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Molybdenum‐based efficient and broadly applicable catalytic system is presented for the dehydrogenative coupling of γ‐/β‐amino alcohols with ketones. In particular, complex Mo(CO)6 in combination with Xantphos and t‐BuOK as base provides direct synthesis of structurally diverse quinoline, pyridine, and pyrrole compounds (total 55 examples) with isolated yields up to 92%. Low catalyst loading, cost‐effective, and high catalytic activities with high tolerance to stand with diverse functional groups are additional features of these molybdenum catalysts exhibiting high efficacy to replace the precious metal catalysts such as ruthenium and iridium. Furthermore, the DFT and experimental studies were explored for the reaction mechanism which can assist in broadening the scope of the molybdenum catalysts for different transformation reactions. [ABSTRACT FROM AUTHOR]

A series of manganese(I) carbonyl complexes bearing structurally related NN- and NNN-chelating ligands have been synthesized and assessed as catalysts for transfer hydrogenation (TH). Notably, the NN-systems based on N-R functionalized 5,6,7,8-tetrahydroquinoline-8-amines, proved the most effective in the manganese-promoted conversion of acetophenone to 1-phenylethanol. In particular, the N-isopropyl derivative, Mn1, when conducted in combination with t-BuONa, was the standout performer mediating not only the reduction of acetophenone but also a range of carbonyl substrates including (hetero)aromatic-, aliphatic- and cycloalkyl-containing ketones and aldehydes with especially high values of TON (up to 17 200; TOF of 3550 h−1). These findings, obtained through a systematic variation of the N-R group of the NN ligand, are consistent with an outer-sphere mechanism for the hydrogen transfer. As a more general point, this Mn-based catalytic TH protocol offers an attractive and sustainable alternative for producing alcoholic products from carbonyl substrates. [ABSTRACT FROM AUTHOR]

Molybdenum complexes ligated with N1,N1-dialkyl-N2-(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines and auxiliary ligands, providing various structural features, were developed: [NNH/NNHN]Mo(CO)4/3 (Mo1–Mo3), [NNHN]Mo(CO)2Br (Mo4–Mo5), [NNH]Mo(CO)(η3-C3H5)Br (Mo6) and [NNHN/S]Mo(CO)(PPh3)2 (Mo7–Mo8). All the complexes were highly active in the transfer hydrogenation (TH) of a model substrate (acetophenone), providing excellent yields of 1-phenylethanol. The structural variation in the ligand framework had a modest effect on the catalyst performance as compared to the changes in the auxiliary ligands Br, PPh3 and CO. This structural evolution provided the complex [Mo(NNH)(η3-C3H5)(CO)2Br] (Mo6) as the most effective catalyst not only for the transfer hydrogenation of acetophenone but also for a wide range of diverse ketones (up to 43 examples). Moreover, easy purification of the products by only removing the acetone byproduct is another noteworthy feature of this environmentally friendly route. [ABSTRACT FROM AUTHOR]

A well-defined PNN-Ru catalyst was revisited to self-condense 2-aminobenzyl alcohol in forming a series of novel aza-crown compounds [aza-12-crown-3 (1), aza-16-crown-4 (2) and aza-20-crown-5 (3)]. All aza-crown compounds are separated and determined by NMR, IR, and ESI-MS spectroscopy as well as X-ray crystallography, indicating the saddle structure of 1 and the twisted 1,3-alternate conformation structure of 3. These aza-crown compounds have been explored to study ferric initiation of transfer hydrogenation (TH) of ketones into their corresponding secondary alcohols in the presence of 2-propanol with a basic t-BuOK solution, achieving a high conversion (up to 95%) by a ferric complex with 2 in a low loading (0.05 mol%). [ABSTRACT FROM AUTHOR]

Reaction of 8-amino-5,6,7,8-tetrahydroquinoline with RuCl2(PPh3)3 at room temperature affords the ruthenium(ii) chelate (8-NH2-C9H10N)RuCl2(PPh3)2 (E), in which the two triphenylphosphine ligands are disposed mutually cis. By contrast, when the reaction is performed at reflux ligand oxidation/dehydrogenation occurs along with cis–trans reorganization of the triphenylphosphines to form the 8-imino-5,6,7-trihydroquinoline-ruthenium(ii) complex, (8-NH-C9H9N)RuCl2(PPh3)2 (F). Complex F can also be obtained in higher yield by heating a solution of E alone to reflux. Comparison of their molecular structures highlights the superior binding properties of the bidentate imine ligand in F over its amine-containing counterpart in E. Both complexes are highly effective in the transfer hydrogenation of a wide range of alkyl-, aryl- and cycloalkyl-containing ketones affording their corresponding secondary alcohols with loadings of as low as 0.1 mol%. Significantly, F can deliver excellent conversions even in bench quality 2-propanol in reaction vessels open to the air, whereas the catalytic efficiency of E is diminished by the presence of air but only operates efficiently under inert conditions. [ABSTRACT FROM AUTHOR]

