The structure of a molecule can change considerably as its energy and thus its electron distribution within the time-domain of dynamic relaxation varies. Based on comparison of approriate measured data of related compounds and supported by quantum chemical calculations, therefore, charge-perturbed and/or sterically overcrowded molecules can be designed. Their preparation, handling, and structural characterization, frequently under extreme and especially largely aprotic conditions, provides some surprises. New structural principles become evident and old-fashioned ones are confirmed. Thus the contact-ion aggregates that form on ultrasonically supported reduction of unsaturated hydrocarbons with sodium metal partly contain dibenzene sodium sandwiches. Vicinal dimethylamino substituents or isoelectronic isopropyl groups cause steric overcrowding and facilitate oxidation to molecular cations by energetically favorable delocalization of the generated positive charge. Molecules and molecular ions in which an even number of II electrons are distributed over a D skeleton containing an odd number of centers preferentially form cyanine subunits. This is demonstrated by the novel ethene dication and dianion salts with central C-C single bonds and molecular halves twisted relative to each other. Altogether in two years well over 50 structures have been determined. Much has been learned from them, especially about electron transfer and contact ion-pair formation in aprotic solvents. Nevertheless, we had to realize that answers to many questions, above all "what crystallizes, how, and why", are still out of reach."ActuuNy, everything is more comjdicuted".calculation of an approximate one-dimensional MNDO energy hypersurface, in which only one of the total of (3n -6) = 30 degrees freedom for the molecule containing n = 12 centers, namely the dihedral angle w(C-SCCS-C) (Scheme 2), is considered. 550@ VCH Verluxsgesellschaft mbH, W-6940 Weinheinz. 1992 OS7O-OS33~92jOSOS-OSSO $3.50+ .2SjO Angew. Chem. Int. Ed. Engl. 31 (1992) 550-581 The Structures of Radical Cations R,NNR;@ in SolutionThe oxidation of silyl-substituted hydrazine derivatives by the oxygen-free and selective one-electron-transfer oxidation reagent AICI,/CH,Cl,[41 yields their radial cations['a' 4 9 61 in their doublet ground states [Eq. (a)].

Die Struktur eines Molekiils kann sich mit seiner Energie und damit seiner Elektronenverteilung innerhalb der Zeitskala dynamischer Relaxation betrlchtlich andern. Ausgehend von Vergleichen geeigneter MeDdaten verwandter Verbindungen und unterstiitzt durch quantenchemische Naherungsrechnungen lassen sich daher ladungsgestorte undloder raumlich iiberfiillte Molekiile entwerfen, deren Synthese, Handhabung und Strukturbestimmung -unter oft extremen und vor allem weitgehend aprotischen Bedingungen -zu manchen Uberraschungen fuhrt. Dabei werden neue Strukturprinzipien deutlich und altbekannte bestatigt. So enthalten einige der bei ultraschallgeforderter Reduktion ungesattigter Kohlenwasserstoffe mit Natriummetall entstehenden Kontaktionenaggregate von Molekiilanionen DibenzolnatriumSandwich-Einheiten. Vicinale Dimethylaminosubstituenten bewirken wie die isoelektronischen Isopropylgruppen sterische Uberfiillung und erleichtern die Oxidation zu Molekiilkationen durch energetisch giinstige Delokalisierung der erzeugten positiven Ladung. Molekiile und Molekiilionen, in denen sich eine gerade Anzahl von .n-Elektronen iiber ein ungeradzahliges cr-Geriist verteilt, bilden bevorzugt Cyanin-Untereinheiten aus; dies belegen auch die erstmals isolierten Salze von Ethen-Dikationen und Ethen-Dianionen mit C-C-Einfachbindung und gegeneinander verdrillten Molekiilhalften. -Insgesamt konnten in zwei Jahren iiber 50 unbekannte Strukturen bestimmt werden. Aus ihnen haben wir vie1 gelernt, insbesondere iiber Elektronentransfer und Kontaktionenpaar-Bildung in aprotischen Losungen. Wir mu& ten aber auch erkennen, daI3 eine Beantwortung vieler Fragen -darunter vor allem ,,was kristallisiert wie und warum?" -noch in weiter Ferne liegt. Eigentlich ist alles vie1 komplizierterVorwort einer Frankfurter Doktorarbeit Ausgangspunkt : Strukturabschatzungen aus MeBdaten und naherungsweisen Energieh yperflachen-BerechnungenAuch der iiberwiegend praparativ engagierte Chemiker profitiert erheblich, wenn er seine Spektren zusatzlich unter dem Gesichtspunkt ,,Molekiilzustands-Fingerabdriicke" betrachtet"]. Je ein Beispiel fur Gasphase und Losung sollen Vorgehen und Erkenntnisgewinn verdeutlichen. Die Struktur von H,CS-CEC-SCH, in der GasphaseDie Gasphasenthermolyse von Bis(methy1thio)cyclobuten-l,2-dion bei 720 K verlauft nach Photoelektronen(PE)-

