Titanium complexes alkylidenes

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

SiO)MNp3 The supported tris(neopentyl)titanium complex easily undergoes decomposition even at low temperatures [42]. Up to 150 °C, only neopentane is released, which can be explained by either a-H elimination, to give an alkylidene species, or y-H elimination, to give a metaUacyclic species (Scheme 11.1). Nevertheless, no such species could be observed by C CP-MAS or H MAS NMR. The... 

Further investigation of the equilibrium between titanacyclobutene and titanium vinyl alkylidene complexes, as discussed in Section 2.12.6.1.4, was reported recently <2007CEJ4074>, along with the incorporation of this reactivity pattern into the synthesis of conjugated dienes, homoallylic alcohols, vinylcyclopropanes, and phosphacyclobutenes from y-chloroallyl sulfides and a source of titanocene(ll). 

Alkyhdene derivatives of titanium and of phosphorus catalyse methylene exchange between olefins. Although exchange of CH2 groups is not useful for synthesis, these systems provide insight into the mechanisms of alkylidene exchange, a basic step in conventional metathesis. Titanacyclobutenes have been isolated from reactions of acetylenes with methylene-titanium complexes but titanacyclobutanes, the assumed intermediate for the case of olefins, have not been isolated. Bis(cyclopentadiene)titanacyclohexane decomposes to produce ethylene as the major product apparently via a-C-C bond cleavage. ... 

The synthesis of an alkylidene bridged binuclear titanium complex (Scheme 654 Section 4.05.5) has been described and its use in ethylene polymerization in the presence of MAO was investigated.1081... 

Cp 2Ti(CH2CMe3)2 (Cp =Cp, CsH4Me) can be prepared as the product of the reaction between CpCTiCL and Mg(CH2CMe3)2-dioxane or LiCH2CMe3 in diethyl ether at low temperature. These compounds can be stored cold and they decompose at room temperature via a-H abstraction to give neopentane and thermally stable alkylidene titanium complexes (Scheme 524 Section 4.05.4.2.4).1275... 

Gp alkylidene titanium complexes have been generally generated by decomposing dialkyl and related titanium derivatives or from treatment of thioacetals with Ti(n) compounds. Thermolyzing diazoalkane complexes permits the synthesis of non-Gp alkylidene titanium derivatives (see Section 4.05.2). 

A plausible mechanism for the Takeda alkylidenation is given here. The first oxidative addition to generate titanocene complex 57 is essentially instantaneous, while the second oxidative addition, giving bimetallic 58 or 59 is slower. By analogy with the Petasis reagent, the rate-determining step is likely to be the generation of the titanium(IV) alkylidene complex 60, a Schrock carbene. 

Very recently, Eisch and co-workers have developed new alkylidene-group IV metal complexes such as methylidene titanium dichloride 67, readily accessible from titanium(iv) chloride and an excess of methyllithium at low temperature (Scheme 24).53 The new methylenating agent 67 can easily convert benzophenone at low temperature into 1,1-diphenylethylene in quantitative yield. 

Another approach to synthetically useful olefin metathesis involves the utilization of higher homologues of titanium-methylidene 15, as shown in Scheme 14.11. If the resulting titanium carbene complex 20 is more stable than the starting alkylidene complex 15, this reaction can be employed for the generation of various titanocene-alkylidenes and as a method for the preparation of unsaturated compounds. 

An alternative means of preparing alkylidene complexes of titanium is by the elimi-nation reaction of dialkyltitanocenes (Scheme 14.14) [25]. The limitation of this method is that only dialkyltitanocenes having no (i-hydrogen, such as 26 or dicyclopropyltitano-... 

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

The reactivity profiles of the boronate complexes are also diverse.43 For example, the lithium methyl-trialkylboronates (75) are inert, but the more reactive copper(I) methyltrialkylboronates (76) afford conjugate adducts with acrylonitrile and ethyl acrylate (Scheme 16).44 In contrast, the lithium alkynylboronates (77) are alkylated by powerful acceptors, such as alkylideneacetoacetates, alkylidene-malonates and a-nitroethylene, to afford the intermediate vinylboranes (78) to (80), which on oxidation (peracids) or protonolysis yield the corresponding ketones or alkenes, respectively (Scheme 17).45a Similarly, titanium tetrachloride-catalyzed alkynylboronate (77) additions to methyl vinyl ketone afford 1,5-diketones (81).4Sb Mechanistically, the alkynylboronate additions proceed by initial 3-attack of the electrophile and simultaneous alkyl migration from boron to the a-carbon. 

Bis(alkyl) complexes, with mercury, preparation, 2, 428 Bis(alkylidene)s, in Ru and Os half-sandwiches, 6, 583 Bis(alkylimido) complexes, with chromium(VI), 5, 346 Bis(rj2-alkyne)platinum(0) complexes, preparation, 8, 640 Bis(alkynyl) complexes in [5+2+l + l]-cycloadditions, 10, 643 with manganese, 5, 819 with mercury, preparation, 2, 426 mononuclear Ru and Os compounds, 6, 409 with platinum, 12, 125 with platinum(II), 8, 539 with titanium(IV), 4, 643 with zirconium, 4, 722... 

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] ... 

A plausible intermediate of this olefination is the titanium-methylene sjtecies 4, which is formed from 1 by removal of AlMe2Cl with a Lewis base, from 2 by fragmentation with elimination of isobutene, and from 3 by a-elimination and release of methane. However, none of these three routes to titanium-carbene complexes of type 4 proved to be generally applicable. Consequently, the use of these reagents in synthesis is essentially limited to the transfer of a methylene unit 18]. From a synthetic viewpoint, a general and easy route to substituted titanium-alkylidene species and their use in carbonyl olefinations would be more desirable. 

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