Rearrangements of carbocations and carbanions Flashcards
What is hyperconjugation?
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
How does the stability of carbanion relate to pKa of conjugate acid?
Increasing pKa of conj acid means increased reactivity of the carbanion
What occurs with sp3 hybridised carbanions (isoelectronic to NH3)?
Pyramidial inversion occurs v quickly so chiral carbanions undergo racemisation
Execptions: vinyl and cyclopropyl anions, and lithium halogen exchange
What occurs in Lithium halogen exchange?
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
Why do cyclopropane carbanions not undergo pyramidial inversion?
120º ideal angle not seen, only 60º in cyclopropane
TS would be highly strained so slow rate of inversion
What are the generic reactions which involve carbocations?
Formed via: hydride loss (C-H attacks E+), standard ionisation (C-LG bond breaks), electrophile reaction
Reacts via: with nucleophile, carbocation rearrangement, deprotonation
What are the generic reactions with carbanions?
Formed via: reduction (grignard to remove halogen), deprotonation, electrophillic substiution (C-X breaks), carbanion addition
Reacts via: hydride loss, reaction with electrophile
What is substiution electrophillic unimolecular (SE1)?
What is β-hydride elimination from carbanions?
Common for transition metals (not grignard), decomposition for organolithiums
H-C-C-PdX ←> C=C + HPdX
Opposite reaction is hydromettalion
What is the purpose of Ag(I) in most reactions?
Always attempting to abstract halides and form the insoluble salt
What occurs in a neopentyl system to a carbocation?
Me3C-C+H2 → Me2C+-CH2Me
The Me group can undergo a Wagner-Meerwin rearrangment (1,2 shift)
Why are neopentyl systems unreactive under SN2?
Nucleophile hindered from attacking C atom (by one of the 3xMe) as staggered conformation
What is the Wagner-Meerwein rearrangement?
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
How are orbital overlaps important in the Wagner-Meerwein rearrangment for ring systems?
Need to check the orbitals allow for the σC-C into an empty p-orbital
Look at angles in the ring systems and consider rotation
What occurs in the Pinacol rearrangment?
1-2 methyl shift with diol, can use expoxides and halohydrins
How can spirocyclic ketones be made?
Diol via the pinacol rearrangment
How can starting diols be formed for a Pinacol reaction?
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
What occurs in the semi-pinacol AKA Tiffeneau–Demjanov reaction?
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*
What is the henry reaction?
Nitroalkane + aldehyde/ketone C-C bond forming reaction
Similar to the aldol reaction
What happens in the Tiffeneau-Demjanov reaction?
HNO2 is added to HO-C-C-NH2
Causes N2+ to form
Then lp from O kicks in and forms C-C migration
What is the dienone-phenol rearrangment?
Reverse of pinacol to rearomatise using acid or base and via 1,2 alkyl shift
What is the reaction for ring annulation via the dienone-phenol rearrangement?
Dienone-phenol formation via SN2 then the rearrangment
What is the transition state of a 1,2 shift?
3-centre-2-electron system, formed as C-C σ attacked empty p-orbital
Why can 1,2-carbocation shifts occur but not carbanions and radicals?
Carbocation TS in the 1,2-shift has net bonding - cyclically conj ring so aromatic
Carbanion TS has e- in antibonding, so less favourable
What occurs to the configuration of a migrating centre and terminus (where the centre migrates to)?
Retention of configuration at migrating centre
Inversion (concerted) or racemisation (SN1) at terminus
Define neighborough group participiation
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
What is anchimeric assistance?
Rate of reaction increase due to NGP
What is a carbonium ion?
Non-classical carbocations
E.g. 3-centre-2-electron bond
What is endo-exo isomerism for noroborane or other bridged-ring systems?
Endo - substitutuent located closest to longest bridge
Exo - substituted located furtherst to longest bridge
Why do noroborane-tosylates undergo solvolysis in different rates for different enantiomers?
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
How do non-classical cations lead to a racemate?
They have a plane of symmetry leading to racemic products
How are π and σ bonds as donors for NGP?
π bonds are better donors
Will participate in NGP more readily
E.g. alkenes or phenyl groups (forms phenonium ions)
How do you synthesise adamantane?
Diels-Alder of 2 x cyclopentadiene, then reduction with H2 and Pd, then treatment with lewis acid
What is a transannnular hydride shift?
Ring allows for the transfer of a hydride from one side of a ring to another
An example of this is the cyclodecyl cation
What is the likeliness of rerrangments of carbanions compared to carbocations?
Carbanions less prone to rearrangment than carbocations
What is an example of a carbanion shift and why does it occur?
1,2-aryl shift
E.g. Ph3C-CH2Cl + 2Li
Delocalised so more stable carbanion
What occurs in the Favorskii rearrangement?
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
What is the quasi Favorskii rearrangment?
Favorskii rearrangement on substrates with no enolisable H atoms, using a base catalyst and follows a semi-pinacol mechanism
Why can a concerted 1,2-shift of a carbanion not occur and what occurs instead?
Geometrically impossible - carbanion cannot reach to perform intramoleuclar SN2 with inversion
Instead they occur by a radical mechanism
What is the Ramberg-Backlund reaction?
Cheletropic extrusion of SO2 from a sulfolane
Name some 1,2 shifts of carbanions via radical mechanisms
1,2 Wittig - Methoxy and BuLi
1,2 Stevens rearrangement
What is the 1,2-wittig rearrangment?
1,2-rearrangement of an ether with an alkyllithium compound
What is the Stevens rearrangements?
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
What is the selecitivity of the 1,2-Wittig rearrangment?
Predominately with retention of config in the migrating group
