[7440-70-2]  · Ca  · Calcium  · (MW 40.08)

[7664-41-7]  · H3N  · Calcium  · (MW 17.03)

(mild reducing agent capable of reducing double and triple bonds; moderately strong base)

Physical Data: Ca: mp 842-848 °C; bp 1240 °C; d 1.55 g cm-3; NH3: mp -77.7 °C; bp -33.35 °C; d-79 0.817 g cm-3.

Form Supplied in: Ca: silver, white, soft metal; NH3: anhydrous gas or liquid in sealed ampules. Drying: ammonia can be dried by passing the gas through a KOH drying tube prior to condensation.

Handling, Storage, and Precautions: calcium metal is a water reactive, flammable solid. In case of contact with water, the heat of the reaction may be sufficient to ignite the product hydrogen as well as the metal. Anhydrous liquid ammonia is corrosive, especially in contact with water. Although classified as a nonflammable gas, it will burn within certain concentration limits. The liquid may cause frostbite in contact with the skin. Use in a fume hood.

Reduction of Aromatic Hydrocarbons.

Although calcium in liquid ammonia, apparently in the form of Ca(NH3)6,2 has long been known to convert aromatic hydrocarbons to unconjugated dienes and even monoalkenes,3 alkali metals in ammonia or in ammonia/alcohol have generally been the reagents of choice, even though calcium is safer to handle. Thus benzene, toluene, and o- and p-xylene afford the corresponding monoalkenes in high yield after treatment with calcium hexamine after 24 hours (eq 1).3 Similar reductions of bi- and terphenyl yield dihydro derivatives, while naphthalene gives 1,4-dihydronaphthalene at -75 °C and a mixture of the 1,4-derivative and 1,2,3,4-tetrahydronaphthalene at -33 °C (eq 2).4

The reductions of aromatic hydrocarbons by calcium in liquid ammonia can be effected under two different conditions.5 In the first case, the calcium is dissolved in ammonia and the latter is removed eventually with the aid of high vacuum to give solid calcium hexamine. In the second procedure, ammonia is added to a mixture of the calcium and the hydrocarbon. While the first method produced substantially higher yields of cyclohexene from benzene, nearly the same results were realized by both methods in the reduction of toluene.

Improved yields and cleaner products were realized by the use of calcium shot in ammonia with ether and HMPA.6 For example, the reduction of t-butylbenzene by this method afforded 1-, 3-, and 4-t-butylcyclohexenes in yields of 56%, 34%, and 10%, respectively, results which are comparable to the better known lithium-amine system.7 Calcium in methylamine/ethylenediamine was found to be an even better system, since substantially higher quantities of the more thermodynamically stable alkenes were formed from reduction of a large number of aromatic hydrocarbons.8 Thus the yield of 1-t-butylcyclohexene in the reduction of the parent hydrocarbon was increased to 87% (eq 3).

Birch-type products were obtained by effecting calcium reductions in butylamine/ethylenediamine in the presence of t-butyl alcohol.9 The authors recommend that all such calcium reductions be run in the presence of white sand using a Hershberg stirrer to remove the insoluble coatings on the surface of the calcium metal. Use of this technique for the reduction of anisole provided the 2,5-dihydro derivative in 86% yield (eq 4).

Reduction of Alkynes and Alkenes.

Dialkylalkynes have been reduced to trans-alkenes by calcium in ammonia5 and by calcium in methylamine/ethylenediamine.10 Thus, 2-nonyne, 3-nonyne, 4-nonyne, and 4-octyne gave the corresponding trans-alkenes in yields of 70-88% and purities of 72-91% (eq 5).10 Similar reduction of 1-heptyne apparently gave substantial metalation unless a large excess of the metal was employed. In the latter case, the major product was heptane.

Arylated alkenes such as 1,1-diphenylethylene and 1,1,2-triphenylethylene have been reduced to the corresponding ethanes by calcium in ammonia.11

Reduction of Other Functional Groups.

Few functional groups other than those above have been reduced by calcium in liquid ammonia. The carbon-carbon double bond of several a,b-unsaturated ketones has been reduced by this reagent to give saturated ketones (eq 6).12 In several cases, though, both the alkene and the ketone have been reduced to saturated alcohols (eq 7).13

Calcium/ammonia solutions have been found to be useful when low basicity is desired, as in the conversion of both stereoisomers of the diacetoxy ketone (1) (R1 = R2 = OAc) to the corresponding ketone (R1 = OAc, R2 = H).14 In this case, calcium was shown to be a better reagent than lithium for this reduction.

