Chromium(II) Sulfate1


[13825-86-0]  · CrO4S  · Chromium(II) Sulfate  · (MW 148.05)

(reducing agent for alkynes, alkenes, organohalides;1 reductions via amine or ammonia complexes2)

Alternate Names: chromium sulfate; chromous sulfate.

Solubility: sol water and aqueous DMF; slightly sol ethanol; insol acetone.

Form Supplied in: not commercially available.

Analysis of Reagent Purity: an aqueous solution of CrSO4 can be standardized by a titration against (CuCl4)2- or K2Cr2O73 as well as FeCl3.4

Preparative Methods: the most common procedure for preparing an aqueous CrSO4 solution employs a reduction of Cr2(SO4)3 with mossy Zn and Hg in water and is used as is.4 Alternate syntheses start with very pure Cr metal5 or K2Cr2O7.3 A method for preparing crystalline blue CrSO4.5H2O has been published.5,6

Handling, Storage, and Precautions: aqueous CrSO4 solutions are stable at temperatures between 20 and 60 °C for several days if stored in an oxygen-free environment.7 The aqueous solution should be stored in a vented container as the powerful reducing agent will reduce water and, over time, liberate hydrogen gas which may lead to an explosion.8 Rigorously dried CrSO4.5H2O can be stored in the presence of air for at least eight months.5 However, it degrades quickly in the presence of oxygen if it is finely ground.5

Comments on Reaction Conditions and Workups.

Most of the described reactions are carried out in deoxygenated solvents and under an oxygen-free nitrogen atmosphere because of the sensitivity of CrSO4 to oxygen. Two common workups for CrSO4 reductions are described in the literature. The aqueous reaction mixture can be saturated with (NH4)2SO4 and extracted with an organic solvent such as ether.9 A DMF/water reaction mixture can be directly extracted with ether/pentane.10 Alternate approaches include oxidation of excess reducing agent by aeration11 and removal of chromium hydroxide by filtration after making the reaction mixture basic.9

Alkynes to (E)-alkenes.

The homogeneous reduction of alkynes in water or aqueous DMF at room temperature selectively yields (E)-alkenes. Substitution by polar functional groups in the propargylic position accelerates the reduction.9 Bulky substituents on the alkyne impede the reaction and terminal alkynes are more reactive than internal ones.9 A study of reductions of a-alkynic ketones provides additional examples for the specific formation of (E)-alkenes (here (E)-alkenones) and for the steric bias of the reaction (increased steric bulk on the alkynes causes longer reaction times and lower yields).10 An advantage of CrSO4 in this type of reduction is its specificity for alkynes in the presence of other functional groups. Epoxides (eq 1), esters, and a,b-unsaturated esters are unreactive under these reaction conditions.10 CrCl2 causes the same transformation to take place but is usually less reactive with the mentioned substrates.10 Certain alkenes can be reduced to alkanes under these conditions as discussed in the next section.

Alkenes to Alkanes.

Alkenes are reduced to alkanes by CrSO4 in water and in aqueous DMF. In general, (Z)-alkenes react more rapidly than the (E)-isomers.12 Conjugated coordination sites (such as hydroxyl groups) and conjugated electron-withdrawing substituents (esters and nitriles, for example) accelerate the rate of reduction.12 Tetrasubstituted alkenes usually yield an even mixture of meso- and (±)-alkanes (eq 2).12

Reduction of Organic Monohalides.

The reduction of organohalides can yield two different products depending on the substrate and the reaction conditions. Benzylic (arylalkyl) and allylic halides produce the reduced alkane or a coupled product.13 The amount of coupled product can be increased by slow addition of the CrSO4 to the halide (eq 3) and by use of anhydrous solvents.13 Aliphatic halides are usually converted to the reduced alkane independent of the reaction conditions (eq 4).13

The nature of the halide affects the rate of this reaction (I > Br > Cl).13 The presence of a hydrogen donor such as mercaptoacetic acid enhances the yield of reduced alkane.14 For both sets of halides their reactivity with CrSO4 can be generalized as follows: benzylic, a-halocarbonyl > allylic > tertiary > secondary > primary > vinylic, aromatic.15

Reduction of Vicinal Dihalides.

