Direct Dynamics Trajectories Demonstrate Dynamic Matching and Nonstatistical Radical Pair Intermediates during Fe-Oxo-Mediated C-H Functionalization Reactions

Origin of Intramolecular versus Intermolecular C-H Arene Activation Selectivity by Cyclopentadienyl-Triphenylphosphine Iridium

Photolysis of (η5-C5Me5)Ir(PPh3)(H)2 in benzene generates the 16-electron (16e-) complex (η5-C5Me5)Ir(PPh3) that undergoes competitive intramolecular ortho-metalation with a phenyl group from -PPh3 and intermolecular C-H activation with benzene. Previous density functional theory (DFT) studies identified the intramolecular π-complex and the intermolecular benzene π-complex intermediates and their corresponding C-H activation transition states. However, neither the mechanism of interconversion between these intermediates nor the origin of intramolecular versus intermolecular pathway selectivity has been established. Here, we characterized the open-shell 16e- iridium species and extensively mapped out the energy landscape for intramolecular ortho-metalation of -PPh3 versus intermolecular benzene C-H activation. Also, we performed DFT-based direct dynamics simulations, and the results suggest that the intramolecular versus intermolecular pathway selectivity is determined dynamically within picoseconds as the 16e- iridium species evolves into a coordinatively saturated structure. During this process, the π-complexes are formed concurrently with, instead of prior to, the iridium hydrides, which could not be explained by kinetic models that assume C-H cleavage as the rate-limiting step. These findings demonstrate that dynamics simulations in addition to DFT calculations are needed for a more complete mechanistic understanding of photoinduced C-H activation reactions, of which the product selectivity can be influenced by atomic motion.

Molecular Dynamics of the Davies Ambimodal C-H Functionalization/Cope Rearrangement Reaction

The mechanism of the dirhodium-catalyzed combined C-H functionalization/Cope rearrangement (CH/Cope) reaction discovered by the Davies group has been investigated with density functional theory (DFT) calculations and quasi-classical molecular dynamics (MD) simulations. Computations from the Davies group previously showed that there is a post-transition state bifurcation leading to a direct CH reaction and also to the CH/Cope product. While this work was in preparation, the Tantillo group and the Ess group independently reported quantum mechanical and molecular dynamics studies on the dirhodium-tetracarboxylate-catalyzed diazoester CH/Cope and CH insertion reactions with 1,3-cyclohexadiene and 1,4-cyclohexadiene, respectively. The Tantillo group cited "dynamic mismatching" to explain the origins of the low yield of CH/Cope products in some experiments; the Ess group explained the origins of product selectivity from the perspective of TS vibrational modes and their synchronization that occurs at the entropic intermediates. We report quasi-classical trajectories for the reaction of the carbene with 1-methylcyclohexene that afford both the CH/Cope and C-H insertion products. After passing through the transition state that involves mostly hydrogen transfer, momentum drives the reaction trajectories toward the CH/Cope products.

Rational Design of the Spatial Effect in a Fe(II)/α-Ketoglutarate-Dependent Dioxygenase Reverses the Regioselectivity of C(sp3)-H Bond Hydroxylation in Aliphatic Amino Acids

The hydroxylation of remote C(sp3)-H bonds in aliphatic amino acids yields crucial precursors for the synthesis of high-value compounds. However, accurate regulation of the regioselectivity of remote C(sp3)-H bonds hydroxylation in aliphatic amino acids continues to be a common challenge in chemosynthesis and biosynthesis. In this study, the Fe(II)/α-ketoglutarate-dependent dioxygenase from Bacillus subtilis (BlAH) was mined and found to catalyze hydroxylation at the γ and δ sites of aliphatic amino acids. Crystal structure analysis, molecular dynamics simulations, and quantum chemical calculations revealed that regioselectivity was regulated by the spatial effect of BlAH. Based on these results, the spatial effect of BlAH was reconstructed to stabilize the transition state at the δ site of aliphatic amino acids, thereby successfully reversing the γ site regioselectivity to the δ site. For example, the regioselectivity of L-Homoleucine (5 a) was reversed from the γ site (1 : 12) to the δ site (>99 : 1). The present study not only expands the toolbox of biocatalysts for the regioselective functionalization of remote C(sp3)-H bonds, but also provides a theoretical guidance for the precision-driven modification of similarly remote C(sp3)-H bonds in complex molecules.

