Main Group Element Analogues of Carbenes, Olefins, and Small Rings

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Abstract
For a long time methylene (:CH2), ethylene (H2CCH2), and cyclopropane [(CH2)3] were considered as compounds whose higher main group homologues or analogues could not be synthesized. One consequence of the failure of earlier experiments was, for example, the “double bond rule” which was introduced over twenty years ago and is still often quoted. Although it does not dispute the existence of multiple bonding between elements, it states that it leads to very kinetically unstable species. This interpretation of the double bond rule has long since been surpassed due to improved experimental and quantum chemical methods which have allowed the synthesis of main group analogues of methylene, ethylene, and cyclopropane that are kinetically stable in solution and the solid state at room temperature. In the following review recent advances in the synthesis of these compounds will be referenced, and the concepts, which were developed by a systematic study, will be laid out. In doing so, a detailed description of the complex reactivity of the individual classes of compounds will be avoided in order to highlight the basic principles. These can then be used to approximately predict reactivity and characteristics of new compounds. There are significant differences between formally and even structurally analogous compounds with elements from the higher rows as compared to their “lighter” homologues. For example, while every chemist is familiar with the planar D2h symmetrical structure of H2CCH2 as the only “significant” isomer, it must be accepted that Sn2H4 has four isomers that are separated by very small energy differences. For cis/trans isomerism of olefins only the rotation of the CC double bond needs to be considered, but there are three possible mechanisms for the disilene derivatives R1RSiSiR1R. In the last century cyclopropane was considered to be not producible and this provided the impetus for Baeyer's strain theory. Investigations into small silicon cycles, which were discovered much later, show that this theory has more validity for these rings. As a result of the collected data, it is slowly becoming clearer that precisely the carbon compounds once seen as mere models are in reality the “exotic” compounds. Rules with broader applications can be developed by studying the compounds made up of main group elements.