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IMPORTANCE OF STEREOCHEMISTRY
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Louis Pasteur In 1848, Louis Pasteur
noticed that a salt of racemic (±)-tartaric acid crystallizes into mirror-image crystals.
Using a microscope and a pair of tweezers, he physically separated the enantiomeric crystals.
Pasteur had accomplished the first artificial resolution of enantiomers.
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Types of stereoisomerism
Atropisomerism Cis-trans isomerism Conformational isomerism Diastereomers Enantiomers Rotamers
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INTRODUCTION RESTRICTED ROTATION GIVING RISE TO
PERPENDICULAR DISYMMETRIC PLANES :
A molecule is chiral if it is not superimposable on its mirrorimage. So far, we believe that a chiral atom is must for thechirality of a molecule. Some compounds are opticallyactive(chiral), eventhough they do not have chiral atoms.The molecule as a whole is chiral in such structures becausethey possess two perpendicular disymmetric planes, e.g.
a
b
ab
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Difference between asymmetry and dissymmetry A dissymmeric molecule may have one axis or more
axes of symmetry (Cn). They lack i, or Sn symmetryelement. On the other hand the chiral or dissymmetricmolecules which lack even Cn axis is termed asasymmetric.
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INTRODUCTION
Neither of the plane can be dissected by a plane ofsymmetry. Therefore, the molecule is notsuperimposable and hence it is optically active. Theexamples of optical isomerism without a chiral centeris given : BIPHENYLS, ALLENES AND SPIRANS. Therequired condition is that the molecule should nothave of either , i or Sn symmetry element to bepresent.
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BIPHENYL ISOMERISM In 1907, Kaufler proposed the “BUTTERFLY” formula
(I) for biphenyl compounds. Later, this argument wasfound to be erroneous. Schultz prepared an acid, forwhich Schmidt et. al. gave the name 6,6’-dinitrodiphenic acid (II). They continued with the samestructure proposed by Kaufler. In 1921, Kenner et. al.prepared 6,6’-dinitrodiphenic acid, for which heproposed co-axial structure (III). If it has butterflyformula, µ ≠ 0 for 4,4’-dinitrodiphenic acid. That ismeasured actually zero which is only possible if it hasco-axial structure.
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Introduction of Biphenyls
NO2
NO2
COOHCOOH
COOH
NO2
O2N
HOOC
(I) (II) (III)
ButterflyKaufler
ButterflyScultz et al
Co-axialKenner
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1. Merer and Watson, J. Mol. Spec. 47, 499 (1973)
z
x
y
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Biphenyls
Acid II & III have different melting points andKenner claimed that these two acids weredistereoisomers. Further, III was resolved becauseof the absence of the any kind of symmetry. It waspointed out that the optical activity is due to theco-axial structure (III). Here, two benzene ringsdo not lie in one plane and rotation about thesingle bond is restricted.
If the biphenyl derivatives have atleast THREEortho-positions, occupied by certain functionalgroups, then such compounds can be resolved. Thetwo conditions for the optical activity in biphenylcompounds have been given :
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Biphenyls (1) Absence of a vertical plane of symmetry : Although, the large groups are present in
ortho-positions, the compound may not be opticallyactive. The important condition is the absence of theplane of symmetry, Consider the following example,which is nonresolvable and inactive because both the ringshave a vertical plane of symmetry. Thus, the plane ofsymmetry is present. e.g.
NO2
NO2
HO2C
HO2C
A
A B
B
vertical planeof symmery
e.g.
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Biphenyls
But 6,6’-dinitrodiphenic acid can exist as nonsuperimpsable mirror images and are resolvablebecause of the absence of the plane of symmetry.
NO2
CO2H
O2N
HO2C
A
B B
A e.g.
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Biohenyl
Nonsuperimposable, optically active
NO2
CO2H
O2N
HO2C
O2N
HO2C
NO2
CO2H
NO2
CO2H
O2N
HO2C
O2N
HO2C
NO2
CO2H
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(2) The substituents in ortho- positions should have alarge size :
When bulky groups are present at ortho- positions inbiphenyl system, the molecule can not remain in plannerform because of the steric hindrance. The steric effectprevents the free rotation about the C-C single bondjoining two benzene rings in co-axial manner.
In the following example, the free rotation isrestricted due to the presence of bulky groups and hencethe molecule is not planner. As a result, the compound isnot superimposable and they are enantiomers withouthaving a chiral centre. The molecule as a whole is chiral dueto restricted rotation, e.g.
A
B
C C A
B
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Optical isomerism in 6,6’-dinitrodiphenic
HO2CCO2H
C O 2 H
C O 2 H
I I
The diphenic acid is not optically active. It has a plane of symmetry.
It has a centre of symmetry. It cannot be resolved.
optically active
NO2
CO2H
O2N
HO2C
NO2 O2N
HO2CCO2H
mirror
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6-Nitro diphenic acid with following structure isoptically active.
