Carbinyl Carbocation

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... 

A chiral homoallyl carbocation/cyclopropyl carbinyl carbocation system has been used to prepare ehiral derivatives of the marine natural product, spiniferin-1. Chiral acetals have been used to prepare optically active diterpenoids in biomimetic cationic cyclizations. ... 

The key features of Pirrung s synthesis of isocomene are outlined retrosynthetically in Scheme 1. Working one step back from 1 gives the tertiary carbocation intermediate 2. The intermediacy of 2 should be brief, for it should readily participate in an Ei-type reaction, in the forward sense, to give isocomene (1). Inspired by the observation that bicyclo[3.3.0] frameworks can be accessed from bicyclo[4.2.0] frameworks through cyclobutyl carbinyl cation rear-... 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

Although radicals are not nearly so prone to rearrangement as are, for example, carbocations, there are a few such rearrangements which have become identified as characteristic of carbon radicals. These include radical cyclizations, particularly the 5-hexenyl radical cyclization, and radical C-C bond cleavages, particularly the cyclopropylcarbinyl to allyl carbinyl radical rearrangement. In hydrocarbon systems, as organic synthetic chemists have learned how to control rapid chain processes, such rearrangements have become important synthetic tools [176-179]. 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

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