2013-carbanions and cations

7/28/2019 2013-Carbanions and Cations

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Acidity of Hydrocarbons: an Introduction

Unlike determining the pKa of a protic acid in an aqueous solution, determination ofthe acidity of hydrocarbons is more difficult.

As most are quite weak acids, very strong bases are required to effect deprotonation.Water and alcohols are far more acidic than nearly all hydrocarbons and areunsuitable solvents for the generation of anions from hydrocarbons. Any strong basewill deprotonate the solvent rather than the hydrocarbon.

For synthetic purposes, aprotic solvents such as diethyl ether, THF, and DME areused, but for equilibrium measurements solvents that promote dissociation of ionpairs and ion clusters are preferred.

Weakly acidic solvents such as dimethyl sulfoxide (DMSO) and cyclohexylamine are

used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitates dissociation of ion pairs so that the equilibriumdata refer to the solvated dissociated ions, rather than to ion aggregates.


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The basicity of a base-solvent system can be specified by a basicity function H.The value of H corresponds essentially to the pH of strongly basic nonaqueoussolutions. The larger the value of H, the greater the proton-abstracting ability of themedium.

Use of a series of overlapping indicators permits assignment of H values to base-

solvent systems, and allows pKs to be determined over a range of 035 pK units. Theindicators employed include substituted anilines and arylmethanes that havesignificantly different electronic (UVVIS) spectra in their neutral and anionic forms.

If the electronic spectra of the neutral and anionic forms are sufficiently different, theconcentration of each can be determined directly in a solution of known H:

For


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When the acidities of hydrocarbons are compared in terms of the relative stabilities ofneutral and anionic forms, the appropriate data are equilibrium acidity measurements,

which relate directly to the relative stability of the neutral and anionic species.

For compounds with pK > ~35, it is difficult to obtain equilibrium data. In such cases, itmay be possible to compare the rates of deprotonation, i.e., the kinetic acidity.

These comparisons can be made between different protons in the same compound orbetween two different compounds by following an isotopic exchange. In the presenceof a deuterated solvent, the rate of incorporation of deuterium is a measure of the rateof carbanion formation.


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It has been found that there is often a correlation between the rate of proton abstraction(kinetic acidity) and the thermodynamic stability of the carbanion (thermodynamic

acidity).

Thus, kinetic measurements can be used to extend scales of hydrocarbon acidities.These kinetic measurements have the advantage of not requiring the presence of ameasurable concentration of the carbanion; instead, the relative ease of carbanion

formation is judged by the rate at which exchange occurs.

This method is applicable to weakly acidic hydrocarbons for which no suitable base willgenerate a measurable carbanion concentration.

The kinetic method of determining relative acidity suffers from one serious complication,related to the fate of the ion pair that is formed immediately on abstraction of the proton.

If the ion pair separates and diffuses rapidly into the solution, so that each deprotonationresults in exchange, the exchange rate is an accurate measure of the rate ofdeprotonation.

However, an ion pair may also return to reactants at a rate exceeding protonation of thecarbanion by the solvent, which is called internal return.


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Thus, is it important that a linear relationship between exchange rates and equilibriumacidity be established for representative examples of the compounds under study. Asatisfactory correlation provides a basis for using kinetic acidity data for compounds ofthat structural type.


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In the ionization of an acid in solution, the acid donates a proton to the medium.The more basic the medium, the larger the dissociation equilibrium. The ability ofthe medium to stabilize the conjugate base also plays an important role in thepromotion of ionization. Let us consider two solvents, HOH and DMSO and theperformance of these solvents in the ionization process.

Although HOH can stabilize anions via H-bonding, DMSO cannot. Hence, a givenacid will show a greater propensity to dissociate in HOH.


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The change in pKa in going from water to DMSO is increasingly diminished asthe conjugate base becomes resonance stabilized (Internal solvation!).


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The nature of the solvent in which the extent or rate of deprotonation is determinedhas a significant effect on the apparent acidity of the hydrocarbon. In general, theextent of ion aggregation is primarily a function of the ability of the solvent to solvatethe ionic species.

In THF, DME, and other ethers, there is usually extensive ion aggregation.

