2.3- Dimethyl-2,3-butanediol, rearrangement

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. 

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 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 ... 

As catalytic tests four reactions, isomerization of 1-butene and methyloxirane, dehydration of 2-propanol and the pinacol rearrangement of 2,3-dimethyl-2,3-butanediol were used. 

The pinacol rearrangement was first described by Fittig in 1860 [10]-treatment of pinacol (2,3-dimethyl-2,3-butanediol, 1), a ditertiary 1,2-diol, with sulfuric acid resulted in the formation of pinacolone (3,3-dimethyl-2-butanone, 2) (Scheme 1). The overall reaction is dehydration with concomitant migration of a methyl substituent. 

Simple diols have been subjected to the pinacol rearrangement in the presence of Amberlyst 15, a polystyrene sulfonic resin with a high concentration of acidic centers. When excess catalyst is used (g catalyst/g diol = 1.25) pinacol gives 3,3-dimethyl-2-butanone in near quantitative yield (batch reactor, 373 K, 2 h) [33]. Of the two isomeric ketones 3,3-diphenyl-2-butanone (10), formed with phenyl migration, is the sole product when 2,3-diphenyl-2,3-butanediol (9) is reacted (Scheme 6). This feature is very similar to that observed under homogeneous conditions in the presence of sulfuric acid [2]. An arylsulfonic silica catalyst prepared by grafting and sulfonation proved to be less selective [33]. 

Photolysis products from ergosterol pro-, pre-, lumi-, and tachy-, 76, 283 Phthalic anhydride 1,3-lsobenzofurandione (85-44-9), 76,123 Pinacol 2,3-Butanediol, 2,3-dimethyl- (76-09-5) 75, 98 Pinacol rearrangement, 76, 296... 

Perhaps the quintessential cationic rearrangement reaction used for synthesis is the Pinacol rearrangement. Fittig was the first to discover that treatment of 2,3-dimethyl-2,3-butanediol (118) with sulfuric acid... 

The reaction was first described for pinacol (2,3-dimethyl-2,3-butanediol), a ditertiary 1,2-diol and catalyzed by sulfuric acid (73). Now it is known that the pinacol rearrangement is characteristic of all types of 1,2-diols, and most electrophilic catalysts are capable of promoting the process. Two possible mechanisms may account for the experimental observations. The stepwise mechanism involves the )8-hydroxy carbocationic intermediate 6 and the pentacoordinate carbocation 7 (Scheme 1). According to the concerted mechanism, the product carbonyl compoimd is formed through the hypercoordinate intermediate 8. 

The Pinacol Rearrangement of 2,3-Dimethyl-2,3-butanediol (Pinacol) (Section 10.7) Chromic Acid Oxidation of an Alcohol (Section to.SA)... 

A vicinal diol reacts with sulfuric acid to give a rearranged dehydration product. The reaction of 2,3-dimethyl-2,3-butanediol (pinacol) to 3,3-dimethyl-2-butanone (pinacolone) is an example of this process, generally known as the pinacol rearrangement. 

---