as well as the possible role of diethyl ether as a reaction intermediate, but they reach no conclusion concerning the relative importance of these mechanisms in the formation of olefins from alcohols. 

Dehydration of tertiary alcohols 16 formed by the addition of organo-metallic species to ketones 15. An example is from Corey s erythronolide synthesis where dehydration of 18 gives only the -enyne319. 

Zeolites are also excellent catalysts in the selective dehydration of tertiary alcohols. Shape selectivity, i. e. selective dehydration of 1-butanol to 1-butene over CaA zeolite in the presence of 2-butanol has also been reported [36]. The bulky 2-methylcyclohexanol, in turn, undergoes dehydration over zeolite Y, although resulting in a mixture of alkenes [37]. 

BF3-OEt2 promotes fast, mild, clean regioselective dehydration of tertiary alcohols to the thermodynamically most stable alkenes. 11/3-Hydroxysteroids are dehydrated by Bp3-OEt2 to give A -enes (eq 21). ... 

Although the dehydration of tertiary alcohol shows strong El elimination character, an alternative mechanism for the dehydration of t-BuOH is presented here. 

An alcohol can be converted to an alkene by dehydration (i.e by the elimination of a molecule of water from adjacent carbon atoms). Dehydration is most often brought about by heating the alcohol with either 85% phosphoric acid or concentrated sulfuric acid. Primary alcohols are the most difficult to dehydrate and generally require heating in concentrated sulfuric acid at temperatures as high as 180°C. Secondary alcohols undergo acid-catalyzed dehydration at somewhat lower temperatures. Add-catalyzed dehydration of tertiary alcohols often requires temperatures only slightly above room temperature. 

In the example, the conjugate base of the acid is the poor nucleophile HS04, and proton loss from the intermediate carbocation is observed. Dehydration of tertiary alcohols is even easier, often occurring at slightly above room temperature. 

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