In dipolar aprotic solvents, especially dimethyl sulfoxide, ion pairing is less significant.The identity of the cation also has a significant effect on the extent of ion pairing. Hardcations promote ion pairing and aggregation. Because of these factors, the numericalpK values are not absolute and are specific to the solvent and cation. Nevertheless,they provide a useful measure of relative acidity.

The two solvents that have been used for most quantitative measurements onhydrocarbons are dimethyl sulfoxide and cyclohexylamine.


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Some of the relative acidities in the table can be easily understood. The order ofdecreasing acidity Ph3CH >Ph2CH2 >PhCH3, for example, reflects the ability of each

successive phenyl group to stabilize the negative charge on carbon. This stabilization isa combination of both resonance and the polar EWG effect of the phenyl groups.

The much greater acidity of fluorene relative to dibenzocycloheptatriene (Entries 5 and6) is the result of the aromaticity of the cyclopentadienide ring in the anion of fluorene.

Cyclopentadiene (Entry 9) is an exceptionally acidic hydrocarbon, comparable in acidityto simple alcohols, owing to the aromatic stabilization of the anion. Note that fusion of abenzene ring decreases the acidity of cyclopentadiene, as illustrated by comparingEntries 6, 7, and 9.

Allylic conjugation stabilizes carbanions and pK values of 43 (in cyclohexylamine) and4748 (in THF-HMPA) were determined for propene. On the basis of exchange rates

with cesium cyclohexylamide, cyclohexene and cycloheptene were found to have pKvalues of about 45in cyclohexylamine. These data indicate that allylic positions havepK ~ 45.The hydrogens on the sp2 carbons in benzene and ethene are more acidic than thehydrogens in saturated hydrocarbons. A pK of45has been estimated forbenzene on

the basis of extrapolation from a series of halogenated benzenes. Electrochemistry hasbeen used to establish a lower limit of about 46for the pK ofethene


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For saturated hydrocarbons, exchange is too slow and reference points are souncertain that determination of pK values by exchange measurements is not feasible.

The most useful approach for obtaining pK data for such hydrocarbons involvesmaking a measurement of the electrochemical potential for the reaction:

From this value and known CH bond dissociation energies, we can calculate the pKvalues. Early application of these methods gave estimates of the pK of toluene ofabout 45 and of propene of about 48. Methane was estimated to have a pK in therange of 5262.

Electrochemical measurements in DMF put the pK of methane at about 48, with

benzylic and allylic stabilization leading to values of 39 and 38 for propene and toluene,respectively. These values are several units smaller than those determined by othermethods. The electrochemical values overlap with the pKDMSO scale for compoundssuch as diphenylmethane and triphenylmethane, and these values are also somewhatlower than those found by equilibrium studies.


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Measurements in the gas phase, which eliminate the effect of solvation, show

structural trends that parallel measurements in solution but have much largerabsolute energy differences.

a S. T. Graul and R. R. Squires, J. Am. Chem. Soc., 112, 2517 (1990).


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Terminal alkynes are among the most acidic of the hydrocarbons. For example, inDMSO, phenylacetylene is found to have a pK near 26.5. In cyclohexylamine, thevalue is 23.2.

The relatively high acidity of acetylenes is associated with the large degree ofscharacter of the CH bond. The s character is 50%, as opposed to 25% in sp3 bonds.The electrons in orbitals with high s character experience decreased shielding from thenuclear charge. The carbon is therefore effectively more electronegative, as viewedfrom the proton sharing an sp hybrid orbital, and hydrogens on sp carbons exhibitgreater acidity.

This same effect accounts for the relatively high acidity of the hydrogens on

cyclopropane rings and other strained hydrocarbons that have increased s character inthe CH bonds. The relationship between hybridization and acidity can be expressedin terms of the s character of the CH bond:

The correlation can also be expressed in terms of the NMR coupling constantJ13CH, which is related to hybridization


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Electron-withdrawing substituents cause very large increases in the acidity of CHbonds. Among the functional groups that exert a strong stabilizing effect on

carbanions are carbonyl, nitro, sulfonyl, and cyano. Both polar and resonanceeffects are involved in the ability of these functional groups to stabilize the negativecharge.

