Pinacol rearrangement, mechanism

DimethyI-2 3-butanediol has the common name piiuicol. On heating with aqueous acid, pinacol rearranges to pinacolone, 3,3-dimethyl-2-butanone. Suggest a mechanism for this reaction. 

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilanc (Peterson olefination, Chapter 10). However, it has recently been shown that aj8-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /J-aldehydo- and /i-kctosilancs (5) ... 

There is also a mechanism that can operate in the absence of an acidic a-hydrogen. This process, called the semibenzilic rearrangement, is closely related to the pinacol rearrangement. A tetrahedral intermediate is formed by nucleophilic addition to the carbonyl group and the halide serves as the leaving group. 

Based on the labeling experiments, a plausible mechanism involving mthenium vinylidene intermediates SS is proposed in Scheme 6.21. Cydization of this vinylidene intermediate leads to the formation of the epoxy carbenium 56, which then undergoes an epoxide opening to form l,4-dien-3-ol 57. A subsequent pinacol rearrangement of this alcohol furnishes ketone 58, providing the required skeleton for the observed phenol product 54. 

The mechanism has been shown to involve a cyclization step followed by a pinacol rearrangement (Hopkins, M. H. Overman, L. E. Rishton, G. M., J. Am. Chem. Soc., 1991, 113, 5354 see also Bach et al., p. 5365, and Woods et al., p. 5378, mentioned in questions 4 and 5, respectively). Provide a detailed mechanism for the reaction and discuss features of the steps and intermediates from the point of view of orbital interactions. 

Isomerization of substituted styrene oxides allows the synthesis of aldehydes in high yields726 [Eq. (5.275)]. Cycloalkene oxides do not react under these conditions, whereas 2,2,3-trimethyloxirane gives isopropyl methyl ketone (85% yield). Isomerization of oxiranes to carbonyl compounds is mechanistically similar to the pinacol rearrangement involving either the formation of an intermediate carbocation or a concerted mechanism may also be operative. Glycidic esters are transformed to a-hydroxy-/3,y-unsaturated esters in the presence of Nafion-H727 [Eq. (5.276)]. 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

The proposed mechanism for the reaction is shown in Scheme 13.75. In the first step, the oxonium cation 208, formed by TfOH-catalyzed condensation of an aldehyde with alcohol 206, undergoes an intramolecular cyclization to form the tertiary carbocation 209. In a subsequent step, cation 209 undergoes a pinacol rearrangement, leading to the observed tetrahydropyran 205. 

Fig. 14.14. Mechanism of the pinacol rearrangement of a symmetric glycol. The reaction involves the following steps (1) protonation of one of the hydroxyl groups, (2) elimination of one water molecule, (3) [l,2]-rearrangement, and (4) deprotonation.

The mechanism for this reaction, shown in Figure 22.5, involves a carbocation rearrangement that occurs by an allowed [1,2] sigmatropic shift. The product of this rearrangement, a protonated ketone, is considerably more stable than the initial carbocation, so the migration is quite favorable. Another example of the pinacol rearrangement is provided in the following equation ... 

The mechanism for the pinacol rearrangement contains no surprises. It consists only of steps that have been encountered previously. 

Fig. 11.12. Mechanism of the pinacol rearrangement of a symmetric glycol. The reaction involves the...

The above-described rearrangement reactions are not the only ones presented within this book. In addition to electrocyclic rearrangements, some rearrangements dependent upon ionic mechanisms were presented. These include the pinacol rearrangement... 

The following reaction involves a starting material with a double bond and a hydroxyl group, yet its mechanism resembles a pinacol rearrangement. Propose a mechanism, and point out the part of your mechanism that resembles a pinacol rearrangement. 

When drawing the mechanism it doesn t matter which hydroxyl group you protonate or which adjacent OC bond migrates—they are all the same. One five-membered ring expands to a six-mem-bered ring but the reason this reaction happens is the formation of a carbonyl group, as in all pinacol rearrangements. 

Diols are converted to carbonyl compounds when treated with strong acids, in a reaction called the pinacol rearrangement. Draw a stepwise mechanism for this reaction. 

Oxiranes cannot be prepared directly from 1,2-diols by dehydration. Formation of the oxirane intermediate has been studied in connection with the mechanism of the pinacolic rearrangement. Oxiranes can be prepared stereoselectively from the acetals and ketals of 1,2-diols. D-(+)-2,3-epoxybutane has been obtained from an optically active diol via conversion of the ketal 64 to a halohydrin ester (Eq. 52). ... 

The first mechanism is a pinacol rearrangement and the compound is symmetrical so it doesn t matter which alcohol is protonated. Both three- and four-membered rings are strained and the cf-bonds are more reactive (higher HOMO energy) than normal. This makes ring contraction an easy reaction even though the strain is not relieved. 

Problem 28.9 The following reactions have all been found to yield a mixture of pinacol and pinacolone, and in the same proportions treatment of 3-amino-2,3-dimethyl-2-butanol with nitrous acid treatment of 3-chloro-2,3-dimethyl-2-butanol with aqueous silver ion and acid-catalyzed hydrolysis of the epoxide of 2,3-dimethyl-2-butene. What does this finding indicate about the mechanism of the pinacol rearrangement ... 

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