Europium camphorate

Determined by HPLC or H NMR in the presence of chiral europium (111) tiis[3-(heptafluoro-propylhydroxy- methylene)-(+)-camphorate]... 

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

The LSR itself consists of a lanthanide ion, commonly europium (Eu3 + ), praseodymium (Pr3+) or ytterbium (Yb3+), and three /1-diketone anions with alkyl or perfluoroalkyl groups Rl and R2 as ligands. Perdeuterated LSRs, which do not give H signals, are commercially available. Incorporation of chiral /J-diketone ligands, mostly derived from substituted camphors, allows chiral recognition. 

Reaction of the chiral lithium enolate of meso-2,6-dimethylcyclohexanone (6), generated by deprotonation with (R)-l-phenylethylamine and (/ )-camphor/(R)-l-phenylethylaniine derived chiral lithium amides (Table 1, entries 17 and 64) with 3-bromopropene, leads to homoallyl ketones of opposite absolute configuration in acceptable yield with poor to modest enantiomeric excess14, which can be determined directly by H-NMR spectroscopy in the presence of tris [3-(heptafluorohydroxymethylene)-D-camphorato]europium(III) [Eu(hfc)3]. 

In accordance with the proportionality (3.8 a), the magnitude of the pseudocontact shift, which predominates for lanthanide shift reagents, decreases with the distance of nucleus i from the paramagnetic ion. Thus, C-9 of camphor is shifted more than C-10 on addition of the europium chelate. Often, the crude approximation (3.8 a) is a valuable assignment aid. 

The H NMR analysis was used for (R)-(+)-2, obtained from (R,S,S)-(+)is6 by acid cleavage of the chiral auxiliary. The ee (>90%) was estimated with shift reagent europium tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (Eu(tfc)3) in CDC13 in the enantiomer, there appeared a doublet at 7.27 ppm with no additional splitting, whereas for the racemate there was a 0.05 ppm shift <2004TA537>. 

Alkylation of 3-mercapto derivatives of 277-benzo-l-thia-2,4-diazine 1,1-dioxides (143 R = H, CH3, OCH3, Cl) with optically pure a- and /3-bromoesters (R = H, GH3 R = H, GH3) occurs exclusively at sulfur to provide 144 <1997TA2199> and 145 <1996TA2703>, respectively (Scheme 9). Both reactions occur mainly with Walden inversion and the ee of inverted products 144 was established by H NMR using the chiral shift reagent Eu(hfc)3 (Europium tris[3-(heptafluoropropylhydroxymethylene)-camphorate]) on the methyl esters of 144. These compounds also display the SIDA phenomenon (i.e., self-induced diastereomeric anisochronism), which results in the enantiomers having different chemical shifts for the ester CH3 and N-2 protons. 

Chiral shift reagents are also tris jS-diketonates based on d-camphor as the chiral moiety. Whitesides and Lewis (1970) introduced the tm[3-(rerf-butyl-hydroxy-methylene)-d-camphorato] europium. It is more soluble in organic solvents than Eu(dpm)3 with otherwise similar chemical properties. Fraser et al. (1971) prepared a fluorinated derivative anticipating advantages similar to those of R(fod)3 relative to R(dpm)3. Indeed their fWs[3-(heptafluoropropyl hydroxy-methylene)-d-camphorato] europium induces larger chemical shift differences between enantiomers in most of the cases. Even larger differences were obtained with fn. 5[3-trifluoromethyl-hydroxymethylene)-d-camphorato] europium (Goer-ing et al., 1971). 

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