Alkenes which are stable to CrSO4 can be obtained from vicinal dihalides in high yields.16 Rapid homogeneous reaction takes place in water or aqueous DMF, consuming most of the reducing agent.16 The stereochemical outcome of the reaction varies as some compounds lead to clean trans elimination (eq 5) while others give isomeric mixtures.16 The reduction of 2,3-dihalopropenes can be influenced by the amount of reducing agent. Excess CrSO4 transforms the initially formed allene into a propene (eq 6).16 In special instances, alkynes have been prepared from the corresponding vinyl dibromides under these conditions if the alkyne is unreactive toward CrSO4.16

Reduction of Geminal Dihalides.

The room temperature reduction of gem-dihalides with CrSO4 leads to mixtures of alkanes, alkenes, and alkanols when carried out in aqueous DMF. Alkanes are the predominant product (eq 7).17 The rate of reaction varies with the halide (I > Br > Cl).17

One report describes the utility of CrSO4 in the reduction of gem-dibromocyclopropanes.11 These cyclopropanes produce mixtures of cyclic allenes, monobromocyclopropanes, cyclopropanes, and allylic alcohols.11 High yields of cyclic allenes can be obtained with certain substrates (eq 8).11

Miscellaneous Reductions.

CrSO4 is capable of reducing aldehydes.18 In a special case it has been utilized selectively to hydrogenolize one of the hydroxyl groups in a 3-hexyne-2,5-diol.19

Reductions with Chromium(II) Complexes.

Complexation of CrII with ammonia or amines enhances its reactivity. Ammoniacal CrII prepared from an aqueous CrSO4 solution readily reduces aldehydes and alkenes.2a Complexation with ethylenediamine enables reduction of alkyl halides which are unreactive or only slowly reactive toward CrSO4 itself.2b

Related Reagents.

Chromium(II) Acetate; Chromium(II) Chloride; Lithium; Lithium Aluminum Hydride; Phenyllithium; Zinc.

1. (a) Hanson, J. R.; Premuzic, E. AG(E) 1968, 7, 247. (b) Hanson, J. R. S 1974, 1. (c) Ho, T.-L. S 1979, 1.
2. (a) Kopple, K. D. JACS 1962, 84, 1586. (b) Kochi, J. K.; Mocadlo, P. E. JACS 1966, 88, 4094.
3. Lingane, J. J.; Pecsok, R. L. Anal. Chem. 1948, 20, 425.
4. Zurqiyah, A.; Castro, C. E. OSC 1973, 5, 993.
5. Lux, H.; Illman, G. CB 1958, 91, 2143.
6. Holah, D. G.; Fackler, J. P. Inorg. Synth. 1970, 10, 26.
7. Kranz, M.; Duczmal, W. Rocz. Chem. 1973, 47, 1823.
8. Bretherick, L. Chem. Br. 1976, 12, 204.
9. Castro, C. E.; Stephens, R. D. JACS 1964, 86, 4358.
10. Smith, A. B., III; Levenberg, P. A.; Suits, J. Z. S 1986, 184.
11. Nozaki, H.; Aratani, T.; Noyori, R. T 1967, 23, 3645.
12. Castro, C. E.; Stephens, R. D.; Moje, S. JACS 1966, 88, 4964.
13. Slaugh, L. H.; Raley, J. H. T 1964, 20, 1005.
14. (a) Heather, J. B.; White, D. R. CA 1982, 96, 163 049u. (b) Heather, J. B.; White, D. R. CA 1982, 97, 92 637s.
15. Castro, C. E.; Kray, W. C., Jr. JACS 1963, 85, 2768.
16. Kray, W. C., Jr.; Castro, C. E. JACS 1964, 86, 4603.
17. Castro, C. E.; Kray, W. C. JACS 1966, 88, 4447.
18. See Ref. 17: description of benzaldehyde reduction under benzotrichloride procedure on p 4454.
19. Willy, W. E.; Thiessen, W. E. JOC 1970, 35, 1235.

Joerg Pfeifer

Eli Lilly & Co., Lafayette, IN, USA

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