Enantioselective construction of six- and seven-membered triorgano-substituted silicon-stereogenic heterocycles

Abstract

The exploitation of chirality at silicon in asymmetric catalysis is one of the most intriguing and challenging tasks in synthetic chemistry. In particular, construction of enantioenriched mediem-sized silicon-stereogenic heterocycles is highly attractive, given the increasing demand for the synthesis of novel functional-materials-oriented silicon-bridged compounds. Here, we report a rhodium-catalyzed enantioselective construction of six- and seven-membered triorgano-substituted silicon-stereogenic heterocycles. This process undergoes a direct dehydrogenative C-H silylation, giving access to a wide range of triorgano-substituted silicon-stereogenic heterocycles in good to excellent yields and enantioselectivities, that significantly enlarge the chemical space of the silicon-centered chiral molecules. Further elaboration of the chiral monohydrosilane product delivers various corresponding tetraorgano-substituted silicon-stereogenic heterocycles without the loss of enantiopurity. These silicon-bridged heterocycles exhibit bright blue fluorescence, which would have potential application prospects in organic optoelectronic materials.

Fig. 1. Construction of silicon-containing π-conjugated molecules.

a Representative silicon-containing π-conjugated molecules. b Asymmetric synthesis of six-membered tetraorgano-substituted Si-stereogenic silanes (previous work). c Enantioselective construction of six- and seven-membered triorgano-substituted Si-stereogenic heterocycles (this work).

Fig. 2. Development of reaction conditions.

Conditions: 1a (0.1 mmol), [Rh(cod)Cl]₂ (1 mol%), ligand (3 mol%), in 1.0 mL solvent under Argon atmosphere. The yield was determined by ¹H NMR using CH₂Br₂ as internal standard, yield in brackets is isolated yield. The ee values were determined by chiral HPLC.

Fig. 3. Scope of six-membered triorgano-substituted silicon-stereogenic heterocycles.

Reaction conditions: 1 (0.1 mmol), [Rh(cod)Cl]₂ (1 mol%), L1 (3 mol%) in toluene (1 mL) at 70 °C for 6 h. a Reaction was performed at 5 mmol scale. b L5 was used as ligand. c [Rh(cod)Cl]₂ (4 mol%), L1 (8 mol%) was used. d [Rh(cod)Cl]₂ (2 mol%), L1 (4 mol%) in toluene (1 mL) at 60 °C for 8 h. e [Rh(cod)Cl]₂ (2 mol%), L1 (6 mol%) was used. f L7 was used as ligand. X-ray crystallographic analysis of 2j allowed to determine the absolute configuration; and configurations of the products 2 were assigned by analogy.

Fig. 4. Scope of seven-membered triorgano-substituted silicon-stereogenic heterocycles.

Reaction conditions: 3 (0.1 mmol), [Rh(cod)Cl]₂ (1 mol%), L3 (3 mol%) in toluene (1 mL) at 100 °C for 12 h.

Fig. 5. Derivatization of the Si-stereogenic heterocycles.

Reaction conditions: a [Rh(cod)Cl]₂ (2 mol%), (racemic)-Josiphos L1 (4 mol%), vinyl ether (2.0 equiv), toluene, 60 °C, 12 h, de = diastereomic excess. b Pt(dvds) (10 µL, 0.1 M in xylene), 1,2-diphenylacetylene (2.0 equiv), toluene, 40 °C, 12 h. c Pd(tBu₃P)₂ (2 mol%), 4-iodoanisole (2.0 equiv), K₃PO₄ (3.0 equiv), NMP, rt, 12 h. d [Rh(cod)Cl]₂ (2 mol%), (racemic)-Josiphos L1 (4 mol%), acetone (2.0 equiv), toluene, 60 °C, 12 h. e [Rh(cod)Cl]₂ (2 mol%), (racemic)-Josiphos L1 (4 mol%), vinyl ferrocene (2.0 equiv), toluene, 60 °C, 12 h. X-ray crystallographic analysis of 6a allowed to determine the absolute configuration; and configurations of the products 4 were assigned by analogy.

Fig. 6. Photophysical properties investigations.

a Fluorescence images of selected silicon-bridged heterocycles (λex = 365 nm). b Absorption spectra of selected silicon-bridged heterocycles in CHCl₃ (10⁻⁵ M). c Emission spectra of selected silicon-bridged heterocycles in CHCl₃ (10⁻⁵ M). d CD (circular dichroism) spectra of compound (R)-4d (purple line, 91% ee) and (S)-4d (green line, 88% ee) in CHCl₃ (2.0 × 10⁻⁴ M) at room temperature. e CPL (circular polarized luminescence) spectra of compound (R)-4d (purple line, 91% ee) and (S)-4d (green line, 88% ee) in CHCl₃ (2.0 × 10⁻³ M) at room temperature, excited at 339 nm. f g_lum (Luminescence dissymmetry factor) values-wavelength curve for (R)-4d (purple line, 91% ee) and (S)-4d (green line, 88% ee).