Asymmetric Aldol Reactions in Caprolactam?Quaternary Ammonium Salt Coordination Ionic Liquid Catalyzed by L‐Pro‐L‐Trp

A direct asymmetric aldol reaction of aldehydes and acetone catalyzed by L ‐Pro‐L ‐Trp was performed in β‐caprolactam? quaternary ammonium salt coordination ionic liquid media in the presence of N‐methylmorpholine (NMM) in high yield and with good enantioselectivity. The approach has the advantages of simple product isolation, and reusable catalyst and coordination ion liquids.     

Theoretical Studies of the Asymmetric Binary‐Acid‐Catalyzed tert‐Aminocyclization Reaction: Origins of the C?H Activation and Stereoselectivity

A mechanistic study of the tert‐aminocyclization reaction was performed by using DFT calculations and labeling experiments. The results showed that the reaction proceeded through a rate‐limiting‐, stereospecific‐, and suprafacial 1,5‐H‐transfer pathway, followed by a barrier‐less C? C bond formation. The mode of stereocontrol for facial selection could be ruled out owing to the high activation energy of C? N bond rotation. The intrinsic feature of this Lewis acid activation was found to be the activation of the LUMO, as well as an intermediate‐stabilization effect. The catalytically active species was believed to be a 1:1 complex of phosphoric acid and MgCl₂, which was stabilized by a H???Cl hydrogen bond. The chiral catalytic complex selectively recognizes and activates one of the two helical conformations of substrate A , required for 1,5‐suprafacial H‐transfer, which dictates the stereoselectivity of the forming products.     

Asymmetric Direct Aldol Reactions Catalyzed by a Simple Chiral Primary Diamine–Br⊘nsted Acid Catalyst in/on Water

Abstract

The direct asymmetric aldol reaction catalyzed by the simple and commercially available chiral primary diamines, (1S,2S)-1,2-diphenylethane-1,2-diamine and (1R,2R)-1,2-diphenylethane-1,2-diamine, is presented. The catalyst system is a primary amine with Br?nsted acid–catalyzed direct aldol reaction of p-nitrobenzaldehyde and cyclohexanone with high chemo- and stereoselectivity on water, which furnishes the corresponding β-hydroxyketone with up to 94% ee.     

Palladium‐Catalyzed Direct Dialkenylation of Cage B?H Bonds in o‐Carboranes through Cross‐Coupling Reactions

Palladium‐catalyzed direct dialkenylation of cage B(4,5) H bonds in o‐carboranes has been achieved with the help of a carboxylic acid directing group, leading to the preparation of a series of 4,5‐[trans‐(ArCHCH)]₂‐ocarboranes in high yields with excellent regioselectivity. The traceless directing group, eliminated during the course of the reaction, is responsible for controlling regioselectivity and dialkenylation. A possible catalytic cycle is proposed, involving a tandem sequence of Pd^II‐initiated cage B H activation, alkene insertion, β‐H elimination, reductive elimination, and decarboxylation.     

Electronic Bond‐to‐Bond Fluxes in Pericyclic Reactions: Synchronous or Asynchronous?

One at a time or all at once? Electronic fluxes during a pericyclic reaction in the electronic ground state--exemplified for the degenerate Cope rearrangement of semibullvalene--may proceed either synchronously or asynchronously. Quantum simulations show that the mechanism is determined by the preparation of the reactants, for example, synchronous at cryogenic temperatures (tunneling) but asynchronous when induced by selective laser pulses (with energy over the barrier).     

Small Peptides Catalyzed Direct Aldol Reactions of Aldehydes with Hydroxyacetone with Regiocontrol in Aqueous Media

Very recently, we[1] found that L-proline amides and dipeptides acted as efficient catalysts for the asymmetric direct aldol reaction. We report here that L-proline-based peptides 1～5 can catalyze the aldol reactions of hydroxyacetone with aldehydes 6 in aqueous media, to give 1,4-diols (7), the disfavored products with either aldolase or L-proline. Both peptides 3 and 4 give good results.     

What Controls the Magnetic Exchange Interaction in Mixed‐ and Homo‐Valent Mn7 Disc‐Like Clusters? A Theoretical Perspective

Density functional theory (DFT) studies have been undertaken to compute the magnetic exchange and to probe the origin of the magnetic interactions in two hetero‐ and two homo‐valent heptanuclear manganese disc‐like clusters, of formula [Mn^II₄Mn^IV₃(tea)(teaH₂)₃(peolH)₄] ( 1 ), [Mn^II₄Mn^III₃F₃(tea)(teaH)(teaH₂)₂(piv)₄(Hpiv)(chp)₃] ( 2 ), [Mn^II₇(pppd)₆(tea)(OH)₃] ( 3 ) and [Mn^II₇ (paa)₆(OMe)₆] ( 4 ) (teaH₃=triethanolamine, peolH₄=pentaerythritol, Hpiv=pivalic acid, Hchp=6‐chloro‐2‐hydroxypyridine, pppd=1‐phenyl‐3‐(2‐pyridyl) propane‐1,3‐dione; paaH=N‐(2‐pyridinyl)acetoacetamide). DFT calculations yield J values, which reproduce the magnetic susceptibility data very well for all four complexes; these studies are also highlighting the likely ageing/stability problems in two of the complexes. It is found that the spin ground states, S, for complexes 1 – 4 are drastically different, varying from S=29/2 to S=1/2. These values are found to be controlled by the nature of the oxidation state of the metal ions and minor differences present in the structures. Extensive magneto–structural correlations are developed for the seven building unit dimers present in the complexes, with the correlations unlocking the reasons behind the differences in the magnetic properties observed. Independent of the oxidation state of the metal ions, the Mn‐O‐Mn/Mn‐F‐Mn angles are found to be the key parameters, which significantly influence the sign as well as the magnitude of the J values. The magneto–structural correlations developed here, have broad applicability and can be utilised to understand the magnetic properties of other Mn clusters.     

