A Highly Diastereoselective Synthesis of (Z)-1-Trimethylgermyl-1-alkenes via the Hydroboration of 1-Trimethylgermyl-1-alkynes Followed by Protonolysis

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. Synth. Commun.  2000,  30:  1903 ; and references cited therein
• 2 Zweifel G. Arzoumanian H. J. Am. Chem. Soc.  1967,  89:  5086 
• 3 Zweifel G. Ayyangar NR. Brown HC. J. Am. Chem. Soc.  1963,  85:  2072 
• 4 Brown HC. Blue CD. Nelson DJ. Bhat NG. J. Org. Chem.  1989,  54:  6064 
• 5 In a mixture of isomers, the vinylic carbons of the (Z)-alkenes can be distinguished from the corresponding carbons of (E)-alkenes. Dorman DE. Jautelat M. Roberts JD. J. Org. Chem.  1971,  36:  2757 

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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. ¹H 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). ¹³C NMR (CDCl₃/without TMS): δ = 0.02, 13.68, 19.60, 21.99, 31.05 (alkyl C), 83.82, 106.16 (alkynyl C) ppm.

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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 Na₂SO₄. 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. ¹H 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). ¹³C NMR (CDCl₃/without TMS): δ = -0.22, 14.08, 22.43, 32.13, 33.45 (alkyl C), 130.2, 146.9 (alkenyl C) ppm.