Alcohols, dehydration isomerization following

Further, a very recent paper has reported a cascade rearrangement, under acidic conditions of the simpler educt, a dispiro[3.0.3.3]undecane derivative 93 to the dehydrated isomeric propellane 94 40). It is somewhat reminiscent of the analogous case of 26 where silver ion is the catalyst10). Treatment of the dispiro-alcohol 78 when heated for 2 hrs at 70 °C with p-toluenesulfonic acid in benzene gives in quantitative yield the [3.3.3]propellene 94. The following cascade is proposed to explain the rearrangement. 

In order to obtain some information on the reaction mechanism, the reaction of propargylic alcohol with acetone in the presence of a catalytic amount of 5a was monitored. The result indicated that the catalytic formation of the hexadienone proceeded via the initial isomerization of propargylic alcohol to dnnamaldehyde followed by aldol condensation between the produced aldehyde and acetone, and then dehydration. In fad, heating of propargylic alcohol in the presence of a catalytic amount of 5a gave only dnnamaldehyde (Scheme 7.41), and the separate reaction ofcinna-... 

Few detailed studies of olefinic dehydration products have been reported. Rapoport ei al. (105) found cis- and major products from n-pentanol dehydration over NaX only traces of 2-methyl-1-butene and 2-methyl-2-butene were reported. Thus, it is evident that double bond isomerization has accompanied or followed the dehydration reaction. Several authors (99,101,106) have suggested that diffusion processes may be rate controlling, or at least of some significance, in zeolite-catalyzed dehydration reactions. Molecular-shape selective alcohol dehydration (7) was discussed earlier. 

When monohydric alcohols undergo dehydration, isomeric alkenes can be formed by the loss of water by f (or 1,2-) elimination. Selective synthesis of certain alkenes can, however, be accomplished-when primary alcohols are treated with appropriate solid catalysts terminal alkenes are formed. In contrast, either 1- or 2-alkenes can be produced by dehydration of secondary 2-alkanols. The reactivity of alcohols follows the sequence tertiary > secondary > primary. 

Coordination-dehydration reactions of propargyl alcohols have been followed in detail on triosmium clusters an example is given in Fig. 26.1 Protonation of triosmium clusters substituted with parallel alkynes leads to propargyl cationic derivatives which, in turn, can isomerize to allenylidene complexes. Fig. 27.1 ... 

To conclude, the mie-pot conversion of cellulose-to-lactic acid (or lactate ester in alcoholic media) thus follows a complex cascade reaction network involving at least six reactions. These reactions have different catalytic needs, but, in general, the presence of both Lewis and Brpnsted acidity are paramount for catalytic success. Br0nsted acidity is key to the hydrolysis of cellulose (step 1) at mild temperatures (<200°C), and to some extent to the dehydration of triose (step 4), whereas Lewis acid sites play a vital role in the isomerization reaction of glucose-to-fructose (step 2), the retro-aldol (step 3), and the 1,2-hydride shift (step 6). Steps 4 and 5 are relatively less demanding they are catalyzed by both acid types. 

Humphries et al. [92,93] gave an overview of different test reactions used to characterize the acidity of zeolites. The reactions included cracking, isomerization, disproportionation, and alcohol dehydration as well as hydride transfer reactions involving cyclohexene conversion. In the following sections, we will discuss each class of these test reactions for their ability to give information about the nature, concentration, and strength of the active sites involved in acid-catalyzed reactions. 

The acid-catalyzed or thermal elimination of water from alcohols is a favorite laboratory method for the preparation of olefins. Isomeric mixtures usually arise with the acid-catalyzed method. The order of reactivity in dehydration usually follows the order of stability of the intermediate (transient) carbonium ion, i.e., tertiary > secondary > primary. The acid-catalyzed procedure is illustrated below, where a 79-87% yield of cyclohexene is obtained [1-3]. 

Elimination under acidic conditions is more successful because the hydroxyl group is first protonated and then it departs the molecule as a neutral water molecule (dehydration) that is a much better leaving group. If different isomeric alkenes are possible, the most substituted alkene will be favoured (Following fig.). The reaction occurs best with tertiary alcohols as the elimination proceeds by the El mechanism. 

Alkaline degradation of codeine methiodide affords a-codeimethine [xxi] [185, 285], which can be isomerized by alcoholic alkali to /3-codeimethine [xxn] [187, 286-7], also obtainable by the degradation of neopine [xm] methiodide [271]. The degradation of codeine ethiodide follows a similar course [288]. These bases suffer dehydration and loss of the basic side-chain when heated with acetic anhydride and sodium acetate (when acetylmethylmorphol [xxm] is formed [187, 289-90]), and when subjected to further Hofmann degradation (which leads to methylmorphenol [xxiv] [290-2]). The resulting aromatic phenan-threne derivatives are of considerable importance in the elucidation of the basic structure of the morphine alkaloids and are discussed in detail in Chapter XXVII. 

PROBLEM 5.13 Each of the following alcohols has been subjected to acid-catalyzed dehydration and yields a mixture of two isomeric alkenes. Identify the two alkenes In each case, and predict which one is the major product on the basis of the Zaitsev rule. 

Lithiation of the methyl of (methylthio)benzene followed by acylation with an acyl chloride and acidification to pH 4-5 yields benzothiophene in good yields, but when aroyl chlorides are used, the mixture has to be heated in benzene. An attempted dehydration of the secondary alcohol (87.4) with hydrobromic acid led to S-demethylation and cyclization. Similar treatment of the isomeric 2-(2-methy thio-4-nitrophenyl)-l-phenylethanol gave a high yield of 6-nitro-2-phenyl-2,3-dihydrobenzo[fi]thiophene [2653]. 

An avalanche of discoveries followed—the dehydration of alcohols over alumina, catalytic isomerization of olefins, conversion of ethanol to butadiene. Another major accomplishment was his introduction in 1905 of an autoclave to contain reactions at high pressures and temperatures. His early training as an artillery officer enabled him to develop a tight seal for the autoclave which could withstand high pressures never before attained, thus opening a new era in the investigation of catalytic reactions. 

Methoxy-2,2-dimethylchromene (23.6) and benzofuran 23.2 were also synthesized by alkylation of 4-methoxyphenol to 2-(3-methyl-2-butenyl)-4-methoxyphenol (22.1) followed by acid catalyzed cyclization of the corresponding epoxide 22.2 to a mixture of isomeric compounds 23.3 and 23.7. Dehydration of alcohol 23.7 with / -TsOH gave 23.6, while NBS dehydrogenation of 23.3 afforded 23.2 140). 

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