Ring compounds, structure four-membered

Prismane is an example of another interesting strained compound. Because it contains three-membered rings fused with four-membered rings, it should be even more strained than cubane. Prismane was prepared in 1973. It is a liquid that is stable at room temperature but explosive under some conditions. In toluene at 90°C its half-life (the time it takes for one-half of the compound to decompose) is 11 h. Note that prismane is isomeric with benzene. In fact, it was one of the structures proposed for benzene in the early days of organic chemistry. 

The structures of a number of compounds containing four-membered rings have been determined by X-ray or electron diffraction methods. The crystal structures of 1-aminocyclobutanecarboxylic acid hydrochloride, the bicyclo[4,2,0]octane derivatives, (7) and (8), and of [4,4,2]propella-3,8-diene-ll,12-dione (9) have been determined by X-ray diffraction. The structure of l,2,3,3-tetrachloro-4,5-dimethyl-spiro-2,3-hexa-1,4-diene (10) in the gas phase has been determined by electron diffraction. Other structures determined by these methods are cited in the body of the text. 

The structure of pyrimidine is shown below. It is a six-membered unsaturated ring compound containing four carbon and two nitrogen atoms. 

Table 1 X-Ray Structures of Compounds with Five-membered Rings and Two, Three or Four Heteroatoms ...

Structure of Small and Large Rings Table 2 Structures of Known Four-membered Heterocyclic Compounds... 

Bimetallic silylene-bridged complexes have been known for a long time and numerous articles related to this subject have appeared. Several compounds have been characterized, some of them also by x-ray structure analysis [165-171], For instance, the complex Mn2(CO)8(Si(C6H5)2)2 shows a distorted (MnSi)2 four-membered ring with a Mn-Mn bond [169], In the following section selected examples which have been described recently or are of particular interest in the present context will be discussed. 

Of particular interest with respect to structure and bonding are the four-membered silaplatinum ring compounds 26abc. 

In nickel and palladium dithiobenzoato complexes, four-membered chelate rings are formed (366), whereas, in the corresponding phenyl-dithio acetates [M2(S2CCH2Ph)4], the dithio ligands act as bridging groups between the two metal atoms, with the formation of binuclear units (367). The molecular structure of the latter compounds shows that each metal atom is coordinated to four sulfur atoms and to the other metal atoms in a square-pyramidal geometry. Other evidence for... 

Cycloadditions of nitrones, nitrile oxides or diazo compounds to thiete dioxides do not show the high stereoselectivity observed with acyclic vinyl sulfones, and mixtures of the two possible adducts are formed . The charge-transfer stabilization energy calculated according to the Klopman-Salem perturbational approach is able to account for the experimental trends of the isomer ratio in terms of the major stereochemical structural differences between the acyclic vinyl sulfones and the four-membered ring sulfones (see Section IV.B.3). 

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