[80422-55-5]  · C7H14Cl2OSi  · 1,3-Dichloro-2-trimethylsiloxy-1-butene  · (MW 213.18)

(1,3-dichloro-2-butanone equivalent; presursor of 1-chloro-3-methyl-2-oxyallyl cation;1 substrate for double alkylation or selective arylation2)

Alternate Name: [2-chloro-1-(chloromethylene)propoxy]trimethylsilane.

Physical Data: colorless oil; bp 54.5-55.5 °C/5 mmHg; IR (neat) 1630 cm-1 (nC=C); 1H NMR (60 MHz, CCl4) d 0.31 (s, 9H, SiMe3), 1.60 (d, J = 6.6 Hz, 3H, allylic methyl), 4.40 (q, J = 6.6 Hz, 1H, allylic), and 5.56 (s, 1H, alkenic).

Solubility: sol common organic solvents (THF, Et2O, CH2Cl2, MeNO2).

Preparative Method: a solution of 1,3-dichloro-2-butanone (14 g, 0.1 mol) in ether (25 mL) is added dropwise into a stirred mixture of Chlorotrimethylsilane (12 g) and Triethylamine (12 g) in ether (100 mL) at ambient temperature. The mixture is stirred under reflux for 7 h and poured into ice water. The organic layer is washed with cold water, dried (MgSO4), and concentrated. Distillation of the resulting oil under reduced pressure (5 mmHg) gives the reagent (18 g, 83%), which is exclusively of (Z) configuration;1 the stereochemistry of the enol silylation depends on the solvent and base employed.3 1-Chloro-2-trimethylsiloxy-1,3-butadiene is sometimes an impurity (less than 5%).

Handling, Storage, and Precautions: the reagent is sensitive to moisture, especially under acidic conditions. If protected from moisture, it can be stored for years without significant decomposition.

2-Oxyallyl Cation.

Silver perchlorate induces ionization of this reagent, generating the 1-chloro-3-methyl-2-(trimethylsiloxy)allyl cation, which reacts with Furan or Cyclopentadiene to give corresponding seven-membered bicyclic ketones.1 Thus a treatment of the reagent (5 mmol) with Silver(I) Perchlorate (2 equiv) in nitromethane in the presence of furan (25 mmol) and calcium carbonate (15 mmol as a buffer) yields 2-chloro-4-methyl-8-oxabicyclo[3.2.1]oct-6-en-3-one as a mixture of two stereoisomers bearing an axial Cl group (eq 1); with cyclopentadiene, a mixture of all four possible stereoisomers is formed (76%). Electrophilic substitution instead of cycloaddition occurs when the reaction with furan is carried out in THF/ether (1:2) in place of nitromethane (eq 2).

Alkylation and Arylation.

Reaction with 2 equiv of alkyl Grignard reagents in the presence of 10% Copper(I) Iodide in THF/HMPA (10:1) results in double alkylation via successive SN2 displacements (eq 3). The allylic chlorine is selectively replaced with an aryl group upon treatment with 2.5 equiv of bis(aryl)copper(I) lithium in ether (eq 4).2

1. Shimizu, N.; Tanaka, M.; Tsuno, Y. JACS 1982, 104, 1330.
2. Shimizu, N. Unpublished results. For related reactions, see: Tanaka, M.; Shimizu, N.; Tsuno, Y. Mem. Fac. Sci., Kyushu Univ., Ser. C 1982, 13, 387 (CA 1983, 98, 54 000s).
3. Shimizu, N.; Matsuno, S.; Tanaka, M.; Tsuno, Y. BCJ 1992, 65, 971.

Nobujiro Shimizu

Kyushu University, Fukuoka, Japan

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