Chiral Molecular Propellers of Triarylborane Ammonia Adducts

Abstract Chiral molecular propeller conformations have been induced to various triaryl structures including trityl derivatives and triaryl boranes. For borane–amine adducts, such induced propeller chirality has not been reported yet due to the low energy barrier for racemization in common triarylboranes such as B(C6H5)3 or B(C6F5)3. Herein, we demonstrate that point chirality in side chains of chiral triarylborane–ammonia adducts, which feature intramolecular hydrogen bonds in addition to the dative N→B bond, can efficiently be transferred to triarylborane propeller chirality. Employing X‐ray crystallography and ECD/VCD spectroscopy for structural characterizations, we investigate three examples with different steric demands of the incorporated chiral alkoxy side groups. We elucidate the conformational preferences of the molecular propellers. Furthermore, we show that computationally predicted conformational preferences obtained for the isolated, only implicitly solvated molecules are actually opposite to the experimentally observed ones.


Conformational analysis
Conformational analysis of 1b   Table S3. Conformational analysis 1d. We list only the first 50 conformers of more than 150 optimized conformers as Boltzmann weights are neglectable.   Table S1 as the ratio given here is calculated over only the first 12 conformers, not over the entire set.

2.
Additional spectra plots Figure S1. Computed UV and ECD spectra of 1b. Figure S2. VCD spectra analysis of 1c. For more information, see main text. Figure S3. Comparison of several computed VCD spectra of 1d with the experimental spectrum. Figure S4. Structures of the ten lowest energy conformers of 1b. Figure S5. Structures of the two lowest energy conformers of 1c. Figure S6. Structures of the two lowest energy conformers of 1d. Figure S7. Similarity analysis of the VCD spectra of 1b/1c simulated with different ratios of P/M-helical form.

Similarity analysis
In order to determine the optimum ratio of P/M-helical propeller conformers of 1b, we simulated VCD spectra with different ratios and computed the corresponding overlap integrals with the experimental VCD. The left panel of Figure S7 shows the changes in the spectra, that are observable when going from a 54:46 ratio towards

X-ray crystallography
Single crystals analyzed on a Rigaku Synergy dual source device, with Co micro focus sealed tube (Cu K α ) using mirror monochromators and a HyPix-6000HE: Hybrid photon counting X-ray detector. The crystals were mounted in Hampton CrypLoops using GE/Bayer silicone grease. Data was recorded and reduced using the CrysalisPro 1 Software. The structure was solved using WinGX 2 in combination with ShelXT 3 and refined with shelXle 4 and ShelXL. Tables for the publication were generated using a modified version of Ciftab. The pictures were generated with Diamond 4 5 Figure S8. Asymmetric unit of 1b.    UV-Vis and CD Spectra were recorded on an Applied Physics Chirascan TM -plus CD Spectrometer in chloroform at room temperature. The quartz cuvettes used had a path length of 1 mm. Concentrations were c(1b) = 1.1 mM, c(1c) = 0.8 mM, c(1d) = 1.0 mM, c(5b) = 6.3 mM, c(5c) = 4.8 mM and c(5d) = 5.8 mM. Spectra were acquired in the spectral range of 240 to 340 nm with a step size of 0.5 nm between measurement points and a sampling time of one second per point. Measurements were repeated eight times and averaged to reduce the signal-tonoise ratio.
The IR and VCD spectra were recorded on a Bruker Vertex 70 equipped with a PMA 50 unit for polarization modulated measurements. Samples were held in BaF 2 cells with 100 µm path length. The IR spectra were accumulated for 32 scans, while the VCD spectra were recorded over a total measurement time of 4 hours each (~16000 scans). Background correction was carried out by subtraction of the solvent recorded under identical conditions.

Computational details.
Conformational searches for all investigated compounds were carried out systematically by preparing individual starting structures for unique combinations of torsional angles (threefold-rotation). All calculations were carried out using Gaussian 09 Rev. E.01 6 employing the B3LYP/6-31+G(2d,p)/IEFPCM(CHCl 3 ) level of DFT. Spectra were simulated by assigning a uniform Lorentzian band shape of 6 cm -1 half-width at half-height to the computed dipole and rotational strength for the IR/VCD and a Gaussian shape of 0.3 eV width for the UV/ECD . The vibrational spectra presented in the main text are scaled with a frequency scaling factor  of 0.98. Relative zero-point energy corrected energies, E ZPC , were used to determine Boltzmann weights. Note: Other DFT functionals such as M06-2X and B3LYP-GD3BJ were evaluated as well but gave worse conformational energies. This may be due to a further overestimation of C-H•• interactions (cf. main text for further discussion).

