Optimization of the BINOL-based polyether catalyzed asymmetric Friedel-Crafts (FC) reactions of indole [a]
Entry Catalyst Indole 2a (equiv.) Solvent Yield (%)[b] ee (%)[c]
1 (R)-1a 1.0 Toluene 68 61
2 (R)-1b 1.0 Toluene 70 98
3 (R)-1c 1.0 Toluene 59 80 4 (R)-1d 1.0 Toluene 24 1 5 (R)-1e 1.0 Toluene 59 70 6 (R)-1b 1.0 CH2Cl2 70 95 7 (R)-1b 1.0 1,4-dioxane 52 91 8 (R)-1b 1.0 THF 61 95 9 (R)-1b 1.0 mesitylene 75 98 10 (R)-1b 1.0 m-xylene 76 99 11 (R)-1b 1.2 m-xylene 83 99 [a] Reaction Conditions: 3a (0.2 mmol), 2a (0.2 mmol), KF (0.4 mmol), catalyst (0.02 mmol) in solvent (0.2 mL) at room temperature for 12 h, unless otherwise specified. [b] Isolated yield. [c] Enantiomeric excess (% ee) was determined by chiral HPLC. A wide range of substituted indoles and imines were then tested under the optimized reaction conditions (1.2 equiv of indole, 2 equiv of KF, and 10 mol% of catalyst in toluene at room temperature for 12 h) to test the generality of the reaction, and the results are summarized in Schemes 2 and 3. This catalytic condition was found to be general with α-amido sulfones 3 bearing different substituents (Scheme 2). Regardless of the electronic and steric nature of the substituents on the aromatic ring, all α-amido sulfones 3 as the imine precursors tested in this study smoothly converted to the corresponding Friedel-Crafts products 4a–4p with high yields and excellent ees after 12 h. Notably, the sterically demanding 1-naphthyl substituted amido sulfone 3b also smoothly converted to the corresponding Friedel-Crafts product 3b in 98% yield with 99% ee. Heteroaromatic substrates such as 2-furanyl (3l) and 3-thienyl (3m) also afforded almost quantitative yields and excellent ees. In addition, various aliphatic substrates 3n–3p were also explored for this reaction. To our delight, the reaction turned out to be highly tolerant to the ene-carbamate[10] formation, and essentially all the investigated alkyl chain substituted substrates including highly challenging primary alkyl substituted substrates afforded good to excellent chemical yields and enantioselectivities (see 4n–4p in Scheme 2). Furthermore, as shown in Scheme 3, this reaction was also found to be very general with respect to indoles 2. The