Chapter 7Carbanions and Other Nucleophilic Carbon Species

R

Li

Li

Li

R

R Li R

+ 4 (CH3)2NCH2CH2N(CH3)2

Li

R

Li

R

NN

N N

CH3

CH3

H3C

H3C

CH3

CH3

H3C

H3C

Tetramethylethylenediamine(TMEDA)

H3C OEt

O O

H3C O

O O

Et

O

C C

OH

H3C OEt

][RH][RlogHpK HR

Advanced Organic Chemistry (Chapter 7) sh.Javanshir

7.1 Acidity of HydrocarbonsDetermination of the relative acidity of most carbon acids is more difficult, because they are so weak acids.

Very weakly acidic solvents such as DMSO and cyclohexyl amine are use as solvent for carbanion generation.

Basicity constant H_: The basicity of base-solvent system. (Analogous to the Hammett acidity function HO.

The value of H_ corresponds essentially to the pH of strongly basic non-aqueous solutions.

The larger values of H_, the greater is the proton abstracting ability of the medium.

Advanced Organic Chemistry (Chapter 7) sh.Javanshir

Assignment of H_ to base-solvent system:

Use of a series of overlapping indicators.

Advanced Organic Chemistry (Chapter 7) sh.Javanshir

Determination the acidity (pK values) of hydrocarbon at a known H_ :

R-H + B- R- + B-H

][RH][RlogHpK HR

If the electronic spectra of the neutral and anionic forms are sufficiently different, the concentrations of each can be determined directly.

If the electronic spectra of the neutral and anionic forms are not sufficiently different, one of the indicators used and its spectrum is monitored.

RH + In- R- + HIn

Thermodynamic Acidity: Acidity of hydrocarbons in terms of the relative stabilities of neutral and anionic forms.

In many cases it is not possible to obtain equilibrium data.

kinetic Acidity: Rate of deprotonation of hydrocarbons.

Isotopic Exchange: In the presence of a source of deuterons, the rate of incorporation of D atom into an organic molecule is a measure of the rate of carbanion formation:

RH + B- R- + BH

R- + S-D R-D + S-

S- + B-H S-H + B-

There is often a correlation between the kinetic acidity and thermodynamic acidity.

Advantage of kinetic measurements: Not requiring the presence of a measurable concentration of the carbanion at any time. The relative ease of carbanion formation is judged from the rate at which exchange occurs. This method is therefore is applicable to very weak acids.

disadvantage of kinetic measurements: complication due to the fate of the ion pair that is formed.

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

Basis for Using Kinetic Acidity DataUnder many conditions, an ion pair may return to reactant at a rate exceeding protonation of carbanion by the solvent (internal return) and exchange has not been resulted.

R3C-H + M Bionization

internalreturn

[R3C M + BH] R3C + M + BH

S-D

Exchange

R3C-D + S

An evidence for occurring internal return:

Racemization without Exchange at Chiral Center

Extent of Ion Pair or Dissociated Forms

Solvent Polarity: Ion pairing is greatest in no-polar

solvents such as ethers. In dipolar solvents , such as

DMSO, dissociated forms are predominant.

Structure: The identity of the cation present have

significant effect if ion pairs are present.

The rate of tritium exchange (kinetic acidity) for a series of related hydrocarbons is linearly related to the equilibrium acidities of these hydrocarbons in the solvent system.

e.g.

H HH H

>>

di-benzofluorene di-benzocycloheptatriene

Ph3CH > Ph2CH2 > PhCH3

Allylic Conjugation:

H

pK=43 (cyclohexyl amine)pK=47-48 (THF-HMPA)

HHHH

pK=45 (cyclohexyl amine)

sp2 Hydrogens of benzene and Ethylene

Ph-H (Benzene): pK = 43

Estimated on the basis of extrapolation from a series

fluorobenzenes.

Ethylene: pK = 46

Estimated by electrochemical methods.

Saturated Hydrocarbons: Exchange is too slow and direct determination of the pK values is not feasible.

Measurement of the electrochemical potential for the reaction:

R + e- R-

From this value and known C-H bond dissociation energies, pK values can be estimated (semi quantitative method).

Isobutane: pK = 71

sp Hydrogens in Acetylenes

PhC C-H pK=26.5 (DMSO)pK=23.2 (cyclohexyl amine)

for

The relative high acidity of acetylenes associated with the large degree of s character (50%) of C-H bond.