Sometimes, only two large substituents in o-ando’- positions are sufficient to produce the restrictedrotation. For example, biphenyl-2,2’-disulphonicacid was resolved by Lesslie and Turner, e.g.
Here, sulphonic acid group is large enough tocollide with H-atoms at ortho positions. Themolecule is racemised on heating.
HO2CCO2H
NO2
SO3H
SO3H
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3D Structure of biphenyl
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Biphenyls Lesslie et. al.(1962) prepared enantiomers of 2,2’-di-t-
butylbiphenyl, which was highly optically stable. Thestability was due to bulky t- butyl group.
CMe3
CMe3
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Biphenyls Another example of only two functionality is 3’-
bromobiphenyl-2-trimethylarsonium iodide. Here, thebulky trimethylarsonium group is too large andcapable of restricted rotation. Moreover , bromine atm-position gives asymmetry to ring. It is opticallyactive but easily racimized.
Br
As(CH3)3I-+
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Atropisomerism In biphenyl, due to steric hindrance, the two isomers
are separable due to restricted rotation about singlebond. Such isomers are called “atropisomers” and theproperty is call “atropisomerism”.
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:Other examples Atropisomerism in aryl amides Rotation is restricted about single bond where X=H
and Y= C, N, O, Cl
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Obstacle theoryEvidence for obstacle theory :
Evidence for obstacle theory provides theinformation about the interference of groups andits non co-planner in nature.
A chemical proof for non co-planner naturewas given by Meisenheimer et al. The “obstaclegroups” in optically active biphenyls were unitedto give cyclic compounds, 2,2’-diamino-6,6’-dimethyl biphenyl was resolved and carried outthe following reaction.
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Obstacle theory6,6’-Dimethyldiphenylamine
NH2
NH2 CH3
CH3 CH3
CH3
NHA c
A cNH
Op tically A ctiv e Op tically A ctiv e
Op tically A ctiv e
A cNH
NHA c
CO2H
HO2CN
N
O
O
H
H
H2SO4
[O]
Op tically In activ e Dilactam
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x-ray, UV and ESR analysis The dilactam is not active and hence it must be planner.
This can be explained in terms of resonance. On the basis of that, it is proved that the non-
planarity is the result of steric repulsion with o-hydrogen atoms.
UV measurements show that biphenyls with o-substitutions are not co-planer. The max of biphenyl = 248(19,000) nm: while benzene has 198(8,000) nm. This is due to resonance hybrid. e.g.
Wheland et al. studied such compounds by means of ESR.
Zwitter Ion
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2. STEREOCHEMISTRY OF ALLENES: Van’t Hoff, as early as in 1875 suggested that an
unsymmetrycally substituted allene should exist intwo enantiomeric forms. e.g. abC=C=Cab; where:a#b.The molecule and its mirror image are notsuperimposable. The groups a and b are inperpendicular planes.
C C Caa
b bC C C
aa
b b
mirror
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In allenes, the two end carbon atoms are in a state oftrigonal hybridization. The center carbon atom is in astate of diagonal hybridization. So, the central carbonforms two bonds, which are perpendicular to eachother. In allenes, C-1 and C-3 are sp2 and central C-2 issp hybridized. Thus, two end groups are non-planner.e.g.
C2 C3C!
SP2
trigonalSPdigonal
SP2
trigonal
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M O Picture of allene, pi bonds are in different planes
=>
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Molecular orbital picture
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End groups in different planes
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Allenes Chiral compounds with no chiral carbon Contains sp hybridized carbon with adjacent double
bonds: >C=C=C<, which are perpendicular. End carbons must have different groups.
Allene is achiral.
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3D ball and stick model of allene
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If a is not equal to b like (or the four groups aredifferent as in or abC=C=Cde), the C2 axis disappearsand molecule is not symmetric. (C2 point group). Intrigonal state, the -bond is perpendicular to the planehaving three -bonds. Thus, the molecule can not havea plane or centre of symmetry. Therefore, it isresolvable. Let us consider following cases.
In case-3, two perpendicular planes are present andtherefore optical active enantiomers are possible.
Enantiomers of 1,3-dichloroallene are as under, wherea=Cl and b=H:
wherea≠b
C C CH
Cl
mirror
CCCH
ClCl
H
Cl
HC C C
b
a
mirror
CCCb
aa
b
a
b
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Penta-2,3-diene Is Chiral
Chapter 5 35
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The first successful resolution of an allene was carriedout by Kohler et al(1935) i.e. 3--naphthyl-1,3-diphenylallene-1-carboxylic acid through its glycolatewith brucine. And other allene carboxylic acid wasresolved by Wotiz using strychnine.
Resolved by brucine
Resolved by strychnine
PhC C C
COOH
Ph PhCCC
HOOC
Ph
mirror
H3CC C C
COOH
(CH2)3CH3
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Synthesis Mills and Maitland provided the experimental proof
to the prediction of Van’t Hoff. They performedcatalytic asymmetric dehydration of 1,3-di-naphthyl-1,3-dipheylprop-2-enol to give dinaphthyldiphenylallene.
