An Improved Synthesis of 3,6-Dihydro-as-indacene

Abstract

This contribution describes an updated synthetic route to 3,6-dihydro-as-indacene along with full characterization of all inter­mediates. The title compound is prepared by Mannich condensation of 2-methylfuran with formaldehyde and dimethylamine hydrochloride, quaternization of the resulting amine with methyl iodide, and conversion into the ammonium hydroxide salt by treatment with silver oxide in water. Subsequent Hoffmann elimination and [6,6]-cycloaddition through pyrolysis produces a furanocyclophane, which after photooxidation, intramolecular cycloaddition, and dehydration with sodium carbonate affords 2,3,6,7-tetrahydro-1,8-dione-as-indacene. Reduction of this diketone gives a mixture of alcohols, which after dehydration under slightly basic or acidic conditions produces 3,6-dihydro-as-indacene. The structure is confirmed by X-ray diffraction, and all intermediates are characterized by means of ¹H and ¹³C NMR spectroscopy.

Dihydro-as-indacene[1] is a rigid hydrocarbon that can be doubly deprotonated to give a dianion consisting of two cyclopentadienyl ligands fused with a benzene ring. The reaction of this dianion with either an organometallic derivative or a metal halide can generate bimetallic metallocene-type complexes, where two metal moieties can bond in either cis or trans configurations with respect to the plane of the tricyclic hydrocarbon, depending on the preponderant role of the ancillary ligands.[2] In recent decades there has been growing interest in the synthesis of such species, since they can result in interesting cooperative effects, as well as novel electronic, magnetic and optical properties.[3] [4] [5] [6] [7] [8]

In contrast to dihydro-s-indacene, where along with the non-substituted hydrocarbon,[9] several alkylated derivatives are also known,[10] [11] [12] [13] the lack of an efficient synthetic procedure for dihydro-as-indacene,[1] as well as the reduced number of known alkylated derivatives,[10] [14] has resulted in a rather slow development of its chemistry, despite the fact that the synthesis of the non-substituted compound was reported by Katz in 1968.[1a]

The Katz synthesis is relatively tedious, and at the time of its publication the characterization of the product was poor,[1a] mainly because NMR spectroscopy had not been developed to the extent it has today. Furthermore, the syntheses of several of the intermediates used in the synthetic route have since been optimized by other investigators for different research purposes. Hence, some of these improvements are either included or mentioned in this article.

The present manuscript describes in detail an updated and convenient synthetic route for the preparation of 3,6-dihydro-as-indacene. The title product is obtained in seven steps and in 32% overall yield.

The synthetic steps involved in the preparation of 3,6-dihydro-as-indacene are shown in Scheme [1]. The synthesis started with the Mannich reaction of 2-methylfuran with formaldehyde and dimethylamine hydrochloride to produce the corresponding substituted furfuryl amine, initially reported by Holdren and Hixon,[15] using a method later modified by Katz.[1a] The tertiary amine 1 was quaternized with methyl iodide, and the resulting quaternary ammonium iodide 2 was converted into the corresponding ammonium hydroxide 3 by treatment with silver oxide in water. Subsequently, the ammonium hydroxide 3 was pyrolyzed to give the furanocyclophane 4 according to the procedure[16] employed by Katz. An alternative method for the preparation of 4 has been published by Vandyck et al.[17] in which the water was removed in the presence of a polymerization inhibitor using a Dean–Stark apparatus. Further transformation of furanocyclophane 4 into the diketone 5 was accomplished by photo-oxidation using methylene blue as the sensitizer (under one atmosphere of oxygen) and further dehydration and aromatization with sodium carbonate, according to Katz’s original procedure. A more expeditious approach for this step has also been published,[18] which employs mCPBA for the oxidation of the furan moiety followed by dehydration–aromatization with Na₂CO₃. Using this latter method, we found that isolation and purification of the endoperoxide intermediate (see spectra S9 and S10 in the Supporting Information) was detrimental to the yield.

