Dicarbonyl(cyclopentadienyl)[(dimethylsulfonium)methyl]iron Tetrafluoroborate1

[72120-26-4]  · C10H13BF4FeO2S  · Dicarbonyl(cyclopentadienyl)[(dimethylsulfonium)methyl]iron Tetrafluoroborate  · (MW 339.93)

(stable precursor of a cationic carbene, or methylene, complex of iron; reacts directly with alkenes to give cyclopropanes2)

Physical Data: mp 129-130 °C; d 1.57 g cm-3.

Solubility: insol or slightly sol most solvents at rt; sol very polar solvents such as nitromethane at rt; sol other polar solvents (e.g. dioxane) at reflux.

Form Supplied in: yellow-orange powder or crystals.

Analysis of Reagent Purity: 1H NMR.

Preparative Methods: the preparation of this reagent has gone through several revisions and improvements since its introduction in 1979. The most recently improved preparations3 are satisfactory for large-scale applications and have been adapted for commercial use. In a one-pot operation, the stable, crystalline, dinuclear complex [Cp(CO)2Fe]2 is reductively cleaved with sodium dispersion to generate a THF solution of the highly nucleophilic sodium ferrate, Na[Cp(CO)2Fe]. This species is immediately alkylated with chloromethyl methyl sulfide, and the resulting iron alkyl, Cp(CO)2FeCH2SMe, is subjected to S-alkylation with methyl iodide. Finally, the resulting sulfonium salt, initially produced as an iodide, is subjected to in situ anion exchange and simultaneous crystallization by treatment with hot aqueous NaBF4, followed by cooling to give the reagent as a yellow-orange, or amber-colored, crystalline solid (eq 1) which has been characterized spectroscopically and by single-crystal X-ray diffraction.4 The anion exchange is performed in order to assure that a nonnucleophilic counterion is present during the subsequent cyclopropanation reactions; a nucleophilic anion such as iodide may apparently intercept reactive intermediates on the pathway leading to the desired cyclopropane products. A remarkable property of the reagent is its unusually high stability compared to most other organometallic reagents. This point is especially clear from the fact that it can be recrystallized from hot water exposed to the air.

Purification: recrystallization from nitromethane, acetone, or other polar solvents.

Handling, Storage, and Precautions: stable indefinitely as a solid, even when exposed to air; slowly decomposes in solution if exposed to air for long periods of time (although reactions with alkenes can be done in the presence of air and even moisture); decomposes upon heating to release dimethyl sulfide (stench). Use in a fume hood.

Alkene Cyclopropanation.

Once this reagent has been prepared (or purchased commercially) it can be used directly in reactions with a wide range of alkenes without the need for using any further activation agents in order to obtain cyclopropanes (eq 2). An iron carbene complex is apparently generated as a reactive intermediate upon dissociation of dimethyl sulfide which ultimately recombines with the iron moiety to give the principal byproduct of the reaction.

The procedure is very simple; the reagent and the alkene are mixed with a polar solvent and then heated in the temperature range of typically 80-100 °C for a few hours. Among the solvents that have been used successfully are 1,4-dioxane, DMF, ethanol, and nitromethane. The last of these, nitromethane, typically and reproducibly gives the highest yields of cyclopropanes. Excess reagent is commonly used to assure complete conversion of alkene to cyclopropane; 2 mol equiv is usually sufficient, but in some cases of less reactive alkene substrates the use of 3 mol equiv is necessary for obtaining optimum yields of cyclopropanes. The products are isolated very easily by adding pentane or another suitable nonpolar solvent to the cooled reaction mixture to precipitate the organometallic byproducts. Filtration of the mixture and use of routine distillation or chromatographic purification techniques then provide the cyclopropanes.

The reaction occurs most efficiently with mono- and disubstituted alkenes. More highly substituted alkenes generally react too slowly and exhibit very poor conversion to cyclopropanes. Some illustrative examples of efficient cyclopropanation are gathered in Table 1. Entries 12 and 22 are representative of relatively simple, nonfunctionalized alkenes. Entries 35 and 46 indicate both compatibility with other simple functional groups (ethers, sulfides, and acetals have also been used2) and the typical selectivity of an electrophilic reagent for more electron-rich alkenes in the presence of electron-deficient double bonds.

Alternative Reagents.

The classical method for the addition of a simple methylene group to an alkene is the Simmons-Smith reaction.7 Indeed, it 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. On the other hand, the Simmons-Smith reagent appears to show reactivity towards both electron-deficient and electron-rich alkenes in some cases; the selectivity is less pronounced than in the case of the iron reagent. More specifically, the conversions shown in entries 3 and 4 of Table 1 were also attempted with the Simmons-Smith reagent, but the iron reagent gave superior results.5,6 A minor disadvantage of the Simmons-Smith reaction is that the active reagent must be prepared freshly before use from Diiodomethane and Zinc/Copper Couple or related combinations of sometimes air-sensitive reagents,8 whereas the iron reagent can be stored as a highly stable solid for indefinite periods of time before use and then be used directly with no further activation being necessary. Diazomethane may also be used as an alternative reagent,1c but the hazards and difficulties of preparing and handling this reagent are well known.

1. (a) Brookhart, M.; Studabaker, W. B. CRV 1987, 87, 411. (b) Helquist, P. In Advances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI: London, 1991; Vol. 2, pp 143-194. (c) Helquist, P. COS 1991, 4, 951.
2. (a) Brandt, S.; Helquist, P. JACS 1979, 101, 6473. (b) O'Connor, E. J.; Brandt, S.; Helquist, P. JACS 1987, 109, 3739.
3. (a) Mattson, M. N.; Bays, J. P.; Zakutansky, J.; Stolarski, V.; Helquist, P. JOC 1989, 54, 2467. (b) Mattson, M. N.; O'Connor, E. J.; Helquist, P. OS 1992, 70, 177.
4. O'Connor, E. J. Helquist, P. JACS 1982, 104, 1869.
5. Wender, P. A.; Eck, S. L. TL 1982, 23, 1871.
6. Bäckvall, J.-E.; Löfström, C.; Juntunen, S. K.; Mattson, M. TL 1993, 34, 2007.
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. Maruoka, K.; Fukutani, Y.; Yamamoto, H. JOC 1985, 50, 4412.

Paul Helquist

University of Notre Dame, IN, USA

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