Isocyanic Acid1

(HNCO)

[75-13-8]  · CHNO  · Cyanic Acid  · (MW 43.03) (HOCN)

[420-05-3]  · CHNO  · Isocyanic Acid  · (MW 43.03) (KOCN)

[590-28-3]  · CKNO  · Potassium Cyanate  · (MW 81.12) (NaOCN)

[917-61-3]  · CNNaO  · Sodium Cyanate  · (MW 65.01) (AgOCN)

[3315-16-0]  · CAgNO  · Silver Cyanate  · (MW 149.89)

(synthesis of isocyanates, allophanates, and some heterocyclic compounds)

Physical Data: mp -86 °C; bp 23.5 °C (by extrapolation); d 1.14 g cm-3; pKa 3.66.

Solubility: poorly sol aliphatic hydrocarbons; sol benzene, toluene, CCl4, ether, THF, etc. Drying: over P2O5.

Analysis of Reagent Purity: see Belson and Strachan.1c

Preparative Methods: by the depolymerization of cyanuric acid,1a,b or by heating urea.1c The gas can be absorbed in ether, THF, CHCl3, or CCl4. Aqueous solutions of HNCO can be prepared in situ by reacting NaOCN or KOCN with 1 equiv of dil HCl or AcOH at temperatures of approx 0-15 °C.

Handling, Storage, and Precautions: isocyanic acid (gas, liquid, or as concentrated solution in anhyd organic solvents) readily polymerizes from -20 °C to rt to give an easily separable mixture of cyanuric acid and cyamelide (a polymer whose structure is as yet unknown), but dilute solutions in ether or CCl4 can be kept at rt without serious decomposition.1c In contact with moisture, even at 0 °C, it hydrolyzes to NH3 and CO2.1c Stored at dry ice or liquid nitrogen temperature, it is quite stable.1c Besides being a strong acid, lacrymator, and vesicant, it possesses an acrid odor. The highly toxic character of isocyanic acid requires that it should be used only in a well-ventilated fume hood.

Introduction.

Of the two acids, cyanic acid (H-O-C&tbond;N) and isocyanic acid (H-N=C=O) (1), that arise from the ambident nucleophile cyanate, (1) is the more stable of the two. Since cyanic acid is kinetically unstable,1a,b it is vital to recognize at the outset that much of the preparative work mentioned in the scientific literature for cyanic acid has, in fact, utilized isocyanic acid. Isocyanic acid, for all practical purposes, whether in the gaseous, liquid, or solid state, has the carbimide structure H-N=C=O, and, although in principle it can tautomerize to give cyanic acid, the observed proportion of cyanic acid in organic solvents is essentially negligible.1c Even in aqueous media, it is the isocyanic acid tautomer that predominates.1c

Addition of Polar Compounds to Isocyanic Acid.

Because of the presence of electrophilic functionality (-N=C=O) in isocyanic acid (1), a wide range of polar reagents2 adds across its HN=C bond, such that their nucleophilic end attacks at carbon, and the positive part at nitrogen. In this way, cyclohexylamine forms N-cyclohexylurea in 75% yield,3a alcohols (primary, secondary, and tertiary ROH) give moderate yields3b-d of crystalline allophanates (2), often contaminated with carbamates, and Diphenylphosphine reacts efficiently with (1) in degassed benzene at rt to form carbamoyldiphenylphosphine (3) in 75% yield.4a With an excess of Trifluoroacetic Anhydride at rt, (1) gives perfluorodiacetamide (4) in 90% yield.4b

Formation of Isocyanates.