Chiral Mn-1 and Mn-2 were prepared and evaluated as catalysts in asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH) of ketones. Pincer complex Mn-1 proved the more effective and indeed it was able to hydrogenate a broad range of pro-chiral ketonic substrates (39 examples), to give their chiral alcohols in high yield and with excellent enantioselectivities (up to 99% ee). [Display omitted] • PNN - and NN -ligands incorporating chiral ferrocenyl units as chelating supports for manganese complexes. • PNN -pincer complex and its NN- bidentate analogue for the asymmetric hydrogenation (AH) and asymmetric transfer hydrogenation (ATH). • The manganese system bearing the tridentate PNN -ligand proving to be more effective than its bidentate counterpart. • Rich library of ketonic substrates differing in their steric and electronic profile. • The importance of π-π interactions and steric hindrance enhancing stereoselectivity. A new type of (R C , S P)-1-(2-diphenylphosphino)ferrocenylethylamine N -substituted with a (R C)-5,6,7,8-tetrahydroquinolinyl group (L PNN -1) was successfully employed as a chiral chelating ligand in both Mn-catalyzed asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH) of a broad range of ketonic substrates (39 examples), leading to high conversions and excellent enantioselectivities for their product alcohols. In particular, PNN -pincer complex Mn-1 and its NN -bidentate analogue Mn-2 have been isolated and their comparative performance as catalysts studied with Mn-1 proving more effective in both ATH and AH. Moreover, Mn-1 generally imparted higher degrees of enantiomeric excess (ee) in both hydrogenation processes which can reach up to 99% in ATH and 93% in AH for propiophenone-type substrates. DFT calculations highlight the importance of π-π interactions and steric hindrance between catalyst and substrate which manifests itself in enhancements in ee for propiophenone over acetophenone substrates. Furthermore, a possible mechanism for the Mn-catalyzed ATH has been proposed on the basis of a joint DFT and experimental investigation. [ABSTRACT FROM AUTHOR]

Cyclooctyl PNN-Mn complex has been employed as a catalyst in the ATH of ketones with outstanding activity (up to 4000 TON) and high enantioselectivity (up to > 99 % ee). [Display omitted] • A new ferrocene-based chiral PNP ligand was successfully applied in Mn-catalyzed asymmetric hydrogenation of ketones. • A broad range of ketones were reduced to chiral alcohols with outstanding catalytic performance and high enantioselectivities. • Compared to PN bidentate ligand and smaller cycloalkyl PNN ligands, L1 enhanced enantioselectivity in ATH reaction. • DFT calculations highlight the importance of π-π stacking and distortion-interactions during the catalytic process. A new ferrocene-based chiral PNN pincer ligand (L1) was successfully applied in Mn-catalyzed asymmetric transfer hydrogenation (ATH) of ketones (37 examples), leading to high activities (up to 4000 TON with 48 h) and excellent enantioselectivities (up to >99 % ee) for their product alcohols. The combined coordination and catalytic studies led to the conclusion that PNN pincer ligand L1 proved higher enantioselectivity in ATH than that observed by PN bidentate analogue L0. Furthermore, DFT calculations and control experiments highlighted the significance of the cyclooctyl group among these cycloalkyl motifs in the Mn-catalysts, resulting in enhanced enantioselectivity (up to 99 % ee), and disclosed the stereocontrol with the assistance of an attractive aromatic π-π stacking interaction between substrate and catalyst in the catalytic process. [ABSTRACT FROM AUTHOR]

A available and robust NN -manganese(I) complex has been developed that can efficiently catalyze the transfer dehydrogenative of a broad range of secondary alcohols to ketones with high TON of 2250. [Display omitted] • Manganese-promoted efficient transfer dehydrogenative oxidation of a wide range of (hetero)aromatic-, cycloalkyl and aliphatic-containing alcohols is reported. • This protocol provides an attractive and sustainable alternative for producing aromatic and aliphatic ketones. • Experimental and DFT studies indicate that this catalytic system follows an outer-sphere bifunctional mechanism. • The NN -bidentate complex Mn1 outperforms its tridentate NNN -Mn analogues due to the configuration and steric hindrance of catalysts. Two types of non-phosphine based manganese(I) complexes, NN -manganese carbonyl complexes (Mn1 – Mn5) and NNN/S -pincer complexes (Mn6 – Mn10), have been assessed as catalysts for the transfer dehydrogenative (TDH) oxidation of secondary alcohols to afford ketones. Among them, Mn1, in combination with KBH 4 , proved the most effective and robust catalyst allowing (hetero)aromatic-, cycloalkyl and aliphatic--containing alcohols to be efficiently converted to their corresponding ketones with particularly high values of TON achievable (TON up to 2550). Furthermore, on the basis of experimental and DFT studies, an outer-sphere bifunctional mechanism for this Mn-catalyzed TDH is proposed. Density functional theory (DFT) computations suggest that the NN -bidentate complex Mn1 outperforms its tridentate NNN -Mn analogues due to the configuration and steric hindrance of catalysts. More importantly, this Mn-based catalytic TDH protocol using acetone as an inexpensive hydrogen acceptor offers an attractive and sustainable alternative for producing aromatic and aliphatic ketones. [ABSTRACT FROM AUTHOR]