[77418-50-9]  · C15H14FeO2S  · Dicarbonyl(cyclopentadienyl)(1-phenylthioethyl)iron  · (MW 314.18)

(stable precursor of a cationic methylcarbene, or ethylidene, complex of iron; for conversion of alkenes into methyl-substituted cyclopropanes2)

Physical Data: mp 59 °C.

Solubility: sol most common solvents; methylene chloride is the solvent of choice.

Form Supplied in: yellow crystals.

Analysis of Reagent Purity: 1H and 13C NMR.

Preparative Methods: the stable, crystalline, dinuclear complex [Cp(CO)2Fe]2 is reductively cleaved with sodium amalgam to generate a THF solution of the highly nucleophilic sodium ferrate, Na[Cp(CO)2Fe]. This species is immediately alkylated with 1-chloroethyl phenyl sulfide to give Cp(CO)2FeCH(Me)SPh, which is purified chromatographically to give the reagent as a yellow, crystalline solid (eq 1) which has been characterized spectroscopically.2 An alternative preparation employs the addition4 of methyllithium or methylmagnesium bromide to the stable, readily available, phenylthiocarbene complex Cp(CO)2Fe=CHSPh+ PF6-.5

Purification: silica gel chromatography.3

Handling, Storage, and Precautions: stable indefinitely as a solid when protected from air; can be exposed to air for short periods of time (weighing, setting up reactions, etc.) without noticeable decomposition; decomposes in solution if exposed to air.

Alkene Cyclopropanation.

Alkenes undergo reaction with the iron reagent in the presence of reactive methylating agents such as Trimethyloxonium Tetrafluoroborate6 (caution: volatile, highly toxic alkylating agent) to give methyl-substituted cyclopropanes (eq 2). The reaction apparently occurs via S-methylation of the iron reagent to produce a sulfonium salt, which in turn undergoes dissociation of thioanisole to generate a carbene complex of iron as the reactive intermediate.

The iron reagent and the alkene are normally used in equimolar amounts, but a slight excess of the methylating agent (total of 1.25 mol equiv) is usually employed to permit optimum conversion to products. The cyclopropane products are isolated very easily by adding pentane or another suitable nonpolar solvent to the reaction mixture to precipitate the organometallic byproducts. Filtration of the mixture and use of routine distillation or chromatographic purification techniques then provides the cyclopropanes.

The product yields are typically modest, usually being in the range of 40-70%. The reaction occurs best with mono- and disubstituted alkenes. More highly substituted alkenes generally react too slowly and exhibit very poor conversion to cyclopropanes. Some illustrative examples are gathered in Table 1.2 Two important stereochemical features of these reactions are: (1) the stereospecific retention of configuration of the initial alkene double bond upon conversion to the cyclopropanes (see entry 4); and (2) the notable preference for endo or syn stereoselectivity with respect to the orientation of the reagent-derived methyl group in the cyclopropanes (see entries 1, 2, and 4).

Alternative Reagents.

The classical method for the cyclopropanation of alkenes is the Simmons-Smith reaction.7 Indeed, this reaction is still probably the most highly used method for effecting this transformation. It is compatible with a wide range of functional groups, and it appears to be less sensitive to the degree of double bond substitution than the present iron reagent. However, the Simmons-Smith reaction has been used most commonly for transfer of the parent methylene group. Extensions to more highly substituted alkylidene groups have been limited in investigation. When ethylidene transfer has been studied,8 poor endo stereoselectivity is seen compared to the present iron reagent. The same observation of lower stereoselectivity has been made for a more recent, aluminum-based modification of the Simmons-Smith reaction.9 Diazoalkanes may also be used as alternative reagents,1c but the hazards and difficulties of preparing and handling these reagents are well-known. Much more closely related to the present reagent is the iron derivative, Cp(CO)2FeCH(Me)OMe.1a,10 Results of cyclopropanations using either of the iron reagents are quite comparable.

1. (a) Brookhart, M.; Studabaker, W. B. CRV 1987, 87, 411. (b) Helquist, P. In Advances in Metal-Organic Chemistry, Leibeskind, L. S., Ed.; JAI: London, 1991; Vol. 2, pp 143-194. (c) Helquist, P. COS 1991, 4, 951.
2. (a) Kremer, K. A. M.; Helquist, P.; Kerber, R. C. JACS 1981, 103, 1862. (b) Kremer, K. A. M.; Helquist, P. JOM 1985, 285, 231.
3. Kremer, K. A. M.; Helquist, P. OM 1984, 3, 1743.
4. Knors, C.; Kuo, G.-H; Lauher, J. W.; Eigenbrot, C.; Helquist, P. OM 1987, 6, 988.
5. Knors, C.; Helquist, P. In Organometallic Syntheses King, R. B.; Eisch, J. J., Eds.; Elsevier: Amsterdam, 1988; Vol. 4, pp 205-209.
6. Meerwein, H. OSC 1973, 5, 1096.
7. Simmons, H. E.; Cairns, T. L.; Vladuchick, S. A.; Hoiness, C. M. OR 1973, 20, 1. See also Ref. 1(c) for a more recent review.
8. Kawabata, N.; Nakagawa, T.; Nakao, T.; Yamashita, S. JOC 1977, 42, 3031.
9. Maruoka, K.; Fukutani, Y.; Yamamoto, H. JOC 1985, 50, 4412.
10. Brookhart, M.; Tucker, J. R.; Husk, G. R. JACS 1983, 105, 258.

Paul Helquist

University of Notre Dame, IN, USA

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