stereochemistry ii - st. paul's cathedral mission college

STEREOCHEMISTRY II

PART-3, PPT-3, SEM-2

Dr. Kalyan Kumar Mandal

Associate Professor

St. Paul’s C. M. College

Kolkata

Stereochemistry II

Part-3: Configurational Descriptors (Ra, Sa) of

Axially Chiral Molecules

CONTENTS

❖ Configurational Descriptors of Chiral Allenes

❖ Configurational Descriptors of Chiral Spiranes

❖ Configurational Descriptors of Chiral Alkylidenecycloalkanes

❖ Configurational Descriptors of Chiral Biphenyls and related Compounds

• Of the three carbons of allene C1 and C3 are sp2 hybridized and

C2 is sp hybridized. The orbital picture of allene (A) is shown in

Figure 1. The shaded p orbitals as well as the unshaded p orbitals

overlap with each other separately to form orthogonal π-bonds

placing the ligands at C1 in a plane orthogonal to that of the

ligands at C3.

• The structure (A) can be projected to a Newman-like projection

formula (B) to R-configuration, as shown, viewed from the left side

with the front ligands in a vertical plane and the rear ligands in a

horizontal plane (Figure 1). The (R,S) nomenclature is independent

of the direction of viewing, and the same specification will follow

when viewed from the right side.

Configurational Nomenclature (Ra, Sa) of

Axially Chiral Allenes

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata


Axially Chiral Allenes

• The allenes of the general formula abC=C=Cab possess a C2 axis,

but no σ-plane, and belong to the point group C2. These molecules

are dissymmetric. If the three or four of the ligands are different as

in abC=C=Cac or abC=C=Ccd, the C2 axis disappears, and the

molecules are totally asymmetric and possess C1 point group. This

is true for all axially chiral molecules.

• For configurational nomenclature of allenes and other axially

chiral molecules, the standard ‘subrule 0’, which states that “the

near groups precede the far groups,” is considered first, ahead of

other subrules.

• R,S nomenclature in cases of some specific examples of a few

optically active allenes and their enantiomers are illustrated in

Figures 2 and 3.

Configurational Nomenclature of Chiral Allenes


• The molecule is viewed from any end of the chiral axis and

Newman-like projections are drawn. The groups near the viewer

are numbered 1 (or a) and 2 (or b), whereas the groups at the far

end are numbered 3 (or c) and 4 (or d), following the priority

sequence rule.

• The order 1 → 2 → 3 (or a → b → c), clockwise or anticlockwise,

gives the configuration as aR (or Ra) or aS (or Sa), respectively

(here, a stands for axial). Interchange of the two geminal groups at

any end in these molecules leads to the enantiomer.

• This type of nomenclature is applicable for other types of

compounds with axial chirality.

Configurational Nomenclature


Configurational Nomenclature of Chiral Allenes

• It is to be noted that if in the resulting Fischer projection, lowest

priority group (4 or d) remains in the horizontal bond, before tracing

a path from 1→ 2 → 3, interchanges of groups need not be done to

put it into any vertical bond. Clockwise path indicates Ra

configuration and anticlockwise path indicates Sa configuration

irrespective of the position of ‘d’ (ligand with lowest priority).


Configurational Nomenclature

• In the example, shown in Figure 3, looking from side A, nearer

groups CH3 and CMe3 get the priority number 1 and 2 and the far

groups Cl and H get priority 3 and 4 respectively. Similarly,

looking from side B, Cl and H of the nearer carbon atom get the

preference in the numbering over the groups CH3 and CMe3.



Axially Chiral Spiranes

• The absolute nomenclature of axially chiral spiranes is determined

by the same method as in the case of axially chiral allenes.

