Reductive Ring-Opening 1,3-Difunctionalizations of Arylcyclopropanes with Sodium Metal

Abstract

Sodium dispersion promotes reductive ring opening of arylcyclopropanes. The presence of a reduction-resistant electrophile, such as methoxypinacolatoborane, epoxide, oxetane, paraformaldehyde, or chlorotrimethylsilane, during the reductive ring opening event leads to the formation of 1,3-difunctionalized 1-arylalkanes by immediate trappings of the resulting two reactive carbanions. In particular, the ring-opening 1,3-diborylations of arylcyclopropanes afford 1,3-diborylalkanes with high syn selectivity.

Key words

carbanions - cleavage - diastereoselectivity - electron transfer - metalation - reduction - ring opening - sodium

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• 9 Experimental ProcedureAn oven-dried 20 mL Schlenk tube was charged with 4,4′-di-tert-butylbiphenyl (DTBB, 53.3 mg, 0.200 mmol), THF (4.0 mL), and trans-1a (191 mg, 0.983 mmol). After the mixture was cooled to –78 °C, MeOBpin (0.97 mL, 6.0 mmol) was added. Sodium dispersion (10.0 M, 0.40 mL, 4.0 mmol) was then added dropwise, and the resulting suspension was stirred at –78 °C for 1.5 h. The resulting mixture was warmed to 0 °C, and the reaction was then quenched with i-PrOH (0.31 mL, 4.0 mmol) and then aqueous NH₄Cl (5 mL). The resulting biphasic solution was extracted with EtOAc (4 × 10 mL). The combined organic layer was dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The residue was purified by column chromatography (hexane to hexane/EtOAc = 30:1) on silica gel to provide 2a as a white solid (81% yield, 359 mg, 0.801 mmol, syn/anti = 89:11); mp 120–130 °C. ¹H NMR (600 MHz, CDCl₃): δ = 7.21 (dd, J = 7.5, 7.5 Hz, 0.89 × 4 H + m, 0.11 × 4 H), 7.13 (d, J = 7.5 Hz, 0.89 × 4 H + m, 0.11 × 4 H), 7.10 (t, J = 7.5 Hz, 0.89 × 2 H + m, 0.11 × 2 H), 2.36 (ddd, J = 14.4, 7.8, 7.8 Hz, 0.89 × 1 H), 2.30 (t, J = 7.8 Hz, 0.89 × 2 H), 2.25 (dd, J = 9.6, 6.6 Hz, 0.11 × 2 H), 2.21 (t, J = 6.6 Hz, 0.11 × 2 H), 2.02 (ddd, J = 14.4, 7.8, 7.8 Hz, 0.89 × 1 H), 1.20 (s, 0.89 × 12 H), 1.18 (s, 0.89 × 12 H), 1.15 (s, 0.11 × 12 H), 1.14 (s, 0.11 × 12 H). ¹³C NMR (151 MHz, CDCl₃): δ (syn isomer) = 143.3, 128.6, 128.4, 125.3, 83.3, 35.2, 31.3 (br), 24.8. ¹¹B NMR (192 MHz, CDCl₃): δ = 33.1 (br). HRMS (APCI-MS, positive): m/z = 448.2957. Anal. Calcd for C₂₇H₃₈B₂O₄: 448.2960 [M]⁺.
• 10 The relative stereochemistry of 2a was unambiguously assigned after oxidation to the corresponding diol with retention of the stereochemistry.

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