Propellane type carbon

Fig. 2.1 Planar- (a), pyramidal- (b) and propellane-type (c) carbon atoms...

The carbon atoms not at the ends of the stack have a half-planar or bntterfly-type disposition of their fonr bonds (Fig. 12.2). This makes them members of the unconventional non-tetrahedral assortment of tetracoordinate carbons to which also belong planar, pyramidal, and propellane (umbrella) carbon (Fig. 12.2 references to these carbon stereochemistries and to half-planar carbon in organometallic compounds, are given in [1]). Thns they excite the same kind of curiosity as that engendered by the planar carbon of Chapter 1 and the pyramidal carbon of Chapter 2. 

One of the most interesting types of polycyclic carbon compounds prepared in recent years is the group of tricyclic substances known as propellanes. A typical example is tricyclo[3.2.2.0 -5]nonane, which sometimes is called [3.2.2]propellane, 12. The physical properties of several of these are included in Table 12-6. A quick look at formula 12 probably does not suggest any great structural difference from the bicyclic compounds we have discussed previously. However, if one tries to construct a ball-and-stick model of 12, one soon concludes that the propellanes are truly extraordinary substances in that all four carbon bonds at the bridgehead carbons extend, not to the comers of a tetrahedron, or even a distorted tetrahedron as for a cyclopropane ring, but... 

Ring expansion reactions of the type shown in equations (17) and (18), in which a more equitable distribution of carbon atoms between two rings of a polycyclic hydrocarbon is produced, provide the link between the decyclization reactions above and the annulation reactions considered later. It would seem that the greater the strain in the reactant, the larger the number of potential catalysts. The complexes of seven different metals catalyze the reaction shown in equation (17). Of the possible catalysts, only the silver (I) salts bring about any byproduct formation.The isomerization of [l.l.l]propellane (equation 18) is catalyzed by [RhCl(PPh3)3]. ... 

As stated earlier, in spite of considerable interest in hydrocarbons possessing inverted carbon atoms, only eight systems (16, 8, 17a, 17b, 4-6, and 18) of this type are known. With the exception of bicyclobutane (16) they are all small-ring propellanes possessing symmetrical pairs of inverted atoms. In the course of the MM study of [k./.m]propellanes (17), with k, /, m equal to 2-4 (55), four questions have arisen ... 

CHAiUNCt The unusual molecule [2.2.2]propellane is pictured below. On the basis of the given structural parameters, what hybridization scheme best describes the carbons marked by asterisks (Make a model to help you visualize its shape.) What types of orbitals are used in the bond between them Would you expect this bond to be stronger or weaker than an ordinary carbon-carbon single bond (which is usually 1.54 A long) ... 

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