Platinum- and Palladium-Catalyzed Sequential Reactions: Regioselective Synthesis of 9-Fluorenylidenes from 9-Ethynylfluoren-9-yl Carboxylates and Furans

Abstract

The transformation of 9-acyloxy-9-ethynylfluorene and furans with platinum and palladium co-catalysts gave 9-fluor­enylidenes in excellent yields with high regioselectivity in a one-pot manner.

Key words

carbene - ring opening - fluorene - platinum - palladium

References and Notes

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11

General Procedure of Ring-Opening Reaction of 2-Methylfuran In a flame-dried Schlenk tube, PtCl₂ (6.5 mg, 0.025 mmol, 5 mol%) was dispersed in toluene (2.0 mL). To the solution were added 1a (124 mg, 0.50 mmol) and 2-methylfuran (90 µL, 1.0 mmol) at r.t. The mixture was stirred at 50 ˚C for 7 h. The yellow suspension was cooled and the solvent was removed under reduced pressure to give 2a. To remove the platinum catalyst, the crude 2a was dissolved in THF (30 mL) and the solution was passed through a short Florisil column. The THF solution was evaporated under reduced pressure to give 2a (99%, Z,Z/Z,E = 63:37). In the case of [RuCl₂(CO)₃]₂, the THF solution obtained after a Florisil column was evaporated in vacuo, and the residue was purified by centrifuge with hexane and a small amount of THF to give a first crop of (E,E)-2a (ca. 65% yield). The hexane and THF solution containing a small amount of (E,E)-2a was evaporated in vacuo, and the residue was purified by column chromatography on SiO₂ with hexane-EtOAc (v/v = 4:1) to give a second crop of (E,E)-2a (ca. 10% yield). The yield in Scheme  [²] was obtained by combining first and second crops of (E,E)-2a.

12

Analytical Data of ( E , E )-2a, ( Z , Z )-2a and ( Z , E )-2a Compound (E,E)-2a: yield 74%; a yellow solid; mp 203.7-204.1 ˚C. IR (KBr): 3054, 1752 (C=O), 1654, 1198, 1176, 728 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 2.34 (s, 3 H), 2.51 (s, 3 H), 6.32 (d, J = 15.2 Hz, 1 H), 6.63 (dd, J = 11.2, 14.6 Hz, 1 H), 7.25-7.34 (m, 5 H), 7.63 (d, J = 14.6 Hz, 1 H), 7.68 (d, J = 8.0 Hz, 1 H), 7.70 (d, J = 8.0 Hz, 1 H), 7.77 (d, J = 8.0 Hz, 1 H), 7.79 (d, J = 8.0 Hz, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 21.1, 27.6, 119.9, 120.2, 125.2, 125.3, 127.3, 127.7, 128.8, 129.2, 130.8, 131.1, 132.3, 132.7, 135.6, 136.7, 140.3, 140.9, 141.6, 144.8, 168.2, 198.0. Anal. Calcd for C₂₂H₁₈O₃: C, 79.98, H, 5.49. Found: C, 79.87; H, 5.66.
Compounds (Z,Z)-2a and (Z,E)-2a: yield 99%; a yellow solid. ^¹H NMR (400 MHz, CDCl₃): δ [(Z,Z)-2a] = 2.27 (s, 3 H), 2.38 (s, 3 H), 6.14 (d, J = 11.7 Hz, 1 H), 7.04 (dd, J = 11.2, 11.7 Hz, 1 H), 7.04 (d, J = 11.7 Hz, 1 H), 7.22-7.37 (m, 4 H), 7.64-7.70 (m, 4 H), 7.77-7.80 (m, 1 H); δ [(Z,E)-2a] = 2.27 (s, 3 H), 2.56 (s, 3 H), 6.17 (d, J = 11.2 Hz, 1 H), 6.68 (dd, J = 11.2, 11.2 Hz, 1 H), 7.22-7.37 (m, 4 H), 7.49 (d, J = 15.1 Hz, 1 H), 7.64-7.70 (m, 2 H), 7.81 (d, J = 8.0 Hz, 2 H), 7.97 (dd, J = 11.7, 15.1 Hz, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 21.1, 21.2, 31.9, 31.9, 119.8, 119.8, 120.0, 125.0, 125.1, 125.1, 125.4, 126.6, 127.1, 127.1, 127.4, 127.6, 127.7, 128.4, 128.5, 128.7, 128.9, 129.0, 130.2, 130.5, 130.8, 132.9, 135.7, 135.7, 136.4, 136.5, 136.8, 140.0, 140.1, 140.3, 140.7, 140.8, 144.0, 145.6, 167.7, 168.4, 198.5, 198.6 (two carbon peaks could not be distinguished with other aromatic carbons).

