Titanium compounds carbonyls with alkenes

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

The role of titanium salt is to activate the carbonyl compounds as Lewis acid. As described above, bis(iodozincio)methane (3) is nucleophilic enough to attack the carbonyl group of aldehydes or ce-alkoxyketones. In the reaction with simple ketones or esters, however, the addition of titanium salt is necessary to facilitate the nucleophilic attack. Instead of this Lewis acid activator, simple heating may induce the nucleophilic attack. Treatment of 2-dodecanone with 3 without titanium salt at higher temperature, however, does not improve the yield of alkene (Scheme 13). The reason for the low reactivity of 3 at higher temperature comes from the structural change of 3 into the polymeric methylene zinc 4 through the Schlenk equilibrium shown in equation 740. 

Bis(adamantylimido) compounds, with monomeric chromium(VI) complexes, 5, 348 Bis(alkene) complexes conjugated, Rh complexes, 7, 214 mononuclear Ru and Os compounds, 6, 401 -02 in Ru and Os half-sandwich rj6-arenes, 6, 538 with tungsten carbonyls and isocyanides, 5, 685 Bis(u-alkenylcyclopentadienyl) complexes, with Ti(II), 4, 254 Bis(alkoxide) nitrogen-donor complexes, with Zr(IV), 4, 805 Bis(alkoxide) titanium alkynes, in cross-coupling, 4, 276 Bis(alkoxo) complexes, with bis-Cp Ti(IV), 4, 588 Bis[alkoxy(alkylamino)carbene]gold complexes, preparation, 2, 288... 

McMurry developed a reduction procedure that is used for alkenation of carbonyl compounds in the presence of low-valent titanium (LVT) reagent. The reagent (thought to be a mixture of Ti(0) and Ti(II) species) is formed by the reduction of TiCU or TiCb with a suitable reducing agent (Zn-Cu alloy, LiAlH4 or alkali metal are the most commonly used). 

Mechanism Two pathways are suggested for this reaction (Scheme 4.49). The titanium-carbene complex A is formed as a key intermediate, which reacts with carbonyl compound to form an alkene via the oxatitanacyclobutane B (Path A). Alternatively, the addition of gem-dimetallic species C to a carbonyl compound gives the adduct D, which eliminates (TiCp2 RS)20 to give an alkene (Path B). 

The metallacycles (4) generate the requisite titanium methylidene thermally. Consistent with this is the observation that reaction with a carbonyl compound is first order in the metallacycle and zero order in the carbonyl compound. Metallacycle stability depends on the alkene moiety that has replaced the aluminum-chlorine portion of (3) and thus could provide a series of reagents whose reactivity is temperature dependent. ... 

A useful alternative to phosphorus ylids are the titanium reagents, such as, 71, prepared from dicyclopentadienyltitanium dichloride and trimethylaluminum. Treatment of a carbonyl compound with the titanium cyclopentadienide complex 71 Tebbe s reagent) in toluene-THF containing a small amount of pyridine " leads to the alkene. Dimethyltitanocene (Me2TiCp2), called the Petasis reagent, is a convenient and highly useful alternative to The mechanism of Petasis... 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

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. 

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