Alkyltitanium compounds

Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

Scheme 14 /3-Hydrogen abstraction reactions of alkyltitanium compounds.

Methyltitaniiim trichloride, CH3TiCl3 Diniethyltitanium dichloride, (CH3),TiCl2. These alkyltitanium compounds are prepared from (CH,) Zn and TiCl4. 

Review. Reetz1 has reviewed newer published reactions of these reagents as well as numerous unpublished results from his own research. The review emphasizes the chemo-, diastereo-, and enantioselective reactions that can be achieved. Drawbacks are that secondary and tertiary alkyltitanium compounds generally are not available, owing to /1-hydride elimination, and that ready ligand exchange makes the preparation of derivatives chiral at titanium difficult. The review includes useful suggestions for experimental techniques. 

Alkyltitanium compounds were found [79] to promote the polymerisation of styrene, leading, however, to an atactic polymer. Also, the polymerisation of styrene employing benzyl derivatives of group 4 metals,... 

Secondary and tertiary alkyltitanium compounds are generally not accessible due to decomposition via P-hydride elimination. 

In subsequent investigations the generality of the above behavior was established for n-alkyltitanium compounds 21,22 77,78). For example, quenching ethyllithium 25 with chlorotitanium triisopropoxide 3 affords 9, which reacts in situ with a 1 1 mixture of 19 and 20 to afford essentially only the aldehyde adduct 23 (Equation 9) 77). 25 itself reacts at 0 °C almost statistically ... 

The amido ligand is isoelectronic with the alkyl and alkoxo groups and has the possibility to exhibit N-M ir-interactions in order to stabilize the systems they form. Different synthetic reactions are used to prepare mono-Cp titanium derivatives containing amido ligands. These compounds are normally synthesized by (i) the action of the corresponding amide salt on Cp TiCl3, (ii) displacement of amine from a homoleptic amido titanium compound by a Cp reagent, (iii) dehalosilylation reactions, and (iv) elimination of alkane by reaction of an alkyltitanium compound with amine. 

Some of the more active catalysts for the polymerization of styrene are shown in Figure 22.17. Monocyclopentadienyltitanium halides, such as CpTiXj, CpTiXj, and CpTiXj, in combination with MAO, were more active catalysts than simple tetrahalides. Of these Cp-ligated trihalide complexes, the fluoride complexes were most active, followed by alkoxides and then chlorides. Moreover, cyclopentadienyl-ligated alkyltitanium compounds, such as Cp TiRj (R = hydrocarbyl) activated by boron compounds B(C Fj)j... 

The active catalyst in a Ziegler-Natta polymerization is thought to be an alkyltitanium compound, which is formed by alkylation of the titanium halide by A1(CH2CH3)2C1 on the surface of a MgCl2/TiCl4 particle. Once formed, this species repeatedly inserts ethylene into the titanium-carbon bond to yield polyethylene. 

Other alkylcobalt and alkyltitanium compounds, especially those with branched alkyl substituents, also decompose by olefin elimination. The isobutyl complex, [(CH3)2CHCH2Co(CN)s] , slowly loses isobutene at room temperature (115). The unique stability of methyl derivatives of the transition metals relative to other alkyl derivatives is partly due to the absence of a j8-hydrogen atom which can be transferred to the metal. The... 

Therefore, methyltitanium compounds and neopentyltitanium compounds not having a -carbon atom are relatively stable as alkyltitanium compounds. The phenyl and other aryl titanium compounds are relatively stable. As described above, titanium compounds having cyclopentadienyl groups are stable and have a high melting point. Cp2TiCl2 forms a 7r-coordination distorted tetrahedral structure [30]. 

In alkyltitanium compounds, the reactivity increases if the number of alkyl group increases. The order of the reactivity is as follows [58] ... 

Several other alkylation reactions may be written. The formation of ethyl free radicals by decomposition of unstable alkyltitanium compounds accounts for the evolution of the hydrocarbon gases. Thus the final product is a complex mixture of organo-aluminium and -titanium compounds, lower titanium chlorides and some organic fragments. The actual composition of the product is dependent on the relative proportions of the starting materials and the time and temperature of reaction. 