Following the procedure of Kuppens et al.,[18] reduction of 2,3,6,7-tetrahydro-as-indacene-1,8-dione (5) was carried out with NaBH₄ in methanol to give 1,2,3,6,7,8-hexahydro-as-indacene-1,8-diol (6) as an inseparable mixture of isomers. Dehydration of this mixture of cis- and trans-diols 6 can be performed either under slightly basic conditions with methyltriphenoxyphosphonium iodide in the presence of HMPA, similar to that previously used for s-indacene,[9] or with 1% p-TSA in refluxing benzene or toluene using a Dean–Stark apparatus,[18] to yield exclusively, in both cases, 3,6-dihydro-as-indacene (7). The ¹H NMR spectrum of 3,6-dihydro-as-indacene (7) is shown in Figure [1].

All the intermediates, with the exception of the inseparable mixture of diols 6, were characterized by means of ¹H and ¹³C NMR spectroscopy (see spectra S1–S14 in the Supporting Information).

Crystals of 7 were obtained by sublimation at 80 °C and 10^–3 Torr. A single-crystal X-ray diffraction experiment allowed solving the crystal structure. Each unit cell consisted of eight molecules of 7, with the structure belonging to the monoclinic C2/c space group (see S22 in the Supporting Information). Table [1] lists the crystal data and refinement parameters. Atom coordinates are given in Table S22.2 and bond distances and angles in Tables S22.4 and S22.5, respectively. Anisotropic displacement parameters were used for non-H atoms (Table S22.3) and the H atoms were positioned in calculated positions and refined riding on their parent atoms (Table S22.6).

Figure [2] shows an ORTEP representation of 3,6-dihydro-as-indacene (7) with atom labelling. The molecule of 7 appears almost planar, as can be seen from the selected torsion angles (Table [2]). Table [2] also shows selected bond distances (Å) and bond angles (°). The bond distances for C(1)–C(2) and C(9)–C(10) are 1.3352(16) Å and 1.3366(15) Å, respectively, and are very close to that of a standard double bond, indicating that they are localized double bonds. In comparison, the bond distances for C(1)–C(12), C(2)–C(3), C(3)–C(4), C(7)–C(8), C(8)–C(9) and C(10)–C(11) [1.4600(14), 1.4993(18), 1.5077(15), 1.5058(14), 1.5019(16) and 1.4624(13), respectively] correspond to single bonds, in agreement with the symmetry observed in the NMR analysis. On the other hand, the torsion angles at the hinges [i.e., 1.46(19)°, defined by C1, C12, C11 and C10] are about 1° (Table [2]), being similar to those of the benzene ring [i.e., –1.25(15)° formed by C12, C11, C7 and C6]. Thus, the ligand is almost planar.

In summary, we have simplified the Katz original synthesis[1a] of dihydro-as-indacene by incorporating several improvements for the preparation of some of the intermediates (as carried out by other investigators with different research purposes). In addition, we have solved the crystal structure of one of the double bond isomers of 3,6-dihydro-as-indacene. This more expedient access to dihydro-as-indacene may help to expand the number of organometallic derivatives, as well as the chemistry of as-indacene.

All manipulations were carried out under an atmosphere of dry N₂ using standard Schlenk techniques or in a glove box under N₂. Solvents were pre-dried and distilled via standard techniques using appropriate drying agents.[15] 2-Methylfuran, formaldehyde, dimethyl amine hydrochloride, methyl iodide, methyltriphenoxyphosphonium iodide and hexamethylphosphoramide (HMPA) were obtained from Aldrich, and used as purchased.

IR spectra were obtained using a Shimadzu model Tracer-100 spectrometer. NMR samples were prepared in a dry glovebox, and the sample tubes were sealed with septa. ¹H and ¹³C NMR spectra were recorded on a Bruker AC-400 spectrometer. Chemical shifts are reported in parts per million relative to TMS, using residual solvent peaks as reference and were assigned using 2D NMR tools. All reported signals are singlets unless otherwise specified. High-resolution mass spectrometry was carried out using an ExactiveÔ Plus Orbitrap, Thermo Fisher Scientific (Bremen, Germany). Scan parameters: resolution = 140000, AGC target = 1e6, max. inject time = 200 seconds HESI source: sheath gas flow = 25, aux gas flow rate = 3, sweep gas flow rate = 0, capillary temp = 250 °C, S-lens RF level = 0, heater temp = 50 °C. Elemental analyses (C and H) were obtained with a Fisons EA 1108 Microanalyzer.

