Carbodiimides 1,3-thiazetidines

Raasch, and Schaumann and co-workers, have studied the reactions of bis (trifluoromethyl)thioketene (83 R1 = CF3) with alkenes, with thioketones, and with carbodiimides or Schiff-base imines. The products were, respectively, thietans, 1,3-dithietans (84 R1 = CF3), and 1,3-thiazetidines (85 R1 = CF3) (from the carbodiimide).114,115 With phenyl azide the 1,2,3,4-thiatriazoline (86 R = CF3) is formed subsequent pyrolysis yields2,l-benzisothiazole.114... 

A successful method for preparing 1,3-thiazetidines is the cycloaddition of carbodiimides with isocyanates. The cycloadduct 151 was derived by the interaction of carbodiimide 152 and tosyl isothiocyanate . Reaction of 152 with methyl or phenyl isothiocyanate preferentially leads to iminotriazine-thiones. Cycloaddition between aryl isothiocyanates and dicyclohexylcar-bodiimide produced 153. ... 

It was originally reported that isothiocyanates added to carbodiimides giving thione derivatives. The products are now recognized to be 2,4-diimino-l,3-thiazetidines which display the expected C=N absorptions between 1620 and 1630 cm"1 (70CC1629,78CCC3258). The UV absorptions, Amax 223 and 260 nm, are reported to be too short for expected C=S 77 — 7r transitions (73BCJ2559). 

It may be mentioned that bis(imino)thiazetidines (68), which are prepared by the reaction of carbodiimides with isothiocyanates, react with hydrazoic acid to give an aminothiatriazole. Although this reaction may be of interest from other points of view, it has no general synthetic application (equation 41) (81BSB63). 

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>. 

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 of sulfenes to Schiff bases (e.g., 434) and carbodiimides yields 1,2-thiazetidine 1,1-dioxides and 3-imino derivatives. A concerted mechanism was suggested. An earlier report of the cycloaddition of a sulfene to a Schiff base lacked proof for the structure of the adduct. Best yields are obtained with two equivalents of imine. The cis isomers can be converted to trans by treatment with triethylamine. ... 

Thioketenes, for example, 463, add to imines, carbodiimides, and azines to give 1,3-thiazetidines with exocyclic double bonds, as exemplified by the synthesis of 464. A novel ring contraction of 464a gives 1,3-thiazetidine 464b. 1,3-Thiazetidlnes are postulated as intermediates in the photochemical addition of thiobenzophenone to imines. They fragment to the two possible thiocarbonyl components and imines. ... 

The cycloaddition of isothiocyanates to imines gives 2-imino-l,3-thiazetid-ines, some of which are unstable.Cycloaddition of carbodiimines to bis(trifluoromethyl) ketene and isothiocyanates " gives 2,4-bis-imino-l,3-thiazetidines. The reaction of carbodiimides with 0,0-dimethyldithiooxalate to give 1,3-dialkyl-5,5-dimethoxy-2,4-imidazolidinedithiones... 

Heating the 3-(4-fluorophenyl)-l,3-thiazetidin-4-one-2-(4-fluorophenyl)imine 770 either neat or in refluxing toluene affords the expected carbodiimide 771 [562] as the sole product (presumably with evolution of 0=C=S). 

Without additional reagents 1,3-Thiazetidines from carbodiimides... 

---