Evidence for Dearomatizing Spirocyclization and Dynamic Effects in the Quasi-stereospecific Nitrogen Deletion of Tetrahydroisoquinolines

Selectivity in organic chemistry is generally presumed to arise from energy differences between competing selectivity-determining transition states. However, in cases where static density functional theory (DFT) fails to reproduce experimental product distributions, dynamic effects can be examined to understand the behavior of more complex reaction systems. Previously, we reported a method for nitrogen deletion of secondary amines which relies on the formation of isodiazene intermediates that subsequently extrude dinitrogen with concomitant C-C bond formation via a caged diradical. Herein, a detailed mechanistic analysis of the nitrogen deletion of 1-aryl-tetrahydroisoquinolines is presented, suggesting that in this system the previously determined diradical mechanism undergoes dynamically controlled partitioning to both the normal 1,5-coupling product and an unexpected spirocyclic dearomatized intermediate, which converges to the expected indane by an unusually facile 1,3-sigmatropic rearrangement. This mechanism is not reproduced by static DFT but is supported by quasi-classical molecular dynamics calculations and unifies several unusual observations in this system, including partial chirality transfer, nonstatistical isotopic scrambling at the ethylene bridge, the isolation of spirocyclic dearomatized species in a related heterocyclic series, and the observation that introduction of an 8-substituent dramatically improves enantiospecificity.

Running Wild through Dirhodium Tetracarboxylate-Catalyzed Combined CH(C)-Functionalization/Cope Rearrangement Landscapes: Does Post-Transition-State Dynamic Mismatching Influence Product Distributions?

A special type of C-H functionalization can be achieved through C-H insertion combined with Cope rearrangement (CHCR) in the presence of dirhodium catalysts. This type of reaction was studied using density functional theory and ab initio molecular dynamics simulations, the results of which pointed to the dynamic origins of low yields observed in some experiments. These studies not only reveal intimate details of the complex reaction network underpinning CHCR reactions but also further cement the generality of the importance of nonstatistical dynamic effects in controlling Rh2L4-promoted reactions.

Dynamical Origin of Rebound versus Dissociation Selectivity during Fe-Oxo-Mediated C-H Functionalization Reactions

The mechanism of catalytic C-H functionalization of alkanes by Fe-oxo complexes is often suggested to involve a hydrogen atom transfer (HAT) step with the formation of a radical-pair intermediate followed by diverging pathways for radical rebound, dissociation, or desaturation. Recently, we showed that in some Fe-oxo reactions, the radical pair is a nonstatistical-type intermediate and dynamic effects control rebound versus dissociation pathway selectivity. However, the effect of the solvent cage on the stability and lifetime of the radical-pair intermediate has never been analyzed. Moreover, because of the extreme complexity of motion that occurs during dynamics trajectories, the underlying physical origin of pathway selectivity has not yet been determined. For the reaction between [(TQA_Cl)FeIVO]+ and cyclohexane, here, we report explicit solvent trajectories and machine learning analysis on transition-state sampled features (e.g., vibrational, velocity, and geometric) that identified the transferring hydrogen atom kinetic energy as the most important factor controlling rebound versus nonrebound dynamics trajectories, which provides an explanation for our previously proposed dynamic matching effect in fast rebound trajectories that bypass the radical-pair intermediate. Manual control of the reaction trajectories confirmed the importance of this feature and provides a mechanism to enhance or diminish selectivity for the rebound pathway. This led to a general catalyst design principle and proof-of-principle catalyst design that showcases how to control rebound versus dissociation reaction pathway selectivity.

Molecular Dynamics of Iron Porphyrin-Catalyzed C-H Hydroxylation of Ethylbenzene

Quasi-classical molecular dynamics (MD) simulations were carried out to study the mechanism of iron porphyrin-catalyzed hydroxylation of ethylbenzene. The hydrogen atom abstraction from ethylbenzene by iron-oxo species is the rate-determining step, which generates the radical pair of iron-hydroxo species and the benzylic radical. In the subsequent radical rebound step, the iron-hydroxo species and benzylic radical recombine to form the hydroxylated product, which is barrierless on the doublet energy surface. In the gas-phase quasi-classical MD study on the doublet energy surface, 45% of the reactive trajectories lead directly to the hydroxylated product, and this increases to 56% in implicit solvent model simulations. The percentage of reactive trajectories leading to the separated radical pair is 98-100% on high-spin (quartet/sextet) energy surfaces. The low-spin state reactivity dominates in the hydroxylation of ethylbenzene, which is dynamically both concerted and stepwise, since the time gap between C-H bond cleavage and C-O bond formation ranges from 41 to 619 fs. By contrast, the high-spin state catalysis is an energetically stepwise process, which has a negligible contribution to the formation of hydroxylation products.