Because of the ionizable H-N bond, isocyanic acid behaves like a pseudo hydrogen halide. In the presence of a suitable catalyst and appropriate conditions, it adds to reactive unsaturated compounds, and undergoes substitution reactions at Si, Sn, and P. Thus several novel isocyanates have been prepared,5a by reaction of (1) with electron-rich alkenes,5b vinyl ethers,5c reactive carbonyl compounds,5d-g chlorosilanes,5f phosphorus halides,5f organometal oxides,5h and pentafluoroguanidine,5i etc. Some selected examples of isocyanates thus obtained (including yields) are:

  • 1)a,a-Dimethylbenzyl isocyanate (PhCMe2NCO, 41%)5b from a-methylstyrene.
  • 2)1-Butoxy-1-methylethyl isocyanate (BuOCMe2NCO, 71%)5c from n-butyl isopropenyl ether.
  • 3)1-(2-Methacryloyloxyethoxy)ethyl isocyanate (5) (83%)5c from vinyloxyethyl methacrylate.
  • 4)Hydroxymethyl isocyanate (HOCH2NCO, 90-95%)5d from formaldehyde at -78 °C. Most carbonyl compounds capable of forming hydrates, e.g. chloral, trifluoroacetaldehyde, hexafluoroacetone, perfluorobutanone, s-dichlorotetrafluoroacetone, give a-hydroxy isocyanates which display their normal reactivity pattern. For example, HOCH2NCO polymerizes explosively above 0 °C, but it reacts with dihydropyran under catalysis with p-Toluenesulfonic Acid at -5 °C to give 2-tetrahydropyranyloxymethyl isocyanate (THPOCH2NCO, 67%) and it has been converted into chloromethyl isocyanate (ClCH2NCO) at -78 °C.5e
  • 5)Acetyl isocyanate (AcNCO, 15%)5f from acetyl chloride.
  • 6)Trimethylsilyl isocyanate (Me3SiNCO, 57%)5f from Me3SiCl.
  • 7)Diphenyl phosphor(isocyanatidite) [(PhO)2PNCO, 70%]5f from diphenyl phosphorochloridite [(PhO)2PCl], and [PhOP(NCO)2, 61%]5f from PhOPCl2.
  • 8)Trifluoroacetyl isocyanate (CF3CONCO 37%)5g from trifluoroacetic anhydride.
  • 9)Tri-n-butyltin isocyanate (Bu3SnNCO, 90%)5h from dibutyltin oxide.
  • 10)Bis(difluoramino)fluoraminomethyl isocyanate (6) (30-50%)5i from pentafluoroguanidine. Compound (6) on fluorination in the absence of NaF gives tris(difluoramino)methyl isocyanate [(F2N)3CNCO] in 80% yield.5j

    Formation of Heterocycles.

    Because of the ambident reactivity of isocyanic acid, when b-propiolactone and (1) are combined (1:1) with a catalytic amount of Boron Trifluoride Etherate at -22 to 0 °C, 3,4,5,6-tetrahydro-2H-1,3-oxazine-2,4-dione is obtained in 95% yield (eq 1).6 Likewise, the vinylogous carbamate (7) upon condensation with (1) and acetaldehyde in ether at rt gives, in 72% yield, dihydropyrimidin-2-one (8) (eq 2), useful in the synthesis of the marine toxin saxitoxin.7 Treatment of (6) with 1 equiv of (1) and 0.1 equiv of urea at -35 °C for 3 h gives the 5-fluorodihydro-s-triazine-2,4(1H,3H)-dione (9) in 90-100% yield (eq 3).5i Finally, the facile addition of the H-N bond of (1) across the HN=C bond of another molecule explains why (1) readily trimerizes to give cyanuric acid and the polymer cyamelide

    Cyanic Acid Salts (Potassium, Sodium, and Silver).

    In contrast to cyanic acid, its alkali metal salts (potassium cyanate, sodium cyanate, and silver cyanate) are stable,1d economical, and convenient starting materials for the synthesis of inorganic and organic isocyanates as well as for the in situ preparation of aqueous (1). In assessing their reactivity pattern, it is important to note that, whereas both Na+ and K+ are hard electrophiles, Ag+ is a soft electrophile.

    Inorganic Isocyanates.