There are two possible sites for the protons in the title compound. For steric reasons the structure in which the protons each interact with three nitrogen atoms is rejected in favor of the structure depicted on the right. The counterion BPh 4− is essential; when Cl− is the counterion, drastic structural changes are induced in the dication. This phenomenon may also be significant in biologically important structures (nitrogen bases!).

Table 2. Preparation of w-ureido D-amino acids Hydantoin o-amino acid Content of ~[ h ] 0 0 n = l '0 n = 2 n = 3 D-alhizziin (101 D-4-ureido-2-aminohutyric acid[ll] 0-ci trulline I1 21 . . n = 4 D-homocitrulline 1131 H 0 H H,N N d S + C O O H I, NH, 0 V 69 73 79 70 > 99 98-99.8 96-99.8 60 m = 1 0 o-w-ureido-AEC [c] 71 98-99.8 m = 2 D-w-carhamoylthialysine 1141 78 98-99.8 [a] Yield of isolated product [YO]. [h] Amount Of D enantiomer [%I, determined by comparison with literature data or by chromatography with the racemic compounds for comparison[l0-14]; the ranges refer to different runs. [c] AEC = S-aminoethylcysteine.Furthermore, we desired to determine the spectrum of substrates accepted by this biocatalyst. Readily obtainable racemic or L-n,w-diamino carboxylic acids were converted into hydantoins analogous to 5['' and subjected to biotransformation (Table 2).This approach provides a series of interesting w-ureido 6-amino acids that are chemically synthesized only with difficulty and are not obtainable at all through fermentation. These examples show the advantages of combining chemical and enzymatic steps in the preparation of enantiomerically pure amino acids unknown in nature. Experimental ProcedureSummary of the synthesis ofo-citrulline (1, C,H,,N,O,, M = 175.19)[15]: A solution of 5 (4 g, 22.8 mmol), prepared according to 191, in water (55 mL) was adjusted to pH 8.4 by addition of concentrated aqueous sodium hydroxide. The solution was then degassed by agitating in a mechanical shaker at 40 "C for 10 min, treated with the bacterial culture (Agrobaclerium radiobacter, 0.5 g) under N,, degassed once again, and then sparged with N,. The vessel was shaken for 24 h at 40 "C under N, at 6 bar and then for 24 h at 40 "C under normal pressure before cooling it and adjusting the pH of the solution to pH9. The solid portion of the culture was separated by centrifugation and the remaining liquid was treated with charcoal to clear the solution. Yield in solution 95%. Isolation was accomplished by filtering the solution through a strongly acidic ion exchanger (Merck, IR 120) and eluting the product with 5 % aqueous NH,. The solution thereby obtained was evaporated and the residue was dissolved in a small amount of water. Addition of ethanol precipitated the product. 3.4g (85%), m.p. 215-218 "C (dec.), [a]:': +22 (c = 2 in 1 N HCI) 'H NMR (250 MHz, D,O): 6 = 3.73 (m, 1 H, Ha), 3.1 (m. 2H, Hy); 1.4-2.0 (2 x m, each 2H, Hfi + H;?.Well over half a century ago, E. Hiickel proposed the (4n + 2)-electron rule for cyclic IT systems. Since then, this simple and fascinating concept"' has stimulated the syntheses of a great variety of novel molecules and molecular ions, which in lab jargon are frequently termed aromatic (Greek: aromatikos = herbal smelling) and which, according to numerous experimental data as well as quantum-chemical results, are distinguished by an energetically favorable electron distribution.['] Deviations from the optimum number of electrons, therefore, should inflict severe perturbations and, as is known f...

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