Potassium Cyanide1

KCN

[151-50-8]  · CKN  · Potassium Cyanide  · (MW 65.12)

(reagent for the synthesis of nitriles,2 cyanohydrins,3 and a-amino nitriles;4 catalyst for benzoin condensation5 and transesterification6)

Physical Data: mp 634 °C; d 1.520 g cm-3.

Solubility: sol 2 parts cold water, 1 part boiling water, 2 parts glycerol, 100 parts ethanol, 25 parts methanol. The aq soln is strongly alkaline (pH = 11.0) and rapidly dec.

Form Supplied in: white granular powder or fused pieces; widely available.

Purification: see Perrin7 for purification and recrystallization procedures.

Handling, Storage, and Precautions: highly toxic; may be fatal if inhaled, swallowed, or absorbed through the skin. Exposure may cause nausea, dizziness, headache, lung irritation, and cyanosis (toxicity data: orl-hmn LDLo 2857 mg kg-1, orl-rat LD50 5 mg kg-1). Air and moisture sensitive; keep in a tightly closed container and store in dry area. Use only in a chemical fume hood. Do not breath the dust or allow it to get in eyes, on skin, or on clothing. In case of fire, water spray must be used with caution since direct contact of KCN with water or steam will cause decomposition, liberating highly toxic HCN gas as well as generating a highly hazardous solution of dissolved cyanide which must be kept out of sewers and watercourses. KCN also forms explosive mixtures, sometimes spontaneously, with chlorates, nitrates, and nitrogen trichloride plus ammonia. Incompatible with acids, strong oxidizing agents, alkaloids, chloral hydrate, iodine, and metallic salts. Thermal decomposition produces toxic fumes of hydrogen cyanide, carbon monoxide, carbon dioxide, and nitrogen oxides.

Nitrile Synthesis.

The transformation of alcohols to nitriles is an important method for the elongation of carbon chains or the synthesis of useful intermediates in organic synthesis.8 The usual procedure involves activation of alcohols by conversion into halides or sulfonates, followed by displacement with a cyano group. Among the methods for direct one-pot conversion of alcohols to nitriles,9,10 the most general and convenient is treatment with KCN in the presence of Tri-n-butylphosphine, CCl4, and 18-Crown-6 (eq 1).10 Displacement of primary, benzylic, or allylic halides with KCN produces nitriles in very good yields; the same reaction proceeds in moderate yield with secondary halides,8,11 while it fails completely for tertiary halides, giving elimination under these conditions. The use of KCN in MeCN containing 18-crown-6 produces cyanide anions (naked anions)12 which are strong nucleophiles and displace halides smoothly and quantitatively (eq 2).13 Mesylates are converted into nitriles in about 70% yield by reaction with KCN in H2O/C6H6 at 5-6 h reflux with phase-transfer catalysis.14 2,3-O-Isopropylidene-5-O-tosylates of nucleosides react with KCN only when the reagent has been complexed with 18-crown-6 (eq 3),15 while reaction of substituted benzylamines with cyanide ions furnish the corresponding benzonitriles.16 Ketones are converted into nitriles by way of tosylhydrazones17a,b or methoxycarbonyl hydrazones.17c Thus the tosylhydrazone derivative of a ketone upon treatment with KCN in MeOH/AcOH is converted into a cyano hydrazide, which is heated at 180 °C for 2 h to give the nitrile (eq 4).18 Hydrocyanation of conjugated double19 or isolated double20 and triple21 bonds proceed smoothly with KCN. Similarly, KCN in combination with NH4Cl results in 1,4-addition of HCN to cholestanone derivatives, affording the 5a- and 5b-cyano ketones in 1:1 ratio.22 The use of organoaluminum cyanides increases the selectivity of the reaction.19

Vinyl bromides are converted into a,b-unsaturated nitriles by reaction with KCN in the presence of Tetrakis(triphenylphosphine)palladium(0) and 18-crown-6. The reaction is highly stereospecific and proceeds in almost quantitative yield (eq 5).23 Vinyl sulfones on exposure to KCN and Dicyclohexano-18-crown-6 in refluxing t-butyl alcohol are smoothly converted to a,b-unsaturated nitriles, providing a particularly useful route for the construction of trisubstituted alkenes.24

Treatment of benzaldoximes with KCN in the presence of a phase-transfer catalyst (18-crown-6 or quaternary ammonium salt) produces a 3:1 mixture of a benzonitrile and benzamide (eq 6),25 thus providing a general, one-pot conversion of aromatic aldehydes to aryl nitriles. Photochemically induced cyanation of arenes26 furnishes only moderate to low yields of aryl nitriles, but the use of KCN in conjunction with 18-crown-6 (naked cyanide ion) doubles the yield.27 Photolysis of arylthallium ditrifluoroacetates in the presence of excess KCN proceeds via in situ formation of the complex ions [ArTl(CN)3]-K+ and produces aryl nitriles in good yields.28 Cyanation of aromatic heterocycles by means of KCN and Benzyltriethylammonium Chloride followed by reaction with an acyl chloride furnishes the corresponding Reissert products (eq 7)29 in better yields than the conventional conditions.30

