1,3-Cyclopentadiene titanium complex

The phosphino-oxazoline copper(II) complex (55) has also been found to be an effective catalyst[136] as have some titanium complexes, such as the extensively researched titanium-TADDOL system (56)[137]. A modified Ti(IV)-TADDOL compound is the catalyst of choice to promote Diels-Alder cycloaddition reactions between cyclopentadiene and alk-2-enyl phenylsulfonylmethyl ketones[138]. 

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). 

An irreversible tt-ct rearrangement has been observed to occur with the treatment of titanocene dicloride (8) with dimethyl sulfoxide (17). The o complex (9) has been isolated, however, treatment of the reaction mixture with benzene produces the titanium complex of dimethyl sulfoxide (10), and treatment of the reaction mixture with maleic anhydride produces the cyclopentadiene addition product (11). Monitoring the reaction mixture by proton NMR spectroscopy demonstrates the ir-a change in the bonding of the cyclopentadienyl ligands. 

Coutts and Wailes (52, 53) synthesized a series of 5-coordinate, mono-cyclopentadiene dithiocarbamate complexes of titanium(III) and tita-nium(IV), [CpTi(S2CNR2)2] (R =Me, Et, or Pr), prepared by ligand-exchange reactions between CpTiCl2 and Na(S2CNR2). Infrared spectra and molecular-weight data suggested (52) structure (VI). CpTiXj... 

C,Hk, 1,3-Cyclopentadiene, 1-methyl-, preparation and purification of, 27 52 titanium complex, 27 52 vanadium complex, 27 54, 55 QH,Oj, Ascorbic acid, platinum complex, 27 283... 

Alkyhdene derivatives of titanium and of phosphorus catalyse methylene exchange between olefins. Although exchange of CH2 groups is not useful for synthesis, these systems provide insight into the mechanisms of alkylidene exchange, a basic step in conventional metathesis. Titanacyclobutenes have been isolated from reactions of acetylenes with methylene-titanium complexes but titanacyclobutanes, the assumed intermediate for the case of olefins, have not been isolated. Bis(cyclopentadiene)titanacyclohexane decomposes to produce ethylene as the major product apparently via a-C-C bond cleavage. ... 

The bis-Cp dichloro titanium complex with the camphor-substituted Gp ring (Scheme 479) is prepared from the corresponding cyclopentadiene compound by reaction with LiBu11, metallating with TiCl3 followed by oxidation with HCl/air. From the initially formed 9 1 mixture of Cz- and Gi-symmetric bis-Gp derivatives, the pure U2-diastereomer... 

More encouraging are the results obtained from the trimethylsilyl-protected D-glucofuranose derivatives 8. The titanium(IV) chloride catalyzed addition of cyclopentadiene (9) to 8 gives 10 with a d.r. 93 7. Poorer results were obtained with 1.3-cyclohexadiene (d.r. 84 16) or anthracene (d.r. 70 30). In order to increase the Lewis acidity of the titanium complex, derivative 11 was examined and a d.r. 98 2 was observed for the ent/o-adduct 122tl. 

The concept of using rigid and reactive (V-acyl-l,3-oxazolidin-2-ones such as 15 shows its value in the cycloadditions to C2-symmetrical chiral diols catalyzed hy titanium complexes. The cycloaddition reactions of 15 to cyclopentadiene (8), catalyzed by 2 equivalents of 18, give adducts with an enantiomeric excess up to 94% ee14. 

Introduction of two methyl groups into the 4-position of the oxazolidinone ring seems to have a negative effect on the selectivity. [4 + 2] Cycloaddition of N-acryloyl and V-crotonoyl-4,4-dimethyl-l,3-oxazolidin-2-ones to cyclopentadiene in the presence of titanium complexes prepared from a broad range of chiral diols, produces adducts with a diastereoselectivity not exceeding a d.r. 64.5 35.59. 

C10H15N, Benzenemethanamine, N,N,4-trimethyl-, lithium complex, 26 152 C10H15P, Phosphine, diethylphenyl-, nickel complex, 28 101 platinum complex, 28 135 CioHigAsi, Arsine, 1,2-phenylenebis(dimethyl-, gold complex, 26 89 nickel complex, 28 103 CioHie, 1,3-Cyclopentadiene, 1,2,3,4,5-pen-tamethyl-, 28 317 chromium complex, 27 69 cobalt complexes, 28 273, 275 iridium complex, 27 19 samarium complex, 27 155 titanium complex, 27 62 ytterbium complex, 27 148 CioH,gBrN04S, Bicyclo[2.2.1]heptane-7-methanesulfonate, 3-bromo-1,7-di-methyl-2-oxo-, U.IRHENDO, ANTPi]-, ammonium, 26 24... 

On the contrary, if the reactivity of the dienophile complex is enhanced, a higher stereoselection can be expected. Actually, the titanium complex 3 of the 3-0-acryloyl-l,2-0-isopropylidene-a-D-xylofuranose, in which the titanium carries three chlorine ligands, with cyclopentadiene forms only one diastereomer 4 of the Diels-Alder adduct. As confirmed by reductive cleavage, only the expected (R)-diastereomer is obtained. The other diastereomer is not detectable by 400 MHz-NMR spectroscopy of the crude hydrolyzed reaction mixture (23). 

Chiral titanocene derivatives can also be used as catalysts in asymmetric DA reaction [150] (Scheme 14.60). The reaction of oxazolidinone derivatives and cyclopentadiene was realized using enantiopure l,2-ethylenebistetrahydroindenyl-Ti(OTf)2 as the catalyst. The bistetrahydroindenyl-derived titanium complex with chiral biphenyl tether was also employed for the same reaction, giving the DA product with moderate enantiomeric excess [151]. 

The reaction of cyclopentadiene with acrolein derivatives in the presence of a chiral helical titanium complex prepared from Ti(O Pr)4 and a BINOL derived tetraol ligand has been reported by Yamamoto. The DA products are obtained with uniformly high enantioselectivity and the endojexo selectivity was found to be sensitive to the constituent pattern of a, -unsaturated aldehydes [154] (Scheme 14.63). 

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