Formation of Carbon–Carbon Double Bond between Cp Ligand and Alkenyl Carbon of Titanacyclopentenes

Abstract

Reactions of bis(cyclopentadienyl)titanacyclopentenes with BiI₃ gave coupling products of one Cp ligand and the alkenyl carbons in moderate yields. An NMR study of the tetracyanoethylene (TCNE) adduct of the product showed that the coupled Cp ring and the alkenyl carbons were connected by one carbon–carbon double bond. This is in sharp contrast to coupling reactions of one Cp ligand with the diene moiety in bis(cyclopentadienyl)titanacyclopentadienes, where two carbon–carbon single bonds were formed between those two components to give dihydroindenes or spiro compounds.

Key words

titanocene - titanacyclopentene - coupling of Cp ligand - three-membered ring - bismuth(III) iodide

Publication History

Received: 31 July 2024

Accepted after revision: 27 August 2024

Accepted Manuscript online:
27 August 2024

Article published online:
16 October 2024

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• 8 A Representative Procedure for the Synthesis of 2 A typical procedure is given for the synthesis of 2a. To a solution of Cp₂TiCl₂ (156 mg, 0.625 mmol) in THF (2.5 mL) was added EtMgBr (1.0 M in THF, 1.25 mL, 1.25 mmol) dropwise at –78 °C, and the mixture was stirred for 1 h at the same temperature. After addition of 5-decyne (0.09 mL, 0.5 mmol) at –78 °C, the mixture was warmed up to –30 °C and stirred for 3 h to form bis(η⁵-cyclopentadienyl)titanacyclopentene 1a. To the deep red solution was added TEMPO (156.2 mg, 1.0 mmol), and the mixture was stirred for 30 min at the same temperature. After addition of BiI₃ (295 mg, 0.5 mmol) at –30 °C, the mixture was heated to 50 °C and stirred for 1 h. After the solution was cooled to –20 °C, the reaction was quenched with 12 M aqueous HCl and extracted with hexane. The organic phase was washed with water, aqueous saturated NaHCO₃ and brine, dried over Na₂SO₄, filtered, then evaporated. The crude product was further purified by column chromatography using silica gel (eluent: hexane) and GPC, which gave 2a as a colorless oil (36 mg, yield 31%). Product 2a was obtained as a mixture of double-bond positional isomers in a ratio of 34:66; NMR yield 56%; isolated yield 31%. 3-[1-(1-Butylcyclopropyl)pentylidene]cyclopent-1-ene (2a, Minor Isomer) ¹H NMR (400 MHz, C₆D₆): δ = 0.48–0.50 (m, 2 H), 0.68–0.71 (m, 2 H), 0.88 (t, J = 7.3 Hz, 3 H), 0.95 (t, J = 7.3 Hz, 3 H), 1.22–1.40 (m, 8 H), 1.47–1.55 (m, 2 H), 2.16–2.21 (m, 2 H), 2.31–2.33 (m, 2 H), 2.38–2.41 (m, 2 H), 5.92 (dt, J = 5.7, 2.8 Hz, 1 H), 6.80 (dt, J = 5.8, 2.2 Hz, 1 H). ¹³C{¹H} NMR (400 MHz, C₆D₆): δ = 13.7 (2 C), 14.3, 14.4, 23.5, 23.9, 25.8, 28.0, 30.0, 31.8, 31.8, 35.0, 40.1, 131.0, 133.0, 135.5, 144.6. HRMS (EI): m/z calcd for C₁₇H₂₈: 232.2191; found: 232.2185. 3-[1-(1-Butylcyclopropyl)pentylidene]cyclopent-1-ene (2a, Major Isomer) ¹H NMR (400 MHz, C₆D₆): δ = 0.46–0.48 (m, 2 H), 0.67–0.70 (m, 2 H), 0.87 (t, J = 7.3 Hz, 3 H), 0.92 (t, J = 7.3 Hz, 3 H), 1.22–1.43 (m, 8 H), 1.47–1.55 (m, 2 H), 2.26–2.30 (m, 2 H), 2.34–2.38 (m, 2 H), 2.59–2.62 (m, 2 H), 5.91 (dt, J = 5.7, 2.8 Hz, 1 H), 6.45 (dt, J = 6.0, 2.2 Hz, 1 H). ¹³C{¹H} NMR (400 MHz, C₆D₆): δ = 13.6 (2 C), 14.3, 14.4, 23.6, 23.8, 26.3, 28.1, 30.0, 32.5, 34.1, 34.5, 39.3, 131.2, 132.8, 135.7, 144.5. HRMS (EI): m/z calcd for C₁₇H₂₈: 232.2191; found: 232.2183.