Cyanates, Isocyanates, Thiocyanates, and Isothiocyanates

7 Cyanates, Isocyanates, Thiocyanates, and Isothiocyanates H Chemical Shifts and Coupling Constants 0 in ppm, J in Hz) 

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As the bonding arrangement of cyanate and isocyanate groups to tellurium is not known (see p. 617), diorgano tellurium halide cyanates, isocyanates, thiocyanates, and isothiocyanates are treated together on the basis of formal tellurium-nitrogen bonding. 

QUINOXALINE CYANATES, ISOCYANATES, THIOCYANATES, ISOTHIOCYANATES, AND NITRONES... 

Reactions of Pyrazine Ketones Pyrazine Cyanates, Isocyanates, Thiocyanates, Isothiocyanates, and Carbonitrile Oxides... 

Emil Fischer, as part of a study of a series of S- and iV-glucosides, prepared 1-D-glucosylurea and 1-D-glucosylthiourea from the corresponding isocyanate (IV) and isothiocyanate (V), respectively. These intermediates were obtained by the condensation of tetra-O-acetyl-a-D-glucopyranosyl bromide (III) with silver cyanate or thiocyanate in anhydrous xylene. The... 

When the reagent is the thiocyanate ion, S-alkylation is an important side reaction (10-43), but the cyanate ion practically always gives exclusive N-alkylation. ° Primary alkyl halides have been converted to isocyanates by treatment with sodium nitrocyanamide (NaNCNN02) and m-chloroperoxybenzoic acid, followed by heating of the initially produced RN(N02)CN. ° When alkyl halides are treated with NCO in the presence of ethanol, carbamates can be prepared directly (see 16-7). ° Acyl halides give the corresponding acyl isocyanates and isothiocyanates. For the formation of isocyanides, see 10-111. 

Several other propargylic substrates have successfully undergone [3,3]-sigmatropic rearrangements. Examples are thiocyanates to isothiocyanates [580], the opposite conversion [581] and the equilibrium between such species as exemplified by 173 (Scheme 1.77) [582], cyanates to isocyanates [582] and the formation of isoselenocya-nates [583] and azides [584]. 

Isocyanate and isothiocyanate carbon nuclei resonate between 120 and 130 ppm [77a]. Isothiocyanate carbons are slightly deshielded relative to comparable isocyanates. Typical nitrile shift values (110-115 ppm) are characteristic of thiocyanates (rhodanides), while the carbon nuclei of rhodanide and cyanate anions shift to lower field due to significant contributions of heterocumulene-type resonance formulae. 

The earliest method of this type was the old Marckwald synthesis (1] in which a suitable a-aminocarbonyl compound is cyclized with cyanate, thiocyanate or isothiocyanatc. More recent modifications have employed the acetals of the a-amino aldehyde or ketone or an a-amino acid ester. The two-carbon fragment can also be provided by cyanamide, a thioxamate, a carbodiimidc or an imidic ester. When cyanates, thiocyanates or isothiocyanates are used, the imidazolin-2-ones or -2-thiones (1) are formed initially, but they can be converted into 2-unsubstituted imidazoles quite readily by oxidative or dehydrogenative means (Scheme 4.1.1). The chief limitations of the method arc the difficulty of making some a-aminocarbonyls and the very limited range of 2 substituents which are possible in the eventual imidazole products. The method is nonetheless valuable and widely used, and typically condenses the hydrochloride of an a-amino aldehyde or ketone (or the acetals or ketals), or an a-amino-)6-ketoester with the salt of a cyanic or thiocyanic acid. Usually the aminocarbonyl hydrochloride is warmed in aqueous solution with one equivalent of sodium or potassium cyanate or thiocyanate. An alkyl or aryl isocyanate or isothiocyanate will give an A-substituted imidazole product (2), as will a substituted aminocarbonyl compound (Scheme 4.1.1) [2-4]. 

Reviews of the Cope reaction, electronic quantum fluxes during the rearrangement of semibullvalene," the chemistry of hetero-substituted isocyanates and isothiocyanates RX-NCY (X = R2N, RO, RS Y = 0, S) and the isomeric cyanates RX-OCN, thiocyanates RX-SCN, nitrile oxides RX-CNO, and nitrile sulfides RX-CNS have appeared. ... 

Interestingly, nucleophiles in which two adjoined atoms have nonbonded electrons are particularly reactive as nucleophiles. Thus, a nucleophile, such as the peroxide anion (HOO ) (Table 7.5a), is about 102 times more reactive than the corresponding hydroxide anion (HO ) and the azide anion ( N=N =N ) (Table 7.5i) is particularly effective. In the same vein, bidentate (literally two toothed ) anions such as thiocyanate (isothiocyanate) ( S-ON <-> S=C=N ) (Table 7.5c) and cyanate (isocyanate) ( O-ON 0=C=N ) (Table 7.5d) are highly reactive with the former yielding mostly thiocyanate (S > N) and the latter mostly isocyanate (N > O) product. 

Retro-ene type [2 r + In + 2a] and [3,3]-sigmatropic shift reactions involving the substituent groups R in heteroatom-substituted cyanates and thiocyanates RX-YCN and the isomeric isocyanates and isothiocyanates of the type RX-NCY (X = CR2, NR, O, or S, Y = O or S) have been investigated computationally (Scheme 3). ... 

Other functional groups that are easily differentiated are cyanide (5c =110-120) from isocyanide (5c = 135- 150), thiocyanate (5c =110-120) from isothiocyanate (5c = 125 - 140), cyanate (5c = 105- 120) from isocyanate (5c = 120- 135) and aliphatic C atoms which are bonded to different heteroatoms or substituents (Table 2.2). Thus ether-methoxy generally appears between 5c = 55 and 62, ester-methoxy at 5c = 52 N-methyl generally lies between 5c = 30 and 45 and. S-methyl at about 5c = 25. However, methyl signals at 5c = 20 may also arise from methyl groups attached to C=X or C=C double bonds, e.g. as in acetyl, C//j-CO-. 

There is virtually no recent literature on quinoxaline cyanates or isocyanates, and most of what little there is on quinoxaline thiocyanates, isothiocyanates, or nitrones has been covered already the preparation of thiocyanatoquinoxalines from halogenoquinoxalines (Section 4.4.1) the conversion of the thiocyanatoquinoxalines into alkoxyquinoxalines (Section 4.4.1), into quinoxalinethiones (Section... 

Related methods may be used to introduce amino functions. For example, thiocyanate 237, made from the 4,6-dimesylate, rearranges to the isothiocyanate 238,225 and 3,4-unsaturated 2-cyanates, easily made in situ from the corresponding carbamates, afford the A-bonded 4-isocyanates of the 2-enes.229 From both types of rearranged products aminodeoxy sugars may be accessed, and in this way perosamine peracetate (239) can be synthesized from the cyanate of a 3,4-unsaturated 2-ol, which is thermally rearranged prior to dihydroxylation and acetylation.229... 

Dielectrophilic species are 1,4-diketones, certain 1,3-dienes, a,B-unsaturated isocyanates, isothiocyanates, cyanates, and thiocyanates bearing electron-withdrawing (e.g., polyfluorinated and perfluorinated) substituents the most frequently used 1,1-dinucleophiles are water, potassium sulfide, primary amines, and ammonia. From this repertoire of building blocks many combinations are possible. Tetrakis(trifluoro-methyl)furans, thiophenes, and pyrroles have been synthesized from per-fluoro-3,4-dimethylhexa-2,4-diene on addition of water, potassium sulfide, or aniline (90CC1127) (Scheme 39). 

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