Bisoxazoline magnesium complexes

Figure 6.20. (a) Acryloyloxazolidinone in bidentate coordination. strain favors the s-cis conformation, (b) Cycloaddition of Ci-symmetric bisoxazoline-magnesium complex [206]. (c) Cycloaddition of C2-symmetric bisoxazoline-copper complex [205]. (d) Rationale for the different topicities of the bisoxazoline complexes, even though both ligands have the same absolute configuration. The dienophile is awn in the plane of the paper, and the favored approach is from the direction of the viewer. 

Enantioselective Cyanohydrin Formation. Magnesium complexes formed with the anionic semicorrin-type ligand (5) catalyze the addition of Cyanotrimethylsilane to aldehydes, leading to optically active trimethylsilyl-protected cyanohydrins. In the presence of 20 mol % of the chloromagnesium complex (9), prepared from equimolar amounts of (5) and BuMgCl, cyclohexanecarbaldehyde is smoothly converted to the corresponding cyanohydrin derivative with 65% ee. Addition of 12 mol % of the bisoxazoline (10) results in a dramatic increase of enantioselectivity to 94% ee (eq 8). Replacement of (10) by its enantiomer reduces the selectivity to 38% ee. This remarkable... 

Experiments described by Corey constitute a noteworthy example of double asymmetric induction where neither participant in the reaction is chiral [95] As illustrated in Figure 4.18 two different catalysts are necessary to achieve the best results. Control experiments indicated that the nucleophile is probably free cyanide, introduced by hydrolysis of the trimethylsilylcyanide by adventitious water, and continuously regenerated by silylation of the alkoxide product. Note that the 82.5% enantioselectivity in the presence of the magnesium complex shown in Figure 4.18a is improved to 97% upon addition of the bisoxazoline illustrated Figure 4.18b as a cocatalyst. Note also that the bisoxazoline 4.18b alone affords almost no enantioselectivity, and that the enantioselectivity is much less when the enantiomer of the bisoxazoline (Figure 4.18b) when used as the cocatalyst. Thus 4.18a and 4.18b constitute a matched pair of co-catalysts and 4.18a and ent-A. %h are a mismatched pair (see Chapter 1 for definitions). The proposed transition structure... 

For the introduction of /j-amino substituents, enantioselective 1,4-additions using 0-benzylhydroxylamine (Equation 43) have been reported by Sibi [162, 165], The magnesium complex of the bisoxazoline ligand 213 was found to be optimal for activation of pyrazole amide derivatives such as 222 [165], Addition of 0-benzylhydroxylamine to 222 is mediated by 30 mol % each of 213 and MgBr2 to furnish 223 in 80% yield and 95% ee. In the course of investigating this catalyst system, the intervention of a kinetic resolution step involving the product was noted. Thus, the minor enantiomer (ent-223) was observed to undergo more rapid amidolysis than the major enantiomer 223 under the reaction conditions. 

In 1993. Corey reported asymmetric cyanohydrin formation mediated by the chiral magnesium bisoxazoline complex 267 (Figure 2.10) [174]. A noteworthy feature is that the enantioselectivity was significantly improved in the presence of bisoxazoline 268 (65 % ee 94% ee). A mechanistic rationale for... 

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