Calcium Hydride1


[7789-78-8]  · CaH2  · Calcium Hydride  · (MW 42.10)

(used as a drying agent for reagents and solvents, as a desiccant in reactions, as a base, and as a catalyst and cocatalyst)

Physical Data: mp 816 °C, d 1.902 g cm-3.

Form Supplied in: gray orthorhombic crystals, fine ground powder, coarse ground powder, and granules/lumps.

Handling, Storage, and Precautions: moisture sensitive and pyrophoric. It reacts vigorously with water, and should be stored and handled under dry conditions (argon or nitrogen).

Calcium hydride reacts with water to form calcium hydroxide and hydrogen gas (eq 1). It reacts much more slowly than the other metal hydrides as a base (toward organic acids), and has therefore found its most widespread use as a drying agent. Thus it has been used to dry hydrocarbons, ethers, amines (including pyridine), esters, phosphites, and alcohols. A typical procedure is to stir the liquid containing approximately 5% w/v calcium hydride for a period of time (1-2 h at least; preferably 24 h), and decanting, filtering, or distilling the liquid away from the calcium salts.2 Traces of water and cyanide may be removed from N,N-Dimethylformamide in this manner.3

Specific compounds dried with calcium hydride and analyzed for residual water content include benzene,2 dioxane,2 acetonitrile,2,4 methanol,5 ethanol,5 2-butanol5 and t-butyl alcohol.5 Benzene is dried to <=0.2 ppm and dioxane to <=50 ppm residual water.5 Simply stirring acetonitrile over calcium hydride for seven days and distilling leaves 1900 ppm water.5 A more rigorous drying protocol for acetonitrile has also been reported for electrochemical use.4 Methanol and ethanol are dried to ~100 ppm residual water content,5 in conflict with an early recommendation against using this drying agent.6 Apparently, calcium hydride reacts with water considerably faster than with alcohols, making it an excellent drying agent for these compounds. On the other hand, perhaps the most convenient drying agent for the lower alcohols is 3°A molecular sieves (powdered sieves are best for all but methanol, where beads are best).5 Molecular Sieves are as good as calcium hydride for methanol and ethanol, and superior for 2-butanol and t-butyl alcohol.5

Calcium hydride has also been used as a desiccant, removing water as it is formed in a reaction. For example, the condensation of methylboronic acid to trimethylboroxin (eq 2) proceeds in 58% yield in the presence of calcium hydride. Note that the reverse reaction is exothermic and quantitative. Other desiccants are not as efficacious.7 Similarly, calcium hydride removes the water formed in the condensation of methylboronic acid with pinacol (eq 3).8

Two instances have been reported where calcium hydride has apparently been used as a desiccant for residual water in reagents such as tetrabutylammonium halides. For example, benzylation of an alcohol with Potassium Hydroxide, Benzyl Bromide, calcium hydride, and Tetra-n-butylammonium Iodide (eq 4) was used in a ganglioside synthesis.9 Another such example is the SN2 ring opening of an allylic ether, which required calcium hydride, probably as a desiccant (eq 5).10

An early report describes calcium hydride as a basic catalyst for the 1,4-addition of 2-nitropropane to benzalacetophenone (eq 6).11 In this reaction the methanol is essential (no reaction without it), and the order of addition is important (the calcium hydride must be added last).

Calcium hydride assists in the acylation of alcohols (acting as a base), including tertiary ones, by anhydrides and acid chlorides (eq 7).12,13

The site of methylation of 2-aminoethanol is influenced by the gegenion of the hydride used: Sodium Hydride affords 96% O-methylation while calcium hydride yields exclusive N-methylation (eq 8).14

The Sharpless asymmetric epoxidation (SAE) is an extremely valuable technique for the synthesis of epoxy alcohols from allylic alcohols.15 Although the asymmetric epoxidation gives good yields (76-80%) and high selectivities (&egt;90% ee), it may require a long reaction time. Wang and co-workers studied the effect of additives on the Sharpless epoxidation of (Z)-2-tridecen-1-ol.16 Addition of silica gel alone did not affect the selectivity, the yield, or the reaction time. Addition of calcium hydride shortened the reaction time but decreased the selectivity. Addition of both, however, resulted in a reaction time shortened from 96 h to 8 h, with no loss of selectivity (eq 9). The calcium hydride/silica gel-modified Sharpless reagent has since been studied on a variety of allylic alcohols,17 and has been used by other groups as well.18

1. (a) Mackay, K. M. Hydrogen Compounds of the Metallic Elements; Spon: London, 1966. (b) Hurd, D. T. An Introduction to the Chemistry of the Hydrides; Wiley: New York, 1952. (c) Wiberg, E.; Amberger, E. Hydrides of the Elements of Main Groups I-IV; Elsevier: New York, 1971.
2. Burfield, D. R.; Lee, K.-H.; Smithers, R. H. JOC 1977, 42, 3060.
3. Fieser, L. F.; Fieser, M. FF 1975, 5, 247.
4. Walter, M.; Ramaley, L. Anal. Chem. 1973, 45, 165.
5. Burfield, D. R.; Smithers, R. H. JOC 1983, 48, 2420.
6. Fieser, L. F.; Fieser, M. FF 1967, 1, 105.
7. Brown, H. C.; Cole, T. E. OM 1985, 4, 816.
8. Brown, H. C.; Park, W. S.; Cha, J. S.; Cho, B. T.; Brown, C. A. JOC 1986, 51, 337.
9. Ito, Y.; Numata, M.; Sugimoto, M.; Ogawa, T. JACS 1989, 111, 8508.
10. Wakamatsu, K.; Kigoshi, H.; Niiyama, K.; Niwa, H.; Yamada, K. T 1986, 42, 5551.
11. Fishman, N.; Zuffanti, S. JACS 1951, 73, 4466.
12. Oppenauer, R. V. M 1966, 97, 62.
13. Helmchen, G.; Wegner, G. TL 1985, 26, 6051.
14. Kashima, C.; Harada, K.; Omote, Y. CJC 1985, 63, 288.
15. Katsuki, T.; Sharpless, K. B. JACS 1980, 102, 5974.
16. Wang, Z.; Zhou, W.; Lin, G. TL 1985, 26, 6221.
17. Wang, Z.; Zhou, W. T 1987, 43, 2935.
18. (a) Schwab, J. M.; Ray, T.; Ho, C. K. JACS 1989, 111, 1057. (b) Prestwich, G. D.; Graham, S. M.; Kuo, J. W.; Vogt, R. G. JACS 1989, 111, 636.

Robert E. Gawley & Arnold Davis

University of Miami, Coral Gables, FL, USA

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