Carbocations alkenyl

Acid sites catalyse reactions by forming carbocations. Alkenyl cations have been seen by NMR and allylic ones by IR. Simple secondary or tertiary carbenium ions have yet to be observed. There are many questions here. For example, why is isobutane formed aiid why is there an induction period in n-hexane cracking ... 

Section 11 16 Addition reactions to alkenylbenzenes occur at the double bond of the alkenyl substituent and the regioselectivity of electrophilic addition is governed by carbocation formation at the benzylic carbon See Table 11 2... 

Alkenyl halides such as vinyl chloride (H2C=CHC1) do not form carbocations on treatment with aluminum chloride and so cannot be used m Friedel-Crafts reactions Thus the industrial preparation of styrene from benzene and ethylene does not involve vinyl chloride but proceeds by way of ethylbenzene... 

Alkenyl sulfoxides 177 and 178, which can be readily prepared from 1-alkynes222, provide synthones for the carbocations 179 and 180. These synthones are useful for the simple construction of cyclopentenones and also in providing an electrophilic precursor for the jS-side-chain on prostanoids223,224. 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

Organomercury reagents do not react with ketones or aldehydes but Lewis acids cause reaction with acyl chlorides.187 With alkenyl mercury compounds, the reaction probably proceeds by electrophilic attack on the double bond with the regiochemistry being directed by the stabilization of the (3-carbocation by the mercury.188... 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Because the carbocations derived from aryl and alkenyl halides are extremely high in energy. 

The last comprehensive review on electrophilic reactions of fluoroolefins was published in 1969 [6], Since then, several reviews and papers dealing with different aspects of this chemistry, such as alkylation and alkenylation reactions [7], addition of halogen fluorosulfates [8], trifluoromethanesulfonates [9] and halogen fluorides [10] to fluoroolefins have been published. Additional information on the reactions involving carbocations could be found in two recent review articles [11,12] some data on the subject are scattered in several books and journals [13-19]. 

The amination of 2-alkenylphenols occurred efficiently compared to 2-allylphenols and -naph-thols69. The mechanism involves a proton exchange equilibrium between the phenolic and amino functions and the photoinduced proton transfer (PPT) from the ammonium ion to the alkenyl group, followed by attack of the amine on the intermediate benzylic carbocation. No photoamination of O-methylated and O-acetylated phenols occurred at all. As a single example of diastereoselective amination, the amine 6 was produced from 5 with good yield and diastereoselectivity, although the configuration was not determined. 

The interaction of an acid with an alkenyl monomer can generate ionic chain carriers, but also covalent products with varying degrees of polarity. It has been shown that in certain systems these ester molecules can propagate the growth of a polymer chain, while in others they are inactive. Another source of covalent species in cationic polymerisation is the collapse (recombination) of the ionic pair or the X displacement from the anion to the carbocation discussed in the previous section. 

The stabilities of most other stable carbocations can also be attributed to resonance. Among these are the tropylium, cyclopropenium, " and other aromatic cations discussed in Chapter 2. Where resonance stability is completely lacking, as in the phenyl (CeH ) or vinyl cations, the ion, if formed at all, is usually very short lived. Neither vinyl nor phenyl cation has as yet been prepared as a stable species in solution. However, stable alkenyl carbocations have been generated on Zeolite Y. ... 

Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Let s compare the isopropyl cation with the iso-propenyl cation. In the latter, the central C has two cr bonds, one it bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp3)-H cr bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp3)-H cr bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp2)-H cr bonds on the left. Because C(sp2) orbitals are lower in energy than C(sp3) orbitals, the... 

C(sp2)-H cationic center. In fact, 2° alkenyl carbocations (R.2C=CR) are only about as stable as 1° alkyl carbocations, and 1° alkenyl carbocations (R2C=CH) are only about as stable as the CH3+ carbocation. 

Common error alert If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect. 

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their involvement in these additions has been questioned. For example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the corresponding additions to alkenes, the difference is not nearly as great as the difference in carbocation stabilities would suggest. 

The approximate relative energies of these structures are illustrated in Figure 3.18. For trivalent carbon, the preferred hybridization is for the positive charge to be located in an unhybridized p orbital. Similarly, the p/sp hybridization is preferred for alkenyl carbocations. These hybridizations place the charge in a less electronegative p... 

The SN1 pathway requires formation of a carbocation. We saw earlier (Section 13-9) that alkenyl halides are unreactive toward displacement reactions. The Ss-2 pathway is poor for the same reason we stated above for benzene compounds. In addition, alkenyl cations are high-energy species because the placement of a positive charge on an -hybridized carbon is very unfavorable. The same issue arises in the case of benzene The phenyl cation is a high-energy species and forms only with difficulty because it contains an s/r-hybridized. positively charged carbon atom. 

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