Titanium metallacycles

Figure 5-41 PM3 vs. Experimental CpTiCp (Centroid) Bond Angles in Titanium Metallacycles, CpCpTiCU... 

A rather novel approach makes use of the reaction between the titanium metallacycle (4) and one double bond of an allene to produce a new metallacycle. That new metallacycle then transfers the allene to a carbonyl compound (equation 26). ... 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

Cp2Ti(PMe3)2 catalyzes the reductive cyclization of the enones 44 to the cyclopentanols 46 via the metallacyclic intermediates 45 (Scheme 27) [64-66]. The cleavage of the titanium-oxygen bond in the metallacycles 45 by a hydrosilane provides a route to the generation of the active catalyst. The net transformation resembles the above-mentioned complementary radical pathway, which affords the opposite isomer. 

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). 

More recently, the doubly lithiated derivatives of [4.1.1]- and [3.1.1]propellasilanes underwent bridging metathesis at the bicyclobutane ends with dihalides of germanium, tin and transition metal titanium, to give the corresponding highly strained metallacyclic propellanes (equation 7)29. 

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 stereochemical outcome was rationalized by a Zimmerman-Traxler type transition state 45.64 Assuming the titanium enolate of 42 has a Z-geometry and forms a 7-membered metallacycle with a chairlike conformation, a model can be proposed where a second titanium metal coordinates to the indanol and aldehyde oxygens in a 6-membered chairlike conformation. The involvement of two titanium centers was supported by the fact that aldehydes that were not precomplexed with titanium tetrachloride did not react (Scheme 24.7).63 Ghosh and co-workers further hypothesized that a chelating substituent on the aldehyde would alter the transition state 46 and consequently the stereochemical outcome of the condensation, leading to. vyn-aldol products 47.64 Indeed, reaction of the titanium enolate of 42 with bidentate oxyaldehydes proceeded with excellent. s v -diastereo-selectivity (Scheme 24.8).65... 

A consensus on or explanation for the influence of the oxidation state of titanium on olefin polymerisation activity has not been reached. The absence of any insertion of the coordinating ethylene into the Ti-C bond in Ti(II) species is noteworthy instead, two ethylene molecules, which coordinate at two coordination sites at Ti(II) species, undergo an oxidative addition, and thus the respective metallacycle, titanacyclopentane, is formed [305], Such a reaction for dimethyltitaniumcomplexed by l,2-bis(dimethylphosphione)ethane [Dmpe] is as follows [305] ... 

As regards stable metallacycle catalysts for cycloolefin polymerisation, catalysts based on Ti (titanacyclobutanes) and Ta (tantallacyclobutanes) are used [47,49]. The first demonstration of a living and well-controlled system concerned the polymerisation of norbornene in the presence of substituted biscy-clopentadienyltitanacyclobutane in which four-coordinate titanium possesses formally a d° 16-electron structure [97,98], The initial titanacycle undergoes a reaction with norbornene at 20 °C to yield the trisubstituted metallacyclobu-tane ... 

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] ... 

The titanium-aluminum methylidene (3), commonly known as the Tebbe reagent,was the first well-characterized compound in this series. The Tebbe reagent (3) and related metallacycles (4) have been used principally as methylidene sources toward carbonyl groups. Both are believed to provide the titanium methylidene (5). ... 

The metallacycles (4) generate the requisite titanium methylidene thermally. Consistent with this is the observation that reaction with a carbonyl compound is first order in the metallacycle and zero order in the carbonyl compound. Metallacycle stability depends on the alkene moiety that has replaced the aluminum-chlorine portion of (3) and thus could provide a series of reagents whose reactivity is temperature dependent. ... 

The titanium methylidene interacts with the carbonyl carbon-oxygen double bond in a sequence that is believed to resemble the metathesis process. With the metallacycle (4), a titanium oxametallacycle is believed to be formed, which then decomposes to generate the new carlxm-carbon double bond and a titanium-oxygen product. The driving force for the reaction has been attributed to the oxophilicity of titanium (equation 18). ... 

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). 

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. 

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