Special Issue: "Advances in Homogeneous Catalysis".

The use of enzymes, organo-catalysts or transition metal catalysts, as opposed to the employment of stoichiometric quantities of other traditional promoters of different organic synthetic processes (like, inorganic/organic bases, Brønsted acids, radicals, etc.) has allowed the discovery of a great number of new synthetic protocols within the toolbox of organic chemists. Moreover, the employment of the aforementioned catalysts in organic synthesis permits: (i) the diminution of the global energy demand and production cost; (ii) the enhancement of both the chemoselectivity and stereoselectivity of the global process; and (iii) the reduction of metal-, organo- or bio-catalyst consumption, thanks to the possible recycling of the catalysts; all these being synthetic concepts closely related with the principles of so-called Green Chemistry. Thus, this Special Issue on “Advances in Homogenous Catalysis” has been aimed to showcase a series of stimulating contributions from international experts within different sub-areas of catalysis in organic synthesis (ranging from metal-, organo-, or bio-catalyzed organic reactions).

Molecules 2020, 25, 1493 2 of 3 new synthetic methodologies, since it not only allowed previously discovered stoichiometric organic reactions to be carried out in a catalytic manner [1][2][3][4], but also unlocked the discovery of new processes in organic synthesis that were unknown until the irruption of catalysis. Finally, it is worth noting that it is difficult to find advanced organic syntheses (both at the academic or industrial level) in which any of the aforementioned catalytic methodologies (metal-, organo-or bio-catalysis) are not used in any of their synthetic stages.
Thus, this Special Issue includes two original research articles and two review articles, covering different topics ranging from transition-metal-catalyzed organic transformations to ionic-liquid-mediated organic reactions. Firstly, in a communication-type article, Song, Xiao and co-workers report an efficient and selective Zn(OTf) 2 -catalyzed cascade cyclization of 2-propynol benzyl azides (1) with different diphenylphoshine oxides (2) in acetonitrile as solvent at 100 • C (see Scheme 1). The desired phosphorylated isoindolines, fused with triazoles (3), were obtained with moderate to excellent yields (57-91%) [8].
Furthermore, it is important to note that, in the field of organic synthesis, the use of metallic complexes, organocatalysts or enzymes represents one of the cornerstones of the development of new synthetic methodologies, since it not only allowed previously discovered stoichiometric organic reactions to be carried out in a catalytic manner [1][2][3][4], but also unlocked the discovery of new processes in organic synthesis that were unknown until the irruption of catalysis. Finally, it is worth noting that it is difficult to find advanced organic syntheses (both at the academic or industrial level) in which any of the aforementioned catalytic methodologies (metal-, organo-or bio-catalysis) are not used in any of their synthetic stages.

Scheme 1. Zn(II)-catalyzed cyclization of 2-propynol benzyl azides (1) with different diphenylphoshine oxides
In the second original research paper, Glotov and co-workers present the synthesis of seven different polyoxometalate based-ionic-liquids (derived from pyridine or nicotinic acid, see Figure 1) and their use as catalyst in the oxidation of organosulfur compounds (like methylphenylsulfide, benzothiophene, 5-methylbenzothiophene, dibenzothiophene, 4-methyldibenzothiophene or 4,6-dimethyldibenzothiophene) in both model and real diesel fuels [9]. Moreover, the authors show that the catalytic system can operate for five consecutive cycles without any decrease in its catalytic activity. Figure 1. Different polyoxometalate-based ionic liquids derived from pyridine or pinacolinic acid, employed by Glotov and co-workers as catalysts for the oxidation of organosulfur compounds in both model and real diesel fuels [9].
Regarding the review-type contributions to this Special Issue, Guo and co-workers present a general overview of the use of cyclic carbonates and carbamates as starting building blocks in organic synthesis via transition-metal-catalyzed decarboxylation for the synthesis of different heteroaromatic organic architectures, ranging from chiral tetrahydroquinolines, functionalized indoles and furanbenzodihydropyran derivatives to chiral 3-indolin malononitriles, among others [10]. Finally, Dr. Noel Nebra reports an outstanding and complete review of high-valent Ni(III) and Ni(IV) organometallic species [11], which is nowadays a hot topic area in the field of transition-metal-catalyzed formation of C-C and C-Heteroatom bonds. Moreover, the author also presents in his review the actual state of the art on the different mechanistic proposals to support the catalytic activity of the aforementioned high-valent Ni(III) and Ni(IV) organometallic species. In the second original research paper, Glotov and co-workers present the synthesis of seven different polyoxometalate based-ionic-liquids (derived from pyridine or nicotinic acid, see Figure 1) and their use as catalyst in the oxidation of organosulfur compounds (like methylphenylsulfide, benzothiophene, 5-methylbenzothiophene, dibenzothiophene, 4-methyldibenzothiophene or 4,6-dimethyldibenzothiophene) in both model and real diesel fuels [9]. Moreover, the authors show that the catalytic system can operate for five consecutive cycles without any decrease in its catalytic activity.
Molecules 2020, 25, x FOR PEER REVIEW 2 of 3 Furthermore, it is important to note that, in the field of organic synthesis, the use of metallic complexes, organocatalysts or enzymes represents one of the cornerstones of the development of new synthetic methodologies, since it not only allowed previously discovered stoichiometric organic reactions to be carried out in a catalytic manner [1][2][3][4], but also unlocked the discovery of new processes in organic synthesis that were unknown until the irruption of catalysis. Finally, it is worth noting that it is difficult to find advanced organic syntheses (both at the academic or industrial level) in which any of the aforementioned catalytic methodologies (metal-, organo-or bio-catalysis) are not used in any of their synthetic stages.

