Palladium Catalyzed Coupling Reaction of an Enol Nonaflate with (Vinyl)tributylstannanes and Acetylenes

Abstract

A novel method for a stereoselective synthesis of trienes and dienynes was developed by palladium catalyzed cross coupling reactions of an enol nonaflate with (vinyl)tributylstannanes and acetylenes in good to excellent yields. Here, the enol nonaflate exhibited a higher reactivity compared with the corresponding enol triflate in the coupling reactions.

Key words

enol nonaflate - palladium catalyst - coupling reaction - alkenyl stannane - acetylene

References

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8

¹H NMR data of enol nonaflate 2a are as follows: (300 MHz, CDCl₃) δ: 0.97 (6 H, s, Me × 2), 1.42-1.50 (2 H, m, CH₂), 1.56-1.62 (2 H, m, CH₂), 1.68 (3 H, s), 2.02 (2 H, t, J = 7 Hz), 6.21 (1 H, d, J = 12 Hz, =CH), 6.46 (1 H, d, J = 16 Hz, =CH).

10

Typical coupling procedure: To a stirred solution of enol nonaflate (2a, 1 mmol),(vinyl)tributylstannane (4, 1.5 mmol), and AsPh₃ (20 mol%, 60 mg) in DMF (2 mL) was added Pd₂dba₃-CHCl₃ adduct (2.5 mol%, 26 mg) all at once under nitrogen. After stirring for an indicated period in Table [1] , the reaction was quenched with saturated aqueous NH₄Cl solution (3 mL) and extracted with diethyl ether (10 mL × 3). The extracts were washed with saturated aqueous NaCl solution (10 mL) and then dried over Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel to afford the coupled product 5.
5a: IR (CHCl₃) cm^-1: 3610, 3446, 2969, 1647; ¹H NMR (300 MHz, CDCl₃) δ: 1.00 (6 H, s, Me × 2), 1.40-1.46 (2 H, m, CH₂), 1.56-1.62 (3 H, m, CH₂ and OH), 1.69 (3 H, s, Me), 2.02 (2 H, t, J = 7 Hz, CH₂), 4.19 (2 H, t, J = 7 Hz, CH₂), 5.78 (1 H, dt, J = 15, 6 Hz, =CH), 6.03 (1 H, dd, J = 16, 10 Hz, =CH), 6.14 (1 H, d, J = 16 Hz, =CH), 6.31 (1 H, ddt, J = 15, 10, 1 Hz, =CH); HRMS (EI) C₁₄H₂₂O: requires 206.1669, found 206.1667.

12

Typical coupling procedure: To a stirred solution of enol nonaflate (2a, 0.5 mmol), acetylene (6, 1 mmol), diisopropylamine (2 mmol, 200 mg), and CuI (15 mol%, 35 mg) in benzene (3 mL) was added Pd(PPh₃)₄ (10 mol%, 58 mg) all at once under nitrogen. After stirring for an indicated period in Table [2] , the reaction was quenched with saturated aqueous NH₄Cl solution (3 mL) and extracted with ether (10 mL × 3). The extracts were washed with saturated aqueous NaCl solution (10 mL) and then dried over Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by column chromatography on a silica gel to produce the coupled product 7.
7a: IR (CHCl₃) cm^-1: 3608, 3450, 2932, 2209, 1604; ¹H NMR (300 MHz, CDCl₃) δ: 1.00 (6 H, s, Me × 2), 1.40-1.47 (2 H, m, CH₂), 1.56-1.61 (3 H, m, CH₂ and OH), 1.70 (3 H, s, Me), 1.99 (2 H, t, J = 6 Hz, CH₂), 4.41 (2 H, d, J = 4 Hz, CH₂), 5.47 (1 H, dt, J = 16.5, 2 Hz, =CH), 6.59 (1 H, d, J = 16 Hz, =CH); HRMS (EI) C₁₄H₂₀O: requires 204.1513, found 204.1520.

13

In the case of ¹³C-labeled aldehyde 1, there was no reappearance of the peak for the conversion to the corresponding enol triflate. On the contrary, there was no trouble in preparation of the enol nonaflate.