Pinacol and Related Rearrangements

Scheme 10.3 gives some examples of pinacol and related rearrangements. Entry 1 is a rearrangement done under strongly acidic conditions. The selectivity leading to ring expansion results from the preferential ionization of the diphenylcarbinol group. Entry 2, a preparation of 2-indanone, involves selective ionization at the benzylic alcohol, followed by a hydride shift. 

Pinacol and related rearrangements (Tiffeneau-Demjanow rearrangement, see Scheme II/5)... 

Closely related to the Wagner-Meerwein reaction is the [1,2] rearrangement of vicinal diols and related substrates collectively referred to as pinacolic rearrangements2,319. Like the Wagner-Meerwein shift, the pinacol rearrangement proceeds via formation of an intermediate cation, followed by alkyl migration and concurrent generation of a carbonyl system. 

When aromatic pinacols are reacted with an acid, products often arise from dehydration and rearrangement.5 This general conversion is known as the pinacol rearrangement. The pinacol rearrangement may be promoted by both Brdnsted and Lewis acids.6 In the procedure described here, superacidic triflic acid is reacted with an aryl pinacol and a dehydrative cyclization occurs to give the substituted phenanthrene product. Related to this conversion, the chemistry of benzopinacol in sulfuric acid and triflic acid is contrasted in Scheme 1. We have proposed that the superacidic triflic acid causes the formation of diprotonated intermediates which promote the dehydrative cyclization.4... 

Recent advances in the semi-pinacol rearrangement of a-hydroxy epoxides and related compounds 07CSR1823. 

A recent review (Collins, 1960) of the pinacol rearrangement covers nearly all of the pertinent isotopic experiments that have been performed. One example of the use of two carbon-14 labels to relate the aldehyde-ketone and pinacol rearrangements will be discussed in detail here. The anomaly of the apparent reversal, in the aldehyde-ketone rearrangement, of the usual order of migratory aptitudes has been mentioned by Wheland (1949) and by Ingold (1953), and relates to the fiict that, for example, 2-methyl-2-phenylpropionaldehyde (67), in concentrated sul-... 

Scheme 1.6 Development of an asymmenic pinacol rearrangement by Antilla and coworkers— and related application to a total synthesis of hopeanol I37L—...

Some representative examples of Pd-induced pinacol-pinacolone-type rearrangement are shown in Scheme 7,P3]-[27] should be mentioned here that all of these reactions involve both rearrangements and oxidation. Other related rearrangement reactions, which do not involve oxidation or reduction, are discussed in Sect. IX.2.1.2. 

There is also a mechanism that can operate in the absence of an acidic a-hydrogen. This process, called the semibenzilic rearrangement, is closely related to the pinacol rearrangement. A tetrahedral intermediate is formed by nucleophilic addition to the carbonyl group and the halide serves as the leaving group. 

Some related ring-expansion reactions utilizing a modified version of the pinacol rearrangement, as shown for the formation of 8,45 9,46 and 10,47 have been performed. In particular,... 

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