Ph
C CH C
naphthyl
OH
Ph
naphthyl-
-H2O
d/l-camphor-10-sulphonicacid
Ph
C C C
naphthyl
Ph
naphthyl-
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Reaction wth boiling 1% benzene solution of (+)-camphorsulphonic acid gives a dextro rotatory and (-)-camphorsulphonic acid gives laevorotatory allene.
1,3-Diphenylpropyne is rearranged over aluminaimpregnated with quinine or brucine, to get active1,3-diphenylallene. Quinine gives (+)-allene whilebrucine gives (-)-allene.
Ph
C C C
H
Ph
H
Al2O3PhC CCH2Ph
brucine
Al2O3
quininePh
C C C
H
Ph
H
(-)-diphenylallene (+)-diphenylallene
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Linder synthesized (1959) (+)-1-chloro-3,4,4—trimethylpenta-1,2-diene using SOCl2.
The naturally occuring antibiotics nemotinic acidalso contains allene grouping.
CCMe3
CMe3
OH
C CH
Me3C
C C C
Cl
H
Me3C
SOCl2
CH C C C CH
C CH CHCH2CH2COOH
OHnemotinic acid
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STEREOCHEMISTRY OF SPIRANS
When both the double bonds in allene structureare replaced by ring systems, the molecule isknown as spiran. Spirans as a whole isasymmetric and shows enantiomerism.
Like allene, in spirans, the groups lie inperpendicular planes and hence they areoptically active. Let us consider a followingspiran:
a
b
x
y
a
b
x
y
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Spirans
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Nomenclature in spirans Carry out nomenclature as a parent chain of alkane as
per total no of carbon atoms present in ring Write number of carbon atoms in each ring except
central atom in big bracket giving priority to higher number before the parent name
Add prefix spiro to the name of compound E.g.
spiro-[2,2]pentane spiro-[5,3]nonane
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Spiro [3,3 ]heptane is the basic structure, in which a & b arethe substituents. C2 and C5 are perpendicular to each other.The absence of symmetry is observed, which makes themolecule as a whole asymmetric. Therefore, two opticalisomers are possible. Here, a,b and C2, C5 are also indifferent planes. Thus, plane of symmetry is absent. e.g. 2,5-Dimethyl spiro[3,3] heptane.
H3C
H
CH3
H
H
H
H3C
H3C
C2 C5
central C
Optically active Optically inactive
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The plane of symmetry in 2,2-dimethylspiro [3,3] heptane is responsible for itsinactive nature.
H3C
H3C
H
H
C2
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SPIRANSWhen penta-erythritol is condensed with pyruvicacid, a spiro compound is obtained. This compoundis resolvable. Other synthesis is from cyclohexanoneand glycol.
CH3COCOOH + C(CH2OH)4
O
O O
OH3C
HOOC
CH3
COOH
-H2O
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Optically active Dilactam of 2,2’,4,4’-benzophenone tetracarboxylic acid hydrolized to inactive 2,2’,4,4’-benzophenone tetracarboxylic acid
CO
C O
OC
O
COOH
COOH
CO
CH2 ONa
OC
ONa
COONa
COONa
NaOH
planarnonplanar
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Spiro-hydantoin and other spirans Spiro-hydantoin with
following formula can also beresolved. Phenatoin, one ofthe epilepsy drug hashydention functionality
Other spiro[3,3]heptane aregiven bellow
COHN
OC NH
HN CO
NHOC
R R
HH
R=NH2 or COOH
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Other examples of optically active spirans are phophonium and ammonium spiro compounds are as follows.
P+COOEt
Ph
EtOOC
Ph
N+COOEt
H
EtOOC
H
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SPIRANS As above discussed optical activity of spirans is due to
the asymmetry of the molecules as a whole and onepair of enantiomers is found. If spiro compoundcontains asymmetric carbon atoms, then the numberof isomers will be more than two.
CHIRAL AXIES Axial chirality refers to stereoisomerism resulting from
the non- planar arrangement of four groups in pairsabout a chiral axis.
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HELICITY This property can also be called helicity, since the axis
of the structure has a helical, propeller, or screw-shaped geometry. P (plus) or Δ is a right-handed helix, where M (minus) or Λ is a left-handed helix.[3][4] The P/M or Δ/Λ terminology is used particularly for molecules that actually resemble a helix, such as hexahelicene. It can also be applied to other structures having axial chirality by considering the helical orientation of the "front" vs "back" Cahn–Ingold–Prelog rankings.
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HELICITY
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CHIRAL AXIES IN BIPHENYLS
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Assingnment of configuration in biphenyls
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Assignment of configuration in allenes
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CHIRAL AXIES IN SPIRANS
B'
A' A
B
A
B
A' B'
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