The X-ray diffraction experiment was performed on a Bruker Smart Apex II diffractometer at 294 K using monochromatic MoKa radiation (wavelength = 0.71073 Å). Frames taken with a 0.3° separation afforded 6759 reflections up to a 2q max of ca. 61°. Data integration was performed using SAINT V6.45A and SORTAV[20] in the diffractometer package. The crystal and collection data and structural refinement parameters are listed in Table [1]. The structure was solved by direct methods using SHELXT-2014[21] and the Fourier difference method, and refined by least squares on F2 using SHELXL-2014/7[21] inside the WinGX program environment.[22]

N,N-Dimethyl-1-(5-methylfuran-2-yl)methanamine (1)[1] [19]

To a solution of dimethylamine hydrochloride (163.0 g, 2.0 mol) and 37% aqueous formaldehyde (162.0 g, 2.0 mol) at 30 °C in a three-necked round-bottom flask equipped with a dropping funnel, a magnetic stir bar and a reflux condenser was added 2-methylfuran (82 g, 1.0 mol) dropwise over 4 h, and stirring was continued until evolution of heat had ceased completely. The mixture was cooled to room temperature and a solution of sodium hydroxide (81.0 g) in water (160 mL) was added. The organic layer was separated and the aqueous phase was extracted with diethyl ether (300 mL). The combined organic phase was washed with water (2 × 150 mL), dried over anhydrous sodium sulfate and evaporated under vacuum. The residue was distilled under reduced pressure to give the title compound as a colorless oil which solidified on standing.

Yield: 112.0 g (80%); bp 71–78 °C (@17 Torr); mp 133–135 °C.

¹H NMR (400 MHz, CDCl₃): δ = 5.87 (dt, J = 15.65, 3.12 Hz, 1 H), 5.71 (dd, J = 12.67, 6.89 Hz, 1 H), 3.24–3.17 (m, 2 H), 2.11 (d, J = 9.65 Hz, 2 H), 2.12–2.03 (m, 6 H).

¹³C NMR (101 MHz, CDCl₃): δ = 151.40, 150.24, 108.88, 105.57, 55.76, 44.68, 13.31.

N,N,N-Trimethyl-1-(5-methylfuran-2-yl)methanammonium Iodide­ (2)

To a round-bottomed flask (1 L) containing a large magnetic stir bar were added compound 1 (70.0 g, 0.50 mol) and diethyl ether (250 mL). Methyl iodide (96.0 g, 0.68 mol) was added at 0 °C with stirring. After about 2 h, a precipitate was observed to start forming, but stirring was continued overnight at the same temperature. The resulting precipitate was filtered, washed with diethyl ether and dried under vacuum. Ammonium salt 2 was obtained as a white powder.

Yield: 130.3 g (93%); mp 120–122 °C.

¹H NMR (400 MHz, D₂O): δ = 6.77 (d, J = 3.30 Hz, 1 H), 6.24 (d, J = 3.31, 1.27 Hz, 1 H), 4.53 (s, 2 H), 3.14 (s, 9 H), 2.37 (s, 3 H).

¹³C NMR (101 MHz, D₂O): δ = 156.32, 140.56, 117.78, 107.25, 61.85, 52.32, 12.74.

HRMS (ESI): m/z [M – I]⁺ calcd for C₉H₁₆NO: 154.1232; found: 154.1205.

N,N,N-Trimethyl-1-(5-methylfuran-2-yl)methanammonium Hydroxide­ (3)

Compound 2 (160.0 g, 0 60 mol) was treated with a solution of silver oxide (81.0 g, 0.35 mol) in water (250 mL) with vigorous stirring for 3 h. The resulting silver iodide precipitate was removed by filtration and washed with water (3 × 50 mL). The water was removed from the solution by vacuum distillation (30 °C, 0.01 Torr) to afford a hygroscopic solid product that was stored in a desiccator until the next step.

