Titanium complexes with alkylidenes

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... 

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. ... 

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). 

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. 

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). 

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. 

Interestingly, the subsequent reactions of the titanium-alkylidene species 12 obtained from dithioacetals are not limited to carbonyl olefina-tions. When the carbene complex is prepared in the presence of olefins, the latter are smoothly cyclopropanated (Scheme 8 13) [14]. Furthermore, the reaction of symmetrically disubstituted acetylenes with dithioacetals containing a methylene unit provides the corresponding trisubsti-tuted 1,3-dienes 14 in a stereoselective fashion 115]. 

Alkene, Alkyne, Alkylidene,m and Carbonyl Complexes. While titanium al-kene complexes are unquestionably involved in polymerizations, relatively few have been isolated. Interactions of TiCL,(dmpe)2 and butadiene under reducing conditions give Ti(T7-C4H6)2(dmpe), which can be converted by CO and PF3 to Ti(CO)2(PF3)-(dmpe)2 and Ti(CO)3(dmpe)2. The latter reacts with K in the presence of biphenyl or naphthalene and then with CO to give Ti(CO) - species which are isolable as K(cryptate)+ salts 102... 

Titanium derivatives containing carbazole (cb) ligands have been isolated and studied. Reaction of Ti(CH2SiMe3)4 with carbazole yields the alkylidene-bridged dimer [Ti(/i-CHSiMe3)(cb)2]2 (Scheme 106). A singlet at 8 14.75 is observed in the 1H NMR spectrum of the alkylidene complex. The reaction of this compound with 2,6-dimethyl-phenyl isocyanide leads to titanium derivatives containing new carbon-carbon bonds (Section 4.05.2.3).109,110... 

The reduction of Cp2TiCl2 with Mg in the presence of P(OEt)3 affords the titanium(n) complex Cp2Ti[P(OEt)3]2 which was used as a catalyst for the carbonyl olefination of thioacetals through a titanium-alkylidene intermediate (see Section 4.05.4.2.4 alkylidene complexes).1148... 

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