Scheme 1. Zn(II)-catalyzed cyclization of 2-propynol benzyl azides (1) with different diphenylphoshine oxides
In the second original research paper, Glotov and co-workers present the synthesis of seven different polyoxometalate based-ionic-liquids (derived from pyridine or nicotinic acid, see Figure 1) and their use as catalyst in the oxidation of organosulfur compounds (like methylphenylsulfide, benzothiophene, 5-methylbenzothiophene, dibenzothiophene, 4-methyldibenzothiophene or 4,6-dimethyldibenzothiophene) in both model and real diesel fuels [9]. Moreover, the authors show that the catalytic system can operate for five consecutive cycles without any decrease in its catalytic activity. Figure 1. Different polyoxometalate-based ionic liquids derived from pyridine or pinacolinic acid, employed by Glotov and co-workers as catalysts for the oxidation of organosulfur compounds in both model and real diesel fuels [9].
Regarding the review-type contributions to this Special Issue, Guo and co-workers present a general overview of the use of cyclic carbonates and carbamates as starting building blocks in organic synthesis via transition-metal-catalyzed decarboxylation for the synthesis of different heteroaromatic organic architectures, ranging from chiral tetrahydroquinolines, functionalized indoles and furanbenzodihydropyran derivatives to chiral 3-indolin malononitriles, among others [10]. Finally, Dr. Noel Nebra reports an outstanding and complete review of high-valent Ni(III) and Ni(IV) organometallic species [11], which is nowadays a hot topic area in the field of transition-metal-catalyzed formation of C-C and C-Heteroatom bonds. Moreover, the author also presents in his review the actual state of the art on the different mechanistic proposals to support the catalytic activity of the aforementioned high-valent Ni(III) and Ni(IV) organometallic species. Regarding the review-type contributions to this Special Issue, Guo and co-workers present a general overview of the use of cyclic carbonates and carbamates as starting building blocks in organic synthesis via transition-metal-catalyzed decarboxylation for the synthesis of different heteroaromatic organic architectures, ranging from chiral tetrahydroquinolines, functionalized indoles and furanbenzodihydropyran derivatives to chiral 3-indolin malononitriles, among others [10]. Finally, Dr. Noel Nebra reports an outstanding and complete review of high-valent Ni(III) and Ni(IV) organometallic species [11], which is nowadays a hot topic area in the field of transition-metal-catalyzed formation of C-C and C-Heteroatom bonds. Moreover, the author also presents in his review the actual state of the art on the different mechanistic proposals to support the catalytic activity of the aforementioned high-valent Ni(III) and Ni(IV) organometallic species.
In summary, the collection of original research and review articles included in this themed issue offer a broad view of the state of the art in "Advances in Homogenous Catalysis", highlighting the enormous scope for advancement and application in this field.