Titanium complexes imines

Titanium-mediated pinacol coupling reactions have been reviewed until 2000.80 81 Since then, various intermole-cular pinacol couplings have been reported with aldehydes, - ketones, a-ketoesters, and imines, as well as asymmetric versions thereof.101-104 Scheme 29 shows one example of an asymmetric pinacol coupling of aromatic aldehydes, promoted and catalyzed by the new chiral titanium complex (A)-75, that has been developed by Riant and co-workers.101 Yields for pinacol products 76 are generally high. Under catalytic conditions, ee is moderate (up to 63%), while stoichiometric conditions allow to obtain up to 91% ee. 

Addition of the (l-silylalkyne)titanium complex to carbonyl compounds and imines occurs at the (3-position to the silyl group, as shown in Fig. 9.2. However, the reaction with sBuOH takes place exclusively at the carbon—titanium bond a to the silyl group to give the (P-silylalkenyl)titanium species, as in Eq. 9.5 (values in square brackets denote the regioselectivity) [24], where the vinyl—titanium bond is visualized by the outcome of the iodi-nolysis. The overall reaction can therefore be regarded as the hydrotitanation of silylace-... 

Equation 9.9 shows a remarkable example of the simultaneous asymmetric construction of three stereogenic centers by the aforementioned reaction of an enyne—titanium complex (see Scheme 9.4) [25], using imines derived from optically active phenylethylamine as the electrophile. 

Snapper and Hoveyda reported a catalytic enantioselective Strecker reaction of aldimines using peptide-based chiral titanium complex [Eq. (13.11)]. Rapid and combinatorial tuning of the catalyst structure is possible in their approach. Based on kinetic studies, bifunctional transition state model 24 was proposed, in which titanium acts as a Lewis acid to activate an imine and an amide carbonyl oxygen acts as a Bronsted base to deprotonate HCN. Related catalyst is also effective in an enantioselective epoxide opening by cyanide "... 

Polyethylene-Wock-poly(clhylcnc-co-norbornene) (PE-fo-P(E-co-NBl ) block copolymer was successfully synthesized by a titanium complex with two non-symmetric bidentate /J-cnaminokclonalo ligands [136,137]. Bis(pyrrolide-imine)titanium complex also has the ability to produce the PE-fo-P(E-co-NBE) block copolymer. PE-fo-PS was synthesized via sequential monomer addition during homogeneous polymerization with bis(phenoxy-imine)metal catalysts [138]. 

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. 

Intramolecular coupling reactions of group 4 imino-acyl complexes yielding the corresponding free imines are rather rare and mainly involve titanium complexes [35 — 38]. The mechanism of these reactions still remains unclear although a concerted reductive elimination step, that initially leads to an -irnine intermediate complex of the type II, has already been postulated [35] (Fig. 4). 

Enyne titanium complexes are synthesized when enynes are treated with Ti(OPr1)2( 72-propene). These species add aldehydes, ketones, and chiral imines to generate multiple stereogenic centers in an acyclic system. Numerous methods for the stereoselective construction of two stereogenic centers are known, although those for more than three in one asymmetric procedure are less common.195... 

The reactivity of the dialkyl complexes TiR2(LL)2 (LL = N,N -dimethylaminotroponiminato) has been widely studied. Reactions with CO and aldehydes or ketones afford unsymmetrical diolato complexes that convert to the corresponding vicinal diols after hydrolysis. CO and acetylene react to form the oxametallacyclopentene complex. Treatment with RNC yields the free imine and low-valent titanium species (Scheme 131). In the reaction with BucNC, free ButN=CMe2 is formed and the addition of benzaldehyde or benzyl reagents affords titanium diolato or enediolato complexes. Thiolato-alkoxo or amido-alkoxo titanium complexes can also be similarly prepared (Scheme 132).123-125... 

The titanium complexes bearing <9-phosphinophenoxo ligands shown in Scheme 380 are prepared by metallation of the corresponding phenol followed by reaction with CpTiCl3. The complexes were found to be fluxional by H NMR analysis at low temperature. The X-ray structure shows that in the solid state the molecule is chiral at the metal center. These complexes are effective catalysts for olefin hydrogenation and imine hydrosilylation.911... 

Hydrosilylation of unsaturated organic molecules is an attractive organic reaction. Asymmetric hydrosilylation of prochiral ketones or imines provides effective routes to optically active secondary alcohols or chiral amines (Scheme 756). These asymmetric processes can be catalyzed by titanium derivatives. The ( A ebthi difluoro titanium complex has been synthesized from the corresponding chloro compound.1659 This compound results in a very active system for the highly enantioselective hydrosilylation of acyclic and cyclic imines and asymmetric hydrosilylation reactions of ketones including aromatic ketones.1661,1666,1926-1929 An analogous l,l -binaphth-2,2 -diolato complex catalyzes the enantioselective hydrosilylation of ketones.1... 

Titanium complexes that are similar to Duthaler s ( 2.5.2) can be generated from TiCl4, Ti(Or-Pr)4 and diacetoneglucose 1.48. These complexes catalyze asymmetric hetero-Diels-Alder reactions, and give high enantiomeric excesses [827], Corey and coworkers [828] also prepared a chiral titanium catalyst derived from cis-/V-sulfonyl-2-amino-1 -indanol and used this to catalyze asymmetric Diels-Alder reactions. Buchwald and coworkers [829, 830] have proposed the use of titanocene-binaphthol catalysts for asymmetric hydrogenation of imines or trisubsti-tuted olefins. 

Fujita and co-workers reported on the catalytic behavior of fluorinated bis (phenoxy-imine) titanium complexes bearing a series of substituents ortho to the phenoxy oxygen for ethylene/higher a-olefin (i.e., 1-hexene, 1-octene, and 1-decene) (co)polymerization (Fig. 3.6) [27]. 

R. Furuyama, M. Mitani, J.-L Mobii, R. Mori, H. Tanaka, T. Fujita, Ethylene/higher a olefin copolymerization behavior of fluminated bis(phenoxy- imine) titanium complexes with methylalumoxane synthesis of new polyethylene-based block copolymers. Macromolecules 38(5), 1546-1552 (2005)... 

R. Fleischer, H. Wanderhch and Braun, Synthesis and structure determination of novel chiral imine -alkoxy titanium complexes. Eur. J. Org. Chem., vol.6, pp. 1063-1070,1998. 

Saito, J. Mitani, M. Mohri, J. Ishii, S. Yoshida, Y Matsugi, T. Matsui, S. et al. Highly syndiospecific living polymerization of propylene using a titanium complex having two phenoxy-imine chelate ligands. Chem. Lett. 2001, 576-577. 

Mitani, M. Furuyama, R. Mohri, J. Saito, J. Ishii, S. Terao, H. Nakano, T. Tanaka, H. Fujita, T. Syndiospecific living propylene polymerization catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate hgands. J. Am. Chem. Soc. 2003,125, 4293 305. 

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