Transfer of Alk-1-enyl Group from Boron to Tin: A Highly Stereoselective Synthesis of (E)-Alk-1-enyltributylstannanes

Abstract

Treatment of (E)-alk-1-enyldicyclohexylboranes with tributyltin methoxide in the presence of galvinoxyl (1 mol%) at room temperature results in transfer of the alk-1-enyl group from boron to tin to give (E)-alk-1-enyltributylstannanes in a highly stereoselective fashion. Subsequent halodestannylation of (E)-alk-1-enyltributylstannanes is allowed to proceed in a one-pot manner to produce the corresponding (E)-1-iodoalk-1-enes and (E)-1-bromoalk-1-enes in good to high yields, respectively.

Key words

transfer - (E)-alk-1-enyltributylstannane - (E)-alk-1-enyldicyclohexylborane - tributyltin methoxide - halodestannylation

References and Notes

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34

The ^¹H NMR spectrum of (E)-hex-1-enyltributylstannane (2a) shows the alkenyl protons at δ = 5.86 (d, J = 18.8 Hz) and 5.94 (dt, J = 18.8, 5.0 Hz) ppm.

35

The ^¹H NMR spectrum of (Z)-hex-1-enyltributylstannane (3a) exhibits the alkenyl protons at δ = 5.78 (dt, J = 12.4, 1.0 Hz) and 6.51 (dt, J = 12.4, 7.0 Hz) ppm.

41

The ^¹H NMR spectrum of (E)-1-iodohex-1-ene (4a) shows the alkenyl protons at δ = 5.97 (dt, J = 14.1, 1.0 Hz) and 6.51 (dt, J = 14.1, 7.3 Hz) ppm.

42

The ^¹H NMR spectrum of (Z)-1-iodohex-1-ene (5a) exhibits the alkenyl protons at δ = 6.14-6.19 (m)ppm.

43

To a solution of BH₃˙SMe₂ (1 mmol) in THF (3 mL) was added cyclohexene (0.164 g, 2 mmol) dropwise at 0 ˚C under argon, and the mixture was stirred for 2 h at this temperature to form a white suspension of dicyclohexyl-borane in THF. To this suspension was added hex-1-yne (0.082 g, 1 mmol) dropwise at 0 ˚C, and the mixture was stirred for 2 h at this temperature to produce a clear solution of (E)-hex-1-enyldicyclohexylborane (1a) in THF. To this solution was added galvinoxyl (0.004 g, 0.01 mmol) under a flow of argon, followed by dropwise addition of tributyltin methoxide (0.321 g, 1 mmol) at 0 ˚C. The resulting mixture was allowed to warm to r.t. and stirred for 1 h to generate (E)-hex-1-enyltributylstannane (2a). The solution of 2a, thus prepared, was cooled to -15 ˚C, and a solution of I₂ (0.279 g, 1.1 mmol) in THF (1 mL) was added dropwise. The resulting mixture was allowed to warm to 0 ˚C and stirred for 0.5 h. The reaction mixture was treated with aq Na₂O₃S₂ to remove excess I₂, and then oxidized by the successive addition of 3 M NaOH (1 mL) and 30% H₂O₂ (0.5 mL) at 0 ˚C. After being stirred for 1 h at this temperature, the mixture was extracted three times with Et₂O. The combined extracts were washed with brine and a 10% aq solution of KF, dried over Na₂SO₄, and concentrated. The residue was purified by column chromatography on silica gel, with pentane as eluent, to give (E)-1-iodohex-1-ene (4a, 0.151 g, 72%).

44

(E)-Oct-1-enyltributylstannane (2c) was prepared in the same manner as described in ref. 43 but using oct-1-yne (0.110 g, 1 mmol) instead of hex-1-yne. To the solution of 2c in THF was added a solution of pyridinium tribromide (0.384 g, 1.2 mmol) in THF (2 mL) dropwise at -15 ˚C, and the mixture was allowed to warm to 0 ˚C and stirred for 0.5 h. The workup procedure was the same as described in ref. 43, except for washing with aq Na₂O₃S₂. Elution with pentane gave (E)-1-bromooct-1-ene (6c, 0.134 g, 70%). The ^¹H NMR spectrum of 6c shows the alkenyl protons at δ = 6.01 (d, J = 13.4 Hz) and 6.15 (dt, J = 13.4, 6.4 Hz) ppm.