Stereochemistry & Asymmetric Synthesis.pptx

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1. Stereochemistry & Asymmetric Synthesis SUBMITTED TO :- DR VIRENDRA KAUR SUBMITTED BY :- SHWETA PANDEY CLASS :- M. PHARMA 2 ST SEM. BRANCH :- PHARMACEUTICAL CHEMISTRY SUBJECT :-ADVANCED ORGANIC CHEMISTRY - II SUBJECT CODE :- MPC 202 T

2. Content Of Presentation Basic concepts in stereochemistry – Optical Activity, Specific Rotation, The Cahn Ingold Prelog (CIP) Sequence Rule, Meso Compounds, Pseudo Asymmetric Centres , Axes Of Symmetry, Fischers D And L Notation, Cis -trans Isomerism, E And Z Notation.

3. Content Of Presentation Methods of asymmetric synthesis using chiral pool, chiral auxiliaries and catalytic asymmetric synthesis, enantiopure separation and Stereo selective synthesis with examples.

4. Stereochemistry

5. Introduction Stereochemistry is the study of the three-dimensional structure of molecules. It also define as the branch of chemistry that concerned with the three-dimensional arrangement of atoms and molecules and the effect of this on chemical reactions.

6. Introduction Starch and cellulose are two polymers that belong to the family of biomolecules called carbohydrates. Cellulose and starch are both composed of the same repeating unit . Starch and cellulose are isomers because they are different compounds with the same molecular formula(C 6 H 10 O 5 ) n They are stereoisomers because only the three-dimensional arrangement of atoms is different.

7. In cellulose, the O atom joins two rings using two equatorial bonds . In starch, the O atom joins two rings using one equatorial and one axial bond.

8. DARVON (painkiller) NOVRAD (anticough agent)

9. History Of Stereochemistry Historical perspective Christiaan Huygens(1629-1695). Dutch astronomer, mathematician, and physicist. He discovered plane polarized light

10. Plane-polarized Light

11. Plane-polarized Light Through Solution Of A Compound X

12. Plane-polarized Light Through Certain Compounds

13. Historical Perspective Jean Baptiste Biot (1774-1862)In 1815, he noted that certain natural organic compounds (liquids or solutions) rotate plane polarized light ( Optical Activity ).

14. Tartaric Acid (Racemic Acid) Obtained From Grape Juice Fermentation) Louis Pasteur (1822-1895)In 1847, he repeated earlier work on Racemic Acid. Crystallization of sodium ammonium salt gives mirror image crystals that he separated by hand. Equimolar solutions of separated crystals have equal but opposite optical activity:

15. Cont…. In 1874, they proposed Carbon with 4 attachments is Tetrahedral . A molecule having a tetrahedral carbon with 4 different attachments may exist as a pair of isomers.

16. The Two Major Classes Of Isomers Constitutional Isomers Differ in the way the atoms are connected to each other Have different IUPAC names Have same or different functional groups Have different physical properties , so they are separable by physical techniques such as distillation Have different chemical properties.

17. Structural Isomerism Ex….

18. The Two Major Classes Of Isomers Stereoisomers Differ only in the way atoms are oriented in space Stereoisomers have identical IUPAC names (except for a prefix like cis or trans), same physical properties . Because they differ only in the 3D arrangement of atoms, stereoisomers always have the same functional group(s).

19. Stereoisomers Ex…

20. Configuration And Conformation Structures that can be interconverted simply by rotation about single bonds are conformations of the same molecule , Structures that can be interconverted only by breaking one or more bonds have different configurations , and are stereoisomers

21. Configuration And Conformation

22. Geometrical Isomerism – GI These isomers result from restricted rotation. Restricted rotation can be caused by either a cyclic structure or a double bond .

23. Geometrical Isomerism – GI Conditions of geometrical isomerism (1) Geometrical isomerism arises due to the presence of a double bond or a ring structure C=N Nitrene 1 methyl cyclobutane

24. Geometrical Isomerism – GI (2) Different groups should be attached at each doubly bonded atom.

25. Condition For GI Case-1 Along C = C bond

26. Condition For GI Case-2 Along C=N- bond

27. Condition For GI Case-3 Along -N=N- bond

28. Condition For GI Case-4 Along a bond of cycloalkane

29. Condition For GI Case-5 Along C=C in ring structures

30. Cis -Trans Isomerism Cis /trans isomerism (also known as geometric isomerism, configuration isomerism) is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted.

31. Cis And Trans Isomers Cis and trans isomers can be separated from each other because they are different compounds with different physical properties for example, they have different boiling points and different dipole moments

32. Requirements For Cis-trans Isomerism 1. There should be a double bond in the molecule .2. The two atoms or groups attached to each doubly bonded carbon atom should be different.

33. Cis And Trans Isomers If the two similar groups are on same side of restricted bond the configuration is cis otherwise trans Which isomer is cis and which is trans?

34. The Cahn Ingold Prelog (CIP) Sequence Rule Absolute configuration refers to the spatial arrangement of atoms within a chiral molecular entity (or group) and its resultant stereochemical description. it was the method given by the Cohn – Ingold- Prelog hence it is known as CIP nomenclature  The purpose of the CIP system is to assign an  R   or   S  descriptor   

35. (CIP) Sequence Rule If the path traced from 1-2-3 clockwise = "R" enantiomer counter clockwise = "S" enantiomer

36. (CIP) Sequence Rule Rule I The group with the first atom having higher atomic number is senior. According to this rule the seniority of atoms is: I > Br > Cl > S > F > 0 > N > C > H Rule II The higher mass isotope is senior. (a) T>-D>-H (b) -C 14 H3 > - C 13 H3 Rule III If the first atom of group is identical then second atom is observed for seniority. CH₃Cl > CH₃OH > CH3NH2

37. (CIP) Sequence Rule Rule IV To assign a priority to an atom that is part of a multiple bond, treat a multiply bonded atom as an equivalent number of singly bonded atoms.

38. Cont…. When the #4 priority is on a wedge you can just reverse the rules. So now we have two sets of rules : If the #4 priority is on a dash : Clockwise = R Counterclockwise = S If the #4 priority is on a wedge, reverse the typical rules : Clockwise = S Counterclockwise = R

39. E Z-naming In G.I. If the two senior groups are on same side of restricted bond the configuration is Z (Z = zusammen = together) otherwise E (E= entgegen opposite). The E isomer has the two higher priority groups on the opposite sides. The Z isomer has the two higher priority groups on the same side.

40. E Z-naming In G.I.

41. E Z-naming In G.I.

42. Physical Properties Of Geometrical Isomers

43. Optical Isomers Ordinary light is an electromagnetic wave , which has oscillation in all the directions perpendicular to the path of propagation. When ordinary light is passed through Nicol prism it has all its oscillations in the same plane and is called plane- polarised light

44. Optical Activity & Plane- polarised Light

45. Optical compounds Certain compounds rotate the plane of polarised light in a characteristic way when it is passed through their solutions. These compounds are referred to as optically active compounds .

46. Rotation Dextrorotatory compounds If the substance rotates plane- polarised towards the right i.e. in clockwise direction it is called dextrorotatory & indicated by 'd ' or (+). Laevorotatory compounds If light is rotated towards left i.e. in anticlockwise direction the substance is said to be laevorotatory and indicated by 'l ‘ or (-).

47. Specific Rotation Specific rotation is the number of degrees of rotation observed if a 1-dm (10-cm) tube is used and the compound has concentration 1 gm/ml [ α ] = Specific rotation θ =observed rotation (degree) l = Pathlength (dm) C=concentration (gm/ml) λ =wavelength (nm) t=temperature (25°C) or

48. Ex The observed rotation of 10.0g of (R)-2-methyl-1-butnaol in 50mL of solution in a 20-cm polarimeter tube is +2.3° at 20 °C, what is the specific rotation of the compound? Answer l= 20-cm ( 1-dm (10-cm) = 2 ‘ α ’= +2.3° C= m/V, m is the mass of the solute dissolved, V is the total volume of the solution. C=10/50=0.2

49. Cause Of Optical Activity Asymmetry of the molecule is responsible for optical activity in organic compounds For any active molecule should not be symmetry

50. Axes Of Symmetry Plane of Symmetry: (Mirror Plan) Centre of symmetry: (Inversion Centre) Alternating Axis of Symmetry (Improper rotation axis)

51. Plane Of Symmetry: (Mirror Plan) A plane of symmetry is a imaginary plane that passes through a molecule such that atoms or groups of atoms on one side of plane form a mirror image of those on the other side. It is particularly useful if the molecule contains two or more chiral centres .

52. Centre Of Symmetry: (Inversion Centre) A centre of symmetry is defined as a point in a molecule from which lines when drawn on opposite sides at equal distance, meet exactly similar points (groups or atoms) in the molecule.

53. Alternating Axis Of Symmetry (Improper Rotation Axis) A molecule is said to possess an n-fold alternating axis of symmetry if, on rotating through an angle of 360° about this axis and followed by a reflection of the resulting molecule in a plane perpendicular to the axis, then the mirror image is exactly identical to the original molecule.

54. Asymmetric And Dissymmetric Compounds A molecule which does not possess any element of symmetry (there are all 23 elements of symmetry) is called asymmetric. A molecule which does not possess plane of symmetry and centre of symmetry is called dissymmetric.

55. Chirality & Enantiomers A chiral object is one that can't be superimposed on its mirror image An achiral object is one that is superimposable on its mirror image

56. Chirality & Enantiomers Nonsuperimposable mirror-image molecules are called enantiomers . A chiral compound always has an enantiomer (a non-superimposable mirror image). Enantiomers always exist in pairs and are optically active . One will be dextrorotatory (+) & other laevorotatory (-) .

57. Meso Compounds, Meso Compounds having atleast two chiral carbons and have plane of symmetry or center of symmetry or both is called meso compounds. Rotation of first half is cancelled by rotation due to second half within the molecule, so it is optically inactive due to internal compensation

58. Plane of symmetry

59. Criteria For Meso Compund They are achiral (optical rotation = 0) They have [a] = 0 due to internal compensation of optical rotation Presence of more than one asymmetric 'C' atoms, no chiral center no meso . They are non resolvable (optically inactive compounds are non- resolvable).

60. Internal Compensation In meso tartaric acid the inactivity is due to effects within the molecule and not external . The amount of rotation due to first half of the molecule is cancelled by the opposite and equal The optical inactivity so produced is said to be due to internal compensation

61. Pseudo Chiral Carbon/Pseudo Asymmetric Centre The carbon atom which is chiral just because of difference in configuration between two identical groups attached to carbon is called PSEUDO CHIRAL CARBON. A pseudo-asymmetric centre is found in a meso molecule where a plane of symmetry runs through a stereogenic centre Pseudo assymetric center is the carbon that show chirality and achirality simultaneously. The r/s (not R/S) nomenclature

62. pseudo-asymmetric centre

63. D & L Nomenclature Used for Carbohydrates & amino acids. Glyceraldehyde is taken as reference unit . If OH is on right hand side D If OH is on Left hand side L

64. D & L Nomenclature Used for Carbohydrates & amino acids . Serine is taken as reference unit . If NH2 is on right hand side D If NH2 is on Left hand side L

65. D-glucose Enantiomers L-glucose Naturally occuring Carbohydrate found in D-form , whereas the naturally occuring amino acids found in L-form .