Chromium(II) Chloride-Haloform


[10049-05-5]  · Cl2Cr  · Chromium(II) Chloride-Haloform  · (MW 122.92) (CHI3)

[75-47-8]  · CHI3  · Chromium(II) Chloride-Iodoform  · (MW 393.78) (CHBr3)

[75-25-2]  · CHBr3  · Chromium(II) Chloride-Bromoform  · (MW 252.77) (CHCl3)

[67-66-3]  · CHCl3  · Chromium(II) Chloride-Chloroform  · (MW 119.39) (Me3SiCHBr2)

[2612-42-2]  · C4H10Br2Si  · Chromium(II) Chloride-(Dibromomethyl)trimethylsilane  · (MW 246.02)

(conversion of aldehydes to (E)-alkenyl halides or (E)-alkenylsilanes with one-carbon homologation; (E)-selective alkylidenation of aldehydes)

Physical Data: see entries for Chromium(II) Chloride, Iodoform, Bromoform, Chloroform, and (Dibromomethyl)trimethylsilane.

Preparative Method: prepared in presence of reactant aldehyde: to a suspension of anhyd CrCl2 (0.74 g, 6.0 mmol) in THF (10 mL) under Ar, add a solution of aldehyde (1.0 mmol) and iodoform (0.79 g, 2.0 mmol) in THF (5 mL) dropwise at 0 °C, at 0 °C for 3 h, pour the reaction mixture into water (25 mL) and extract with ether; dry the combined extracts over Na2SO4 and concentrate; purification by column chromatography gives an alkenyl halide. Low-valent chromium derived by reduction of CrCl3 (6.0 mmol) with Lithium Aluminum Hydride (3.0 mmol) can be used instead of CrCl2.

Handling, Storage, and Precautions: use in a fume hood; chromium(II) chloride should be used under an inert atmosphere (argon or nitrogen).

Preparation of (E)-Alkenyl Halides.1

The reagent prepared by treatment of CHI3 or CHCl3 with CrCl2 gives iodo- and chloroalkenes, respectively (eq 1). When bromoform is employed, a mixture of bromo- and chloroalkenes is produced. This side-reaction is overcome by using a combination of CrBr3 and LiAlH4 instead of CrCl2.1 The rates of reaction of the haloforms are in the sequence I > Br > Cl. Reflux of a mixture of CHCl3 and CrCl2 in THF before addition of an aldehyde and sonication are reported to accelerate the reaction. An iodoalkene having a terminal 13C can be prepared by using 13CHI3 (eq 2).2 An ene-reaction product is obtained as a byproduct in the case of an aldehyde having a suitable alkenic bond.1,3

(E)-Isomers of alkenyl halides are produced selectively. The (E):(Z) ratios of the alkenyl halides increase in the order I < Br < Cl. A mixed solvent of dioxane and THF is employed to improve the (E):(Z) ratios in certain cases.4 Isomerization using NaOH in t-BuOH is also effective to obtain high (E):(Z) ratios (eq 3).5

Although ketones are also converted into the corresponding alkenyl halides, they are less reactive than aldehydes. Selective conversion of an aldehyde into an (E)-iodoalkene can thus be accomplished without affecting the coexisting ketone group.1 The following functional groups are also tolerated during the reaction: ester, lactone, amide, nitrile, 1,3-diene, alkyne, alkene, bromide, chloride, and acetal of ethylene glycol. A hydroxyl group can be protected by using the following groups: OMe, OCH2Ph, OSiMe3(t-Bu), OAc, OCOPh, OTHP, and OMPM. Because the basicity of the reagent is not strong, epimerization at the a-position of aldehydes does not generally take place. The alkenyl iodides (RCH=CHI) (1),6 (2),7 (3),8 and (4)9 are prepared from the corresponding aldehydes (RCHO).

Preparation of (E)-Alkenylsilanes.10

When Me3SiCHBr2 is used instead of a haloform, (E)-alkenylsilanes are produced from aldehydes stereoselectively (eq 4). Because the reaction proceeds under mild conditions, selective transformation of an aldehyde to an (E)-alkenylsilane can be performed in the presence of ketone, cyano, ether, acetal, and ester groups (eq 5).10-12

(E)-Selective Wittig-Type Alkylidenation of Aldehydes.13

Treatment of aldehydes in THF with the reagent derived by reduction of 1,1-diiodoalkane with CrCl2-DMF produces (E)-alkenes with high stereocontrol (eq 6).

Related Reagents.

Bromoform; Chloroform; Chromium(II) Chloride; 1,1-Diiodoethane; Diiodomethane; Iodoform.

1. Takai, K.; Nitta, K.; Utimoto, K. JACS 1986, 108, 7408.
2. Baker, K. V.; Brown, J. M.; Cooley, N. A. J. Labelled Compd. Radiopharm. 1988, 25, 1229.
3. Jung, M. E.; D'Amico, D. C.; Lew, W. TL 1993, 34, 923.
4. Evans, D. A.; Black, W. C. JACS 1993, 115, 4497.
5. (a) Wulff, W. D.; Powers, T. S. JOC 1993, 58, 2381. (b) Hayashi, T.; Konishi, M.; Okamoto, Y.; Kabeta, K.; Kumada, M. JOC 1986, 51, 3772.
6. Pontikis, R.; Randrianasolo, L. R.; Merrer, Y. L.; Nam, N. H.; Azerad, R.; Depezay, J.-C. CJC 1989, 67, 2240.
7. Kende, A. S.; Kawamura, K.; DeVita, R. J. JACS 1990, 112, 4070.
8. Roush, W. R.; Brown, B. B. JACS 1993, 115, 2268.
9. Kanda, Y.; Fukuyama, T. JACS 1993, 115, 8451.
10. Takai, K.; Kataoka, Y.; Okazoe, T.; Utimoto, K. TL 1987, 28, 1443.
11. Burke, S. D.; Takeuchi, K.; Murtiashaw, C. W.; Liang, D. W. M. TL 1989, 30, 6299.
12. Yoshida, J.; Maekawa, T.; Morita, Y.; Isoe, S. JOC 1992, 57, 1321.
13. Okazoe, T.; Takai, K.; Utimoto, K. JACS 1987, 109, 951.

Kazuhiko Takai

Okayama University, Japan

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