• Rule of Nomenclature: One has to look along the chiral axis of

any axially chiral spirane molecule and project on a plane

orthogonal to the chiral axis. Now, after assigning the priority

sequence of ligands 1 (or a) and 2 (or b) to the near ligands

(shown by a solid line), and 3 (or c) and 4 (or d) to the far ligands

(shown by a dotted line), the (R,S) specification can be made as

shown in the examples given in Figure 4.


Configurational Nomenclature of Chiral Spiranes

Configurational Nomenclature (Ra, Sa) of Axially

Chiral Alkylidenecycloalkanes

• The absolute nomenclature of alkylidenecycloalkanes, on the basis

of axial chirality, is determined by the same method as in the case

of axially chiral allenes and spiranes.


any axially chiral alkylidenecycloalkane molecule and project on a

plane orthogonal to the chiral axis. After assigning the priority

sequence 1 (or a) and 2 (or b) to the near ligands (shown by a solid

line), and 3 (or c) and 4 (or d) to the far ligands (shown by a dotted

line) the (R,S) specification can be made as shown in the examples

given in Figure 5.


Configurational Nomenclature of Axially Chiral

Alkylidenecycloalkanes and related Compounds


Configurational Nomenclature of Chiral Biphenyls• Selection rules for axial chirality has been introduced in case of

atropisomers. The absolute nomenclature of axially chiral

biphenyls, on the basis of axial chirality, is determined by the

same method as in the case of axially chiral allenes, spiranes, etc.

• Ordering of groups: In the new convention only the four atoms

C2, C6, C2', and C6' which correspond to the four vertices of the

elongated tetrahedron and which contribute more to the chirality

properties of the molecule are considered for sequencing.


the chiral biaryl molecule and project on a plane orthogonal to the

chiral axis. After assigning the priority sequence 1 (or a) and 2 (or

b) to the near ligands (shown by a solid line), and 3 (or c) and 4 (or

d) to the far ligands (shown by a dotted line), the (R,S)

specification chiral biphenyls can be made.


Configurational Nomenclature Chiral Biphenyls

• Subrule 0: Proximity rule: Groups about the near end of the

chiral (molecular) axis precede over groups about the far end

(same as in other axially chiral molecules). The (R,S)

nomenclature of chiral biphenyls is shown in the examples given

in Figure 6


• In the case of biphenyl it is important to note that the ring

substituents are to be explored from the center on outward. Thus, in

the biphenyl in Figure 7, in the left ring the sequence is

C(2′)-OCH3 > C(6′)-H; the chlorine atom is too far out to matter, a

decision being made before it is reached in the outward

exploration. The fiducial atoms (i.e. those that determine the

configurational symbol) are the same when the molecule is viewed

from the right.

• The molecule (A; Figure 7), according to the new system when

viewed from the left hand side along the 1′-1 bond gives the

projection (B) and when viewed from the right hand side along the

1-1' bond gives the projection (C). Both the projection confirms to

S-configuration. It does not matter which end is taken as the near.



• When C-2 and C-6 in a ring are attached to identical atoms and C-3

and (or) C-5 are also substituted as in A (Figure 8), ambiguity

arises and the priority order of the ortho carbon atoms has to be

determined through exploration around the ring or side chain.

• Thus in A (Figure 8), C-2 with a methyl group gets precedence

over C-6 with a hydroxymethyl (CH2OH) group.



• This is because an exploration from C-2 around the ring (along the

route providing highest precedence) leads to the phenolic hydroxyl

group whereas similar exploration from C-6 along the side chain

leads to a primary hydroxyl group. The structure (A) confirms to

R-configuration from whichever side the molecule is viewed.


Atropisomerism in Compounds

other than Biphenyls

• Molecule of this type may give rise atropisomers if sufficient steric

hindrance is created around the pivotal bond. Thus the substituted

stilbene (A; Figure 9) is capable of resolution. The substituted

naphthylamine (B) provides another example in which the peri

nitro group prevents the substituents at nitrogen to cross the plane

of the naphthalene ring.


stereochemistry ii - st. paul's cathedral mission college

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