Chiral Chalcogenyl‐Substituted Naphthyl‐ and Acenaphthyl‐Silanes and Their Cations

Abstract Cyclic silylated chalconium borates 13[B(C6F5)4] and 14[B(C6F5)4] with peri‐acenaphthyl and peri‐naphthyl skeletons were synthesized from unsymmetrically substituted silanes 3, 4, 6, 7, 9 and 10 using the standard Corey protocol (Chalcogen Ch=O, S, Se, Te). The configuration at the chalcogen atom is trigonal pyramidal for Ch=S, Se, Te, leading to the formation of cis‐ and trans‐isomers in the case of phenylmethylsilyl cations. With the bulkier tert‐butyl group at silicon, the configuration at the chalcogen atoms is predetermined to give almost exclusively the trans‐configurated cyclic silylchalconium ions. The barriers for the inversion of the configuration at the sulfur atoms of sulfonium ions 13 c and 14 a are substantial (72–74 kJ mol−1) as shown by variable temperature NMR spectroscopy. The neighboring group effect of the thiophenyl substituent is sufficiently strong to preserve chiral information at the silicon atom at low temperatures.


Experimental part 1.1 General remarks
All experiments were carried out under argon or nitrogen atmosphere using Schlenk techniques. The glass equipment was stored in an oven at 120°C and evacuated prior to use.
The solvents n-pentane, n-hexane, benzene, tetrahydrofuran and diethyl ether were dried over sodium-potassium alloy and distilled under nitrogen atmosphere. The deuterated solvents were first dried over NaK and then either condensed before use or stored over molecular sieve (4 Å). Commercially available solid materials were stored and weighted in a glove box or dried at high vacuum before use. The n-butyl lithium was used as a 1.6 M solution in n-hexane. 1,8-Dibromonaphthalene, 5,6-dibromoacenaphtene, 1-Bromo-8-dimethylsilylnaphthalene, trityl borate [Ph3C][B(C6F5)4] were synthesized according to literature procedures. [S1-S4] Thin-layer chromatography was performed using commercial available aluminium foil ( Chiral HPLC was performed on a Thermo Scientific Dionex Ultimate 3000 with a Lux 5µm Cellulose-3, 250 x 4.6 mm column and a flow rate of the eluent of 1mL/min at 25°C. The determination of the optical rotation was performed using the Polartronic M from Schmidt+Haensch with Na-D-line (589 nm) at 20°C (cell lengths: 1 dm).
Infrared spectra were performed on a Bruker Tensor 27 spectrometer with a MKII Reflection Golden Gate Single Diamond ATR system.
For silanes, combustion analysis values for carbon show often too low values, which we attribute to the formation and incomplete combustion of silicon carbide, although vanadium pentoxide as combustion aid was used. Satisfactory combustion analyses could not be obtained from all silyl borates due to their high reactivity.

5-Mesityltellanyl-6-phenylmethylsilylacenaphthene 7
A solution of 1.77 g (5.00 mmol) 5-bromo-6-(methylphenylsilyl)acenaphthene 5 in diethyl ether (45 mL) was cooled to -10 °C. 3.1 mL (5.00 mmol) n-butyl lithium were added dropwise to the solution via syringe and the reaction mixture was stirred for four hours. In a round-bottom flask 2.47 mg (5.00 mmol) dimesityl ditelluride were suspended in diethyl ether (150 mL) and added to the reaction mixture at -80 °C via Teflon tube. The reaction mixture was allowed to warm to room temperature overnight. Then saturated ammonium chloride solution (200 mL      Then the solution of the trityl borate was added to the silane at r.t. and the biphasic reaction mixture was stirred for 30 min. Subsequently, the phases were separated, the upper, nonpolar phase was removed and the polar phase was washed with benzene three times. After removing the solvent under low pressure, the residue was dissolved in a deuterated solvent and analyzed by NMR spectroscopy.

