Ligand aromatic dihydroxylated

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. 

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). 

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. 

A polymeric cinchona alkaloid-derived ligand 44 was prepared and used to catalyze the asymmetric dihydroxylation of olefins (see the diagram below).66 Both aliphatic and aromatic olefins afforded diols with good enantioselectivities. 

Jew and Park have also utilized the dimerization effect, as observed in the development of Sharpless asymmetric dihydroxylation, where ligands with two independent cinchona alkaloid units attached to heterocyclic spacers led to a considerable increase in both the enantioselectivity and scope of the substrates, to design dimeric and trimeric cinchona alkaloid-derived phase-transfer catalysts 12 [12] and 13 [13]. These authors investigated the ideal aromatic spacer for optimal dimeric catalysts, and found that the catalyst 14 with a 2,7-bis(bromomethyl) naphthalene spacer and two cinchona alkaloid units exhibited remarkable catalytic and chiral efficiency (Scheme 11.3) [14]. 

In order to predict facial selectivity, Sharpless and co-workers invoke a mnemonic device.25 To an approaching olefin, the greatest steric constraints are presented by the NW, and to an even greater extent, the SE quadrants. The SW and NE quadrants are more open and, in addition, the SW quadrant contains what is described as an attractive area . The attractive area is particularly well suited to accommodate flat aromatic groups. The olefin positions itself according to the constraints imposed by the ligand and is dihydroxylated from above (p-facc), in the case of dihydroquinidine derivative, or from below (a-face) in the case of dihydroquinine derivatives. The commercially available AD-mix-a and AD-mix-P are chosen according to this mnemonic. 

Dihydroquinine [2, (8a,9/ )-10,l l-dihydro-6 -methoxy-9-cinchonanol] is obtained by reduction of the double bond of quinine and is commercially available. It has been used as a calalyst for the addition of zinc alkyls to aldehydes (Section D. 1.3.1.4.) and as its 4-chlorobenzoic acid ester as a chiral ligand for enantioselective osmium tetroxide catalyzed dihydroxylations (Section D.4.4.). The ester, together with several aromatic ethers, is also commercially available. 

More recently a new ligand scaffold of [di-(2-pyridyl)methyl]benzamide has been introduced by Que [81]. This novel fadal N,N,0-ligand arrangement mimics that found for the Rieske dioxygenase. The iron(II) complex from this ligand efHdently catalyzes the dihydroxylation of various alkenes, including aliphatic and aromatic ones (Table 1.5, entry 6). It should be noted that the ds-diol to epoxide ratio is significantly increased. With a,p-unsaturated alkenes, even no epoxide was reported. 

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