Aryl pinacol

In a similar way, Guiles and co-workers immobilized an aryl pinacol boronate on resin via an ester linkage, and then a small number of aryl halides were coupled to this (Scheme 31). The products were cleaved and immediately transformed into their methyl esters, presumably for ease of analysis and separation from unreacted boronate. The reaction was found to be slower than that of the opposite polarity (i.e., with the resin-bound iodide), and only moderate yields were obtained after as much as 48 h. Yields could be considerably improved upon a second round of coupling, but only one example was given of this. The aryl bromides were found to react only upon heating. 

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... 

Aryl pinacols are readily obtained from the photolysis of substituted benzophenones in isopropyl alcohol. Photochemical coupling of benzophenones in isopropyl alcohol has even been reported to occur in direct sunlight.7 It is known however, that some aryl ketones do not give the pinacol from photolysis in isopropyl alcohol.8 Nevertheless, aryl pinacols have also been prepared from electrochemical reduction,9 hydroxylation of olefins,10 photoreduction of ketones by amines,11 metal reduction,12 and by other routes.13 The phenanthrene ring system has often been... 

TABLE. SUBSTITUTED PHENANTHRENES FROM THE DEHYDRATIVE CYCLICATION OF ARYL PINACOLS IN TRIFLIC ACID... 

Aryl pinacol prepared by the photolysis of the appropriately substituted benzophenone in isopropyl alcohol. Yields reported for the crude product. 3QQ... 

Aryl pinacol has been known to undergo the pinacol rearrangement to yield a pinacolone-like product. When arylpinacol reacted with TfOH, 9,10-disubstituted phenanthrene was produced in high yield (Scheme 2.15) [40]. 

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