Carbanions - 11.pdfCarbanions CH 3-, 8-e structure ... Favorskii rearrangement Br O H H H H H H H H 1) OH-, H 2 O 2) Acid H H H H H H H H O OH α-halocarbonyl Reduced ring ... Slide 1 ...

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  • Carbanions

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

    R2

    R1 R3

    R1 R2

    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 -delocalization.

    -substituent interacts better with pure p-orbital of planar structure than with - 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 HH

    H 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

  • 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

  • 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

    -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 C3vCH3 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 -donor (the N-atom of the NH2group) and a -acceptor (the carbonyl group)

    Formation of radicals: homolytic cleavage (thermal or photochemical)

    CH3NNCH3 N2 + 2 CH3hv

    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

    CH2NN 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)

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