Chiral Resolution and Chiral Memory Experiments
General Procedure F: A Schlenk flask was charged with copper(I)chloride and triphenylphosphane in a ratio of 1:2. Toluene was added and the mixture was stirred until triphenylphosphane was dissolved. Subsequently sodium tert-butoxide (equimolar to CuCl) was added at r.t. and the mixture was stirred until the color turned yellow (5 -10 min). The pyridyl alcohol (R)-E was dissolved in toluene and added at r.t. to the catalyst mixture. The mixture turned orange. Subsequently the silane was added either as a solid in one portion or dissolved in toluene whereupon the mixture turned brown-red. After stirring for approximately 16 h, the reaction mixture was filtrated through a thin layer of silica gel to remove the Cu(I) species, the solvent was removed and the crude product was purified by a two-step column chromatography if not mentioned otherwise in the details. The first column chromatography (eluent petroleum ether/ethyl acetate 100:0  50:50) with a short column resulted in two fraction: 1. (+)-Silane + Ph3P, 2. Siloxanes + impurities. Both fractions needed further purification which is specified for each compound in detail below.

6-Phenoxy-5-methylphenylsilylacenaphthene 3a
The kinetic resolution of the title compound 3a was performed according to General Procedure F using 305.6 µmol copper(I) chloride, 1.61 mmol of the pyridyl alcohol and 3.06 mmol of silane 3a. The catalyst was prepared as usual at r.t., then the mixture was cooled with an ice bath, first the alcohol/toluene and subsequently the silane/toluene mixture was added dropwise, the mixture was warmed slowly to r.t. over night. The crude product was purified by a short column chromatography (eluent petroleum ether/ethyl acetate 100:0  0:100) resulting in two fractions. Fraction 1 ((+)-silane 3a + Ph3P) was further purified by oxidation of the phosphane with H2O2. Therefore, the solid were dissolved in petroleum ether and 0.6 mL H2O2 (30w% in H2O) was added at r.t.. After stirring the mixture for 16 h the solid which precipitated was filtered off and the phases were separated. The solvent of the organic layer was removed and the residue was purified via recrystallization from hexanes. (+)-Silane 3a was obtained with a yield of 52 % and an ee of 56 % ([] = +12° (0.01 mol L -1 )). Fraction 2 (siloxanes 12a) was not further purified (yield 61 %). The crude fraction 2 was used for the reduction of siloxane to obtain (-)silane 3a.

Reduction of siloxanes 12
The siloxanes 12 were dissolved in 2-20 mL diethylether and di-iso-butylaluminiumhydride (1 M in n-hexane) was added at r.t.. The mixture was stirred for 16 h and afterwards quenched by addition of 1-20 mL 1 M hydrochloric acid. The two phasic mixture was stirred for 20-60 min, the phases were separated and the product was extracted from the aqueous layer (3 x 5-20 mL Et2O). After removal of the solvent, the product was purified via column chromatography or preparative TLC (eluent PE). Details see

Chiral Memory
A Schlenk tube was charged with 1.0 equiv. of silane (-)-3a, (-)-4a or (+)-9 and a second Schlenk tube was charged with 1.0 equiv. of trityl borate [Ph3C][B(C6F5)4]. The solids were dissolved in DCM or chlorobenzene, respectively. The silane was cooled to the temperature indicated in Table S2 and trityl borate was added. The mixture was stirred for the time indicated in Table S2. Then, sodium triethyl borohydride in toluene was added and the mixture was stirred overnight. The solvent was removed and the residue was suspended in petroleum ether. The mixture was filtrated through a thin layer of silica, the solvent was removed and the crude product was purified via preparative TLC (eluent petroleum ether). After the purification, the formation of silanes was confirmed by NMR spectroscopy and then their optical rotation was measured and their ee was determined via chiral HPLC (Table S3)

Computational Details
All quantum chemical calculations were carried out using the Gaussian09 package. [S7] The molecular structure optimizations were performed using the M06-2X functional [S8] along with the def2-TZVP basis set for the elements Te, Se, S, Si, O, C, H and using the corresponding pseudopotential for Te. [S9] Every stationary point was identified by a subsequent frequency calculation either as minimum (Number of imaginary frequencies (NIMAG): 0) or transition state (NIMAG: 1). The SCF energies (E(SCF)) and the absolute computed Gibbs free energies at T = 298.15 K and p = 0.101 MPa (1 atm) in the gas phase (G298) are given in Table S7 for all optimized molecular structures. The optimized molecular structures of all compounds of interest are given as cartesian coordinates in the structure file (Computed_Molecular_structures.xyz). Table S7. Calculated absolute energies, E(SCF), and free enthalpies at 298 K, G 298 for the compounds of interest (at M06-2X/def2-TZVP). "Cation isodes" and "silane isodes" are silyl cations and silanes used for the calculation of the isodesmic reactions given in Scheme 5.