Study of the Kinetics, Intermediates and Product Distribution in the Synthesis and Decomposition Reactions of Thiourea Derivatives

Abstract

The solvent-effect together with the influence of substituents in the aryl group on the yield of isothiocyanates in the decomposition of N-mono aryl thioureas can partly be explained by solvent-and substituent-effects on a consecutive reaction of two decomposition products amine and isothiocyanate. A more detailed investigation of this step, which is representative for the reaction scheme of thioureas, shows that reactions of aromatic amines with aryl isothiocyanates are characterized by an equilibrium limiting the yield of the product NN' diaryl thiourea. Both amine and product seem to effect about equal catalytic influences on both steps of the equilibrium. From a study of the solvent-effect on the reaction rate, it can be concluded that besides the unspecific factors, polarity and polarisability, also association-effects and catalytic activities of the solvent must be taken into account.The considerable influence of the basicity of the amine on the reaction rate together with the pronounced effect of substituents in the phenyl isothiocyanate shows that the nucleophilic attack of the amine on the isothiocyanate must be the rate determining step; this view is in agreement with the low value for the isotope-effect. The whole of the data can be correlated by assuming a reaction scheme wherein amine attacks the isothiocyanate both as a dimeric form and a monomeric form complexed with reaction product or solvent.

Exploring the Conversion of Macrocyclic 2,2′-Biaryl Bis(thioureas) into Cyclic Monothioureas: An Experimental and Computational Investigation

“…Although 10 would be present in undetectable amounts by 1 H NMR, the formation of the oligomeric 9 would be driven by its low solubility in C 2 D 2 Cl 4 . The reversible nature of the process R-NH-CS-NH-R′ ⇆ R-NH 2 + R′-NCS would allow the existence of 10 in a certain amount …”

“…The above results allow us to reach several conclusions: Both, the formation of the bis­(thioureas) 1 and 8 from the respective diamine and bis­(isothiocyanate) and their splitting to the corresponding monothioureas take place at much lower temperatures or shorter reaction times in DMF- d 7 compared to C 2 D 2 Cl 4 . This fact is probably due to the higher polarity and the basic and good hydrogen-bond-acceptor character of the DMF which would stabilize the putative polar transition states The monothioureas 2 and 6 can be obtained directly from the respective diamine and bis­(isothiocyanate) under heating for longer reaction times in DMF- d 7 and C 2 D 2 Cl 4 .…”

“…Both, the formation of the bis­(thioureas) 1 and 8 from the respective diamine and bis­(isothiocyanate) and their splitting to the corresponding monothioureas take place at much lower temperatures or shorter reaction times in DMF- d 7 compared to C 2 D 2 Cl 4 . This fact is probably due to the higher polarity and the basic and good hydrogen-bond-acceptor character of the DMF which would stabilize the putative polar transition states …”

“…The reversible nature of the process R-NH-CS-NH-R′ ⇆ R-NH 2 + R′-NCS would allow the existence of 10 in a certain amount. 49 The straightforward reversion of the last process in C 2 D 2 Cl 4 became evident from the unexpected conversion of 1 into diamine 3 and bis(isothiocyanate) 4 at 140 °C for 6 h (left side of Scheme 5, Figure S16). An analogous transformation of the bis(thiourea) 8 into the diamine 5 and bis(isothiocyanate) 7 was observed, but it takes half the time (left side of Scheme 5, Figure S17).…”

“…This fact is probably due to the higher polarity and the basic and good hydrogenbond-acceptor character of the DMF which would stabilize the putative polar transition states. 49 (2) The monothioureas 2 and 6 can be obtained directly from the respective diamine and bis(isothiocyanate) under heating for longer reaction times in DMF-d mine and bis(isothiocyanate), probably due to a more strained structure of the corresponding bis(thiourea) with origin in the higher biaryl angle, while disfavors the formation of the corresponding monothiourea, probably by the same reason.…”