Perhaps the best basis for comparing these groups is the data on the various

substituted methanes.

Relative acidities of the substituted methanes with reference to aromatichydrocarbon indicators in DMSO: ordering NO2 >C=O>CO2R~SO2 ~CN>CONR2for anion stabilization


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The presence of two EWGs further stabilizes the negative charge. Pentane-2,4-

dione, for example, has a pK around 9 in water. Most -diketones are sufficientlyacidic that their carbanions can be generated using the conjugate bases ofhydroxylic solvents such as water or alcohols, which have pK values of 1520.

Stronger bases are required for compounds that have a single stabilizing functional

group. Alkali metal salts of ammonia or amines and sodium hydride are sufficientlystrong bases to form carbanions from most ketones, aldehydes, and esters. The Li+

salt of diisopropylamine (LDA) is a popular strong base for use in syntheticprocedures.


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NMR characterization of reactive intermediates

Certain nuclei such as 1H, 7Li, 13C, 19F, 15N, 31P have allowed spin statesof +1/2 and 1/2; this property allows them to be studied by NMR spectroscopy


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Beff= B0(1-). In fact, is responsible for different resonances of the nuclei.

= dia + para + I, where I = m + r+ e + s

m anisotropic factors; r ring currents; e electric field; s solvent effect

s-orbitals p-orbitals (absent for protons)


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Range of NMR signals (chemical shifts)

109Ag19F15N13C7Li1H

20008005002001010


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The chemical shift is independent of the operating frequency of the spectrometerBUT, remember that for different nuclei the operating frequencies are different !

400 MHz NMR machine means 400 MHz for1H only !

It will be ca. 100 MHz for13C (that is, 100 Hz in 1 ppm !)

Nuclei split each other if they are close to each other J-coupling constant


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JHH range is ca. 50 Hz

2

JHH ~ 20-40 Hz3JHH ~ 0-20 Hz3.5JHH ~ 1-3 Hz

JCH range is ca. 100 300 Hz and is dependent on the hybridization of the carbon

Generally, 1JCH 500 / (1 + 2), where 2 is the p-character of the hybridization

Sp3

2

= 3 125 HzSp2 2 = 2 166 HzSp 2 = 1 250 Hz

Experimental:

CH4 125 HzCyclobutane 134 HzEthylene 157 HzCyclopropane 161 HzAcetylene 250 Hz


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Carbon atoms also split each other in the 13C NMR spectra(but the splitting is generally of very low intensity)

Generally, 1JCiCj 550 Hz (si) (sj), where s is the s-character of the hybridization

Sp3 sp3 550 (1/4) (1/4) = 34 HzSp2 sp2 550 (1/3) (1/3) = 61 HzSp sp 550 (1/2) (1/2) = 137 Hz


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Integration (not for13C) number of equivalent nuclei

Depends on the relaxation time of the nuclei. Good integration is observed whentime between pulses is 3-5 T1

Where T1 is spin-lattice relaxation - most common for typical organic molecules

Aliphatic protons usually have shorter T1 than aromatic and often appear with largeintegration. This might mislead in the interpretation of the NMR spectra.

Carbons with hydrogen atoms have shorter T1 than quaternary carbons andrequire longer times to be observed.


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Org. Magn. Res. 1974, 6, 580

Consider carbanions at the benzylic position

CH2-

M+

A

C1Co

CmC

p

C

For C6H5CH2-

JHC = 132 Hz (M= Li)

JHC = 153 Hz (M= K)


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Carbocations


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Acidity functions return

100% sulfuric acid is very strong. Its acidity function stands at -12But, there are yet stronger acids around


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George Olah (Nobel prize 1994)

Superacids:

Acids that are stronger than 100% H2SO4

Superacids can reach acidities of Ho= -28, or 1016 times acidity of anhydrous sulfuric

acid.


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Magic acid FSO3HSbF5 is a convenient medium for studies of carbocations.

The fluorosulfonic acid acts as a proton donor and antimony pentafluoride isa powerful Lewis acid that assists ionization.

The solution is essentially non-nucleophilic, so carbocation of even moderate

stability can be generated and observed by NMR spectroscopy.

