Titanium complexes hydrides

A synthetically useful diastereoselectivity (90% dc) was observed with the addition of methyl-magnesium bromide to a-epoxy aldehyde 25 in the presence of titanium(IV) chloride60. After treatment of the crude product with sodium hydride, the yy -epoxy alcohol 26 was obtained in 40% yield. The yyn-product corresponds to a chelation-controlled attack of 25 by the nucleophile. Isolation of compound 28, however, reveals that the addition reaction proceeds via a regioselective ring-opening of the epoxide, which affords the titanium-complexed chloro-hydrin 27. Chelation-controlled attack of 27 by the nucleophile leads to the -syn-diastereomer 28, which is converted to the epoxy alcohol 26 by treatment with sodium hydride. 

Magnesium—nickel hydride, 4458 Plutonium(III) hydride, 4504 Poly(germanium dihydride), 4409 Poly(germanium monohydride), 4407 Potassium hydride, 4421 Rubidium hydride, 4444 Sodium hydride, 4438 f Stibine, 4505 Thorium dihydride, 4483 Thorium hydride, 4535 Titanium dihydride, 4484 Titanium—zirconium hydride, 4485 Trigermane, 4415 Uranium(III) hydride, 4506 Uranium(IV) hydride, 4536 Zinc hydride, 4486 Zirconium hydride , 4487 See COMPLEX HYDRIDES, PYROPHORIC MATERIALS See entry LANTHANIDE—TRANSITION METAL ALLOY HYDRIDES... 

The titanium complex is diamagnetic which, if the titanium is present as Tin, indicates a strong exchange interaction. It reacts with C02 forming THF.Ti(OOCH)2MgCl15, which yields formic acid on hydrolysis and ethyl formate with C2H5I. This indicates that the hydrogen is bound to the carbon atom of the co-ordinated C02 molecule as in (7) or (8). The formation of C—H bonds implies the presence of reactive metal-hydride intermediates.76... 

Complex (2) is believed to be a titanium(in) hydride and has not been isolated or characterized (no H NMR signals for any titanium species are observable, probably as a result of the paramagnetic nature of the complex). This complex is extremely air sensitive and must be handled under rigorously oxygen-free conditions. Solutions of complex (2) are stable under inert atmosphere for at least 24 h and exhibit no sensitivity to light. For synthetic purposes it is most convenient to generate the active catalyst from complex (1) immediately prior to use. 

Chiral titanium complexes are also employed as effective asymmetric catalysts for other carbon-carbon bond-forming reactions, for example addition of diketene (Sch. 66) [154c,162], Friedel-Crafts reaction (Sch. 67) [163] (Sch. 68) [164], iodocar-bocyclization (Sch. 69) [165], Torgov cyclization (Sch. 70) [166], and [2 -i- 1] cycloaddition (Sch. 71) [167]. Asymmetric functional group transformations can also be catalyzed by chiral titanium complexes. These transformations, for example the Sharpless oxidation [168] or hydride reduction [169] are, however, beyond the scope of this review because of space limitations. Representative results are, therefore, covered by the reference list. 

Replacement by hydrogen of vinylic chlorine without reduction of the double bond can be accomplished by complex hydrides. The effrcacy of lithium aluminum hydride is increased by titanium tetra-... 

Examination of the enantioselectivities in Table 7.5 indicates a striking difference in selectivity achieved in the reduction of cyclic (entries 1-8) vs. acyclic imines (entries 9-11). The former is very nearly 100% stereoselective. The simple reason for this is that the acyclic imines are mixtures of E and Z stereoisomers, which reduce to enantiomeric amines vide infra). The mechanism proposed for this reduction is shown in Scheme 7.11 [86]. The putative titanium(III) hydride catalyst is formed in situ by sequential treatment of the titanocene BINOL complex with butyllithium and phenylsilane. The latter reagent serves to stabilize the catalyst. Kinetic studies show that the reduction of cyclic imines is first order in hydrogen and first order in titanium but zero order in imine. This (and other evidence) is consistent with a fast 1,2-insertion followed by a slow hydrogenolysis (a-bond metathesis), as indicated [86]. Although P-hydride elimination of the titanium amide intermediate is possible, it appears to be slow relative to the hydrogenolysis. 

Titanium, tetrakis(trimethysilyl)oxy-, 334 Titanium complexes alloy hydrides, 353 amino adds, 342 antimony, 345 arsenic, 345 bromides, 357 chlorides, 355, 356 fluorides, 354 Group IV derivatives, 352 halides, 354 electron spectra, 358 hexamethylphosphoramide, 335 iodides, 357... 

Group 4 metallocene complexes can also be used as catalysts in the reduction of C=N bonds. Willoughby and Buchwald employed the titanium-based Brintzinger catalyst (3.54) for the asymmetric reduction of imines. The catalyst is activated by reduction to what is assumed to be the titanium(III) hydride species (3.55). The best substrates for this catalyst are cyclic imines, which afford products with 95-98% ee. Various functional groups including alkenes, vinyl silanes, acetals and alcohols were not affected under the reaction conditions. For example, the imine (3.56) was reduced with excellent enantioselectivity, without reduction of the alkene moiety. 

Preparation.—Alcohols can be obtained from terminal alkenes by hydro-alumination with lithium aluminium hydride in the presence of a titanium complex, followed by oxidative cleavage of the adduct (Scheme 1). The sequence provides anti-Markovnikov alcohol. 

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