Guanidines: from classical approaches to efficient catalytic syntheses

Abstract

From organosuperbases capable of base-catalyzing organic reactions, through versatile 'ligand-sets' for use in coordination chemistry, to fundamental entities in medicinal chemistry, guanidines are amongst the most interesting, attractive, valuable, and versatile organic molecules. Since the discovery of these compounds, synthetic chemists have developed new methodologies that are mainly based on multi-step and stoichiometric reactions. Despite the fact that these methodologies are still being used by the interested scientific and industrial communities, drawbacks such as the poor availability of precursors, low yields, and use and production of undesirable substances highlight the need for safe, simple and efficient syntheses of these entities. This review focuses on the metal-mediated catalytic addition of amines to carbodiimides as an atom-economical alternative to the classical synthesis.

Half‐Sandwich Guanidinate–Osmium(II) Complexes: Synthesis and Application in the Selective Dehydration of Aldoximes

“…Interestingly, the electronic nature of the N-aryl substituents of the guanidinate ligands plays a significant role in both the efficiency and selectivity of the process. Thus, the best results were obtained with complexes 3d-g bearing the strong electron-withdrawing CF 3 and CN groups, which led to the almost quantitative consumption (≥96 % by GC) of the starting (E)-benzaldoxime after only 2 h (entries 4-7). By employing these complexes, the quantity of benzamide formed never exceeded 1 %.…”

Half‐Sandwich Guanidinate–Osmium(II) Complexes: Synthesis and Application in the Selective Dehydration of Aldoximes

“…Interestingly, the electronic nature of the N-aryl substituents of the guanidinate ligands plays a significant role in both the efficiency and selectivity of the process. Thus, the best results were obtained with complexes 3d-g bearing the strong electron-withdrawing CF 3 and CN groups, which led to the almost quantitative consumption (≥96 % by GC) of the starting (E)-benzaldoxime after only 2 h (entries 4-7). By employing these complexes, the quantity of benzamide formed never exceeded 1 %.…”

Tandem Radical Fragmentation/Cyclization of Guanidinylated Monosaccharides Grants Access to Medium-Sized Polyhydroxylated Heterocycles

“…The fact that only the endocyclic guanidines were obtained this time underscores the preferential formation of bigger sized rings. However, the almost null stereodiscrimination exhibited by the nucleophilic attack does suggest that, contrary to the photolysis of model 14, in this case a transient sp 2 -hybridized oxocarbenium-like intermediate must have formed upon oxidation, thus rendering compounds 27 and 28 almost in equal rates.…”

Guanidinates as Alternative Ligands for Organometallic Complexes

“…Although, as mentioned above, the most common coordination mode of guanidinato ligands is the chelating k 2 -N,N' form, in the case of bulky lithium guanidinates, additional metal-carbon bond interactions can occur due to the presence of aromatic groups. These M-arene interactions have also been found for other group 1 elements, which are used to It is noteworthy that the great interest in the study of these compounds provides a broad palette of synthetic methods, involving both stoichiometric and catalytic processes [33][34][35][36][37].…”

Half‐Sandwich Guanidinate–Osmium(II) Complexes: Synthesis and Application in the Selective Dehydration of Aldoximes

“…Interestingly, the electronic nature of the N-aryl substituents of the guanidinate ligands plays a significant role in both the efficiency and selectivity of the process. Thus, the best results were obtained with complexes 3d-g bearing the strong electron-withdrawing CF 3 and CN groups, which led to the almost quantitative consumption (≥96 % by GC) of the starting (E)-benzaldoxime after only 2 h (entries 4-7). By employing these complexes, the quantity of benzamide formed never exceeded 1 %.…”

Tandem Radical Fragmentation/Cyclization of Guanidinylated Monosaccharides Grants Access to Medium-Sized Polyhydroxylated Heterocycles

“…The fact that only the endocyclic guanidines were obtained this time underscores the preferential formation of bigger sized rings. However, the almost null stereodiscrimination exhibited by the nucleophilic attack does suggest that, contrary to the photolysis of model 14, in this case a transient sp 2 -hybridized oxocarbenium-like intermediate must have formed upon oxidation, thus rendering compounds 27 and 28 almost in equal rates.…”

Guanidinates as Alternative Ligands for Organometallic Complexes

“…Although, as mentioned above, the most common coordination mode of guanidinato ligands is the chelating k 2 -N,N' form, in the case of bulky lithium guanidinates, additional metal-carbon bond interactions can occur due to the presence of aromatic groups. These M-arene interactions have also been found for other group 1 elements, which are used to It is noteworthy that the great interest in the study of these compounds provides a broad palette of synthetic methods, involving both stoichiometric and catalytic processes [33][34][35][36][37].…”

Half‐Sandwich Guanidinate–Osmium(II) Complexes: Synthesis and Application in the Selective Dehydration of Aldoximes

“…Interestingly, the electronic nature of the N-aryl substituents of the guanidinate ligands plays a significant role in both the efficiency and selectivity of the process. Thus, the best results were obtained with complexes 3d-g bearing the strong electron-withdrawing CF 3 and CN groups, which led to the almost quantitative consumption (≥96 % by GC) of the starting (E)-benzaldoxime after only 2 h (entries 4-7). By employing these complexes, the quantity of benzamide formed never exceeded 1 %.…”

Tandem Radical Fragmentation/Cyclization of Guanidinylated Monosaccharides Grants Access to Medium-Sized Polyhydroxylated Heterocycles

“…The fact that only the endocyclic guanidines were obtained this time underscores the preferential formation of bigger sized rings. However, the almost null stereodiscrimination exhibited by the nucleophilic attack does suggest that, contrary to the photolysis of model 14, in this case a transient sp 2 -hybridized oxocarbenium-like intermediate must have formed upon oxidation, thus rendering compounds 27 and 28 almost in equal rates.…”

Guanidinates as Alternative Ligands for Organometallic Complexes

“…Although, as mentioned above, the most common coordination mode of guanidinato ligands is the chelating k 2 -N,N' form, in the case of bulky lithium guanidinates, additional metal-carbon bond interactions can occur due to the presence of aromatic groups. These M-arene interactions have also been found for other group 1 elements, which are used to It is noteworthy that the great interest in the study of these compounds provides a broad palette of synthetic methods, involving both stoichiometric and catalytic processes [33][34][35][36][37].…”