Carbocations are inherently high-energy species. The ionization of t-butyl chlorideis endothermic by 153 kcal/mol in the gas phase.

An activation energy of this magnitude would lead to an unobservably slow reactionat normal temperature. Carbocation formation in solution is feasible because of

solvation of the ions that are produced.


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13C shift was 335.2 ppm about 300 ppm downfield from the starting t-Bu-F(NB: more on the NMR technique will follow)

Superacids also prevent proton elimination that quenches carbocations


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St bilit f b ti


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Stability of carbocations

The relative stability of the carbocation can be expressed in terms of its pKR+ , which

is defined as

where HR is an acidity function defined for the medium.

In dilute aqueous solution, HR is equivalent to pH, and pKR+ is equal to the pH at

which the carbocation and alcohol are present in equal concentrations.


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Carbocation stability is more often expressed in terms ofhydride affinity.

Hydride affinity values based on solution measurements can be derived fromthermodynamic cycles that relate pKa and electrochemical potentials.The hydride affinity, G, for the reaction

is a measure of carbocation stability. This quantity can be related to anelectrochemical potential by summation with the energy for hydrogen atom removal,i.e., the homolytic bond dissociation energy.


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where H./H and R+/R. are one-electron oxidation potentials for H and R..

The former potential is about 0.55 V in DMSO. Measurement of R+/R. can be

accomplished by cyclic voltammetry for relatively stable carbocations and by othermethods for less stable cations.

The hydride affinity can also be calculated (in vacuum) and their comparison is

related to the TD stability of carbocations.


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The data acquired in gas phase were found to be in reasonable correlation with


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The data acquired in gas phase were found to be in reasonable correlation withhydride affinities obtained from solution phase experiments (calorimetricmeasurements of the ionization enthalpy).

The solution values are smaller but the relative stability trends remain

For simple carbocations the increase in carbocation stability with additional alkyl


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For simple carbocations, the increase in carbocation stability with additional alkylsubstitution is one of the most important and general trends in organic chemistry.

Hyperconjugation is the principal mechanism by which alkyl substituentsstabilize carbocations.

There is considerable evidence of the importance of hyperconjugation on thestructure of carbocations, including NMR data, crystallographic data, andcomputational studies. The tert-butyl cation has been studied by each method.

The NMR results indicate shortening of the CC bonds, as would be predicted byhyperconjugation.

The crystal structure gives a value of 1.44 .

A t ti l t d f th t B ti h li ht l ti f th C H b d


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A computational study of the t-Bu cation shows a slight elongation of the CH bondsaligned with thep orbital, and the CCH bond angles are slightly reduced

J. Am. Chem. Soc. 1993, 115, 259

Importance of hyperconjugation can be verified using isotope labeling


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The K value is 1.97.

Thus, 2 is more stable than 1 (-secondary isotope effect).

Importance of hyperconjugation can be verified using isotope labeling

The relationship of C H (and C C) hyperconjugation and the reactivity as well as the


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The relationship of CH (and CC) hyperconjugation and the reactivityas well as thestructure of carbocations is very important.

Hyperconjugation represents electron sharing with an empty orbital and can lead tostructural changes or reactions.

If the electron density is substantially shared between the two atoms, the structure is

bridged. If the electron sharing results in a shift of the donor group, rearrangementoccurs. These rearrangements are the culmination of electron donation by formationof a new bond.

Wagner-Meerwein rearrangement mechanism


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Hyperconjugation also makes carbocations susceptible to proton removal, as occursin elimination reactions.

The weakened CH bond and increased positive charge make hydrogen susceptibleto removal as a proton.

Thus, there is a preference for the removal of the proton from the most highly

substituted carbon, which is the one that is most engaged in hyperconjugation.

Within any given series of carbocations, substituents affect stability in predictable


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Within any given series of carbocations, substituents affect stability in predictableways. ERG substituents stabilize carbocations, whereas EWG substituentsdestabilize them.

Careful attention must be paid to both resonance and polar effects.

The resonance effect is very strong for substituents directly on the cationic carbon.

Benzylic cations are strongly stabilized by resonance interactions with the aromaticring. Substituent effects can be correlated by the Yukawa-Tsuno equation.