The sp orbital is more electronegative than sp2 and sp3.

Ab initio calculations: CH3- and Et- have

pyramidal shape.

The optimum bond angle of H-C-H is 97-100°.

Carbanions are predicted to be pyramidal.

In planar carbanion, the LP would occupy a p orbital.

In pyramidal geometry, the orbital would have substantial s character.

Since, the LP would be of lower energy in an orbital with some s character.

Stereochemistry of H-Exchange

CH3CH2 C C CH2CH3

CH3Ph

O-HCH3

CH3CH2CCH3

Ph

S-HB-H CH3CH2 C CH3

CH3

H

CH3CH2 C CH3

O

+

Base

Optically Active

Low dielectric constant solvents: Retention of configuration.

Increasing the amount of inversion with increasing the proton-donating ability and dielectric constant of the solvent.

Base: t-BuOK

Solvent: Benzene

93% net retention of configuration

Short lifetime for the carbanion in a tight ion pair. Carbanion does not symmetrically solvated before protonation by H-B or ketone.

Base: KOH

Solvent: Ethylene Glycol

48% net inversion of configuration

Solvent is a good proton source and the protonation must be occurring on an unsymmetrically solvated species that favor back-side protonation.

Base: t-BuOK

Solvent: DMSO

100% Racemization

Sufficient lifetime for the carbanion to become symmetrically solvated.

The Stereochemistry of H-D exchange in 2-phenyl butane:

Base: t-BuOK

Solvent: t-BuOH

retention of configuration

Ion pair formation in which a solvent molecule coordinated to the metal ion acts as the proton donor.

Base: t-BuOK

Solvent: DMSO

Racemization

Symmetrical salvation is achieved prior to deuteration.

H3C C H

Et

Ph

+ K+ O

R

D

ORD

OR

H3C C

Et

Ph

K+ O

R

D

ORD

O

R

H

H3C C H

Et

Ph

+ K+ O

R

D

ORD

ORH

The most preparative method of organo lithium compounds:

CH3I + 2Li CH3Li + LiI

n-BuBr + 2Li n-BuLi + LiBr

PhBr + 2Li PhLi + LiBr

Although these compounds have some covalent character, but they react as would be expected of the carbanions derived from simple hydrocarbons.

The order of basicity and reactivity in H-abstraction:

CH3Li > n-BuLi > t-BuLi

Deprotonation of Ph-CH3 by t-BuLi is thermodynamically favor, but the reaction is quite slow in hydrocarbon as solvent.

Organo lithiums exist as tetramer, hexamer and higher aggregation in hydrocarbons and other solvents. These species can be studied by low temperature NMR spectroscopy.

[(BuLi)4.(THF)4] + 4 THF 2[(BuLi)2.(THF)4]

major minor

Increasing the reactivity of organo lithiums

R

Li

Li

Li

R

R Li R

+ 4 (CH3)2NCH2CH2N(CH3)2

Li

R

Li

R

NN

N N

CH3

CH3

H3C

H3C

CH3

CH3

H3C

H3C

Tetramethylethylenediamine(TMEDA)

PhLi is tetrameric in 1:2 ether - cyclohexane, but dimeric in 1:9 TMEDA - cyclohexane.

Li

Li

Li

Li

CH2CH3

H3CH2C CH2CH3

H3CH2C

tetrameri structure(distorted cubic)

Li N

N

Li

N

N

CH3

CH3

CH3

CH3H3C

H3C

2,2'Dilithiobiphenyl(complexed with HMEDA)

Li

N N

Li

NN

CC CC PhPh

Lithium phenylacetylide (complexed with a diamine)

7.2 Carbanion Stabilized by Functional Groups

Negative charge delocalization by functional groups to more electronegative element cause stabilization of the carbanion and increases the C-H bond acidity.

Order for anion stabilization:

NO2 > C=O > CO2R ≈ SO2 ≈ CN > CONR2

C NO

O

H

HC N

O

O

H

H

C CO

R

H

HC C

O

R

H

H

C S

O

O

H

HR C S

O

O

H

HR C S

O

O

H

HR

C C NH

HC C N

H

H

Both dipolar and resonance effects are involved:

Enolate Ions

RCCH2R'

O

RC=CHR'

OH

RC=CHR'

O

RCCHR'

O

EnolKeto

Enolate

R2CHCR' + B

O

R2C=CR' + BH

Oslow

R2C=CR' + X2

O

R2CCR' + X

O

X

fast

Measuring the kinetic Acidity of C=O Compounds:

Measuring the rate of halogenation of C=O compounds.

