Pinacolones Pinacol rearrangement

Synthesis Ketone A is just pinacolone, the product of the pinacol rearrangement (frames 154-5). 

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. 

A reaction that follows this pattern is the acid-catalyzed conversion of diols to ketones, which is known as the pinacol rearrangement.60 The classic example of this reaction is the conversion of 2,3-dimethylbutane-2,3-diol(pinacol) to methyl /-butyl ketone (pinacolone).61... 

A number of experiments have been carried out to determine the relative migratory aptitude of groups in pinacol/pinacolone type rearrangements, and in general the relative ease of migration is found to be ... 

The presence of a quaternary atom, for instance, may be traced back to either a "pinacol" or a "Wagner-Meerwein-type" rearrangement. In the classical "pinacol rearrangement", pinacol 1 (a 1,2-diol) under strong acid conditions rearranges to pinacolone 2 which bears a quaternary atom at the a-position of the carbonyl group ... 

Dehydration of diols pinacol rearrangement. Preparation of pinacolone... 

Pinacol rearrangement is a dehydration of a 1,2-diol to form a ketone. 2,3-drmethyl-2,3-butanediol has the common name pinacol (a symmetrical diol). When it is treated with strong acid, e.g. H2SO4, it gives 3,3-dimethyl-2-butanone (methyl r-butyl ketone), also commonly known as pinacolone. The product results from the loss of water and molecular rearrangement. In the rearrangement of pinacol equivalent carbocations are formed no matter which hydroxyl group is protonated and leaves. 

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 pinacol rearrangement is a useful reaction that proceeds via a carbocation rearrangement. Treatment of 2,3-dimethyl-2,3-butanediol, also known as pinacol, with acid results in the formation of a ketone, pinacolone ... 

Many common rearrangement reactions are related to the rearrangement of 1,2-dihydrojqr compounds to carbonyl compounds. Often these reactions are called pinacol rearrangements, because one of the first examples was the transformation of pinacol to pinacolone ... 

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 ... 

All classes of vicinal diols (primary, secondary, tertiary, alkyl- or aryl-substituted) will undergo the pinacol rearrangement, and many acids and solvents have been used for this purpose. Various procedural modifications have been introduced over the years for particular glycols, but sulfuric acid remains the most commonly employed catalyst. The use of 25% H2SO4, as recommended in the procedure of Adams, affords pinacolone in essentially quantitative yield. In some instances better results are obtained when cold concentrated acid is used as the solvent. The choice of reagents and conditions is important, and can completely alter the course of the reaction. For example, pinacol also serves as the starting material for the synthesis of 2,3-dimethylbutadiene, formed by slow distillation of a mixture of the diol and catalytic HBr. ... 

From Table 2 (entry 5) and Table 3 (entry 5) it is apparent that the Ti-Al-Beta catalyzed pinacol rearrangement results in a higher yield of pinacolone 4 than the direct rearrangement starting from the epoxide. Consequently, in order to promote the rearrangement of epoxide 2 to pinacolone 4 via pinacol 3, 2 equivalents of water (relative to the amount of epoxide) were added at the start of the reaction (Scheme 4 conditions A). Furthermore, after formation of the diol a Dean-Stark apparatus was used to remove the excess water (conditions B). 

The diol 20 (pinacol) rearranges to pinacolone 22 in the presence of acid in the second step the methyl group migrates with its bonding electrons to give the stabilized cation 21, which loses a proton to give the final ketone 22 (reaction 5.17). 

A change of a polarity from a polar to nonpolar state (reverse polarity change) can be accomplished by the pinacol-pinacolone rearrangement and has been exploited in chemically amplified lithographic imaging [151, 348-350]. The pinacol rearrangement involves conversion of vie-diols to ketones or aldehydes with an acid as a catalyst (Fig. 115). 

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