Aldol zirconium catalyzed

Scheme 8.11 Zirconium-catalyzed aldol-Tishchenko reaction of ketone aldol products [17].

Table 8.4 Zirconium-catalyzed aldol-Tishchenko reactions as developed by Schneider et al. [17b].

Group 4 Titanium and Zirconium. The titanium tetrachloride mediated aldol reaction of silyl enol ethers with aldehydes was first reported in early of 1970s (16,17). It proceeds in a highly regioselective manner for cross or direct aldol reactions in high yields (18-20). Since the pioneer contribution by Mukaiyama s group, numerous synthetically useful procedures were developed in titanium- and zirconium-catalyzed aldol reaction of broad substrates (21-23). 

Zhang, G., Hattori, H. and Tanabe, K. Aldol addition of acetone, catalyzed by solid base catalysts magnesium oxide, calcium oxide, strontium oxide, barium oxide, lanthanum-(III) oxide and zirconium oxide, Appl. Catal., 1988, 36, 189-97. 

Not all aldol additions exhibit a dependence of product configuration on enolate geometry. Acid catalyzed aldols [45], some base catalyzed aldols [58], and aldols of some transition metal enolates [63,64] show no such dependency. For example, zirconium enolates afford syn adducts ( / topicity) independent of enolate geometry for a number of propionates [63,64]. As shown in Scheme 5.9, two explanations have been proposed to explain the behavior of zirconium enolates. One explanation (Scheme 5.9a) is that the closed transition structure changes from a chair for the Z(0)-enolate to a boat for the (0)-enolate [16,63,65]. Another hypothesis is that these additions occur via an open transition structure. Although the original authors... 

After the first reports of the above-mentioned highly eflident catalytic enantioselective aldol reaction, some groups independently reported catalytic symmetric aldol reactions of silicon enolates vith aldehydes using chiral boron [72], titanium [73], zirconium [74], and copper Le vis acids [75], or by transmetalation to chiral Pd(II) enolates [44]. Chiral phosphoramide-promoted aldol reactions of trichlorosilyl enol ethers have been reported as Le vis base-catalyzed asymmetric aldol reactions [76]. 

Scheme 5.114 Rhodium-catalyzed enantioselective conjugate addition of vinyl zirconium compounds 446 and subsequent diastereoselective aldol addition.

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