    Inorganic isocyanates are prepared by the displacement of halogen attached to the element of interest (B, Si, Ge, Sn, Pb, N, P, As, Sb, O, S, Cl, Br, and I) with a salt of cyanic acid. However, not all elements (N and O) give stable isocyanates, and some (e.g. I-NCO) can only be prepared in situ and utilized accordingly.8 Thus reaction of AgOCN with Bromodimethylborane gives Me2B-NCO in 90% yield.9 Similarly, isocyanates have been prepared by the displacement of a halogen at Si, Ge, Sn, S, and P with AgOCN.8 Isocyanates of Si (TMS-NCO), S (e.g. PhSO2-NCO, p-MeC6H4SO2-NCO, and ClSO2-NCO), and I (I-NCO) are covered separately in this encyclopedia. More recently, methyl phenylsulfenylcarbamate (PhS-NHCO2Me) and N-phenyl-N-(phenylsulfenyl)urea (PhS-NH-CO-NHPh) have been obtained, in 47% and 36% yield, by trapping the in situ formed phenylsulfenyl isocyanate (PhSCl + AgOCN -> PhSNCO) in benzene, with MeOH or PhNH2, respectively.10 With NaOCN, displacement of a chloro group at P in MeP(O)ClF, MeP(O)ClN3, and PhPCl2, takes place to give the expected isocyanates.8,11

    Organic Isocyanates.

    Although conversion of a simple alkyl halide, sulfate, tosylate, or phosphate into its respective isocyanate using cyanic acid salts can be satisfactorily achieved in the laboratory, this topic is a subject of numerous patents. Thus while Et-NCO can be obtained in good yields by simply heating Et2SO4, ethyl p-toluenesulfonate, or triethyl phosphate12 with KOCN, the reaction of EtBr with KOCN is best performed8 in dimethyl sulfone as a solvent. In some cases, the choice of the solvent employed is very critical to the success of the reaction. Use of an aprotic solvent such as DMF alone should be avoided, since organic isocyanates readily trimerize in DMF to give isocyanurates.13 Diluting DMF with xylene essentially overcomes this problem, and in this way MeOCH2-NCO has been obtained from NaOCN and MeOCH2Cl.14 AgOCN in ether and 2,3-dichloro-1,4-dioxane (10) give the diisocyanate (11) in 40-50% yield (eq 4).15

    When crotyl bromide is treated with AgOCN in ether for 3 h at rt, 2-butenyl isocyanate (67%) and 1-methylallyl isocyanate (28%) are obtained in 84% yield.16 The reaction of AgOCN with 3-bromopropanoyl bromide in benzene under sonication in an ice-water bath gives 3-bromopropanoyl isocyanate (BrCH2CH2CO-NCO) in 55% yield.17 This procedure, when extended to 4-bromobutanoyl chloride, gives 4-bromobutanoyl isocyanate (BrCH2CH2CH2CO-NCO) in only 10% yield.17 The displacement of a halogen at carbon with metal cyanates has also been accomplished in the presence of a phase transfer catalyst.18 Thus when bromoalkyl (meth)acrylate (12) (15 mmol) in 5 mL of anhyd MeCN is added to a slurry of NaOCN (22.5 mmol) and Bu4NBr (3 mmol) in 15 mL of dry MeCN at rt and the mixture is heated at 55 °C for 21 h, the isocyanate (13) (59%) and the isocyanurate (14) (32%) are readily obtained. When the above mixture is heated at 65 °C for 62 h, the isocyanurate (14) is obtained in 75% yield. However, when the above reaction is performed in the presence of water (1.2 equiv), and the mixture is heated at 60 °C for 24 h, the symmetrical urea (15) is obtained in 81% yield. Substituting the water with 6 equiv of MeOH gives the carbamate (16) in 90% yield.18 Finally, it should be noted that treatment of alkyl halides with alkali metal or silver cyanate is of no preparative value in the synthesis of cyanates.19

    N-Carbamates and Polyurethanes.

