Titanium insertion reactions

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

T. J. Groshens, 1. Molecular Vapor Synthesis The Use of Titanium Monoxide and Vanadium Monoxide. 2. Oxidative Insertion Reactions of First Row Early Transition Metal Atoms, Ph. D. Thesis, Kansas State University, 1988. 

Transition metal-carbon a bonds are notoriously labile, although stable cr-organo-titanium or -zirconium compounds have been isolated from insertion reactions". ... 

Finally, vanadium alkoxides and dialkylamides, such as [V(OEt)4]2 or V(NR2)4, can be obtained by methods similar to those used for titanium the dialkylamides react with CS2, undergoing a V—N insertion reaction (see page 777) to give eight-coordinate Viv compounds, V(S2CNR,)4.24... 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

It should be noted that the monomer coordination step shown in Eq. (2.82) may not be a distinct step as discussed previously. An important feature of this mechanism which affects the stereospecificity of olefin polymerizations using these types of soluble catalysts is the fact that the insertion of the monomer into the transition metal-carbon bond involves a secondary insertion reaction, i.e., the more substituted carbon of the double bond in the monomer becomes bonded to the transition metal (Corradini et al., 1985). In contrast, a primary insertion mechanism to form a transition metal bond to the less substituted carbon on the double bond of the monomer Ti-CH2CHR-P is involved in polymerizations using typical heterogeneous catalysts, e.g., from titanium halides and alkylaluminum compounds (Boor, 1979). 

Phillips supported chromium (II) catalyst, the most commonly used for high density polyethylene (HDPE) manufacture, possibly behaves in a similar manner, but the olefin insertion reaction is faster by several orders of magnitude. In the original Zeigler catalyst systems for HDPE, an aluminium alkyl is used to reductively alkylate the primary component, most frequently a titanium compound, to give the true catalytic species. 

Titanium and zirconinm tetra-alkoxides undergo insertion reactions of both organic isocyanates and carbodiimides into M-0 bonds, as shown by Eqs (2.290) and (2.291) ... 

Insertion Reactions Involving Zirconium and Titanium 5.2.1 Hydrozirconation and Carbozirconation... 

This model is modified by Pino [300,301], Corradini [302], Kissin [303], Keii [304], Terano [305], Cecchin [306] to other titanium complexes. Bimetallic models between the titanium compound and the cocatalyst were discussed by Sinn and Patat [137], Pino [301], and Zakharov [307]. Others suggest that the growing polymer chain is bound to the transition metal through a double bond (carbene complex) and that the insertion reaction occurs through formation of a metal-cyclobutane intermediate [308,309]. 

With organotitanium compounds, especially carbometalation, substitution and insertion reactions tend to proceed. Carbometalation, as described in the reaction mechanisms of the Ziegler-Natta catalyst in Chapter 7, contains olefin polymerization with titanium compounds. Namely, titanium reacts with organoalumi-num compounds and the titanium atom forms a bridged structure (Ti—R—Al) with an alkyl group of organoaluminum compounds and this Ti—R adds to a carbon-carbon double bond. 

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