Triflate, Lanthanoid

Ytterbium triflate [Yb(OTf)3] combined with TMSG1 or TMSOTf are excellent reagents for the conversion of a-methyl styrene and tosyl-imines into homoallylic amides 32 (Equation (19)) (TMS = trimethylsilyl).29 These conditions produce the first examples of intermolecular imino-ene reactions with less reactive imines. Typically, glyoxalate imines are necessary. A comprehensive examination of the lanthanoid metal triflates was done and the activity was shown to directly correlate with the oxophilicity scale. The first report used preformed imines, and subsequently it was found that a three-component coupling reaction could be effected, bypassing the isolation of the intermediate imine.30 Particularly noteworthy was the successful participation of aliphatic aldehydes to yield homoallylic amines. 

The asymmetric conjugate addition of thiophenol to ( >3-crotonoyloxazolidin-2-one, catalysed by the scandium(III) triflate complex of Ph-PYBOX, gave the corre- sponding adduct in 66% ee. Lanthanoid triflates gave lower enantioselectivities (<28% ee).132... 

Asymmetric cyanosilylation of ketones and aldehydes is important because the cyanohydrin product can be easily converted into optically active aminoalcohols by reduction. Moberg, Haswell and coworkers reported on a microflow version of the catalytic cyanosilylation of aldehydes using Pybox [5]/lanthanoid triflates as the catalyst for chiral induction. A T-shaped borosilicate microreactor with channel dimensions of 100 pm X 50 pm was used in this study [6]. Electroosmotic flow (EOF) was employed to pump an acetonitrile solution of phenyl-Pybox, LnCl3 and benzal-dehyde (reservoir A) and an acetonitrile solution of TMSCN (reservoir B). LuC13-catalyzed microflow reactions gave similar enantioselectivity to that observed in analogous batch reactions. However, lower enantioselectivity was observed for the YbCl3-catalyzed microflow reactions than that observed for the batch reaction (Scheme 4.5). It is possible that the oxophilic Yb binds to the silicon oxide surface of the channels. 

Lanthanoids As mentioned in several cases above, some lanthanoid triflates displayed more or less catalytic activity for alkenol cyclizations. In one optimized case, several alkenol cyclizations were performed with Ln(OTf)3, Sm(OTf)3, and Yb(OTf)3 as catalysts (1 mol%) at 120°C in an ionic liquid as solvent (W-ethyl-W-methyl-imidazolium triflate) [73]. Addition of 2,6-di-ferf-butylpyridine blocked the activity of Yb(OTf)3 nevertheless, the authors argue that triflic acid, even though it catalyzed the reaction in a blind test, could not have been the hidden active catalyst, because triflic acid was not found by NMR spectroscopy in a vacuum transfer of volatiles from the reaction solution. 

Most of the known monocyclopentadienyl compounds of trivalent lanthanoids are dichloride complexes of CpLnCl2(THF) type. Besides these compounds, the ytterbium p-diketonates, lanthanum and cerium iodides (Me5C5)Lnl2 and the lutetium triflate complex CpLu(0S02CF3)2(THF)3 have been obtained (Table 111.14)... 

This method allows to obtain the compounds with substituted and unsubstituted cyclopentadienyl ligands practically for all lanthanoids, including hardly accessible compounds of early REM [270], and the triflate complex CpLn(0S02CF3)2(THF)3 as well [241]. 

Ligation in aqueous solution and the nature of complexes isolated from aqueous solution have relevance to the behaviour of lanthanoid corrosion inhibitors. In the presence of weakly coordinating anions such as triflate, bromate and perchlorate, Ln ions in aqueous solution are considered to be either 9-coordinate (La Eu ), tricapped trigonal prismatic [Ln(H O) ] , or 8-cooitiinate (Dy Lu ), square... 

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