Deciphering the origin of cooperative catalysis by dirhodium acetate and chiral spiro phosphoric acid in an asymmetric amination reaction

The mechanism of asymmetric amination of diazo-acetate by tert-butyl carbamate catalyzed by dirhodium tetra(trifluoro)acetate and chiral SPINOL-phosphoric acid is examined using DFT (M06 and B3LYP) computations. A cooperative participation of both catalysts is noticed in the stereo-controlling transition state of the reaction

Asymmetric Cooperative Catalysis in a Three-Component Reaction: Mechanism and Origin of Enantio- and Diastereoselectivities

Mechanistic insights gained through density functional theory (DFT M06 and B3LYP) computations on a three-component cooperative asymmetric catalytic reaction between a diazo ester, a carbamate, and an imine, catalyzed by dirhodium acetate and chiral phosphoric acid (Brønsted acid), are presented. The addition of the dirhodium-bound enol to the imine yielding an α,β-diamino ester is energetically more preferred over a potentially competitive protonation of the same enol leading to an α-amino ester

Insights on the Origin of Regiodivergence in the Parallel Kinetic Resolution of <i>rac</i>-Aziridines Using a Chiral Lanthanum–Yttrium Bimetallic Catalyst

Parallel kinetic resolution of racemic mixtures is an important method used in asymmetric synthesis of chiral compounds. In a recent example, a <i>rac</i>-<i>cis</i>-2,3-substituted chiral <i>N</i>-benzoyl aziridine was reacted with dimethyl malonate, in the presence of a La–Y heterobimetallic chiral BINAM Schiff base (L) catalyst, to form enantiomerically pure (ee > 98%) γ-amino acid derivatives through a ring-opening reaction in near-quantitative yields from both the enantiomers (∼48%). High regio- and enantioselectivities even with a <i>rac</i>-aziridine, having C2 and C3 substituents as similar as ethyl and <i>n</i>-propyl. Through a comprehensive computational investigation, we delineate the origin of regio-divergent and enantioselective formation of γ-amino ester derivatives. The Gibbs free energy of the transition state for the ring-opening at the propyl substituted C2 carbon leading to 3-benzamidoheptan-4-yl malonate is found to be 7.2 kcal/mol lower than that at the ethyl substituted C3 carbon in the case of (2<i>R</i>,3<i>S</i>)-aziridine. A reversal of the regio-chemical preference for its enantiomeric (2<i>S</i>,3<i>R</i>)-aziridine is noted where the ring-opening occurs at the ethyl substituted C3 carbon. The La–Y catalyst is found to initially “recognize” both the enantiomers of the <i>rac</i>-aziridine rather indiscriminately. The activation barriers for the most-preferred ring-opening for each enantiomer are found to be closely similar, suggesting that both enantiomers would react. The high regio-selectivity in the addition of lanthanum-bound malonate to the aziridine anchored onto the yttrium center is due to a unique geometric disposition of the aziridine in the stereocontrolling ring-opening transition state. The lowest-energy ring-opening transition state for each enantiomer of aziridine exhibited very similar geometries, while notable geometric distortions is identified in the malonate addition to less-preferred site of the same enantiomer

Axial Coordination Dichotomy in Dirhodium Carbenoid Catalysis: A Curious Case of Cooperative Asymmetric Dual-Catalytic Approach toward Amino Esters

One of the most recent developments in asymmetric catalysis is to employ two or more catalysts under one-pot reaction conditions. This article presents some interesting mechanistic insights on a cooperative dual-catalytic protocol relying on the catalytic ability of dirhodium carbenoid (derived from rhodium­(II) tetracarboxylate and a diazo compound) and a chiral spirophosphoric acid ((<i>R</i>)-SPA) in an asymmetric N–H insertion reaction. We have employed DFT­(M06 and B3LYP) computational methods to identify the stereocontrolling transition states wherein a chiral (<i>R</i>)-SPA protonates a dirhodium-bound enol intermediate. A true cooperative action elicited by both catalysts has been noted in the enantioselective protonation. More importantly, whether the second axial ligand on the remote rhodium atom could influence the energetic features of the reaction has been probed for the first time. In all steps (such as nitrogen extrusion, addition of amine to the dirhodium carbenoid, and the enol formation), except that in the stereocontrolling event, no major effect of axial ligation has been noticed. However, the presence of the axial ligand helps in stabilizing the protonation transition state and reduces the activation barrier for protonation, suggesting a vital role in stereoselectivity. The predicted sense of stereoselectivities is in good agreement with the experimental results

