Stereochemistry (Tutorial Chemistry Texts)

179 Pages·2002·16.31 MB·English

by  Morris

St ereochemistry
DAVID G. MORRIS
University of Glusgow
RSeC
ROYAL SOCIl3Y OF CHEMISTRY
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ISBN 0-85404-602-X
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0 The Royal Society of Chemistry 2001
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TITLES IN THE SERIES FORTHCOMING TITLES
Stereochemistry D G Morris Molecular Interactions
Reactions and Characterization of Solids Reaction Kinetics
S E Dann Basic Spectroscopy
Main Group Chemistry W Henderson X-ray Crystallography
d- and f-Block Chemistry C J Jones Lanthanide and Actinide Elements
Structure and Bonding J Barrett Maths for Chemists
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Organotransition Metal Chemistry A F Hill Chemistry of Solid Surfaces
Quantum Mechanics for Chemists Biology for Chemists
D 0 Hayward Multi-element NMR
Thermodynamics and Statistical Mechanics
J D Gale and J M Seddon
Mechanisms in Organic Reactions
R A Jackson
Heterocyclic Chemistry M Sainsbury
Atomic Structure and Periodicity J Barrett
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Preface
Stereochemistry, first recognized by van? Hoff and Le Bel, now perme-
ates organic chemistry. For a full appreciation of the subject, an under-
standing of stereochemistry is necessary in terms of both the relevant
conventions and definitions that are in use, and also what is happening at
a molecular level during a reaction. It is hoped that this self-contained
text will provide a means by which undergraduate students (and maybe
others) become conversant with stereochemistry. The text being limited in
size, it has been necessary to be selective in the choice of topics.
Particular thanks are due to one of my former teachers, Professor
Alwyn G. Davies FRS, than whom no-one could have been more helpful
and courteous, for reading the manuscript and for his comments. Thanks
are likewise due to Mr Martyn Berry. I am grateful to Professor Keiji
Gamoh, Dr Sean Higgins, Dr Susan Armstrong and Dr David Rycroft,
who read selected chapters and for discussions. I also thank Professor Zvi
Rappoport and Dr David Procter for a discussion. The diligence of all the
above prevented some errors and infelicities from featuring in the text.
For any that remain, I alone am culpable. Gratitude is due to Dr Colin
Drayton for help with editing and to Mrs Janet Freshwater of the Royal
Society of Chemistry for overseeing the project with benign efficiency.
Space considerations meant that some material had to be omitted.
Accordingly, a Glossary of useful stereochemical terms has been placed
on the RSC web site (http://www.chemsoc.org/pdf/tct/stereochemgloss.
pdf). A further such list that derives from IUPAC may be found at the web
site address of Dr G. P. Moss ([email protected]).
Finally, I thank my wife Brenda for her assistance with the preparation
of the manuscript, during which she sacrificed several months of her life
on the altar of Stereochemistry.
David G. Morris
Glasgow
Contents
1.1 Introduction 1
1.2 Hybridization: Methane 2
1.3 Hybridization: Ethene and Alkenes 3
1.4 Hybridization: Ethyne 5
1.5 Bonding and Anti-bonding Orbitals 6
1.6 Conformation: Ethane 7
1.7 Conformation of Propane and n-Butane 9
1.8 Cyclohexane: Chair Conformation 10
1.9 Cyclohexane: Boat Conformation 12
1.10 Inversion of Cyclohexane 14
1.11 Monosubstituted Cyclohexanes 15
2.1 Chirality, Enantiomers and Optical Activity 19
2.2 How to Specify a Configuration 23
2.3 Enantiomeric Excess, Enantiomeric Ratio 27
2.4 Racemization 29
2.5 Homochiral Molecules 30
3.1 Enantiomers and Diastereoisomers 37
3.2 Meso Configuration 39
3.3 ErythrolThreo and SynlAnti Configurations 41
V
vi Contents
3.4 Caged Compounds with Two Stereogenic Bridgehead
Carbons 43
3.5 Epimers, and the Nomenclature of Bicyclic Compounds 44
3.6 Separation of Enantiomers: Resolution 46
3.7 Separation of Enantiomers by Chromatography 48
3.8 Resolution with Enzymes 49
3.9 Structure of Polypropene 52
4.1 Configuration and Relative Stability of Alkenes and Dienes 60
4.2 Cyclohexene 62
