Rational Design and Synthesis of [5]Helicene-Derived Phosphine Ligands and Their Application in Pd-Catalyzed Asymmetric Reactions

A series of novel optically active [5]helicene-derived phosphine ligands (L1, with a 7,8-dihydro[5]helicene core structure- and L2, with a fully aromatic [5]helicene core structure) were synthesized. Despite their structural similarities, L1 and L2 exhibit particularly different characteristics in their use as chiral ligands. L1 was highly effective in the asymmetric allylation of indoles with 1,3-diphenylallyl acetate (up to 99% ee), and in the etherification of alcohols (up to 96% ee). In contrast, L2 was highly effective in the stereocontrol of helical chirality in Suzuki–Miyaura coupling (SMC) reaction (up to 99% ee). Density functional theory analysis was employed to propose a model that accounts for the origin of the enantioselectivity in these reactions.

The precipitate was collected by filtration and purified by flash column chromatography on silica gel with hexane−EtOAc (1:1) to afford rac-7 as a pale yellow solid (16 mg). The mother liquor was concentrated under reduced pressure and purified over silica gel to give (M)-(−)-7 (45 mg, >99% ee).

DFT calculation methods and data
All calculations were carried out with the D.01 revision of the Gaussian 09 program package. 8,9 The three dimensional structures presented in Figure S1 and S2 were visualized utilizing CYLview. 10

12-1. Computational methods for (π-ally)palladium intermediates with (M)-L1
The geometries of all intermediates (IM-endo and -exo) were optimized at the DFT level by using the B3PW91. The standard 6-31G* basis set has been applied for atoms except Pd, which has been described by the LANL2DZ basis set. 11 Frequencies were analytically computed at the same level of theory to give gas-phase free energies and to confirm whether the structures are minima (no imaginary frequencies) or transition states (only one imaginary frequency). The Gibbs free energies were in all cases computed by adding to the single-point energies both zero-point vibrational energies and thermal corrections (298 K, 1 atm).

12-2. Computational methods for transitions states (TSs) for the reductive elimination steps
The geometries of all transition states were optimized at the DFT level by using the dispersion-corrected functional B97-D. The standard 6-31G* basis set has been applied for atoms except Pd, which has been described by the LANL2DZ basis set. Frequencies were analytically computed at the same level of theory to give gas-phase free energies and to confirm whether the structures are minima (no imaginary frequencies) or transition states (only one imaginary frequency).
All transition state structures were confirmed to connect the proposed reactants and products by intrinsic reaction   Figure S3. X-ray structure of L1 Figure S4. X-ray structure of L2

Isolation and X-ray structure of Pd(dba)[L1] 10
A solution of L1 (2.2 mg, 0.005 mmol) and Pd 2 (dba) 3 •CHCl 3 (2.0 mg, 0.002 mmol, [Pd]:L1 = 1:1) in THF was stirred for 1h at 50 ºC under an argon atmosphere. After cooling to room temperature, the solvent was removed under reduced pressure. The residue was purified by flash column chromatography on silica gel eluting with hexane−EtOAc (1:1), followed by recrystallized from acetone to afford complex 10 as a red X-ray quality single crystals ( Figure S5).