The catalytically active species is assumed to be a Ti(III) hydride formed by alkylation-reduction of a titanocene dichloride. As mentioned above, the course of the reaction is governed either by Markovnikov or antf-Markovnikov addition to the double bond that produces the intermediates 97a and 97b. Intramolecular carbometallation gives the alkyltitanium compounds 98a and 98b that after j0-hydrogen elimination afford the corresponding products 92 and 93 (Scheme 40). 

The key step of the reaction mechanism is reaction of the titanium complex 118 with the a,co-enone to give the titanaoxacyclopentane 140, which reacted with diphenylsilane by the cleavage of the Ti-0 bond to afford the alkyltitanium compound 141. Reductive elimination furnished the siloxane 138 and the catalyst entered the cycle again (Scheme 62). 

Addition to carbonyl groups. These alkyltitanium(IV) compounds (and related reagents) add readily to both aldehydes and ketones to form alcohols in high yield. Addition to aldehydes is so much faster than addition to ketones that selective addition to an aldehyde is possible. Diastereoselective addition to ketones is a useful feature. Thus the reaction affords an excellent route to the axial alcohol (2). 

How does the anionic alkyl of the original trialkylaluminum or of the dialkylaiuminum chloride, which has sufficient anionic character to undergo anionic hydride exchange or CH3OT reaction, form a catalyst which becomes cationic under certain polymerization conditions No studies of this have been reported. One possibility is an internal oxidation-reduction reaction that converts an anionic alkyltitanium trichloride to a cationic alkyltitanium trichloride (Equation 10). Basic and electrophilic catalyst components would determine the relative contributions of the anionic and cationic forms. This type of equilibrium or resonance structures could also explain the color in transition metal compounds such as methyltitanium trichloride (73). 

When more than the stoichiometric amount of AlEt3 is used to reduce TiCl4, in particular at Al/Ti ratios higher than 0.45, the reaction product is not brown or purple and crystalline anymore but black and amorphous (4). Under such conditions TiCl4 is overreduced with formation of miscellaneous titanium compounds of valence < 3 containing TiCl2 and possibly also some alkyltitanium chloride (10,18). 

In the alkylation of a-chiral aldehydes with no ability to chelate with organometal-lic compounds such as Grignard reagents, erythro alcohols are usually obtained preferentially according to the Cram s rule [127], and high Cram selectivity can be achieved with alkyltitanium reagents developed by Reetz [128]. In contrast, application of amphiphilic alkylation to a-chiral aldehydes enables one to achieve the hitherto difficult anti-Cram selectivity, affording threo alcohols selectively as shown in Sch. 91 [125]. 

Alkyltitanium(IV) complexes having N -dialkylamino ligand systems, RTi(NR 2)3, fail to give nucleophilic additions to carbonyl compounds (Section 1.5.3.1.1). Their reaction with aldehydes leads instead to tertiary amines by addition of both the alkyl moiety of the reagent and one of the N, -dialkylamino ligands (equation 59). The synthetic interest of the reaction is restricted to noneno-lizable aldehydes, since enolizable carbonyl compounds lead to enamines. ... 

The field of polymer chemistry was revolutionized in 1953 with the development of Ziegler-Natta catalysts. These catalysts are organometallic complexes that can be prepared by treating an alkyl aluminum compound, such as Et3Al, with TiC. Although the mechanism is not entirely understood, it is widely believed that the active form of the catalyst is an alkyltitanium intermediate with a vacant coordination site. 

The other reason that the organotitanium compounds are not stable is, in particular, because alkyltitaniums are liable and its -elimination tends to proceed. For example, titanium compounds having an ethyl group form a four electron adduct Ti(Me2PCH2CH2PMe2)(EtCl3) as shown in Figure 12.1 [26]. 

In further studies on nucleophilic alkylation of aldehydes and ketones (to give secondary and tertiary alcohols, respectively) using alkyltitanium(iv) compounds, the e lier work on methyl tri-isopropoxytitanium (5,155) has been extended to other alkyl tri(alkoxy)titanium reagents. The observed chemoselec-tive addition to the aldehyde in benzaldehyde-acetophenone mixtures [equation (2)] illustrates that these systems are less reactive but more selective... 

Alkyltitanium halides are prepared using a variety of alkylating agents. Methyltitanium trichloride may be prepared from methyl aluminium compounds. 

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