Titanium triflate

A titanium triflate, TiCl(OTf)3, proved to be an excellent catalyst for equimolar esterification of a carboxylic acid and an alcohol in the presence of (Me2SiO)4 as a dehydrating agent (Eq. 215) [510]. The catalyst is required only in very small amounts (0.1 mol %) and a variety of combinations of carboxylic acids and alcohols enter this esterification. 

Triflates of titanium and tin are effective catalysts for various condensations of carbonyl compounds [I2I, 122, 123, 124, 125] Claisen and Dieckmann type condensations between ester functions proceed under mild conditions in the presence of dichlorobis(trifluoromethanesulfonyloxy)titaiiiuin(rV) and a tertiary amine (equations 59 and 60) These highly regio- and stereoselective condensations were used successfully m the synthesis of carbohydrates [122]... 

The cyclopentadienyl triflate complexes of zirconium and titanium 51 and 52 (Figure 3.7) are also active catalysts [51]. Their activity has been tested in a wide variety of dienes and dienophiles. It is noteworthy that even at low catalyst loadings, rate accelerations between 10 and > 10 times have been observed. No special precautions were taken to dry the solvents or the substrates, in contrast with the traditional Lewis acids which require either predried solvents or high catalyst loadings. 

These oxazolidinones can be acylated and converted to the lithium, boron, tin, or titanium enolates by the same methods applicable to ketones and esters. For example, when they are converted to boron enolates using di-n-butylboron triflate and triethyl-amine, the enolates are the Z-stereoisomers.125... 

Titanium derivatives were shown to serve as cross-coupling partners for aryl triflates chlorides or bromides, with better tolerance to functional groups than Grignard reagents although specific expensive N,P-chelating ligands are required (Equation (7)) 171... 

A short five-step synthesis of a bifuran, namely ( )-2,2 -bis(diphenylphosphino)-3,3 -binaphtho[2,l-I>]furan (BINAPFu) from naphthofuranone via a low-valent titanium-mediated dimerization was reported. The newly developed resolution procedure for phosphines was utilized to provide the optically active bidentate phosphine ligands (BINAPFu), which consistently outperforms BINAP in the asymmetric Heck reaction between 2,3-dihydrofuran and phenyl triflate . Another way in which a benzofuranone can be converted into benzo[7 ]furan is by treatment of the former with 1-BU2AIH at -78°C followed by an acidic work up <00TL5803>. 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

Boron trifluoride diethyl etherate Sodium bicarbonate Palladium on charcoal Sodium periodate Hydroperoxide, 1,1-dimethylethyl Titanium tetraisopropoxide Ethyl diisopropylamine Triethylamine Ammonium sulfate Trimethylsilyl triflate d-3-oxo-a-D-gluco-pyranoside -oxo-a-D-erythro-hexopyranoside... 

Ti-F bond [182.5(5) pm], which as expected is shorter than the Ti-Cl bond of 17 [228.4(3) pm] [12]. We were of course interested in the catalytic properties of 24 in comparison with triflate 23. In the presence of 23, allylsilane 5 adds smoothly to benzaldehyde the product is, however, racemic. Most probably the catalyst is tri-methylsilyl triflate, which is formed according to pathway B (Scheme 2, cf. [4a]). On the contrary, no reaction between benzaldehyde and allyl silane 5 is observed in the presence of fluoride 24. As we observed earlier, all cyclopentadienyltitanium TADDOLates studied catalyzed the conversion of benzaldehyde 25 with salt-free (i-PrO),TiCH3 26 to 2-phenylethanol 27 [11]. In this case the titanium fluoride effect was indeed very pronounced, as at -78 °C 0,5 % of 24 afforded 60 % of product 27 in 17 h with 78 % ee with 2 % of 24 the induction is 93 % ee. 

S708>, and 2-phenyl-l,3-dioxolanes are cleaved cleanly to diols by the use of catalytic erbium triflate <20050BC4129>. A titanium silicate molecular sieve is effective in catalyzing methanolysis of l,3-dioxolan-2-one to give ethanediol and dimethyl carbonate <1996CC2281>. 

A number of methods that utilize enolsilanes directly in the aldol process with either aldehydes or acetals have been developed recently. These reactions are usually performed either in the presence of Lewis acids such as titanium tetrachloride (67) or with fluoride ion (68). Recently trimethylsilyl triflate (CF3S03SiMe3) was found to be an efficient acid catalyst for condensation... 

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