II PROSTEREOISOMERISM Topicity of Ligands and Faces

SEM-2, CC-3 PART-2, PPT-6

Dr. Kalyan Kumar Mandal Associate Professor St. Paul’s C. M. College Kolkata Prostereoisomerism Topicity of Ligands and Faces Part-2 CONTENTS

❖ Introduction ❖ Homotopic Ligands ❖ Homotopic Faces Topicity of Ligands and Faces: Introduction • In certain molecules, such as propionic acid (A; Figure 1), a

nonstereogenic center (here Cα) can be transformed into a stereogenic center by replacement of one or other of two apparently identical ligands by a different one. Such ligands are called “homomorphic” from Greek homos meaning same and morphe meaning form. They are identical only when separated from the rest of the molecule.

• Thus the replacement of HA at Cα in propionic acid by OH generates the chiral centre of (S)-lactic acid, whereas the

analogous replacement of HB gives rise to the enantiomeric (R)-lactic acid. The Cα centre in propionic acid has, therefore, been called a “prochiral” as well as “prostereogenic centre.”

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction

• HA and HB at such a centre are called “heterotopic ligands” from Greek heteros meaning different and topos meaning place. Prochiral axes and planes may similarly be defined in relation to chiral axes and planes. • Substitution is one of the common ways of interconverting organic molecules, another is addition. The chiral centre in lactic acid (B and C; Figure 2) can also be generated by the addition of hydride (e.g., from sodium borohydride or lithium aluminium hydride ) to the carbonyl group of pyruvic acid (A; Figure 2).

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction

• Depending on the face of the keto acid (pyruvic acid) the hydride adds to, either (S)- or (R)-lactic acid is obtained. The addition of hydride ion (H-) to the front/top face of the keto acid as depicted in Figure 2 will give rise to (R)-lactic acid (B), whereas (S)-lactic acid is obtained by addition of the nucleophile to the rear face of the C=O group. Thus the carbonyl group in pyruvic acid is also said to be prochiral and to present two heterotopic faces.

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction • A prochiral axis (in chloroallene A; Figure 3) can be converted

into the chiral allenes, B and C by replacement of HA and HB by C1 separately.

• Ligands (atoms or groups in a molecule) and faces may be homotopic or heterotopic. Heterotopic ligands and faces may be either enantiotopic or diastereotopic. It may be pointed out that topicity describes the relationships of two or more homomorphic ligands (or faces) which together constitutes a set.

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction • In view of the interrelationship between topicity of ligands and isomerism in general, it may be instructive to draw a classification diagram (Figure 4) for topicity and to compare it with that drawn for isomerism.

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands and Faces • Two criteria, namely, a substitution (or addition) criterion and (or) a symmetry criterion are employed to determine the topic relationships of homomorphic ligands and faces (only one test suffices).

Substitution and Addition Criteria

• Two homomorphic ligands are homotopic if substitution (or replacement) of first one and then the other by a different ligand leads to the same structure. (The replacement ligand must be different not only from the original one but also from all other ligands attached to the same atom.). Examples of homotopic ligands are shown in Figure 5.

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Substitution Criterion <