Advances in Copper‐Catalyzed C?C Coupling Reactions and Related Domino Reactions Based on Active Methylene Compounds

Active methylene compounds are a major class of reaction partners for C? C bond formation with sp² C? X (X=halide) fragments. As one of the most‐classical versions of the Ullmann‐type coupling reaction, activated‐methylene‐based C? C coupling reactions have been efficiently employed in a large number of syntheses. Although this type of reaction has long relied on noble‐metal catalysis, the renaissance of copper catalysis at the end of last century has led to dramatic developments in Ullmann C? C coupling reactions. Owing to its low cost, abundance, as well as excellent catalytic activity, the exceptional atom economy of copper catalysis is gaining widespread attention in various organic synthesis. This review summarizes the advances in copper‐catalyzed intermolecular and intramolecular C? C coupling reactions that use activated methylene species as well as in tandem reactions that are initiated by this transformation.     

Can Enantioselectivity be Computed in Enthalpic Barrierless Reactions? The Case of CuI‐Catalyzed Cyclopropanation of Alkenes

An extensive computational study has been carried out on different catalytic systems for cyclopropanation reactions based on copper. Most DFT schemes used present drawbacks that preclude the calculation of accurate absolute kinetic properties (energy barriers) of such systems, excepting the M05 and M06 suites of density functionals. On the other hand, there is a wide range of DFT methods capable of reproducing relative energy values, which can be easily translated into selectivities. Most of the theoretical levels used tend to overestimate activation barriers, allowing the location of the transition state (TS) on the potential-energy surface (PES) of the most reactive systems, which are probably artifacts of the method. However, after a thorough analysis of the calculated PES, and the origin of the energy differences obtained for the different alkene approaches in chiral systems, it is found that energy differences are almost constant over a wide range of geometries covering the reaction channel zone in which the true TS on the Gibbs free-energy surface (GFES) lies. Therefore, many computational schemes can still be used confidently to explain and predict enantioselectivities in these systems.     

Rh‐Catalyzed Aldehyde?Aldehyde Cross‐Aldol Reaction under Base‐Free Conditions: In Situ Aldehyde‐Derived Enolate Formation through Orthogonal Activation

The chemoselective generation of aldehyde‐derived enolates to realize an aldehyde? aldehyde cross‐aldol reaction is described. A combined Rh/dippf system efficiently promoted the isomerization/aldol sequence by using primary allylic, homoallylic, and bishomoallylic alcohols; secondary allylic and homoallylic alcohols; and trialkoxyboranes that were derived from primary allylic and homoallylic alcohols. The reaction proceeded at ambient temperature under base‐free conditions, thus giving cross‐aldol products with high chemoselectivity. Mechanistic studies, as well as its application to double‐aldol processes under protecting‐group‐free conditions, are also described.     

What Controls the Magnetic Interaction in bis‐μ‐Alkoxo MnIII Dimers? A Combined Experimental and Theoretical Exploration

The synthesis and magnetic characterisation of a series of bis‐μ‐alkoxide bridged Mn^III dinuclear complexes of general formula [Mn^III₂(μ‐OR)₂(biphen)₂(ROH)_x(L)_y] (where R=Me, Et; H₂biphen=2,2′‐biphenol and L=terminally bonded N‐donor ligand) is described, doubling the literature basis set for this type of complex. Building on these findings we have categorised all known μ‐OR bridged Mn^III dinuclear complexes into one of three classifications with respect to their molecular structures. We have then employed DFT and MO calculations to assess all potential magneto‐structural correlations for this class of compound in order to identify the structural requirements for constructing ferromagnetic family members. Our analysis indicates that the most influential parameter which governs the exchange interaction in this class of compounds is the relative orientation of the JT axes of the Mn^III atoms. A perpendicular orientation of the JT axes leads to a large ferromagnetic contribution to the exchange. These results also suggest that a large ferromagnetic interaction and a large anisotropy are unlikely to co‐exist in such structural types.     

Chiral Phosphate in Rhodium‐Catalyzed Asymmetric [2+2+2] Cycloaddition: Ligand,Counterion, or Both?

Investigations based on NMR spectroscopy, mass spectrometry, and DFT calculations shed light on the metallic species generated in the rhodium‐catalyzed asymmetric [2+2+2] cycloaddition reaction between diynes and isocyanates with the chiral phosphate TRIP. The catalytic mixture comprising [{Rh(cod)Cl}₂], 1,4‐diphenylphosphinobutane (dppb), and Ag(S)‐TRIP actually gives rise to two species, both having an effect on the stereoselectivity. One is a rhodium(I) complex in which TRIP is a weakly coordinating counterion, whereas the other is a bimetallic Rh/Ag complex in which TRIP is a strongly coordinating X‐type ligand.     

Aerobic Asymmetric Dehydrogenative Cross‐Coupling between Two C?H Groups Catalyzed by a Chiral‐at‐Metal Rhodium Complex

A sustainable C C bond formation is merged with the catalytic asymmetric generation of one or two stereocenters. The introduced catalytic asymmetric cross‐coupling of two C H groups with molecular oxygen as the oxidant profits from the oxidative robustness of a chiral‐at‐metal rhodium(III) catalyst and exploits an autoxidation mechanism or visible‐light photosensitized oxidation. In the latter case, the catalyst serves a dual function, namely as a chiral Lewis acid for catalyzing enantioselective enolate chemistry and at the same time as a visible‐light‐driven photoredox catalyst.