Allyl cation resonance-stabilized formation

How can we account for the formation of 1,4-addition products The answer is that allylic carbocations are involved as intermediates (recall that allylic means "next to a double bond"). When 1,3-butadiene reacts with an electrophile such as H+, two carbocation intermediates are possible a primary nonal-lylic carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 11.5), it is more stable and forms faster than a nonallylic carbocation. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Although both Sn2 and SnI mechanisms might be formnlated for such reactions, all the available evidence favours an Sn 1 process. This is rationalized in terms of formation of a favourable resonance-stabilized allylic cation by loss of the leaving gronp. In the majority of natnral prodnct structures, the nucleophile has attacked the allylic system on the same carbon that loses the diphosphate, bnt there are certainly examples of nncleophilic attack on the alternative tertiary carbon. 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

The key to formation of these two products is the presence of a double bond in position to form a stabilized allylic cation. Molecules having such double bonds are likely to react via resonance-stabilized intermediates. 

With the positive charge on the more stable secondary carbon, carbocation 25 is an allylic cation. The presence of the conjugating C=C unit leads to formation of a resonance-stabilized cation intermediate. Because... 

For the primary carbocation in the reaction shown in Figure 9.10, the positive charge is localized at the end of the molecule and has no resonance stabilization. Protonation at the end carbon gives an ion (the allylic cation) that can be represented by more than one resonance structure. The choice is easy once we have looked carefully at the structures. Formation of the lower energy delocalized allylic cation will be favored over formation of the higher energy localized primary cation (Fig. 9.13). 

We saw this reaction briefly in the discussion of resonance in Chapter 9 (p. 369). The source of the two pathways becomes clear as we work through the mechanism step by step. The first reaction must be protonation of butadiene by hydrogen chloride (Fig. 12.36). There is a choice of two cations, one of which is a resonance-stabilized allylic cation, and the other an unstabilized primary carbocation. The stabilized cation is much lower in energy and its formation is greatly preferred. 

FIGURE 12.36 Formation of a resonance-stabilized allylic cation is preferred. 

FIGURE 12.42 The 1,2- and 1,4-addition products equilibrate through the formation of a resonance-stabilized allylic cation. 

The fragmentation patterns of alkenes also reflect a tendency to break weaker bonds and to form more stable cationic species. The bonds one atom removed from the alkene function—the so-called allylic bonds— are relatively easily broken, because the result is a resonance-stabilized carbocation. For example, the mass spectra of terminal straight-chain alkenes sudi as 1-butene reveal formation of the 2-propenyl (allyl) cation, at m/z = 41, the base peak in the spectrum (Figure 11-29 A). 

Branched and internal alkenes fragment similarly at allylic bonds. Figure 11-29B shows the mass spectrum of 2-hexene, in which the base peak at m/z = 55 corresponds to formation of the resonance-stabilized 2-butenyl cation. 

Suitable salts were those of perchloric acid or, preferably, of hexafluorophosphoric acid." " Experiments carried out with bromine-terminated polystyrene or polybutadiene gave a rapid precipitation of silver bromide even at low temperatures, due to the resonance stabilization of the benzylic or the allylic carbenium ions thus generated. The efficiency of block copolymer formation was found to be determined by factors such as the structure of the carbenium ion, the nature of the gegenion, the experimental conditions and, of course, the nature of the second monomer. Under carefully chosen conditions, THF could be polymerized to form living cationic block copolymers... 

Our first step must be to react bromine with the diene to form the allyl cation (because the allyl cation is very well stabilized by resonance, the formation of bromonium ions is not favored). As usual, we must consider both resonance forms, and the tertiary allyl cation will contribute more to the overall structure. 

Substituents at the 3-position, if capable of stabilizing an adjacent positive charge by resonance and/or inductive effects, direct strongly to the adjacent (i.e. 2) position (equation 3). Stabilization of the allylic carbonium ion intermediate is obviously involved. This effect can be quite pronounced for R = Ph in equation (3) the ratio of 2- to 5-bromo derivatives for electrophilic bromination is ca. 660 (68JOC2902). Even more striking is the exclusive formation of the 2,2 -dibrominated product (19) from 3,3 -dithienyl (equation 4) (69JOC343). As expected, substituents not capable of stabilizing the cationic intermediate direct substitution to the 5-position. 

Step 1 Make a new bond between a nucleophile (it bond) and an electrophile—add a proton. Electrophilic addition is initiated by the reaction of a terminal carbon of one of the double bonds with HBr to give an allylic carbocation intermediate (Section 9.3B), which can best be represented as a resonance hybrid of two contributing structures. Formation of this stabilized cation is the rate-determining step. 

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