8 Rearrangements of Carbanions and Free Radicals.

8 Rearrangements of Carbanions and Free Radicals

Carbanion Rearrangements
Sigmatropic Shifts in Hydrocarbon Anions Rearrangements of carbanions are far less common than that of carbocations. 1,2-Shifts of alkyl groups or hydrogen atoms in alkali or alkali earth derivatives of hydrocarbon do not occur. (2) Suprafacial [1,4] migration in allylic carbanions does not seem to occur. (3) Several examples of [1,6] hydrogen migrations in pentadienyl anions showed intramolecular processes instead of intermolecular processes, in which hydrogen shifts occur readily at temperature as low as 35 oC. (4) Thermal rearrangements of pentadienyl anions proceed by antarafacial paths.

Sigmatropic Shifts in Hydrocarbon Anions

Sigmatropic Shifts in Hydrocarbon Anions Rearrangements of carbanions are far less common than that of carbocations. (5) [1,8] Alkyl group migration occurs in the rearrangement of carbanion 3, although the possibility of a two-step elimination-addition mechanism has not been eliminated.

Sigmatropic Shifts in Hydrocarbon Anions Rearrangements of carbanions are far less common than that of carbocations. (6) Migration of allylic group in carbanions proceed by allowed [2,3] shifts

Sigmatropic Shifts in Hydrocarbon Anions However, as the reaction temperature are raised, increasing amount of 1,2-migration products are formed by free-radical dissociation-recombination mechanisms.

Addition-Elimination Mechanisms Migrations of vinyl groups In contrast to alkyl groups, vinyl groups and aromatic rings do undergo 1,2-migration in carbanions. C1 and C2 of 3-butenyl (homoallylic) Grignard reagents slowly interchange positions via initial addition of the “carbanionoid” carbon to the double bond. This rearrangement are slow at room temperature.

Addition-Elimination Mechanisms Migrations of vinyl groups Similar rearrangements of lithium reagents can be quite rapid.

Addition-Elimination Mechanisms Aryl group migrations Phenyl groups do undergo slow 1,2-migrations in lithium reagents, and rapid 1,2-migrations in organosodium, organo potassium, or organocesium reagents provided that at least one aryl group remains at the migration origin to stabilize the resulting anoin.

Addition-Elimination Mechanisms Aryl group migrations Rearrangements proceed by addition-elimination mechanism involving three-membered ring intermediates rather than by [1,2] sigmatropic shifts.

Addition-Elimination Mechanisms Aryl group migrations The cesium reagent 4 reacts with carbon dioxide to yield a cyclopropyl derivative as the principal product.

Addition-Elimination Mechanisms Aryl group migrations The relative migratory aptitudes of different aromatic rings are consistent with migrations in carbanions rather than in free radicals. Phenyl group migrate more rapidly than para-methylphenyl groups

Addition-Elimination Mechanisms Rearrangements via homoenolate anions Similar reactions which the carbanionic sites could add to the carbonyl groups to form cyclopropoxide anions occur much more rapidly in homoenolate anoins.

Rearrangements of a-Hetero Carbanions 1,2- and 1,4-Shifts in Wittig rearrangements 1,2-Shifts of alkyl groups are common when the negatively charged carbons are substituted with oxygen, nitrogen, or sulfur atoms.

Rearrangements of a-Hetero Carbanions 1,2- and 1,4-Shifts in Wittig rearrangements Rearrangements proceed by homolytic dissociation of the carbon oxygen bonds, followed by recombination of the resulting alkyl radicals and ketyl radical anions.

Rearrangements of a-Hetero Carbanions 1,2- and 1,4-Shifts in Wittig rearrangements Evidence 1 for the proposed mechanism 5-Hexenyl radicals are known to cyclize rapidly to cyclopentylmethyl radicals, but no such cyclization occurs with 5-hexenyl carbanions.

Rearrangements of a-Hetero Carbanions 1,2- and 1,4-Shifts in Wittig rearrangements Evidence 2 for the proposed mechanism Alkyl groups from the alkyllithium reagents can be incorporated into the rearrangement products, if excess amounts of alkyllithium reagents are employed. Electron exchange reactions of the lithium reagents with alkyl radicals from The ethers

Rearrangements of a-Hetero Carbanions 1,2- and 1,4-Shifts in Wittig rearrangements In Wittig rearrangements of allylic carbanions, 1,4-shifts can accompany 1,2-migrations 1,4-Wittig migrations proceed by free-radical dissociation-recombination mechanisms

Rearrangements of a-Hetero Carbanions [2,3] Wittig rearrangements Migration of allylic groups in Wittig rearrangements proceed via [2,3] paths and result in inversion of the allylic structures. 1,2-Migrations may also occur at relatively high temperature which would favor homolytic dissociation steps.

Rearrangements of a-Hetero Carbanions [2,3] Wittig rearrangements Since it is often difficult to form the precursors for [2,3] Wittig rearrangements by abstraction of protons from allyl alkyl ethers, some other methods can be used. Electron-transfer mechanism

Rearrangements of a-Hetero Carbanions [2,3] Wittig rearrangements Since it is often difficult to form the precursors for [2,3] Wittig rearrangements by abstraction of protons from allyl alkyl ethers, some other methods can be used.

