Calcium in liquid ammonia

A solution of lithium, sodium, potassium or calcium in liquid ammonia can reduce a wide variety of unsaturated groups. Thus when aromatic rings are reduced by such metals in liquid ammonia, non-conjugated cyclohexadienes are produced. The reaction is called Birch reduction. 

Benzene and its homologs can be converted to the corresponding cyclo-hexadienes and cyclohexenes, and even cyclohexanes, by treatment with dissolving metals lithium, sodium, potassium or calcium in liquid ammonia or amines. Conversions are not complete, and the ratio of cyclohexadienes to cyclohexenes depends on the metal used, on the solvent, and on the presence of hydrogen donors (alcohols) added to the ammonia or amine [392, 393, 394]. 

Addition of benzene in methanol to an excess (25-100%) of sodium in liquid ammonia afforded 84-88% yields of 1,4-cyclohexadiene [392]. o-Xylene under the same conditions gave 70%-92% yield of 1,2-dimethyl-1,4-cyclohexadiene [392, 395]. Lithium in neat ammonia at 60° gave 91% of 1,4-cyclo-hexadiene and 9% of cyclohexene (conversion 58.4%), while calcium under the same conditions yielded 21% of cyclohexadienes and 79% of cyclohexene in conversions of 13-60% [393]. If benzene dissolved in ether was added to the compound Ca(NH3)s, preformed by dissolving calcium in liquid ammonia and evaporating ammonia, a 75% conversion to pure cyclohexene was achieved [394]. 

Similar results were achieved when benzene was reduced with alkali metals in anhydrous methylamine at temperatures of 26-100°. Best yields of cyclohexene (up to 77.4%) were obtained with lithium at 85° [396]. Ethylamine [397] and especially ethylenediamine are even better solvents [398]. Benzene was reduced to cyclohexene and a small amount of cyclohexane [397, 398] ethylbenzene treated with lithium in ethylamine at —78° gave 75% of 1-ethyl-cyclohexene whereas at 17° a mixture of 45% of 1-ethylcyclohexene and 55% of ethylcyclohexane was obtained [397], Xylenes m- and p-) yielded non-conjugated 2,5-dihydro derivatives, l,3-dimethyl-3,6-cyclohexadiene and 1,4-dimethyl-1,4-cyclohexadiene, respectively, on reduction with sodium in liquid ammonia in the presence of ethanol (in poor yields) [399]. Reduction of diphenyl with sodium or calcium in liquid ammonia at —70° afforded mainly 1-phenylcyclohexene [400] whereas with sodium in ammonia at 120-125° mainly phenylcyclohexane [393] was formed. 

Cyclic flve-membered mono- and dltiiioacetab were reduced with calcium in liquid ammonia to give )3-alkoxy- and )3-thioalkoxymercaptans, respectively RCH2OCH2CH2SH and RCHjSCHjCHjSH. Benzaldehyde ethylenedi-thioacetal, on the other hand, gave toluene (75%), and phenylacetaldehyde ethylenedithioacetal gave ethylbenzene (71% yield) [795]. 

Reduction of cyclic five-membered ethylene monothioketals with calcium in liquid ammonia cleaves the bond- between carbon and sulfur and yields alkyl -mercaptoethyl ethers (7-88%) [795]. Cyclic five-membered ethylene dithioketals (ethylenemercaptoles) afford, analogously, alkyl -mercaptoethyl thioethers (yields 85%) [795]. 

Calciferol, 416, 418-419 Calcium, in liquid ammonia, 56 Camphor, Baeyer-Villiger oxidation, 347... 

In contrast reduction of (I) by calcium in liquid ammonia gives, almost exclusively, the E isomer (3). Stereoselectivity is decreased using either lithium or sodium as the metal. 

Table 13 Reductions of Oxathiolanes and Oxathianes with Calcium in Liquid Ammonia " ...

