Inter conversion of cis trans isomers

Inter conversion of cis trans isomers

Inter conversion of cis trans isomers

  1. a)Thermal, b) catalytic or c) photochemical means.
  2. Directed fashion (valuable synthetically in cases where     the most readily accessible isomer is not the one wanted as a synthetic target),

a) Thermal equilibration not easy to overcome rotation barrier, normally gives mixture of isomers. In some cases one isomer predominates over the other at equilibrium e.g. stilbene C6H5CH = CHC6H5 ( cis-trans) due to low lying resonance delocalized triplet  biradical.Inter conversion of cis trans isomers

  • formed by intersystem crossing; push-pull ethylenes

( where extensive decocalization possible) e.g below virtually almost no  C = C double bond , hence single

bond rotation , then isomerisationInter conversion of cis trans isomers

e.g. Maleic and fumaric acid conversion at high temp.

Inter conversion of cis trans isomers

Another example of thermal isomerization is the following

Inter conversion of cis trans isomers

15,15 – cis-β- Carotene to all-trans isomer

b) Catalyzed equilibration:Catalysts are free radicals and radical generators  e.g. oxides of nitrogen,halogen in presence of light  that is not  absorbed by double bond i.e., not photochemical isomerisation, iodine , diphenyldisulfide  etc.Also acids e.g. BF3,H2SO4, alkali metals general mechanism is shown

Inter conversion of cis trans isomers
Cis-trans isomerization (A* represents a cation, radical , or anion)

c) Photochemical Isomerization

The cis-trans  isomerization of alkenes occurs under  irradiation with light that they absorb. Isomerization takes place through a singlet excited state. Isomerization can also be effected via triplet state. Triplet  excited state is attained either by intersystem crossing or by using a photo sensitizer that can transfer triplet energy to the compd to be isomerized. A  typical sensitizer is  C6H5COC6H5 which by  irradiation , is excited from ground state  Bo to  a long –lived singlet Bs* which readily undergoes  intersystem crossing to lower lying  triplet  Bt* this triplet can be transferred to the molecule  to be isomerized.

Inter conversion of cis trans isomers

Directed cis-trans Interconversion

A directed interconversion can be effected in Principle by

  1. An anti addition followed by syn elimination
  2. Syn addition followed by anti elimination
  3. Syn elimination (to an alkyne) followed by anti addition
  4. Anti elimination followed by syn addition

When elements added are different from elements eliminated then anti-anti e.g. Addition of  Cl2 anti; elimination of HCl anti. At the final stage reduction of chloroalkene to an alkene with retention of configuration

Inter conversion of cis trans isomers

Conversion of trans to cis alkene: erythro means same group in the same side

  • A sequence of anti addition followed by apparent syn elimination ( direct anti elimination of BrCl by iodide is so slow that SN2 displacement of Br by I ( with inversion of  configuration  takes place)  subsequent anti elimination of ICl leads  to cis alkene in high purity;  i.e., SN2 inversion plus anti elimination is equivalent to syn elimination

[R’= (CH3)2CH(CH2)4-   ;   R= -(CH2)9CH3]

Inter conversion of cis trans isomers

              [R’= (CH3)2CH(CH2)4-   ;   R= -(CH2)9CH3]

-An attractive cis-trans interconversion  sequence would involve conversion of the alkene into a 1,2-diol  by syn addition followed by an anti elimination – [Hydroxylation of alkenes]

Inter conversion of cis trans isomers

A = KMnO4 or OsO4 or CH3CO2Ag, I2, CH3CO2H-H2O

B = RCO3H followed by hydrolysis or CH3CO2Ag, I2 (dry) or epoxidation followed by hydrolysis

Syn Elimination:   Sequence of diol

Inter conversion of cis trans isomers

Next Post Topics: Determination of configuration of cis-trans Isomers

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