Adj t t ith h d i f l t t l t bili


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Adjacent atoms with one or more unshared pairs of electrons strongly stabilize acarbocation.

Alkoxy and dialkylamino groups are important examples of this effect.

Halogen substituents also stabilize carbocations as a result of resonance donationfrom the halogen electron pairs.

A fluorine or chlorine substituent is nearly as stabilizing as a methyl group in the gasphase.

Remember: Fluorine is the strongest pi-donor and stabilizes carbocations betterthan any other halogen

When an sp2 hybridization cannot be achieved, the formation of carbocationsf


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is extremely unfavorable

1-chloroapocamphane is inert to nucleophilic substitution.

The structure of the bicyclic system prevents rehybridization to a planar sp2 carbon.

Backside nucleophilic attack is precluded because of the bridgehead location of the

CCl bond.

More flexible bridgehead structures


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Relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, 1-bromobicyclo[2.2.2]octane, and 1-bromobicyclo[2.2.1]heptane in 80% ethanol at 25oC

Carbocation stabilization modes


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-resonance assistancelone pair assistance hyperconjugation

The history of a big controversy


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Ruzicka, L. Helv. Chim. Acta, 1918, 1, 110.

Meerwein and Van Emster (1922)

Proposed mechanism nothing unusual

Only exo isomer is formed


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Only exo-isomer is formed

Halide isotope exchange suggests that there is Cl- dissociation

but no isomerization

Solvolysis of norbornyl brosylates


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relative rates

Winstein, Trifan. J. Am. Chem. Soc., 1949, 71, 2953.

Bs =

1 Enantiomer 1 : 1 racemic mixture

Isotope labeling experiments (Roberts) showed that racemization does not occur viasimple hydride migration


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simple hydride migration

"It is attractive to account for these results by way of the bridged (nonclassical)f l ti f th b l ti i l i l t d t f f ti f


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formulation for the norbornyl cation involving accelerated rate of formation fromthe exo precursor by anchimeric assistance." -Saul Winstein

Proposed Nu-substitution pathway

"The norbornyl cation does not possess sufficient electrons to provide a pair for all ofthe bonds required by the proposed bridged structures. One must propose a new


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the bonds required by the proposed bridged structures. One must propose a newbonding concept, not yet established in carbon structures. - H. C. Brown

2 rapidly interconverting classical cations

Rapid equilibration proposed by Brown


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exo-norbornyl brosylate undergoes solvolysis at a rate approximately 350-344 times that of theendo isomer. If this factor is the result of participation, which should be essentially absent in the 2-

phenyl and 2-methyl tertiary derivatives, we should expect to find an increase in the exo-norbornylto cyclopentyl brosylate rate ratio to a value in the neighborhood of 2000 (5X 400).However, the observed factor, exo-norbornyl to cyclopentyl derivative, is of the same order ofmagnitude as observed with the tertiary derivatives : 3.9 for methanolysis, 3.6 for ethanolysis, and13 for acetolysis.

HC Brown, J.Am.Chem.Soc. 1964


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If the norbornyl cation were a resonance-stabilized species, we should anticipate that the

effects of the methyl and phenyl substituents would be very similar to their effects in the -phenylethyl and benzhydryl systems, and very much smaller than their effects in the othersecondary systems.However, the effect of methyl is to produce a rate enhancement of 55,000,with phenyl bringing about a further increase of 5260. Clearly, these results do not accord withthe postulated presence of non-classical resonance in the norbornyl cation or transition state.

HC Brown, J.Am.Chem.Soc. 1964


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Previous treatments of high exo-endo rate ratios in norbornyl systems have assumed thatthese arise from enhanced exo rates. However, it has been suggested that the high exo-endoratios which are observed may be the result, not of an unusually high exo rate with a

normal endo rate, as is commonly assumed, but of a normal exo rate with an unusually slowendo rate.

If solvolysis proceeds via an unassisted ionization, than exo- and endo-isomers shouldreact differently: endo would be very slow to react due to steric repulsions while


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react differently: endo would be very slow to react due to steric repulsions whileexo will behave normally.

