Nonlinear effects titanium complexes

A similar nonlinearity is seen in the ene reaction of methyl glyoxylate and a-methylstyrene (Scheme 43) (69). Thus, the reaction catalyzed by a complex in situ formed from dibromo(diisopropoxy)titanium(IV) and (/ )-binaphthol in 33% ee affords the chiral adduct in 91% ee with the same enantioselectivity as would have been obtained had enantiomerically pure binaphthol been used. Molecular weight measurements suggest the catalyst is a dinuclear titanium compound, although the structure has not been elucidated. This nonlinear effect is interpreted by the difference in the dissociation constant of the diastereomeric dimers as... 

K. Mikami, Y. Motoyama, and M. Terada, Asymmetric catalysis of Diels-Alder cycloadditions by an MS-free binaphthol-titanium complex dramatic effect of MS, linear vs positive nonlinear relationship and synthetic applications, J. Am. Chem. Soc., 116 (1994) 2812-2820. 

Matsuoka, T., Harano, K., Uemura, T., Hisano, T. Hetero Diels-Alder reaction of N-acyl imines. I. The reaction of N -thiobenzoyl-N,N-dimethylformamidine with electron-deficient dienophiles. Stereochemical and mechanistic aspects. Chem. Pharm. Bull. 1993, 41, 50-54. Mikami, K., Motoyama, Y., Terada, M. Asymmetric Catalysis of Diels-Alder Cycloadditions by an MS-Free Binaphthol-Titanium Complex Dramatic Effect of MS, Linear vs Positive Nonlinear Relationship, and Synthetic Applications. J. Am. Chem. Soc. 1994, 116, 2812-2820. 

Uemura et al. found that the combination Ti(OPr%/binaphthol/water in ratio 1 2 >10 acts as a catalyst for oxidation of aryl methyl sulfides into the corresponding sulfoxides by Bu OOH (see also Section 1.4.1) [159]. A mechanistic study showed that the titanium complex was a sulfoxidation catalyst (initial ee -50%) as well as a catalyst for the overoxidation into sulfones, with an enhancement of the ee of the residual sulfoxides (because the minor sulfoxide enantiomer is preferentially oxidized). In a subsequent paper, the authors reported the kinetic resolution of racemic aryl methyl sulfoxides by the same catalyst [160]. A stereoselectivity factor s of 2.6 was calculated for the kinetic resolution of racemic methyl p-tolyl sulfoxide. For example, methyl p-tolyl sulfoxide (<99% ee) could be recovered from oxidation at about 75% conversion. Using partially resolved l,l -binaphthol, a positive nonlinear effect was established. 

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. 

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