Alkyne-alcohols, dehydration

Moving one step further back in the synthetic sequence towards cyclopentenoftes identifies the alkynic alcohols as viable precursors. Since the subsequent dehydration-hydration-cyclization steps are all acid-... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

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). 

The reactions are those of functionalized 1-alkynes, the first of which were described for alkynes bearing substituted hydroxymethyl groups, such as substituted propargyl alcohols, HC=CCRR (OH), and often proceed further to form metal allenylidene complexes, by spontaneous dehydration of a (usually unobserved) hydroxy-vinylidene complex (Equation 1.22) ... 

Consideration of the oxidation level reveals diat while one carbon is reduced (the one to which hydrogen adds), die other is oxidized (die one to which the oxygen adds). There is no net change in oxidation level of the alkene functional group. Likewise die reverse processes of these addition reactions, namely, elimination of HX from alkyl halides and dehydration of alcohols to give alkenes, are not redox processes. Additions of water to alkynes is analogous. In this case, however, the product is a ketone, the oxidation level of the ketone is seen to be the same as the alkyne, and so no net change in oxidation level has occurred. 

We seem not to be getting closer to 58, but 58 would actually result by dehydration of 62. In real life, the alcohol 62 is turned into the chloride and a double elimination with NaNth gives 58 after acidification.14 Though we saw elimination reactions used to make alkenes in chapter 15, this is the first we have seen to make alkynes. 

A similar reaction applied to propargyl alcohols in place of terminal alkynes led to the one-step catalytic head-to-head cyclodimerization of propargyl alcohols and to the formation of alkylidenecyclobutene derivatives [90] (Eq. 69). It was shown that the reaction occurs via cyclobutadieneruthenium and cy-clobutenylruthenium intermediates, dehydration and carboxylate addition. 

Alkenes and alkynes obviously don t fit easily into these categories as they have no bonds to heteroatoms. Aikenes can be made from alcohols by dehydration without any oxidation or reduction so it seems sensible to put them in the alcohol column. Similarly, alkynes and aldehydes are related by hydration/dehydration without oxidation or reduction. 

Dehydrogenation of alkanes such as ethane (1.38) relates them to alkenessuch as ethene (ethylene, 1.39). The same functional group may be obtained by dehydration of ethanol (1.40). Further dehydrogenation of ethene would generate an alkyne, ethyne (acetylene, 1.41). In terms of oxidation level, the alkene is related to the alcohol and the alkyne is related to the ketone. 

Dehydration of allylic alcohols with strong acid leads to allenes if an isomerization to an alkyne is not possible. This method has been used for the synthesis of tetraarylallenes and tetra-r-butylallene. This method was also used for the first synthesis of an optically active allene by an enantioselective dehydration using (-l )-camphorsulfonic acid. ... 

No doubt joining the alkyne to the alkene could also have been done by a coupling reaction in the coordination sphere of a metal but an alternative is to imagine the alkene as coming from the dehydration of an alcohol 251. This allows disconnection to the known lactone 250. The synthesis of the alkyne uses DIBAL for partial reduction and the differential protection of the two OH groups by more or less hindered silyl groups. 

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