Titanium complexes with boranes

In case of borate as cocatalyst, the catalytic activity of the titanium complex with a pentamethylcyclopentadienyl hgand is high, but a titanium complex with a cyclopentadienyl ligand without any substituents is not active for the syndiospecific styrene polymerization. The reason is that the reaction product of the borate and the cycopentadienyltitanium compound is unstable. The stability of the active site with the borate compound is lower in comparison to that with MAO. The reaction of CH2(Cp)2Ti(Me)2 with dimethylanilinium tetrakis(pentafiuorophenyl)borate or tris(pentafluorophenyl)borane in an equimolar mixture has been examined by Miyashita, Nabika, and Suzuki [11]. Two types of methylene bis(cyclopentadienyl)titanium ion complexes were isolated (see Fig. 3.6). These complexes were active in the polymerization of styrene, but only atactic polystyrene was formed. 

To date, the hydrofunctionalization route has mainly been used to prepare PBs. Both conceivable strategies, namely the hydroboration of unsaturated phosphines and the hydrophosphination of unsaturated boranes have been reported (Scheme 25). With dialkyl and diarylboranes, the reactions proceed spontaneously under mild conditions, while the addition of boronates HB(OR)2 is catalyzed by a titanium complex, and the... 

Sodium acetoxytrihydroborate is also effective,while certain titanium, rhodium and cobalt complexes catalyze hydroborations with sodium tetrahydroborate." In the latter case the facial selectivities can be quite different from, even opposite to, those observed with borane-THF. ... 

Insertion into metal boryl bonds is represented by two examples described for a late and an early transition metal complexes. Platinum diboryl complexes react with alkynes to give the diborylated alkene (Scheme 6.61, a) [191]. A titanium(II) metallocene with a coordinated ethylene reacts with borane to give, via insertion of the alkene into a M-B bond and subsequent j6-H ehmination, a complex with a coordinated vinylborane (Scheme 6.61, b) [192]. Both processes can be made catalytic. 

Reactions catalyzed by titanocene and lanthanocene complexes occur by different mechanisms (Schemes 16.13 and 16.14). The mechanism of the hydroboration of vinylarenes catalyzed by titanium complexes is shown in Scheme 16.13. In this mechanism, a titanocene bis-borane complex dissociates borane to generate a 16-electron complex that coordinates the alkene or alkyne. Coupling of one carbon of the resulting metallacycle with the boron of the coordinated borane forms the final product. The mechanism of the lanthanocene-catalyzed reactions, shown in Scheme 16.14, relates to the mechanism of... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

The complex shown in Scheme 511 is formed by the reaction of Cp 2Ti(CH2=CH2) with the corresponding borane in appropriate conditions. These complexes are intermediates in the dehydrogenative borylation of ethylene through the selective conversion of a titanium-olefin bond to vinyl borate esters.1303... 

Carbonyl compounds include isobut3raldehyde, phenyl isopropyl ketone, glyoxylic acid and pyruvic acid. Diaryl ketones do not react. Modifications of the method consist in the use of a borohydride exchange-resin, of sodium triacetoxyborohydride NaBH(OAc)3 or in treating a mixture of a ketone and an amine with an equivalent of titanium(IV) chloride and Hiinig s base (ethyldiisopropylamine) in dichloromethane, followed by a methanolic solution of sodium cyanoborohydride. The borane-pridine complex and hydrogen telluride are excellent replacanents for sodium cyanoborohydride. 

Styren . Mono-Cp complexes of titanium are especially usefial in the S5m-diotactic polymerization of styrene (first reported in Ref 221, a material with high melting point ( 278°C) and other useful features such as a rapid crystallization rate (222). The catalysts are believed (223) to be cationic Ti(III) monoalkyl species, which then undergo predominantly 2,1-insertions (Scheme 10). This is true even in the case of systems in which the activator is the fluorinated borane B(CeF5)3. The mechanism of reduction is unclear, but appears to be accelerated in the presence of styrene. 

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