Carbocation rearrangements predicting

ThomsonNOW Click Organic Interactive to use a web-based palette to predict products from simple carbocation rearrangements. 

Clearly, we must be able to predict when to expect a carbocation rearrangement. There are two common ways for a carbocation to rearrange either through a hydride shift or through a methyl shift. Your textbook will have examples of each. Carbocation rearrangements are possible for any reaction that involves an intermediate carbocation (not just for addition of HX across an alkene). Later in this chapter, we will see other addition reactions that also proceed through carbocation intermediates. In those cases, you will be expected to know that there will be a possibility for carbocation rearrangements. 

Figure 11.8. Isotope effects (162.5 l<) on a carbocation rearrangement studied by Saunders and Cline [64]. The rule of the geometric mean predicts that the three-isotope equilibrium isotope effect should be the cube of the single-isotopic-site effect (1.1 778) = 1.6338.

The addition of a proton to the alkene forms a secondary alkyl carbocation. A carbocation rearrangement occurs because a 1,2-hydride shift leads to a more stable secondary benzylic cation (Section 4.6). It is electron delocalization that causes the benzylic secondary cation to be more stable than the initially formed secondary carbocation. Had we neglected electron delocalization, we would not have anticipated the carbocation rearrangement, and we would not have correcdy predicted the product of the reaction. 

Because the reaction of a secondary or a tertiary alcohol with a hydrogen halide is an SnI reaction, a carbocation is formed as an intermediate. Therefore, we must check for the possibility of a carbocation rearrangement when predicting the product of the substitution reaction. Remember that a carbocation rearrangement will occur if it leads to formation of a more stable carbocation (Section 4.6). For example, the major product of the reaction of 3-methyl-2-butanol with HBr is 2-bromo-2-methylbutane, because a 1,2-hydride shift converts the initially formed secondary carbocation into a more stable tertiary carbocation. 

Early attempts to verify the stereochemical predictions of orbital symmetry control were hampered by carbocation rearrangement reactions/ such as Wagner-Meerwin shifts, although the very presence of these anomalous pathways is consistent with a cationic pathway. It is now well established that the Nazarov cyclization occurs via a pentadienyl cation 10,... 

At the end of this chapter, we will learn to predict when rearrangements are likely to occur. For now, we will just focus on recognizing carbocation rearrangements. There are two common ways in which carbocation rearrangements are accomplished via either a hydride shift or a methyl shift. A hydride shift involves the migration of H ... 

Throughout this course, we will encounter many examples of carbocation rearrangements, so we must be able to predict when these rearrangements will occur. Recall that the two common types of carbocation rearrangement are hydride shifts and methyl shifts (Figure 6.32). 

In both cases, a secondary carbocation is converted into a more stable, tertiary carbocation. Stabihty is the key. In order to predict when a carbocation rearrangement will occur, we must determine whether the carbocation can become more stable via a rearrangement. For example, consider the following carbocation ... 

In Section 6.11, we discussed the stability of carbocations and their ability to rearrange via either a methyl shift or a hydride shift. The problems in that section focused on predicting when and how carbocations rearrange. That skill will be essential now, because the mechanism for HX addition involves formation of an intermediate carbocation. Therefore, HX additions are subject to carbocation rearrangements. Consider the following example, in which, the it bond is protonated to generate the more stable, secondary carbocation, rather than the less stable, primary carbocation ... 

Predict whether the following carbocation will rearrange, and if so, draw the curved arrow showing the carbocation rearrangement ... 

Less stable carbocations rearrange to more stable ones through hydride (H ) shifts. Stay alert—from now on, every time you see a carbocation, you have to consider whether it will rearrange into a lower-energy carbocation. Once again, you have to be able to judge and predict stability. 

WORKED EXAMPLE 9.2 Predicting the Product of a Carbocation Rearrangement... 

The extent of carbocation rearrangement is difficult to predict It depends on the alkene structure, the solvent, the strength and concentration of the nucleophile, and the temperature. In general, rearrangements are favored under strongly acidic, nucleophile-deficient conditions. 

The pinacol rearrangement is frequently observed when geminal diols react with acid. The stmcture of the products from unsymmetrical diols can be predicted on the basis of ease of carbocation formation. For example, l,l-diphenyl-2-metltyl-l,2-propanediol rearranges to... 

Carbocations, as we learned in Chapter 4 of Part A, can readily rearrange to more stable isomers. To be useful in synthesis, such reactions must be controlled and predictable. This goal can be achieved on the basis of substituent effects and stereoelectronic factors. Among the most important rearrangements in synthesis are those directed by oxygen substituents, which can provide predictable outcomes on the basis of electronic and stereoelectronic factors. 

Rearrangements are an unexpected complication, and it is sometimes difficult to predict when they might occur. We need to look carefully at the structure of any proposed carbocation intermediate and consider whether any such rearrangements are probable. In most cases we shall only need to rationalize such transformations, and will not be trying to predict their possible occurrence. 

There are inan common fragmentation processes which can help to explain and predict ion fragments found in El spectra. In general, these processes are promoted by the stability of the carbocation fragments produced and by six-membered transition stales in the rearrangements of ions. 

We summarize here a procedure to predict the feasibility and the stereochemistry of thermally concerted reactions involving cyclic transition states. The 1,2 rearrangement of carbocations will be used to illustrate the approach. This is a very important reaction of carbocations which we have discussed in other chapters. We use it here as an example to illustrate how qualitative MO theory can give insight into how and why reactions occur ... 

Solved Problem 11-3 shows how these rules are used to predict the products of dehydrations. The carbocations are drawn to show how rearrangements occur and how more than one product may result. 

Draw the carbocation, look for. possible rearrangements, then consider all the ways that the original carbocation and any I rearranged carbocation might lose protons to give alkenes. Zaitsev s rule usually predicts the major product. 

In another approach Borodkin et al. (1976c) established linear relations between the rates (log k) of two different migrating groups (CHj and H) in several carbocations with different core structure. Interestingly, the rates of such different reactions as 1,2-shifts in simple alkyl carbocations and sigma-tropic rearrangements of cyclopentadienes could be satisfactorily predicted by these relations. 

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