Bridged Ring Systems, Cage Molecules

The systematic treatment of such structures is beyond the scope of the present review. The majority of examples selected belongs to bonding type 12, constructed of two or more three-membered rings and other structural elements. Propellanes are of the 11 type. A monograph on cage molecules has appeared recently308. 

The electron-density distribution of a bicyclo[1.1.0]butane (186) derivative (201) has been determined at 98 K by This molecule represents a transition from 186 to 

In related tricyclo[2.1.0.0 jpentane (202) systems substituted at C1, C5 and C3, the dihedral angle between the three-membered rings is about 95 to 99°. The bridge bond Cl—C5 shortens as the bond angles C5—Cl—R and Cl—C5—R to the R substituents open, following a linear correlation . Irngartinger and Lukas have also found a linear relationship with the fold angle of the bicyclobutane unit  

The structure of 203 has been redetermined by refinements of the XD data in the centrosymmetric orthorhombic space group Pnca. The molecule has C2 symmetry in the crystal. The side bonds in the bicyclobutane unit are longer. Cl—C2 1.540 (6) A, the bridge bond is shorter. Cl—C6 1.442 (5) A, and the fold angle of 110.7 ° (calculated from the coordinates) is narrower than in free 186. 

Parameters of the octabisvalene derivative 204 also fit the correlations for bicyclobutanes. The bridge bonds are 1.505 (3) and 1.501 (3) A, the dihedral angles 118.0 (1) and 

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In compound 91 the diborahexasiloxane ring system 89 can be identified, which has been expanded to a three-dimensional structure by an additional (R2Si0)20 fragment bridging now, in contrast to compound 90, two boron atoms [127], This cage can be obtained in yields of 45% from tetraphenyldisiloxanediol and boric acid when reacted in a 6 1 stoichiometry. The molecule contains a... 

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