Rearrangements of a-Hetero Carbanions Use in synthesis If a carbon-oxygen bond to a chiral center is broken in a [2,3] Wittig rearrangement, the new carbon-carbon bond will be formed with nearly complete transfer of chirality.

Rearrangements of a-Hetero Carbanions Use in synthesis The rearrangement proceeds with complete inversion at the anionic carbon. Therefore, [2,3] Wittig rearrangement proceeds by concerted and cyclic mechanism instead of free-radical one.

Rearrangements of Yields Stevens rearrangements Generation of ylides (common)

Rearrangements of Yields Stevens rearrangements Generation of ylides (less common)

Rearrangements of Yields Stevens rearrangements alkyl and benzyl groups in ammonium and sulfonium ylides can migrate from the positively charged nitrogen or sulfur atoms to the adjacent negatively charged carbon atoms. These reactions are often so rapid that the ylides cannot even be detected before they rearrange. Stabilized ylides by carbonyl groups have longer lifetimes and may be isolated as crystalline salts.

Rearrangements of Yields Stevens rearrangements These rearrangements have proven to be very useful methods for the formation of new carbon-carbon bonds.

Rearrangements of Yields Stevens rearrangements Ylides containing allylic anions can undergo 1,4- as well as 1,2-Stevens rearrangements. 1,2- rearrangements often proceed with nearly complete retention of the configurations of chiral migrating groups. However, it was later shown that rearrangements proceed by dissociation-recombination mechanism.

Rearrangements of Yields Stevens rearrangements Rearrangements of sulfonium ylides appear to proceed by free-radical dissociation-recombination processes. They result in significant racemization of chiral migrating groups. Intermolecular cross-products from rearrangements of mixtures of ylides and recombination product of free radicals

Rearrangements of Yields Sommelet-Hauser rearrangements [2,3] Migrations of allylic groups in ylides in preference to Stevens rearrangements +

Rearrangements of Yields Sommelet-Hauser rearrangements These rearrangements take precedence over Stevens rearrangements even when the double bond participating in the reaction is part of an aromatic ring. Proceed by a [2,3] migration +

Rearrangements of Yields Sommelet-Hauser rearrangements In some instances, Stevens rearrangements accompany Sommelet-Hauser rearrangements, but the products from Sommelet-Hauser rearrangements usually predominate. The final tautomerism cannot take place

Rearrangements Resulting from Intramolecular Substitution Reactions Favorskii rearrangements The reactions of a-haloketones or a,b-epoxyketones with hydroxide, alkoxide, or amide anions yield carboxylic acid derivatives resulting from the migration of an alkyl group from the carbonyl group to the a-carbon.

Rearrangements Resulting from Intramolecular Substitution Reactions Favorskii rearrangements Proposed mechanism

Rearrangements Resulting from Intramolecular Substitution Reactions Favorskii rearrangements Bulky substituent Less bulky substituent

Rearrangements Resulting from Intramolecular Substitution Reactions Favorskii rearrangements The reactions of a,a’-dibromoketones with triethylamine yield stable cyclopropenone derivatives.

Rearrangements Resulting from Intramolecular Substitution Reactions Favorskii rearrangements Internal SN2 displacements of halide ions result in inversions of configurations of the halogenated carbons.

Rearrangements Resulting from Intramolecular Substitution Reactions Favorskii rearrangements In the absence of resonance stabilization of one of the two possible anions, the less-substituted alkyl anion is formed.

Rearrangements Resulting from Intramolecular Substitution Reactions Quasi-Favorskii rearrangements Many ketones lacking hydrogens on their unhalogenated a-carbon undergo reactions that yield products of Favorskii rearrangements.

Rearrangements Resulting from Intramolecular Substitution Reactions Quasi-Favorskii rearrangements

Rearrangements Resulting from Intramolecular Substitution Reactions The Ramburg-Backlund reaction An a-halosulfone reacts with base to yield an alkene formed by joining the two alkyl groups of sulfone by a double bond.

Rearrangements Resulting from Intramolecular Substitution Reactions The Ramburg-Backlund reaction Proposed mechanism After the initial formation of a-sulfonyl carbanions, internal nucleophilic displacements of halide anions result in the formation of three-membered sulfone rings and then remove the SO2 to give cycloalkene.

Rearrangements Resulting from Intramolecular Substitution Reactions The Ramburg-Backlund reaction Dihalosulfones can result in the formation of alkynes, vinyl halides, and salts of sulfonic acids.

Rearrangements Resulting from Intramolecular Substitution Reactions The Ramburg-Backlund reaction If tertiary amines are employed as the bases, however, unsaturated cyclic sulfones can be isolated.

Rearrangements Resulting from Intramolecular Substitution Reactions The Neber reaction The reactions of oxime tosylates or of N-chloroimines with strong bases result in migrations of nitrogen atoms to form a-amino ketones.

Rearrangements Resulting from Intramolecular Substitution Reactions The Neber reaction When these reactions are carried out at low temperatures or in the absence of hydroxylic solvents, it is possible to isolate azirines.

8 Rearrangements of Carbanions and Free Radicals.

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8 Rearrangements of Carbanions and Free Radicals.

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