Hildegard Keller

Zur theorie der chiralitätsfunktionen—III: Chiralitätsbeobachtungen im absorptionsgebiet von [2.2]metacyclophanen

Zusammenfassung Die Unzulanglichkeit von Naherungsanaitzen zur Beschreibung von Molrotationen optisch akriver 4,14-homodisubstituierter [2.2]Metacyclophane gibt Anlass zur Analyse ihres Circulardichroismus (CD). Die aus den langstwelligen elektronischen Ubergangen ( 1 L b - bzw. 1 W-Bande) erhaltenen Chiralitatsbeobachtungen zeigen an allgemeinen gute Ubereinstimmung mit den verwendeten Naherungsfunktionen. Im Gegensatz dazu ist eine adaquate Beschreibung energetisch hoherer Ubergange nicht mehr moglich. Diese Nichtaquivalenz von Observablen der optischen Aktiviat im Rahmen der Naherungsansatze erlaubt eine Lokauslisierung der zwischen berechneten und gemessenen Molrotationen gefundenen Diskepanzen auf bestimmte elektronische Ubergange. Eine physikalische Deutung ist im Hinblick auf Wechselwirkingen der beiden aromatischen Chromophore in hoher angeregten Zustǎnden moglich.

Synthesen 2-substituierter 3-Indol-alkansäuren

A synthetic approach to 2-alkoxycarbonyl-3-indolealkanoic acidsvia partial saponification of the corresponding diesters is presented. 2-Ethoxycarbonyl-3-indoleacetic acid as well as the homologous propionic and butyric acids may conveniently be prepared by this method. A different synthesis of 3-indoleacetic acids functionalized at position 2 can be accomplished by decomposition of ethyl diazoacetate in the presence of the indole derivative at elevated temperature. Cyclization of 2-hydroxymethyl-3-indoleacetic acid, thus prepared, yields the corresponding δ-valerolactone. With 1-acyl-indoles and ethyl diazoacetate addition of the carbene to the C-2=C-3 double bond prevails, resulting in the formation of cyclopropanecarboxylic esters.

Concerning the theory of chirality functions: Part I. Construction and discussion of chirality functions by means of [2.2] metacyclophanes

Abstract By means of [2.2]metacyclophane (skeleton symmetry C 2h ) the construction of qualitatively-complete chirality functions via the first and second “Naherungsansatze” is demonstrated. In this context, a detailed comment concerning the algebraic theory of chirality functions is given. As the first term of the chirality function (“abridged Ansatz”), both methods furnish a simple superposition rule (“quadrant rule”). The second component as deduced via the method of polynomials is of third degree in ligand specific parameters. A particular property of the second approximation method is derived. The consequences with respect to observables of the chirality phenomenon, in the case of the approximations being valid, are discussed.

Rotamere 2-Phenyl-1,3-dioxane

By means of dipole moment measurements, NMR-spectroscopical methods and force field calculations of o-, m- and p-substituted 2-phenyl-1,3-dioxanes a systematic study concerning the rotameric behaviour of the phenyl group was performed.In o-substituted 2-phenyl-1,3-dioxanes the preferred conformation of the phenyl ligand was found to be displaced about 60 degs. from thecisoid-bisectional conformer. Introduction of an additional substituent in the second o-position gives rise to a change in the rotameric state, the preferred conformation now being bisectional. In contrast, the barriers resisting rotation about the Car−C-2-bond in 2-phenyl-1,3-dioxane and its m- and p-substituted derivates turn out to be much lower. Thus at room temperature an ensemble of approximately bisectional rotamers exists.The conformations and rotational barriers determined are discussed in terms of non bonded and dipole-dipole interactions.

Isomerisierung und Ringinversion von Tribenzaspiro[5.5]undecaphanen

For the two possible ring inversions in tribenzaspiro[5.5]undecaphanes—corresponding to the isomerizations7→9 and9→7 resp.—no mutual influence is found. Thus the degenerate isomerizations (“topomerizations”)7→7 and9→9 can proceedvia two consecutive (non-concerted) reversals. A ring inversion path similar to that for [2.2]metacyclophane (1) is postulated. The considerable ease of the isomerization of9 as compared to1 can be understood on the basis of thermodynamic arguments.Moreover it is confirmed that the free enthalpy difference ΔG360=3.84 kcal/mol of7 and9 at equilibrium originates almost entirely from the different stabilities of boat and chair shaped benzene rings.These conclusions are drawn from a kinetic study of the thermal isomerization of9 and equilibration experiments in comparison with the corresponding data of1. The activation parameters for the process9→7 are: ΔG360isomA≠=26.52 kcal/mole, ΔHisomA≠=25.35 kcal/mole, ΔSisomA≠=−3.28 cal/deg·mole (isomerization) and ΔG360invA≠=27.03 kcal/mole, ΔHinvA≠=25.35 kcal/mole, ΔSinvA≠=−4.67 cal/deg·mol (ring reversal); the corresponding data for the degenerate isomerizations of7 and9—identical with those for the ring reversal in7—were deduced to be ΔG360invB≠=30.87 kcal/mole, ΔHinvB≠=27.50 kcal/mole and ΔSinvB≠=−9.5 cal/deg·mole.

