Rearrangements of diols

Pinacol rearrangement of 1,2-diols Zeolites NaX, NaY, Carbonyl Fine chemicals... 

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) ... 

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 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... 

The isomerization of endo-trimethylenenorbomane to the exo isomer is efficiently catalyzed by superacid systems in cyclohexane or Freon-113 (1,1,2-tri-chloro-l,2,2-trifluoroefhane) [65]. The fd (SO,l l-Sbl -, system is also effective for isomerization of trimefhylenenorbornane to adamantane, although a stoichiometric amount of the acid is necessary to obtain good results [66]. SbCl and SbClj-AgSbFg promote the pinacol rearrangement of 1,2-diols and their trimethylsilyl ethers (Scheme 14.27) [67]. 

F IT T I G Pinacolone Rearrangement Acid catalyzed carbocation rearrangement of 1,2-diols to ketones. 

Cyclization of halohydrins 0-16 Cyclization of 1,2-diols 0-18 Payne rearrangement of 2,3-epoxy alcohols... 

The MNDO method has been employed405 to study the reaction pathway and to optimize the structures of reactant, product, and transition state of the acid-catalysed rearrangement of 1,2-propylene glycol, and the unimolecular dehydration of protonated a,co-diols in the gas phase has been examined406 by tandem mass spectrometric experiments. It has been shown that the reaction of l,2-diarylcyclopropane-l,2-diols (342) with acids yields primarily the a,//-unsaturated ketones (343) in which the aryl... 

A chiral Lewis acid-catalysed method for the 1,2-migration of (dichloromethyl)borate complexes to provide synthetically useful (a-chloroalkyl)boronates has been developed,557 and the diastereoselective rearrangement of the a,a-dichloromethylboronate derivatives of 1,2-diols, (427) —> (429), has been explained558 on the basis of a bidentate interaction between the catalytic Lewis acid and the substrate, leading to a favoured transition state (428). 

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. 

An efficient pinacol rearrangement mediated by trialkyl orthoformate has been developed [83]. The reactions of various types of 1,2-diol with a catalytic amount of SnCU in the presence of trimethyl orthoformate afford the rearranged product in good yields via a cyclic ortho ester intermediate (Eq. 50). This combined system is applicable not only to cyclic and acyclic tri- and tetrasubstituted diols but also to diols forming acid-sensitive acetals. 

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