Yield: 85.0 g (90%); mp 162–164 °C.

¹H NMR (400 MHz, D₂O): δ = 6.79 (d, J = 3.20 Hz, 1 H), 5.99 (d, J = 3.18 Hz, 1 H), 4.86 (s, 2 H), 3.35 (s, 9 H), 2.24 (s, 3 H).

¹³C NMR (101 MHz, D₂O): δ = 156.32, 140.56, 117.78, 107.25, 61.85, 52.32, 12.74.

HRMS (ESI): m/z [M – OH]⁺ calcd for C₉H₁₆NO: 154.1232; found: 154.1225.

(13,14-Dioxatricyclo[8.2.1.1^4,7]tetradeca-1(12),4,6,10-tetraene)- (Furanocyclophane) (4)

Compound 3 (66.0 g, 0.35 mol) was pyrolyzed by heating under vacuum in an oil bath (150–155 °C, 0.2 Torr) for 3 h. The volatile product was trapped at –80 °C in a solution of hydroquinone (0.60 g) in ethanol (50 mL). This solution from the cold trap was boiled, filtered through Grade 1 filter paper, and washed with cold ethanol to yield a crude white solid. The solid was crystallized from CH₂Cl₂ to give a off-white solid.

Yield: 28.0 g (71%); mp 191–192 °C.

¹H NMR (400 MHz, CDCl₃): δ = 6.09 (s, 4 H), 2.85–2.75 (m, 4 H), 2.73–2.62 (m, 4 H).

¹³C NMR (101 MHz, CDCl₃): δ = 157.16, 108.06, 31.10.

HRMS (ESI): m/z [M]⁺ calcd for C₁₂H₁₂O₂: 188.0837; found: 188.0929.

2,3,6,7-Tetrahydro-as-indacene-1,8-dione (5)

Compound 4 (5 g, 0.026 mol) in methanol (1 L) and methylene blue (0.65 mg) were added to a 2-necked round-bottomed flask. The resulting solution was stirred and irradiated with a 650 W lamp under oxygen (1 atm). The reaction was monitored for 2 h by TLC until the starting material had disappeared. The solvent was evaporated in vacuo (to an approximate volume of 250 mL, or one fourth of the original volume). The concentrate was transferred to a beaker (500 mL) and treated with potassium iodide (10 g, 0.060 mol) in water (12 mL) and acetic acid (4 mL), and left to stir for 2 h at room temperature during which time a brown solution was obtained. Sodium thiosulfate was added in small portions until the color of iodine had disappeared completely and the solution had turned green. At this point, Na₂CO₃ (23 g, 0.22 mol) in water (15 mL) was added and the reaction was stirred for 30 min (in order to neutralize the acetic acid present), giving the unstable endoperoxide intermediate 2,3,6,7,8a,8b-hexahydro-3a,5a-epoxy-as-indacene-1,8-dione, which was used in the next step without purification (a small sample was purified for characterization purposes, see S9 and S10 in the Supporting Information). To complete the dehydration, a suspension of sodium carbonate (23 g of Na₂CO₃ in 100 mL of water) was added and the mixture was stirred for an additional 1 h at room temperature. Water (several portions to complete a total of 100 mL) was added gradually to the solution to dissolve the excess carbonate. The product was extracted with chloroform (3 × 50 mL) and the combined organic phases were dried for 24 h over anhydrous sodium sulfate. The solvent was evaporated under vacuum and the blue residue was purified by chromatography on silica gel (eluent: chloroform) to give the pure product 5 as a brown solid.

Yield: 3.6 g (81%), mp 206–207 °C.

¹H NMR (400 MHz, CDCl₃): δ = 7.62 (s, 2 H), 3.19–3.11 (m, 4 H), 2.75–2.67 (m, 4 H).

