Titanium-oxygen bond

Cp2Ti(PMe3)2 catalyzes the reductive cyclization of the enones 44 to the cyclopentanols 46 via the metallacyclic intermediates 45 (Scheme 27) [64-66]. The cleavage of the titanium-oxygen bond in the metallacycles 45 by a hydrosilane provides a route to the generation of the active catalyst. The net transformation resembles the above-mentioned complementary radical pathway, which affords the opposite isomer. 

Titanium alkoxyesters and their derivatives are of considerable importance as a result of their ability to modify the properties of various kinds of plastics and coatings. The ease of formation and stability of the titanium-oxygen bond is the basis for these phenomena, which in the literature are summarized under the terms coupling agents or crosslinking agents . The applications include... 

If titanium-nitrogen and titanium-oxygen bonds arc available in the same molecule, CO insertion yields carbamates [104]. If carbon dioxide has the capability (0 insert into a metal-carbon or a metal-nitrogen bond, the insertion into M-N is preferred hafnium dialkyl diarnides react with CO at room temperature to yield the carbamato derivatives HfRjfOjCNR ) [105). 

The active species in the Tebbe olefination is believed to be the nucleophilic (Schrock-type) titanocene methylidene, which is formed from the Tebbe reagent upon coordination of the aluminum with a Lewis base (e.g., pyridine). This methylidene in its uncomplexed form, however, has never been isolated or observed spectroscopically owing to its extreme reactivity. The same intermediate can also be generated by other means." The titanocene methylidene reacts with the carbonyl group to form an oxatitanacyclobutane intermediate that breaks down to titanocene oxide and the desired methenylated compound (alkene). The driving force is the formation of the very strong titanium-oxygen bond. 

The crucial aspect for catalytic turnover is the stability of radicals, epoxides, and the titanium(III) reagent under buffered protic conditions. C-C bond-forming reactions have also been realized using the same concept. In these cases, both titanium-oxygen bonds (addition to a,)5-unsaturated esters) or a titanium-oxygen... 

The structure of HaTiCL was determined by X-ray diffraction. The Ti—O bond length [176.7(1) pm] and the Ti—O—C bond angle [146.1(1)°] are in accord with a titanium—oxygen bond order close to 2. 

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],... 

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

As in the reductive ring-opening, titanocene—oxygen bonds have to be protonated. Here, a titanium enolate, which is generated after reductive trapping of an enol radical, has to be protonated, in addition to a simple titanocene alkoxide. As before, 2,4,6-collidine hydrochloride constitutes a suitable acid to achieve catalytic turnover, but here zinc dust turned out to be the reductant of choice [31c], The features of the stoichiometric reaction are preserved under our conditions. Acrylates and acrylonitriles are excellent radical acceptors in these reactions. Methyl vinyl ketone did not yield the desired addition product. Under the standard reaction conditions, a-substituted acceptors are readily tolerated, but (3-substitution gives the products only in low yields. 

The key step in achieving catalytic turnover is protonation of the titanium—oxygen and titanium—carbon bonds, which is readily achieved by employing collidine hydrochloride as a protic acid. An interesting feature of the cyclization shown above is its diastereocon-vergent nature. From a diastereomeric mixture of the epoxides, the cyclization product is obtained as essentially a single isomer. Unfortunately, this is not always the case, as shown in Table 12.1 [36],... 

This problem has been partially overcome by elimination of the phosphorus-oxygen bonds, as, for example, in the poly(phosphinoisocyanates), which have the structure shown in 6.47.42 It is also possible to form poly(metal phosphinates) with repeat unit -M(0PR20)2- by allowing a metal alkoxide to react with a phosphinic acid.43 Typical metal atoms are aluminum, cobalt, chromium, nickel, titanium, and zinc.43 Polymeric phosphine oxides can be prepared by the reactions... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

If the 1,5-diearbonyl compound is required, then an aqueous work-up with either acid or base cleaves the silicon-oxygen bond in the product but the value of silyl enol ethers is that they can undergo synthetically useful reactions other than just hydrolysis. Addition of the silyl enol ether derived from aeetophenone (PhCOMe) to a disubstituted enone promoted by titanium tetrachloride is very rapid and gives the diketone product in good yield even though a quaternary carbon atom is created in the conjugate addition, This is a typical example of this very powerful class of conjugate addition reactions. 

Products isolated from the insertion of SO2 into transition metal-carbon bonds have been shown to adopt structures (I), (III), and possibly, (IV). The first mode of bonding, expected of class b (la) or soft metals (105), is by far the most common one encountered. In fact, oxygen-bonded insertion products have been isolated only for titanium and zirconium (131, 132). However, recent spectroscopic studies have demonstrated that (III) is the kinetic product of the insertion with a number of metal carbonyl alkyls and aryls it then isomerizes to (I) (72, 73) ... 

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