BINOL-derived phosphoric acids catalyze the asymmetric allylboration of aldehydes. DFT and QM/MM hybrid calculations showed that the reaction proceeds via a transition state involving both a hydrogen-bonding interaction from the catalyst hydroxyl group to the pseudoaxial oxygen of the cyclic boronate and a stabilizing interaction from the phosphoryl oxygen of the catalyst to the formyl hydrogen of the aldehyde. These interactions lower the energy of the transition structure and provide extra rigidity to the system. This mechanistic pathway is consistent with the experimentally observed enantioselectivity except in one case. We have used our model's predictions to guide our own experimental work. The conflict is resolved in favor of our calculations.

Aurovertins are fungal polyketides that exhibit potent inhibition of ATP synthase. Aurovertins contain a 2,6-dioxabicyclo[3.2.1]-octane ring that is proposed to be derived from a polyene precursor through regioselective oxidations and epoxide openings. In this study, we identified only four enzymes are required to produce aurovertin E 4. The core polyketide synthase produces a polyene α-pyrone 10. Following pyrone O-methylation by a methyltransferase, a flavin-dependent monooxygenase (FMO) and an epoxide hydrolase can iteratively transform the terminal triene portion of the precursor into the dioxabicyclo[3.2.1]-octane scaffold. We demonstrate that a tetrahydrofuranyl polyene 12 is the first stable intermediate in the transformation, which can undergo epoxidation and anti-Baldwin 6-endo-tet ring opening to yield the cyclic ether product. Our results further demonstrate the highly concise and efficient ways in which fungal biosynthetic pathways can generate complex natural product scaffolds.

Modern density functional theory and powerful contemporary computers have made it possible to explore complex reactions of value in organic synthesis. We describe recent explorations of mechanisms and origins of stereoselectivities with density functional theory calculations. The specific functionals and basis sets that are routinely used in computational studies of stereoselectivities of organic and organometallic reactions in our group are described, followed by our recent studies that uncovered the origins of stereocontrol in reactions catalyzed by (1) vicinal diamines, including cinchona alkaloid-derived primary amines, (2) vicinal amidophosphines, and (3) organo-transition-metal complexes. Two common cyclic models account for the stereoselectivity of aldol reactions of metal enolates (Zimmerman-Traxler) or those catalyzed by the organocatalyst proline (Houk-List). Three other models were derived from computational studies described in this Account. Cinchona alkaloid-derived primary amines and other vicinal diamines are venerable asymmetric organocatalysts. For α-fluorinations and a variety of aldol reactions, vicinal diamines form enamines at one terminal amine and activate electrophilically with NH(+) or NF(+) at the other. We found that the stereocontrolling transition states are cyclic and that their conformational preferences are responsible for the observed stereoselectivity. In fluorinations, the chair seven-membered cyclic transition states is highly favored, just as the Zimmerman-Traxler chair six-membered aldol transition state controls stereoselectivity. In aldol reactions with vicinal diamine catalysts, the crown transition states are favored, both in the prototype and in an experimental example, shown in the graphic. We found that low-energy conformations of cyclic transition states occur and control stereoselectivities in these reactions. Another class of bifunctional organocatalysts, the vicinal amidophosphines, catalyzes the (3 + 2) annulation reaction of allenes with activated olefins. Stereocontrol here is due to an intermolecular hydrogen bond that activates the electrophilic partner in this reaction. We have also studied complex organometallic catalysts. Krische's ruthenium-catalyzed asymmetric hydrohydroxyalkylation of butadiene involves two chiral ligands at Ru, a chiral diphosphine and a chiral phosphate. The size of this combination strains the limits of modern computations with over 160 atoms, multiple significant steps, and a variety of ligand coordinations and conformations possible. We found that carbon-carbon bond formation occurs via a chair Zimmerman-Traxler-type transition structure and that a formyl CH···O hydrogen bond from aldehyde CH to phosphate oxygen, as well as steric interactions of the two chiral ligands, control the stereoselectivity.

