Yttrium enolates

Thus, lanthanide (Lu and Yb) and yttrium enolates, which were prepared from the lithium enolate of acetaldehyde and the corresponding dimeric dicyclopenta-dienyl metal chlorides [Cp2MCl2]2> were studied by NMR and IR spectroscopy, and clear evidence for the O-bound tautomer was provided in all cases. This was confirmed by a crystal structure of dimeric yttrium enolate 17 with a Y2O2 core unit > [51]. The chelate complex 18 of scandium, on the other hand, has been shown by a crystal structure to be monomeric. Here again, the metal is bound to the enolate oxygen atom (Scheme 3.7) [52]. 

Scheme 3.7 O-metal bound dimeric yttrium enolate 17 and monomeric scandium enolate 18, both derived from acetaldehyde.

Routes to monomeric , mononuclear , monolanthanide alkoxides, enolates, siloxides and aryloxides - an expanded title which will put the scope of the article in a more concrete form. The synthesis of mononuclear alkoxides, in particularly homoleptic derivatives [1], was decisively stimulated by the discovery of high temperature superconducting ceramics based on YBa2Cu307<, where yttrium represents the lanthanide elements [2]. The support of volatile and highly soluble molecular precursors is a prerequisite for synthesizing thin films of these materials by means of MOCVD [3] and sol gel processes [4], respectively. More recently, lanthanide alkoxide reagents became established in... 

In Chapter 17 you saw epoxides acting as electrophiles in Sn2 reactions. They can be used to alkylate enolates providing epoxide opening is assisted by coordination to a Lewis acidic metal ion in this case the lanthanide yttrium(III). The new C-C bond in the product is coloured black. Note that the ketone starting material is unsymmetrical, but has protons only to one side of the carbonyl group, so there is no question over which enolate will form. The base is one of the LDA variants we showed you on p. 668—LHMDS. 

Yttrium triflate catalyzes the reaction of lithium enolates with epoxides to form y-hydroxy ketones. 

Lutetium enolate chemistry mimics that of yttrium Lu(III) is the smallest of all of the trivalent lanthanide cations, and so the chemistry of Lu(III) is more similar to that of its Ad congener Y(III). 

The H NMR spectra (471, 472) of yttrium, lutetium, and titanium derivatives are consistent with structures in which the oxygen atom of the enolate bonds to the metal center. Resonances due to the =CH group appear as doublets of doublets. The signals due to the =CH2 protons appears as two doublets... 

The reactions of imines with silyl enolates were tested in the presence of 5 mol% of Ln(OTf)3, and selected examples are shown in Table 12 [41]. In most cases the reactions proceeded smoothly in the presence of 5 mol% of Yb(OTf)3 (a representative lanthanide triflate) to afford the corresponding b-amino ester derivatives in good to high yields. Yttrium triflate (Y(OTf)3) was also effective, and the yield was improved when Sc(OTf)3 was used instead of Yb(OTf)3 as a catalyst. Not only silyl enolates derived from esters, but also that derived from a thioester, worked well to give the desired b-amino esters and thioester in high yields. In the reactions of the silyl enolate derived from benzyl propionate, anti adducts were obtained in good selectivities. In addition, the catalyst could be recovered after the reaction was completed and could be reused. 

Primary and secondary alcohols reacted with vinyl or isopropenyl acetate at room temperature in he presence of a catalytic amount (0.05-1 mol%) ofY5(OTr)i30 to give the corresponding esters (Scheme 12.84) [183]. In selected cases, the yttrium catalyst promoted selective O-acylation of amino alcohols without the formation of an amide. Enol esters also reacted with a-amino acid esters in the absence of a catalyst at room temperature to give the corresponding amides. 

Related Y(OTf)3-catalyzed alkylations of lithium enolates with epoxides and a one-pot synthesis of 1,2,4-oxadiazole derivatives from yttrium-derived enolates have been reported. ... 

The yttrium triflate also promotes in an efficient way many other important reactions such as the addition of enolates to epoxides, leading to the corresponding hydroxyl ketones in almost quantitative yield (Equation (8.26)) [54], or the one-step synthesis of 1,2,4-oxadiazole from ketones, nitriles, and nitric acid (Equation (8.27)) [55]. 

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