The classical method for the synthesis of acyl cyanides is the reaction of an acyl halide with heavy metal cyanides at high temperatures.31 Recent modifications using aryl iodide32 or a phase-transfer catalyst33 have led to some improvements. On the contrary, aroyl cyanides are prepared by reaction of aroyl chlorides with KCN in MeCN. The presence of trace amounts of water markedly accelerates the rate and improves the yield (eq 8).34 However, the use of too much water lowers the yield of the product due to hydrolysis and dimer formation.34 Aromatic or heteroaromatic acyl cyanides may be prepared at lower reaction temperatures by ultrasonic treatment of the heterogeneous mixture of an aroyl chloride and KCN in MeCN (eq 9).35 Several phenylacetonitriles are obtained in good yield by reaction of the appropriate benzyl halide with the cyanide form of an anion exchange resin.36 The direct conversion of aromatic iodides into aroyl nitriles in good yields is accomplished by their carbonylation catalyzed with Iodo(phenyl)bis(triphenylphosphine)palladium(II)37 and in situ reaction with KCN (eq 10).38 Cyanoformates are obtained from the corresponding chloroformates by reaction with KCN in the presence of 18-crown-6 as catalyst (eq 11).39

a-Cyanoenamines are obtained either by conversion of amides to a-chloroenamines and subsequent treatment with KCN in MeCN (eq 12),40 or starting with aldehydes and proceeding via a-chloroaldimines and reaction with KCN in MeOH (eq 13).41 These compounds are recognized as reactive intermediates since they can easily be converted to the corresponding a-diketones or dihydropyrazine derivatives.40

Cyanotributylstannane, which is an efficient cyanation agent for acyl chlorides, is obtained in very good yield by treatment of Tri-n-butylchlorostannane with KCN in MeCN (eq 14).42 Similarly, Cyanotrimethylsilane is prepared by treatment of Chlorotrimethylsilane with H2SO4 and KCN (eq 15).43

Cyanohydrin Synthesis.

Cyanohydrins are versatile starting materials for the synthesis of several classes of compounds.44 They are prepared by treatment of aldehydes or ketones with KCN and AcOH in various organic solvents.45 This is an equilibrium reaction which proceeds smoothly except with sterically hindered ketones or aromatic aldehydes, in which case the benzoin condensation competes. Cyanohydrin synthesis was first applied to carbohydrates by Kiliani46 in 1885 as the key step of a route (modified later by Fischer47) which allows the construction of two epimeric aldoses having one carbon atom more than the parent aldose. More recent studies have confirmed that the ratio of epimers (eq 16) is pH dependent.48 Thus reaction of D-arabinose with KCN at acidic pH furnishes D-glucononitrile as the predominant epimer, while under alkaline conditions D-mannonitrile is the predominant epimer. Similarly treatment of formaldehyde with KCN gave glyconitrile (eq 17).49 Cyanohydrin synthesis is important in the construction of steroid sidechains.50 Thus 17-keto steroids are transformed to corticoids via their 17b-cyanohydrins (eq 18).51 Treatment of b-hydroxy ketones with KCN/TMSCN in the presence of Zinc Iodide produces syn-b-hydroxy cyanohydrins in high distereomeric excess (eq 19).52

Optically active cyanohydrins are obtained using KCN and enzymatic catalysis.53 Thus treatment of benzaldehyde with oxynitrilase (extracts of ground defatted almonds) in 1N KCN/AcOH buffer (pH 5.4) furnishes exclusively (R)-(+)-a-hydroxyphenylacetonitrile (eq 20).54 The other enantiomer is obtained by kinetic resolution of racemic cyanohydrins either by using a lipoprotein lipase from Pseudomonas species55 or by incubation with Pichia miso (IAM 4682), which hydrolyzes selectively the acetate of the (R)-enantiomer, leaving the (S)-enantiomer intact.56 Silylated cyanohydrins are obtained in high yields by a one-pot reaction using KCN in combination with TMSCl (eq 21).57,58 The presence of ZnI2 enhances the rate of cyanosilylation, while the presence of 18-crown-6 has very little effect on the yield or the rate.57

a-Amino Nitriles.

