Aromatics dihydroxylation

The aromatic dihydroxylated ligands (represented by catechol, 2,3-dihydroxybenzoate, and 4,5-dihydroxynapthalene 2,7-disulfonate) stabilize the 3+ and 2+ oxidation states of manganese in alkaline media. The data for salicylate indicate that it forms a less stable Mn(III) complex than the dihydroxy ligands. Formation of the 4+ complex is precluded because the ligands are more easily oxidized than are the Mn(III) complexes. 

Resorcinol diols represent a new class of aromatic dihydroxylic compounds with the general formula (15.33) ... 

SCHEME 36.20. Examples of aromatic dihydroxylations catalyzed by toluene dioxygenase and naphthalene dioxygenase. 

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.). 

In 2008, Que and coworkers reported an asymmetric version of the dihydroxylation with a new type of ligands bearing bipyrrolidine as the chiral backbone [71]. The corresponding iron(II) complex showed general activity in the dihydroxylation of various olefins using H202- Satisfactory results are obtained with aliphatic as well as with aromatic olefins. For example, dihydroxylation of styrene gave styrene oxide and 1-phenylethane-1,2-diol in <1% and 65% yield, respectively (Scheme 10). 

Dioxygenases usually contain a tightly bound iron atom and catalyze hydroperoxidation of allylic molecules or carboxylic acids, and dihydroxylation of aromatics (Figure 1.5) [33]. 

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. 

Wordy Over the past few years, we have encountered numerous examples of water as the perfect solvent. We observed this first in osmium-catalyzed dihydroxylation reactions and also in nucleophilic ring-opening reactions of epoxides. We also observed this in cycloaddition reactions and in most oxime ether, hydrazone, and aromatic heterocycle condensation processes.Finally, we observed it in formation reactions of an amide from a primary amine and an acid chloride using aqueous Schotten-Baumann conditions. ... 

In previous work, Corey used the free base form of 34 as an effective chiral ligand in the Os04-promoted dihydroxylation of olefins [90]. He later found that ammonium salt 34 catalyzed the addition of HCN to aromatic N-allyl imines (Scheme 5.50) [91]. The U-shaped pocket of the catalyst is essential in fixing the orientation of the hydrogen-bonded activated aldimine via n-n interactions. 

Hudlicky, T., Tian, X., Konigsberger, K., Mauiya, R., Rouden, J. and Fan, B. (1996c) Toluene-dioxygenase-mediated as-dihydroxylation of aromatics in enantioselective synthesis. Asymmetric total syntheses of pancratistatin and 7-deoxypancratistatin, promising antitumor agents. J. Am. Chem. Soc., 118, 10752-10765. 

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