Electrophilic addition to strained alkenes

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

Identify which protonation reaction alkene A protonated alkene A, alkene B protonated alkene B) 

Both cis and tran -cyclohexene have been synthesized, but only one of them can be isolated. Electrophilic addition of ROH to one isomer occurs spontaneously, while addition to the other isomer occurs only in the presence of a strong acid, such as sulfuric acid. Calculate the energy of protonation for each isomer cyclohexene protonated cyclohexene, trans-cyclohexene protonated trans-cyclohexene), and identify the more reactive isomer. Also examine electrostatic potential maps. Suggest an explanation to account for both the reactivity difference and the structural changes. (See also Chapter 7, Problem 5.) 

Electrostatic potential map for alkene A shows negatively-charged regions (in red) and positively-charged regions (in blue). 

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In addition to the 1,3-ally lie strain concept, Houk has employed a model for 7t-facial stereoselection of electrophilic additions to chiral alkenes, such as hydroboration, epoxidation, and dihydroxylation, with similar predictive success. ... 

Epoxidation appears to involve electrophilic addition to the alkene, since the reaction is favored by electron-withdrawing groups on the peracid and electron-donating groups on the alkene. The epoxidation reaction is highly exothermic, with an experimental heat of reaction of -38 kcal/mol. The kinetic expression is overall second order, first order in the alkene and first order in the peracid. Steric effects do not appear to be important. The rate constant for the reaction increases with the number of alkyl substituents on the double bond, but the location of the alkyl groups is not important. For example, cis-2-butene, fra s-2-butene, and isobutene have nearly the same reactivity.The rate constant for the reaction is sensitive to strain, with faster rates observed for alkenes that produce greater relief of strain upon epoxidation. For example, frans-cyclooctene is epoxidized about 100 times faster than cis-cyclooctene. ... 

To achieve diastereoselectivity in electrophilic additions to a double bond in acyclic compounds, there must be a facial preference for attack. An A strain provides such an element for conformational control, as exemplified by hydroboration of the alkene shown below.The hydration of a double bond via hydroboration involves (1) anti-Markovnikov addition of the B-H bond, (2) cis addition of the B-H bond, (3) addition of the B-H bond from the less hindered side of the double bond, and (4) oxidation with retention of configuration. 

In the case of electrophilic addition, the reactions of tricyclic dienes 1 with several electrophilic reagents have been investigated.1 7 Interestingly, some of these compounds undergo addition reactions with remarkable syn stereoselectivity. For example, the reaction of dimethyl tricy-clo[4.2.2.02,5]deca-3,9-diene-7,8-dicarboxylate with iodine azide solution, prepared in situ from an excess of sodium azide and iodine monochloride, in acetonitrile at — 5 C provided the. yyn-4-azido-3-iodo derivative 2 (Table 1) in 90% yield.1,2,4,6 The formation of the 5,>,n-4-azido-3-iodo derivative 2 is thought to be the first example of a syn addition of iodine azide to an alkene.1,2 The formation of the syn-product is best explained by the twist strain theory,8 according to which the syn transition structure A is favored over the an/7-coplanar transition structure B.1... 

Alkynes undergo the same set of reactions as alkenes but are slightly less reactive because the intermediates involved are less stable. For the Ade2 process, the vinyl cation intermediate formed is less stable than the alkyl carbocations formed when electrophiles attack alkenes. Addition to the vinyl cation produces a mixture of syn and anti addition. Stabilizing the vinyl cation by bridging is less favorable since the bridged ion is more strained and may have some antiaromatic character. 

Arynes react readily with simple alkenes to give either benzocyclobutenes or substituted benzenes (Scheme 7.31). The formation of benzocyclobutenes by [2+2] cycloaddition reaction of the aryne to the alkene proceeds best for strained and electron-rich carbon-carbon (C=C) double bonds. For example, dicyclopentadiene reacts to give the ex o-isomer of the corresponding four-membered ring in good yield. The addition to cyanoethene (acrylonitrile) and the reaction with the electron-rich ethoxyethene (ethyl vinyl ether) gives the cyano- and ethoxy-benzocyclobutenes in 20% and 40% yields, respectively. The latter reaction almost certainly involves nucleophilic addition of the enol ether to the electrophilic aryne followed by coUapse... 

The transition state of singlet carbene cycloaddition to alkenes involves an electrophilic approach of the vacant p orbital to the n bond of alkenes. By contrast, the first step of the triplet addition process may involve the in-plane a orbital of the carbene. As in the case of C—H insertion (see Section 5.1), the difference in the transition structure between the singlet and triplet cycloaddition becomes important in the intramolecular process, especially when approach to a double bond is restricted by ring strain. Direct photolysis of ( )-2-(2-butenyl)phenyldiazomethane (99) in the presence of methanol gives l-ethenyl-l,la,6,6fl-tetrahydrocycloprop [fljindene [100, 29%, (E/Z)= 10 1] and l-(2-butenyl)-2-(methoxymethyl)benzene (101, 67%). Triplet-sensitized photolysis results in a marked increase in the indene (52%, EjZ) = 1.3.T) at the expense of the ether formation (4%) (Scheme 9.30). On the other hand, direct photolysis of phenyldiazomethane in an equimolar mixture of... 

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