Enolate ions rearrangement

This reaction, called the oxy-Cope rearrangement has proved highly useful in synthesis." The oxy-Cope rearrangement is greatly accelerated (by factors of 10 -10 ) if the alkoxide is used rather than the alcohol.In this case the direct product is the enolate ion, which is hydrolyzed to the ketone. 

Another example of the formation of a rearranged product is the palladium(0)-catalysed reaction of the enolate ion of 2-methylcyclohexanone with 3-methyl-3-nitro-l -butene... 

Dufraisse announced that 138 was formed when an alkaline solution of hydroxyindanone 185 is treated with air or oxygen up to 80% 138 could be obtained in this way. This observation can be explained either by oxidation of an enolate ion to an oxiranol, with subsequent rearrangement, or by oxidative cleavage of the enolate ion to a benzilic acid (186), which after ring closure will lose CO2 and HjO to give 138 (Scheme 8). Compound 138 is also formed when n-benzoylbenzil (187) is treated with potassium hydroxide in ethanol a benzilic acid rearrangement may be the first step of the sequence. 

Platinacyclobutane complex 118 undergoes equilibrium heterolytic scission of the exocyclic carbon-carbon bond to form a cationic allyl complex and the organic enolate ion (Equation 35) <1993OM3019>. Similar dissociative ionization was previously reported for rearrangements of iridium and rhodium metallacyclobutane complexes formed by nucleophilic alkylation < 1990JA6420>. This carbon-carbon bond activation is generally associated with reversible central carbon alkylation of Jt-allyl complexes (Section 2.12.9.3.3), but the homolytic equivalent has recently been... 

Another base-catalyzed side reaction is the enediol rearrangement, which moves the carbonyl group up and down the chain, as shown in Mechanism 23-3. If the enolate ion (formed by removal of a proton on C2) reprotonates on the Cl oxygen, an enediol intermediate results. Removal of a proton from the C2 oxygen and reprotonation on Cl gives fructose, a ketose. 

We call this disconnection enolate allylation because we want to alkylate an enolate ion with an allyl halide. We have already discussed (chapter 19) the regioselectivity problems inherent in reactions of allyl derivatives and you should recall that we want to react at the correct (less substituted) end of the allyl system and we want to make an f-alkcnc. The solution is to use a [3,3]-sigmatropic rearrangement. This is the reaction sequence ... 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

When an alkyne undergoes the acid-catalyzed addition of water, the product of the reaction is an enol. The enol immediately rearranges to a ketone. A ketone is a compound that has two alkyl groups bonded to a carbonyl (C=0) group. An aldehyde is a compound that has at least one hydrogen bonded to a carbonyl group. The ketone and enol are called keto-enol tautomers they differ in the location of a double bond and a hydrogen. Interconversion of the tautomers is called tautomerization. The keto tautomer predominates at equilibrium. Terminal alkynes add water if mercuric ion is added to the acidic mixture. In hydroboration-oxidation, H is not the electrophile, H is the nucleophile. Consequently, mercuric-ion-catalyzed addition of water to a terminal alkyne produces a ketone, whereas hydroboration-oxidation of a terminal alkyne produces an aldehyde. 

Claisen rearrangement. The ester enolate Claisen rearrangement of allyl esters of protected amino acids proceeds after replacing the lithium ion with Zn. 

The authors propose the following mechanism (Scheme 47) The benzoxepine is protonated, yielding the enol. Subsequent proton transfer gives the unstable oxonium ion. Rearrangement via C—O bond cleavage and formation of a new C—C bond generates an aromatic compound, ethyl 3,4-dihydroxy-2-naphthoate. This intermediate then reacts with o-phenylenediamine to yield the respective phenazine. 

Recently reported rearrangements of linear triquinanes (244) to their angularly fused isomers (247) have been envisaged to involve the formation and equilibration of the enolates (245) and (246) followed by an intramolecular Michael addition of the enolate ion in (246) to the cyclopentenone moiety see Scheme 62. It has been reported that treatment of l,10-dibromobicyclo[8.1.0]undecane-3,8-dione (248) with triethylamine in dichloromethane results in the formation of a novel cyclopenta[(>]pyran derivative (249). The mechanism outlined in Scheme 63 has been postulated to account for this unusual transformation. 

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