Synthesis, Structure, and Catalytic Properties of Bis[bis(sulfonamido)] Titanium Complexes

Abstract

Reaction of the titanium tetraamide Ti(NMe2)4 with 2 equiv of racemic or resolved bis(sulfonamide) ligand resulted in the formation of the [bis(sulfonamido)]2Ti complexes (TiL2) and N,N-dimethylamine. We have characterized the resulting TiL2 complexes by X-ray crystallography and variable-temperature NMR spectrometry to explore differences in the bonding and geometry of the bis(sulfonamido) ligand in these complexes. When racemic ligand was used, the resulting TiL2 complexes consisted of the homochiral and heterochiral diastereomers. One of these heterochiral diastereomers was characterized by X-ray crystallography (7hetero). When resolved ligand was employed, it was found that the product composition was dependent on the size of the bis(sulfonamide) ligand employed in the synthesis. When the aryl group of the bis(sulfonamide) ligand was tolyl (1res), two diastereomeric compounds were observed that were in equilibrium. These complexes were the C2-6homo and Cs-6homo diastereomers. When the aryl group was 2,5-dimethylphenyl (Xyl) (3rac), the major product was determined to be the homochiral diastereomer, C2-8homo, by X-ray crystallography. The homochiral diastereomer generated from the bis(sulfonamide) ligand with the aryl group 5-chloro-2-methoxy phenyl (4res) also gave the homochiral diastereomer C2-9homo. By NMR spectrometry both C2-8homo and C2-9homo were found to be single diastereomers, and no signals that could be attributed to the Cssymmetric counterparts to Cs-6homo were observed. The structures of C2-6homo, 7hetero, C2-8homo, and C2-9homo were determined. Although the geometries between the homochiral and heterochiral diastereomers are very different, the metrical parameters are not. Each of the titanium centers in these structures is eight-coordinate. The ligand was found to bind through one of the sulfonyl oxygens of each sulfonyl group as well as the sulfonamido nitrogens. The Ti-N distances of the sulfonamido groups range from 2.070(3) to 2.095(3) A, while Ti-O sulfonyl distances range from 2.159(2) to 2.328(3) A. The bis(sulfonamide) ligands used in this work have also been used in the asymmetric addition of diethylzinc to aldehydes. This reaction is very efficient and gives excellent enantioselectivity with a wide variety of aldehydes. To determine if the TiL2 complexes were involved in the catalysis, an equilibrium mixture of the diastereomers C2-6homo/Cs-6homo was used in the asymmetric addition and compared with the catalyst generated under the reaction conditions from the ligand 1res. Although the mixture of C2-6homo/Cs-6homo gave high enantioselectivity, it exhibited different enantioselectivity and turnover frequency from the catalyst generated from 1res.