Substitution tetrasubstituted ureas

The reaction of phosgene with substituted ureas is well known. Phosgene reacts with tetrasubstituted ureas as a chlorinating agent to afford chloroformamidinium chlorides in high yields. This topic will be discussed in volume 2. 

In contrast, dipolar aprotic solvents possess large relative permittivities (sr > 15), sizeable dipole moments p > 8.3 10 ° Cm = 2.5 D), and average C.f values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C—H bonds are not sufficiently polarized. However, they are usually good EPD solvents and hence cation sol-vators due to the presence of lone electron pairs. Among the most important dipolar aprotic solvents are acetone, acetonitrile [75], benzonitrile, A,A-dimethylacetamide [76, 77], A,A-dimethylformamide [76-78], dimethylsulfone [79], dimethyl sulfoxide [80-84], hex-amethylphosphoric triamide [85], 1-methylpyrrolidin-2-one [86], nitrobenzene, nitro-methane [87], cyclic carbonates such as propylene carbonate (4-methyl-l,3-dioxol-2-one) [88], sulfolane (tetrahydrothiophene-1,1-dioxide) [89, 90, 90a], 1,1,3,3-tetramethylurea [91, 91a] and tetrasubstituted cyclic ureas such as 3,4,5,6-tetrahydro-l,3-dimethyl-pyr-imidin-2-(l//)-one (dimethyl propylene urea, DMPU) [133]. The latter is a suitable substitute for the carcinogenic hexamethylphosphoric triamide cf. Table A-14) [134]. 

In another approach [77] (Scheme 31) 4-nitro substituted dimethylphthalate 118 was reacted with fluorinated alcohol in DMF in the presence of NaH at room temperature to give 4-fluoroalkoxy substimted diester 119. It was then converted to 4-fluoro-alkoxy substituted phthalic anhydride 120, which was directly used in template cyclotetramerization with metal acetates (Mg", Co", Zn") or chlorides (Cu, Fe" , Al ") by urea melt method (urea, ammonium molybdate, NH4CI) to give directly the corresponding p-tetrasubstituted metal phthalocyanine complexes (yields 7-26 %). 

Several recent papers deal with the restricted C—rotations in unsubstituted/" monosubstituted, NlsT-disubstituted, NN-disubsti-tuted, N,N,N -trisubstituted, " and tetrasubstituted " thioureas, together with the conformational aspects involved. The barriers to C—N rotation of thio- and seleno-urea were equal (AF = 12.8 kcal mol" ) and above that of urea (AF = 11,0 kcal mol" ). Restricted-rotation phenomena have been observed for substituted N-(tosylmethyl)thioureas, thiourea S-trioxides, " and 3-aryl-2-thiohydantoins. ... 

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