Titanium complexes macrocyclic ligands

The macrocyclic chemistry of tetradentate Schiff base complexes has been known for long time. However, the successful use of such a complex as an enantioselective catalyst in epoxidation reactions is a relatively recent finding. In these reactions complex 9.9 or an analogue is used. One of the possible routes for the synthesis of intermediate 9.2 of Table 9.1 involves the use of a similar catalyst. While complex 9.9 works well with unfunctionalized alkenes, for the epoxidation of allylic alcohols, dialkyl tartarates, 9.10, are the preferred ligands. As we shall see, the mechanisms of epoxidation in these two cases are different. Also for the tartarate-based system titanium is the metal of choice (see Section 9.3.3). 

The macrocyclic calix[4]arene complex (Ti(Bu -calix[4]arene))2 was prepared from Ti(NMe2)4 and the neutral ligand. One of the four O atoms of each ligand bridges the two titanium centers.673 The related complex TiCl(But-calix[4]arene(0Me)(0)3) (61) was synthesized by a salt metathesis reaction. Subsequent displacement of the Cl by alkyl or aryl groups was readily accomplished.674... 

Macrocyclic mono-Cp titanium(iv) complexes with peptide-derived ligands coordinated to the titanium atom have been described by the reaction of the pseudo-nonapeptide with 3 equiv. of NaOMe in CH2C12 and subsequent reaction with Cp TiCl3 (Cp = Cp, Cp ) (Scheme 395). The molecular structure of the Cp derivative has been determined by X-ray diffraction and shows a distorted pseudo-octahedral titanium environment.939... 

An example of a macrocycle with oxygen donors to support a Ti catalyst for lactide polymerization has been reported by Frediani et a/. " The authors described several titanium chloride complexes bearing calix[4]arene ligands, which act as catalysts for solvent-free lactide polymerization (Figure 18). These complexes acted as dual-site catalysts with two polymer chains growing from one metal center. 

Dimeric titanium-salen, prepared from macrocyclic chiral salen ligand 26, was used in the preparation of acyl cyanohydrins from aldehydes and ethylcyanoformate (Scheme 7.18). A ligand/metal ratio of 1 2 was found to be optimum for the enantioselectivity, confirming the dimeric titanium-salen structure as the catal34ically active complex. This was strengthened by the observation of a linear correlation between the enantiomeric excesses of... 

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