Enders and Hundertmark [9] recently reported that debenzylation of 5 with calcium in liquid ammonia gives the corresponding alcohol 6 in quantitative yield [10]. Similar debenzylation of 7 produces 8 in 83% yield. The use of calcium, in contrast with the more reactive lithium, circumvents reduction of the existing phenyl ring. They also found that the calcium method is far more reliable than palladium-catalyzed hydrogenolysis, which is very sensihve to catalyst poisoning by traces of sulfur and tin by-product. 

In a total synthesis of sterols, van Tamelen et al. [11] developed a new method for selective removal of a benzyl group in an alkyne bearing a non-terminal C-C triple bond. Treatment of polyenyne benzyl ether 9 with 4.4 equiv. calcium in liquid ammonia gives the corresponding alcohol 10 in 92% yield (Scheme 4.2). 

In the reductive cleavage of y-lactone 17, Naito et al. [17] reported that use of calcium in liquid ammonia at -70 °C enables the production of the desired carboxylic acid 18 as the sole product in 62% yield (Scheme 4.5). Other procedures including use of aluminum amalgam, chromium(II) chloride, and zinc in acetic acid led to complete recovery of the starting y-lactone 17. 

On the other hand, reduction of cyclopropyl hahdes 61 with dissolved calcium in liquid ammonia at low concentration and at the boiling point of ammonia gives two ring-opened products 64 (78%) and 65 (22%). This reaction is believed to involve a solvated electron-calcium cation pair [40—43]. 

Calcium-Liquid NH3 [1, 106-107, before references]. Deaceloxylation of (1) was effected by reaction with calcium in liquid ammonia but could not be brought about with zinc in refluxing acetic anhydride.3... 

The second synthesis is based on the acid-catalysed fission of the Dieb-Alder adduct (60) to give the 4-substituted cyclohexenone (61). Further elaboration and chromatographic separation yielded the ester (62) (stereochemistry of the isobutyl group unassigned). Reduction of this compound with calcium in liquid ammonia followed by oxidation gave racemic juvabione. 

Studies have continued on the intercalation of the alkaline-earth metals into lattices of the transition-metal sulphides. Calcium and strontium are intercalated into M0S2 from liquid ammonia solution. X-Ray data reveal a lowering of the crystal symmetry of the sulphide and an increase in the complexity of the structure on intercalation. The intercalation compounds begin to superconduct at ca 4 K (for Ca) and 5.6 K (for Sr), and they show considerable anisotropy with respect to the critical magnetic field. Calcium in liquid ammonia also intercalates with TiS2- Two Ca,TiS2 phases have been identified. The limits of the first are 0.03 0.50, for which a relationship between x and cell parameters a and c... 

Trityl ethers are easily deprotected using sodium in liquid ammonia [Scheme 4.205] or calcium in liquid ammonia but catalytic hydrogenolysis appears to be unreliable. ... 

A note has been published on the conversion of an acetamido branched-chain sugar derivative to the corresponding nitro analogue (L-decilonitrose) by sequential N-deacetylation using calcium in liquid ammonia followed by m-chloroperbenzoic acid oxidation. Syntheses of D- and L-rubranitrose by Brimacombe s group are mentioned in Chapter 10, and the synthesis of 3 -C-methyldauno-rubicin (containing vancosamine) is covered in Chapter 19. 

In order to study a bis[8]annulene compound with a divalent central metal atom we have synthesized and characterized the divalent ytterbium complex K2[Yb(CgHg)2]F K2[Yb(CgHg)2]r and its calcium analogue, K2[Ca(CgHg)2].51 These compounds were prepared by a reaction which utilizes the solubility of ytterbium and calcium metals in liquid ammonia. Reaction of cyclooctatetraene, potassium and either ytterbium or calcium, in liquid ammonia solution, gives the desired potassium salt of the complex dianion. 

Oligosaccharides containing H-acetylamino groups can be quantitatively de-fi-acetylated by use of calcium in liquid ammonia. Polysaccharides were not fully il-deacetylated. ... 

---