Titanium catalysis complex

Vorstenbosch, M. L. W. Alkene Epoxidation with Silsesquioxane-Based Chromium and Titanium Complexes, Schuit Institute of Catalysis, Eindhoven, 2002. 

Ziegler-Natta catalysts are primarily complexes of a transition metal halide and an organometallic compound whose structure is not completely understood for all cases. Let us use as an example TiCU and R3AI. The mechanism of the polymerization catalysis is somewhat understood. This is shown in Fig. 14.6. The titanium salt and the organometallic compound react to give a pentacoordinated titanium complex with a sixth empty site of... 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

The asymmetric oxidation of sulphides to chiral sulphoxides with t-butyl hydroperoxide is catalysed very effectively by a titanium complex, produced in situ from a titanium alkoxide and a chiral binaphthol, with enantioselectivities up to 96%342. The Sharpless oxidation of aryl cinnamyl selenides 217 gave a chiral 1-phenyl-2-propen-l-ol (218) via an asymmetric [2,3] sigmatropic shift (Scheme 4)343. For other titanium-catalysed epoxidations, see Section V.D.l on vanadium catalysis. 

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. 

Since cationic titanium complexes have been proposed as intermediates in aUcene polymerization by soluble Cp2TiCl2/AlR3-based Ziegler-Natta catalysts (see Ziegler-Natta Catalysts), there has been considerable interest in the synthesis and reactivity of cationic titanium complexes (see Oligomerization Polymerization by Homogeneous Catalysis). 

Ene Cyclization, " The asymmetric catalysis of the intramolecular carbonyl-ene reaction not only of type (3,4) but also (2,4) employs the BINOL-derived titanium complexes [(I )-BINOL-TiX2 X = C104 or OTf], modified by the perchlorate and trifluoromethanesulfonate ligands. The tmns-... 

We reported the first examples of asymmetric catalysis of intramolecular carbonyl-ene reactions of types (3,4) and (2,4) using the BINOL-derived titanium complex (1) [80,83]. The catalytic 7-(2,4) carbonyl-ene cyclization gives the oxepane with high ee, and gem-dimethyl groups are not required (Sch. 27). In a similar catalytic 6-(3,4) ene cyclization, the fram-tetrahydropyran is preferentially produced, with high ee (Sch. 28). The sense of asymmetric induction is exactly the same as observed for the glyoxylate-ene reaction—the (f )-BINOL-Ti catalyst provides the (R)-cyclic alcohol. 

Keck also investigated asymmetric catalysis with a BINOL-derived titanium complex [102,103] for the Mukaiyama aldol reaction. The reaction of a-benzyloxyalde-hyde with Danishefsky s dienes as functionalized silyl enol ethers gave aldol products instead of hetero Diels-Alder cycloadducts (Sch. 40) [103], The aldol product can be transformed into hetero Diels-Alder type adducts by acid-catalyzed cyclization. The catalyst was prepared from BINOL and Ti(OPr )4, in 1 1 or 2 1 stoichiometry, and oven-dried MS 4A, in ether under reflux. They reported the catalyst to be of BINOL-Ti(OPr% structure. 

The Lewis acid-catalyzed conjugate addition of silyl enol ethers to a,y3-unsaturated carbonyl derivatives, the Mukaiyaraa Michael reaction, is known to be a mild, versatile method for carbon-cabon bond formation. Although the development of catalytic asymmetric variants of this process provides access to optically active 1,5-dicarbonyl synthons, few such applications have yet been reported [108], Mukiyama demonstrated asymmetric catalysis with BINOL-Ti oxide prepared from (/-Pr0)2Ti=0 and BINOL and obtained a 1,4-adduct in high % ee (Sch. 43) [109]. The enantioselectiv-ity was highly dependent on the ester substituent of the silyl enol ether employed. Thus the reaction of cyclopentenone with the sterically hindered silyl enol ether derived from 5-diphenylmethyl ethanethioate proceeds highly enantioselectively. Sco-lastico also reported that reactions promoted by TADDOL-derived titanium complexes gave the syn product exclusively, although with only moderate enantioselectiv-ity (Sch. 44) [110]. 

As shown above, asymmetric catalysis of Diels-Alder reactions has been achieved by use of chiral titanium complexes bearing chiral diol ligands. Yamamoto has reported a chiral helical titanium complex derived from Ti(OPr )4 and a BINOL-derived tetraol ligand (Sch. 54) [134], The Diels-Alder products are obtained with uniformly high enantioselectivity, irrespective of the substituent pattern of a,/3-unsaturated aldehydes. Corey has also reported a new type of chiral titanium complex derived from an amino alcohol ligand (Sch. 55) [135]. The chiral titanium complex serves as an efficient asymmetric catalyst for the reaction of 2-bromoacrolein the Diels-Alder product is obtained with high enantioselectivity. 

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. 

Mikami, K., Terada, M. Chiral titanium complexes for enantioselective catalysis. Lewis Acid Reagents 199S, 93-136. 

Aoki, S., Mikami, K., Terada, M., Nakai, T. Enantio- and diastereoselective catalysis of addition reaction of allylic silanes and stannanes to glyoxylates by binaphthol-derived titanium complex. Tetrahedron 1993, 49,1783-1792. 

Oguni has reported asymmetric amplification [12] ((-i-)-NLE) in an asymmetric carbonyl addition reaction of dialkylzinc reagents catalyzed by chiral ami-noalcohols such as l-piperidino-3,3-dimethyl-2-butanol (PDB) (Eq. (7.1)) [13]. Noyori et al. have reported a highly efficient aminoalcohol catalyst, 2S)-3-exo-(dimethylamino)isobomeol (DAIB) [14] and a beautiful investigation of asymmetric amplification in view of the stability and lower catalytic activity of the het-ero-chiral dimer of the zinc aminoalcohol catalyst than the homo-chiral dimer (Fig. 7-5). We have reported a positive non-linear effect in a carbonyl-ene reaction [15] with glyoxylate catalyzed by binaphthol (binol)-derived chiral titanium complex (Eq. (7.2)) [10]. Bolm has also reported (-i-)-NLE in the 1,4-addition reaction of dialkylzinc by the catalysis of nickel complex with pyridyl alcohols [16]. 

---