Titanium complexe chiral ligand

The advantages of titanium complexes over other metallic complexes is high selectivity, which can be readily adjusted by proper selection of ligands. Moreover, they are relative iaert to redox processes. The most common synthesis of chiral titanium complexes iavolves displacement of chloride or alkoxide groups on titanium with a chiral ligand, L ... 

The chemistry of complexes having achiral ligands is based solely on the geometrical arrangement on titanium. Optically active alcohols are the most favored monodentate ligands. Cyclopentadienyl is also well suited for chiral modification of titanium complexes. 

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

The most commonly used traditional Lewis acids are halides of aluminum, boron, titanium, zinc, tin, and copper. However, there are also more complex Lewis-acids that are quite effective catalysts that can be easily modified for carring out enantioselective processes, by incorporating chiral ligands. These can overcome some limitations associated with the use of classical Lewis acids [47]. 

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

Figure 1.25 Minimum-energy diastereoisomeric monomer free intermediates for butadiene polymerization catalyzed by titanium complexes presenting Cp group as ancillary ligand. Chiralities of coordination of allyl groups (assumed to be si) and back-biting double bonds (si or re) are indicated, in order to easily visualize possible stereoregularity (iso or syndio) of model chains. In fact, like and unlike chiralities would possibly lead to isotactic and syndiotactic enchainments, respectively.

In the context of asymmetric catalysis, titanium silsesquioxanes containing the chiral ligand (lR,2S,5R)-(-)-menthoxo ligand (MentO) (figure 14.3) have been synthesized from the monosilanol la (R = cyclopentyl) and Ti(OPr )4 [70]. The molecular complexes were tested as asymmetric homogeneous catalysts for the epoxidation of cinammic alcohol with tert-butyl hydroperoxide and compared... 

We reported a catalytic enantioselective cyanosUylation of ketones that produces chiral tetrasubstituted carbons from a wide range of substrate ketones [Eq. (13.31)]. The catalyst is a titanium complex of a D-glucose-derived ligand 47. It was proposed that the reaction proceeds through a dual activation of substrate ketone by the titanium and TMSCN by the phosphine oxide (51), thus producing (l )-ketone cyanohydrins ... 

Chiral titanium complexes with a, a, a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) ligands are versatile auxiliaries in the Lewis acid catalyzed alcoholysis of racemic 4-(arylmethyl)-2-phenyl-5(477)-oxazolones 234, providing the corresponding enantiomerically enriched N-protected amino acid esters 235 (Scheme 7.73). The enantioselectivity of the reaction is dependent on the solvent, temperature, and chiral ligand. Selected examples of the alcoholysis of saturated 5(477)-oxazolones are shown in Table 7.21 (Fig. 7.23). 

Jeong and co-workers utilized a cobalt-alkyne complex to enhance enantioselectivity of the addition of bis (homoallyl)zinc to propargyl aldehydes 68 by the exaggeration of steric environment. The reaction provided optically enriched propargyl alcohol 69 in the presence of a chiral ligand and titanium tetra(isopropoxide) in excess. Adduct 69 was subjected to PKR to yield optically enriched bicyclic compounds 70 (Equation (39)). ... 

Lewis acid-promoted asymmetric addition of dialkylzincs to aldehydes is also an acceptable procedure for the preparation of chiral secondary alcohol. Various chiral titanium complexes are highly enantioselective catalysts [4]. C2-Symmet-ric disulfonamide, chiral diol (TADDOL) derived from tartaric acid, and chiral thiophosphoramidate are efficient chiral ligands. C2-Symmetric chiral diol 10, readily prepared from 1-indene by Brown s asymmetric hydroboration, is also a good chiral source (Scheme 2) [17], Even a simple a-hydroxycarboxylic acid 11 can achieve a good enantioselectivity [18]. 

Bis(phoshacyclopentadienyl)titanium(II) complexes, preparation and reactivity, 4, 265 Bisphosphanes on DIOP modification, 10, 7 in hydrogenations, 10, 7 in hydrogenations, P-chiral ligands, 10, 11 Bisphosphinidenes, with platinum(II), 8, 453 -54 Bisphosphinites, in hydrogenations, 10, 14 Bis(phosphinoalkyl-thioether)arenes, in ruthenium isocyanides, 7, 138... 

A number of stereospecific non-enzyme catalysts have been developed that convert achiral substrates into chiral products. These catalysts are usually either complex organic (Figure 10.8(a)) or organometallic compounds (Figure 10.8(b)). The organometallic catalysts are usually optically active complexes whose structures usually contain one or more chiral ligands. An exception is the Sharpless-Katsuki epoxidation, which uses a mixture of an achiral titanium complex and an enantiomer of diethyl tartrate (Figure 10.8(c)). 

The solvent-free asymmetric Hetero-Diels-Alder reaction of 14 different aldehydes with Danishefsky s diene was carried out with 0.1-0.005 mol% of chiral titanium complexes to afford dihydropyrones with up to quantitative yields and 99.8% ee. A library of chiral metal complexes was generated by combining one or two different chiral diol ligands e.g. 5 (13 different diols were applied) with titanium isopropylate (Scheme 16) [46],... 

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