Allene und Heteroallene als Substrate zinkvermittelter, biomimetischer Additionsreaktionen

Die vorliegende Dissertation beschreibt die systematische Übertragung der biochemischen Prinzipien der Reaktion des Enzyms Carboanhydrase (CA) auf rein chemische Fragestellungen. CAs katalysieren die reversible Hydratisierung von Kohlendioxid und besitzen enorme Bedeutung bei unterschiedlichsten biochemischen Prozessen in jedem Organismus. Nicht zuletzt aufgrund ihres evolutionsbiologischen Alters gehören CAs zu den Enzymen mit den höchsten Wechselzahlen und Beschleunigungsfaktoren. Mittels geeigneter Modelle für experimentelle Umsetzungen und quantenchemische Berechnungen konnten die Reaktionsmechanismen von Allen und den Heteroallenen Carbodiimid, Keten, Ketenimin und Thioketen an biomimetischen Zinkkomplexen aufgeklärt werden. Die Berechnungen dieser Arbeit zeigen u.a., dass nicht Cyanamid, sondern das Isomer Carbodiimid das aktive Substrat der mechanismusgestützten Inhibition von CA darstellt. Auch Keten, Ketenimin und Thioketen können als Inhibitoren für CA wirken, da analog zu der Umsetzung mit Carbodiimid freie Carbonylgruppen zu einer hohen thermodynamischen Stabilität der Intermediate führen. Intensiv wurde die biomimetische Hydratisierung von Allen untersucht. Die durch kostengünstige Zinkkatalysatoren vermittelte Reaktion mit Nukleophilen erscheint als durchaus realisierbar und besitzt demzufolge einen deutlichen ökonomischen Aspekt. Die Ergebnisse aus der Berechnung der Aktivierungsbarrieren der Modellreaktion mit Wasser zeigen einen starken katalytischen Effekt, der sich durch geeignete Substitutionen verstärken lässt

Push-pull-allenes: The influences of substituents on the activation of allenes by biomimetic zinc complexes

The influence of substituents at the allene skeleton on the rate-determining step of the reaction with nucleophiles catalyzed by biomimetic zinc complexes was investigated with quantum chemical (especially DFT) methods. Additional examinations were applied to derivatives of the zinc hydroxide complex modeled in analogy to the catalytic center of carbonic anhydrase. Especially suitable substituents in the allene moiety can lead to a significant lowering of the activation barrier. Further we demonstrate that by the application of this principle of a bioanalogous enhancement of reactivity other nucleophiles instead of the biological substrate can also be reactants in completely closed catalytic reaction cycles. © 2010 Verlag der Zeitschrift für Naturforschung

Triplet state homoaromaticity : concept, computational validation and experimental relevance

Cyclic conjugation that occurs through-space and leads to aromatic properties is called homoaromaticity. Here we formulate the homoaromaticity concept for the triplet excited state (T1) based on Baird's 4n rule and validate it through extensive quantum-chemical calculations on a range of different species (neutral, cationic and anionic). By comparison to well-known ground state homoaromatic molecules we reveal that five of the investigated compounds show strong T1 homoaromaticity, four show weak homoaromaticity and two are non-aromatic. Two of the compounds have previously been identified as excited state intermediates in photochemical reactions and our calculations indicate that they are also homoaromatic in the first singlet excited state. Homoaromaticity should therefore have broad implications in photochemistry. We further demonstrate this by computational design of a photomechanical “lever” that is powered by relief of homoantiaromatic destabilization in the first singlet excited state

Computational Investigation of Brook-Type Silabenzenes and Their Possible Formation through [1,3]-Si→O Silyl Shifts

Quantum chemical calculations with the M06-2X, B3LYP, and B3LYP-D2 density functional theory methods were performed in order to examine the formation of Brook-type silabenzenes <b>4a</b>–<b>n</b> through [1,3]-trimethylsilyl (TMS) and [1,3]-triisopropylsilyl (TIPS) shifts from a tetrahedral silicon atom to an adjacent carbonyl oxygen of cyclic conjugated acylsilane precursors. All Brook-type silabenzenes, having a 2-trialkylsiloxy substituent, are at lower relative energies than their precursors. The free energy of activation at the M06-2X/6-311+G­(d,p) level for the thermal [1,3]-silyl shifts leading to the smallest Brook-type silabenzene (<b>4a</b>) is 30.2 kcal/mol, and it is 27.5 kcal/mol for a silabenzene (<b>4l</b>) with TIPS, OTIPS, and <i>tert</i>-butyl substituents. The geometries and nucleus-independent chemical shifts (NICS) of the Brook-type silabenzenes indicate aromatic character. The [4 + 2], [2 + 2], and [4 + 4] cycloaddition dimers were also studied. At the M06-2X/6-311+G­(d)//M06-2X/6-31G­(d) and B3LYP-D2/6-31G­(d) levels, i.e., two DFT methods which accurately describe nonbonded dispersive interactions, most Brook-type silabenzene dimers studied herein are lower in energy than two silabenzenes. The activation energies for dimerization of <b>4l</b> to either of two [4 + 2] cycloadducts (25.7 and 29.6 kcal/mol with M06-2X/6-31G­(d)) suggest that this silabenzene potentially can exist as a monomer at ambient temperature. However, the transition state structures for the dimerization of <b>4l</b> reveal where further bulk should be added, leading to silabenzene <b>4n</b>, a species for which dimerization is endothermic or only slightly exothermic

