William von Eggers Doering’s Many Research Achievements during the First 65 Years of his Career in Chemistry

Abstract

This Account highlights William von Eggers Doering's important discoveries in many fields of chemistry. His synthetic and mechanistic studies have contributed to areas including non-benzenoid aromatics, carbenes, pericyclic reactions, and diradical intermediates. Doering's synthesis with L. H. Knox of the highly stable tropylium ion and their investigation of its reactivity were the starting point for the development of the field of non-benzenoid aromatics. Working with A. K. Hoffmann, Doering demonstrated the synthesis of dichloro- and dibromocarbene by base-induced alpha elimination of HCl or HBr from CHCl3 or CHBr3 under anhydrous conditions. These results allowed for the synthesis of a variety of cyclopropanes and derivatives including allenes. Using 14C labeling experiments, Doering and Prinzbach showed that the mechanism of insertion of singlet methylene into a C-H bond was a concerted process.In their work on the Cope rearrangement, Doering and Roth's outstanding stereochemical analysis showed that the rearrangement of acyclic 1,5-hexadienes proceeds concertedly, passing over a chairlike transition state. This work has had an enormous impact on the understanding of stereochemical control in synthetic organic chemistry, and many fruitful applications in synthesis have stemmed directly from this finding. Transition-state resonance structures analogous to those for ground-state aromatics can qualitatively explain the relatively large substituent effects on the rate of the Cope rearrangement. However, quantum chemical calculations have quantitatively described these effects. The rapid degenerate Cope rearrangements in the cis-divinylcyclopropane units of 3,4-homotropilidene, barbaralone, and bullvalene establish these molecules as having fluxional structures. The unique molecule bullvalene has more than 1.2 million possible structures interconnected by degenerate Cope rearrangements, which average all H and all C atoms. Doering has also examined stepwise thermal reorganizations that pass through intermediary 1,3- or 1,4-diradicals and do not show conformational equilibration as would be expected for classical intermediates. Doering calls these processes "not-obviously concerted". He discusses "continuous diradicals" as transition states and rationalizes the course of these reactions through the concept of a "caldera" (a flat surface with small energy wells as found on the top of volcanoes).The understanding of fundamental chemical reactions remains the focus of Doering's research. In his terms, "understanding" means not only gaining deep insight but also the intellectual control that allows researchers to predict a reaction's course. Because the interplay between theory and experiment has led to great progress in this predictive ability, Doering's experimental work has provided an important input for computational chemistry and to the essential understanding of chemical reactions.

Recent applications of the divinylcyclopropane–cycloheptadiene rearrangement in organic synthesis

“…The first rotation of a vinyl-moiety into the endo -orientation requires 0.8 kcal/mol giving 9xn , the endo -orientation of both vinyl-moities requires 2.9 kcal/mol represented as 9nn [17–19]. The necessary energy for the only possible transition state 9nn' results in 19.7 kcal/mol, which is in good agreement with the experimental values of von Doering [20–21]. The rearranged cycloheptadiene 10 is favoured by −20.1 kcal/mol compared to the corresponding cis -divinylcyclopropane ( 9 ).…”

The Niecke Biradicals and Their Congeners – The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties

“…Accordingly, for A (Figure ) the transition state (TS conc ) for the Cope reaction emerges from a concerted process, the shifting electrons are delocalized over all carbon atoms. However, energetically close by is a 1,4‐biradical, 9.9 kcal higher in energy . The findings on the Cope reaction can easily applied to analysis of bonding in the 1,3‐diphospha‐cyclobutane‐1,3‐diyl system, as illustrated in B .…”

Doppelrotoren mit fluktuierenden Achsen: Domino‐Rotation und Katalyse der Azid‐Alkin‐Huisgen‐Cycloaddition

“…Während fluktuierende Moleküle mit Bindungsspaltung/‐bildung eine lange Geschichte aufweisen, ist eine ähnliche Dynamik innerhalb multipler entarteter Strukturen in supramolekularen Mehrkomponentengeräten selten . Erstmals werden hier Mehrkomponentenrotoren mit fluktuierenden Achsen ausgestattet, die eine Domino‐Rotation ermöglichen.…”