Carbocations, -sigmatropic rearrangement

The rearrangements of carbocations that were first encountered in Chapter 8 can be classified as [ 1,2] sigmatropic rearrangements ... 

Although this carbocation is tertiary, it rearranges to a cation that is even more stable by migration of a methyl group by an allowed [1,2] sigmatropic rearrangement... 

When we come to use the Woodward-Hoffmann rules on these [2,3]-sigmatropic rearrangements, we find something new. We have a K bond and a o bond and a carbanion. How are we to represent a carbanion (or a carbocation) that is just a p orbital on an atom The new symbol we use for a simple p orbital is to. A carbanion is an component and a carbocation is an m0 component as it has zero electrons. If the two new bonds are formed to the same lobe of the p orbital of the carbanion, we have an m2s component but, if they are formed to different lobes, we have an m2a component. 

Sigmatropic rearrangements in a carbocation are very common processes. However, there are stereospecific hydrogen or methyl shifts in ben-zenonium cations that may be regarded as [1,2] or [1,6] shifts (Scheme 3.38). 

N-Acyliminium ions are hot and will even add to unactivated olefins. For example, N, 0-acetal 20 reacts with formic acid to provide 23. The stereochemistry of the process is relatively clean and can be rationalized by an twft -periplanar addition of electrophilic carbon and nucleophilic oxygen across the carbon-carbon double bond with a transition state that resembles a chair N-acylpiperidine. In reality, this process is mechanistically more complex than this simple model (for example, possible carbocation intermediates and in some cases, some underlying sigmatropic rearrangements), but the model is simple, easy to remember, and has excellent predictive value. How was this chemistry used to prepare perhydrohistrionicotoxin ... 

Electrocyclic reactions. Sigmatropic rearrangements. Reactions of carbocations. 

The mechanism for this reaction, shown in Figure 22.5, involves a carbocation rearrangement that occurs by an allowed [1,2] sigmatropic shift. The product of this rearrangement, a protonated ketone, is considerably more stable than the initial carbocation, so the migration is quite favorable. Another example of the pinacol rearrangement is provided in the following equation ... 

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

In a sigmatropic reaction, movement of a cr bond takes place, producing rearrangement (tropic is from the Greek word tropos, to turn). Common types of sigmatropic reactions are the familiar 1,2-hydride or alkyl shifts in carbocations and the Cope rearrangement. 

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