Synthetic procedures
General procedure for the synthesis of chiral alkyl-phenyl ethers 4b to 4d In a heat dried schlenk tube, 2-bromophenol (519.0 mg, 3.0 mmol, 1.5 eq.) triphenylphosphine (524.6 mg, 2.0 mmol, 1 eq.) and the chiral alcohol (2.0 mmol, 1 eq.) were mixed under argon atmosphere. Dry THF (3 mL) was added and the mixture was cooled to 0 °C, before DIAD (0.39 mL, 404.4 mg, 2.0 mmol, 1 eq.) was added dropwise via syringe. The mixture was then allowed to warm to r.t. and stirred for another 0.5 h, before the schlenk tube was fitted with a reflux condenser and the reaction mixture was stirred under reflux. After completion of the reaction, as judged by TLC, the solvent was removed, yielding a yellow oil. The crude product was purified by column chromatography (eluent: cyclohexane) to afford 4b to 4d as colourless, viscous liquids.

Tris-(o-alkoxyphenyl)-borane ammonia complexes 1b to 1d
Tris-(o-2-butoxyphenyl)-borane ammonia complex 1b In a heat dried schlenk flask, the 2-butylphenylether 4b (450.0 mg, 1.96 mmol, 3 eq.) was dissolved in dry diethyl ether (5 ml) under argon atmosphere. The mixture was cooled to 0 °C and a solution of n-butyl lithium in hexanes (2.39 mol/L, 0.82 mL, 1.96 mmol, 3 eq.) was added dropwise. After complete addition, the mixture was allowed to warm to r.t. and was stirred for 1 h at that temperature. Subsequently, the mixture was again cooled to 0 °C before BF 3 ×Et 2 O (80.0 µL, 92.0 mg, 0.65 mmol, 1 eq.) was added dropwise. The mixture was stirred an additional 16 h at r.t. Quenching with a saturated, aqueous solution of ammonia afforded a white precipitate, which was re-dissolved by adding diethyl ether. The aqueous phase was then separated and extracted with diethyl ether (3 × 10 mL) and the combined organic phases were dried with anhydrous magnesium sulfate.

Tris-(o-2-phenylethoxyphenyl)-borane ammonia complex 1c
In a heat dried schlenk flask, the 2-benzylphenylether 4c (544.3 mg, 1,96 mmol, 3 eq.) was dissolved in dry diethyl ether (5 ml) under argon atmosphere. The mixture was cooled to 0 °C and a solution of n-butyl lithium in hexanes (2.39 mol/L, 0.82 mL, 1.96 mmol, 3 eq.) was added dropwise. After complete addition, the mixture was allowed to warm to r.t. and was stirred for 1 h at that temperature. Subsequently, the mixture was again cooled to 0 °C before BF 3 ×Et 2 O (80.0 µL, 92.0 mg, 0.65 mmol, 1 eq.) was added dropwise. The mixture was stirred an additional 16 h at r.t. Quenching with a saturated, aqueous solution of ammonia afforded a white precipitate, which was re-dissolved by adding diethyl ether. The aqueous phase was then separated and extracted with diethyl ether (3 × 10 mL) and the combined organic phases were dried with anhydrous magnesium sulfate.
After removal of the solvent, a pale yellow oil was obtained. The crude product was purified by column chromatography on silica (eluent: cyclohexane/ethylacetate: 20/1) to afford a slightly yellow solid which was washed with ethanol to give 1c as a colourless solid.
The mixture was stirred for additional 16 h at r.t. and subsequently quenched with a saturated aqueous solution of ammonia. The resulting white precipitate was re-dissolved in diethyl ether and the aqueous phase was separated and extracted with diethyl ether (3 × 10 mL). The combined organic phases were dried with anhydrous sodium sulfate. Removal of the solvent gave a yellowish oil. The crude product was purified by column chromatography on silica (eluent: hexane/ethylacetate: 10/1) to afford a slightly yellow solid which was crystallized from ethanol to give 1c as a colourless crystalline needles (18.9 mg, 36.5 µmol, 6%).