Phenylsilane-Cesium Fluoride


[694-53-1]  · C6H8Si  · Phenylsilane-Cesium Fluoride  · (MW 108.23) (CsF)

[13400-13-0]  · CsF  · Phenylsilane-Cesium Fluoride  · (MW 151.91)

(mild nucleophilic non-Lewis acidic hydride donor1 used under aprotic conditions)

Physical Data: phenylsilane: bp 120 °C; d 0.877 g cm-3. Cesium fluoride: mp 683 °C.

Solubility: phenylsilane: sol THF, MeCN. Cesium fluoride: sol H2O; slightly sol THF, MeCN.

Form Supplied in: phenylsilane is a clear liquid; cesium fluoride is a white solid; both widely available. Drying: anhydrous cesium fluoride is prepared by flame drying under vacuum, taking care not to fuse the salt.

Handling, Storage, and Precautions: no particular requirements.

4-Oxazoline Formation.

The PhSiH3-Cesium Fluoride reagent1,2 is a nonnucleophilic hydride donor without Lewis acid character with characteristics similar to the Diphenylsilane-Cesium Fluoride,2 a-NaphSiH3-CsF,2 and (RO)3SiH-CsF2 reagents on which it is based. In aprotic solvents, it has been used to reduce 4-oxazolium salts to 4-oxazolines.1 The 4-oxazolines open at -40 °C to azomethine ylides, which are trapped with dipolarophiles to give 3-pyrrolines (2) (eq 1). For isolation purposes the 3-pyrrolines are oxidized to pyrroles (3). The mildness of the reagent allows for the selective reduction of the oxazolium salt without further reduction of the 4-oxazoline, azomethine ylide, or dipolarophile which are present at different stages of the reaction. Of the reducing agents tried for (1), PhSiH3 gave the highest yield (95%), although Ph2SiH2 (93%) worked nearly as well.1b Other reducing agents gave much poorer yields. Propiolates, acrylates, N-Phenylmaleimide, and vinyl sulfone dipolarophiles have been used in this [3 + 2] cycloaddition without dipolarophile reduction.

Dipolarophiles such as Ethyl Acrylate exhibit regiochemical preference for meta acyl or carboxyl groups (eq 2).1 With one exception (entry 6), all cases are consistent with FMO approximation in which the largest dipole HOMO coefficient is at the acceptor-substituted carbon (Table 1).1b

A comparison of methods to generate azomethine ylides via 4-oxazoline ring opening vs. aziridine thermolysis has shown that the oxazoline method can produce the same azomethine dipole under kinetic control as the aziridine method under thermodynamic control.3 Since the 4-oxazoline method allows improved regiochemical selectivity in many cases, the synthesis of 4-oxazolines from PhSiH3-CsF-induced reduction of oxazolium salts possesses greater potential for azomethine ylide applications in stereocontrolled synthesis.

Intramolecular trapping of an azomethine ylide generated from an oxazolium salt has been accomplished (eq 3).4 This approach allows a new entry into the mitosene skeleton.

Related Reagents.

Diphenylsilane-Cesium Fluoride.

1. (a) Vedejs, E.; Grissom, J. W. JACS 1986, 108, 6433. (b) Vedejs, E.; Grissom, J. W. JACS 1988, 110, 3238.
2. Boyer, J.; Corriu, R. J. P; Perz, R.; Reye, C. JOM 1979, 172, 143 (CA 1979, 91, 108 039n).
3. Vedejs, E.; Grissom, J. W. JOC 1988, 53, 1882.
4. Vedejs, E.; Piotrowski, D. W. JOC 1993, 58, 1341.

Thomas J. Fleck

The Upjohn Company, Kalamazoo, MI, USA

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