Lanthanum triflate

In the next step of the sequence, the authors sought to introduce a hydroxy-methylene substituent at the unsubstituted 7-position of the enone. This bond construction can be carried out by conducting a Baylis-Hillman reaction with formaldehyde. In this instance, the authors used a modification of the Baylis-Hillman reaction which involves the use of a Lewis acid to activate the enone [26]. Under these conditions, the enone 42 is treated with excess paraformaldehyde in the presence of triethylphosphine (1 equiv), lanthanum triflate (5 mol%), and triethanolamine (50 mol%). It is proposed that the lanthanum triflate forms a complex with the triethanolamine. This complex is able to activate the enone toward 1,4-addition of the nucleophilic catalysts (here, triethylphosphine). In the absence of triethanolamine, the Lewis acid catalyst undergoes nonproductive complexation with the nucleophilic catalyst, leading to diminution of catalysis. Under these conditions, the hydroxymethylene derivative 37 was formed in 70 % yield. In the next step of the sequence, the authors sought to conduct a stereoselective epoxidation of the allylic... 

Benzyl and substituted benzyl protecting groups are ubiquitous in organic synthesis. For base-sensitive substrates, the benzyl imidate, e.g. 6, is often used to install this group. Amit Basu of Brown University reports (Tetrahedron Lett. 44 2267,2003) that in situations such that the usual acidic promoters do not work, metal triflates can be effective. Lanthanum triflate worked particularly well, giving both high yield and high conversion in five minutes at room temperature. 

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... 

Other RE(OTf)3 were also examined as catalysts in the reaction of 1 with acetic anhydride (Table 2). Catalytic amounts of all the RE(OTf)3 listed effectively mediated the acylation of 1. Among these, scandium trifluoromethanesulfonate (scandium triflate, Sc(OTf)3)[6] or Yb(OTf)3 was superior to other RE(OTf)3 and afforded the acylation product 2 quantitatively. When, on the other hand, lanthanum trifluoromethanesulfonate (lanthanum triflate, La(OTf)3) was used, the yield of 2 was relatively low. The yields shown in Table 2 may reflect the catalytic activity of respective RE(OTf)3. 

In most Diels-Alder reactions, no catalyst is needed, but Lewis acids are effective catalysts in many cases, particularly those in which Z in the dienophile is a C=0 or C=N group. A Lewis acid catalyst usually increases both the regioselec-tivity of the reaction (in the sense given above) and the extent of endo addition, °° and, in the case of enantioselective reactions, the extent of enantioselectivity. It has been shown that InCla is an effective catalyst for aqueous Diels-Alder reactions,which is suitable for ionic Diels-Alder reactions, and there are other Lewis acid catalysts that are effective in water. °° Brpnsted acids have also been used to accelerate the rate of the Diels-Alder reaction.Lanthanum triflate [La(OTf)3] has been reported as a reusable catalyst ° ° and Me3SiNTf2 has been used as a green Lewis acid catalyst. ° Cationic Diels-Alder catalysts have been developed, particularly oxazaborolidine catalysts. ° Some Diels-Alder reactions can also be catalyzed by the addition of a stable cation radical, for... 

The use of Ln(OTf)3 in the activation of aldehydes other than formaldehyde was also investigated [18]. The model reaction of 1-trimethylsiloxycyclohexene (2) with benzaldehyde under the influence of a catalytic amount of Yb(OTf)3 (10 mol%) was examined. The reaction proceeded smoothly in H20-THF (1 4), but the yields were low when water or THF was used alone. Among several Ln(OTf)3 screened, neodymium triflate (Nd(OTf)3), gadolinium triflate (Gd(OTf)3), Yb(OTf)3, and lutetium triflate (Lu(OTf)3) were quite effective, while the yield of the desired aldol adduct was lower in the presence of lanthanum triflate (La(OTf)3), praseodymium triflate (Pr(OTf)3) or thulium triflate (Tm(OTf)3) (Table 2). 

Parvulescu, A. N., Gagea, B. C., Parvulescu, V, Parvulescu, V. L, Poncelet, G., and Grange, P. 2002. Comparative behavior of silica-embedded ferf-butyldim-ethylsilyltrifluoro-methanesulfonate and lanthanum triflate catalysts. Catal. Today 73 177-185. 