An examination of the preferred path for the leaving group in the endo derivative reveals thatthis path brings the departing group very close to the opposite side of the rigid endocyclic

system. Indeed, if one constructs an intimate ion pair with a representative anion, such aschloride, situated along the perpendicular to the face of the carbonium ion at C-2, at distancesequal to the sum of the two ionic radii, the anion is observed to be severely crowded againstthe 5,6-bridge.

It is apparent that the chloride ion must depart along some other path which is more favorable

sterically, but more costly in requiring greater separation of charges in the ion pair.

So, is the nonclassical structure a minimum or maximum in energy ?


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Superacids come to the rescue


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J. Am. Chem. Soc. 1964, 5679


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At low T, the spectrum resolves to 3 singlets (4 : 1 : 6 ratio) showing that2-norbornyl ion stable in solution as equilibrating Wagner-Meerwein shifts

i i t di t


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or mesomeric intermediate

The low-temperature spectrum is consistent with the assumption that (1) and (2) are proceedingrapidly and (3) slowly.The protons on carbons 1, 2, and 6 would interconvert rapidly, and they appear as the low-fieldpeak of area 4. The protons on carbons 3, 5, and 7also would be equivalent, giving the high-fieldpeak of area 6. The single bridgehead proton gives the signal at intermediate field.


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Just for comparison, the 13C shifts of classical planar carbocations are in the area of300 ppm (t-Bu cation 335 ppm) !

At very low temperatures, the 6,2- and 3,2- hydride shifts become very slow andthe positive charge becomes delocalized between 1 and 2 positions as evidentfrom the NMR shifts for protons and carbon atoms


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from the NMR shifts for protons and carbon atoms.

This means that the stable structure is either the symmetrically bridged ion orcarbon migration is too fast, corresponding to ca.2-4 kcal/mol

Yannoni, C. S.; Myhre, P. C. J. Am. Chem. Soc. 1982, 104, 7380

Solid-state 13C NMR spectrum of 2-nornbornyl cationat -268 oC (!) showed that still there is no resolution between

the 1 and 2 positions.

At this temperature, the barrier for the Wagner-Meerwein shiftshould be less than 0.2 kcal/mol, which is very unlikely.

Thus, there is an additional support for the bridged non-classicalstructure.

X-ray structural analysis


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Laube,Angew. Chem. Int. Ed. Engl. 26 (1987), 6, 560

X-ray photoelectron spectroscopy

The method shows charge distribution as it measures


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carbon 1s electron binding energies.

The timescale of this method is 1018, much faster than NMR and chemicaltransformations (Wagner-Meerwein)

In localized cations, the formal charge is unequally shared by different atoms.Consequently, the core electrons of these atoms are differently screened and showincreasing binding energies with increasing positive charge localization.

The photoelectron spectrum of tert-butyl cation exhibits two

clearly separate peaks with Eb = 285.2 and 288.6, respectively.

Trityl and tropylium cations show extensive positive charge delocalization


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The 2-methylnorbornyl cation (Figure 1, upper trace) shows aseparation of 3.7 eV (with an intensity ratio of ~1 :7) whichindicates that in spite of the stabilizing effect of the methylgroup, there is some delocalization in the bicyclo[2.2.1]heptylsystem.

For the norbornyl cation, there is a single broad line with apronounced shoulder on the higher binding energy side(corresponding to C6 and C2). A curve resolver analysis gavean approximate intensity ratio of 2:5 and a maximum

separation of 1.7 eV. These results clearly suggest that this ionis of nonclassical nature since no high-binding energy linecharacteristic of a carbenium center is found.


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An equilibrating classical structure 1a 1b should give an electron spectrumidentical with a static classical carbenium ion, even under conditions of extremelyrapid equilibration. For example, the rapidly equilibrating, degenerate cyclopentylcation clearly shows the carbenium center line separated from the methylenecarbons.

Olah, G. A.; Mateescu, L. A. J. Am. Chem. Soc. 1970, 92, 7231.Olah, G. A.; Mateescu, L. A. J. Am. Chem. Soc. 1972, 94, 2529.


