Carbanions

CH3-, 8-e structure isoelectronic to NH3 : must be pyramidal

R2

R1 R3

R1R2

R3

The barrier to inversion is only 1-2 kcal/molIt is very fast and simple cabonanions act as planar

If we would like to stabilize the pyramidal structure, the barrier to inversion should be increased. How?

1. More electronegative substituents

2. Sterically. Put the anion in a small ring

sp3- 109.5sp2- 120.0

Ring- 600

sp3 is closer to the ring angle

Stabilization of planar structure by π-delocalization.

π-substituent interacts better with pure p-orbital of planar structure than with σ- out orbital of pyramidal structure

Carboanions below are planar

C

N

H

HC

N

H

H

N+

O

OH

H

N+

O

OH

H

OH

HOH

H

Allyl and benzyl anions are also planar- draw the resonance structures

Allyl anion and its analogs

H

H

H

H

H

H

H

H

H

H

O OH(R)O

+ OH(R)

Resonance in carboxylic acids and esters

H H H HH H

Benzyl anion

Resonance stabilization of carboanions

Which proton will go?

H

HHH

O OH-

H H

HO

H

H

H

O

+ H2O

O

H

H

H

H

C

H

H

HO

C

-O

H

H

H

Stability of carboanions due to substituents at C

30 < 20 <10 < CH3-

Reverse to stability of carbocations

Carboanions are less prone to rearrangements than carbocations unless the resulting structure is more stable

CH2

CH2

Ph

PhCH2

Ph

Ph

Stability of carboanions

30 < 20 <10 < CH3-

Reverse to stability of carbocations

Note that resonance stabilization may inverse the trend in stabilization as shown at the above sample

Reactions involving carboanions: Favorskii rearrangement

Br

O

H

HH

H HH

H

H

H 1) OH-, H2O

2) Acid

HH

H

H

H H

H

H

H

O OH

α-halocarbonyl

Reduced ring

Proposed mechanism:

Br

O

HH

HO O

Br

O

H

HH

H HH

H

HBr-

Oxallyl

OHO OH O

O

OH

H

O

OH

H

acid

OH

O

Radicals

CH3• D3h but energetically very close to C3v

CH3 is only the planar radical- substituents shift to pyramidal form, e.g., CF3 is strongly pyramidal

Pyramidalization and stability

30 > 20 > 10 > CH3

Pyramidalization decreases eclipsing interactions:

H

H

H

HH

H

H 60

Radicals are stabilized by delocalization of unpaired electron

Capto-dative radicals: radical center resides between a donor and an acceptor- exceptional stability

NH

H

H

O

OHNH2

O

OH

H

The unpaired electron resides between a π-donor (the N-atom of the NH2group) and a π-acceptor (the carbonyl group)

Formation of radicals: homolytic cleavage (thermal or photochemical)

CH3N≡NCH3 N2 + 2 CH3•hv

Typical chemistry- chain reactions

Radical cations

Remove electron from the system

Distonic radical cations (RC)- charge and spin are separated

O

H

HH

H-e O

HH

H

H

O+

H

H

H

H

Distonic RC

Distonic captodative RC

NH

H

H

O

OH

H

NH2

O

O

H

H

H

The unpaired electron is in fact delocalized over the main chain

The C-H of radical cations is acidic. The carboxyl group is basic!

Carbenes

py pz

E

E

py

E

E

CH2- 6 e systembent

Closed-shell singlet

In fact, triplet CH2 carbene is 9 kcal/mol lower in energy than singlet

H

H

H

H

TripletHCH=1360

singletHCH=1050

How to stabilize singlet? Increase bending using substituents

CF2 – singlet is 50 kcal/mol lower than triplet

Lone-pair donation (Y=N, O..) stabilizes singlet

Y

H

Y

H

Y

H H

Y

Stable carbenes

N

N+

R

R

Hbase

N

N

R

R

N

N

R

R

Orbital overlap- electronic effectR- bulky groups- steric protection

Good ligands for Ru-based olefin metathesis catalysis

Generation of carbenes: In singlet state

Mostly from diazoalkanes, for example

CH2N≡N CH2 + N2

In triplet state:

Sensitization. Use a photoexcited triplet molecule S

S↑↑ + CH2N2↑↓ S↑↓ + CH2↑↑ + N2↑↓

Reactions of carbens- depend on the electronic state

Singlet carbene has empty orbital as a cation and a lone pair like an anion- combined properties

Triplet has two singly-occupied orbitals and acts more like biradical

Major reaction of carbenes- cycloaddition

H

HR

R'+ CH2

H H

HR H

R'

Singlet adds in one step and streospecifically, i.e., trans- alkene gives ‘trans’-cyclopropane

Triplet cannot add in one step as spin flip is needed

Stepwise mechanism:

HH

H

R H

R'

HH

H

R R'

H

RR'

RR'

Spin-flip closure

Enough time for rotation about a C-C bond

H

H

H

H

CH2 CH2

1,2-hydrogen shift. The barrier is only about 0.6 kcal/mol for carbenes

Singlet Biradicals

Open-shell triplet may be considered as triplet biradical

Singlet biradical has two weakly coupled electrons with opposite spin

Ozone molecule is a singlet biradical in its ground state O

O

O

Many reactions go via a singlet biradical transition state

Those transition states are formed when (a) a single bond is homolitically broken, (b) a double bond is twisted; (c) a triple bond is bent

O O O O

a) b) c)