Dehydration regioselectivity, alcohols

In addition to being regioselective alcohol dehydrations are stereoselective A stereo selective reaction is one m which a single starting material can yield two or more stereoisomeric products but gives one of them m greater amounts than any other Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene Dehydration of 3 pentanol for example yields a mixture of trans 2 pentene and cis 2 pentene m which the more stable trans stereoisomer predominates... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

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. 

Dehydration. Tertiary alcohols are dehydrated in refluxing dioxane in the presence of Mo02(acac)2. However, regioselectivity is not observed. 

Dehydration of Alcohols. The combination of POCI3 and Pyridine is an effective dehydrating agent for alcohols (eq 24) and cyanohydrins. The stereochemistry of elimination is anti, although the regioselectivity is often not high, particularly for tertiary alcohols. ... 

Dehydration of this alcohol is selective m respect to its direction Elimination occurs m the direction that leads to the double bond between C 2 and C 3 more than between C 2 and C 1 Reactions that can proceed m more than one direction but m which one direction is preferred are said to be regioselective... 

Chloral or benzaldehyde in the presence of dehydrated alumina48 and Al(OtBu)3 in the presence of f-BuOOH,49 are oxidizing systems reminiscent of Oppenauer oxidations that can perform regioselective oxidations of secondary alcohols. 

An improved route to the key intermediate 326 was also developed (165). Namely, 322 was converted to the monoprotected 1,4-dione 327 by sequential addition of the Grignard reagent derived from 2-(2-bromoethyl)-2-methyl-l,3-dioxolane followed by oxidation of the resulting benzylic alcohol with pyridin-ium dichromate (PDC). The ketone 327 was then smoothly transformed to the 2-azadiene 328 by olefination with BAMP. The regioselective addition of n-butyllithium to 328 as before followed by alkylation of the resulting metalloenamine with benzyl A-(2-bromoethyl)-A-methylcarbamate and acid-catalyzed hydrolysis furnished 325, which was converted to the cyclohexenone 326 by base-induced cycloaldolization and dehydration. 

When an alcohol has two or three different P carbons, dehydration is regioselective and follows the Zaitsev rule. The more substituted alkene is the major product when a mixture of constitutional isomers is possible. For example, elimination of H and OH from 2-methyl-2-butanol yields two constitutional isomers the trisubstituted alkene A as major product and the disubstituted alkene B as minor product. 

The metallation of diethyl 3-(trimethylsilyl)-l-propenylphosphonate with LDA at low temperature gives the delocalized allylphosphonate carbanion, which reacts regioselectively in the a-posi-tion with aliphatic or aromatic aldehydes to afford the corresponding P-alcohol as a mixture of diastereomers. " Subsequent dehydration with DCC in CH2CI2 at reflux in the presence of catalytic CUCI2 stereospeciflcally produces the desired 2-phosphonylated 1,3-dienes. A one-pot approach is based on the P-elimination reaction of an intermediate enol phosphate using z-BuOK. ... 

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