Titanium complexes metallacycles

Another example for a new titanium complex [Cp2Ti(As2Me4)]2[AsFg]4 contains a six-membered metallacycle. The temperature-dependent H NMR spectra indicate dynamic behaviour and a chair conformation (40). 

Ind-amido titanium complexes with o -alkenyl functions in position 2 of the indenyl ring have been synthesized and characterized. After activation with MAO, these complexes were used as homogeneous and heterogeneous catalysts for the homopolymerization of ethylene and propylene and the co-polymerization of ethylene and 1,7-octadiene.406 A series of alkyl-, u -alkenyl-, and u -phenylalkyl-substituted Cp- and Ind-amido dichloro titanium complexes have been synthesized and characterized. The cj-phenylalkyl-substituted complexes react with LiBu to give metallacycles via a CH activation reaction on the ortho-position of the phenyl group (Scheme 305).741 742 After activation with MAO, these complexes catalyze ethylene polymerizations. The substituents on the aromatic system influence the polymerization activity of the catalysts and the properties of the polyethylene. The u -alkenyl-substituted catalysts show self-immobilization in ethylene polymerization. 

Reactions of the anti-van t Hoff/le Bel titanium complex Cp2Ti(PhC)2(HBEt2) with ethylene, acetylenes, phospa-cetylenes, acetone, and nitriles give metallacycle or bimetallic compounds (Scheme 521). In some cases, hydroboration of the G-C or C-heteroatom multiple bond in the substrates is observed.1331 (C5Me5)2Ti[r/2-CH2=CHB(Cat)]1303... 

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... 

There is some material of relevance in a review on chiral titanium complexes for the enantioselective addition of nucleophiles to carbonyl groups , in a short review on stereoselective transformations mediated by chiral CpM (M = Ti, Zr, Hf) complexes, and in a review on the preparation of organoaluminium and organomagnesium metallacycles from titanium and zirconium... 

Titanium complexes of diphenylacetylene react with carbon dioxide to give similar five-membered ring metallacycles . 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

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. 

Metallacyclic complexes containing two molecules of diyne per MCp2 group have also been isolated, that with titanium containing 2,4-alkynyl substituents (225) while with zirconium, the unusual seven-membered metallacumulene structure 226 is adopted, which has only one alkynyl substituent. The bi- and tricyclic 2/2 complexes 227 and 228 have so far been obtained only from reactions of TiCp2 with PhC=CC=CPh.30 ... 

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). 

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

Compound 1 exhibits significantly different reactivity than the acyclic analogue. The metallacycle is more stable than the acyclic complex and whereas Cp2Ti"Bu2 decomposes via the expected (3-1I elimination pathway to produce butenes and butane, the thermal decomposition products of 1 are ethylene and 1-butene. In addition, the metallacycle is observed to be significantly more reactive towards CO than Cp2Ti"Bu2 Reaction of 1 with carbon monoxide at —55 °C yields the titanium acyl species, based on infrared data, which then rapidly converts to cyclopentanone at 0 °C (Scheme l).13... 

The metallacycle formed according to scheme (11) is in equilibrium with a small (unobservable) amount of titanium alkylidene complex formed by the opening of the titanacycle ring the alkylidene complex is then trapped by norbornene to give a new titanacycle, and thus the polymer chain is propagated [49] ... 

Treatment of the acetylene complexes Cp2Ti( 72-C2R2) (R = SiMe3 or Ph) with C02 at room temperature resulted in the formation of the dimeric cr-alkenylcarboxylate complexes 131 and 132 (Equation (35)).102 The magnetic moment of both complexes was 1.6 /xB per titanium atom indicating that they contained trivalent Ti centers. The structure of 132 was elucidated by X-ray diffraction, and the presence of two fused metallacyclic rings was confirmed. 

The synthesis and characterization of the neutral homoenolate mono-Gp CpTiC CF CF COOEt) and cationic bis-Cp titanium [Cp2TiCFl2CF[2COOEt]+Znl3 derivatives have been reported. The molecular structure of the cationic complex shows ester coordination to give a five-membered metallacycle (Scheme 499).53... 

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