A First Asymmetric Diamination of Olefins

Abstract

An asymmetric diamination of olefins employing stable imidoosmium(VIII) complexes is described for the first time. The reaction is based on the use of chiral acrylic esters and related substrates with enantiopure menthol and 8-phenyl menthol as chiral auxiliaries and gives rise to osmaimidazolidines with good to excellent diastereomeric ratios (up to 95:5). Removal of the osmium centre and the chiral auxiliary leads to novel enantiopure 1,2,3-trisubstituted compounds.

Key words

diamines - asymmetric synthesis - imido complexes - osmium - diastereoselectivity

References

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10

Details on the full characterisation of new compounds, further substrates including studies on functional group tolerance, and X-ray structures of various racemic compounds will be given in a full account.

11

Typical experimental procedure for the diamination of olefins: (-)-8-Phenylmenthyl cinnamate (181 mg, 0.5 mmol) was added to a solution of bis(N-t-butylimido)dioxo-osmium(VIII) (183 mg, 0.5 mmol) in 5 mL of freshly distilled THF. The resulting orange solution was stirred at r.t. for 11 h during which it turned dark red. The solvent was removed under reduced pressure to leave a red-brown oil which was passed through a small pad of silica gel (hexanes-ethyl acetate, 4:1). The pure mixture of the two diastereomers (248 mg, 0.34 mmol, 68% yield) was separated by semipreparative HPLC (Knauer Eurospher 100CN, t-BuOCH₃-n-hexane, 15:85, 14 mL/min, 254 nm). Retention times: 12.9 min for 4b and 15.6 min for 4a. Selected data for the major diastereomer 4a: ¹H NMR (300 MHz, benzene-d ₆, 25 °C): δ = 0.79 (d, J = 6.4 Hz, 3 H), 0.80-0.95 (m, 4 H), 1.08 (s, 3 H), 1.18 (s, 9 H), 1.19 (s, 9 H), 1.22 (s, 3 H), 1.43-1.55 (m, 1 H), 1.60-1.71 (m, 1 H), 2.05-2.20 (m, 2 H), 3.64 (s, 1 H), 4.77 (dt, J = 4.1, 10.7 Hz, 1 H), 4.90 (s, 1 H), 6.85-6.93 (m, 1 H), 7.01-7.28 (m, 9 H). ¹³C NMR (75 MHz, benzene-d ₆, 25 °C): δ = 21.9, 23.8, 26.7, 29.4, 30.2, 30.9, 31.3, 34.81, 39.5, 41.8, 50.2, 66.8, 67.4, 76.4, 81.3, 84.1, 125.4, 125.7, 127.2, 127.7, 127.9, 128.3, 128.6, 171.2. MS (EI, 70 eV): m/z (%): 728 (2) [M⁺], 469 (17), 413 (11), 279 (39), 146 (100). HRMS: calcd for C₃₃H₄₉N₂O₄ ¹⁸⁸Os: 724.3173, found: 724.3166.

14

At present we refrain from any mechanistic discussion. Both a [2+2] or [3+2] mechanism [9] might be operating. Experimental and theoretical investigations addressing this question are underway.

15

Apparently, this way of amide formation is not a general reaction. While it worked well for the present purpose, preliminary reactions indicated that it is not suitable for conventional N-alkylated α-amino esters. In these cases, the reaction proceeds very sluggishly and with complete racemization (K. Muñiz, unpublished results).

17

Typical experimental procedure for LiAlH ₄ reduction: Osmaimidazolidine 4b (365 mg, 0.5 mmol) was dissolved in absolute THF (4 mL), LiAlH₄ (57 mg, 1.5 mmol) was added in one portion and the resulting mixture was stirred for 2 hours at r.t. It was treated with methanol and water, extracted with ethyl acetate, dried over MgSO₄ and filtered over celite. The solvent was removed under reduced pressure to leave the desired compound 9 as a colourless oil (131 mg, 0.47 mmol, 94% yield). Selected data for 9: [α]_D ²² = -22 (CH₂Cl₂, c = 0.1). ¹H NMR (300 MHz, DMSO-d ₆, 25 °C): δ = 0.93 (s, 9 H), 0.97 (s, 9 H), 3.16 (dd, J = 2.6, 10.7 Hz, 1 H), 3.19-3.42 (m, 5 H), 3.80 (d, J = 7.0 Hz, 1 H), 7.19-7.47 (m, 5 H). ¹³C NMR (75 MHz, DMSO-d ₆, 25 °C): δ = 29.9, 30.0, 49.9, 50.4, 57.58, 58.8, 61.5, 125.9, 127.4, 128.0, 147.0. MS (EI, 70 eV): m/z (%): 278 (3) [M⁺], 259 (11), 222 (37), 201 (66), 166 (49), 77 (100), 65 (95). Elemental analysis: calcd for C₁₇H₃₀N₂O: C 73.33; H 10.86; N 10.06; found: C 73.62; H 11.01; N 9.89.

18

Typical experimental procedure for Os removal: Osmaimidazolidine 10b (177 mg, 0.35 mmol) was dissolved in absolute ethanol (3 mL), sodium borohydride (30 mg, 0.79 mmol) was added in small portions and the resulting mixture was stirred for 60 min at r.t. It was filtered over celite and the solvent was removed under reduced pressure to leave the desired compound 11b as a colourless oil (89 mg, 0.31 mmol, 89% yield). Selected data for 11b: [α]_D ²² = -89 (CH₂Cl₂, c = 0.1). ¹H NMR (300 MHz, CDCl₃, 25 °C): δ = 0.87 (d, J = 7.3 Hz, 3 H), 1.16 (s, 9 H), 1.26 (s, 9 H), 1.43 (s, 9 H), 2.91 (dq, J = 7.3, 9.4 Hz, 1 H), 3.47 (d, J = 9.4 Hz, 1 H), 7.52 (br s, 1 H). ¹³C NMR (75 MHz, CDCl₃, 25 °C):
δ = 22.7, 28.4, 29.7, 29.7, 51.8, 51.8, 52.9, 56.9, 59.3, 171.5. MS (EI, 70 eV): m/z (%): 285 (6) [M⁺], 228 (17), 174 (66), 125 (49), 73 (100). Elemental analysis: calcd for C₁₆H₃₅N₃O: C 67.32; H 12,36; N 14.72; found: C 66. 95; H 12.22; N 15.01.