¹³C NMR (101 MHz, CDCl₃): δ = 203.63, 156.35, 134.64, 132.38, 36.85.

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₂H₁₁O₂: 187.0681; found: 187.0763.

1,2,3,6,7,8-Hexahydro-as-indacene-1,8-diol (6)

A suspension of compound 5 (3.6 g, 0.021 mol) in methanol (119 mL) was stirred at 0 °C, and then NaBH₄ (0.81 g, 0.021 mol) was added in several portions, at such a rate that the internal temperature did not exceed 10 °C. The resulting mixture was stirred at room temperature for 2 h, after which it was carefully poured into H₂O (300 mL) and ethyl acetate (320 mL). The aqueous layer was separated and extracted with ethyl acetate (480 mL). The organic extract was washed with saturated aqueous sodium chloride, dried over anhydrous Na₂SO₄, filtered and evaporated to dryness under vacuum. An inseparable mixture of yellow solid isomeric alcohols was obtained which was used in the next step without further purification.

Yield: 3.5 g (97%); mp 110–111 °C.

IR (KBr): 3336 (OH) cm^–1.

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₂H₁₅O₂: 191.0994; found: 191.1072.

3,6-Dihydro-as-indacene (7)

(a) Methyltriphenoxyphosphonium Iodide/HMPA

Compound 6 (3.0 g, 0.016 mol) was heated with methyltriphenoxyphosphonium iodide (44 g) in dry HMPA (130 mL) in a one-necked flask containing a magnetic stir bar. The well-stirred mixture was heated at 75 °C for 3 h, cooled down to room temperature, diluted with 2 M potassium hydroxide (850 mL), extracted with cyclohexane (3 × 100 mL), and sequentially washed with water and brine. Drying over anhydrous MgSO₄ and evaporation of the solvent under vacuum gave a yellow solid. The solid was purified by chromatography on silica gel (eluent: n-hexane) to give the product as a yellow crystalline solid.

Yield: 2.2 g (85%); mp 61–62 °C.

IR (KBr): 3048 (C–H, Ar), 2898 (C–H, CH₂), 1608 (C=C, five-membered ring), 1704 (C=C, Ar), 891 (C–H, Ar).

¹H NMR (400 MHz, CDCl₃): δ = 7.23 (s, 2 H), 7.00 (dt, J = 5.59, 1.94 Hz, 2 H), 6.50 (dt, J = 5.60, 1.97 Hz, 2 H), 3.35 (t, J = 2.00 Hz, 4 H).

¹³C NMR (101 MHz, CDCl₃): δ = 142.04, 137.53, 134.12, 130.04, 119.87, 39.05.

(b) p-Toluenesulfonic Acid in Refluxing Benzene or Toluene

Compound 6 (3.5 g, 0.018 mol) was dissolved in either dry benzene or toluene (200 mL) in the presence of p-TSA·H₂O (51 mg, 0.268 mmol) in a flask fitted with a Dean–Stark apparatus and a condenser. The mixture was refluxed for 3 h until TLC analysis showed that no starting material remained. The mixture was cooled to room temperature and poured over aqueous NaHCO₃ (160 mL, 5% solution). The organic layer was separated, dried over anhydrous Na₂SO₄, filtered and evaporated to dryness under vacuum. The slightly yellowish solid residue was dissolved in n-hexane/dichloromethane (2:1) and purified by chromatography on silica gel (eluent: n-hexane) to give the product as a yellow crystalline solid.

Yield: 2.4 g (83%).

