Organocatalytic Trapping of Elusive Carbon Dioxide Based Heterocycles by a Kinetically Controlled Cascade Process

A conceptually novel approach is described for the synthesis of six-membered cyclic carbonates derived from carbon dioxide. The approach utilizes homoallylic precursors that are converted into five-membered cyclic carbonates having a β-positioned alcohol group in one of the ring substituents. The activation of the pendent alcohol group through an N-heterocyclic base allows equilibration towards a thermodynamically disfavored six-membered carbonate analogue that can be trapped by an acylating agent. Various control experiments and computational analysis of this manifold are in line with a process that is primarily dictated by a kinetically controlled acylation step. This cascade process delivers an ample diversity of six-membered cyclic carbonates in excellent yields and chemoselectivities under mild reaction conditions.     

Metal-directed self-assembly of a Zn(II)-salpyr complex into a supramolecular vase structure

A new type of supramolecular building block, Zn(II)-salpyr [salpyr = N,N'-3-pyridylenebis(salicylideneimine)], is described that contains both a pyridyl donor and a Lewis acidic Zn(II) acceptor site in the salen framework. As a consequence, this building block self-organizes into a stable tetrameric vase structure via cooperative intermolecular Zn-N(pyr) interactions.     

Multiple scaling replica exchange for the conformational sampling of biomolecules in explicit water

A multiple scaling replica exchange method for the efficient conformational sampling of biomolecular systems in explicit solvent is presented. The method is a combination of the replica exchange with solute tempering (REST) technique and a Tsallis biasing potential. The Tsallis biasing increases the sampling efficiency, while the REST minimizes the number of replicas needed. Unbiased statistics can be obtained by reweighting of the data using a weighted histogram analysis technique. The method is illustrated by its application to a ten residue peptide in explicit water.     

Diketopyrrolopyrrole-based acceptor polymers for photovoltaic application

Developing new acceptor materials as alternatives to fullerene acceptors remains a challenge in the field of organic photovoltaics. We report on the synthesis and optoelectronic properties of three acceptor polymers bearing diketopyrrolopyrrole units in the main chain (PA, PB and PC). Their performance as the acceptor material in bulk heterojunction solar cells using P3HT as the donor material has been tested. The solar cells show relatively high open-circuit voltages (≥0.9 V) but low fill factors and short-circuit current densities limit the photovoltaic device performance. Formation of free charge carriers and low electron mobility are identified as the major obstacles. In blends of P3HT with PA or PB charge formation is limited, while for the P3HT:PC blend photogenerated charges recombine into the PC triplet state before they can separate, unless assisted by a reverse electric field.     

A graph-theoretical approach for the analysis and model reduction of complex-balanced chemical reaction networks

In this paper we derive a compact mathematical formulation describing the dynamics of chemical reaction networks that are complex-balanced and are governed by mass action kinetics. The formulation is based on the graph of (substrate and product) complexes and the stoichiometric information of these complexes, and crucially uses a balanced weighted Laplacian matrix. It is shown that this formulation leads to elegant methods for characterizing the space of all equilibria for complex-balanced networks and for deriving stability properties of such networks. We propose a method for model reduction of complex-balanced networks, which is similar to the Kron reduction method for electrical networks and involves the computation of Schur complements of the balanced weighted Laplacian matrix.     

A DFT Study on the Mechanism of the Cycloaddition Reaction of CO2 to Epoxides Catalyzed by Zn(Salphen) Complexes

The reaction mechanism for the Zn(salphen)/NBu₄X (X=Br, I) mediated cycloaddition of CO₂ to a series of epoxides, affording five‐membered cyclic carbonate products has been investigated in detail by using DFT methods. The ring‐opening step of the process was examined and the preference for opening at the methylene (C_β) or methine carbon (C_α) was established. Furthermore, calculations were performed to clarify the reasons for the lethargic behavior of internal epoxides in the presence of the binary catalyst. Also, the CO₂ insertion and the ring‐closing steps have been explored for six differently substituted epoxides and proved to be significantly more challenging compared with the ring‐opening step. The computational findings should allow the design and application of more efficient catalysts for organic carbonate formation.     

A short desymmetrization protocol for the coordination environment in bis-salphen scaffolds

A remarkable short preparation of desymmetrized bis-salphen scaffolds is presented. The protocol consists of an hydroxide-mediated hydrolysis of Lewis acidic bis-Zn(salphen) complexes yielding C(s)-symmetric diimine/amine salts that can be selectively transformed into bis-salphens with dissymmetric substitution patterns within each salphen unit under mild conditions. These isolated nonsymmetrical bis-salphen derivatives do not show signs of imine scrambling or decomposition due to a metal template effect. A possible rationale is provided for the formation and isolation of one of the intermediate bis-phenolate salts, and the hypothesis involves H-bond directed hydrolysis of the nearest located imine bond across the bis-salphen scaffold.     

Bicyclic Guanidine Promoted Mechanistically Divergent Depolymerization and Recycling of a Biobased Polycarbonate

We here report the organocatalytic and temperature-controlled depolymerization of biobased poly(limonene carbonate) providing access to its trans-configured cyclic carbonate as the major product. The base TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) offers a unique opportunity to break down polycarbonates via end-group activation or main chain scission pathways as supported by various controls and computational analysis. These energetically competitive processes represent an unprecedented divergent approach to polycarbonate recycling. The trans limonene carbonate can be converted back to its polycarbonate via ring-opening polymerization using the same organocatalyst in the presence of an alcohol initiator, offering thus a potential circular and practical route for polycarbonate recycling.     

Silver-Mediated Cascade Synthesis of Functionalized 1,4-Dihydro-2H-benzo-1,3-oxazin-2-ones from Carbon Dioxide

A conceptually novel catalytic domino approach is presented for the synthesis of highly functional 1,4-dihydro-2H-1,3-benzoxazine-2-one derivatives. Key to the chemoselectivity is a proper design of the precursor to override thermodynamically favored parasitic cyclization processes and empower the formation of the desired product through Thorpe–Ingold effects. The synthetic diversity of these CO₂-based heterocycles is further demonstrated, and the isolation of a reaction intermediate supports an unusual ring-expansion sequence from an α-alkylidene, five-membered cyclic carbonate to a six-membered cyclic carbamate by N-induced isomerization.