Titanium carbon bond

The monometallic mechanism is illustrated in Fig. 7.13a. It involves the monomer coordinating with an alkylated titanium atom. The insertion of the monomer into the titanium-carbon bond propagates the chain. As shown in... 

The bisfunctionalization of alkynes by both C02 and another electrophile can also be achieved, as shown in Scheme 9.17,17a The titanium-carbon bond in the titanacycle complex 31, which was formed by reaction of C02 with the titanacyclopropene 30, can be substituted with various electrophiles. For example, its reaction with NBS or I2 afforded the synthetically useful vinyl bromide or iodide 32, respectively, while the reaction with D20 yielded the /3-deuterated a,/ -unsaturated carboxylic acid. When an aldehyde such as PhCHO was used as an electrophile, butenolide 33 was produced after acidic workup. 

The key step in achieving catalytic turnover is protonation of the titanium—oxygen and titanium—carbon bonds, which is readily achieved by employing collidine hydrochloride as a protic acid. An interesting feature of the cyclization shown above is its diastereocon-vergent nature. From a diastereomeric mixture of the epoxides, the cyclization product is obtained as essentially a single isomer. Unfortunately, this is not always the case, as shown in Table 12.1 [36],... 

The special aspect of this mechanism is that monomers get added up in a stepwise manner into polarised titanium carbon bond and the polymer is given out of the active centre. 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

Ti—13Cp 40.1 ppm), and only weak signals of 13C-labeled alkyl groups at the aluminium were observed (see Fig. 6). These results indicate that an insertion of ethylene takes place into a titanium-carbon bond of a titanium-aluminum complex and no alkyl exchange between the bonds of titanium-alkyl and aluminum-alkyl occurs. 

Fig. 16. 13C-Fr-NMR spectrum at 200K, and after 15 min at 210K, of the system (ij5-CsHs)2Ti(Et)Cl-AlEtCI2 with, 3C-enriched ethylene. Al Ti C2H4 = 1 1 1 solvent toluene-( strong signals of Cl3-enriched atoms are only observed for titanium-carbon bonds and polyethylene. From Fink el al. (160).

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

Sinn and Patat (59) drew attention to the electron-deficient character of those main group alkyls that afford complexes with the titanium compound. Fink et al. (51) showed by 13C NMR spectroscopy with 13C-enriched ethylene at low temperatures (when no alkyl exchange was observed) that, in the more highly halogenated systems, insertion of the ethylene takes place into a titanium-carbon bond of a titanium-aluminum complex. 

Photolysis of Cp2Ti(CH3)2 and Cp2Ti(CH3)Cl in the presence of another transition metal complex has been shown to result in methyl transfer from titanium to the second transition metal and also to other ligand transfer reactions (32a). Photolysis in the presence of certain transition metal halides gave simple metathetical exchange of halide for methyl [e.g., Eq. (26)]. The authors postulated titanium-carbon bond... 

According to ref the Ti—C bond is mainly of covalent character with Ti —C polarization. The length of the titanium-carbon bond, when passing from the methyl to the ethyl complex, changes insignificantly, and it can be supposed that the properties of this bond are independent of the chain length. In the formation of a rt-complex an appreciable proportion of the electron density (as 0.25 e) is transferred from ethylene to the titanium ion. 

Sulfur dioxide insertion into a titanium-carbon bond was first reported in 1971 Reaction (h) is too vigorous at dry-ice temperature with neat sulfur dioxide, and must be moderated by using a solvent such as a saturated hydrocarbon or dichloromethane. 

Titanocene alkyne complexes serve as useful synthons for a range of transformations including acetylene loss to yield titanocene -type chemistry or insertion into the titanium-carbon bond. Two recent reviews have been published on the scope of this reactivity with the parent cyclopentadienyl complexes, (77S-CsHs)2Ti( 72-RCCR). Reactivity not covered in these reviews is presented here.66 89... 

The mechanism of this transformation is unclear at the present time, but two possibilities are pictured below. In the first (Fig. 5), loss of a CO ligand and binding of the acetylene initially provides the T -alkyne complex 17. Subsequent loss of a second equivalent of CO allows for coordination of the alkene to give 17a. Insertion of the olefin into the titanium-carbon bond of the alkyne complex produces metallacyclopentene 18. The insertion of CO generates acyl complex 19 which, upon reductive elimination, yields the observed cyclopentenone product. A second plausible mechanism (Fig. 6) involves initial formation of metal-... 

The basic feature of proposals for the monometallic mechanism is that propagation occurs entirely at one metal center. A monometallic mechanism involving titanium in a lower valence state, for example, RTiCl, has been proposed (63) to be an active site for ethylene polymerization with propagation occurring by coordination and insertion into the titanium-carbon bond (Reaction 12). 

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