R2CHCR'

O

R2C=CR' + BH

OB

R2C=CR' + S-D

O

R2CCR' + S

O

D

Rate of Deprotonation: Isotope exchange using D or T

Rate of deuteration of simple alkyl ketones:

CH3 > RCH2 > R2CH

Steric hindrance to the approach of the base is probably the major factor.

Structural Effects on The Rate of Deprotonation

Very strong bases such as LDA or HMDS in polar aprotic solvents such sc DME or THF gives solutions of the enolates whose composition reflect the rate of removal of the different protons in the unsymmetrical C=O compounds (kinetic control). The least hindered proton is removed most rapidly under these conditions.

For unsymmetrical ketones the kinetic product is less substituted one.

Thermodynamic Control: Establishing the equilibrium between the various enolates of a ketone and formation the more stable enolate highly substituted.

Ideal conditions for kinetic control of enolates formation are those in which deprotonation is rapid, quantitative, and irreversible.Experimentally:

a) Using very strong base such as LDA

b) Aprotic solvent

c) Absence of excess ketone

d) Low temperature

Kinetic and Thermodynamic Acidity in Nitroalkanes show opposite responses to alkyl substitution.

Umpolung Reactions: Formal reversal of the normal polarity of a functional group.

Conjugate base of 1,3-dithiane (pK=31 in cyclohexyl amine):

S S

H H

+ n-BuLiTHF

S S

H Li

+ n-Bu-H

a) Negative charge delocalization involving 3d orbitals.

b) MOT: Negative charge delocalization involving * orbital of C-S bond.

Carbanion Derived from Sulfoxides

O R'H

R

O R'H

HR

RCHSR'

O

RCH2SR'

O

removed preferentially

Phosphorous and Sulfur Ylides Ylides: Molecules for which one of the contributing structures has opposite charges on adjacent atoms when the atoms have octet of electrons.

R2C PR'3 R2C PR'3

Phosphonium Ylide

R2C SR'2

Sulfoxonium Ylide

O

R2C SR'2

O

R2C SR'2 R2C SR'2

Sulfonium Ylide

Formation of Ylides Deprotonation of onium salts:

Phosphonium Salt Phosphonium Ylide

RCH2 PR'3 RCH PR'3base

R'2SCH2Rbase

CHRR'2S

Sulfonium Salt Sulfonium Ylide

CH2RR'2S

Obase

CHRR'2S

O

Sulfoxonium YlideSulfoxonium Salt

The stability of the resulting species is increased by substituents groups that can help to stabilize the electron-rich carbon. In the absence of any stabilizing group, the onium salts are much less acidic and strong bases such as amide ion is required.

The addition of O atom in the sulfoxonium salts stabilizes these ylides considerably relative to the sulfonium ylides.

RCH2 PR'3strong

baseRCH PR'3

CH2RR'2S

Obase

CHRR'2S

O

Sulfoxonium YlideSulfoxonium Salt

7.3 Enols and EnaminesCarbonyl compounds as nucleophile in acidic media:

Enol form:

RCCH2R + H+

O

RC=CHR + H+

OH

RCCH2R

OH

RC=CHR + E+

OH

RC

OH

CH2R

E

RC

OH

CH2R

E

RC

O

CHR + H+

E

Enols are not as reactive as enolate ions.

Enolization Mechanism: Isotope Exchange

O

+ HAfast

OH

HA

slow

OH

+ HA

Measuring The Rate of Enolization: Halogenation

RCCH2R

O k1HA

k-1

RC=CHR

OH k2X2

fastRC

O

CHR + HX

Xk2 >> k-1, k-1

kH / kD ≈ 5

In contrast with base catalyzed removal of proton, the acid catalyzed enolization to result in preferential formation of the more substituted enol.

The amount of enol present in equilibrium with a C=O group is influenced by other substituents groups. In single ketones, aldehydes, or esters, there is very little of the enol present at equilibrium.

