Aluminum triflate

P,P] The Michael addition of silyl ketene acetals to enoates requires more strongly Lewis acidic conditions. Both aluminum triflate (98) and clay montmorillonite (78) have been used to catalyze stereoselective reactions (Table 18, Scheme 49). In general, the selectivities obtained for ketene acetals... 

Metal triflates can be easily prepared from metal halides and triflic acid at -78 C. They show several unique properties compared with the corresponding metal halides. In an early study, Olah reported the use of boron-, aluminum-, and gallium triflates [M(OTf)J as effective Friedel-Crafts catalysts. In the benzoylation and acetylation of toluene and benzene with acyl chlorides, the relative reactivity is boron triflate > gallium triflate > aluminum triflate, in agreement with the relative acidity strength. 

Funabashi, M. and Kunioka, M. (2005) Biodegradable composites of poly(lactic acid) with cellulose fibers polymerized by aluminum triflate. Macromolecular Symposia, 224,309-321. 

Wan C, Li Y, Shahbazi A, Xiu S (2008) Succinic acid production from cheese whey using ActinobacUlus succinogenes 130 Z. Appl Biochem Biotechnol 145 111-119 Wang Y, Kunioka M (2005) Ring-opening polymerization of cyclic monomers with aluminum triflate. Macromol Symp 224 193-205... 

Another example of polymer-supported A1 based Lewis acid is cross-linked polystyrene-supported aluminum triflate (79). Cross-linked polystyrene-supported AICI3 (72) was easily converted into (79) by treatment with triflic acid. This catalyst was applied to dithioacetalization of carbonyl compounds and transdithioacetal-ization of acetals (Scheme 19.19) [44]. From benzaldehyde the corresponding dithioacetal (84) was obtained in the presence of the polymeric catalyst (79) in 98% yield in 30 minutes. The same product was also obtained from the dimethyl acetal (85) in 94% yield. Chemoselectivity of the polymeric catalyst was also demonstrated in Scheme 19.19. Aldehydes reacted faster in the presence of ketone (34) to give the dithioacetals with (79). Aliphatic ketone (89) exclusively reacted with dithiol (81) in the presence of aromatic ketone (34). These chemoselectivities were higher than those obtained from the reactions using nonsupported Al(OTf)3. 

Mesoporous MCM-type materials incorporating aluminum triflate were also considered for the synthesis of solketal, which is an acid-catalyzed reaction of acetone with glycerol, also known as ketalization reaction (Equation (8.50)) [119]. The selectivity to the desired product (solketal) was total, not being affected on varying the various reaction variables. On these catalysts the reaction does not produce other products such as hemiketal or six-membered cyclic ketal. 

An interesting approach to synthesizing benzopyrans starting form 3,4,6,-tri-O-acetyl-D-galactal was reported by Williams and coworkers [13]. The reaction proceeds via aluminum triflate-catalyzed C-coupling of various functionalized phenols with unsaturated D-galactal system. The synthetic route to this new class of C-functionalized benzopyrans is illustrated in Scheme 6.9. 

The polymerization of PDL by using aluminum triflate (trifluoromethane sulfonate) has been reported recently [40]. Using glycerol as an initiator, the bulk polymerization was carried out at 100 °C for 6 h to produce a polymer (yield 49%) of M 12400gmol and PDl 2.24. 

Triflates of aluminum, gallium and boron, which are readily available by the reaction of the corresponding chlorides with triflic acid, are effective Fnedel-Crafis catalysis for alkylation and acylation of aromatic compounds [119, 120] Thus alkylation of toluene with various alkyl halides m the presence of these catalysts proceeds rapidly at room temperature 111 methylene chloride or ni-tromethane Favorable properties of the triflates in comparison with the correspond mg fluorides or chlorides are considerably decreased volatility and higher catalytic activity [120]... 

The branched oligo(arylene)s 37 and 40 can undeigo a further oxidative cyclization with copper(ll) chloride or triflate/aluminum trichloride leading to the formation of large, hitherto unknown polycyclic aromatic hydrocarbons PAHs 41 and 42. 

Related to the nitrile oxide cycloadditions presented in Scheme 6.206 are 1,3-dipolar cycloaddition reactions of nitrones with alkenes leading to isoxazolidines. The group of Comes-Franchini has described cycloadditions of (Z)-a-phenyl-N-methylnitrone with allylic fluorides leading to enantiopure fluorine-containing isoxazolidines, and ultimately to amino polyols (Scheme 6.207) [374]. The reactions were carried out under solvent-free conditions in the presence of 5 mol% of either scandium(III) or indium(III) triflate. In the racemic series, an optimized 74% yield of an exo/endo mixture of cycloadducts was obtained within 15 min at 100 °C. In the case of the enantiopure allyl fluoride, a similar product distribution was achieved after 25 min at 100 °C. Reduction of the isoxazolidine cycloadducts with lithium aluminum hydride provided fluorinated enantiopure polyols of pharmaceutical interest possessing four stereocenters. 

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

Boron tris(trifluoromethanesulfonate). This triflate is obtained by reaction of BC1, with triflic acid in S02C1F at -78°. Distillation at reduced pressure provides a solid, m. p. 45°, b. p. 68-73°/0.5 mm. It is extremely hygroscopic, and is soluble m CRiC, CHjNOi, CH CN. Aluminum and gallium triflate are poorly soluble in the common solvents. All three triflates can function as Friedel-Crafts catalysts, but the boron triflate is the most effective as a soluble catalyst. ... 

Varela et al. [40] obtained similar results from 2,3,4,5-tetra-O-methyl-D-galac-tono-1,6-lactone (20). The homopolymerization of this lactone, promoted by aluminum isopropoxide [Al(0 Pr)3] or scandium triflate [Sc(OTf)3], was attempted. 

Triflates of boron, aluminum, and gallium were found to be efficient catalysts in Friedel-Crafts acylations.46 However, these are water-sensitive materials and were required to be used in equimolar quantities to be effective. More recently various water-tolerant and recyclable triflate salts, which were also tested in alkylation, were found to exhibit similar good characteristics in Friedel-Crafts acylations. Although benzene cannot be acylated, Sc triflate,47 48 lanthanum triflates,48-51 and Hf triflate52 usually give high yields of aryl ketones in acylation with acid anhydrides. In many cases, Li perchlorate was found to accelerate the reactions.48 52... 

Oxasilacyclopentenes were shown to be competent substrates for a scandium triflate-catalyzed Mukaiyama aldol process (Scheme 7.35).104 Exposure of silacy-clopentene 121 and benzaldehyde to substoichiometric amounts of scandium triflate produced ketone 122 diastereoselectively.105 This ketone was proposed to form by addition of enolate 123, resulting from desilylation of 121,106 to benzaldehyde. A 1,3-Brook rearrangement then afforded 122 from 124.107 This ketone could be further functionalized through lithium aluminum hydride reduction followed by deprotection to afford triol 125 containing four contiguous stereocenters. Thus, the molecular complexity of silyloxyalkynes can be increased dramatically in just three operations. 

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