Synthesis of Silylcyclopropanes through the Catalytic Generation of Zinc Silylcarbenoids from Enynones

Dedicated to Professor Antonio Echavarren on the occasion of his 60^th birthday

Abstract

A simple methodology for the preparation of 1-furyl-1-(silyl)cyclopropanes is reported. ZnCl₂ serves as inexpensive, low-toxicity catalyst for the generation of silylcarbenes from enynones, which can be trapped by alkenes under mild reaction conditions. These cyclopropanations take place in high yields and selectivities, showing a remarkable scope.

Key words

carbenes - cyclopropanation - silylcyclopropanes - silyl carbenes - zinc catalysis

• References and Notes

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• 16 Representative Procedure: To a solution of enynone 1 (0.30 mmol) and alkene 2 (1.8 mmol, 6.0 equiv) in 1,2-dichloroethane (3.0 mL, 0.10 M), ZnCl₂ (4.0 mg, 10 mol%) was added at ambient temperature. The Schlenk flask was placed in a preheated oil bath at 50 °C and the reaction mixture was stirred until consumption of 1 (TLC analysis). After elimination of the solvent, purification by flash column chromatography (SiO₂, hexane–EtOAc) afforded the corresponding cyclopropanes 3. Characterization data for compound 3a: ¹H NMR (300 MHz, CDCl₃): δ = 7.05–7.23 (m, 3 H), 6.92–7.03 (m, 2 H), 5.86 (s, 1 H), 2.37 (s, 3 H), 2.34 (dd, J = 7.9, 6.1 Hz, 1 H), 2.25 (s, 3 H), 1.63 (dd, J = 6.0, 4.8 Hz, 1 H), 1.39 (dd, J = 7.8, 4.8 Hz, 1 H), 0.09 (s, 9 H). ¹³C NMR (75 MHz, CDCl₃): δ = 194.4 (C), 156.6 (C), 153.9 (C), 138.3 (C), 127.9 (CH), 127.6 (CH), 125.9 (CH), 121.6 (C), 107.5 (CH), 29.0 (Me), 26.9 (CH), 17.7 (C), 15.1 (CH₂), 14.2 (Me), –2.9 (Me). HRMS (ESI): m/z [M]⁺ calcd for C₁₉H₂₄O₂Si: 312.1546; found: 312.1543.
• 17 The trans configuration of cyclopropanes 3 was established by NOESY experiments and was not affected during the purification.
• 18 Higher temperatures or longer reaction times did not influence the yield.
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• 21b Signals attributable to the silylcyclopropane were not observed by ¹H NMR analysis. Nevertheless, formation of cyclopropane followed by ring expansion cannot be ruled out.

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• 23 Unfortunately, attempts to obtain cyclopropanols by Tamao–Fleming oxidation (on compounds 3w–x) through various standard procedures failed, leading to complex mixtures or hydrodesilylation product. Various attempts to perform Hiyama couplings were also unsuccessful.