Isothiocyanates 4+2 cycloaddition reactions

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). 

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). 

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... 

Cycloaddition reactions of sulfonyl isothiocyanate 76 and, -disubsti-tuted enamines were studied by Schaumann et The mechanism of cycloaddition is postulated to consist of two steps, originating with the formation of the zwitterionic adduct 77. 

Thermal and photochemical cycloaddition reactions of 27r-electron species represent an important synthetic approach to four-membered rings. The reactions summarized in this section include 2 + 2 cycloaddition reactions of thioketones, thioketenes, isothiocyanates, sulfenes and iminosulfenes with alkenes, allenes, ketenes, ketenimines and alkynes. 

R CO-5-R-ttaH) tetrazoles, respectively (114). The azide Pd(N3)2-(PPh3)2 also undergoes 1,3-cycloaddition reactions with organic isonitriles (RNC) (15) and isothiocyanates (RNCS) (114) to afford the complexes 42 and 43 (M = Pd), respectively. N-l-Coordinated tetrazolate chelate complexes 44 and 45 are formed by treatment of... 

Furthermore, isothiocyanate complexes can be prepared starting from the metal-azido complexes and reacting these with CS2. These reactions most probably proceed via 1,3-dipolar cycloaddition reactions.354 Syntheses of isothiocyanato complexes via this route are known for several metals (see, for example, ref. 355 for a synthesis involving Co). In these reactions formation of a cyclic intermediate (cf. equation 24) could be established. 

A cycloreversion mechanism is suggested for the transformation of the nonisolable cycloadduct 90 to the aldehyde 91 and isothiocyanate 92 <1996BCJ719> and for the spiro-1,4,2-oxathiazole intermediates 94 to the dioxothiazoline 95 and the aryl isothiocyanate 92 <2001MOL510>. Both cycloadducts are obtained by cycloaddition reactions of nitrile oxides 88 to thiocarbonyl compounds (Scheme 12). 

Regiospecific, Pd(II) catalysed, cycloaddition reactions of azetidines to yield ring-enlarged products include the formation of tetrahydro-2-iminopyrimidines (e.g. 10) by reaction with carbodi-imides (95JOC253) and tetrahydro-l,3-thiazin-2-imines (e.g. 11) from isothiocyanates (95JOC3092). 

The [2+2] cycloaddition reactions of various 4-dialkylamino-3-butyn-2-ones with substituted phenyl isothiocyanates in refluxing tetrahydrofuran gave access to a series of thietimines 92a-j in poor to satisfactory yields (Table 6) <2001SL361>. As it may be concluded from Table 6, when diethylamine derivatives were replaced by dimethylamine... 

Z + Z] Cycloaddition reactions are important in the construction of 1,3-thiazetidine rings . Isothiocyanates add carbodiimides across the carbonsulfur bond < 1975J(P2) 1475 >. For example, the reaction of the 2-thioxo-l,3-thiazepan-4-one derivative 208 with A,A-dicyclohexylcarbodiimide affords the 1,3-thiazetidine system 209 (Scheme 100) <1999JHC1167>. 

S,N,N-trimethyl-N -arylisothiourea 30, upon reaction with an isothiocyanate, produces a carbodiimide 31. This reaction involves a [2 + 2] cycloaddition reaction, followed by a cycloreversion to give the observed products. 

The [2+2] cycloaddition reaction of isothiocyanates with carbodiimides generally proceeds across the C=S bond of the isothiocyanate to give thiazetidine derivatives 218. However, in the reaction of methyl isothiocyanate with N-(4-dimethylaminophenyl)-N -methylcarbodiimide a mixture of the thiazetidine derivative 218 and the isomeric iminodi-azetidinethione derivative 219 is obtained. The structure of 219 was confirmed by X-ray crystallography. ... 

Cycloaddition Reactions. In [2+2] cycloaddition reactions with car-bodiimides sometimes [2+4] cycloadducts are produced as coproducts. Examples include the reaction of phenylcarbonyl isocyanate, phenylcarbonyl isothiocyanate and thiocarbamoyl isothiocyanate with carbodiimides to give [2+4] cycloadducts, discussed in Section 5.3.1. In the current section, mainly [2+4] cycloaddition of carbodiimides as dienophile with dienes derived from oxoketenes, generated in situ or masked oxoketenes, especially 2,3-diones investigated by Kollenz and his coworkers are discussed. 

Carbodiimides also play a role as building blocks of numerous N-heterocycles. Examples are the cycloaddition reactions of carbodiimides. In the formation of [2-1-2] cycloadducts N-methyl-N -t-butylcarbodiimide is used as a marker to determine the mode of addition of isocyanates or isothiocyanates to carbodiimides. In these reactions addition across the C=N or the C=0 bond of the isocyanate or across the C=N or the C==S bond of the isothiocyanate can occur. Addition to the N-methyl-N -t-butylcarbodiimide always proceeds across the N-methyl double bond. Also, theretroreaction of the [2-1-2] cycloaddition reaction of carbodiimides can afford a different set of heterocumulenes. ... 

A representative 1,3-dipolar cycloaddition process occurs with yV-aryl-C-(trifiuoromethyl)-nitrilimines, generated from the corresponding hydrazonoyl bromides, c.g. 4. under basic conditions. which can react with dimethyl fumarate and maleate,bicyclic olefins. and dipolarophiles containing cumulative double bonds. With sodium isocyanates as the dipolarophilc the cycloaddition reaction occurs across the C = N bond, while with potassium isothiocyanate it occurs through the C = S bond. ... 

CSI also reacts with diaryl azines (83) to afford the bicyclic triazolotriazolones (84), which are probably formed by 1,3-dipolar cycloaddition of CSI (two equivalents) to the azine (Scheme 39). Phenyl isothiocyanate undergoes a similar 1,3-dipolar cycloaddition reaction with a diaryl azine (83) to give the adduct (85) (Scheme 40). The main difference is that phenyl isothiocyanate is much less reactive than CSI, and consequently the cycloaddition requires more drastic conditions, namely prolonged boiling in xylene. 

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