Lanthanide chemoselectivity

Thorimbert and coworkers have reported the use of complexes (TBA)sH2[al-RE(H20)4 P2W17O61] (RE = La, Sm, Eu, Yb TBA=tetrabutylammonium) as Lewis acid catalysts. These complexes are soluble in organic solvents, and the water molecules on the lanthanide ions are labile, thus providing the metal centers with available coordination sites for organic substrates. These catalysts show high chemoselectivity for the competition reactions between... 

Figure 5.31 Comparison of the chemoselectivity for RE-POM catalysts and RE triflates [90]. (Reproduced with permission from C. Boglio, et al., Lanthanide complexes of the monovacant Dawson polyoxotungstate [al-PiWnOei] as selective and recoverable Lewis acid catalysts, Angewandte Chemie International Edition, 2006, 45, no. 20, 3324-3327. Wiley-VCH Verlag GmbH Co. KGaA.)...

Atkinson, R, Bretonniere, Y, and Parker, D. (2004) Chemoselective signalling of selected phospho-anions using lanthanide luminescence. Chemical Communications, 438 39. 

LnCpa and Cp2LnCl complexes initiate the dehalogenation of aiyl and vinyl halides by NaH. Chemoselectivity is observed in the reduction of m-bromochloro-benzene and p-iodochlorobenzene to chlorobenzene [192], The asymmetric reduction of methylphenyl glyoxylate to methyl mandelate by NADH models is catalyzed by chiral lanthanide )6-diketonates [193],... 

While selective reaction of aldehydes takes place with the typical Lewis acids TiCL, SnCl4, TMSOTf, etc., lanthanide triflates [Ln(OTf)3] are unique Lewis acids that change the reaction course dramatically aldimine reacts selectively in the coexistence of aldehydes [70]. Among a series of Ln(OTf)3 tested, Yb(OTf)3 exhibited the most prominent chemoselectivity in addition to high chemical yields. The silyl enol ethers of ketones, allyltributylstannane and Me3SiCN are all applicable as chemoselective nucleophiles (Table 2-9). Preferential formation of Yb(OTf)3-aldimine complexes was postulated by C NMR spectral analysis in the presence of PhCHO and Y-benzylideneaniline. 

Transition metal catalysis of the Michael reaction of 1,3-dicarbonyl compounds with acceptor activated alkenes has been known since the early 1980 s 2>3 It is a valuable alternative to the classic base catalysis of the reaction. Because of the mild and neutral conditions, the chemoselectivity of these reactions is superior to that provided by base catalysis, since the latter suffers from various unwanted side or subsequent reactions, such as aldol cyclizations, ester solvolyses or retro-Claisen type decompositions. A number of transition metal and lanthanide compounds have been reported to catalyze the Michael reaction, but FeCb 6 H20 is one of the most efficient systems to date. A number of 3-diketones or p-oxo esters and MVK are cleanly converted to the corresponding Michael reaction products within a few hours at room... 

Lanthanide triflates and Sc(OTf)3 effectively catalyze conjugate addition of SEE, KSA, and ketene silyl thioacetals under mild conditions (0°C to room temperature, 1-10 mol% catalyst) (Scheme 10.86) [69, 238]. After an aqueous work-up these Lewis acids can be recovered almost quantitatively from the aqueous layer and can be re-used without reduction of fheir catalytic activity. Eu(fod)3 also is effective in not only aldol reactions but also Michael addition of KSA [239]. The Eu(fod)3-catalyzed addition of KSA is highly chemoselective for enones in the presence of ketones. 

Trialkyl borohydrides such as Lithium Tri-s-butylborohydride and Potassium Tri-s-butylborohydride are superior reagents for the chemoselective 1,4-reduction of enones. On the other hand, 1,2-reduction can be obtained by using NaBHj in the mixed solvent MeOH-THF (1 9), or with NaBHj in combination with CeCH or other lanthanide salts. ... 

Kobayashi et al. disclosed reversed chemoselectivity of lanthanide Lewis acid over conventional Lewis acid in cyanation reaction. The comparitive reaction was performed with equimolar mixture of aldehydes, imines, and TMSCN, affording Strecker-type product exclusively with 20mol% of Yb(OTf)3 [10]. In sharp contrast, SnCU required more than a stoichiometric amount of reagent and gave cyanohydrin preferentially (Table 13.15). 

Interesting chemoselectivity was observed in this addition reaction. HBF4-catalyzed addition reaction selectively proceeded toward an aldimine in the presence of an aldehyde (Scheme 3.2). In general, common Lewis acids except for some lanthanide triflates or transition metals activate aldehydes rather than aldimines preferentially. The high chemoselectivity was realized because the more basic nitrogen was activated more effectively by HBF4 than the carbonyl oxygen. 

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