Synthesis of a Polymerizable Benzocyclobutene that Undergoes Ring-Opening Isomerization at Reduced Temperature

Abstract

1-Ethoxyvinylbenzocyclobutene is a substituted benzocyclobutene that undergoes radical polymerization to produce polymers that can be crosslinked at 100–150 °C. The 4- and 5-vinyl isomers are synthesized in a 1:4 ratio via a halogenated benzyne intermediate produced from anthranilic acid, followed by cycloaddition with ethyl vinyl ether and replacement of the halogen atom with a vinyl group.

Key words

benzocyclobutene - cycloaddition - ring-opening isomerization - Kumada coupling - benzyne

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• 31 4- and 5-Vinyl-1-ethoxybenzocyclobutene via Kumada Coupling 1-Ethoxyvinylbenzocyclobutene was prepared via a Kumada coupling in 50–75% yield as in the following example. A solution of iodinated 1-ethoxybenzocyclobutene (3.7 g, 14 mmol; 1:4 ratio of the 4- and 5-iodo isomers) in anhydrous Et₂O (20 mL) was added dropwise over 30 min to Mg turnings (0.35 g, 14 mmol) in anhydrous THF (25 mL) at r.t. The solution was stirred at r.t. until ¹H NMR analysis of MeOH-quenched aliquots confirmed that the formation of the Grignard reagent was complete (18 h). The solution was then transferred to the glass sleeve of a Parr reactor, followed by the addition of Ni(dppp)Cl₂ (20 mg, 37 μmol) and vinyl bromide (2.0 g, 19 mmol). After stirring at r.t. for 12 h, the mixture was neutralized with 2% aq HCl (50 mL) and then extracted with Et₂O (3 × 25 mL). The combined organic layers were washed with sat. aq NaHCO₃ solution (2 × 30 mL) and H₂O (30 mL), and dried over MgSO₄. The product was purified³² by column chromatography using silica gel as the stationary phase (hexanes–Et₂O, 4:1, R_f = 0.60) to yield 1.5 g (64%) of a colorless oil composed of a 1:4 ratio of the 4- and 5-vinyl-1-ethoxybenzocyclobutene isomers. 5-Vinyl-1-ethoxybenzocyclobutene ¹H NMR (300 MHz, CDCl₃): δ = 1.29 (t, ³ J = 7.0 Hz, 3 H, CH ₃), 3.10 (dd, ² J = 14.4 Hz, ³ J = 1.6 Hz, 1 H, CHHAr), 3.44 (dd, ² J = 14.4 Hz, ³ J = 4.4 Hz, 1 H, CHHAr), 3.66 (dq, ² J = 9.1 Hz, ³ J = 7.0 Hz, 1 H, OCHH), 3.73 (dq, ² J = 9.1 Hz, ³ J = 7.0 Hz, 1 H, OCHH), 5.04 (dd, ³ J = 4.4 Hz, ³ J = 2.0 Hz, 1 H, CHOEt), 5.18 (dd, ³ J _BX = 10.9 Hz, ² J _AB = 0.8 Hz, 1 H, HCH _B,trans =), 5.68 (dd, ³ J _AX = 17.6 Hz, ² J _AB = 0.9 Hz, 1 H, HCH _A,cis =), 6.70 (dd, ³ J _AX = 17.6 Hz, ³ J _BX = 10.9 Hz, 1 H, CH _X=), 7.09 (d, ³ J_ortho = 8.4 Hz, 1 H, aromatic C3H), 7.24 (s, aromatic C6H), 7.35 (d, ³ J_ortho = 8.5 Hz, 1 H, aromatic C4H). Anal. Calcd for C₁₂H₁₄O: C, 82.72; H, 8.10. Found: C, 82.46; H, 8.11.
• 32 Alternatively, an inhibitor such as pyrogallol can be added to the crude product, which can then be purified by distillation at 77 °C/1 mmHg.