Titanium complexes titanocene

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

A novel route to 2,3-dihydrothiophenes involved a titanocene-promoted carbene formation and subsequenct intramolecular cyclization onto a thiol ester <99SL1029>. Treatment of thioacetal 9 with the low-valent titanium complex 10 gave 2,3-dihydrothiophene 12 by intramolecular olefination of the thiol ester of titanium-carbene intermediate 11. Another metal-mediated cyclization onto the thiophene ring system involved the palladium-catalyzed cyclization of 1,6-diynes <99T485>. For example, treatment of thioether 1,6-diyne 13 with Pdlj in the presence of CO and Oj in methanol followed by treatment with base gave 14. 

Subsequently, direct incorporation of GO by titanocene(ii) catalyst, Gp2Ti(GO)2, under a GO atmosphere was reported.This catalytic system showed substantially higher TON and broader functional group compatibility. However, this catalyst fails to react with sterically hindered olefins and alkynes. In a recent contribution from the same group, a series of aryloxide titanium complexes 22 (figure 4) are prepared and shown to promote PKR with some sterically hindered enynes." ... 

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. 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

Alkenes without functional groups are difficult to hydrogenate enantioselec-tively with noble-metal catalysts. Titanium complexes with a C2 symmetric chiral ligand framework, as in the ansa-titanocene (22-XVI), reduce aryl-substituted C=C bonds in very high isolated and optical yields 19... 

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. 

Another example of a titanium complex is the binuclear titanocene complex 18 which is obtained by the insertion of titanocene dicarbonyl into the S-S bond of a fused ring compound 17, involving elimination of CO (Scheme 5)... 

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