Aziridines. The highly cis-selective cycloaddition between iinines and diazoacetates in protic media uses lanthanum triflate as catalyst. 

Lanthanum complex 139 was obtained in 92% yield by treatment of lanthanum triflate with ligand 82.1n addition to their spectroscopic data, both complexes were characterized by X-ray crystallography. Besides 137, gallium and indium complexes 140— 143 are the only known chelated examples with main group metals (Chart 4). [2-(Di-terf-butylphosphanyl)-... 

Recently, the You group reported the ehiral NHC/Lewis acid catalysed [4 + 2] annulation of 2-bromoenals bearing a y-hydrogen with isatin derivatives. In the presence of lanthanum triflate, the reaction went smoothly (Scheme 20.52). 

Sorensen s synthesis of abyssomicin C relied on an intramolecular Diels-Alder reaction catalyzed by lanthanum triflate. Heating 131 in the presence of the Lewis acid catalyst promotes both elimination of the silyl... 

On the other hand, the couphng of methyl (Z)-2-(bromomethyl)-2-butenoate with the aldehyde under the same conditions generated the desired homoallylic alcohol in 80% yield with a high 93 7 syn/anti selectivity (Eq. 4.21). The indium-mediated aUylation of triflu-oroacetaldehyde hydrate (R = H) or trifluoroacetaldehyde ethyl hemiacetal (R = Et) with an aUyl bromide in water yielded a-trifluoromethylated alcohols. Lanthanum triflate-promoted indium-mediated allylations of aminoaldehydes in aqueous media generated j8-aminoalcohols stereoselectively. Indium-mediated intramolecular carbocychzation in aqueous media generated fused Q -methylene-y-butyrolactones. An advanced intermediate for azaspiracids was only accessible by the indium-mediated aUylation. Other potentially reactive functionalities, such as azide, enone, and ketone, did not interfere with the reaction. 

Lanthanum Triflate-Catalyzed Rearrangement-Cycloaddition of 2,6-Diazasemibullvalenes and Azides Synthesis of Triazabrexadienes... 

Scheme 6.4 Lanthanum triflate-catalyzed ring opening-rearrangement-cycloaddition of 2,6-diazasemibullvalenes and azides...

Scheme 6.5 Proposed mechanism for lanthanum triflate-catalyzed cycloaddition of 2,6-diazasemibuUvalenes and azides...

Lanthanum is the first element of the sixth-period transition metals. Its properties are close to those of the other rare earth elements and, as a consequence, the catalytic behavior of the lanthanum triflate is also very close to that of the entire series. An important example concerning the stability of these compounds in water is that indicating the capability of metal triflates, such as Yb(OTf)3, Eu(OTf)3, Sc(OTf)3 or La(OTf)3, to catalyze the hydration of alkynes to the corresponding ketones [56]. However, the interest in water-compatible lanthanide triflate-based catalysts is much larger and mainly includes the carbon-carbon bond-forming reactions. To increase the utility of these catalysts understanding of their aqueous mechanism is very important. For such a purpose, dynamic measurements of the water... 

The coexistence of the Lewis and Br0nsted-type OH groups of the support was also demonstrated from NHj-FITR spectra of the sol-gel rcrf-butyldimethylsilyltrifluoromethane-sulfonate and lanthanum triflate encapsulated catalysts (Figure 8.1). 

Figure 8.1 NH3-FTIR spectra of the tert-butyidimethylsilyltrifluoromethanesulfonate (a) and lanthanum triflate (b) encapsulated catalysts after evacuation at 100°C.

Scheme 8.8 Synthesis of SBA-15 Tethered Lanthanum Triflate Lewis Acid. With Permission...

Lanthanum triflate immobilized onto the mesoporous SBA support [105] affords the allylation reactions of carbonyl compounds with tetraallytin, which are among the most fundamental and important C-C bond-forming reaction (Equation (8.44)) [135]. 

BERNSON A, LINDGREN J, HUANG w and FRECH R, Coordination and conformation in PEO, PEGM and PEG systems containing Uthium or lanthanum triflate . Polymer, 1995,36(23), 4471-4478... 

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