The preparation of a-amino nitriles is the key reaction of the Strecker synthesis,59 which is considered to be the most convenient experimental protocol for the preparation of a-amino acids4 and 1,2-diamines60 on the preparative scale. The classical procedure in which an aldehyde or ketone is treated with KCN, NH4Cl,59 is sensitive to the nature of the ketone substitution, involves lengthy reaction times and tedious work-up procedures, and leads to considerable amounts of cyanohydrin byproduct.61 Modifications using Al2O3 and ultrasound in MeCN have improved the yield slightly.62 On the contrary, stepwise synthesis either by displacement of the hydroxy group of a-amino alcohols with cyanide63 using KCN, AcOH, or via cyanohydrins64 by treatment with ammonia or NaN3 have greatly improved the yields. Recently, however, two simple one-pot procedures have been reported. These produce a-amino nitriles efficiently, in almost quantitative yields, by treatment of carbonyl compounds with KCN and either alkyl- or benzylamines in an aqueous solution of NaHSO3 (eq 22)65 or benzylamine in methanolic AcOH (eq 23).66 It is noteworthy that the a-amino nitrile synthesis (Strecker method) with substituted cyclic ketones furnishes preferentially a-amino acids of one geometrical isomer (eq 24),67 while the opposite isomer is obtained via the spirohydantoin synthesis (Bucherer method) (eq 25).67 Optically active a-amino nitriles are obtained when a chiral auxiliary is used. Thus reaction of R-(-)-2-phenylglycinol with isobutylaldehyde furnishes the two diastereomers in an 84:16 ratio (eq 26).68

Benzoin Condensation.

Aromatic69 or heteroaromatic aldehydes70 are condensed in a KCN-catalyzed process to form benzoins (a-hydroxy ketones) (eq 27). The reaction mechanism involves the formation of a cyano-stabilized carbanion,71 facilitated by the use of an aprotic solvent. KCN is a highly specific catalyst, since it performs three functions:72 (i) acts as a nucleophile, (ii) permits loss of the aldehyde proton with its electron-withdrawing ability, and (iii) at the end, acts as a leaving group. The benzoin-type reaction has been extended to include a catalyzed addition of aldehydes to a,b-unsaturated nitriles.73 Thus 4-keto nitriles are obtained by addition of aromatic aldehydes to a,b-unsaturated nitriles under KCN catalysis (eq 28). Similarly, benzoin-type condensation of tetraphthalaldehyde bisbisulfite with benzaldehyde under KCN catalysis furnishes the bisbenzoin product (eq 29).74

Transesterification.

KCN is a mild and effective catalyst for the transesterification of a,b-unsaturated esters without isomerization of the conjugated double bond. Thus methyl trans,trans-farnesoate is readily converted into the ethyl ester with very slight cis/trans isomerization (eq 30).75 Similarly, KCN in combination with 18-crown-6 is an effective catalyst for cyanoesterification.76 On the other hand, treatment of d-lactones with methanol in the presence of KCN results in lactone ring opening and methyl ester formation (eq 31).77

Ring Formation and Ring Opening.

Reaction of a-chloro ketimines with KCN in MeOH results in nucleophilic addition and subsequent intramolecular nucleophilic substitution, yielding the corresponding a-cyano aziridines (eq 32).78 Treatment of 2-bromodeoxybenzoin with KCN in the presence of solid adsorbent (silica gel or alumina) gives stereoselectively cis-2,3-diphenyl-2-cyanooxirane in good yield (eq 33).79 Cold aqueous ethanolic KCN converts N-methyl-C-phenyl nitrone into 1-methyl-4,5-diphenylimidazole via the intermediate cyanoimine (eq 34).80 N-Allyl-N-3-butenyl-N-Cbz-amine on treatment with thexylborane, followed by cyanidation, affords an azacyclone, which can be transformed into d-coniceine by catalytic reduction (eq 35).81 On the other hand, alkaline solutions of KCN in the presence of Tetra-n-butylammonium Bromide open 1,2-epoxides, yielding b-hydroxy nitriles (eq 36).82 Similarly activated cyclopropane rings are cleaved by reaction with KCN.83

Aromatization.

KCN in DMF is an effective reagent for the conversion of 2,4-cyclohexadien-1-ols to benzene derivatives.84 The same conditions were applied for the aromatization of 2,4-cyclohexadiene-6-imine derivatives (eq 37).85

Reduction.

0.1 N aqueous KCN solutions of aromatic nitro compounds upon UV irradiation undergo deoxygenation to the corresponding nitroso compounds (eq 38).86

Related Reagents.

Acetone Cyanohydrin; Acetyl Cyanide; t-Butyldimethylsilyl Cyanide; Cyanotributylstannane; Cyanotrimethylsilane; Diethylaluminum Cyanide; Hydrogen Cyanide; Tetraethylammonium Cyanide; Zinc Cyanide.


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Serkos A. Haroutounian

Agricultural University of Athens, Greece



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