Titanium olefin metathesis

Cydopentadienyl Titanium Derivatives for Carbonyl Olefination/Olefin Metathesis... 

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]. 

Scheme 17. Titanium-mediated metathesis strategy for the conversion of olefinic esters (118) to cyclic enol ethers (123) (Nicolaou et al.) [34]...

Scheme 19. Titanium-mediated metathesis strategy for the conversion of olefinic esters to 6- and 7-mem -bered cyclic enol ethers, (a) 4.0 equiv of Tebbe reagent (93), THF, 25°C, 20 min then reflux, 2-8 h, 64% (129), 45% (131a), 32% (131b), 45% (133) (Nicolaou etal.) [34a]...

Titanium-Based Olefin Metathesis and Related Reactions... 

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-... 

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. 

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. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

Olefin metathesis technology, in polymer synthesis, 26 944-948 Olefin oligomerization, 16 111 Olefin oxides, alkanolamines from (with ammonia), 2 122-140 Olefin polymerization, organic titanium compounds in, 25 122 Olefin(ic) polymers, 17 699-709 ethylene-propylene elastomers, 17 705-707... 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

Complex 4 can be regarded as titanium carbene Cp2Ti = CH2 coordinated by Me2AlCl. This is the first example of a well-defined metal carbene that catalyzes olefin metathesis and is re-isolated in high yield at the end of the reaction, Eq. (10). In a comparatively slow degenerative metathesis reaction (near equilibrium after 47 h at 51 °C). The 13C label of isobutylene was shown to grow into the methylene group of methylene cyclohexane [29]. 

Switching from olefin metathesis to Ziegler Natta polymerization is of interest in order to prepare block copolymers and to establish the relationship between these two related modes of olefin polymerization. Model studies for this purpose included the conversion of titancyclobutanes 10 and 7a into titanium alkyl compounds Eq. (30a and b) by the addition of 1 equivalent of ethanol [43],... 

However, once again, the main obstacle to further development was one of limited substrate scope resulting from the oxophilic titanium, molybdenum, and tungsten metal centers. The problem is illustrated in Figure 6.2, which summarizes the reactivity of early and late transition metal olefin metathesis catalysts with common... 

Takeda, T. Titanium-based olefin metathesis and related reactions. Titanium and Ziivonium in Organic Synthesis 2002, 475-500. 

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