Catalysts bisoxazoline

An enantioselective nitro-Mannich reaction of alkyl- and aryl-benzylimines gives /3-nitroamines in high ee, using a chiral copper(II)-bisoxazoline catalyst, with the products affording 1,2-diamines by reduction.30... 

Intramolecular reactions using (5,5)-/-Bu-box and [Cu(MeCN)4]PFe complexes as catalysts have emerged as remarkably effective for the synthesis of macrocycles from membered ring lactones can be obtained in high enantiomeric purity with high efficiency. If two double bonds are present in the molecule, the unique preference of copper-bisoxazoline catalysts to promote the formation of the larger ring is demonstrated (eq 4). ... 

The low face-selectivity in the previous example can be partially attributed to single-point coordination of the aldehyde to the Lewis acid. Evans and co-workers have used a-alkoxy aldehydes in aldol reactions with much success (Sch. 35) [70]. Thus a variety of silylketene acetals add to 144 in the presence of a copper(bisoxazoline) catalyst with very high selectivity and chemical yields. Reaction with 147, which contains an additional stereoelement, gave a single aldol product 148 with high syn selectivity. The dienol silane 149 also provides high selectivity in the aldol reaction. A square pyramidal model accounts for the observed selectivity. In this model, the substrate binds... 

Siegel and Schmalz [79] reported a new approach to optically active ferrocenes with planar chiraHty, based on a Cu-catalyzed C-H insertion (Scheme 17). Both a five- and a six-membered ring could be formed in this way. Using a copper-bisoxazoline catalyst, good yields and ees in the range of 60-80% could be obtained, whereas chiral dirhodium catalysts proved to be ineffective in this case. 

Fig. 22. Turnover in selectivity for Mg(bisoxazoline) catalysts with the addition of coordinating ligands (tetrahedral octahedral transposition)...

This explanation is described as a mnemonic rule [228], which can only be taken as a first approximation of reality. The same rule can be used to rationalize the topicity of other asymmetric Diels-Alder reactions, such as those employing titanium BINOLate catalysts (Figure 6.18i, [230]), or iron bisoxazoline catalysts (Figure 6.18j,k [206,215]). Although the explanation seems reasonable, the picture is not complete, since it does not account for a number of observations, including the fact that the dioxolane substituents exert an extraordinary effect on catalyst efficiency (c/ Table 6.6, entries 2 and 5). Additionally, both titanium TADDOLate [228] and BINOLate [230] complexes show a nonlinear relationship between enantiomeric purity of the catalyst and that of the product, which suggests that some sort of dimerization phenomenon is involved. 

The bisoxazoline catalysts, sometimes abbreviated to [Cu-(5,5)-t-Bu-box]X2, are suitable for inter- and intramolecular asymmetric Diels-Alder reactions with... 

Alkene Epoxidation/Aziridination. Olefin epoxidation, principally of styrene derivatives, using a chiral Ru(II)-bisoxazoline catalyst and PhI(OAc)2 takes place with modest enantiomeric induction (eq 56). The rate of this reaction is enhanced by the presence of H2O. An electron-deficient Fe-porphyrin complex also serves as a catalyst for O-atom transfer using PhI(OAc)2 as the terminal oxidant. ... 

Scheme 8,71 Enantioselective a-arylation of W-acyloxazolidinones with copper(ll)-bisoxazoline catalysts and diaryliodonium salts, as described by Gaunt and coworkers [122].

The enantioselective Diels-Alderreactionbetweencyclopentadiene and acryloyl-2-oxazolidinone substrates has become a benchmark test for the effectiveness of a new catalyst system. In addition to the widely employed Cu(II)/bisoxazoline catalysts, a number of additional chiral Cu(II) catalysts have been developed for and/or applied to enantioselective Diels-Alder reactions. Scheme 17.58 summarizes the application of chiral Cu(II) catalysts other than Cu(II)/bisoxazoline complexes in benchmark Diels-Alder reactions. 

A hybrid strategy that involves synergism between metal catalyst and BB organo-catalyst has been applied to the aza-Henry reaction [62], Thus, the aza-Henry reaction between tert-butyl 2-nitropropionate and iminoesters is catalyzed by combined use of quinine (16) and a Cu-bisoxazoline catalyst (20mol%). The approach is based on chiral molecular recognition of the organic and the metallic catalysts and produces the densely functionalized adduct 48 in good diastereo- and enanti-oselectivity (Scheme 29.29). 

In addition to their use with the traditional enoxysilanes derived from ketones and esters, the bisoxazoline catalysts have been shown to mediate aldol-like additions between chelating aldehydes and 5-alkoxy-substituted ox-azoles to afford adducts such as 328 (Scheme 4.46) [165], Interestingly, the identification of aluminum salen complexes 330 as a viable catalyst expands the scope of the process to include aromatic aldehydes (Equation 31). The products are isolated as isoxazolines (cf. 331) with a high degree of relative and absolute stereocontrol. 

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