Controlling Catalyst Behavior in Lewis Acid-Catalyzed Carbonyl-Olefin Metathesis.

Lewis acid-catalyzed carbonyl-olefin metathesis has introduced a new means for revealing the behavior of Lewis acids. In particular, this reaction has led to the observation of new solution behaviors for FeCl that may qualitatively change how we think of Lewis acid activation. For example, catalytic metathesis reactions operate in the presence of superstoichiometric amounts of carbonyl, resulting in the formation of highly ligated (octahedral) iron geometries.

Glycosyl Exchange of Unactivated Glycosidic Bonds: Suppressing or Embracing Side Reactivity in Catalytic Glycosylations.

While developing boron-catalyzed glycosylations using glycosyl fluoride donors and trialkylsilyl ether acceptors, competing pathways involving productive glycosylation or glycosyl exchange were observed. Experimental and computational mechanistic studies suggest a novel mode of reactivity where a dioxolenium ion is a key intermediate that promotes both pathways through addition to either a silyl ether or to the acetal of an existing glycosidic linkage. Modifications in catalyst structure enable either pathway to be favored, and with this understanding, improved multicomponent iterative couplings and glycosyl exchange processes were demonstrated.

Combined Theoretical and Experimental Investigation of Lewis Acid-Carbonyl Interactions for Metathesis.

The coordination of a carbonyl to a Lewis acid represents the first step in a wide range of catalytic transformations. In many reactions it is necessary for the Lewis acid to discriminate between starting material and product, and as a result, how these structures behave in solution must be characterized. Herein, we report the application of computational modeling to calculate properties of the solution interactions of acetone and benzaldehyde with FeCl.

Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Reactions: Interrupted Metathesis vs Carbonyl-Ene Reaction.

Lewis acid catalysts have been shown to promote carbonyl-olefin metathesis through a critical four-membered-ring oxetane intermediate. Recently, Brønsted-acid catalysis of related substrates was similarly proposed to result in a transient oxetane, which fragments within a single elementary step via a postulated oxygen-atom transfer mechanism. Herein, careful quantum chemical investigations show that Brønsted acid (triflic acid, TfOH) instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive.

Fluoride Migration Catalysis Enables Simple, Stereoselective, and Iterative Glycosylation.

Challenges in the assembly of glycosidic bonds in oligosaccharides and glycoconjugates pose a bottleneck in enabling the remarkable promise of advances in the glycosciences. Here, we report a strategy that applies unique features of highly electrophilic boron catalysts, such as tris(pentafluorophenyl)borane, in addressing a number of the current limitations of methods in glycoside synthesis. This approach utilizes glycosyl fluoride donors and silyl ether acceptors while tolerating the Lewis basic environment found in carbohydrates.

In Pursuit of Sustainable Hydrogen Storage with Boron-Nitride Fullerene as the Storage Medium.

Using well calibrated DFT studies we predict that experimentally synthesized B24 N24 fullerene can serve as a potential reversible chemical hydrogen storage material with hydrogen-gas storage capacity up to 5.13 wt %. Our theoretical studies show that hydrogenation and dehydrogenation of the fullerene framework can be achieved at reasonable rates using existing metal-free hydrogenating agents and base metal-containing dehydrogenation catalysts.

Theoretical investigation on the chemistry of entrapment of the elusive aminoborane (H2 N=BH2 ) molecule.

Aminoborane (H2 N=BH2 ) is an elusive entity and is thought to be produced during dehydropolymerization of ammonia borane, a molecule of prime interest in the field of chemical hydrogen storage. The entrapment of H2 N=BH2 through hydroboration of exogenous cyclohexene has emerged as a routine technique to infer if free H2 N=BH2 is produced or not during metal-catalyzed ammonia borane dehydrogenation reactions. But to date, the underlying mechanism of this trapping reaction remains unexplored.

Unfolding the crucial role of a nucleophile in Ziegler-Natta type Ir catalyzed polyaminoborane formation.

formation through dehydropolymerization of ammonia-borane by Brookhart's iridium pincer catalyst has been under intense scrutiny but a sound molecular level understanding has remained elusive. Herein, using DFT the mechanism outlined by us for IrH2POCOP catalyzed formation underscores the importance of generation of nucleophiles, in particular that of the metal bound NH2BH2 moiety armed with a nitrogen lone pair for chain initiation and chain propagation steps.

Dramatic reduction in the activation barrier for dinitrogen splitting using amine-borane as a hydrogen carrier: insights from the DFT study.

DFT based mechanistic investigations on a silica surface supported Tantalum system reveal that there is a significant reduction in the free energy activation barrier for N≡N bond dissociation upon using a suitable amine-borane as a hydrogenating agent as compared to that of using molecular H2 for the same purpose, which suggests that dinitrogen dissociation can be achieved in surface chemistry at much lower temperatures than those in the presently used systems.

The role of solvent and of species generated in situ on the kinetic acceleration of aminoborane oligomerization.

The unexpected role of nucleophilic assistance of solvents and intermediates generated in situ in catalyzing NH2BH2 oligomerization is revealed in a computational study. The rate-determining free-energy barrier E(A) that is due to solvent participation for conversion of NH2BH2 to cyclotriborazane (NH2BH2)3 is only 12.7 kcal  mol(-1), whereas without nucleophilic assistance it is as high as 29.

Copper(II) complexes of 3,4,5-trisubstituted pyrazolates: in situ formation of pyrazole rings from different carbon centers.

The synthesis and characterization of two pyrazolate-bridged dicopper(II) complexes, [Cu(2)(L(1))(2)(H(2)O)(2)](ClO(4))(2) (1, HL(1)=3,5-dipyridyl-4-(2-keto-pyridyl)pyrazole) and [Cu(2)(L(2))(2)(H(2)O)(2)](ClO(4))(2) (2, HL(2)=3,5-dipyridyl-4-benzoylpyrazole), are discussed. These copper(II) complexes are formed from the reactions between pyridine-2-aldehyde, 2-acetylpyridine (for compound 1) or acetophenone (for compound 2), and hydrazine hydrate with copper(II) perchlorate hydrate under ambient conditions. The single-crystal X-ray structure of compound 1·2H(2)O establishes the formation of a pyrazole ring from three different carbon centers through C-C bond-forming reactions, mediated by copper(II) ions.