Ketones pinacol-pinacolone rearrangement

No. Pinacolone has a trimethylated a C. Acetoacetic ester is used for preparing mono- or dialkylated methyl ketones. The compound is made by a pinacol-pinacolone rearrangement of (CH,)2COHCOH(CH,)2. 

The conversion of pinacol (Expt 5.35) to t-butyl methyl ketone (pinacolone, Expt 5.98) under acid conditions exemplifies a general reaction of 1,2-diols (the pinacol-pinacolone rearrangement). The mechanism, formulated below, involves loss of water from the protonated 1,2-diol accompanied by a 1,2-nucleophilic shift of a methyl group. 

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). 

Those compounds, which fit the type R R COH COH R R where R may also be R, are known as pinacols. Pinacols, under acidic conditions, form ketones by what is called a "pinacol-pinacolone rearrangement. This rearrangement is shown in the following reaction ... 

In the reaction of the hydroxy ester (87) with methylmagnesium iodide the process stops at the ketone (91) stage. The pinacols were investigated under various pinacolone rearrangement conditions.144 By a Grignard reaction of 2-cyanoquinuclidine (92), 2-benzoyl-quinuclidine (93) has been prepared.49... 

Scheme 8 1. A representation of the prnacol-pinacolone rearrangement. Pinacol (2,3-dunethlbutan-23-diol) on treatment with acid (snlfnric add, H2SO4) is protonated, loses water, and a methyl gronp migrates (as methide) to the carbon from which water was lost with participation of the remaining hydroxyl gronp. Proton loss from the resulting protonated ketone generates pinacolone (3-methyl-2-butanone).

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

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... 

Another example of migration of a group, in the original case Me, to a cationic carbon atom occurs in the acid-catalysed rearrangement of 1,2-diols, e.g. pinacol (cf. p. 218) Me2C(OH)C(OH)Me2 (34) to ketones, e.g. pinacolone, MeCOCMe3 (35) ... 

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 dehydration of ditertiary alcohols in the presence of hydrobromic acid may lead to dienes (e.g. pinacol to 2,3-dimethylbuta- 1,3-diene, cognate preparation in Expt 5.12), although in this case some concomitant rearrangement to t-butyl methyl ketone (pinacolone, Expt 5.98) occurs under the acidic conditions employed. 

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 ... 

Heating 2,3-dimethyl-2,3-diol, pinacol (24), at 80°C in the presence of Nafion-H gave a 92% yield of pinacolone (methyl-tert-butyl ketone) (25) (Eqn. 22.18). Similar yields were obtained with other ditertiary vicinal diols. Reaction of pinacol with either La-HY or H-ZSM-5 or aluminum exchanged montmorillonite 2 gave both pinacolone (25) and 2,3-dimethylbutadiene (26), resulting from the dehydration of the diol, in nearly equal quantities. Only a small amount of dehydration was observed when the rearrangement was run over... 

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). 

Rearrangements of pinacol, l,l -dihydroxy-l,l -dicyclopentyl, and 1,1 -dihydroxy-1,1 -dicyclohexyl to the corresponding ketones, with negligible alkene formation, occurred in superheated D2O in 60 min at 275 °C [103]. Although a classical method for the pinacolone from pinacol required boiling in 25%... 

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