Bipyridyl/carbazolate silver(I) and gold(I) N-heterocyclic carbene complexes: A systematic study of geometric constraints and electronic properties

A series of silver(I) and gold(I) carbene complexes of the type [M(L)(2,2 &apos;-bipyridine)][PF6] (L = 1-benzyl-3-(2-pyridylmethyl)benzimidazolylidene; M = Ag (1); M = Au (3)) and [M(L)(carbazole)] (M = Ag (2); M = Au (4)) were synthesized and analyzed using a range of spectroscopic and crystallographic techniques. Inspection of the solid-state structures of 1, 2 and 4 revealed a number of intermolecular noncovalent interactions. In the solid-state structure adopted by 1, pi-pi and Ag-Ag interactions directed the complexes to orient in a head-to-tail fashion. The photophysical properties were found to be influenced by the ancillary ligands in solution as well as in the solid-state. Calculations were performed to support the aforementioned structural and optoelectronic assignments

Synthesis and Study of Palladium(II) and Platinum(II) Complexes Supported by a Common &quot;Wingtip&quot; N-Heterocyclic Carbene

Two annulated imidazolium salts, 2-(phenyl)imidazo[1,5-a]pyridin-4-ylium hexafluorophosphate 1 center dot H(PF6) and 1-methyl-2-(phenyl)imidazo[1,5-a]pyridin-4-ylium hexafluorophosphate 2 center dot H(PF6), were synthesized via formylative cyclization of the corresponding Schiff bases followed by anion metathesis with KPF6. Independently treating 1 center dot H(PF6) or 2 center dot H(PF6) with silver oxide and then palladium chloride in acetonitrile led to the formation of the complexes [Pd(1)(2)Cl(CH3CN)]PF6 (1a) and [Pd(2)(2)Cl(CH3CN)]PF6 (2a), respectively. Likewise, [Pt(1)(2)Cl(CH3CN)]PF6 (1b) and [Pt(2)(2)Cl(CH3CN)]PF6 (2b) were synthesized using similar transmetallation chemistry. The complexes were characterized using various spectroscopic techniques and the solid state structures of 1-H(PF6) as well as 2a were elucidated using X-ray diffraction analyses. A series of DFT calculations were also performed to gain further insight into the respective structures of the complexes. Complexes 1a and 2a were found to facilitate Suzuki coupling reactions under relatively mild conditions

Synthesis and Study of Palladium(II) and Platinum(II) Complexes Supported by a Common "Wingtip" N-Heterocyclic Carbene

Two annulated imidazolium salts, 2-(phenyl)imidazo[1,5-a]pyridin-4-ylium hexafluorophosphate 1 center dot H(PF6) and 1-methyl-2-(phenyl)imidazo[1,5-a]pyridin-4-ylium hexafluorophosphate 2 center dot H(PF6), were synthesized via formylative cyclization of the corresponding Schiff bases followed by anion metathesis with KPF6. Independently treating 1 center dot H(PF6) or 2 center dot H(PF6) with silver oxide and then palladium chloride in acetonitrile led to the formation of the complexes [Pd(1)(2)Cl(CH3CN)]PF6 (1a) and [Pd(2)(2)Cl(CH3CN)]PF6 (2a), respectively. Likewise, [Pt(1)(2)Cl(CH3CN)]PF6 (1b) and [Pt(2)(2)Cl(CH3CN)]PF6 (2b) were synthesized using similar transmetallation chemistry. The complexes were characterized using various spectroscopic techniques and the solid state structures of 1-H(PF6) as well as 2a were elucidated using X-ray diffraction analyses. A series of DFT calculations were also performed to gain further insight into the respective structures of the complexes. Complexes 1a and 2a were found to facilitate Suzuki coupling reactions under relatively mild conditions.© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei