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Topicity
- 1. C X Y A B Relation between X &Y TOPICITY By Dr. G. Krishnaswamy Faculty DOS & R in Organic Chemistry Tumkur University Tumakuru Bottom face Top face 10/6/2019 1
- 2. Earlier part ofstereochemistry was concentrated on the stereo center. (TOPOs in Greek means place) C X Y A B Stereo center * C X Y A B Relation between X & Y Now we start to see the relationship between the ligands attached to stereo center if the attached ligands are homomorphic in nature. 10/6/2019 2
- 3. Homomorphic Groups /Ligands / Atoms The Groups / ligands / atoms which are in isolation look the same or super imposable mirror images of each other are called homomorphic groups / ligands / atoms. C CH3 CH3 H H Homomorphic groups Homomorphic atoms Homo in greek means same Morph in greek means form 10/6/2019 3
- 4. In case ofatoms, they must be of same element example two H’s or two Br atoms. C CH3 CH3 HH H H Isolated Isolated Identical and super imposable If we isolate the two H’s, then they are same and super imposable to each other hence they are called homomorphic hydrogen's. 10/6/2019 4
- 5. In case ofgroups, they must have same constitution and configuration. Example two methyl or two Ph groups of same chirality R or S. They are called homomorphic groups / ligands / atoms. TOPICITY can be defined as geometrical or sterochemical relationship between homorphic groups / ligands / atoms and structure of the molecule. Different types of relationships are possible for homorphic ligands / groups / atoms. 1. Homo topic (Homo-same; topo-place) 2. Hetero topic (Hetero-different; topo-place) 10/6/2019 5
- 6. Ligands can notby itself be called homotopic or heterotopic, in order to use this terminologies a comparison with other homomorphic ligand or ligands present either in the same molecule (internal comparison) or in a different molecule (external comparison) is necessary. Two criteria are used to decide whether the ligands / groups are equivalent or not 1. Substitution-addition criteria 2. Symmetry criteria Are employed to determine the topic relationship of homomorphic ligands 10/6/2019 6
- 7. 1. (a) Homotopic ligands Two or more ligands that are identical when viewed in isolation but individual replacement of two identical ligands by another give rise to identical molecule, then they are called homotopic ligands. 1. Substitution-addition criteria Two homomorphic ligands are homotopic if substitution (replacement) of first one and other by different test ligand leads to homomers or identical product. 10/6/2019 7
- 8. Br Br Ha Hb Ha D HbD Br Br D H Br Br H D Identical product HOMOTOPIC Hence, Ha & Hb are homotopic atoms 10/6/2019 8
- 9. Ha X Hb X Identical productCC C Hb HaH H C C C H XH H C C C X HH H Hence, Ha & Hb are homotopic atoms 10/6/2019 9
- 10. Hence, Ha, Hb& Hc are homotopic atoms Ha F Hb F Identical products COOH Hc Ha Hb Hc F COOH H F H COOH H H F COOH F H H 10/6/2019 10
- 11. Ha D Identical products COOH Ha OH HbD COOH D OH COOH H OH HbHO COOH HO D COOH HO H COOH Hence, Ha & Hb are homotopic atoms Turn the molecule 180o in plane 10/6/2019 11
- 12. 1. (b) Homotopic faces Two faces of a pi system or a double bond are homotopic if addition to either face gives same or identical product. Bottom face Top face Backface Frontface OR 10/6/2019 12
- 13.
- 14. Identical productsO H3C H3C NaBH4 NaBH4 H H H3C H3C H OH H3C H3C OH H Two products arehomomers. Hence, acetone has homotopic face 10/6/2019 14
- 15. Identical products H H Br2 Br2 H H Br H H Br H H Br H H Br H H Br2 Br2 Hence, ethylene hashomotopic face 10/6/2019 15
- 16. 2. Symmetry criteria 2.(a) Homo topic ligands Two homomorphic ligands are homotopic if they can interchange position by rotation around Cn axis. HbHa H3C CH3 C2 HaHb H3C CH3 180o rotation They are identical and hence homotopic ligands10/6/2019 16
- 17. C H3C CH3 C2 180o rotation C C Ha Hb C H3CCH3 C C Hb Ha They are identical and hence homotopic ligands 10/6/2019 17
- 18. 2. (b) Homotopic faces C2 180o rotationCC H3C H CH3 H CC H3C H CH3 H Two faces of pi system are homotopic if they can interchange face result in same structure by rotation around C2 axis. They are identical and hence it has homotopic face10/6/2019 18
- 19. H H H H O C2 HH H H O 180o rotation They are identical and hence it has homotopic face 10/6/2019 19
- 20. NMR Spectroscopy ofHomotopic Hydrogen If the hydrogen atoms in the molecule are homotopic, then they are chemically equivalent. Hence they will resonate at same chemical shift values. H C H Cl Cl C O CC H H H H H H 10/6/2019 20
- 21. SUMMARY Between homotopic groupsand faces no differentiation is possible either by enzyme or by NMR or by human being because they are homomers or identical. Topicity Substitution- addition criteria Symmetry criteria Reactivity Homotopic groups and faces Homomers / Identical Cn or C2 No differentiation possible 10/6/2019 21
- 22. 2. Hetero topicligands Two or more ligands that are identical when viewed in isolation but individual replacement of two identical ligands by another ligand give rise to two structurally different (isomeric) molecule, then they are called heterotopic ligands. 10/6/2019 22
- 23. Heterotopic ligands Same connectivity Stereochemically heterotopicConstitutionally heterotopic Enantiomers Diastereomers Enantiotopic Diastereotopic YESNO 10/6/2019 23
- 24. Two or moreligands in a molecule that are identical on individual replacement by another ligand give rise to two molecule that constitutional isomers of each other, then the original two ligands are said to be constitutionally heterotopic ligands. Constitutionally Hetero topic ligands 10/6/2019 24
- 25. C C H3C H CH3 COOH C C X H CH3 COOH CC H3C H X COOH CH3 X CH3 X 1 23 1 23 10/6/2019 25
- 26. H F HF NO2 H H NO2 F H NO2 H F10/6/2019 26
- 27. Stereo chemically heterotopic Twoor more ligands in a molecule that are identical on individual replacement by another ligand give rise to two molecule that are enantiomers / super imposable mirror images of each other, then the original two ligands are said to be enantiotopic ligands. Two or more ligands in a molecule that are identical on individual replacement by another ligand give rise to two molecule that are diastereomers / non super imposable not mirror images of each other, then the original two ligands are said to be diastereotopic ligands. 10/6/2019 27
- 28. (a) Enantiotopic ligands 1.Substitution-addition criteria Two homomorphic ligands are enantiotopic if substitution (replacement) of first one and other by different test ligand leads to enantiomers. COOH C CH3 Ha Hb COOH C CH3 Br H COOH C CH3 H Br Ha Br Hb Br 1 2 3 4 1 2 3 4 (S) (R) They are enantiomers and hence enantiotopic ligands 10/6/2019 28
- 29. C CC (R)(S) H Cl Ha Hb Ha ClHb Cl C CC H Cl Cl H CC C H Cl Cl H 1 23 41 23 4 They are enantiomers and hence enantiotopic ligands 10/6/2019 29
- 30. (R) (S) OH O Hb Ha HaD Hb D OH O H D OH O D H They are enantiomers and hence Ha & Hb are enantiotopic ligands10/6/2019 30
- 31. (R)(S) Ha D HbD H3C CH3 Ha H Hb H H3C CH3 D H H H H3C CH3 H H D H They are enantiomers and hence Ha & Hb are enantiotopic ligands10/6/2019 31
- 32. (b) Enantiotopic faces Twofaces of a pi system or a double bond are enantiotopic if addition to either face gives enantiomeric product. (R) (S) O H Ph Top face Bottom face H Ph Et OH H Ph OH Et EtMgBr EtMgBr Addition reaction from either face leads to formation of enantiomers and hence two faces are enantiotopic 10/6/2019 32
- 33. O H Ph Top face Bottom face H Ph H Ph OH CN CN OHHCN HCN (S) (R) Additionreaction from either face leads to formation of enantiomers and hence two faces are enantiotopic 10/6/2019 33
- 34. Molecules having stereoheterotopic ligands (enantiotopic) exhibit prostereoisomerism or prochirality Prochiral molecules are those which are achiral can be converted into chiral molecule in a single step. Prostereoisomerism or Prochirality Prochirality may be the result of substitution reaction of Sp3 carbon substituent (usually hydrogen) with other substituent results in chiral center. OR Prochirality may be the result of addition reaction of an Sp2 carbon to a chiral Sp3 carbon with nucleophile. 10/6/2019 34
- 35. (R) (S) Ha ClHb Cl 12 3 1 2 3 4 C CH3 Hb Ha 4 C CH3 H Cl C CH3 Cl H 4th group on wedge bond hence clock wise "S" configuration PROCHIRAL CHIRAL CHIRAL PROCHIRAL HYDROGENS Prochirality may be the result of substitution reaction of Sp3 carbon substituent. 10/6/2019 35
- 36. Prochirality may bethe result of addition reaction of an Sp2 carbon. (R)(S) NaBH4 1 2 3 1 2 3 4 CH2H3C C H3C 4 View the molecule through C-H bond for assigning the configuration PROCHIRAL CHIRAL CHIRAL O C H OH H3C C H2 C OH H H3C C H2 H3C H3C NaBH4 10/6/2019 36
- 37. 2. Symmetry criteria (a)Enantiotopic ligands Two homomorphic ligands are enantiotopic if they can interchangeable through plane of symmetry or center of inversion or Sn axis. COOH Ha OH Hb OH COOH plane of symmetry 10/6/2019 37
- 38. H3C OH Ha Hb Rotation Reflection HOCH3 Hb Ha H3C OH Hb Ha Cl Cl OH OH Inversion center 10/6/2019 38
- 39. O H Ph Top face Bottom face O Ph H Topface Bottom face (b) Enantiotopic faces Two faces are enantiotopic if they can interchangeable through plane of symmetry or center of inversion or Sn axis. Structure is not same upon rotation hence mirror plane exists. 10/6/2019 39
- 40. O Ph H Top face Bottom face Molecularmirror plane exists 10/6/2019 40
- 41. NMR Spectroscopy ofEnantiotopic Hydrogen If the hydrogen atoms in the molecule are enantiotopic, then they are chemically equivalent. Hence they will resonate at same chemical shift values. 10/6/2019 41
- 42. SUMMARY Between enantiotopic groupsand faces differentiation is possible either by enzyme or by NMR in chiral reagent or catalyst. Topicity Substitution- addition criteria Symmetry criteria Reactivity Enantiotopic groups and faces Enantiomers σh or Sn Differentiatio n possible 10/6/2019 42
- 43. (a) Diastereotopic ligands Substitution-additioncriteria Two homomorphic ligands are diastereotopic if substitution (replacement) of first one and other by different test ligand not already attached to the molecule leads to diastereomers / non super imposable not mirror images. 10/6/2019 43
- 44. Substitution of Ha& Hb by Cl leads to formation of trans and cis products which are diastereomers and hence two hydrogens are diastereotopic H3C C C H Hb Ha H3C C C H H Cl H3C C C H Cl H Ha Cl Hb Cl -CH3 & -Cl are Cis -CH3 & -Cl are Trans 10/6/2019 44
- 45. Ha Cl Hb Cl -Br & -Cl are Trans -Br& -Cl are Cis Br H Ha Hb Br H Cl H Br H H Cl Substitution of Ha & Hb by Cl leads to formation of trans and cis products which are diastereomers and hence two hydrogens are diastereotopic10/6/2019 45
- 46. Geminal methylene protonsadjacent to a stereocenter on substitution test by other ligands not already present in the molecule usually leads to diastereomers and are usually diastereotopic. O Hb Ha Stereo center Adjacent to a stereo center hence they are usually diastereotopic 10/6/2019 46
- 47. Ha Cl Hb Cl O Hb Ha O Cl H O HCl (S) (R) (R) (R) RR SS RS SR Enantiomers Enantiomers Diastereomers Diastereomers Diastereomers Substitution of Ha & Hb by Cl leads to formation of diastereomers and hence two hydrogens are diastereotopic 10/6/2019 47
- 48. Ha D Hb D (S) (R) (R) (R) CH3 H Cl HbHa Cl CH3 H Cl D H Cl CH3 H Cl H D Cl (R) Substitution of Ha & Hb by D leads to formation of diastereomers and hence two hydrogens are diastereotopic 10/6/2019 48
- 49. (b) Diastereotopic faces Twofaces of a carbonyl group adjacent to a stereo center on addition reaction leads to diastereomers and possess diastereotopic face. Stereo center CH3 O H3C H C6H5 Two faces of a carbonyl group adjacent to a stereo center 10/6/2019 49
- 50. HCNHCN (S) (R) (R) CH3 O H3C H C6H5 Topface Bottom face (R) CH3 OH H3C H C6H5 (R) CH3 CN H3C H C6H5 NC HO Additon of HCN two face of carbonyl adjcent to stereo center leads to formation of diastereomers and hence two faces are diastereotopic. 10/6/2019 50
- 51. NMR Spectroscopy ofDiastereotopic Hydrogen If the hydrogen atoms in the molecule are diastereotopic, then they are chemically and magnetically non equivalent. Hence they will resonate at different chemical shift values. H H HO H CH3 Diastereotopic hydrogens Due to non equivalent nature of protons it splits into multiplet. 10/6/2019 51
- 52. SUMMARY Between diastereotopic groupsand faces differentiation is possible either by enzyme or by reagent or by NMR. Topicity Substitution- addition criteria Symmetry criteria Reactivity Diastereotopic groups and faces Diastereomers Not applicable Differentiation possible 10/6/2019 52
- 53. EXAMPLES H Br Ha Hb H Br D H H Br H D trans Cis Substitution of Ha& Hb by D leads to formation of diastereomers and hence two hydrogens are diastereotopic. 10/6/2019 53
- 54. Ha Hb D H H D Substitution of Ha& Hb by D leads to formation of homomers and hence two hydrogens are homotopic. MeO OMe H H MeO OMe D H MeO OMe H D Substitution of H & H by D leads to formation of homomers and hence two hydrogens are homotopic. 10/6/2019 54
- 55. Substitution of H& H by D leads to formation of homomers and hence two hydrogens are homotopic. Cl Cl H H Cl Cl D H Cl Cl H D R R R R R R O O H Ph CH3 H3C Ph H O O H Ph CH3 H3C Ph H C2 H & H are interchangeable by C2 rotation and hence two hydrogens are homotopic. 10/6/2019 55
- 56. H H D H H D Substitution of H& H by D leads to formation of homomers and hence two hydrogens are homotopic. H H D H H D Substitution of H & H by D leads to formation of enantiomers and hence two hydrogens are enantiotopic. 10/6/2019 56
- 57. H H Cl D H Cl HD Cl Substitution of H & H by D leads to formation of enantiomers and hence two hydrogens are enantiotopic. H H D H H D Exo Endo Substitution of H & H by D leads to formation of diastereomers and hence two hydrogens are diastereotopic. 10/6/2019 57
- 58. H H D H H D Substitution of H& H by D leads to formation of diastereomers and hence two hydrogens are diastereotopic. 10/6/2019 58