Abstract
1-Trimethylgermyl-1-alkynes easily prepared by deprotonation of terminal alkynes with n -butyllithium followed by treatment with trimethylgermanium chloride, readily react with dicyclohexylborane (freshly prepared by reacting cyclohexene with borane-methyl sulfide complex) in tetrahydrofuran. The resulting (Z )-1-trimethylgermyl-1-alkenyl dicyclohexylboranes are readily protonolysed in n -pentane by acetic acid. The monoethanolamine workup, followed by vacuum distillation provides the corresponding (Z )-1-trimethylgermyl-1-alkenes in high stereochemical purities (98%) and in excellent yields (78-92%).
Key words
diastereoselective - alkynes -
n -butyllithium - dicyclohexylborane - protonolysis
References
1 Huang X.
Liang CG.
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2000,
30:
1903 ; and references cited therein
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Ayyangar NR.
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Nelson DJ.
Bhat NG.
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5 Z )-alkenes can be distinguished from the corresponding carbons of (E )-alkenes. Dorman DE.
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6
The Preparation of 1-Trimethylgermyl-1- hexane is Representative : In an oven-dried 100 mL round bottom flask equipped with a magnetic stirring bar and a septum inlet, 1-hexyne (50 mmol) was reacted with n -butyllithium (20 mL, 2.5 M solution in hexane) at -78 °C for 1 h. To this reaction mixture, trimethylgermanium chloride (50 mmol) was added slowly via syringe while stirring and allowed to stir at -78 °C for an additional hour. The reaction was then allowed to stir overnight at r.t. The resulting product was isolated and the structure of 1-trimethylgermyl-1-hexyne was confirmed by NMR data. An 84% yield was obtained for 1-trimethylgermyl-1-hexyne. IR(neat): ν = 1240, 2172, 2865, 2874, 2913, 2934, 2959 cm-1 . 1 H NMR (CDCl3 /without TMS): δ = 0.30 (s, 9 H), 0.89 (m, 3 H), 1.47 (m, 4 H), 2.20 (m, 2 H). 13 C NMR (CDCl3 /without TMS): δ = 0.02, 13.68, 19.60, 21.99, 31.05 (alkyl C), 83.82, 106.16 (alkynyl C) ppm.
7
The Preparation of (
Z
)-1-Trimethylgermyl-1-hexene is Representative : In an oven-dried 100 mL round bottom flask equipped with a magnetic stirring bar and a septum inlet, tetrahydrofuran (50 mL) was added using a double-ended needle under nitrogen atmosphere. Cyclohexene (100 mmol) was also added to the flask using the same technique. The flask was cooled to 0 °C using an ice-water bath followed by the dropwise addition of BMS (27.8 mL, 1.8 M solution in tetrahydrofuran). The reaction mixture was allowed to stir for 2 h. 1-Trimethygermyl-1-hexyne (50 mmol) was added to the mixture via syringe through the septum inlet and allowed to stir at 0 °C for 3 h followed by 1 h at r.t. The resulting solution was treated with HOAc (50 mmol) via syringe and stirred for 6 h at r.t. All of the tetrahydrofuran was completely removed under vacuum and replaced with n -pentane (ca. 50 mL). Monoethanolamine (50 mmol) was added dropwise to remove the boron by-product. Shortly upon the addition of monoethanolamine, a white precipitate formed inside the flask. The supernatant solution was isolated by filtration over glasswool and Na2 SO4 . A preliminary thin-layer chromatography plate was run to ascertain the purity of the compound. The solution was evaporated to remove the n -pentane and was then chromatographed over alumina. Pentane (10 mL) was used to wash the solution into the column and then to elute the column followed by a 2% acetone in pentane (100 mL) mixture. The solution was further purified using high vacuum distillation. An 84% yield was obtained for (Z )-1-trimethylgermyl-1-hexene. IR(neat): ν = 1263, 1650, 1700, 2335, 2360, 2928, 2968 cm -1 . 1 H NMR (CDCl3 /without TMS): δ = 0.26 (s, 9 H), 0.91 (m, 5 H), 1.36 (m, 6 H), 2.11 (m, 2 H), 5.64 (m, 1 H), 6.31 (m, 1 H). 13 C NMR (CDCl3 /without TMS): δ = -0.22, 14.08, 22.43, 32.13, 33.45 (alkyl C), 130.2, 146.9 (alkenyl C) ppm.
