Isothiocyanates cycloaddition

The chemistry of isothiocyanates resembles very much the chemistry of isocyanates. In addition to the widely known nucleophilic reactions of isothiocyanates, cycloaddition reactions are also prominent. The cycloaddition reactions can occur across either the 0=N or the C=S double bond but the latter is often preferred. The polarization of the N=0=S bond is shown in the following resonance forms ... 

Acylisocyanates or isothiocyanates undergo cycloaddition with 5-hydroxy-THISs under so mild conditions that isolation of the initial adducts becomes possible (23). In cycloaddition reactions the 5-hydroxy-THISs can be replaced by their precursors (23). 

A similar regiospecific [2 -I- 2] cycloaddition across a C=S group occurred when benzoyl isothiocyanate (436) and 2,3-diphenyl-1-azirine were heated in refiuxing benzene for 12 hours. The product obtained was shown to be (438) and an intermediate such as (437) could also be involved in this cycloaddition (74JOC3763). In contrast, thiobenzoyl isocyanate added in a [4-1-2] fashion, and after ring expansion gave a thiadiazepine derivative. 

Cycloaddition of 2-nitrosopyridine 48 with nitrile oxides can give either di-A -oxides such as 49 or 3-mono-A -oxides such as 50 (93JHC287). In general, greater electron withdrawing character in the aromatic substituent appears to favor formation of the di-A -oxides. Sulfur ylides such as compound 51 are obtained from aryl isothiocyanates and l-amino-2-methylthiopyridinium iodides (84JCS(P1)1891) nitrogen ylides can be obtained from a similar reaction (86H(24)3363). 

Cycloadditions of isocyanates and their derivatives with vinyiaziridines were first reported by Alper and coworkers. From their previous studies of cydoadditions to vinylepoxides or alkylaziridines, they investigated cydoadditions to vinyiaziridines and found that such reactions with isocyanates, carbodiimides, or isothiocyanates in the presence of catalytic amounts of Pd(OAc)2 (2 mol%) and PPh3 (10 mol%) at room temperature afforded five-membered ring products 249 in 34—97% yields (Scheme 2.61) [91]. When an aziridine 247 possessing an alkyl substituent at the... 

Thioazadienes 90, formed in situ by the reaction of trimethylsilylimines and isothiocyanates, underwent cycloaddition reactions with nitriles bearing electron-withdrawing groups, to afford 1,3,5-thiadiazines [94] in excellent yield (Equation 2.26). 

Flamers R. J., Coulter S. K., Ellison M. D., Flovis J. S., Padowitz D. F., Schwartz M. P., Greenlief C. M., Russell J. N. Jr Cycloaddition Chemistry of Organic Molecules With Semiconductor Surfaces Acc. Chem. Res. 2000 33 617 624 Keywords carbonyi group, semiconductor materiais, surface reaction, aikenes, aromatic compounds, azo compounds, cycioaikadienes, isothiocyanates, unsaturated compounds... 

The 1,3-dipolar cycloaddition of a nitrone to a C=N species remains (see CHEG-II(1996) for earlier examples) a popular route to 1,2,4-oxadiazolidines. The use of isocyanates and isothiocyanates as the dipolarophile allows access to l,2,4-oxadiazolidin-5-ones and 5-thiones, as the examples in Equations (75) <1995HCO307> and (76) <2006SC997> show, giving access to l,2,4-oxadiazolidin-5-ones 373 and 374, respectively. 

Organometallic dithiocarbimates are also known. Thus, a diarylgermanium phenyldithiocarbimate, Tbt(Tip)GeS2C=N-Ph, was obtained from the kinetic-ally stabilized diarylgermylene Tbt(Tip)Ge in a reaction with phenyl isothiocyanate, via [2+2] cycloaddition of a diarylgermanethione [Tbt(Tip)Ge=S] intermediate, evidenced by electronic spectroscopy.425... 

Dipolar cycloaddition of 3-cyano-4// -1 -benzopyran-4-thione 187 with benzonitrile oxide proceeded regioselectively to give cycloadduct 188 (involving the thione function). The unstable cycloadduct fragmented to yield 3-cyano-chromone 189 and phenyl isothiocyanate (Scheme 1.32) (355). 

Cyclic a-methoxynitrones (225f) and (223 g) react respectively with isocyanates and isothiocyanates at ambient temperature, giving 1,3-dipolar cycloaddition products (763) and (764) (Scheme 2.314). Under similar conditions, the reaction of aldonitrone (223a) proceeds much more slowly to give cycloadducts... 

Itoh and co-workers reported the ruthenium(n)-catalyzed [2 + 2 + 2]-cycloaddition of 1,6-diynes with isocyanates to afford the corresponding bicyclic pyridones 163 (Scheme 72).356 357 For previously reported ruthenium-catalyzed [2 + 2 + 2]-cycloaddition of 1,6-diynes see Refs 358 and 358a, and for theoretical calculations of the cyclocotrimer-ization of alkynes with isocyanates, isothiocyanates, and carbon disulfide see Refs 359 and 359a. 

For a related example of a ruthenium(II)-catalyzed cycloaddition of 1,6-diynes with isothiocyanates and carbon disulfide, see Yamamoto, Y. Takagishi, H. Itoh, K. J. Am. Chem. Soc. 2002, 124, 28-29. 

Dipolar cycloaddition reactions, of nitrones to olefins, 46, 97 of 3-phenylsydnone, 46, 98 Dispiro[5.1.5.1]tetradecane-7,14-dione, photolysis to cyclohexylidene-cyclohexane, 47, 34 preparation from cyclohexanecarbonyl chloride and triethylamine, 47, 34 Displacement of bromine from 1-bromo-2-fluoroheptane to give 2-fluoro-heptyl acetate, 46, 37 N,N -Disubstituted formamidines from triethyl orthoformate and primary amines, 46, 41 N,N-Disubstituted thioureas from secondary amines and silicon tetra-isothiocyanate, 45, 69 N,N-Disubstituted ureas from secondary amines and silicon tetraiso-cyanate, 45, 69... 

However, when pyridyliminophosphorane (306a) is treated with phenyl isocyanate or isothiocyanate (Scheme 110), mixed carbodiimides are obtained, which are capable of an intermolecular Diels-Alder reaction resulting in triazine 308. The cycloaddition occurs specifically with one C = N double bond of the carbodiimide serving as the dienophile (77ZC371). 

Scheme 12 shows synthesis of 1,2,3-thiadiazoles by the Wolff, Hurd-Mori and Pechmann-Nold methods. Pechmann s and Wolffs are the oldest of the methods. The Pechmann-Nold synthesis involves the [3 + 2] cycloaddition of diazo-compounds to isothiocyanates or thiocarbonyl compounds (modified Pechmann synthesis). The use of thiocarbonyl compounds in the [3 + 2] cycloaddition step has broadened the scope of this reaction and made the starting materials more readily accessible. Wolffs method requires the synthesis of diazoketones that are treated with a thionating reagent to produce 1,2,3-thiadiazoles. With the development of new methods of diazotransfer reactions, the diazoketone precursors have become easily attainable and with further attention to the thionating reagents, this reaction is also useful for the synthesis of 1,2,3-thiadiazoles. 

Oxathiazolidin-3-imine (93) possesses a thioimidate moiety similar to 4-methyl-5-phenyl-imino-l,2,3,4-thiatriazoline (70) and is capable of undergoing cycloaddition-elimination reactions with isothiocyanates to give identical products, however at a much higher rate. Acetone can therefore be used as solvent to catalyze this kind of reaction of thiatriazolines. 

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