Dienes allylic cation formation

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

The products obtained from addition to conjugated dienes arc always consistent with the formation of the most stable intermediate carbonium ion an allyl cation. This requires the first step to be addition to one oj the ends of the conjugated system. 

These reactions formally involve addition of HX to the n-bond and ring opening of an intermediate cyclopropyl cation to an allyl cation, although the details of the mechanism are unclear and may, at least in some cases, involve direct attack at the cr-bond. In some cases, the intermediate allylic cation may be trapped intramolecularly to produce methylenefurans or dihy-dropyrans [see Sections 3.2.3.4., 3.3.2.5. and l.B.2.4.1.3. (E17c, p2313fi)]. In other cases, dienes are produced, e.g. formation of 6. ... 

Cycloaddition of allyl cations to conjugated dienes provides a route to seven-membered carbocycles. One of several methods can be used to generate the allyl cation, such as from an allyl halide and silver trifluoroacetate, or from an allyl alcohol by way of its trifluoroacetate or sulfonate. Cycloaddition of the allyl cation proceeds best with a cyclic diene, particularly for intermolecular reactions. Thus, cyclohexadiene and methylallyl cation gave the bicyclo[3.2.2]nonadiene 187 (3.125). Many intramolecular examples are known, such as the formation of the... 

Oxythallation of optically active cyclonona-1,2-diene has been studied and compared with acetoxymercuration and acetoxyplumbation. The formation of oxythallation adducts was shown to be irreversible and to proceed in an nnti-fashion a planar allylic cation was not the sole product precursor during this addition step. A proposed mechanism involves formation of a thallinium ion (166) which may be in equilibrium with the planar allylic cation (167). ... 

Dienes react with HX (HCl, HBr, HI) to give 1,2-addition and 1,4-addition products via formation of an allylic cation. The major product is determined by the temperature of the reaction, where low temperature favors 1,2-addition and high temperature favors 1,4-addition. Halogens such as CI2, Br2, and I2 also add to dienes to give a mixture of 1,2-and 1,4-addition products. Both HX and X2 react with alkenes with the C=C unit conjugated to a benzene ring in essentially the same manner as with alkenes that do not contain a benzene ring. 

Under thermodynamic (relatively high energy) conditions, the more stable product is the major compound formed in addition reactions with conjugated dienes. Under kinetic conditions, the major product comes from 1,2-addition—even if it is the less stable product. Why Proximity In the formation of the allyl cation, the nucleophile is born closer to the 2-position than to the 4-position. 

How can we account for the formation of the 1,4-addition product The answer is that an allylic carbocation is involved as an intermediate, where the word allylic means next to a double bond. When buta-1,3-diene reacts with an electrophile such as H" ", two carbocation intermediates are possible—a primary carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 2.4), it is more stable and forms faster than a nonallylic carbocation. 

Electrophilic addition of HCI to a conjugated diene involves the formation of allylic carbo-cation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then allow each resonance form to react with Cl , generating a maximum of four possible products. 

In Summary Conjugated dienes are electron rich and are attacked by electrophiles to give intermediate allylic cations on the way to 1,2- and 1,4-addition products. These reactions may be subject to kinetic control at relatively low temperatures. At relatively higher temperatures, the kinetic product ratios may change to thermodynamic product ratios, when such product formation is reversible. 

The products 18 and 19 arise from different regiochemis-tries associated with the reaction of diene 17 with the oxy-allylic cation generated from 16 (Scheme 19.9). Indeed, regiochemical control in intermolecular [4- -3] cycloaddition reactions still remains an issue that must be faced. The cyclopentanone 20 presumably arises from a formal [3-1-2] cycloaddition, likely through nucleophilic attack of the diene on the oxyallylic cation, followed by collapse of the resulting zwitterion 22 (Scheme 19.10). The formation of these... 

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. 

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