Electrocyclization Reactions of Annulated 1,2-Diazahepta-2,4-dien-6-ynyl Anions

Abstract

Hydrazones 12a,b,f furnish annulated indenone derivatives (6-azafulvenes) 13a-d upon deprotonation with t-BuOK in DMF at 50 °C. In contrast, deprotonation of a similar N-methyl ­hydrazone 12d unexpectedly leads to the isoquinoline derivative 15. Mechanisms for both transformations are proposed, supported by quantum chemical density functional theory (DFT) and ab initio calculations.

Key words

anionic electrocyclic reactions - indenes - heterocycles - hydrazones - quinolines - 6-azafulvenes

References and Notes

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• 17b

  Details of the quantum chemical calculations may be obtained from E.-U. Würthwein upon request.

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7

Preparation of 1-[2-(Hex-1-ynyl)benzylidene]-2-phenyl-hydrazine (12a): A mixture of 2-(1-hexynyl)benz-aldehyde (0.93 g, 5.00 mmol) and phenylhydrazine (0.54 g, 5.00 mmol) in anhyd CH₂Cl₂ (20 mL) was stirred in the presence of molecular sieves (4 Å) at r.t. for 16 h. Molecular sieves were removed by filtration through Celite and washed with CH₂Cl₂ (20 mL). The solvent was removed in vacuo to afford the hydrazone 12a (1.10 g, 4.00 mmol, 80%) as a yellow oil. The obtained hydrazone was pure enough to be used without further purification. FTIR (film): 3313 (br, w), 2931 (s), 2871 (s), 2862 (m), 2225 (m), 1575 (m), 1494 (s), 1448 (m), 1257 (s), 1145 (m) cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 0.98 (t, J = 7.3 Hz, 3 H), 1.47-1.68 (m, 4 H), 2.48 (t, J = 7.4 Hz, 2 H), 6.87 (tt, J = 7.3, 1.2 Hz, 1 H), 7.11-7.14 (m, 2 H), 7.19 (dd, J = 7.4, 1.4 Hz, 1 H), 7.25-7.30 (m, 3 H), 7.38 (dm, J = 7.8 Hz, 1 H), 7.72 (br s, 1 H, NH), 8.04 (dm, J = 8.0 Hz, 1 H), 8.19 (s, 1 H, CH=N). ¹³C NMR (100 MHz, CDCl₃): δ = 13.7, 19.4, 22.2, 30.9, 78.1, 95.7, 112.8, 120.2, 122.4, 124.5, 127.8, 127.9, 129.3, 132.6, 136.0, 136.1, 144.6. HRMS (ESI): m/z calcd for C₁₉H₂₀N₂H: 277.1705; found: 277.1699. Anal. Calcd for C₁₉H₂₀N₂: C, 82.57; H, 7.29; N, 10.14. Found: C, 82.87; H, 7.48; N, 9.57.

8

Preparation of 1-(2-Butyl-1 H -inden-1-ylidene)-2-phenylhydrazine (13a): Hydrazone 12a (0.28 g, 1.00 mmol) was added under argon to the solution of t-BuOK (0.22 g, 2.00 mmol) in DMF (5 mL) and the mixture was stirred at 50 °C until all starting hydrazone had disappeared (approx. 5 h, TLC monitoring). H₂O (20 mL) was added and the mixture was extracted with Et₂O (3 × 20 mL). The combined organic extracts were dried with MgSO₄ and the residue was purified by flash chromatography (Et₂O-pentane-Et₃N, 0.5:10:0.2) giving 13a (0.07 g, 0.25 mmol, 25%) as a yellowish oil. FTIR (film): 2956 (s), 2927 (s), 1600 (s), 1566 (s), 1504 (s), 1494 (s), 1255 (s), 1207 (s), 1168 (s), 1151 (w), 1122 (m), 1093 (w) cm^-1. ¹H NMR (500 MHz, CDCl₃): δ = 1.00 (t, J = 7.1 Hz, 3 H), 1.43-1.52 (m, 2 H), 1.68-1.75 (m, 2 H), 2.66-2.69 (m, 2 H), 6.56 (s, 1 H), 7.03 (tt, J = 7.3, 1.0 Hz, 1 H), 7.17-7.40 (m, 7 H), 7.68 (d, J = 7.2 Hz, 1 H), 8.92 (s, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 14.1, 22.7, 26.2, 31.4, 113.6, 121.0, 121.8, 122.7, 124.9, 126.1, 127.4, 128.8, 129.4, 143.9, 144.0, 144.1, 144.3. HRMS (ESI): m/z calcd for C₁₉H₂₀N₂H: 277.1705; found: 277.1699. Anal. Calcd for C₁₉H₂₀N₂: C, 82.57; H, 7.29; N, 10.14. Found: C, 82.64; H, 7.54; N, 9.62.

9

CCDC 658782 contains the supplementary crystallographic data of picrate 13d·2,4,6-(O ₂ N) ₃ C ₆ H ₂ ( OH). These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: [phone], E-mail: [e-mail]].

10

Preparation of 3-Butylisoquinoline (15): Hydrazone 12d (0.21 g, 1.00 mmol) was added under argon to the solution of t-BuOK (0.22 g, 2.00 mmol) in DMF (5 mL) and the mixture was stirred at r.t. until all starting hydrazone had disappeared (approx. 0.5 h, TLC monitoring). H₂O (20 mL) was added and the mixture was extracted with Et₂O (3 × 20 mL). The combined organic extracts were dried with MgSO₄ and the residue was purified by flash chromatography (tert-butyl methyl ether) giving 15 (0.09 g, 0.47 mmol, 47%) as a yellow oil. All spectroscopic and physical data were in accordance with published data (see ref. 11).