Rearrangements of carbocations and carbanions

Question 1

What is hyperconjugation?

Answer 1

Donation of C-H σ bond or C-C σ bond electrons into an empty p-orbital

Energy of bonding e- reduced and p-orbital raised

Question 2

How does the stability of carbanion relate to pKa of conjugate acid?

Answer 2

Increasing pKa of conj acid means increased reactivity of the carbanion

Question 3

What occurs with sp³ hybridised carbanions (isoelectronic to NH₃)?

Answer 3

Pyramidial inversion occurs v quickly so chiral carbanions undergo racemisation

Execptions: vinyl and cyclopropyl anions, and lithium halogen exchange

Question 4

What occurs in Lithium halogen exchange?

Answer 4

Alkenyl iodides/bromides and Li-tBu

Li replaces the halogen bonded to alkene C, C-Li bond undergoes SN2 with bromoalkane

Stereospecific: E -> E, Z -> Z

Question 5

Why do cyclopropane carbanions not undergo pyramidial inversion?

Answer 5

120º ideal angle not seen, only 60º in cyclopropane

TS would be highly strained so slow rate of inversion

Question 6

What are the generic reactions which involve carbocations?

Answer 6

Formed via: hydride loss (C-H attacks E+), standard ionisation (C-LG bond breaks), electrophile reaction

Reacts via: with nucleophile, carbocation rearrangement, deprotonation

Question 7

What are the generic reactions with carbanions?

Answer 7

Formed via: reduction (grignard to remove halogen), deprotonation, electrophillic substiution (C-X breaks), carbanion addition

Reacts via: hydride loss, reaction with electrophile

Question 8

What is substiution electrophillic unimolecular (S_E1)?

Answer 8

Question 9

What is β-hydride elimination from carbanions?

Answer 9

Common for transition metals (not grignard), decomposition for organolithiums

H-C-C-PdX ←> C=C + HPdX

Opposite reaction is hydromettalion

Question 10

What is the purpose of Ag(I) in most reactions?

Answer 10

Always attempting to abstract halides and form the insoluble salt

Question 11

What occurs in a neopentyl system to a carbocation?

Answer 11

Me₃C-C⁺H₂ → Me₂C⁺-CH₂Me

The Me group can undergo a Wagner-Meerwin rearrangment (1,2 shift)

Question 12

Why are neopentyl systems unreactive under SN2?

Answer 12

Nucleophile hindered from attacking C atom (by one of the 3xMe) as staggered conformation

Question 13

What is the Wagner-Meerwein rearrangement?

Answer 13

1,2 shift of a Me group to the site of a carbocation

Occurs to form a more stable carbocation

Best orbital overlap important in determining migrating group

Question 14

How are orbital overlaps important in the Wagner-Meerwein rearrangment for ring systems?

Answer 14

Need to check the orbitals allow for the σ_C-C into an empty p-orbital

Look at angles in the ring systems and consider rotation

Question 15

What occurs in the Pinacol rearrangment?

Answer 15

1-2 methyl shift with diol, can use expoxides and halohydrins

Question 16

How can spirocyclic ketones be made?

Answer 16

Diol via the pinacol rearrangment

Question 17

How can starting diols be formed for a Pinacol reaction?

Answer 17

Use a pinacol reaction - use a carbonyl and Mg via a single electron transfer process

Mg donates 2e-, one to each C of two reagent molecules

Two radicals react with eachother as Mg2+ coordinates

Question 18

What occurs in the semi-pinacol AKA Tiffeneau–Demjanov reaction?

Answer 18

1,2 shift to a C⁺ not using a diol

Can include C-C migration (ring expansion or contraction), hydride shift, and epoxide formation

Antiperiplanar required as gives best overlap of σ and σ*

Question 19

What is the henry reaction?

Answer 19

Nitroalkane + aldehyde/ketone C-C bond forming reaction

Similar to the aldol reaction

Question 20

What happens in the Tiffeneau-Demjanov reaction?

Answer 20

HNO₂ is added to HO-C-C-NH₂

Causes N₂⁺ to form

Then lp from O kicks in and forms C-C migration

Question 21

What is the dienone-phenol rearrangment?