Zur Theorie der Chiralitätsfunktionen

The Algebraic Theory of Chirality Functions is derived by means of exclusively qualitative considerations. Hence, the significance of quantitative results is questionable. Moreover the construction of “Näherungsansätze” (”Approximation-Ansatz”), which may be interpreted as semiempirical methods, is achieved on the basis of plausibility and mathematical simplicity. Since physical arguments are not included, the consistency or inconsistency of “Näherungsansätze” with experimental results do not justify direct physical conclusions.

Zur Anwendbarkeit von Ringstrommodellen in der Konformationsanalyse

The classical (Johnson-Bovey) and quantum mechanical (Haigh-Mallion) approach accounting for the shielding contribution to protons in the proximity of aromatic nuclei are compared with respect to their applicability in conformational analysis. This is accomplished by means of the1H-NMR spectra of various cyclophanes their geometries being fairly well known from X-ray data or other independent methods. The chemical shifts of the monitor protons (all of them being situated above or below the plane of the benzene ring considered) are consistent with shielding effects predicted by the classical theory notwithstanding the fact that the benzene hexagons are not strictly planar in any of these systems. In contrast, the quantum mechanical treatment considerably underestimates the shielding of protons in those regions.

Zum rotameren Verhalten des Phenyl- und Phenyltricarbonylchrom-liganden in 2-substituierten 1,3-Dioxanen: Ein Vergleich

Dipole moment measurements, NMR-spectroscopical methods and dipole-dipole interaction calculations applied to 2-benchrotrenyl-1,3-dioxanes and a comparison with o-substituted 2-phenyl derivatives furnish evidence, that in both cases the rotameric state is mainly determined by the steric demand of the substituent being attached to phenyl: the declination angle (π) of the Cr(CO)3-group from the bisectional cisoid relation (with respect to H on C-2) is identical with that for the ortho substituent and amounts to about 60 degs. The influence of the Cr(CO)3-fragment on the conformational behaviour of the benzene moiety in geminal 2,2-disubstituted derivatives is consistent with these results: the assumption of an axial and gonal conformation for the phenyl ring, which had recently been established for 2-alkyl-2-phenyl-1,3-dioxanes, is no longer valid for the 2-alkyl-2-phenyltricarbonylchromium compounds.

Grenzen der Anwendbarkeit regionaler Symmetrieregeln: Der aromatische1Lb-Cottoneffekt

A critical examination concerning the validity of symmetry rules, which correlate theCD of the inherently achiral, chirally perturbed benzene chromophore in the1Lb wavelength region with the absolute configuration is presented. It is shown, that the derivation of generally applicable symmetry rules is interdicted by fundamental arguments as is deduced by studying the origin of optical activity in the above mentioned electronic transition.Further evidence is given, that the generation of parallel components of the electric and magnetic moment is due to a vibronic perturbation. While the electric transition moment can roughly be estimated using spectroscopic partial moments, the magnetic transition moment will depend on the properties of the ▪-skeleton including both conformational rigidity and the mass distribution (which latter involves the substitution pattern). Thus the uncertainity concerning the direction of the magnetic moment prevents the deduction of a symmetry rule applicable to different molecular frames. Moreover, the relevance of such rules is shown to be limited even within distinct frames. A method is developed to find the conditions which imply the establishment of an unequivocal sector rule for a definite substitution pattern. Moreover, this may afford a tool for a rough localization of the magnetic transition moment.

Concerning the theory of chirality functions

Within the scope of the theory of chirality functions, qualitatively complete chirality functions are subject to restrictions concerning both generality and applicability. In contrast thereto, the concept of qualitative supercompleteness results in less restrictive requirements for chirality functions. Consequently, the applicability of qualitatively supercomplete chirality functions is unlimited with respect to the number of ligand kinds. Given this concept, a group theoretical treatment is performed supplying the formal conditions of qualitative supercompleteness. Subsequently a construction rule for qualitiatively supercomplete chirality functions is presented, which is elaborated in detail in the appendix. On combining physical considerations with the requirement of qualitative supercompleteness the resulting chirality functions appear to include all the possible interactions within and/or between ligands and skeleton. From both a mathematical and a physical point of view these chirality functions should be adequate for describing the chiroptical properties of molecules belonging to a given skeletal class. Nevertheless, all the other critical objections to the theory of chirality functions remain.