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Hydride Ion Affinities


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HIA for the classical 2-Me-2-norbornyl cation is 225 kcal/molHIA for the 2-norbornyl cation is 231 kcal/mol

The difference is relatively small, suggesting additional stabilization due tonon-classical delocalization

Typically, the HIA difference between 2o and 3o carbocations is ca. 20 kcal/mol

225 kcal/mol 231 kcal/mol

So, what was it all good for ?


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Clearly, there was no lack of devoted adversaries (perhaps a more proper term thanenemies) on both sides of the norbornyl ion controversy. It is to their credit that wetoday probably know more about the structure of carbocations, such as the norbornylcation, than about most other chemical species. Their efforts also resulted not only in

rigorous studies but also in the development or improvement of many techniques.

Although many believe that too much effort was expended on the futile norbornylion controversy, I am convinced that it eventually resulted in significant new insightsand consequences to chemistry.

It affected in a fundamental way our understanding of the chemical bonding ofelectron deficient carbon compounds, extending Kekules concept of the limitingability of carbon to associate with no more than four other atoms or groups.

George Olah, J. Org. Chem. 2001, 66, 5943

If we are now comfortable with the 2e-3c cations, lets continue further


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Protonated hydrocarbons


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Mechanistically, mainly linear TS is involved, although non-linear can also occur


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The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, thatis, the reaction of trivalent carbocations with molecular hydrogen giving their parenthydrocarbons, involves the same five coordinate carbonium ions.

In addition to C-H, C-C bonds can also be broken


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Preparation of branched C8 hydrocarbons by dimerization of isobutylenewith isobutane (industrial process)


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C4 compounds are obtained as by-products of petroleum refining

In this process, isobutane acts as a hydride transfer agent and a source of thetert-butyl cation, formed via intermolecular hydride transfer.

In fact, there is often no need for the highly reactive isobutylene as tert-butyl cationcan react with isobutane under superacidic conditions


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This is the reverse of the superacid-catalyzed cracking of hydrocarbons (a significantpractical application)


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RON = research octane numbers

Not only protolytic reactions but also a broad range of reactions with variedelectrophiles (alkylation, formylation, nitration, halogenation, oxygenation, etc.) were


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found to be feasible when using superacidic, low-nucleophilicity reaction conditions.

Alkane formylation to aldehydes and ketones


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Additionalproposed mechanism

Cyclopropylmethyl cations


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Roberts, J.D.; Mazur, R.H. J. Am. Chem. Soc. 1951, 73, 3542

The stabilization of carbocations by cyclopropyl substituents results from theinteraction of the cyclopropyl bonding orbitals with the vacant carbonp-orbital.The electrons in these orbitals are at relatively higher energy than normal -electrons


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and are therefore particularly effective in interacting with the vacantp-orbital of thecarbocation.

Once a cyclopropylmethyl cation is formed it can rearrange to two other isomericcyclopropylmethyl cations:

The rearrangement most likely proceeds via a nonplanar cyclobutyl cation as anintermediate or a transition state.


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The tricyclopropylmethyl cation is more stable than the triphenylmethyl cation !

The cation is completely bridged and can react with nucleophiles from at all carbon

atoms

Different stabilization modes :


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H3C H

H

OTsH2C

OTs

H

OTs

Relative solvolysis rates

(Brown) 1 36 500

H

H

Hyperconjugation -delocalization -bridging

H3C CH3

CH3OPNB

CH3

CH3OPNB

CH3

CH3OPNB


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Relative solvolysis rates(Brown)

1 970 503,000

Cl

Relative solvolysis rates

(Hart)

Cl Cl Cl

1 250 25,000 2,5 x 106

Solvolysis of rigid cyclopropylmethyl tosylates:


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7-norbornenyl cation


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Norbornenyl tosylate solvolysis occurs with complete retention of stereochemistry !

Winstein J. Am. Chem. Soc. 1956, 78, 592

Winstein S Brookhart M J Am Chem Soc 1972 94 2347 l (10 )


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Winstein, S.; Brookhart, M. J. Am. Chem. Soc. 1972, 94, 2347 -scale (10 ppm)

The -electrons are donated to the anti-bonding orbital of the C-OTs bond.This facilitates ionization and helps to delocalization the positive charge.


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In the case of the syn-isomer, the substitution proceeds 107 times more slowly thanwith the anti-isomer.

