[69875-71-4]  · C8H17ClOSi  · 3-Chloro-3-methyl-2-trimethylsilyloxy-1-butene  · (MW 192.76)

(allyl halide, precursor of 2-oxyallyl cation with the aid of AgClO4; precursor of silyl vinyl ether)

Physical Data: bp 74-75 °C/90 mmHg.

Solubility: sol common organic solvents; insol and slowly decomposed in H2O.

Form Supplied in: oil.

Analysis of Reagent Purity: 1H NMR (CDCl3-Me4Si) d 0.18 (s, 9H), 1.95 (br s, 3H), 4.23 (br s, 1H), 4.40 (s, 1H), 4.87 (br s, 1H), 5.30 (m, 1H).

Preparative Method: 1 a mixture of 3-chloro-3-methyl-2-butanone (1),2 Chlorotrimethylsilane, and Triethylamine in DMF is heated at 85-90 °C for 18 h. The mixture is diluted with petroleum ether, washed with cold water, and dried over Na2SO4. Solvent is evaporated, and the remaining oil is fractionated under reduced pressure to give 3-chloro-3-methyl-2-trimethylsilyloxy-1-butene (2) in 52% yield (eq 1). A byproduct, 2-methyl-3-(trimethylsilyloxy)-1,3-butadiene, is isolated in foreruns (54-55 °C/95 mmHg, 9%).1

Purification: distillation.

Handling, Storage, and Precautions: moisture sensitive; use only in a chemical fume hood; recommended to distill just prior to use.

2-Trimethylsilyloxyallyl Cation.1,3-5

Reaction of 3-chloro-3-methyl-2-trimethylsilyloxy-1-butene (2) with Silver(I) Perchlorate produces 3,3-dimethyl-2-trimethylsilyloxyallyl cation (3), which reacts with cyclopentadiene, furan, or alkenes to give 2,2-dimethylbicyclo[3.2.1]oct-6-en-3-one (4), 2,2-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (5), or 2,2-dimethylcyclopentanone derivatives (6), respectively (eqs 2 and 3). The reaction with furan in nitromethane is assumed to be a concerted reaction which gives the tricyclic ketone as the sole product. The one in THF/Et2O results in a stepwise reaction to give 2-(1,1-dimethyl-2-trimethylsilyloxy-3-chloro-2-butenyl)furan.

In general, the reaction of 2-oxyallyl cation with furans is either concerted or stepwise (Scheme 1). Concerted reaction of the 2-oxyallyl cation species (3) with the furan follows the [3 + 4] manner to give 8-oxabicyclo[3.2.1]oct-6-en-3-ones (9). Stepwise reaction of the allyl cation first produces a 2-substituted furyl cation intermediate (8) which then cyclizes to give the same products (9) or undergoes intermolecular reaction with a nucleophile to give a mixture of dihydrofurans (10) and (11). Alternatively, deprotonation of the intermediate cationic species affords 2-substituted furans (12).

Similar 2-oxyallyl cation species are generated from a-halo ketones with Triethylamine or from a,a-dihalo ketones with Zinc/Copper Couple or Pentacarbonyliron. Cycloaddition reactions of 2-oxyallyl cation species have been reviewed.3

The electrophilicity of the oxyallyl intermediates decreases in the order of M = FeX > SiR3 > ZnX > Li > Na. The allyl cation of high electrophilicity favors stepwise reaction pathways, while less electrophilic 2-oxyallyl cations needs more nucleophilic furans to afford [3 + 4] cycloaddition products.

Regioselective Synthesis of Silyl Vinyl Ethers.6

Reaction of (2) and its derivatives with lithium dialkyl- or diarylcuprate proceeds in an SN2 fashion to give 4-substituted 2-methyl-3-trimethylsilyloxy-2-butenes (13) (eq 4). This method can be applied to the synthesis of the silyl vinyl ethers that are inaccessible by other methods.

1. Shimizu, N.; Tanaka, M.; Tsuno, Y. JACS 1982, 104, 1330.
2. Wyman, D. P.; Kaufman, P. R. JOC 1964, 29, 1956.
3. Hoffman, H. M. R. AG(E) 1984, 23, 1.
4. Shimizu, N.; Tsuno, Y. CL 1979, 103.
5. Shimizu, N.; Tsuno, Y. Mem. Fac. Sci. Kyusyu Univ. Ser C. 1979, 12, 105, (CA 1980, 92, 94 008).
6. Tanabe, M.; Shimizu, N.; Tsuno, Y. Mem. Fac. Sci. Kyusyu Univ. Ser C. 1982, 13, 387 (CA 1983, 98, 54 000).

Tamejiro Hiyama & Manabu Kuroboshi

Tokyo Institute of Technology, Japan

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