Electron-withdrawing groups carbocation reactivity

Extensive studies by Gorman and Gassman have shown that an allyl cation can be a 27r-electron component in a normal electron-demand cationic Diels-Alder reaction and, since a carbocation is a very strong electron-withdrawing group, the allyl cation is a highly reactive dienophile [19a, 21]. 

Ketones and aldehydes can also be converted to enol ethers if, after the loss of H2O, the carbocation fragments with loss of H+ to give the alkene. Because enol ethers are extremely reactive toward H+, they are usually isolated only when the double bond is conjugated to an electron-withdrawing group, as in the conversion of /3-diketones to vinylogous esters. 

An electron-withdrawing group attached to the ring decreases the electron availability at the ortho and para positions (making the meta position the least electron poor), decreases reactivity, and destabilizes the carbocation formed upon attack at the ortho and para positions, so meta attack is preferred. 

Alkenes that are directly attached to lone-pair-bearing heteroatoms, especially N and O, are particularly reactive toward electrophiles. A resonance structure that shows a formal negative charge on the /3 C can be drawn, and the intermediate carbocation is especially well stabilized. Conversely, alkenes substituted with electron-withdrawing groups are less reactive toward electrophiles. 

Both the 9-methyl and 9-trifluoromethyl-9-fluorenyl cations (7 and 8) have been generated by laser flash photolysis of corresponding alcohol precursors. The product ions were then studied by time-resolved spectroscopy. Consistent with previous studies related to carbocation-bearing electron-withdrawing groups, ion (8) exhibits a significant bathochromic shift in the UV absorption compared to (7). The ions were quenched with methanol and reaction rates showed cation (8) to be a more reactive species. 

One model for bonding in a diazo compound would be die ylide (89 equation 37)." Unlike many other ylides, diazoalkanes are stable to air and water. With acid, however, protonation can lead to the highly reactive salt (90), the functional equivalent of the corresponding carbocation. As the substituents on the diazo group are made increasingly electron withdrawing, the ylide becomes less basic, and thus more stable to acid. Reaction of a diazo compound with a transition metal can also often be understood as proceeding via initial donation of electron density by (89) to a coordinatively unsaturated metal center. 

We mentioned above that adjacent n systems increase the rate of the 5 2 reaction by stabilizing the transition state, and likewise increase the rate of S f reactions by stabUlzing the carbo-cation. The effect on the 5 2 reaction applies to both C=C (electron-rich) and C=0 (electron-deficient) n systems, but only C=C n systems increase the rate of 5, 1 reactions. Adjacent C=0 groups in fact significantly decrease the reactivity of afkyl halides towards S(jl reactions because the electron-withdrawing effect of the carbonyl group greatly destabilizes the carbocation. 

The electron-withdrawing nitrogen atom makes the intermediate obtained from electrophilic aromatic substitution of pyridine less stable than the carbocation intermediate obtained from electrophilic aromatic substitution of benzene. Pyridine, therefore, is less reactive than benzene. Indeed, it is even less reactive than nitrobenzene. (Recall from Section 19.14 that an electron-withdrawing nitro group strongly deactivates a benzene ring toward electrophilic aromatic substitution.)... 

The electron-withdrawing NO2 group will destabilize the carbocation so the benzylic halide will be less reactive, while the electron-donating OCH3 group will stabilize the carbocation, so the benzylic halide will be more reactive. 

The monosubstituted alkenes 2, 3 and 5 are aU less reactive than 1 because their corresponding mercurinium ions involve resonance structures with a secondary carbocation, thus resulting in higher energy than the mercurinium ion obtained from compoimd 1. Alkene 3 reacts slower than 2 due to electron-withdrawal from the -OMe group, which would destabilize the mercurinium ion by further reducing the electron density of the resulting secondary carbocation resonance contributor. A similar inductive effect would also destabilize the mercurinium ion from alkene 5, but an... 

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