SummaryIn this brief review, we have attempted to point up the importance of chemical equilibria in structural inorganic chemistry and to show how the kind and yield of various molecules may be treated quantitatively. Because of the importance of kinetic control in organic chemistry and because the chemistry of carbon compounds is by far the most advanced branch of general and descriptive chemistry, preparative inorganic chemists have often overemphasized the value of reactionmechanism theory to their studies and have thereby neglected the powerful quantitative approach of thermodynamics which may be used in equilibrated systems. The general theorems for treating in terms of small molecular segments the multitude of equilibria present in families of compounds have been surveyed superficially with the hope that the interested reader will turn to the original papers for details. In these days of high-speed computers, not only physical-inorganic chemists, but also those interested solely in preparative work can advantageously employ the theoretical ideas discussed above. Future Developments a) T h e o r yAtomic-interaction theories are being developed for correlation and prediction of the deviations from randomness in the equilibria involving chains. Work is also under way on the theoretical basis of ring-chain equilibria and the time needed to reach equilibrium in various systems. The concept of order of environment within molecules can be extended to the interpretation of what might be called atomically additive properties, such as density, refractive index: or bulk magnetic susceptibility. In addition, we are studying several examples of the effect of solution nonideality on deviations from randomness in equilibria. b) AnalysisNew and improved analytical techniques for assaying molecules in mixtures are surely to be expected in a reasonably near future. In particular, nuclear magnetic resonance spectrometers of very high resolution will become available with improvements in field-strength uniformity of superconducting magnets. These should allow one to see molecules in considerably greater depth than is possible today, as well as to extend practical high-resolution NMR to samples having the natural abundance of chemically interesting nuclei such as 13C, 15N, 29Si, 77Se, 111Cd or 113Cd, 119Sn, 125Te, 195Pt, 199Hg, and 207Pb, all of which have spin 112. Moreover, with very high field strengths, it will be possible to measure much more rapid exchange rates and to observe uncoalesced resonances under conditions where either exchange or quadrupole broadening now makes the spectra difficult and sometimes impossible to interpret. Another way of increasing the range of elements that can be studied by NMR is to use isotopic enrichment of those magnetically suitable nuclei which are naturally present in low abundance (e.g. 29Si).

Vor einigen Jahren wurde von Wittig [I] darauf hingewiesen, daJ sich at-Komplexe charakteristisch durch eine anionische Lockerung der am Zentralatom haftenden Substituenten auszeichnen. Diese Koordinationsverbindungen stehen somit in ihrem Verhalten den vonMeerwein untersuchten onium-Komplexen [21 mit kationischer Lockerung der Liganden gegeniiber, erganzen aber zu einem Gesamdbild der organischen Komplexcheniie 151.

Das intermediare Auftreten von 4.5-Dehydro-2.3 ; 6.7-dibenzo-cycloheptatrienon (11) wird anhand kinetischer und praparativer Untersuchungen diskutiert. 11 l5Rt sich mit Dienen und nucleophilen Partnern in guten Ausbeuten abfangen. C = C-Dreifachbindungen in Siebenringen beanspruchen besonderes Interesse, da alle Versuche zur Isolierung von Cycbheptin bislang fehlschlugen, wiihrend das bereits merklich gespannte Cyclooctinz) noch in Substanz erhalten werden konnte. Wenn auch das siebengliedrige Cycloalkin als solches noch nicht gefaat wurde, so dad auf Grund der Arbeiten von G. WIITIG doch als gesichert angesehen werden, daB Cycloheptin3) und neuerdings auch Benzocycloheptenin4) sowohl bei der Oxydation des entsprechenden 1 .ZBis-hydrazons als auch bei der Enthalogenierung des 1.2-Dibrom-cycloheptens als kurzlebige Zwischenstufen auftreten.In diesem Zusammenhang interessieren ebenfalls die Arbeiten japanischer Autoren, die bei der Einwirkung von Basen auf 3-Halogen-tropolone ,,cine"-Substitutionen~) nachwiesen und j.q-Dehydro-rropolon6) diskutierten, wobei ein Vergleich mit Dehydrobenzol besonders reizvoll erscheint. Aus der uns zuganglichen Literatur geht jedoch nicht eindeutig hervor, inwieweit andere, in diesem besonderen Fall denkbare Mechanismen mit Sicherheit ausgeschlossen werden konnten7). Einfache Halogentropone durften dagegen wegen ihrer leichten Isomerisierbarkeit zu Benzoedurederivaten fur entsprechende Umsetzungen wenig geeignet sein7).Die vorliegende Arbeit, die sich mit dem Nachweis des 4.5-Dehydro-2.3; 6.7-dibenzo-cyclohepfafrienons (It) befafit, ergab sich ursprunglich aus einem synthetischen Problem, das beim 1)

The readily available and highly reactive title compounds represent versatile building blocks in organic synthesis. Recent examples of 1,3‐dipolar cycloadditions and Diels–Alder reactions and their applications are described. This microreview is especially devoted to cycloadducts formed between 1 and 2 and furans, and their transformations to give a variety of different classes of compounds, including derivatives of natural products. Here the conversion of oxanorbornadienes into bridged oxepines plays an important role in many cases. Different routes to enantiomerically pure compounds are also presented. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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