4.3 Carbon-Nitrogen Double Bonds 63
4.4 Amides 64
4.5 Cis Hydroxylation of Alkenes 67
4.6 Trans Hydroxylation of Alkenes 67
4.7 Addition of Bromine to Alkenes 69
4.8 Hydration of Alkenes 72
5.1 Allenes and Related Molecules 80
5.2 Biphenyls 83
5.3 Absolute Configuration of Allenes and Biphenyls 85
5.4 Hexahelicene 86
5.5 Silicon, Germanium and Tin Compounds 87
5.6 Amines, Ammonium Salts, Phosphorus and Arsenic
Compounds 88
5.7 Sulfoxides, Sulfonium Salts and Selenoxides 91
6.1 Cyclic Molecules, Configurational Assignment and Strain 99
6.2 Cy clo pro pane 100
6.3 Cyclobutane 101
6.4 Cyclopent ane 102
6.5 Cyclohexanes 102
6.6 Decalins 111
6.7 Steroids 113
6.8 Anomeric Effect 114
Contents vii
7.1 Nucleophilic Substitution 125
8.1 Prochiral Molecules, Enantiotopic Groups and Faces 140
8.2 Diastereotopic Compounds 143
8.3 Reactivity Difference of Enantiotopic Hydrogens and
in Enzyme-mediated Reactions 144
8.4 Non-enzymatic Selectivity in Addition to C=O and
C=C Bonds 145
8.5 Stereochemical Information from NMR Spectroscopy 147
Simple Molecules:
Hybridization, Conformation
and Configuration
I. I Introduction
Chemistry, like everyday life, takes place overwhelmingly in three dimen-
sions. Stereochemistry embraces the spatial aspects of chemistry and can
be considered in two parts. The first deals with the shapes and proper-
ties of mainly three-dimensional molecules and involves a knowledge of
the terms and which are intro-
duced in the first two chapters. The second aspect deals with reactivity
and includes the preferred or obligatory direction of approach of
reagents, and also the consequences for the nature of the products. In
respect of reactivity, it can be said that except for spherical reactants,
e.g. H+ and Cl-, there is almost always a preferred direction of approach
of one molecule or ion toward another.
We consider first the simplest organic molecule methane, CH,, in
I
2 Stereochemistry
particular its geometry, its representation in two dimensions and its
make-up from the component atoms. In methane, all four carbon--
hydrogen bonds are equivalent and are directed toward the corners of a
regular tetrahedron with carbon at the centre. The HCH bond angles
are all 109”28’,a value that is found only in molecules with ‘tetrahedral’
symmetry. The normal depiction of methane is given in 1 and the
convention, which is applied generally, for representing this three-
dimensional molecule in two dimensions is now described. The central
carbon is taken to lie in the plane of the paper, together with any two
hydrogens. Most commonly these are the upper and left hydrogens and,
as shown in 1, bonds between each of these two hydrogens and.carbon
are represented by lines of ‘normal’ thickness. The thick ‘wedge’ repre-
sents a bond between carbon and the hydrogen in front of the page, and
the dashed line indicates a bond between carbon and the hydrogen
H
I behind the page.
Note that any three atoms must lie in a plane. One can always draw
,/Cp,
a plane through three points; in the everyday world, for example, a three-
H
legged stool never rocks. The three atoms that lie in the plane of the
1
paper in 1 are chosen for convenience.
1.2 Hybridization: Methane
When a bond is formed between, say, C and H, the electronic distribu-
tion in the orbitals is perturbed and two atomic orbitals that normally
each contain an electron are said to overlap. In the case of a simple mol-
ecule such as in methane, the question arises: how does one reconcile the
equivalence of the four C-H bonds in methane with the outer shell
electron configuration, 2s22p2,o f carbon? Hydrogen contributes its one
electron in a 1s orbital, whereas carbon has four atomic orbitals of appro-
priate energy available for bonding. These four orbitals are one 2s and
three 2p orbitals, which are designated 2p,, 2py and 2pz according to
their different directions in space; x, y and z refer to the directions
of a Cartesian coordinate system. A 2s and a 2py orbital are shown in
Figure 1.1.
The 2s orbital is spherically symmetrical, and its electron density is
highest at the nucleus. Each 2p orbital is cylindrically symmetrical
Figure 1. I