Regioselectivity of Intramolecular Rhodium-Catalyzed C–H Insertion Reactions of α-Aryl-α-diazocarboxylates: Influence of the Aryl Substituent

Abstract

It was found that α-aryl-α-diazocarboxylates react with variable regioselectivity in intramolecular rhodium-catalyzed C–H insertion reactions. 3-(Trialkoxysilyl)propyl esters underwent a clean cis-γ-lactone formation (62–69%) if the aryl rest was a phenyl group while the analogous α-(2-bromophenyl)-α-diazocarboxylates produced the respective β-lactones. In general β-lactone formation was shown to be the only detectable C–H insertion pathway for the latter substrate class irrespective of the ester substituent. The β-lactone products (eight examples) were obtained in yields of 41–68% with a diastereomeric ratio (dr) of 75:25 to 96:4 in favor of the trans diastereoisomer. The 2-bromo substituent could be displaced by an aryl group in a subsequent Suzuki cross-coupling reaction.

Key words

carbene complexes - diastereoselectivity - insertion - lactones - regioselectivity - rhodium - ring closure

• References and Notes


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• 14 Representative Procedure for 8c: A 50-mL Schlenk flask was charged with benzene (12 mL) and Rh₂(OAc)₄ (0.5 mol%) and the solution was heated to 80 °C. Diazo compound 7c (567 mg, 1.91 mmol), dissolved in benzene (4.0 mL), was added to the catalyst solution at 80 °C over 3 h employing a syringe pump. After complete addition the reaction mixture was stirred at 80 °C for additional 30 min. After removing the solvent under reduced pressure the crude reaction mixture was purified directly by flash column chromatography (silica gel, pentane–EtOAc 98:2 → 90:10) yielding compound 8c (340 mg, 1.26 mmol, 66%) as an inseparable mixture of diastereoisomers in a ratio of 88:12 (trans/cis). ¹H NMR: (360 MHz, CDCl₃): δ = 0.91 [t, ³ J = 7.4 Hz, 0.36 H, Me (cis)], 1.00 [t, ³ J = 7.4 Hz, 2.64 H, Me (trans)], 1.32–1.40 [m, 0.24 H, CH ₂CH₃ (cis)], 1.49–1.65 [m, 1.76 H, CH ₂CH₃ (trans)], 1.93–2.01 (m, 1 H, CHCH₂CHH), 2.12–2.21 (m, 1 H, CHCH₂CHH), 4.43–4.47 [m, 0.88 H, CHCH₂ (trans)], 4.80 [d, ³ J = 4.3 Hz, 0.88 H, CHAr (trans)], 4.89–4.93 [m, 0.12 H, CHCH₂ (cis)], 5.26 [d, ³ J = 6.6 Hz, 0.12 H, CHAr (cis)], 7.14 [virt. t, ³ J ≅ 7.4 Hz, 0.12 H, CCH_arCH _ar (cis)], 7.21 [virt. t, ³ J ≅ 7.4 Hz, 0.88 H, CCH_arCH _ar (trans)], 7.28 [virt. t, ³ J ≅ 7.4 Hz, 0.12 H, CCH_arCH_arCH _ar (cis)], 7.36 [virt. t, ³ J ≅ 7.4 Hz, 0.88 H, CCH_arCH_arCH _ar (trans)], 7.44 [d, ³ J = 7.8 Hz, 0.88 H, CCH _ar (trans)], 7.48 [d, ³ J = 7.8 Hz, 0.12 H, CCH _ar (cis)], 7.57 [d, ³ J = 7.8 Hz, 0.12 H, CH _arCBr (cis)], 7.60 [d, ³ J = 7.8 Hz, 0.88 H, CH _arCBr (trans)]. ¹³C NMR (90.6 MHz, CDCl₃): δ (major diastereoisomer) = 13.7, 18.4, 30.6, 60.8, 80.1, 123.8, 128.3, 128.7, 129.9, 133.0, 133.2, 168.8. HRMS: m/z calcd for C₉H₆ ⁷⁹BrO₂: 224.0195; found: 224.0191. HRMS: m/z calcd for C₉H₆ ⁸¹BrO₂: 226.0175; found: 226.0177.
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