Acid-catalyzed dehydration condition

Terpigorev and Rudakova have employed the condensation of the diol 331 with nitroguanidine 332 under acid-catalyzed dehydrating conditions to generate the tetrahydroimidazole ring 333 in 55% yield (Equation 89) <1998RJO1026>. 

Levuhnic acid is one of the products of selective dehydration of cellulosic biomass feedstocks. Levuhnic acid is produced when six-member ring carbohydrates derived from ceUulose are subjected to acid-catalyzed dehydration conditions (Fig. 9.6) [4, 43]. The other main product of this reaction is formic acid. Although levulinic acid has some potential use as a solvent or in the production of industrial and pharmaceutical chemicals, its current market is minimal [4]. Therefore, the conversion of levulinic acid to a directly usable biofuel has become an important area of interest. In particular, esterification, oxidation, hydrogenation, reductive amination, condensation, and enzymatic conversion have been tested as potential methods to produce useful compounds from levulinic acid [4,44,45]. 

As a method for the preparation of alkenes a weakness in the acid catalyzed dehydration of alcohols IS that the initially formed alkene (or mixture of alkenes) sometimes isomenzes under the conditions of its formation Write a stepwise mechanism showing how 2 methyl 1 butene might isomenze to 2 methyl 2 butene in the presence of sulfuric acid... 

Acid catalyzed dehydration of benzyhc alcohols is a useful route to alkenylben zenes as is dehydrohalogenation under E2 conditions... 

The reaction is an F.1 process and occurs through the three-step mechanism shown in Figure 17.6). As usual for El reactions (Section 11.10), only tertiary alcohols are readily dehydrated with acid. Secondary alcohols can be made to react, but the conditions are severe (75% H2S04,100 °C) and sensitive molecules don t survive. Primary alcohols are even less reactive than secondary ones, and very harsh conditions are necessary to cause dehydration (95% H2S04,150 °C). Thus, the reactivity order for acid-catalyzed dehydrations is... 

FeCl3 6H20 turned out to be the catalyst of choice for this reaction, since the presence of water improved the yield. However, high yields of the desired ketones were obtained for electron-rich alkynes with anhydrous FeCla at room temperature. Alcohols that are sensitive to acid-catalyzed dehydration were also tolerated under the present conditions (R = Me or Et). Based upon experimental observations a mechanism for this reaction was proposed (Scheme 14). 

Allenyl cations generated from silver dehalogenations6 or acid-catalyzed dehydration of prop-2-yn-l-ols7 also undergo cycloaddition to alkenes under mild conditions. 

Depending upon the conditions, the reaction of benzylmagnesium chloride with benzal-dehyde can yield benzylphenylmethanol, dibenzoylphenylmethane and 1,3-diphenyliso-chroman. The last product arises from the acid-catalyzed dehydration of the 1,5-diol (296), which is the predominant product when the Grignard reagent is added to benzaldehyde at room temperature (Scheme 81) (51 3163). There was no evidence for the formation of o-methylbenzhydrol, consistent with the lack of this type of product with citronellal and various simple aliphatic aldehydes (44JA354,51JA3237). 

Alcohols can undergo acid-catalyzed dehydration to give either the corresponding alkenes or the corresponding ethers. The product distribution of the dehydration of alcohols over Nafion-H catalyst shows temperature dependence187 (Table 5.40). Alcohols are thus efficiently dehydrated in the gas phase over Nafion-H under relatively mild conditions with no evidence for any side reactions such as dehydrogenation or decomposition. At higher temperature, olefin formation predominates. 

Cyclodehydration of hexane-1,6-diol with tin phosphate as a catalyst (230°C) gives oxepane in 32% yield <2001GC143>. Acid-catalyzed dehydration of 2,2 -bis(hydroxydiphenylmethyl)biphenyl by an ion-exchange resin allows one to prepare a sterically hindered 2,2,7,7-tetraphenyldibenzo[c, ]oxepine in 70% yield under mild conditions (CH2C12, rt) <2002J(P1)2673>. 

Under aqueous conditions dimethylamine, piperidine, and triethylamine catalyze the ring-opening reaction of 3a,5,6a-triaryl-3,3a-dihydro-2//-furo[3,2-6]pyrrole-2,6(6a//)-diones (178) <84S663> giving benzoylhydroxy-l-pyrrolines (179), which afford benzoylpyrroles (180) upon acid-catalyzed dehydration (92BCJ2611). 

The first category includes furan compounds, water, and char—all of which are the expected products of the better understood acid-catalyzed dehydration reactions of carbohydrates under aqueous acid conditions (J). The second category includes glyoxal, acetaldehyde, and other low molecular weight carboxyl compounds, which are sim-... 

Experimental data are presented which describe the acid catalyzed dehydration chemistry of 1-propanol and 2-propanol in supercritical water at 375 C and 34.5 MPa. The data for 1-propanol dehydration are kinet-ically consistent with the acid catalyzed E2 mechanism, but not consistent with the related El mechanism. Neither the Ad 3 mechanism nor the Ad 2 mechanism is able to mimic the kinetic behavior of 2-propanol formation. The steady state idealization of the E2 mechanism does not represent the true kinetic behavior of the E2 mechanism over the range of experimental conditions presented in this paper. 

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