Bridgehead positions carbocations

The hydrolytic stability of the [Co(diCLsar)]3+ cation toward conversion to the [Co(diHOsar)]3+ dihydroxysarcophaginate is due to the difficulty of forming a carbocation at a planar bridgehead position. It was noted that the nitrosation of [Co(diAMsar)]3+ sarcophaginate occurring without rearrangement involves compe-... 

Further evidence for the S l mechanism is that reactions run under S l conditions fail or proceed very slowly at the bridgehead positions of [2.2.1] (norbornyl) systems" (e.g., 1-chloroapocamphane, 8). If S l reactions require carbocations... 

The preferred structure of a carbocation is planar, and the nature of the bridgehead position is such that any carbocation formed at the bridgehead is unable to become planar. This raises the question of how easy is it to form a bridgehead carbocation The role of kinetics is significant... 

With some structures, the geometry imposed by a molecular skeleton leads to greater steric strain in the carbocation than in the reactant. This effect is particularly notable in 1-substituted bridgehead systems in which the molecular framework forces the carbocation to be nonplanar. In 1, for example, the carbocation is decidedly nonplanar, with C—C—C bond angles at the carbocation center calculated to be 110°. The strain associated with formation of this carbocation means that 4-tricyclyl derivatives are very resistant to solvolysis. The relative reactivities of several structures substituted at the bridgehead position (Figure 8.3) confirm that solvolytic reactivity decreases as the size of the bridges decreases. ... 

Valence tautomerism of the bullvalene skeleton appears to be implicated in the rearrangement processes that occur during the formation of potential bullvalenyl-carbinyl carbocations. " Thus, for example, solvolysis of the esters [(154) and (155)] in aqueous acetone affords the alcohol (156) as the exclusive product. This result is consistent with the mode of attack pictured in (154), or the equivalent alternative formulation which places the carbinyl ester function at the cyclopropane bridgehead position [i.e. the valence tautomer of (154)]. In the case of the positionally isomeric... 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

The nitration occurred with ease and high positional selectivity at the bridgehead to give 1-nitroadamantane as the major product (Equation 5.56). The best result was obtained at -78 °C in the presence of a large excess of NO, when tertiary/secondary positional selectivity was as high as 100. Skeletal rearrangement was not observed. Formation of nitrates (4-5%) in Equations (5.58) and (5.59) may be attributed to the oxidation of the initially formed adamantyl radicals to carbocations, followed by coupling with the nitrate ion. 

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