Dihydroxylation of aromatic rings

The uncatalysed p-coumaric acid oxidation led to the formation of intermediates (not shown here) almost similar to those of the catalysed reaction, without formation of dihydroxylated aromatic compounds, such as 3,4- dihydroxybenzaldehyde. This result shows that the catalyst may promote the hydroxylation of aromatic ring by enhancing the formation of hydroxyl radicals in the reaction mixture. 

An example is the study by Lambert9 of the influence of aromatic rings and neighbouring electron-withdrawing groups on Sn2 reactions. He needed the few-tosylate 50. This comes from the diol 51 and now he had a choice. He could epoxidise an -alkene or dihydroxylate a Z-alkene. He chose the latter as Z-52 could be made by a Wittig reaction. 

The diastereomeric excess (d.e.) of 76a reached 72% (epoxidation) and 98% (dihydroxylation). Nitro substitution on the aromatic ring (as in 77a) significantly reduced the selectivity (increased the syn proportion), although anti preference was stiU retained in epoxidation (20% d.e.) and in dihydroxylation (68% d.e.). 

We have recently broadened those investigations to study the origin of the enantioselectivity in the dihydroxylation of terminal aliphatic n-alkenes. The dihydroxylation of the series from propene to 1-decene was studied by means of the IMOMM method [97]. Experimental studies on propene, 1-butene, 1-pentene, 1-hexene and 1-decene showed that the reaction was enantioselec-tive in all cases, leading to the R product. Moreover, the results show a dependence of the enantioselectivity on the chain length it sharply increases from propene to 1-pentene, and after that the enantioselectivity remains practically constant for 1-hexene and 1-decene. The explanation for this dependence of the enantioselectivity with the chain length remained elusive. On the other hand, the -stacking interactions that were found to be critical for styrene cannot be responsible for the observed enantioselectivity for these terminal aliphatic n-alkenes because they do not have aromatic rings. 

Hydroxylation of Benzenic Rings. Intermediate products corresponding to the monohydroxylation of the aromatic ring have been identified in many cases (43,76,77,83,84,88-90). Electron-donating substituents exert the expected orientation effects to para and ortho positions. No orientation dominates for nitrobenzene (90). Dihydroxylated intermediates have also been identified, as well as the corresponding quinones. 

This guideline predicts that dihydroxylation of the diene 141 would give the diol 142 as the main product.30 This is indeed the case but, as we shall see, this selection is easy to overturn. In this case, if the bulk of the aromatic ring is increased then the selectivity is reversed. 

Accordingly, the synthesis of phthalide 44 began from methyl ketone 27 (Scheme 18). In order to avoid interference of the olefin moiety with haloge-nation of the aromatic ring, asymmetric dihydroxylation was conducted first. Treatment of alkene 27 with AD-mix a in tcrt-butanol/water (1 1) provided diol 27 in a pleasing 87% yield. Inspection of the and NMR spectra did not indicate the presence of a diastereomeric mixture. However, although alkene 27 is structurally well suited to the Sharpless mnemonic, we thought it... 

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