A New Radical-Ionic Allylation Sequence

Abstract

Fluoride induced elimination of β-xanthylsilanes, formed by the radical addition of xanthates to allylsilanes, leads to the formation of diversely substituted allyl derivatives.

Key words

radical addition - xanthates - silanes - olefins - elimination

References

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13

Typical Experimental Procedures:
To a solution of xanthate 1c (0.63 g, 2.53 mmol, 1 equiv) and trimethylallylsilane 2a (1.39 mL, 8.75 mmol, 3 equiv) in refluxing degassed 1,2-dichloroethane (5.8 mL) was added lauroyl peroxide (DLP) (0.050 g, 0.13 mmol, 0.05 equiv) under N₂ atmosphere. DLP (0.03 equiv) was added every 2 h until complete consumption of the starting material. 5% DLP was needed to complete this reaction. The reaction was allowed to cool to r.t. and the solvent removed in vacuo. Purification by flash chromatography (EtOAc-petroleum ether 3:17) gave xanthate 3c (0.75 g, 82%) as a light yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 4.58 (2 H, m, OCH ₂ CH₃), 4.47 (2 H, t, J = 8.1 Hz, OCH ₂ CH₂N), 3.97 (2 H, t, J = 8.1 Hz, OCH₂CH ₂ N), 3.86 [1 H, m, CHS(CS)OEt], 3.07 [1 H, m, N(CO)CHaHb], 2.93 [1 H, m, N(CO)CHaHb], 2.14 [1 H, m, N(CO)CH₂CHaHb], 1.89 [1 H, m, N(CO)CH₂CHaHb], 1.47 (3 H, t, J = 7.1 Hz, OCH₂CH ₃ ), 1.08 (1 H, dd, ² J = 14.9 Hz and J = 7.2 Hz, CHaHbSiMe₃), 1.01 (1 H, dd, ² J = 14.9 Hz and J = 8.1 Hz, CHaHbSiMe₃), 0.03 (9 H, s, SiMe₃). ¹³C NMR (400 MHz, CDCl₃): δ = 214.1, 172.3, 153.2, 69.4, 61.8, 47.6, 42.2, 32.1, 31.1, 22.9, 13.4, -1.1. MS (CI): m/z (%) = 242 (100) [M - S(CS)OEt].
To a solution of β-silylxanthate 3c (0.11 g, 0.33 mmol, 1 equiv) in THF (1.1 mL) at r.t. under a N₂ atmosphere was added a solution of TBAF (0.65 mL, 1 M in THF, 0.65 mmol, 2 equiv). After 3 h the solvent was removed in vacuo and the residue was purified by column chromatography (EtOAc-petroleum ether 2:8) to afford olefin 4c (0.042 g, 75%) as a light yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 5.88-5.78 (1 H, m, CH=CHtHc), 5.06 (1 H, dq, J = 17.1 Hz, ² J and ⁴ J = 1.6 Hz, CH=CHtHc), 4.99 (1 H, dq, J = 10.2 Hz, ² J and ⁴ J = 1.4 Hz, CH=CHtHc), 4.40 (2 H, t, J = 8.2 Hz, OCH ₂ CH₂N), 4.00 (2 H, t, J = 8.2 Hz, OCH₂CH ₂ N), 3.01 [2 H, t, J = 7.4 Hz, N(CO)CH ₂ ], 2.40 [2 H, m, N(CO)CH₂CH ₂ ]. ¹³C NMR (400 MHz, CDCl₃): δ = 172.3, 153.2, 136.4, 115.3, 61.8, 42.1, 34.1, 27.8. MS (CI):
m/z (%) = 187 (100) [MNH₄ ⁺], 170 (30) [M + H].