Reaction of CS2 and CO2 with Amidines and Guanidines

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Alshamrani, Aliyah
CS2, CO2, Amidines, and Guanidines
Neutral organic bases such as amidines and guanidines are very useful in studies of the reactivity of heterocumulenes such as carbon disulfide (CS2) and carbon dioxide (CO2) to build C-C or C-N bonds. Reaction of amidines with CS2 in the absence of water and without adding desulfurization reagents or solvents is less documented. In this research I have investigated different types of sulfur and nitrogen containing products obtained upon reaction of carbon disulfide with cyclic and acyclic amidines. The reactivity behaviour and the resulting products depend on the structure of the starting amidines. Our observations for the contrasting reactivity were supported by calculations of the free energy of the reaction. 1,4,5,6-Tetrahydropyrimidine derivatives, as examples of cyclic amidines, follow different reactivity compared to acyclic amidines. They also follow different patterns of reaction with CS2 depending on their structures. Possessing an N-H bond allows the cyclic amidine to form a dithiocarbamate salt {[BH+] [(B-CS-2)-(H)]}, where B is the starting amidine, analogous to those formed by secondary amines [R2NH2][R2NCO2] and [R2NH2][R2NCS2] with CO2 or CS2, respectively. If the molecule does not possess an N-H bond then observed reactivity depends on whether there is a methylene alpha to the amidine carbon. Having such a methylene group promotes the formation of trithioanhydride. A tetrahydropyrimidine having neither an N-H bond nor an alpha methylene forms an unstable zwitterionic adduct (B-CS2). The acyclic amidines and their analogues (e.g., acetamidines and TMG) have the ability to rotate around the (C=N) bond to form a four-membered cyclic transition state which cleaves to form two molecules: isothiocyanates (RNCS) and thioacetamide or tetramethylthiourea (TMTU) in the case of the guanidine. Such rotation cannot happen in cyclic amidines due to their rigidity. Alkylation reactions performed on the products of the reaction of 1,4,5,6-tetrahydropyrimidines and CS2 lead to the formation of dithiocarbamate salts and dithiocarbamate esters and other interesting compounds. Compounds structurally related to these products have significant medical applications. The alkylation products also serve to confirm the proposed structures of some of the products obtained from the initial reactions with CS2. Both cyclic and acyclic amidines and their analogues were exposed to different CO2 pressures (1 or 60 bar) under various temperatures (room temperature or 60 oC) in order to form carbamic acid or carbamate products, but either bicarbonate salts or no reactivity were observed.
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