(Polyallyl)scandium triflylamide ditriflate (PA-Sc-TAD)
(a polymer-supported scandium catalyst)
Solubility: PA-Sc-TAD is gummy, but is dispersed and partially soluble in a CH2Cl2-CH3CN mixed solvent. The dispersed catalyst assembles again when hexane is added.
Preparative Methods: the synthetic route of PA-Sc-TAD is shown below. Polyacrylonitrile was treated with BH3·SMe2 in diglyme for 36 h at 150 °C. The resulting amine 1 was reacted with triflic anhydride in the presence of triethylamine in 1,2-dichloroethane for 10 h at 60 °C to afford sulfonamide 2 (1). After 2 and KH were combined, scandium trifluoromethanesulfonate was added and the mixture was stirred in THF for 48 h at room temperature. PA-Sc-TAD thus obtained was characterized by elemental analysis.
PA-Sc-TAD can be handled under air, but should preferentially be stored under inert atmosphere.
One of the drawbacks of polymer-supported catalysts is their low reactivity, which may be ascribed to the insolubility of the catalyst. PA-Sc-TAD was designed to be partially soluble in an appropriate solvent, to be precipitated after completion of the reaction, and to be recovered quantitatively by filtration.
Quinoline Synthesis via Three-Component Reactions
PA-Sc-TAD catalyzes three-component reactions of aldehydes, aromatic amines, and alkenes to give quinoline derivatives in high yields (2). The procedure is very simple: just mix the catalyst, an aldehyde, an aromatic amine, and an alkene (alkyne). After the reaction is complete, the catalyst can be recovered by addition of hexane and filtration, and the filtrates are concentrated to give almost pure quinoline derivatives in most cases. Many combinations of aldehydes, amines, and alkenes are used in this reaction, and a large quinoline library could be prepared on the basis of these combinations. A characteristic feature of this method compared to conventional combinatorial synthetic technology using polymer-supported reagents is that >100 mg scale syntheses with a large array of diverse molecular entries have been achieved with high purities (high yields and high selectivities).
Since PA-Sc-TAD is water-tolerant, substrates having water of crystallization can be used directly. Moreover, the continuous use of the recovered catalyst is possible without any loss of activity (3).1
Synthesis of b-Amino Carbonyl Compounds and a-Amino Nitriles
PA-Sc-TAD also catalyzes three-component reactions of aldehydes, amines, and silylated nucleophiles to afford b-amino ketones, b-amino esters, and a-amino nitriles in high yields (4).2 The reaction proceeds a little slower compared to that using lanthanide trifluoromethanesulfonate (Ln(OTf)3) as a catalyst, but the reaction proceeds smoothly at room temperature. Heterocyclic and aliphatic aldehydes and a glyoxal also work well with various amines.
PA-Sc-TAD shows high aldimine-selectivity in the presence of aldehydes.3 In the competition reaction of benzaldehyde and N-benzylideneaniline with the silyl enol ether of propiophenone, PA-Sc-TAD provides higher aldimine-selectivity than Ln(OTf)3 (Ln = Yb or Sc) (1). When Sc(OTf)2(NTf2) etc. was used as catalyst, lower selectivities were obtained. It is suggested that the high selectivity obtained by the PA-Sc-TAD catalyst could not be caused by the effect of counter anions. The aldimine-PA-Sc-TAD catalyst complex might be more stable than aldimine-nonpolymer Lewis acid complexes due to the polymer effect.
- 1. Kobayashi, S.; Nagayama, S., J. Am. Chem. Soc. 1996, 118, 8977.
- 2. Kobayashi, S.; Nagayama, S.; Busujima, T., Tetrahedron Lett. 1996, 37, 9221.
- 3. Kobayashi, S.; Nagayama, S., Synlett 1997, 653.
Masaharu Sugiura & Shu Kobayashi
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
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