Skeletal Rearrangements of Carbocation Intermediates

Carbocations initially formed upon addition of an electrophile to an alkene may be able to undergo skeletal rearrangement depending on whether or not a more stable cation exists and, if it does exist, whether or not it can be reached via a low-energy pathway. Consider addition of HBr to 3-methyl-1-butene, the product of which is 2-methyl-2-butyl bromide. 

Draw Lewis structures for the possible carbocations resulting from protonation of the double bond in 3-methyl-1-butene, and decide which is favored. (Check your result using available energy data for C5H11 carbocations.) What would be the product of bromide addition to the more stable cation Is this the observed product  

Draw a Lewis structure for the carbocation which would result from a 1,2-hydride shift in the more stable (initially-formed) cation. Is this carbocation more stable than the initially-formed ion What would be the product of bromide addition to this cation Is this the observed product  

Examine the transition state for the hydride shift. Calculate the barrier from the more stable initial carbocation. Is the process more facile than typical thermal rearrangements of neutral molecules (.05 to. 08 au or approximately 30-50 kcal/mol) Is the barrier so small ( .02 au or approximately 12 kcal/mol) that it would be impossible to stop the rearrangement even at very low temperature Where is the positive charge in the transition state Examine atomic charges and the electrostatic potential map to tell. Is the name hydride shift appropriate If not, propose a more appropriate name. 

Addition of hydrogen halides to alkenes is not stereospecific. In contrast, addition of Br2 proceeds exclusively with anti stereochemistry. 

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Taking into account the catalytic results during olefin oxidation, it is observed that methacrolein is again mainly obtained over MoV-based ° or VPO catalysts, " while MA is also observed with the latter catalyst. In this way, it has been suggested that a skeletal rearrangement of a carbocation or radical intermediate could occur due to the presence of strong Br0nsted acid sites on the VPO catalyst surface, which could explain the formation of maleic anhydride. 

Studies reveal an advantage to using boron trifluoride in dichloromethane at reduced temperatures instead of Brpnsted acids in the organosilicon hydride reductions of a number of dialkylbenzyl alcohols.126 129 The use of Brpnsted acids may be unsatisfactory under conditions in which the starting alcohol suffers rapid skeletal rearrangement and elimination upon contact with the acid, and also in which the alcohol does not yield a sufficient concentration of the intermediate carbocation when treated with protic acids.126... 

A long-established feature of the carbocation intermediates of reactions, such as SnI solvolysis and electrophilic aromatic alkylation, is a skeletal rearrangement involving a 1,2-shift of a hydrogen atom, or an alkyl, or aryl group. The stable ion studies revealed just how facile these rearrangements were. Systems where a more stable cation could form by a simple 1,2-shift did indeed produce only that more stable ion even at very low temperatures (see, e.g., Eq. 3). 

Dehydrations produce olehns from alcohols by the acid-catalyzed elimination of a water molecule from between two carbons. Acid-catalyzed dehydrations often give mixtures of products because the intermediate carbocation is prone to cationic rearrangements to more stable carbocations prior to formation of the olefin product. Moreover, even when the intermediate carbocation is not subject to skeletal rearrangement, as in file case of tertiary alcohols, mixtures of regioisomers are often produced during file loss of a proton from file carbocation. As a consequence, the acid-catalyzed dehydration of alcohols is generally not a viable synthetic method. 

For skeletal rearrangements over zeolite, the nonclassical protonated cyclopropane intermediate could account for the experimental observations. Theoretical studies of the reaction mechanism indicated that protonated cyclopropane-type species do not appear as intermediates but rather as transition states. Considering all zeolite-catalyzed hydrocarbon reactions (hydride transfer, alkylation, disproportionation, dehydrogenation), only carbocations in which the positive charge is delocalized or sterically inaccessible to framework oxygens can exist as free reaction intermediates. In theoretical studies on the mechanism of the superacid-catalyzed isomerization of n-alkanes (ab initio and DFT calculations), protonated cyclopropanes were found to be transition states for the branching of both the 2-butyl cation and the 2-pentyl cation. ... 

Free-radical addition of HBr to syn-7-methoxynorbornene yields mixtures of exo- and en io-2-bromo-syn-7-methoxynorbornanes under ionic conditions, products of skeletal rearrangement are also formed. Partial retention of optical configuration has been observed in the low-temperature addition of HCl tol-methyl-2-methylenenorbornanes. Thus, reaction of (— )-(403) with HCl in pentane at — 78 °C for a few minutes gave (-)-(404) with 27% retention of configuration presumably equilibration of the carbocation intermediates is sufficiently slow to allow for the interception of the primary intermediate by Cl ion. Product studies indicate that the radical addition of iodine to dimethyl norbornadiene-2,3-dicarboxylate and to dimethyl quadricyclanedicarboxylate mainly occurs by exo-attack, the (interconvertible) radicals (405) and (406) being the initial respective intermediates. ... 

A synthesis of 4-alkyl-2-methoxy-1-phenylthio-1,3-butadienes by a simple -elimination of thiophenol from a thioacetal is not possible owing to skeletal rearrangement that is fostered by stabilization of a cyclopropylcarbinyl carbocation intermediate by the alkyl substituent (eq 51). Interconversion of an initial a-phenylthio carbocation to a more stable a-methoxy carbocation intermediate leads to the generation of a 4-alkyl-l-methoxy-2-phenylthio- 1,3-butadiene instead. 

Elimination El (Sections 9.6, 9.7) An El reaction occurs in two steps slow, rate-detennirung breaking of the C— Lv bond to form a carbocation intermediate followed by rapid proton transfer to solvent to form an alkene. An El reaction is first order in haloalkane and zero order in base. Skeletal rearrangements are common. 

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