13

We reported that the Ru-catalyzed reaction of 3-methylbut-1-yn-3-yl acetate with 2-methylfuran gave the corresponding Z,E-isomer regioselectively. [9b] Although we cannot explain the difference at present, we presume that the reaction of 1a using the ruthenium catalyst would proceed via a completely different pathway.

15

General Procedure of Isomerization Reaction In a flame-dried Schlenk tube, a mixture of (Z,Z)-2a and (Z,E)-2a (66 mg, 0.20 mmol, Z,Z/Z,E = 79:21) and a transition-metal compound (0.010 mmol) were dispersed in toluene (2.0 mL). The mixture was stirred at 50 ˚C for 7 h. The yellow suspension was cooled, and the solvent was removed under reduced pressure. The purification step using column chromatography on SiO₂ was necessary for Pd(PPh₃)₄-catalyzed reaction. PdCl₂ was purchased from Wako Pure Chemicals Inc. (Japan) and used without further purification.

16

Without PtCl₂, no (E,E)-2a was detected, and 1a was recovered in 70% yield.

17

Although THF is one of the suitable solvents for the isomerization reaction of 2a, the reaction of 1a and
2-methylfuran with PtCl₂/PdCl₂ in THF gave (E,E)-2a selectively but in a lower yield than that in toluene because of unidentified byproduct formation.

18

Since the reaction using [RuCl₂ (CO)₃]₂ gave (E,E)-2a regioselectively, as shown in Scheme  [²] , we also examined co-catalytic system using [RuCl₂ (CO)₃]₂ and PtCl₂. When the reaction of 1a with 2-methylfuran (2 equiv) in 5 mol% of PtCl₂ and 5 mol% of [RuCl₂(CO)₃]₂ was carried out at 50 ˚C, we obtained 2a with a mixture of three regioisomers (Z,Z/Z,E/E,E = 27:28:45) in 93% yield. This indicates that the ruthenium complex has comparatively lower catalytic activity for isomerization reaction than palladium compounds shown in Table  [¹] .

19

General Procedure of Ring-Opening Reaction and Sequential Isomerization Reaction
In a flame-dried Schlenk tube, PtCl₂ (1.3 mg, 0.0050 mmol) and PdCl₂ (0.9 mg, 0.0050 mmol) were dispersed in toluene (4.0 mL). To this solution were added 1a (124 mg, 0.50 mmol) and 2-methylfuran (50 µL, 0.55 mmol) at r.t. The mixture was stirred at 50 ˚C for 7 h. The yellow suspension was cooled, and the solvent was removed under reduced pressure to give 2a. To remove the transition-metal catalysts, the crude 2a was dissolved in THF (30 mL), and the solution was passed through a short Florisil column. The THF solution was evaporated under reduced pressure to give (E,E)-2a in 99% yield. For reactions using other furans, the THF solution obtained after a Florisil column was evaporated in vacuo, and the residue was purified by centrifuge with hexane and a small amount of THF to give a first crop of (E,E)-2. The hexane and THF solution containing a small amount of (E,E)-2 was evaporated in vacuo, and the residue was purified by column chromatography on SiO₂ with hexane-EtOAc (v/v = 4:1) to give a second crop of (E,E)-2. The yield in Table  [²] was obtained by combining first and second crops of (E,E)-2. The analytical data of (E,E)-2 are summarized in the Supporting Information.