    The reaction of alkali metal cyanates with simple alkyl halides in DMF, in the presence of a suitable alcohol, provides a direct synthesis of N-substituted carbamates.20 The initial reaction is an SN2 process, which exhibits the expected reactivity, i.e. allylic, benzylic, and primary halides react faster than secondary halides, and the reaction fails with tertiary halides. In a typical experiment, an appropriate alkyl halide (50 mmol) in 5 mL of DMF is added to a well stirred slurry of KOCN (75 mmol) and EtOH (85 mmol) in DMF (40 mL) at 100-120 °C. After heating for an additional 4.5-45 h (depending upon the substrate), the N-carbamate can be isolated readily. In this way, 2-chlorobutane gives s-BuNHCO2Et (55%); the yields of n-BuNHCO2Et and BnNHCO2Et) from n-BuCl and PhCH2Cl are 77% and 88%, respectively. By appropriately increasing the molar ratio of KOCN and EtOH, both the chlorine atoms in a,a-dichloro-p-xylene or (E)-1,4-dichloro-2-butene can be displaced to obtain the expected N-biscarbamates in 70% and 77% yield.20 Substituting EtOH with a diol [HO(CH2)nOH] in the above reaction is the key to the synthesis of the poly(polymethylene tetramethylenedicarbamate) type of polyurethanes (eq 5).21

    A simple synthesis of methyl N-(1-adamantyl)carbamate [1-(methoxycarbonylamino)adamantane] has been achieved in 54% yield by reaction of 1-adamantanol or its nitrate with KOCN, MeOH, and concd H2SO4 in CHCl3.22 The reaction probably proceeds by the alkylation of the 1-adamantyl cation with in situ formed carbamic acid (H2NCO2H) or methyl carbamate.

    O-Carbamates.

    Addition of alcohols to isocyanic acid in ether gives mixtures of O-alkyl carbamates and allophanates (2). However, when Trifluoroacetic Acid (2 equiv) is added to a suspension of 1 equiv of alcohol and 2 equiv of NaOCN (curiously enough, KOCN cannot be substituted for NaOCN) in benzene or methylene chloride and slowly stirred at rt, excellent yields of O-alkyl carbamates are obtained.23 This method is applicable to the synthesis of carbamates from primary, secondary, or tertiary alcohols24 (2 h reaction time affords 60-90% yield), including propargyl alcohol (eq 6), cyclic and acyclic 1,3-diols, phenols, aldoximes, ketoximes, n-BuSH, and t-BuSH.23

    Reaction with Allenyl Cations.

    Potassium cyanate in 70:30 acetone-water (v/v) reacts at rt, under solvolytic conditions,25 with 1,3,3-triphenylchloroallene (17) to form an unstable propargylcarbamic acid which, upon treatment with base, gives the propargylamine (18) in 45% yield (eq 7).

    Reaction with Photogenerated Arylvinyl Cations.

    When a two-phase mixture of 1-bromo-1,2,2-tris(p-methoxyphenyl)ethylene (19) (1 mmol), KOCN (10 mmol), and n-Bu4NCl (2 mmol) in CH2Cl2 (90 mL) and H2O (10 mL) is irradiated through a Pyrex filter under N2 at 10-15 °C, the photogenerated arylvinyl cation (20) is captured by the cyanate ion via N-attack to form the b-aryl isocyanate (21) (eq 8). This readily cyclizes to give 7-methoxy-3,4-bis(p-methoxyphenyl)-1-isoquinolone (22) in 93% (at 70% conversion) yield.26 The presence of the p-MeO group in the aromatic ring helps stabilize the vinyl cation and thus is responsible, in part, for the excellent yields obtained in this reaction.

    Reaction with a a-Chlorocarbenium Ion.

    Potassium cyanate in DMF reacts with the Vilsmeier-Arnold reagent (23) to form initially a highly reactive intermediate, 4-(dimethylamino)-1-oxa-3-azabutatrienium chloride (24), which reacts with DMF to give the Gold salt (25) in 86% yield (eq 9).27 Similarly, when bis(4-chlorophenyl)chlorocarbenium hexachloroantimonate (26) is refluxed with KOCN in 1,2-dichloroethane, and the intermediate 1-oxa-3-azabutatrienium salt (27) is treated with 4,4-dichlorobenzophenone, tetrakis(4-chlorophenyl)-2-azaallenium hexachloroantimonate (28) is obtained in 67% yield (eq 10).28

    Reaction with Amines.

