Titanium with olefins

The formation of the active metal-carbon bond as a result of the interaction of low-valent ions of the transition metal with olefin is the most intriguing step of the polymerization process by one-component catalysts. The possibility of the formation of the transition metal-carbon bond resulting from the reaction of titanium low-valent ions with ethylene is shown in Dzsabiev et al. (182) ... 

Alternating Copolymerization. In the last part of this paper we would like to refer briefly to our findings in connection with the alternating copolymerization of dienes with olefins. The alternating copolymerization of butadiene with propylene was first investigated in 1969 by Furukawa and others (15, 16, 17). They used catalyst systems based on titanium or vanadium compounds. 

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-... 

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. 

Bis(pyrrolide-imine) complexes, with titanium, for olefin copolymers, 4, 1145... 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

Scheme 8. Reactions of the titanium-alkylidene species 12, prepared from dithioacetals, with olefins and acetylenes.

Selective generation of free radicals from epoxides promoted by (cyclopentadienyl) titanium (III) chloride, followed by trapping, usually with olefin. 

Other titanium-based olefination reagents have been developed. Eisch used a zinc analog of the Tebbe reagent (688) in a reaction with benzophenone to give 1,1-diphenylethene in 78% yield. Similarly, Clawson et al. used 689 in olefination reactions with ketones and aldehydes. Alkoxytitanium reagents such as 690 have been employed, as in the conversion of cyclohexane carboxaldehyde to 1-cyclohexyl-1,3-butadiene (691), in 86% yield.In this olefination reaction, the (Z)-isomer predominated over the ( ) (96 4 Z/E). 

The reaction of 17 with 18 is essentially irreversible, but the reaction of 17 with acyclic olefins is reversible and leads to the expected metathesis reactions, for example, the cis/trans isomerization of HDC=CMePh (Lee, J.B. 1982). Many isotopic labelling and kinetic experiments have been carried out in an attempt to discover whether a titanium-carbene-olefin complex plays a significant kinetic role in these reactions. The general conclusion is that this is unlikely and it is thought that complete dissociation to Ti(=CH2)Cp2 must occur before reaction takes place with an olefin or acetylene (Gilliom 1986a Anslyn 1987 Hawkins 1988, see ref 4 therein). If such a complex does have a finite existence, it probably corresponds only to a very shallow minimum in the energy profile for the reaction. Stereochemical evidence for this conclusion comes from a study of the isomerization reaction (17). 

Recently the oxidative coupling of olefins and alkynes, not only with late transition metal complexes, but also with low valent early transition metal complexes to give metallacyclopentane or metallacyclopentadiene complexes is attracting increasing attention. For example, titanium(II) and zirconium(II) complexes react with olefins and form metallacycles. Various stoichiometric processes to produce useful compounds that are otherwise unavailable can be produced by exploit-... 

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... 

Titanium- and tungsten-based catalysts tend to react with olefinated ketones and esters, whereas molybdenum-based counterparts are more reactive toward olefins (though they can also react with aldehydes and other polar or protic groups). Ruthenium-based catalysts react preferentially with C=C double bonds over most other moieties, a feature that makes them unusually stable toward alcohols, amides, aldehydes, and carboxylic acids. Because of this trend, the functional-group tolerance of an olefin metathesis catalyst can be increased by focusing on a later class of transition metal such as ruthenium [8]. 

Titanium tetrachloride s. under n-CgHjMgBr Phosphoric acid boron fluoride C-Alkylation of aryl derivatives with olefins... 

Kinetic studies and the application of various methods have helped to define the nature of the active centers, to explain the aging effects of Ziegler catalysts, to establish the mechanism of interaction with olefins, and to obtain quantitative evidence of some elementary steps [9,112-115]. It is necessary to differentiate between the soluble catalyst system itself and the polymerization system. Unfortunately, the well-defined bis(cyclo-pentadienyl)titanium system is soluble, but it becomes heterogeneous when polyethylene is formed [116]. 

Lithium aluminium hydride reacts with olefins in the presence of titanium tetrachloride to give the aluminate (112), which in the presence of a copper(ii) catalyst adds to enones in good yield [equation (69)]/ " ... 

The research by Beerman also demonstrated the relative stability of the Ti-methyl bond as compared to the relatively less stable Ti-ethyl bond that contains a hydrogen on the beta carbon and can, therefore, undergo beta-hydride transfer to the titanium metal and eliminate an ethylene molecule. This early research eventually lead in the 1970s to the identification of transition metal carbene complexes (M=CH2), which when reacted with olefins provide metallacyclobutanes [29]. 

Titanium vinylidene complexes also react with olefins to give four-membered ring met-allacycles . 

The results presented in Section 6.4.3 revealed that the McMurry reactions of ster-ically hindered ketones can in fact be viewed as Wittig-like olefination reactions. While these two reactions were for a long time thought to be mechanistically different, formation of carbenoid species by reduction of ketones with low-valent titanium complexes is not really surprising in view of the highly oxophilic and reducing character of titanium. Carbonyl olefinations by means of titanium carbe-... 

The highly regio- and diastereoselective addition of an alkyl and an arylthio group to an olefinic double bond ( carbosulfenylation ) is achieved with arenesulfenyl chlorides and alkyl-chloro-titanium(IV) species (Reetz reagent, from R2Zn/TiCU 5 1 M. T. Reetz, 1987, 1989), Use of the more bulky 2,4,6-triisopropylbenzenesulfenyl chloride improves the yield of the highly versatile alkyl aryl sulfide products. 

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