Organic Single Molecular Structures for Light Induced Spin-Pump Devices

We present theoretical results on molecular structures for realistic spin-pump applications. Taking advantage of the electron spin resonance concept, we find that interesting candidates constitute triplet biradicals with two strongly spatially and energetically separated singly occupied molecular orbitals (SOMOs). Building on earlier reported stable biradicals, particularly <i>bis</i>(nitronyl nitroxide) based biradicals, we employ density functional theory to design a selection of potential molecular spin-pumps which should be persistent at ambient conditions. We estimate that our proposed molecular structures will operate as spin-pumps using harmonic magnetic fields in the MHz regime and optical fields in the infrared to visible light regime

Metal-mediated reaction modeled on nature: the activation of isothiocyanates initiated by zinc thiolate complexes

On the basis of detailed theoretical studies of the mode of action of carbonic anhydrase (CA) and models resembling only its reactive core, a complete computational pathway analysis of the reaction between several isothiocyanates and methyl mercaptan activated by a thiolate-bearing model complex [Zn(NH ) SMe]+ was performed at a high level of density functional theory (DFT). Furthermore, model reactions have been studied in the experiment using relatively stable zinc complexes and have been investigated by gas chromatography/mass spectrometry and Raman spectroscopy. The model complexes used in the experiment are based upon the well-known azamacrocyclic ligand family ([12]aneN , [14]aneN , i-[14]aneN , and [15]aneN ) and are commonly formulated as ([Zn- ([X]aneN )(SBn)]ClO . As predicted by our DFT calculations, all of these complexes are capable of insertion into the heterocumulene system. Raman spectroscopic investigations indicate that aryl-substituted isothiocyanates predominantly add to the CdN bond and that the size of the ring-shaped ligands of the zinc complex also has a very significant influence on the selectivity and on the reactivity as well. Unfortunately, the activated isothiocyanate is not able to add to the thiolate-corresponding mercaptan to invoke a CA analogous catalytic cycle. However, more reactive compounds such as methyl iodide can be incorporated. This work gives new insight into the mode of action and reaction path variants derived from the CA principles. Further, aspects of the reliability of DFT calculations concerning the prediction of the selectivity and reactivity are discussed. In addition, the presented synthetic pathways can offer a completely new access to a variety of dithiocarbamates. © 2011 American Chemical Society. © 2011 American Chemical Society

Polyfulvenes: Polymers with “Handles” That Enable Extensive Electronic Structure Tuning

The fundamental electronic structure properties of substituted poly­(penta)­fulvenes and pentafulvene-based polymers are analyzed through qualitative molecular orbital (MO) theory combined with calculations at the B3LYP and HSE06 hybrid density functional theory (DFT) levels. We argue that the pentafulvene monomer unit has a unique character because electron density in the exocyclic CC double bond can be polarized into and out of the five-membered ring, a feature that is not available to other more commonly used monomers. It is investigated how the energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively), as approximate band gaps, are influenced by exocyclic substitution, introduction of linker groups, benzannulation, and ring substitution. In particular, the exocyclic positions of the fulvene act as “handles” by which the electronic structure of the polymer can be tuned between the quinoid and fulvenoid valence bond isomers; electron-withdrawing exocyclic substituents lead to polyfulvenes in the quinoid form while those with electron-donating substituents prefer the fulvenoid. Taken together, the HOMO–LUMO gaps of polyfulvenes can be tuned extensively, varying in ranges 0.77–2.44 eV (B3LYP) and 0.35–2.00 eV (HSE06) suggesting that they are a class of polymers with highly interesting, yet nearly unexplored, properties

New Class of Molecular Conductance Switches Based on the [1,3]-Silyl Migration from Silanes to Silenes

On the basis of first-principles density functional theory calculations, we propose a new molecular photoswitch which exploits a photochemical [1,3]-silyl­(germyl) shift leading from a silane to a silene (a SiC double bonded compound). The silanes investigated herein act as the OFF state, with tetrahedral saturated silicon atoms disrupting the conjugation through the molecules. The silenes, on the other hand, have conjugated paths spanning over the complete molecules and thus act as the ON state. We calculate ON/OFF conductance ratios in the range of 10–50 at a voltage of +1 V. In the low bias regime, the ON/OFF ratio increases to a range of 200–1150. The reverse reaction could be triggered thermally or photolytically, with the silene being slightly higher in relative energy than the silane. The calculated activation barriers for the thermal back-rearrangement of the migrating group can be tuned and are in the range 108–171 kJ/mol for the switches examined herein. The first-principles calculations together with a simple one-level model show that the high ON/OFF ratio in the molecule assembled in a solid state device is due to changes in the energy position of the frontier molecular orbitals compared to the Fermi energy of the electrodes, in combination with an increased effective coupling between the molecule and the electrodes for the ON state