The reaction proceeds via rearrangement and stabilization of the partially delocalizedcation as an allylic system.

1,5 PARTICIPATION IN THE SOLVOLYSIS OF -(3-CYCLOPENTENYL)-ETHYL p-NITROBENZENESULFONATE


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R. G. Lawton, J. Am. Chem. Soc., 83, 2399 (1961).

H3CO H O2NOBs OBsOBs

55 1 1.4 x 10-4

Relative solvolysis ratesBrown

Phenonium ion

Solvolysis of 3-phenyl-2-butyl tosylates (Cram, 1949, 1952)


93/109

erythro erythro(enantiomerically pure)

threo threo(racemic mixture)

X- X-

Proposed mechanism


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phenonium ion

CH3H3CH3C CH3

from erythro(chiral)

from threo(achiral)

Solvent- and substituent-dependent competition

The substituent(s) and nucleophile (solvent) can influence the reaction path:


95/109

The substituent(s) and nucleophile (solvent) can influence the reaction path:

When EWG are present in the aromatic ring, the reaction usually proceeds with little participation

of the phenonium cation. In such case, the reaction follows a typical substitution pattern (ks) withlow -value (ca. -0.7).

When EDG are present, the aromatic ring is heavily involved in the cation stabilization via thephenonium cation (k).


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Ethanol is a good nuceophile, acetic acid is worse and formic acid is even worse

Hence, aromatic ring participation is the smallest for ethanol and largest for formic acid

The more e-donating substituent the larger is the ring participation

Cl

**

*

SbF5

Isotope scrambling


97/109

Complete isotope scrambling

*SbF

+

Bridged phenonium

Ea = 13 kcal/mol

-methylbenzyl cation


98/109

Cyclopropyl assistance:


99/109

Tsuji, J. Am. Chem. Soc. 1967 , 89, 1953.Wells, Tetrahedron, 1966, 22, 2007.

Metal-stabilized carbocations


100/109

Milstein, J.Am.Chem.Soc. 1998, 477

Stabilized methylene arenium cation


101/109

HO

PtBu2

PtBu2

Rh Cl

CR2

13

C NMR: (ppm)

+

Methylene Carbon Atom

+R= CH3

R= adamantyl

+ 255

+ 286.5

+ 44.15

(Practically the same as that in the starting material)

Benzyl Cation

Olah, G. A. et. al

J. Org. Chem. 1993, 58, 4851

Even stronger phenolic acid


102/109

PtBu2

Ir ClCH3OO

HOTf CH3O

OP

tBu2

Ir Cl+

Iridium Quinones and Quinone Methides via -2 C=O Bond

- H2HO

OP

tBu2

Ir Cl+H2O, 10 min

Metal-stabilized phenoxonium cations :


103/109

HOTf

NEt3CH3O

OP

tBu2

PtBu2

Ir Cl

O

OP

tBu2

PtBu2

Ir Cl

o-Quinone Methide

p-Quinone

CH3O

OP

tBu2

PtBu2

Ir Cl+

OTf-

HO

OP

tBu2

PtBu2

Ir Cl+

OTf-

HOTf

NEt3

PtBu2H

PtBu2OTf

-

H2P

tBu2OTf

-

The positive charge is indeed in the ring :

Intermolecuraly stabilized methylene arenium cations


104/109

Milstein, J. Am. Chem. Soc. 2006, 16450

Just remember, the majority of carbocations arenormal tri-valent carbenium ions

Stabilization of carbocations (summary):


105/109

1. Inductive effect

Relatively electropositive groups such as alkyls can stabilize a neighboringcarbocation center through inductive effect

2. Hyperconjugation


106/109

2. Resonance

a) Lone pair


107/109

Donation strength : N > O > F >> S, Cl

b) Conjugation with a -bond or aromatic ring

3. Aromaticity


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cyclopropenyl cation, 2 e, tropylium ion, 6 e, aromaticaromatic, stable aromatic, stable

4. Non-classical type cations

+O


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5. Metal-coordinated (otherwise elusive) cations

CH3OO

PtBu2

PtBu2

Ir Cl+

OTf-

2013-carbanions and cations

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