O

CC

C

OH

H

H3C CH3H3C CH3

O O

H H

Effect of Solvent on the Extent of Enol FormEthyl acetoacetate:

H3C OEt

O O

H3C O

O O

Et

O

C C

OH

H3C OEt

SolventEnol form (%)CCl415-20

Acetone5Water1

The strong intramolecular H-bond in the enol form minimize the molecular dipole by reducing the negative charge on the oxygen of the C=O group.

In the more polar solvents is less important, and in protic solvent such as water, H-bonding by the solvent is dominant.

Generation of Enols of Simple Ketones in high Concentration: Metastable species

RCO2C

H

OCH3

OCH=CH2H2O, CH3CN

-20 °CRCOOH + HCOOMe + HO-CH=CH2

NMR: Half life at -20 °C is several hours

Half life at +20 °C is 10 minutes

In DMSO and DMF, in which the rate of exchange by

H-binding is slow, metastable enols have increased lifetime.

Generation of Enols of Simple Ketones in Water:Addition of enolate solution to water: The initial protonation takes place on O atom, generating the enol form. Ketonization rate depends on pH.Acid Catalyzed Ketonization: C-protonation concerted with O-deprotination (General acid catalysis)

C CH

H

O

H

HH2OHA HCCH3 + H3O + A

O

C CH

H

O

H

H+ B C C

H

H

O

H

H2OC CH3 + OH

O

H

Base-catalyzed Ketonization: C-protonation of the enolate

Enols are more acidic than ketones

CH2=CPh

O

Ph CH3

OH

CH2=CPh

O

K=10-10.5

K=10-18.4K=10-7.9

CH2=CCH3

O

H3C CH3

OH

CH2=CCH3

O

K=10-11

K=10-19.2K=10-9.2

Enamines

C CR

R

R2N

RC C

R

R

R2N

Renamine imine

Enamines of 2-Alkylcyclohexanones

N

CH3

N

CH3 H

Strongly Favored

Steric Repulsion

R

H

N

HH

N

H

H

R

H

favored disfavored

Preference for the formation of less substituted isomer.

7.4 Carbanions and Nucleophiles in SN2 ReactionCarbanions are soft nucleophiles.

Evidences for SN2 type mechanism:

Reaction of 2-bromobutane: a) Allyl and benzyl lithium

Complete inversion of configuration

b) BuLi

Racemization

Complicating process: the reaction of organo lithium reagents with alkylating reagents conceivably occur at any of the aggregation stages present in solution.

(RLi)4 4 RLi2(RLi)2

R'X R'XR'X

R-R' R-R'R-R'

Ph-Li + Cl-CH2CH=CH2

Cl

Li

*

**Ph-CH2CH=CH2

Ph-Li + Cl-CH2CH=CH2*

Li

Li

Cl*

*Ph-CH2CH=CH2

Alkylation of Enolate Ions: C-Alkylation vs. O-Alkylation

O O O

Soft electrophiles prefer C-alkylation.

HOMO of enolates have -character: Attack of electrophile approximately perpendicular to the plane of the enolate.

O

X

O

X

O

X

Both tetrameric and dimeric clusters can exist.

Sensitivity of the reaction rate to the degree of aggregation.

a) Addition of HMPA, crown ethers or similar complexing agents:

The rate acceleration of enolate alkylation reaction.b) Use of dipolar aprotic solvents (e.g. DMF, DMSO in place of THF):

The rate acceleration of enolate alkylation reaction.c) Effect of metal cation: Reactivity order

BrMg+ > Li+ > Na+ > K+

According the order of dissociation of ion pair and aggregates.

C- versus O-alkylation

a) Addition of HMPA, crown ethers or similar complexing agents:

Increasing the O-alkylation product.

b) With the soft leaving groups such as Br- and I- C-alkylation is the major pathway.

Steric and Stereo electronic EffectsEnolates that are exocyclic to cyclohexane ring:

X

O-

axial

equatorialPreference for equatorial attack.

t-Bu

OHH

R'X

more favorable

R'X

lessfavorable

O

R

R'

t-Bu

R'

ROt-Bu

Endocyclic cyclohexanone Enolates that are to ring.

The Enolates of 1-Decalone.

H

O-

disfavored

favored

R'X

H R'

O

END OF CHAPTER 7