Titanium complexes stereochemistry

Titanium, tetrakis(trimethysilyl)oxy-, 3, 334 Titanium, tetranitrato-stereochemistry, 1,94 Titanium, triaquabis(oxalato)-structure, I, 78 Titanium, tris(acetylacetone)-structurc, 1,65 Titanium alkoxides oligomeric structure, 2,346 synthesis ammonia, 2, 338 Titanium chloride photographic developer, 6,99 Titanium complexes acetylacetone dinuclear, 2, 372 alkyl... 

BINOL-derived titanium complex was found to serve as an efficient catalyst for the Mukaiyama-type aldol reaction of ketone silyl enol ethers with good control of both absolute and relative stereochemistry (Scheme 8C.24) [57]. It is surprising, however, that the aldol products were obtained in the silyl enol ether (ene product) form, with high syn-diastereoselec-tivity from either geometrical isomer of the starting silyl enol ethers. 

Indium-mediated allylation of 4-acetoxy-2-azetidinones affords 4-allyl-substituted azetidinones with retention of the stereochemistry (Equation (82)).320 An aminoalkoxy titanium complex is readily allylated with allylindium reagents to give homoallylic amines (Scheme 86).321... 

Mukaiyama Aldol Condensation. The BINOL-derived titanium complex BINOL-T1CI2 is an efficient catalyst for the Mukaiyama-type aldol reaction. Not only ketone silyl enol ether (eq 25), but also ketene silyl acetals (eq 26) can be used to give the aldol-type products with control of absolute and relative stereochemistry. 

Mukaiyama Aldol Condensation. As expected, the chiral titanium complex is also effective for a variety of carbon-carbon bond forming processes such as the aldol and the Diels-Alder reactions. The aldol process constitutes one of the most fundamental bond constructions in organic synthesis. Therefore the development of chiral catalysts that promote asymmetic aldol reactions in a highly stereocontrolled and truly catalytic fashion has attracted much attention, for which the silyl enol ethers of ketones or esters have been used as a storable enolate component (Mukaiyama aldol condensation). The BINOL-derived titanium complex BINOL-TiCl2 can be used as an efficient catalyst for the Mukaiyama-ty pe aldol reaction of not only ketone si ly 1 enol ethers but also ester silyl enol ethers with control of absolute and relative stereochemistry (eq 11). ... 

Ethylene-bridged bis(indenyl) titanium complexes in which the Ind ligands are attached at the 2-position and containing alkyl substituents in 1-position have been obtained as a mixtures of meso- and racemo-isomers (Scheme 649). The assignment of the stereochemistry for these compounds has been established on the basis of spectroscopic data and crystal structure determinations. These compounds act as catalysts for the epoxidation of unfunctionalized alkenes. The catalytic activity of this new class of complexes in epoxidation reactions was in some cases found to exceed that of known bis-Cp titanium complexes.1670... 

Titanium complexes are often encountered in Lewis acid-catalysed reactions. This is certainly true for catalysed aldol reactions. Mikami and Matsukawa demonstrated that titanium/BINOL complexes e.g. complex (7.20) afforded high yield and enantioselectivity in the aldol reactions of thioester ketene silylacetals with a variety of aldehydes. In contrast to some of the aldol reactions described above, the stereochemistry of the adducts is dependant on the geometry of the enol ether. Thus, reaction of the (B)-enol ether (7.21) with aldehyde (7.22) yields the sy -aldol adduct (7.23) predominantly while the (Z)-e.no ether (7.24) results in isolation of the anti-adduct (7.25) as the major product. The authors invoke a closed silatropic ene transition state (structure (7.26) for syn-transition state), substantiated by suitable crossover experiments, to explain the diastereoselectivities... 

Mikami reported that BINOL derived titanium complex efficiently catalyzed the aldol reaction of silyl enol ether with excellent control of both absolute and relative stereochemistry [106] (Scheme 14.37). The reaction was proposed to proceed via a prototropic ene reaction pathway that is different from that of Mukaiyama aldol condensation. A cyclic antiperiplanar transition-state model was proposed to explain the pref erential formation of the syn diastereomer from either (E)- or (Z)-silyl enol ethers [106]. Further modifications of the catalyst system include the use of perfluorophenols and other activating additives [107], or performing the reaction in supercritical fluids [108]. Furthermore, the nucleophile could be extended to enoxysilacyclobutane derivatives [109]. 

In the envisaged titanium oxo complex, the Ti atom is side-bound to the peroxy moiety (02H), consistent with all the spectroscopic results mentioned in Section III in Scheme 27, between the two O atoms that are side-bound to Ti4+, the O atom attached to both the Ti and H atoms is expected to be more electrophilic than the O atom attached to only the Ti atom and is likely to be the site of nucleophilic attack by the alkene double bond. The formation of the Ti-OH group (and not the titanyl, Ti=0, as proposed by Khouw et al. (221)) after the epoxidation and its subsequent condensation with Si-OH to regenerate the Ti-O-Si links had been observed (Section III.B) by FTIR spectroscopy by Lin and Frei (133). Because this is a concerted heterolytic cleavage of the 0-0 bond, high epoxide selectivity and retention of stereochemistry may be expected, as indeed has been observed experimentally (204). 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

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. 

The reactivity of such compounds has been studied in detail 201-212). Substitution of the 7r-cyclopentadienyl group has been investigated for monocyclopentadienyl halogen and alkoxytitanium derivatives, dicyclo-pentadienyltitanium halides and alkoxides 201-211), and for (77-a llyl) (77-cyclopentadienyl)palladium 212a). The stereochemistry of these complexes has been confirmed unequivocally by physical methods including X-ray studies. The structures of (77-allyl)(7T-cyclopentadienyl)palladium 213) and of (77-cyclopentadienyl)titanium trichloride 214) are shown. 

---