The mechanisms of Diels-Alder reactions between 1,2,3-triazines and enamines have been explored with density functional theory computations. The focus of this work is on the origins of the different reactivities and mechanisms induced by substituents and by hexafluoroisopropanol (HFIP) solvent. These inverse electron-demand Diels-Alder reactions of triazines have wide applications in bioorthogonal chemistry and natural product synthesis. Both concerted and stepwise cycloadditions are predicted, depending on the nature of substituents and solvents. The nature of zwitterionic intermediates and the mechanism by which HFIP accelerates cycloadditions with enamines are characterized. Our results show the delicate nature of the concerted versus stepwise mechanism of inverse electron-demand Diels-Alder reactions of 1,2,3-triazines, and that these mechanisms can be altered by electron-withdrawing substituents and hydrogen-bonding solvents.

C-H activation and functionalization are on the forefront of modern synthetic chemistry. Imagine if any C-H bond of a molecule could be converted to a C-X bond, where X is the target functionality-this would alter the synthetic blueprints for complex target molecules since it would provide novel disconnections in retrosynthetic analysis. Collaborations between many experimental and computational groups have led to rapid developments of new C-H functionalization methods. Our groups represent an example of this; we were brought together as part of the NSF-supported Center for Selective C-H Functionalization. Many examples of experimental-computational synergy for selective Pd(II)-catalyzed C-H activation of aryl and alkyl groups are described in this Account. We describe computations by the Houk group made in response to experimental stimuli by the Yu group. The first section discusses the experimental and computational investigations of oxazoline-directed stereoselective Pd(II)-catalyzed C(sp3)-H bond activation that occurs through the concerted metalation-deprotonation (CMD) pathway involving a monomeric Pd(II) complex. The second section involves two types of bidentate ligands, mono-N-protected amino acid (MPAA) and acetyl-protected aminoethyl quinoline (APAQ) ligands that promote the C-H activation reactions with the ligand as the internal base. In the MPAA-assisted remote C-H bond activation, the basic dianionic amidate ligand participates in the deprotonation of a specific C-H bond. This mechanism accounts for the improved reactivity and selectivity in C-H activation reactions with MPAA ligands. The chiral APAQ ligands enable asymmetric palladium insertion into prochiral C-H bonds on a single methylene carbon center. The dianionic amidate of the APAQ ligand acts as an intramolecular base to deprotonate the methylene C-H asymmetrically and facilitate chiral Pd-C bond formation. The origins of the dramatic differences of five-membered (relatively inactive) and six-membered (highly active) chelation in β-methylene C(sp3)-H activation reactions by a Pd(II) catalyst were explained with density functional theory (DFT) calculations. This is mainly due to the steric repulsions between the ArF group of the substrate and the quinoline group of the ligand. The steric repulsion between the ArF group of the substrate and the quinoline group of the APAQ ligand destabilizes the five-membered chelate transition structure, increasing the energy of the transition state. The third section discusses a mechanism involving a Pd-Ag heterodimeric complex intermediate in the template-directed, Pd(II)-catalyzed remote meta functionalization of toluene derivatives and benzoic acid derivatives. The nitrile directing group of the template coordinates with Ag while the Pd is placed adjacent to the meta-C-H bond in the transition state, leading to the observed high meta selectivity. The selective activation of remote meta-C-H bonds at various distances can be achieved by tuning the template. The dual role of AgOAc as both an oxidant and part of the heteronuclear active species in the mechanism involving PdAg(OAc)3 was determined by DFT calculations and is in accord with literature information about complexes. For the systems discussed in these three sections, the similarity is that they all proceed via the CMD mechanism. The differences lie in the proton acceptors and the active Pd species. Common CMD involves a monomeric Pd mechanism with acetate as the proton acceptor. Both MPAA and APAQ ligands react via monomeric Pd mechanisms with a ligand moiety (the amidate oxygen) as the proton acceptor. Nitrile-containing template-mediated meta-C-H activations proceed via a Pd-Ag heterodimeric mechanism, still with acetate as the proton acceptor. The interaction between our two groups, experts in experiment and computation, and the discoveries made possible by that interplay are highlighted in this Account.

Copper-catalyzed reactions of glycine ester arylimines and methacrylonitrile provide selective access to either the endo or exo pyrrolidine cycloadducts. DFT calculations have elucidated the origins of ligand-controlled diastereoselectivity.

