Solid pinacol rearrangement

Selective hydroxylation with osmium tetroxide (one equivalent in ether-pyridine at 0 ) converts (27) to a solid mixture of stereoisomeric diols (28a) which can be converted to the corresponding secondary monotoluene-sulfonate (28b) by treatment with /7-toluenesulfonyl chloride in methylene dichloride-pyridine and then by pinacol rearrangement in tetrahydrofuran-lithium perchlorate -calcium carbonate into the unconjugated cyclohepte-none (29) in 41-48 % over-all yield from (27). Mild acid-catalyzed hydrolysis of the ketal-ketone (29) removes the ketal more drastic conditions by heating at 100° in 2 hydrochloric acid for 24 hr gives the conjugated diketone (30). 

Not only the photoreaction but also a pinacol rearrangement (Scheme 3) and a m-chloroperbenzoic acid oxidation (Table 7) occurred in the solid state, selectively and acceleratively. 

The MNDO method has been employed " to study the acid-catalysed rearrangement of propylene 1,2-glycol. Propanaldehyde was found to be the major product with a small amount of acetone also being produced. The solid-state pinacol rearrangement of l,l,2-triphenylethane-l,2-diol has been performed over various solid... 

Acid Form - Pseudoliquid Phase Behavior. Owing to a high affinity for polar molecules, large quantities of molecules such as alcohols and ether are absorbed within the bulk phase of crystalline heteropolyacids. The amounts of pyridine, methanol, and 2-propanol absorbed correspond to 50-100 times that which can be adsorbed on the surface, while nonpolar molecules like ethylene and benzene are adsorbed at the surface only. Catalytic reactions of polar molecules occiu both on the surface and in the bulk, so that the solid heteropolyacid behaves as a highly concentrated solution, called a pseudoliquid phase . The dehydration of alcohols, various conversions of methanol and dimethyl ether to hydrocarbons in gas-solid systems, and the alkylation of phenol and pinacol rearrangements can all occur in the pseudoliquid. The transient response using isotopically labeled 2-propanol provides evidence for the pseudoliquid phase behavior of H3PW12O40. This behavior influences the selectivity, for example, the aUcene/aUcane ratio, in the conversion of dimethyl ether. 

The more environmentally benign solid acid Cs2 5H0.5 PW12O40 has been ground in a mortar with a diol for 5 min, then left 9 h at room temperature to give the pinacol rearrangement (8.11).56... 

Of the solid electrophilic catalysts an acidic clay [13] and alumina were applied in the early history of the pinacol rearrangement. The last two decades, with the development of a variety of new classes of solid acid has, however, brought an upsurge in research activity. Almost all types of solid acid have been tested and found to have high catalytic activity in the transformation of vicinal diols, although selectivity is not always satisfactory. 

The idea of solid-phase acid-catalyzed processes was originally developed by Toda et al. [46,47]. The pinacol rearrangement of aryl-substituted secondary and tertiary diols in the presence of solid p-toluenesulfonic acid (333 K) and trichloroacetic acid (293 K) was found to proceed faster and more selectively than in solution (batch reactor) [46]. 

A polymeric pinacol,poly[3-methyl-2-(4-vinylphenyl)-2,3-butanediol],has been prepared by radical polymerization of the styrenic diol monomer and shown to be cleanly and quantitatively converted to a non-conjugated ketone in the solid state by reaction with a photochemically-generated acid [151,348, 350]. The rearrangement reaction can be readily monitored by IR spectroscopy as the disappearance of the hydroxyl OH absorption is accompanied by appearance of a new ketone carbonyl absorption (Fig. 116). Since a polar alcohol (isopropanol) dissolves the polar diol polymer in the unexposed regions but cannot dissolve the less polar ketone polymer produced in the exposed regions, the resist functions as a negative system with alcohol as a developer. The diol polymer is stable thermally to 225 °C in the absence of acid. 

Pinacol-pinacolone rearrangements proceed faster and more selectively in solid state. A 1 3 molar ratio of the pinacol and p-toluene sulphonic acid (p-TsOH) (powered mixture) on keeping at 60° C give the products (A) and (B). It is found that the hydride migrates more easily than the phenyl anion and the yield of A is higher than that of B in all the reactions (Scheme 22). 

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