Transition-Metal-Free Conjugate Stannyl Transfer to α,β-Unsaturated Carbonyl and Carboxyl Compounds in Basic Aqueous Media

 

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Abstract

An unpretentious procedure for the conjugate stannylation of several α,β-unsaturated acceptors using silyl stannanes is reported. Competing reaction pathways of conjugate addition and reduction are discussed.

References and Notes

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General Procedure for Conjugate Stannylation: Under an inert atmosphere, a flame-dried Schlenk tube was charged with the indicated α,β-unsaturated acceptor A (0.10 mmol, 1.0 equiv) and the THF-H₂O solvent mixture (20:1, 2.1 mL). The base MX (0.1 mmol, 1.0 equiv) as well as B (1, 0.25 mmol, 2.5 equiv) were added, and the reaction mixture was heated to 50 °C. Conversion was monitored by TLC. After full consumption of A the reaction mixture was diluted with tert-butylmethylether (10 mL) at r.t. A small portion of silica gel was added and the solvents were removed in vacuo. The crude product C on silica gel was then subjected to flash chromatography on Et₃N-deactivated silica gel using cyclohexane-tert-butylmethylether solvent mixtures. Stannanes C (22-95%) were isolated as colorless oils.

Reagent 1 is not particularly volatile, stable towards GLC measurements, and provides easy-to-interpret NMR spectra.

Preparation of Dimethylphenyl(trimethylsilyl)stannane (1): Neat dimethylphenylstannyl chloride [19] (3.00 g, 11.5 mmol, 1.00 equiv) was added dropwise to Me₃SiCl-activated lithium (large excess) in THF (40 mL) at 0 °C under ultrasonic irradiation, and the reaction mixture was then maintained at these conditions for 2 h. The supernatant blue-black-colored solution was subsequently transferred to another flask and slowly treated with trimethylsilyl chloride (3.10 mL, 24.3 mmol, 2.10 equiv) at 0 °C. After stirring at ambient temperature for additional 3 h, the solvent was removed under reduced pressure. Crude 1 was purified by flash chromatography on silica gel using cyclohexane as solvent. The title compound (3.22 g, 10.8 mmol, 94%) was obtained as a colorless oil. IR (ATR): 3062 (w), 2950 (w), 2893 (w), 1427 (m) cm^-1. ¹H NMR (300 MHz, CDCl₃): d = 0.26 [t, ² J _H,Sn = 24 Hz, 6 H, SnMe₂], 0.29 [t, ³ J _H,Sn = 3.0 Hz, 9 H, SiMe₃], 7.27-7.55 (m, 5 H, Ph). ¹³C NMR (75 MHz, CDCl₃): d = -11.3 [SnMe₂], 1.3 [SiMe₃], 128.2, 128.5, 137.1, 142.0 (Ph). ²⁹Si NMR (60 MHz, CDCl₃): d = -9.3. ¹¹⁹Sn NMR (112 MHz, CDCl₃): d = -138.9. Anal. Calcd for C₁₁H₂₀SiSn: C, 44.18; H, 6.74. Found: C, 44.30; H, 7.01.