Answer 21

Reverse of pinacol to rearomatise using acid or base and via 1,2 alkyl shift

Question 22

What is the reaction for ring annulation via the dienone-phenol rearrangement?

Answer 22

Dienone-phenol formation via SN2 then the rearrangment

Question 23

What is the transition state of a 1,2 shift?

Answer 23

3-centre-2-electron system, formed as C-C σ attacked empty p-orbital

Question 24

Why can 1,2-carbocation shifts occur but not carbanions and radicals?

Answer 24

Carbocation TS in the 1,2-shift has net bonding - cyclically conj ring so aromatic

Carbanion TS has e- in antibonding, so less favourable

Question 25

What occurs to the configuration of a migrating centre and terminus (where the centre migrates to)?

Answer 25

Retention of configuration at migrating centre

Inversion (concerted) or racemisation (SN1) at terminus

Question 26

Define neighborough group participiation

Answer 26

Direct interaction of the reaction centre with e- contained in the parent molecule but not conjugated with reaction centre

Examples of NG : lone pair, π bond, σ bond

Usually an incipient carbenium centre

Question 27

What is anchimeric assistance?

Answer 27

Rate of reaction increase due to NGP

Question 28

What is a carbonium ion?

Answer 28

Non-classical carbocations

E.g. 3-centre-2-electron bond

Question 29

What is endo-exo isomerism for noroborane or other bridged-ring systems?

Answer 29

Endo - substitutuent located closest to longest bridge

Exo - substituted located furtherst to longest bridge

Question 30

Why do noroborane-tosylates undergo solvolysis in different rates for different enantiomers?

Answer 30

Exo-Ts reacts faster as NGP of antiperiplanar C-C σ bond to form non-classical carbocation

endo-Ts ionises slower (no NGP) to form classical carbocation, followed by non-classical formation

Question 31

How do non-classical cations lead to a racemate?

Answer 31

They have a plane of symmetry leading to racemic products

Question 32

How are π and σ bonds as donors for NGP?

Answer 32

π bonds are better donors

Will participate in NGP more readily

E.g. alkenes or phenyl groups (forms phenonium ions)

Question 33

How do you synthesise adamantane?

Answer 33

Diels-Alder of 2 x cyclopentadiene, then reduction with H₂ and Pd, then treatment with lewis acid

Question 34

What is a transannnular hydride shift?

Answer 34

Ring allows for the transfer of a hydride from one side of a ring to another

An example of this is the cyclodecyl cation

Question 35

What is the likeliness of rerrangments of carbanions compared to carbocations?

Answer 35

Carbanions less prone to rearrangment than carbocations

Question 36

What is an example of a carbanion shift and why does it occur?

Answer 36

1,2-aryl shift

E.g. Ph₃C-CH₂Cl + 2Li

Delocalised so more stable carbanion

Question 37

What occurs in the Favorskii rearrangement?

Answer 37

Rearrangement of an alkyl group from one side of the carbonyl to another, replacing a halogen on the other alpha carbon with R

Using a base catalyst

Question 38

What is the quasi Favorskii rearrangment?

Answer 38

Favorskii rearrangement on substrates with no enolisable H atoms, using a base catalyst and follows a semi-pinacol mechanism

Question 39

Why can a concerted 1,2-shift of a carbanion not occur and what occurs instead?

Answer 39

Geometrically impossible - carbanion cannot reach to perform intramoleuclar SN2 with inversion

Instead they occur by a radical mechanism

Question 40

What is the Ramberg-Backlund reaction?

Answer 40

Cheletropic extrusion of SO₂ from a sulfolane

Question 41

Name some 1,2 shifts of carbanions via radical mechanisms

Answer 41

1,2 Wittig - Methoxy and BuLi

1,2 Stevens rearrangement

Question 42

What is the 1,2-wittig rearrangment?

Answer 42

1,2-rearrangement of an ether with an alkyllithium compound

Question 43

What is the Stevens rearrangements?

Answer 43

Converting quaternary ammonium salts and sulfonium salts to the corresponding amines or sulfides in presence of a strong base

Via 1,2-shift in a radical mechanism

Question 44

What is the selecitivity of the 1,2-Wittig rearrangment?

Answer 44

Predominately with retention of config in the migrating group