Organic Chemistry à la Genevoise

This article describes the situation of Department of Organic Chemistry in the year of the 450th anniversary of the University of Geneva

Artificial tongues and leaves

The objective with synthetic multifunctional nanoarchitecture is to create large suprastructures with interesting functions. For this purpose, lipid bilayer membranes or conducting surfaces have been used as platforms and rigid-rod molecules as shape-persistent scaffolds. Examples for functions obtained by this approach include pores that can act as multicomponent sensors in complex matrices or rigid-rod π-stack architecture for artificial photosynthesis and photovoltaic

Ordered and Oriented Supramolecular n/p-Heterojunction Surface Architectures: Completion of the Primary Color Collection

In this study, we describe synthesis, characterization, and zipper assembly of yellow p-oligophenyl naphthalenediimide (POP-NDI) donor−acceptor hybrids. Moreover, we disclose, for the first time, results from the functional comparison of zipper and layer-by-layer (LBL) assembly as well as quartz crystal microbalance (QCM), atomic force microscopy (AFM), and molecular modeling data on zipper assembly. Compared to the previously reported blue and red NDIs, yellow NDIs are more π-acidic, easier to reduce, and harder to oxidize. The optoelectronic matching achieved in yellow POP-NDIs is reflected in quantitative and long-lived photoinduced charge separation, comparable to their red and much better than their blue counterparts. The direct comparison of zipper and LBL assemblies reveals that yellow zippers generate more photocurrent than blue zippers as well as LBL photosystems. Continuing linear growth found in QCM measurements demonstrates that photocurrent saturation at the critical assembly thickness occurs because more charges start to recombine before reaching the electrodes and not because of discontinued assembly. The found characteristics, such as significant critical thickness, strong photocurrents, large fill factors, and, according to AFM images, smooth surfaces, are important for optoelectronic performance and support the existence of highly ordered architectures

Ab initio investigations of perhydrotriquinacenyl and other bridgehead carbocations

Ab initio MO calculations at the SCF level using 6–31 G* basis sets show that only 2.1 kcal mol–1 more energy is needed to form perhydrotriquinacenyl cation 1 from the corresponding hydrocarbon than to form the tert-butyl cation from 2-methylpropane; such a stabilisation energy is comparable to those computed for 1-noradamantyl cation 2, 3-noradamantyl cation 3, and bicyclo[3.2.1]octan-1-yl cation 4, which are shown to be stabilized efficiently by C–C hyperconjugation

Ab initio calculations of the conformation and difference electron density of dithio-oxamide

The conformation and difference electron density maps for dithio-oxamide were calculated by an ab initio method using a 4-31G basis set. The quasi absence of p conjugation through the central C---C bond is revealed

Steric effects on reaction rates - V. An updated version of the foote-schleyer correlation based on molecular mechanics

The strain changes occurring during solvolysis of secondary tosylates are calculated by a molecular mechanics programme (MM2) using an empirical force-field for carbenium ions. The rate constants for acetolysis of kC-substrates correlate with these strain changes, defining a straight line representative for kC behaviour

Protonated Cyclopropane as an Intermediate in Cation-Olefin Cyclizations. Ab initio and Density Functional Theory Investigations

In order to investigate the mechanism of cation–olefin cyclizations, the model calculations for the cyclization of cis-5,6-dimethyl-5-hexenyl cation were studied with both ab initio and density functional theory methods. The geometry optimization of the cis-5,6-dimethyl-5-hexenyl cation reveals its spontaneous rearrangement into protonated cyclopropane intermediates, for which four conformers were identified on the potential energy surface of C8H15+ ions. The present study demonstrates that two of these conformers are suitably preorganized to undergo chair- and boat-like cation–olefin cyclizations. The result is the formation of the chair cyclohexyl cation and the protonated cyclopropane moiety embedded into the cyclohexyl cation in boat conformation. The analysis of the intrinsic reaction coordinates show that the formation of the latter species via the boat-like pathway is preferred by 2.0 kcal/mol. To appreciate the influence of the leaving group, the cyclization reactions of protonated cis-1,2-dimethyl-6-hydroxyhexene were also studied by the MP2 and MP4 methods, using the 6-31G* and 6-311+G* basis sets. In this model system, the neutral water molecule was used to simulate the leaving group during the cyclization step in which the cyclohexyl cation–water complexes were generated. The MP4(SDQ)/6-31G*//MP2/6-31G* computed energy barriers are almost identical for the chair- and boat-like cyclization reactions of the appropriately preorganized reactants. The boat-like cyclization pathway, preferred only by 0.2 kcal/mol, again leads to the incorporation of the protonated cyclopropane into the cyclohexyl cation–water complex in boat conformation. To evaluate the cyclization energy barriers when the preorganization of the reactant is not taken into account, the lowest conformer of protonated alcohol has to be considered. In this case, the chair-like cyclization pathway is preferred by a small margin of 0.8 kcal/mol for the formation of the C–C hyperconjugated chair cyclohexyl cation–water complex. The use of a larger basis set and the inclusion of polarization functions has little, if any, effect on the relative stability of cation–water complexes. The MP2 and MP4 methods appear to be well suited for investigations of cation–olefin cyclizations for the model systems proposed in this study. In contrast to intermolecular cation–olefin additions, which are barrier-less processes, the present study demonstrates that the MP2 and, to an even greater extent, the MP4 method, reveal distinct energy barriers for intramolecular cation–olefin cyclizations

Steric Effects on Reaction Rates. Part IX. Force-Field Parameters for Bridgehead and Rigid Tertiary Carbenium Ions

Molecular-mechanics calculations for strain of carbenium ions are tested using Bentley's unified reactivity scale for bridgehead solvolysis as reference. Excellent correlations are obtained for solvolytic bridgehead reactivity with the calculated steric-energy difference (?Est) between substrate (R--H or R--OH) and cation (R+). After adjustment of appropriate force-field parameters, the approach is successfully extended to the rigid, but planar cations derived from structures 15-20; however, the general set of parameters cannot be applied to highly strained systems such as the cation formed from 17. With all of the 18 sets of parameters tested, the 2-endo-norbornyl derivative 16, is adequately correlated, while the exo isomer 15 exhibits enhanced reactivity by a factor of ca. 102 to 103

Steric Effects on Reaction Rates. VI. Application of a New MM2 Force-Field for Carbenium Ions to Solvolysis Rates of Secondary p-Toluenesulfonates

An empirical force-field for carbenium ions has been incorporated in Allinger's MM2 programme. Structural parameters of secondary carbenium ions calculated by this method are compared with those obtained with Schleyer's BIGSTRN calculations. The strain changes occurring upon solvolysis of secondary p-toluenesulfonates are evaluated by means of this force-field and correlated with the rate constants for solvolysis. The equation for correlation of acetolysis, relative to cyclohexyl p-toluenesulfonate, of 28 kc substrates is ΔG+(rel) = 0.67 ΔEst - 0.20 (r = 0.958)