Transition metal catalysts titanium complexes

With chiral ligands, the transition-metal catalyst-hydroperoxide complex yields optically active oxiranes. " One of the most significant advances in the formation of chiral epoxides from allyl alcohols has recently been reported by the Sharpless group. Using (-l-)-tartaric acid, ferf-butylhydroperoxide, and titanium isopropoxide, they were able to obtain chiral epoxides in very high enantiomeric excess. The enantiomeric epoxide can be obtained by employing (—)-tartaric acid (Eq. 33a). 

The high oxophilicity of early transition metal catalysts (titanium, zirconium, or chromium) causes them to be poisoned by most functionalized olefins, particularly the commercially available polar comonomers. However, there are examples of copolymerizations with special substrates or with very high levels of a Lewis acid incorporated into the polymerization system to protect the polar functionality through complexation. " Alternative routes to polar copolymers involving metathesis of cyclic olefins and functionalization of the resulting unsaturated polymer or metathesis of polar cycloolefins followed by hydrogenation to remove the resulting unsaturation have been published.The cost of these multistep... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Transition metals and their complexes can be immobilized in the mesopores or incorporated in the structure to make silica-supported metal catalysts. For instance, titanium catalysts for selective oxidation can be formed by modifying the mesoporous structure with either Ti grafted on the surface (Tif MCM-41) or Ti substituted into the framework (Ti->MCM-41). The grafted version makes the better catalyst for the epoxidation of alkenes using peroxides, and has good resistance to leaching of the metal. 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

As far as the role of the transition metal catalysts in the hydromagnesiation is concerned, other hydrometallation reactions such as hydroa-lumination 1108] and the recently reported hydro-zincation 1109) are catalyzed by the same class of titanium complexes, so all of these reactions can be grouped together. In fact, these reactions show quite similar applicabilities to particular unsaturated compounds and similar rcgioselcctivi-ties with unsymmetrical substrates (eqs. 3.64 and 3.65). 

Lewis acids act as electron pair acceptors. The proton is an important special case, but many other compounds catalyze organic reactions by acting as electron pair acceptors. The most important Lewis acids in organic reactions are metal cations and covalent compounds of metals. Metal cations that function as Lewis acids include the alkali metal monocations Li+, Na+, K+, di- and trivalent ions such as Mg +, Ca, Zn +, Sc, and Bi + transition metal cations and complexes and lanthanide cations, such as Ce + and Yb. Neutral electrophilic covalent molecules can also act as Lewis acids. The most commonly employed of the covalent compounds include boron trifluoride, aluminum trichloride, titanium tetrachloride, and tin(IV)tetrachloride. Various other derivatives of boron, aluminum, titanium, and tin also are Lewis acid catalysts. 

Reaction of alkenes with tert-butyl hydroperoxide (t-BuOOH) in the presence of a transition metal catalyst, for example, a vanadium(V), molybdenum(VI) or titanium(IV) complex, provides an excellent method for the preparation of epoxides. The molybdenum catalysts are most effective for the epoxidation of isolated double bonds, and the vanadium or titanium catalysts are most effective for aUylic alcohols. Even terminal alkenes can be epoxidized readily. For example, 1-decene was converted into its epoxide with t-BuOOH and Mo(CO)6 on heating in 1,2-dichloroethane. 

As indicated by the title, these processes are largely due to the work of Ziegler and coworkers. The type of polymerisation involved is sometimes referred to as co-ordination polymerisation since the mechanism involves a catalyst-monomer co-ordination complex or some other directing force that controls the way in which the monomer approaches the growing chain. The co-ordination catalysts are generally formed by the interaction of the alkyls of Groups I-III metals with halides and other derivatives of transition metals in Groups IV-VIII of the Periodic Table. In a typical process the catalyst is prepared from titanium tetrachloride and aluminium triethyl or some related material. 

Ziegler-Natta catalysts-—there are many different formulations—are organometallic transition-metal complexes prepared by treatment of an alkyl-aluminum with a titanium compound. Triethylaluminum and titanium tetrachloride form a typical preparation. 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

Colona and coworkers oxidized a variety of alkyl aryl and heterocyclic sulfides to the sulfoxides using t-butyl hydroperoxide and a catalytic amount of a complex (97) derived from a transition metal and the imines of L-amino acids. Of the metals (M = TiO, Mo02, VO, Cu, Co, Fe), titanium gave the highest e.e. (21%), but molybdenum was the most efficient catalyst. The sulfoxides were accompanied by considerable sulfone125. 

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