7-Membered rings three endocyclic double bonds

This similarity between five- and six-membered cyclic hydrocarbons is not surprising, but it is not any longer observed for the three-membered ring analogs, probably because a considerable ring strain introduced by short endocyclic double bonds. 

In all circumstances, the a-carbon is shielded, and the attack of CHjO" is regarded as unlikely. It should be emphasized that the so-called onium mechanism (Delavarenne and Viehe, 1969) involving the peculiar three-membered ring species [104], containing an endocyclic double bond and oxonium and carbanion centres, appears to be unlikely in the present case. 

In this section we compare the enthalpy of formation of isomeric pairs of cyclic olefins, one with an exocyclic double bond and the other with the double bond endocyclic. We start with 1-methylcycloalkenes and related methylenecycloalkanes, species genetically 18a and 18b, respectively. From prior experience with cyclopropanes and the thermochemical consequences of replacement of sp3 carbons by sp2 carbons in three-membered rings52, we expect 1-methylcyclopropene to be considerably less stable than methylenecyclopropane because the former compound has two trigonal carbons within the ring while the latter has but one. And so it is found56 the former has a gas-phase enthalpy of formation 43.1 2.2 kJ mol-1 more positive than the latter. 

Molecules with endocyclic Si=Si double bonds, like the tetrasilabutadiene 139, have also only recently become accessible and their chemistry has not yet been reviewed. In the less than four years since 1996 two three-membered, two four-membered and three five-membered ring compounds containing an Si=Si double bond have been prepared and, in most cases, their structures have been characterized by X-ray crystallography. Although two synthetic routes dominate in the synthesis of acyclic disilenes, the cyclic disilenes have mostly been prepared by the special methods summarized below. 

Summary The synthesis, structural characteristics, and physical properties of unsaturated three-membered rings containing heavier group 14 elements with endocyclic metal-metal double bonds, including cyclotrigermenes, cyclotrisilenes, cyclotristannene, and disilagermirenes, is summarized. 

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