We report the generation of the first 3,4-piperidyne and its use as a building block for the synthesis of annulated piperidines. Experimental and computational studies of this intermediate are disclosed, along with comparisons to the well-known 3,4-pyridyne. The distortion/interaction model is used to explain the observed regioselectivities.

We report the first catalytic method for activating the acyl C-O bonds of methyl esters through an oxidative-addition process. The oxidative-addition adducts, formed using nickel catalysis, undergo in situ trapping to provide anilide products. DFT calculations are used to support the proposed reaction mechanism, to understand why decarbonylation does not occur competitively, and to elucidate the beneficial role of the substrate structure and the Al(OtBu)3 additive on the kinetics and thermodynamics of the reaction.

C-H functionalization represents a promising approach for the synthesis of complex molecules. Instead of relying on modifying the functional groups present in a molecule, the synthetic sequence is achieved by carrying out selective reactions on the C-H bonds, which traditionally would have been considered to be the unreactive components of a molecule. A major challenge is to design catalysts to control both the site- and stereoselectivity of the C-H functionalization. We have been developing dirhodium catalysts with different selectivity profiles in C-H functionalization reactions with donor/acceptor carbenes as reactive intermediates. Here we describe a new dirhodium catalyst capable of the functionalization of non-activated primary C-H bonds with high levels of site selectivity and enantioselectivity.

Amides are common functional groups that have been studied for more than a century. They are the key building blocks of proteins and are present in a broad range of other natural and synthetic compounds. Amides are known to be poor electrophiles, which is typically attributed to the resonance stability of the amide bond. Although amides can readily be cleaved by enzymes such as proteases, it is difficult to selectively break the carbon-nitrogen bond of an amide using synthetic chemistry. Here we demonstrate that amide carbon-nitrogen bonds can be activated and cleaved using nickel catalysts. We use this methodology to convert amides to esters, which is a challenging and underdeveloped transformation. The reaction methodology proceeds under exceptionally mild reaction conditions, and avoids the use of a large excess of an alcohol nucleophile. Density functional theory calculations provide insight into the thermodynamics and catalytic cycle of the amide-to-ester transformation. Our results provide a way to harness amide functional groups as synthetic building blocks and are expected to lead to the further use of amides in the construction of carbon-heteroatom or carbon-carbon bonds using non-precious-metal catalysis.

Effective differentiation of prochiral carbon-hydrogen (C-H) bonds on a single methylene carbon via asymmetric metal insertion remains a challenge. Here, we report the discovery of chiral acetyl-protected aminoethyl quinoline ligands that enable asymmetric palladium insertion into prochiral C-H bonds on a single methylene carbon center. We apply these palladium complexes to catalytic enantioselective functionalization of β-methylene C-H bonds in aliphatic amides. Using bidentate ligands to accelerate C-H activation of otherwise unreactive monodentate substrates is crucial for outcompeting the background reaction driven by substrate-directed cyclopalladation, thereby avoiding erosion of enantioselectivity. The potential of ligand acceleration in C-H activation is also demonstrated by enantioselective β-C-H arylation of simple carboxylic acids without installing directing groups.

The hallmark of enzymes from secondary metabolic pathways is the pairing of powerful reactivity with exquisite site selectivity. The application of these biocatalytic tools in organic synthesis, however, remains under-utilized due to limitations in substrate scope and scalability. Here, we report how the reactivity of a monooxygenase (PikC) from the pikromycin pathway is modified through computationally guided protein and substrate engineering, and applied to the oxidation of unactivated methylene C-H bonds. Molecular dynamics and quantum mechanical calculations were used to develop a predictive model for substrate scope, site selectivity and stereoselectivity of PikC-mediated C-H oxidation. A suite of menthol derivatives was screened computationally and evaluated through in vitro reactions, where each substrate adhered to the predicted models for selectivity and conversion to product. This platform was also expanded beyond menthol-based substrates to the selective hydroxylation of a variety of substrate cores ranging from cyclic to fused bicyclic and bridged bicyclic compounds.

We report the first observed Pd-catalyzed isomerization ("cage-walking") of B(9)-bromo-meta-carborane during Pd-catalyzed cross-coupling, which enables the formation of B-O and B-N bonds at all boron vertices (B(2), B(4), B(5), and B(9)) of meta-carborane. Experimental and theoretical studies suggest this isomerization mechanism is strongly influenced by the steric crowding at the Pd catalyst by either a biaryl phosphine ligand and/or substrate. Ultimately, this "cage-walking" process provides a unique pathway to preferentially introduce functional groups at the B(2) vertex using B(9)-bromo-meta-carborane as the sole starting material through substrate control.