• References

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Corresponding Author

Publication History

Received: 11 June 2020

Accepted after revision: 11 August 2020

Article published online:
02 September 2020

© 2020. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

• References

• 1a Katz TJ, Balogh V, Schulman J. J. Am. Chem. Soc. 1968; 90: 734
  CrossrefSearch in Google Scholar
• 1b Hafner K. Angew. Chem., Int. Ed. Engl. 1964; 3: 165
  CrossrefSearch in Google Scholar
• 1c Erden I, Xu FP, Sadoun A, Smith W, Sheff G, Ossun M. J. Org. Chem. 1995; 60: 813
  CrossrefSearch in Google Scholar
• 1d Wightman RH, Wain RJ, Lake DH. Can. J. Chem. 1977; 49: 1360
  CrossrefPubMedSearch in Google Scholar
• 2 Ceccon A, Bisello A, Crociani L, Gambaro A, Ganis P, Manoli F, Santi S, Venzo A. Organomet. Chem. 2000; 600: 94
  CrossrefSearch in Google Scholar
• 3 Ceccon A, Santi S, Orian L, Bisello A. Coord. Chem. Rev. 2004; 248: 683
  CrossrefPubMedSearch in Google Scholar
• 4 Manríquez JM, Ward MD, Reiff WM, Calabrese JC, Jones NL, Carroll PJ, Bunel EE, Miller JS. J. Am. Chem. Soc. 1995; 117: 6182
  CrossrefPubMedSearch in Google Scholar
• 5 Santi S, Orian L, Durante C, Bencze EZ, Bisello A, Donoli A, Ceccon A, Benetollo F, Crociani L. Chem. Eur. J. 2007; 13: 7933
  CrossrefPubMedSearch in Google Scholar
• 6 Santi S, Ceccon A, Carli F, Crociani L, Bisello A, Tiso M, Venzo A. Organometallics 2002; 21: 2679
  CrossrefPubMedSearch in Google Scholar
• 7 Aguirre-Etcheverry P, O’Hare D. Chem. Rev. 2010; 110: 4839
  CrossrefPubMedSearch in Google Scholar
• 8 Iijma S, Motoyama I, Sano H. Chem. Lett. 1979; 1349
  Crossref
• 9 Trogen L, Ulf E. Acta Chem. Scand. 1979; B33: 109
  CrossrefSearch in Google Scholar
• 10 Bell WL, Curtis CJ, Eigenbrot CW, Pierpont CG, Robbins JL, Smart JC. Organometallics 1987; 6: 266
  Thieme ConnectSearch in Google Scholar
• 11 Dahrouch MR, Jara PL, Portilla Y, Abril D, Alfonso G, Chavez I, Manriquez JM, Riviere BM, Castel A. Organometallics 2011; 20: 5591
  CrossrefSearch in Google Scholar
• 12 Barlow S, O’Hare D. Organometallics 1996; 15: 3483
  CrossrefSearch in Google Scholar
• 13 Ceccon A, Crociani L, Santi S, Venzo A, Biffisc A, Boccaletti G. Tetrahedron Lett. 2002; 43: 8475
  CrossrefSearch in Google Scholar
• 14 Faúndez R, Castillo F, Preite M, Schott E, Zarate X, Manriquez JM, Molins E, Morales-Verdejo C, Chávez I. Synthesis 2019; 51: 441
  CrossrefPubMedSearch in Google Scholar
• 15 Holdren RF, Hixon RM. J. Am. Chem. Soc. 1946; 68: 1198
  CrossrefSearch in Google Scholar
• 16 Wasserman H. H., Doumaux A. R. Jr. 1962; 84: 4611
  CrossrefPubMedSearch in Google Scholar
• 17 Vandyck K, Matthys B, Van der Eycken J. Tetrahedron Lett. 2005; 46: 75
  CrossrefPubMedSearch in Google Scholar
• 18 Kuppens T, Vandyck K, Van der Eycken J, Herrebout W. J. Org. Chem. 2005; 70: 9103
  CrossrefPubMedSearch in Google Scholar
• 19 Armarego WL. F, Chai CL. L. In Purification of Laboratory Chemicals, 6th ed. Elsevier; Oxford: 2009
  CrossrefSearch in Google Scholar
• 20 Blessing RH. Acta Cryst. 1995; A51: 33
  CrossrefPubMedSearch in Google Scholar
• 21 Sheldrick GM. Acta Cryst. 2015; C71: 3
  CrossrefSearch in Google Scholar
• 22 Farrugia LJ. J. Appl. Cryst. 2012; 45: 849
  CrossrefSearch in Google Scholar

• Supplementary Material