N,N′-Dibenzosuberyl-1,1′-Binaphthyl-2,2′-diamine: A Highly Effective Supporting Ligand for the Enantioselective Cyclization of Aminoalkenes Catalyzed by Chelating Diamide Complexes of La(III) and Y(III)

Abstract

Enantioselective hydroamination/cyclization of representative aminoalkenes catalyzed by chelating diamide complexes of La(III) and Y(III) are described. It is noteworthy that the La(III) complex derived from the sterically demanding (R)-N,N′-dibenzosuberyl-1,1′-binaphthyl-2,2′-diamine proligand provides enantioselectivities that are in many cases significantly higher than those obtained with the corresponding Y(III) analogue. In addition, the presence of LiCl was typically found to suppress both the rates and the enantioselectivities obtained with the Y(III) complex when compared to its La(III) counterpart, in addition to completely suppressing the bicyclization of 7b. The amide complexes employed in the latter study were prepared by ‘amine elimination’ using the new, highly active bases La[N(TMS)(t-Bu)]₃ and Y[N(TMS)(t-Bu)]₃.

Key words

hydroamination/cyclization - asymmetric - lanthanum

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• 3 The complex (TMS-CH₂)₃La(THF)_1.5 (3b) undergoes slow decomposition at 23 °C and must be used immediately upon its generation.
• 4 Alternatively, the complexes 4a,b could be prepared in THF. In these cases the THF was removed in vacuo and replaced with C₆D₆ prior to the addition of the aminoalkene.
• 5 Enantiomeric excesses were determined by conversion of the pyrrolidine products to the corresponding Mosher’s amides followed by ¹⁹F NMR spectroscopic evaluation, and comparison to authentic racemates.
• 6 We have previously observed a related suppression of activity when a Lu(III) complex derived from similar proligand type was used for hydroamination/cyclization (see ref. 2e).
• 7 Preparative scale (1 mmol) reactions were conducted with the aminoalkenes 7b and 7d. In these cases the pyrrolidines 8b (see ref. 10) and 8d were isolated as the corresponding N-tosylamides in 68% and 71% yields, respectively after purification by column chromatography.
• 8 Tris(amide)s 5a and 5b were readily prepared from preformed LiN(TMS)(t-Bu) and YCl₃ or LaCl₃(THF)_1.5 in Et₂O or THF, respectively, followed by removal of the solvent and sublimation.
• 9 For a highly enantioselective Y(III) catalyst, see: Manna K, Kruze ML, Sadow AD. ACS Catal. 2011; 1: 1637
• 10 (S)-4,4-Diallyl-2-methyl-1-tosylpyrrolidine: In an argon-filled glove box, YCl₃(THF)_3.5 (8.21 mg, 0.02 mmol) and THF (0.5 mL) were added into a J. Young NMR tube equipped with teflon screw cap. (Trimethylsilyl)-methyllithium (5.8 mg, 0.06 mmol) was added and reactant mixture was kept at r.t. for 30 min. N,N′-Bis(10,11-dihydro-5H-dibenzo[a,d][7]annulen-5-yl)-[1,1′-binaphthalene]-2,2′-diamine (2; 13.84 mg, 0.021 mmol) was added and the solution was maintained at r.t. overnight. Removal of THF in vacuo was then followed by addition of C₆D₆ (0.5 mL). The 2,2-diallylpent-4-en-1-amine (7b; 0.18 g, 1.1 mmol) was added to the preformed complex and the reactant mixture was subsequently kept at 21 °C for 21 h. Following pyrrolidine formation, the teflon screw cap was removed, the contents were diluted with CH₂Cl₂ (4 mL) and then transferred into a 5-mL, round-bottomed flask. TsCl (0.36 g, 1.6 mmol) and pyridine (0.2 mL, 1.64 mmol) were added in succession. The reactant mixture was stirred at 21 °C for 12 h. The reactant mixture was diluted with Et₂O (20 mL), washed with sat. NaHCO₃ (10 mL) and brine (10 mL). The organic phase was subsequently dried with MgSO₄. Concentration in vacuo followed by flash chromatography on silica gel (hexane–EtOAc, 20:1) afforded (S)-4,4-diallyl-2-methyl-1-tosylpyrrolidine (0.24 g, 0.75 mmol, 68%). ¹H NMR (500 MHz, CDCl₃): δ = 7.73 (d, J = 8.2 Hz, 2 H, 2 × ArH), 7.32 (d, J = 7.9 Hz, 2 H, 2 × ArH), 5.68–5.73 (m, 1 H, CH₂CH=C), 5.52–5.58 (m, 1 H, CH₂CH=C), 4.97–5.09 (m, 3 H, CH₂CH=CH ₂), 4.80–4.83 (m, 1 H, CH₂CH=CH ₂), 3.61–3.63 (m, 1 H, CH₂CHCH₃), 3.17 (dd, J = 41.7, 10.6 Hz, 2 H, CH₂N), 2.34–2.52 (m, 3 H, ArMe), 2.12 (d, J = 7.3 Hz, 2 H, CH ₂CHCH₃), 1.86 (dd, J = 12.8, 7.3 Hz, 1 H, CHCH ₃), 1.72 (dd, J = 13.9, 6.8 Hz, 1 H, CHCH ₃), 1.57 (dd, J = 13.9, 7.8 Hz, 1 H, CHCH ₃), 1.40 (d, J = 6.1 Hz, 4 H, 2 × CH ₂CH=C). ¹³C NMR (126 MHz, CDCl₃): δ = 143.6, 135.7, 134.2, 133.9, 129.9, 127.8, 118.7, 58.6, 55.7, 44.9, 43.6, 41.3, 39.9, 30.1, 22.9, 21.9. IR (film): 3072, 2972, 2920, 1638, 1601, 1442, 1342, 1158, 1095, 917, 658, 585 cm^–1. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₅NO₂S: 320.1679; found: 320.1660.