: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Iron Catalysis in Organic Synthesis

The effective use of ring strain has been applied to considerable advantage for the construction of complex systems. The focus here is directed towards cyclopropanes as building blocks for organic synthesis. Although thermodynamics should take the side of synthetic chemists, only a specific substitution pattern at the cyclopropane ring allows for particularly mild, efficient, and selective transformations. The required decrease in the activation barrier is achieved by the combined effects of vicinal electron-donating and electron-accepting moieties. This Review highlights the appropriate tools for successfully employing donor–acceptor cyclopropanes in ring-opening reactions, cycloadditions, and rearrangements.

A new golden age for donor-acceptor cyclopropanes.

The irruption of polymers from renewable resources on the scene of macromolecular science and technology

Cyclization reactions of donor–acceptor (D–A) cyclopropanes are recognized as versatile methods for construction of carbocyclic and heterocyclic scaffolds. In the literature, many examples of these polarized cyclopropanes' reactivity with nucleophiles, electrophiles, and radicals are prevalent. Although intermolecular reactivity of donor–acceptor cyclopropanes is widely reported, reviews that center on their intramolecular chemistry are rare. Thereupon, this tutorial review focalizes on new intramolecular transformations of donor–acceptor cyclopropanes for cycloisomerizations, formal cycloadditions, umpolung reactions, rearrangements and ring-opening lactonizations/lactamizations from 2009 to 2013. Furthermore, the role of D–A acceptor cyclopropanes as reactive subunits in natural product synthesis is underscored.

Intramolecular donor–acceptor cyclopropane ring-opening cyclizations

Ringspannung gewinnbringend zu nutzen, ist seit jeher fur den Aufbau komplexer Systeme von grosem Vorteil. Schnell richtet sich somit der Blick auf Cyclopropane als Bausteine fur die organische Synthese. Auch wenn die Thermodynamik dabei auf der Seite des Synthesechemikers steht, ermoglicht erst ein spezielles Substitutionsmuster am Cyclopropan besonders milde, effiziente und selektive Umsetzungen. Die erforderliche Absenkung der Aktivierungsbarriere wird durch die Kombination von zueinander vicinal stehendem Elektronendonor und -akzeptor erreicht. Dieser Aufsatz beleuchtet die richtigen Hilfsmittel, um mit Donor-Akzeptor-substituierten Cyclopropanen Ringoffnungen, Cycloadditionen und Umlagerungen durchzufuhren.

Ein neues goldenes Zeitalter in der Chemie Donor‐Akzeptor‐substituierter Cyclopropane

anti-Oligoannelated THF moieties: synthesis via push-pull-substituted cyclopropanes

Multiple displacement of oxygen: Electron-rich oligopyrroles and pyrrolo[3,2-e]indoles are generated by a domino process induced by donor-acceptor-substituted cyclopropanes. Up to seven molecules of water are eliminated, thus allowing the introduction of nitrogen and aromaticity.

Domino reactions of donor-acceptor-substituted cyclopropanes for the synthesis of 3,3'-linked oligopyrroles and pyrrolo[3,2-e]indoles.

The synthesis of oligocyclic oligoacetals using five-membered rings as repetitive unit is described. Furan was used as the starting material, which is converted by a three-step procedure consisting of twofold cyclopropanation, reduction, and oxidative ring enlargement into a tricyclic bis(enol ether). A repetition of this synthetic procedure leads to the formation of extended oligoacetal systems. Insights into the structures were gained by X-ray crystallographic investigations and revealed helical arrangements of the subunits in the solid-state. DFT (B3LYP) calculations have been carried out to elucidate the transition state of the ring enlargement and the flexibility of the annelated oligocyclic systems. Strain energies and topologies of potential cyclically condensed oligoacetals are predicted.

Johannes Kaschel

Papers

A new golden age for donor-acceptor cyclopropanes.

Ein neues goldenes Zeitalter in der Chemie Donor‐Akzeptor‐substituierter Cyclopropane

anti-Oligoannelated THF moieties: synthesis via push-pull-substituted cyclopropanes

Domino reactions of donor-acceptor-substituted cyclopropanes for the synthesis of 3,3'-linked oligopyrroles and pyrrolo[3,2-e]indoles.

From Furan to Molecular Stairs: Syntheses, Structural Properties, and Theoretical Investigations of Oligocyclic Oligoacetals