C2-P bond analogs

Carbon-7 is located on a plane that bisects the 2,3-bond and is in fact homoallylic to both sides of the double bond. Therefore a developing p orbital on it is in a position to overlap equally with each of the p orbitals of the double bond as is shown in Figure 6.7. No attack of solvent at C2 or C3 occurs in this system to give a three-membered ring analogous to t-cholesteryl derivatives and to 49 because the resulting carbon skeleton would be too strained. The fact that the product is 100 percent awh -acetate is a result of the hack side of Ct being hindered bv the three-center bond that is fully formed in the intermediate carbonium ion. 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

Some of these trends are exemplified by the selection of molecules and complex ions in Table 1. They have been classified by (i) the total number of valence electrons (VE), and (ii) the steric number of the central atom (SN), which is calculated by adding the number of lone-pairs to the number of bonded atoms and used for interpreting molecular geometries in the VSEPR model (see Topic C2). The species listed in Table 1 illustrate the wide variety of isoelectronic relationships that exist between the compounds formed by elements in different groups and periods. Species with SN=4 are found throughout the p block, but ones with lower steric numbers and/or multiple bonding are common only in period 2. In analogous compounds with heavier elements the coordination and... 

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