1.2- Diols pinacolone rearrangement

F I T T I G Pinacolone Rearrangement Acid catalyzed cartxjcation reanangement o< f, 2-diols to ketones... 

A solvent-free pinacol-pinacolone rearrangement has been reported using micro-wave irradiation. The process involves the irradiation of the gem diols with Al3+-mon-tmorillonite K 10 clay for 15 min to afford the rearrangement product in excellent yields (Scheme 6.25) [24]. The comparative studies performed by conventional heating in an oil bath showed that the reaction times are too long (15 h). 

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

Many carbonyl compounds can be synthesized by acid-catalyzed rearrangements of 1,2-diols (a type of reaction often called the pinacol-pinacolone rearrangement). 

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. 

In the pinacol-pinacolone rearrangement a 1,2-diol rearranges to give a carbonyl group that is adjacent to a quaternary carbon. There are many variations of this reaction for example, the semipinacol rearrangement, in which a (P-amino alcohol is treated with nitrous acid. This reaction may be modified so that a ring expansion takes place, in which case it is called the Tiffeneu-Demyanov ring expansion. 

In the previous chapter, one of the first reactions that we studied was the pinacol-pinacolone rearrangement, in which a symmetrical diol gave a carbonyl compound. There are many ways to make a vicinal diol, e g. the addition of H202 to a carbon/carbon double bond. Such reactions were... 

Pinacol-pinacolone rearrangement The rearrangement of a 1,2-diol when treated with concentrated sulphuric acid. Named after the rearrangement of pinacol to pinacolone. 

The pinacol-pinacolone rearrangement of 1,2-diols may be interpreted by two possible mechanisms. The concerted mechanism (Scheme 6.15, route a) involves the 49 pentacoordinate intermediate, whereas in the stepwise mechanism (Scheme 6.15, route Z>) the intermediate [i-lix droxy carbenium ion... 

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). 

Diols give dinitrates under the reaction conditions. No pinacolone rearrangement was observed. Glycerol gives trinitroglycerin in quantitative yield. 

The pinacolone rearrangement of diols to their corresponding carbonyl compounds (e.g. equation 4.16) is promoted by faujasite and pentasil zeolites [91]. HY zeolite performed best at a temperature of 105°C to give yields of up to 83%. 

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).

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... 

Rearrangements may also proceed via intermediates that are essentially cations, anions, or radicals, though those involving carbocations, or other electron-deficient species, are by far the most common. They may involve a major rearrangement of the carbon skeleton of a compound, as during the conversion of 2,3-dimethylbutan-2,3-diol (pinacol, 42) into 2,2-dimethylbutan-3-one (pinacolone, 43, cf. p. 113) ... 

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) ... 

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 ... 

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. 

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