Titanocenes, metathesis reaction

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

Scheme 18. Titanocene-mediated photocatalytic butadiyne metathesis reaction.

The olefin metathesis reaction as applied to carbohydrate dienes has been reviewed, as has the use of titanocene(III) and zirconocene(III) chlorides as mild reagents for generating 1-glycosyl radicals from glycosyl halides, and converting them into glycals. ... 

One of the peculiarities of the dehydrogenating coupling reaction is that it was initially discovered for titanium but details of the o-bond metathesis reaction steps were worked out for Zr and Hf. The reason that this had not been done for Ti was the difficulty to generate suitable precursor compounds such as Cp2Ti(Cl)Si (SiMc3)3. Recent studies have shown that the Si-Ti bond is much weaker than the corresponding bonds to Zr and Hf [110]. As a consequence reductive elimination of silylated titanocenes is a facile process. The thus formed Ti(ll) species can... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

One of the synthetically useful titanium-based olefin metatheses is the reaction of titanocene-methylidene with terminal allenes 8. Productive olefin metathesis occurs when titanacyclobutanes are treated with 8 (Scheme 14.7) [17] and the resulting a-alkyli-... 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

Despite the successful reactions mentioned above, olefin metathesis utilizing titanocene-methylidene is not necessarily regarded as a useful synthetic tool. Indeed, the steric interaction between the substituent at the carbon a to titanium and the bulky cyclopentadienyl ligand disfavors the formation of the titanocene-alkylidene 15. Hence, cleavage of the titanacycle affords only titanocene-methylidene and the starting olefin (Scheme 14.9). 

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. 

The organotitanium compounds produced by desulfurization of the diphenyl thioacetals of aldehydes 28 with the titanocene(II) species Cp2Ti[P(OEt)3]2 29 react with carbon—carbon double bonds to form the olefin metathesis-type products. Thioacetals 28 may be transformed into terminal olefins by desulfurization with 29 under an ethene atmosphere (Scheme 14.15) [27]. This reaction is believed to proceed through a titanacyclobutane intermediate, formed by cycloaddition of the titanocene-alkylidene with ethene. 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

Formation of Titanocene-Methylidene and its Reaction with Olefins 476 Formation of Trtanocene-Alkylidenes and their Application to Olefin Metathesis 479... 

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... 

A common theme in many reactions is the generation of an equivalent of CP2M through the reduction of a tetravalent precursor metallocene with a metal alkyl such as n-butyllithium. The chemistry of olefin complexes of CP2M is characterized by a facile interconversion between formally M(II) and M(IV) manifolds, as shown in equation 34. Another central motif of metallocene chemistry, especially that of titanocenes, is the accessibility of pathways connecting metallacycles and metal alkylidene complexes, eg in alkene metathesis (eq. 35). 

Cross-metathesis of the allyl-Cp substituted titanocene dichloride leads to the formation of dinuclear titanium complexes (Scheme 12.25, M = Ti, n= 1) [33]. The composition of the reaction mixture depends on the catalyst used. Treatment of the substrate with Ru-I (3 mol%) in benzene, toluene, or dichloromethane gave a mixture of Z and E isomers in a 1 1 ratio, while application of the Ru-II catalyst resulted with the formation of the pure E isomer. 

Titanocene derivatives catalyze reductive cyclization of an alkene with a hetero-atom-containing functional group and the cleavage of the titanium-oxygen bond in these metallacycles was promoted by reaction with silanes, with concomitant formation of Ti-H and Si-O bonds via a <7-bond metathesis process (Scheme 12.45) [65],... 

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