Titanocene hydride

If 13 was allowed to stand at room temperature in an inert atmosphere over a prolonged period, or if a saturated hexane solution of Cp2TiMe2 was treated with excess hydrogen at low temperature with efficient stirring, a gray-green polymeric titanocene hydride 14 formed which exhibited reactivity similar to the violet hydride 13 (24). Thus, exposure of a toluene... 

Figure 1.31 illustrates a mechanism proposed for this hydrogenation. The titanocene hydride 31A is expected to be a catalytic species. The imine substrate is inserted into the Ti—H bond of 31A with a 1,2-fashion to form a titanocene amide complex 31B. Then the hydrogenolysis of 31B through a a-bond metathesis produces the amine product with regeneration of 31A. The enantioface selection... 

The nucleophilic 7i-allyltitanium complex 67 is prepared by the reaction of the conjugated diene 65 with titanocene hydride 66, generated in situ by the treatment of titanocene dichloride with 2 moles of z-PrMgCl [21]. The complex is nucleophilic and reacts with aldehydes regio- and stereoselectively to give homoallylic alcohols [22]. 

Polymer-attached Cp2TiCl2 has been reduced by sodium naphthalide, and the resultant species, which may contain a mixture of Ti(IV), Ti(III), and Ti(II), are more active hydrogenation catalysts for olefins than is the unsupported Cp2TiCl2 (72). Although distinct Ti-H-containing species were not identified, it has been suggested that the complete reaction occurs at one metal center, in contrast to earlier suggestions that such reductions involve a bimolecular reaction of an intermediate titanocene alkyl and a titanocene hydride (30). 

Figure 7.6. Transition structures for titanocene hydride imine reduction [86] (a) Front view of heterocycle reduction, (b) Top view of heterocycle reduction, (c) Front view of acylic imine reduction, (d) Top view of imine reduction.

Scheme 7.11 Asymmetric reduction of imines using titanocene hydride complex 16.

Titanocene difluorides also catalyze the hydrosilation of imines, presumably through the formation of the trivalent titanocene hydride Cp 2Ti—H. This reaction can be made enantioselective, as shown in (equation 43) (255). 

It is believed that the transition state (88) is responsible for the observed stereochemistry, and that bases utilizing a metal which bonds ti tly to oxygen favour (88) hence the selectivity of the zirconium base. This approach to trans-hydrindanes has been applied to a short stereoselective synthesis of adreno-sterone. A mixture of cis- and rran -hydrindanes is obtained by treating 1,2-divinylcyclohexane with a titanocene hydride derivative. ... 

Barrero, Oltra and coworkers reported on the use of epoxygeranyl acetate in titanocene-mediated cyclizations and found that the termination of the reaction took place via a /i-hydride elimination after trapping of the radical by the second equivalent of Cp2TiCr [94,95]. This finding together with Takahashi s tandem cyclization [96] (see below) marks the first example of extremely interesting developments in epoxypolyene cyclizations via radicals that are discussed separately in the following section. 

A highly reactive form of titanocene could be obtained when a suspension of the gray-green hydride 14 was stirred in ethyl ether for 2 hours at room temperature. The solid gradually disappeared concurrent with the evolution of 0.5 equivalent of H2 per equivalent of Ti. Molecular weight measurements showed this metastable form of titanocene (15) to be dimeric. Treatment of a cold ethereal solution of (Cp2Ti)2 (15) with CO resulted in a quantitative yield of Cp2Ti(CO)2 (1) (24,36). 

The major obstacle to this approach is that there are few reagents capable of generating higher homologues of titanocene-methylidene. Although the procedure is not straightforward, the titanacycle 21 formed by the addition of diisobutylaluminum hydride to the double bond of (l-propenyl)titanocene chloride 22 serves as a titanocene-propylidene 23 equivalent in carbonyl olefmation (Scheme 14.12) [22]. 

It is also essential that competing radical pathways are excluded. The radical intermediates should therefore be relatively persistent. This is the case here, because tertiary radicals are relatively slowly trapped by hydrogen atom donors, e.g., THF, which is usually applied as solvent in titanocene-mediated or -catalyzed reactions, or a second equivalent of Cp2TiCl. Flowever, in the absence of other pathways this reduction, which was followed by a -hydride elimination, was observed [75,76]. Our results with 10 are summarized in Table 5. 

Hydroalumination. Titanocene dichloride is an effective catalyst for hydro-uluminution of alkenes and alkynes with his(dialkylamino)alancs5 and various complex aluminum hydrides. The adducts can be quenched with water or iodine. The reaction is satisfactory for terminal alkenes and internal alkynes, but is not clcun for internal alkenes and terminal alkynes. 

High enantioselectivity (ca. 95-99% ee) is observed in this system, better than that revealed in previous reports of the hydrosilylation of imines. The mechanism is as yet unclear however, the authors propose that an active catalyst may be formed by cleavage of the Ti-F bond and generation of a Ti(III) hydride species. Insertion of an imine into the Ti-H bond, followed by a (r-bond metathesis with the silane in a four-centered transition state, may lead to the observed products. Another report on the activity of titanocene complexes as catalysts for the hydrosilylation of aid- and keti-mines also indicates formation of a Ti-H species as catalyst.188 Hydrosilylation proceeds to yield silylamines, with dependence on substitution at nitrogen and on the nature of the ligand bound to the metallocene precursor. 

There has been no recent comprehensive review of this area, although a book on the organometallic chemistry of titanium, zirconium, and hafnium deals, in part, with some of the hydride derivatives (1). In the present review, the first part of the discussion reflects the fact that much of the early work on organotitanium hydrides was, often unknowingly at the time, interwoven with attempts to prepare titanocene, Cp2Ti (Cp = tj3-C5H5). Subsequent sections deal with similar compounds containing an additional metal (e.g., aluminum), miscellaneous titanium hydride compounds, and a summary of the main properties of the above species. 

The Ti-Ti separation (3.195 A) leaves open the question of how much Ti-Ti interaction is present, and it is significant that the compound is weakly paramagnetic. The presence of a 7r-bonded fulvalene has also been found in other systems, namely, the mixed aluminotitanium hydrides (CpTWHXHsAlEtsXCxoHs) (26) (VII), and [(CgH TiHAlEt,], CjoHg) (26, 27), which are discussed further below, and the niobocene (CpNbH)2(CioH8) (28). In addition the l3C-NMR spectrum of titanocene... 

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