1,1'-Bi-2-naphthol (BINOL)-derived phosphoric acids catalyze the asymmetric propargylation of aldehydes. Density functional theory (DFT) calculations showed that the reaction proceeds via a six-membered transition structure (TS) in which the catalyst Brønsted acidic site interacts with the pseudoaxial cyclic boronate oxygen and the phosphoryl oxygen interacts with the formyl proton. This model accurately predicts the stereochemical outcome observed experimentally. Replacement of the phosphoric acid hydroxyl group with an N-triflyl moiety has been included in the model by calculation and a broader understanding achieved by qualitative assessment of similar reactions. We present a qualitative guide to rationalizing the experimental outcome and use this to make a prediction which was confirmed experimentally.

A practical, catalytic entry to α,α,α‐trisubstituted (α‐tertiary) primary amines by C−H functionalisation has long been recognised as a critical gap in the synthetic toolbox. We report a simple and scalable solution to this problem that does not require any in situ protection of the amino group and proceeds with 100 % atom‐economy. Our strategy, which uses an organic photocatalyst in combination with azide ion as a hydrogen atom transfer (HAT) catalyst, provides a direct synthesis of α‐tertiary amines, or their corresponding γ‐lactams. We anticipate that this methodology will inspire new retrosynthetic disconnections for substituted amine derivatives in organic synthesis, and particularly for challenging α‐tertiary primary amines.

The cinchona alkaloid-derived urea-catalyzed asymmetric conjugate addition of aromatic thiols to cycloalkenones was studied using density functional theory (DFT). Deprotonation of the thiol gives a protonated amine that activates the electrophile by Brønsted acid catalysis, while the urea group binds the nucleophilic thiolate by hydrogen bonding. These results demonstrate the generality of the Brønsted acid-hydrogen bonding transition state (TS) model for cinchona alkaloid catalysis that we recently showed to be favored over Wynberg's widely accepted ion pair-hydrogen bonding model and represent the first detailed mechanistic study of a cinchona urea-catalyzed reaction. The conformation of the catalyst methoxy group has a strong effect on the TS, an effect overlooked in previous mechanistic studies of reactions catalyzed by cinchona alkaloids.

The catalyst generated in situ from RuH2(CO)(PPh3)3, (S)-SEGPHOS, and a chiral phosphoric acid promotes asymmetric hydrohydroxyalkylation of butadiene and affords enantioenriched α-methyl homoallylic alcohols. The observed diastereo- and enantioselectivities are determined by both the chiral phosphine and chiral phosphate ligands. Density functional theory calculations (M06/SDD-6-311G(d,p)-IEFPCM(acetone)//B3LYP/SDD-6-31G(d)) predict that the product distribution is controlled by the kinetics of carbon-carbon bond formation, and this process occurs via a closed-chair Zimmerman-Traxler-type transition structure (TS). Chiral phosphate-dependent stereoselectivity arising from this TS is enabled through a hydrogen bond between the phosphoryl oxygen and the aldehyde formyl proton present in TADDOL-derived catalysts. This interaction is absent in the corresponding BINOL-derived systems and the opposite sense of attack on the aldehyde occurs. Additional factors influencing stereochemical control are determined.

Wynberg's report from 1977 that natural cinchona alkaloids catalyze the asymmetric conjugate addition of aromatic thiols to cycloalkenones is a landmark discovery in hydrogen bonding organocatalysis. Wynberg proposed that this reaction proceeded via the formation of a thiolate-alkylammonium tight ion pair and activation of the enone electrophile by a hydrogen bond from the catalyst's hydroxyl group. This reaction model provided the mechanistic basis for understanding Wynberg's reaction and many other asymmetric transformations since. Our quantum mechanical calculations reveal a different model should be used to explain the results: the alkylammonium ion activates the enone by Brønsted acid catalysis, and the catalyst's hydroxyl group orients the thiolate nucleophile. The new model rationalizes the stereoselective outcome of Wynberg's reaction and provides a new, general model for asymmetric cinchona organocatalysis.