Triol ligands

A number of chiral catalysts originally devised for aldol reactions (see Chapter 4) have also successfully been deployed in enantioselective Mannich additions. One such example is the dinuclear zinc complex 220 developed by Trost (Equation 18) [154]. The complex is generated in situ from Et2Zn and the corresponding chiral diamino triol ligand derived from prolinol. Excellent enantio- and diastereoselectivity were obtained in reactions with a-hydroxyketones such as 218. From imine 219, the syn a-hydroxy-/l-amino ketone 221 was isolated in > 99% ee and > 15 1 dr. Additional investigations revealed that appropriate choice of the N-substituent on the imine could selectively provide either the anti or the syn a-hydroxy- -amino ketones [155]. 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Uemura and coworkers utilized (R)-binaphthol 85 as chiral ligand in place of DET in association with Ti(IV)/TBHP, which not only mediated the oxidation of sulfides to (R)-configurated sulfoxides, but also promoted the kinetic resolution of sulfoxides (equation 50). In this latter process the two enantiomers of the sulfoxide are oxidized to sulfone by the chiral reagent at different rates, with decrease of the chemical yield, but increase of the ee values. Interestingly, the presence of ortho-nilro groups on the binaphthol ligand lead to the reversal of enantioselectivity with formation of the (5 )-configurated sulfoxide. Non-racemic amino triols and simple 1,2-diols have been successfully used as chiral mediators. 

The first tridentate amine may be the same or different from the second ammonia (with activated charcoal in water) may even replace the latter to produce salts such as [Co(den, dpn or dtn)(NH3)3] , where dtn = 1, 5, 9-triazanonane. The series [Codtn2] had to be prepared by Equation 3 because Method 1 gave only the red binuclear [Co2dtn2(OH)3] . This is a /x-triol complex of which the hexammine analog is long known 49). The same result was obtained with the new ligand medtn or 5-methyl-l, 5, 9-triazanonane. 

With the intent to investigate the ability of polyhydroxy silanes to act as supporting anionic ligands for metals, we focused our studies on the synthesis of novel Ti and Zr siloxide complexes from triol 8a (Scheme 2). When Ti(OEt)4 was reacted with one equivalent of 8a in heptane for 3 h, a yellow microcrystalline material could be isolated in a yield of 35%. An X-ray Structure analysis revealed the formation of the bimetallic Ti-siloxide 14, in which both Ti atoms are connected to each other... 

Without tach as auxiliary ligand Cs2[Si(cj5 -InsH 3)2] (Cs298) [92] shows a bis-tridentate 1,3-diolate binding-cw-inositol complex. In contrast to the finding that a pyranoidic 1,2,3-triol like C4-Me- 8-D-Ribp, with its large torsional angles, is not so efficient an Si-chelator as a furanoidic diol, the significant enrichment of aqueous alkaline silicate solutions by hexaco-ordinate species on addition of cw-inositol is remarkable. 

Ligands with three different phosphine donors (39)-(47) (Table 2) are accessible on the routes shown in Scheme 1. The starting materials for routes A and B are generally obtained from the corresponding triol (see Equation (3)). 

Sulfur donors may also be introduced by the primary transformation of the triol into a functionalized thiethane (Equation (10)). Subsequent addition of two potentially different phosphorus nucleophiles leads to tripod ligands containing two phosphane donors together with an SH function ((84), (85), (87), (88)). 

Figure 4.11 The effect of some ligands of potassium cation on the unsaturation of the polyether triols. Temperature 110 °C Pressure 0.4 MPa catalyst concentration 0.0056 mol/1 Ligands polyethylene glycol MW = 2000 ( ) and 18-crown 6 (())...

A number of interesting applications of cycloisomerization to natural product syntheses have been carried out by Trost. As an example, total synthesis of picro-toxinin has been achieved based on cycloisomerization (Alder-ene reaction) of the 1,6-enyne system 141 as a key reaction. No satisfactory cyclization of 141 occurred when phosphine ligands such as P(o-Tol)3, DPPB, and triisopropyl phosphite were used. However, smooth cyclization took place to give the Alder-ene product in a quantitative yield at 50 °C when A,A -bis(benzylidene)ethylenediamine (BBEDA) was used as a ligand, and the triol 142 was obtained in 75 % yield after... 

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