Guanidine-thiourea-based catalyst

Organocatalysts As mentioned earlier, organocatalytic ROP was first reported applying pyridines as the nucleophilic catalyst. Subsequently, guanidine, amidine, and thiourea-based organocatalysts were found to be highly active for the ROP of lactones [40-42],... 

Some bifunctional hydrogen-bond donor/Brpnsted base catalysts are shown in Figures 2.39 and 2.40. They comprise chiral amino alcohols and amino phenols, chiral amine-thiourea derivatives, and chiral guanidines, among others. In the absence of detailed experimental NMR or kinetic studies [179], most of our... 

Conformationally flexible guanidine-thiourea catalyst 174 has unique solvent-dependent properties in the catalytic Mannich-type reaction. Based on detailed kinetic analyses, the origin of solvent-dependent stereo-discrimination is controlled by enthalpy-entropy compensation. The stereoselectivities of (S)-selective Mannich-type reactions in non-polar solvents such as m-xylene are predominantly due to differences in the entropies of activation (Table 28.9, entries 1-3), whereas the stereo-discrimination of (R)-selective reactions in polar solvents such as acetonitrile result from differences in the enthalpies of activation (Table 28.9, entries 4-6) [89]. 

Where R4 is a hydrogen or carbon atom, 10.15 is simply an amidine. However, urea 10.16, thiourea 10.17, or guanidine 10.18 and their derivatives may be used. These nucleophiles may be condensed with ester and nitrile functionalities as well as with aldehydes and ketones. Such condensations to afford pyridimidine derivatives are usually facilitated by acid or base catalysis, although certain combinations of reactive electrophilic and nucleophilic compounds require no catalyst at all. Some examples are shown below. 

Recent developments in organocatalytic pathways for the ROP of lactide and several lactones, without adverse transesterification creating polymers that are metal-free and therefore perfect candidates for biomedical and microelectronic applications, have been developed using N-heterocyclic carbenes, thiourea-tertiary amines, and amidine and guanidine bases. Here, the exquisite control, the absence of metal ions, the ready synthetic availability of the catalysts, and the mild reaction conditions are of major importance for tailor-made polyesters, and also have high potential for functional polycarbonates [91, 92]. 

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