1‐Methyl‐1‐(trimethylsilyl)allene

(1a; R3 = Me3, R1 = Me, R2 = H)4b,9 [74542-82-8] C7H14Si (MW 126.30) InChI = 1S/C7H14Si/c1-5-6-7-8(2,3)4/h5,7H,1-4H3 InChIKey = SBPLHANFNZGLJQ-UHFFFAOYSA-N (1b; R3 = t-BuMe2, R1 = Me, R2 = H)7 [99035-24-2] C10H20Si (MW 168.39) InChI = 1S/C10H20Si/c1-8-9(2)11(6,7)10(3,4)5/h1H2,2-7H3 InChIKey = ZDJOEJQCVQKQAK-UHFFFAOYSA-N (1c; R3 = (i-Pr)3, R1 = Me, R2 = H)7 [120789-53-9] C13H26Si (MW 210.48) InChI = 1S/C13H26Si/c1-9-13(8)14(10(2)3,11(4)5)12(6)7/h10-12H,1H2,2-8H3 InChIKey = ZLYUYKJQRQJIBU-UHFFFAOYSA-N (1d; R3 = Me3, R1 = H, R2 = Me)4b [74542-82-8] C7H14Si (MW 126.30) InChI = 1/C7H14Si/c1-5-6-7-8(2,3)4/h5,7H,1-4H3 InChIKey = SBPLHANFNZGLJQ-UHFFFAOYAF (propargylic anion equivalents;2, 3 three-carbon synthons for [3 + 2] annulations leading to five-membered compounds including cyclopentenes,4 dihydrofurans,5 pyrrolines,5 isoxazoles,6 furans,7 and azulenes8) Physical Data: (1a) bp 111 °C; bp 54–56 °C/90 mmHg; (1b) bp 62–65 °C/30 mmHg; (1d) bp 107–110 °C. Solubility: sol CH2Cl2, benzene, THF, Et2O, and most organic solvents. Form Supplied in: colorless liquid; not commercially available. Analysis of Reagent Purity: (1a) IR (neat) 2955, 2910, 2900, 2860, 1935, 1440, 1400, 1250, 935, 880, 830, 805, 750, and 685 cm−1; 1H NMR (250 MHz, CDCl3) δ 0.08 (s, 9H), 1.67 (t, 3H, J = 3.3), and 4.25 (q, 2H, J = 3.3); 13C NMR (67.9 MHz, CDCl3) δ −2.1, 15.1, 67.3, 89.1, and 209.1.4b,9 Preparative Methods: 1-methyl-1-(trialkylsilyl)allenes can be conveniently prepared by the method of Vermeer.9, 10 Silyl-substituted propargyl mesylates thus undergo SN2′ displacement by the organocopper reagent generated from methylmagnesium chloride, Copper(I) Bromide, and Lithium Bromide. 1-Methyl-1-(trimethylsilyl)allene is produced in 52% yield from commercially available (trimethylsilyl)propargyl alcohol in this fashion (eq 1).4b,9 The t-butyldimethylsilyl and triisopropylsilyl analogs are synthesized by the same method in 90% and 58% yield, respectively.7 Propargyl alcohols bearing these and other trialkylsilyl groups can be prepared by treatment of Propargyl Alcohol with n-Butyllithium and the appropriate trialkylsilyl chloride.7 Allenylsilanes bearing other C-1 substituents can be prepared in an analogous manner by using the appropriate Grignard reagents. (1) 1-Methyl-1-(trialkylsilyl)allenes can be alkylated at C-3 with a variety of alkyl halides by treatment of the allenylsilane with n-butyllithium and the desired alkyl halide (e.g. eq 2).7 In addition to ethyl bromide, alkylating agents such as n-heptyl bromide,6 1,2-dibromobutane,7 and 5-bromo-1-pentene8 have been employed in this reaction. (2) 3-Methyl-1-(trimethylsilyl)allene (1d) is prepared by the direct silylation of the lithium derivative of 1,2-butadiene.4b Sequential treatment of a THF solution of 1,2-butadiene with 1.0 equiv of Lithium 2,2,6,6-Tetramethylpiperidide (−78 °C, 3 h) and 1.05 equiv of Chlorotrimethylsilane (−78 °C to 25 °C, 12 h) affords (1d) in 41% yield after distillation (eq 3). (3) Purification: allenes (1a), (1b), and (1c) are purified by distillation at reduced pressure or by column chromatography. Allene (1d) is distilled at atmospheric pressure. The allenylsilanes obtained by the Vermeer method typically contain up to 7–8% of the trialkylsilyl-1-butyne isomer produced by SN2 reaction. This mixture can be used directly in most subsequent reactions without further purification. If desired, however, the alkynyl contaminant can be selectively removed by treatment of the mixture with Silver(I) Nitrate in (10∶1) methanol–water at room temperature for one hour.9 1-Methyl-1-(trimethylsilyl)allene is obtained in 79% yield after pentane extraction and distillation. Handling, Storage, and Precautions: 1-methyl-1-(trimethylsilyl)allene is stable indefinitely when stored under nitrogen in the refrigerator.