Allyl-Allyl Coupling Promoted by Catalyst Systems with two Palladium Atoms - A Plethora of Potentially Pericyclic Processes.

Recently, Huang and co-workers reported a catalytic reaction that utilizes H2 as the sole reductant for a C-C coupling of allyl groups with yields up to 96%. Here we use computational quantum chemistry to identify several key features of this reaction that provide clarity on how it proceeds. We propose the involvement of a precatalyst Pd-Pd bound dimer, demonstrate the importance of ligand π-π interactions and counterions, and identify a new, energetically viable, mechanism involving two dimerized, outer-sphere reductive elimination transition structures that determine both the rate and selectivity. Although we rule out the previously proposed transmetalation step on energetic grounds, we show it to have an unusual aromatic transition structure in which two Pd atoms support rearranging electrons. The prevalence of potential metal-supported pericyclic reactions in this system suggests that one should consider such processes regularly, but the results of our calculations also indicate that one should do so with caution.

Entropic Effects on Diamine Dynamics and CO2 Capture in Diamine-Appended Mg2(dopbdc) Metal-Organic Frameworks.

Recent measurements [Xu, J.; J. Phys. Chem. Lett. 2019, 10 (22), 7044-7049] have reported temperature-dependent rates of detachment of diamine from Mg sites in diamine-appended Mg2(dobpdc) [dobpdc4- = 4,4'-dihydroxy(1,1'-biphenyl)-3,3'-dicarboxylic] metal-organic frameworks, a process hypothesized to be a precursor for cooperative CO2 adsorption, leading to step-shaped isotherms or isobars. Here, we compute the rate of diamine exchange in this system for different diamines using metadynamics simulations based on a density functional theory-derived neural network potential. Reanalyzing recent measurements accounting for entropic effects, we find a positive correlation between the previously reported CO2 adsorption step pressure and the free energy at room temperature and show that the experiments and simulations are in good qualitative and quantitative agreement. The rates obtained here provide new insight into the chemical dynamics of CO2 adsorption in a class of materials that are promising for carbon capture and a lower bound on the time scale of cooperative adsorption.

Catalytic asymmetric cationic shifts of aliphatic hydrocarbons.

Asymmetric catalysis is an advanced area of chemical synthesis, but the handling of abundantly available, purely aliphatic hydrocarbons has proven to be challenging. Typically, heteroatoms or aromatic substructures are required in the substrates and reagents to facilitate an efficient interaction with the chiral catalyst. Confined acids have recently been introduced as tools for homogenous asymmetric catalysis, specifically to enable the processing of small unbiased substrates1. However, asymmetric reactions in which both substrate and product are purely aliphatic hydrocarbons have not previously been catalysed by such super strong and confined acids. We describe here an imidodiphosphorimidate-catalysed asymmetric Wagner-Meerwein shift of aliphatic alkenyl cycloalkanes to cycloalkenes with excellent regio- and enantioselectivity. Despite their long history and high relevance for chemical synthesis and biosynthesis, Wagner-Meerwein reactions utilizing purely aliphatic hydrocarbons, such as those originally reported by Wagner and Meerwein, had previously eluded asymmetric catalysis.

The Construction of Highly Substituted Piperidines via Dearomative Functionalization Reaction.

Nitrogen heterocycles play a vital role in pharmaceuticals and natural products, with the six-membered aromatic and aliphatic architectures being commonly used. While synthetic methods for aromatic N-heterocycles are well-established, the synthesis of their aliphatic functionalized analogues, particularly piperidine derivatives, poses a significant challenge. In that regard, we propose a stepwise dearomative functionalization reaction for the construction of highly decorated piperidine derivatives with diverse functional handles. We also discuss challenges related to site-selectivity, regio- and diastereoselectivity, and provide insights into the reaction mechanism through mechanistic studies and density functional theory computations.

Metal-organic frameworks as O2-selective adsorbents for air separations.

Oxygen is a critical gas in numerous industries and is produced globally on a gigatonne scale, primarily through energy-intensive cryogenic distillation of air. The realization of large-scale adsorption-based air separations could enable a significant reduction in associated worldwide energy consumption and would constitute an important component of broader efforts to combat climate change. Certain small-scale air separations are carried out using N2-selective adsorbents, although the low capacities, poor selectivities, and high regeneration energies associated with these materials limit the extent of their usage. In contrast, the realization of O2-selective adsorbents may facilitate more widespread adoption of adsorptive air separations, which could enable the decentralization of O2 production and utilization and advance new uses for O2. Here, we present a detailed evaluation of the potential of metal-organic frameworks (MOFs) to serve as O2-selective adsorbents for air separations. Drawing insights from biological and molecular systems that selectively bind O2, we survey the field of O2-selective MOFs, highlighting progress and identifying promising areas for future exploration. As a guide for further research, the importance of moving beyond the traditional evaluation of O2 adsorption enthalpy, ΔH, is emphasized, and the free energy of O2 adsorption, ΔG, is discussed as the key metric for understanding and predicting MOF performance under practical conditions. Based on a proof-of-concept assessment of O2 binding carried out for eight different MOFs using experimentally derived capacities and thermodynamic parameters, we identify two existing materials and one proposed framework with nearly optimal ΔG values for operation under user-defined conditions. While enhancements are still needed in other material properties, the insights from the assessments herein serve as a guide for future materials design and evaluation. Computational approaches based on density functional theory with periodic boundary conditions are also discussed as complementary to experimental efforts, and new predictions enable identification of additional promising MOF systems for investigation.