Titanium, cationic carbonyls

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

Density functional theory studies arene chromium tricarbonyls, 5, 255 beryllium monocyclopentadienyls, 2, 75 chromium carbonyls, 5, 228 in computational chemistry, 1, 663 Cp-amido titanium complexes, 4, 464—465 diiron carbonyl complexes, 6, 222 manganese carbonyls, 5, 763 molybdenum hexacarbonyl, 5, 392 and multiconfiguration techniques, 1, 649 neutral, cationic, anionic chromium carbonyls, 5, 203-204 nickel rj2-alkene complexes, 8, 134—135 palladium NHC complexes, 8, 234 Deoxygenative coupling, carbonyls to olefins, 11, 40 (+)-4,5-Deoxyneodolabelline, via ring-closing diene metathesis, 11, 219... 

If the reaction between enol silyl ethers and a,/ -unsaturated ketones is attempted in the presence of a titanium Lewis acid, the mode of the reaction switches to 1,4-addition with reference to the unsaturated ketone [109-113]. The reaction of an enol silyl ether is shown in Eq. (30) [114]. Ketene silyl acetals react with a,j8-unsaturated ketones in similar 1,4-fashion, as exemplified in Eq. (31) [115]. Acrylic esters, which often tend to polymerize, are also acceptable substrates for a, -unsaturated carbonyl compounds [111]. A difluoroenol silyl ether participated in this cationic reaction (Eq. 32) [116], and an olefinic acetal can be used in place of the parent a-methylene ketone [111] to give the 1,5-diketone in good yield (Eq. 33) [117]. More results from titanium-catalyzed 1,4-addition of enol silyl ethers and silyl ketene acetals to a,f -unsaturated carbonyl compounds are summarized in Table 4. 

The first step of the mechanism involves the initial complexation of titanium tetrachloride to the carbonyl group of the electron-deficient alkene (enone) to give an alkoxy-substituted allylic carbocation. The allylic carbocation attacks the (trimethylsilyl)allene regiospecifically at C3 to generate vinyl cation I, which is stabilized by the interaction of the adjacent C-Si bond. The allylic Ji-bond is only coplanar with the C-Si bond in (trimethylsilyl)allenes, so only a C3 substitution can lead to the formation of a stabilized cation. A[1,2]-shift of the silyl group follows to afford an isomeric vinyl cation (II), which is intercepted by the titanium enolate to produce the highly substituted five-membered ring. Side products (III - V) may be formed from vinyl cation I. 

Na-, La-, and Re-exchanged zeolites have also been used as catalysts of the Michael reaction between silyl enol ethers and a, 6-unsaturated carbonyl compounds. This study, performed by Sasidharan et al. [87], focused mainly on the catalytic activity of titanium silicalite molecular sieves (TS-1 and TS-2). They found that TS-1 and TS-2 catalyze 1,4-Michael addition of silyl enol ethers and a,y5-unsatu-rated carbonyl compounds under anhydrous conditions. The zeolites tested as catalysts of this reaction, e. g. ReY, LaY, steamed zeolite Y, and cation-exchanged ZnZSM-5, were less active (or inactive). 

Zeotype materials containing metal cations, for example ions of titanium, vanadium, chromium, iron, or tin, in the tetrahedral positions of their frameworks have been explored as solid Lewis acid catalysts (Ig). Such materials have been shown to be active in the Meerwein—Pormdorfr-Verley reduction of carbonyl compounds (151,233), and the BV oxidation (also called BV rearrangement) (234). 

Like the carbonylation of epoxides, the carbonylation of aziridines occurs with increased rates and scope in the presence of catalysts containing a Lewis acidic cation and Co(CO) " anion. A particularly active version of fliis catalyst for the carbonylation of aziridines is the uncommon species [CpjTi(THF)2]""[Co(CO)J . As shown in Equation 17.56, this catalyst is substantially more active tiian COj(CO)g for the carbonylation of N-benzyl cyclohexene imine. The carbonylation of N-tosyl-2-methylaziridine has also been accomplished (Equation 17.57), and ttiis reaction is important because of the ability to prepare optically active N-tosyl aziridines. Although the reaction catalyzed by the titanium and cobalt system occurred to only 35% conversion, the carbonylation of the N-tosyl-2-methylaziridine catalyzed by the aluminum and cobalt system occurred to completion under the same reaction conditions. 

This group seldom manages more than a handful of papers for this report and 1993 was no exception. Ellis and coworkers, as part of a long series on highly reduced organometallics, have published a study on the carbonyl hydrides of titanium new cationic Zr(IV) carbonyl complexes are reported and carbonyl adducts of open titanocenes are the subject of a paper by Wang et ai. ... 

The coordination and organometallic chemistry of zirconium and hafnium are surveyed for the year 1991.107,108 There is useful material in a review of homogeneous Group 4 metallocene Ziegler-Natta catalysts, a review of the coordination chemistry of cyclopentadienyl titanium carboxylate and related complexes O and a review of bis(cycIopentadienyl)zirconium(IV) or hafnium(IV) complexes with Si-, Ge-, Sn-, N-, P-, As-, Sb-, 0-, S-, Se-, Te- or transition metal-centred anionic ligands. Cationic zirconocene or hafiiocene complexes serve as Lewis acids with unique reactivities, they are active for C-F bond activation, coordinative activation of ether linkages, carbonyl activation and C-O bond cleavage. New synthetic methods based on the... 

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