Lanthanum(III) Triflate1


[52093-26-2]  · C3F9LaO9S3  · Lanthanum(III) Triflate  · (MW 586.10)

(Lewis acid catalyst useful for aldol, Michael, Diels-Alder, and other C-C bond forming reactions; also useful for preparing organolanthanum reagents, which react with hindered tertiary amides to afford ketones)

Alternate Name: lanthanum trifluoromethanesulfonate.

Physical Data: mp >360 °C.

Solubility: sol H2O; partially sol THF, CH2Cl2; sparingly sol Et2O; insol hexane.

Preparative Methods: to an aqueous solution of triflic acid (50% v/v), a slight excess of lanthanum oxide (La2O3) is added; the resulting mixture is stirred for 15 min at rt and then filtered to remove the unreacted oxide; the filtrate is concentrated in vacuo at about 60 °C; residual water is removed by azeotropic distillation with toluene in vacuo to dryness; the resulting hydrate is placed in a flask equipped with a stirring bar, a stopcock filled with a glass wool plug, and a NaOH trap, and is then dehydrated by heating under vacuum (140 °C/0.1 mmHg or 180 °C/1.0 mmHg) for 16 h. The anhydrous La(OTf)3 is a white to gray-white solid.2-4

Handling, Storage, and Precautions: anhydrous La(OTf)3 is hygroscopic and should be stored and weighed in a glove-box. Use in a fume hood.

Preparation of Organolanthanum Reagents.1,4

Organolanthanum triflates RLa(OTf)2 (R = alkyl, aryl), readily available by reaction of organolithium compounds with equimolar anhydrous La(OTf)3 in ethereal solvents, are very effective nucleophiles in reactions with carbonyl derivatives but exhibit relatively low basicity. Generally these reagents behave in a parallel manner to their organocerium chloride counterparts,5 but are soluble in ethereal solvents and thus are more reactive. In particular, the addition reaction of RLa(OTf)2 to the highly enolizable compound 1,3-diphenylacetone is complete within a few minutes at -78 °C, affording the carbinol product in essentially quantitative yield (eq 1). The analogous reaction utilizing RCeCl2 requires a higher reaction temperature (0 °C) and a longer reaction time (1.5 h).5 One application of MeLa(OTf)2 in organic synthesis is depicted in eq 2 in which efficient 1,2-addition is achieved.6 In this case, the use of MeLi or MeMgBr reagents gives a mixture of the starting material and mono- and di-1,2- and 1,4-addition products.

A unique characteristic of organolanthanum triflate reagents is that they smoothly react with sterically hindered aliphatic, aromatic, and heterocyclic tertiary amides to provide the corresponding ketones in excellent yields (eq 3); unsatisfactory results are obtained when employing organocerium or conventional RLi or RMgX reagents.4 If amides possess additional donor functionality (i.e. MeO), an excess of the RLa(OTf)2 reagent is needed to ensure complete conversion. Even so, further addition to the ketone product is not usually observed due to the formation of stable tetrahedral adducts prior to aqueous workup.

Lewis Acid Catalyst for Aldol, Michael, and Diels-Alder Reactions.

La(OTf)3 has been used as a Lewis acid catalyst to promote aldol condensations between silyl enol ethers and formaldehyde under aqueous conditions (eq 4).7 La(OTf)3 can catalyze the Michael addition of silyl enol ethers to a,b-unsaturated ketones at rt in CH2Cl2 to afford the expected products in high yields (eq 5). In some of the cases studied, as little as 1 mol % of La(OTf)3 is sufficient, and other lanthanide triflates are also effective. Enol silanes derived from ketones, thioesters, and esters are suitable, and no 1,2-addition products are obtained.8 La(OTf)3 is also an effective catalyst for the Diels-Alder cycloaddition of carbonyl-containing dienophiles with cyclopentadiene.8 For all three catalytic processes mentioned here, La(OTf)3 can be almost quantitatively recovered upon aqueous workup and can be reused.7,8

Synthesis of Amidines.

In the presence of 1 mol % of La(OTf)3, reaction of primary amines (2 equiv) with nitriles results in the formation of N,NŽ-disubstituted amidines in good yields (eq 6);3 when primary diamines (1 equiv) are employed, cyclic amidines are obtained in good to excellent yields (eq 7).3

1. Molander, G. A. CRV 1992, 92, 29.
2. Smith, P. H.; Raymond, K. N. IC 1985, 24, 3469.
3. Forsberg, J. H.; Spaziano, V. T.; Balasubramanian, T. M.; Liu, G. K.; Kinsley, S. A.; Duckworth, C. A.; Poteruca, J. J.; Brown, P. S.; Miller, J. L. JOC 1987, 52, 1017.
4. (a) Collins, S.; Hong, Y. TL 1987, 28, 4391. (b) Collins, S.; Hong, Y.; Hoover, G. J.; Veit, J. R. JOC 1990, 55, 3565.
5. Imamoto, T.; Takiyama, N.; Nakamura, K.; Hatajima, T.; Kamiya, Y. JACS 1989, 111, 4392.
6. Collins, S.; Hong, Y.; Ramachandran, R.; Taylor, N. J. OM 1991, 10, 2349.
7. Kobayashi, S. CL 1991, 2187.
8. Kobayashi, S.; Hachiya, I.; Takahori, T.; Araki, M.; Ishitani, H. TL 1992, 33, 6815.

Yaping Hong

Sepracor, Marlborough, MA, USA

Scott Collins

University of Waterloo, Ontario, Canada

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