Calcium in ammonia offers better selectivity than lithium in the debenzylation of a variety of benzyl ethers containing other functional groups.15 For example, in eq 8 the benzyl group of the alkynic ether is removed without concomitant reduction of the triple bond.

Epoxides have also been reduced to alcohols by calcium in ammonia (eq 9).16 The results are similar to those realized by the use of lithium metal.

Calcium/Ammonia as a Base.

Although little used for many years, solutions of calcium in ammonia have been employed to convert certain active hydrogen compounds to their calcium salts. For example, calcium salts have been prepared from terminal alkynes,17 alcohols,18 nitrogenous heterocycles such as 1,2,4-triazole,19 and other organonitrogen compounds, illustrated by dicyanodiamide.20

1. (a) Campbell, K. N.; Campbell, B. K. CRV 1942, 31, 77. (b) Watt, G. W. CRV 1950, 46, 317. (c) Birch, A. J. QR 1950, 4, 69. (d) Smith, H. Organic Reactions in Liquid Ammonia, Chemistry in Non-aqueous Ionizing Solvents; Wiley: New York, 1963; Vol. 1, Part 2. (e) Birch, A. J.; Smith, H. QR 1958, 12, 17. (f) Augustine, R. L. Reduction; Dekker: New York, 1968. (g) Kaiser, E. M. S 1972, 391. (h) Birch, A. J.; Subba Rao, G. Advances in Organic Chemistry, Methods and Results, Taylor, E. C., Ed; Wiley: New York, 1972. (i) House, H. O. Modern Synthetic Reactions, 2nd ed.; Benjamin: Menlo Park, CA, 1972; Chapter 3. (j) Caine, D. OR 1976, 23, 1. (k) Hudlicky, M. Reductions in Organic Chemistry; Ellis Horwood: Chichester, 1984.
2. Kraus, C. A. JACS 1908, 30, 653.
3. (a) Kazanskiy, B. A.; Smirnova, N. V. CA 1938, 32, 2090. (b) Kazanskiy, B. A.; Glushnev, N. F. CA 1939, 33, 1279.
4. Hückel, W.; Bretschneider, H. LA 1939, 540, 157.
5. Campbell, K. N.; McDermott, J. P. JACS 1945, 67, 282.
6. Benkeser, R. A.; Kang, J. JOC 1979, 44, 3737.
7. Benkeser, R. A.; Robinson, R. E.; Sauve, D. M.; Thomas, O. H. JACS 1955, 77, 3230.
8. (a) Benkeser, R. A.; Belmonte, F. G.; Kang, J. JOC 1983, 48, 2796. (b) Benkeser, R. A.; Belmonte, F. G.; Yang, M.-H. SC 1983, 13, 1103.
9. Benkeser, R. A.; Laugal, J. A.; Rappa, A. TL 1984, 25, 2089.
10. Benkeser, R. A.; Belmonte, F. G. JOC 1984, 49, 1662.
11. Gilman, H.; Bailie, J. C. JACS 1943, 65, 267.
12. (a) Howell, F. H.; Taylor, D. A. H. JCS 1958, 1248. (b) Weiss, M. J.; Schaub, R. E.; Allen, Jr., G. R.; Poletto, J. F.; Pidacks, C.; Conrow, R. B.; Coscia, C. J. T 1964, 20, 357. (c) Fieser, L. F.; Fieser, M. FF 1967, 1, 106. (d) Barton, D. H. R.; Lier, E. F.; McGhie, J. F. JCS(C) 1968, 1031.
13. (a) Church, R. F.; Ireland, R. E.; Shridhar, D. R. JOC 1962, 27, 707. (b) Angibeaund, P.; Riviere, H. CR(C) 1966, 263, 1076.
14. Chapman, J. H.; Elks, J.; Phillips, G. H.; Wyman, L. J. JCS 1956, 4344.
15. Hwu, J. R.; Chua, V.; Schroeder, J. E.; Barrans, Jr., R. E.; Khoudary, K. P.; Wang, N.; Wetzel, J. M. JOC 1986, 51, 4731.
16. Benkeser, R. A.; Rappa, A.; Wolsieffer, L. A. JOC 1986, 51, 3391.
17. Moissan, H. CR(C) 1898, 127, 911; CR(C) 1903, 136, 1217.
18. Chablay, E. CR(C) 1911, 153, 953.
19. Strain, H. H. JACS 1927, 49, 1995.
20. Franklin, E. C. JACS 1922, 44, 486.

Edwin M. Kaiser

University of Missouri-Columbia, Columbia, MO, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.