    When 1 equiv of NaOCN or KOCN is added to an ice-cold solution of dilute HCl or AcOH, the in situ generated (1) reacts with aromatic and aliphatic amines to give ureas (RNH2 -> RHNCONH2 and R2NH -> R2NCONH2). With excess isocyanic acid, biuret (R2NCONH2 -> R2NCONHCONH2) can also be formed.1b Hence, the temperature, pH, and the stoichiometry of the reagents employed should be carefully chosen.29 More recently, 2-ureido-3,4,5-trimethoxybenzoic acid has been prepared from 2-amino-3,4,5-trimethoxybenzoic acid on >100 kg scale, with yields approaching nearly 100%.30 Also, N-(2-sulfoethyl)urea (H2NCONHCH2CH2SO3H) has been obtained from taurine (H2NCH2CH2SO3H), KOCN, and HCl in 88% yield.31a Ureido derivatives of amines have been utilized in an efficient entry to the synthesis of cinodine antibiotics,31b and in a model study aimed at the synthesis of the marine hepatotoxin cylindrospermopsin.31c

    Heterocyclic Synthesis.

    The cyclization reactions of various bifunctional compounds with isocyanic acid or cyanate salts provide a general method for heterocyclic ring formation. Representative examples of these reactions are illustrated in eqs 11-18 by syntheses of 5,6-dimethoxy-4-methyl-2(1H)-quinazolinone from an o-aminoacetophenone (eq 11),32 4-methyl-5-pentyl-4-imidazolin-2-one from 3-bromo-2-octanone (eq 12);33 N3-substituted hydantoins (30) from chloroacetanilides (29) (eq 13);34 3-aryl-1,2,4-oxadiazol-5(4H)-ones (33) from aryl hydroxamoyl chlorides (31), presumably via the intermediate nitrile oxide (32) (eq 14);35 1-phenyl-3-trifluoromethyl-D2-1,2,4-triazolin-5-one (36) from N-phenyltrifluorohydrazonoyl bromide (34), probably by [3 + 2] dipolar cycloaddition with the nitrilimine (35) (eq 15);36 7-methyl-2-phenyl-1,2,4,7-tetraazaspiro[4.5]decan-3-one (38) from the phenylhydrazone (37) (eq 16);37 2-morpholinothieno[2,3-d]pyrimidin-4(3H)-one (40) from the carboximidoyl chloride (39) (eq 17);38 and 3,3-bis(1,2,4-oxadiazol-5(4H)-one) potassium salt from dichloroglyoxime (eq 18).39 Similar reactions of (1) occur with azines to form bitriazoles40 and with Schiff bases to form four- or six-membered heterocyclic structures.41

    Silver(I) Catalyzed Cyclization of O-Propargyl and O-(2,3-Butadienyl) Carbamates.

    Silver cyanate, while serving as a source of Ag+, can be efficiently utilized as a catalyst for the amino cyclization of O-propargyl42 and O-(2,3-butadienyl)43 carbamates according to eqs 19 and 20. Success of the cyclization according to eq 19 depends upon the presence of t-BuOK, while in eq 20, Et3N is necessary. When R1 = t-Bu (eq 20), the trans isomer is the predominant product, but with R1 = Me or Et, mixtures of trans and cis isomers are produced. The reaction works best with R2 = Ts, and poorly with R2 = Ac.

    Related Reagents.

    Benzenesulfonyl Isocyanate; Chloroacetyl Isocyanate; Chlorosulfonyl Isocyanate; Iodine Isocyanate; Methyl Isocyanate; Phenyl Isocyanate; p-Toluenesulfonyl Isocyanate; Trichloroacetyl Isocyanate; Isocyanatotrimethylsilane.


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    2. Polar reagents which add to isocyanic acid, and are discussed in Ref 1c, include among others the following: H2O, HCl, H2SO4, carboxylic acids, primary, secondary, and tertiary alcohols, ammonia, primary and secondary amines, phenols, hydroperoxides, oximes, H2NOH, HNCS, HNCO, and pyrazolines.
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    Acharan S. Narula & Kishore Ramachandran

    University of North Carolina at Chapel Hill, NC, USA



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