b-Eliminations
4.1 Concepts of Elimination Reactions 4
4.1.1 The Concepts of a,b- and 1,n-Elimination
Reactions in which two atoms or atom groups X and Y are removed from a
B
compound are referred to as eliminations (Figure 4.1). In many eliminations X and Y
are removed in such a way that they do not become constituents of one and the
same molecule. In other eliminations they become attached to one another such
that they leave as a mol- ecule of type X¬Y or X“Y or as N‚N. The atoms or
groups X and Y can be bound to C atoms and/or to heteroatoms in the substrate.
These atoms can be sp3 or sp2 hy- bridized.
Elimination
X X Y X Y X Y
1 3 1 4
C C C C C C C C C
Y C
-elimi- -elimi- 1,3-elimination 1,4-elimi-
nation nation nation
C C C C C C C C C
C
carbene olefin 1,3-diradical cyclopropane 1,3-diene
Reaction examples
H I Br Br
Cl H Br Br
N N 1 4
C Ph C C H 1 3 1 3 H2C CH CH CH2
Cl Cl H H
NaOMe
Ph H CH
NaOH Mg
Cl
C H H H
2
Cl
C C C
,Zn
Fig. 4.1. 1,n-Eliminations
(n 1–4) of two atoms or
groups X and Y, which
CH CH2
are bound to sp3-
hybridized C atoms.
,
, 130 4 b-Eliminations
Depending on the distance between the atoms or groups X and Y removed
from the substrate, their elimination has a distinct designation. If X and Y are
geminal, their removal is an a-elimination. If they are vicinal, it is a b-
elimination. If X and Y are separated from each other by n atoms, their removal is
called 1,n-elimination, that is, 1,3-, 1,4-elimination, and so on (Figure 4.1).
Chapter 4 is limited to a discussion of the most important eliminations, which
are the olefin-forming B-eliminations.
In this book, A-eliminations are described only in connection with their
applications: the a-elimination of HCl from CHCl3 base (Figure 3.11) and the a-
elimination of XZnI from carbenoids X¬CH 2¬ZnI (Figure 3.12) as well as the a-
elimination of N2 from diazomalonic ester (Figure 3.13) in cyclopropanations, the a-
elimination of LiBr from carbenoids Br¬CR1R2¬Li in thermal rearrangements
(Figures 11.23, 11.29), the a-elimination of N 2 from a-diazoketones in the
photochemical Wolff rearrangement (Section 11.3.2), and the related a-elimination
of N2 from azides in the Curtius degra- dation (Section 11.4.5).
B-Eliminations in which at least one of the leaving groups is removed from a
het- eroatom are considered to be oxidations. Eliminations of this type are
therefore not treated here but in the redox chapter (mainly in Section 14.3.1).
1,3-Eliminations are mentioned in the preparation of 1,3-dipoles such as
diazoal- kanes or a-diazoketones (Section 12.5.3) and nitrile oxides (Section
12.5.4), in con- nection with the decomposition of primary ozonides to carbonyl
oxides (Section 12.5.5) and the decomposition of phenylpentazole to phenyl azide
(Section 12.5.6).
4.1.2 The Terms syn- and anti-Elimination
In various eliminations the mechanism implies a well-defined stereorelationship
B
be- tween the eliminated atoms or groups X and Y and the plane of the
resulting C“C double bond (Figure 4.2). For example, X and Y may leave into
one and the same half-space flanking this double bond. Their removal is then
called a syn- or cis- elimination. (Be careful when using the second term: cis-
eliminations can also give trans- or E-olefins!) There are other eliminations
where group X leaves the substrate in the direction of one half-space and group Y
leaves in the direction of the other half- space, both flanking the C“C double
bond produced. These are so-called anti- or
R1 R3 R1 R3
X Y syn-Elimination
R2 R4 R2 R4
is identical with
4.1 Concepts of Elimination Reactions 4
4.1.1 The Concepts of a,b- and 1,n-Elimination
Reactions in which two atoms or atom groups X and Y are removed from a
B
compound are referred to as eliminations (Figure 4.1). In many eliminations X and Y
are removed in such a way that they do not become constituents of one and the
same molecule. In other eliminations they become attached to one another such
that they leave as a mol- ecule of type X¬Y or X“Y or as N‚N. The atoms or
groups X and Y can be bound to C atoms and/or to heteroatoms in the substrate.
These atoms can be sp3 or sp2 hy- bridized.
Elimination
X X Y X Y X Y
1 3 1 4
C C C C C C C C C
Y C
-elimi- -elimi- 1,3-elimination 1,4-elimi-
nation nation nation
C C C C C C C C C
C
carbene olefin 1,3-diradical cyclopropane 1,3-diene
Reaction examples
H I Br Br
Cl H Br Br
N N 1 4
C Ph C C H 1 3 1 3 H2C CH CH CH2
Cl Cl H H
NaOMe
Ph H CH
NaOH Mg
Cl
C H H H
2
Cl
C C C
,Zn
Fig. 4.1. 1,n-Eliminations
(n 1–4) of two atoms or
groups X and Y, which
CH CH2
are bound to sp3-
hybridized C atoms.
,
, 130 4 b-Eliminations
Depending on the distance between the atoms or groups X and Y removed
from the substrate, their elimination has a distinct designation. If X and Y are
geminal, their removal is an a-elimination. If they are vicinal, it is a b-
elimination. If X and Y are separated from each other by n atoms, their removal is
called 1,n-elimination, that is, 1,3-, 1,4-elimination, and so on (Figure 4.1).
Chapter 4 is limited to a discussion of the most important eliminations, which
are the olefin-forming B-eliminations.
In this book, A-eliminations are described only in connection with their
applications: the a-elimination of HCl from CHCl3 base (Figure 3.11) and the a-
elimination of XZnI from carbenoids X¬CH 2¬ZnI (Figure 3.12) as well as the a-
elimination of N2 from diazomalonic ester (Figure 3.13) in cyclopropanations, the a-
elimination of LiBr from carbenoids Br¬CR1R2¬Li in thermal rearrangements
(Figures 11.23, 11.29), the a-elimination of N 2 from a-diazoketones in the
photochemical Wolff rearrangement (Section 11.3.2), and the related a-elimination
of N2 from azides in the Curtius degra- dation (Section 11.4.5).
B-Eliminations in which at least one of the leaving groups is removed from a
het- eroatom are considered to be oxidations. Eliminations of this type are
therefore not treated here but in the redox chapter (mainly in Section 14.3.1).
1,3-Eliminations are mentioned in the preparation of 1,3-dipoles such as
diazoal- kanes or a-diazoketones (Section 12.5.3) and nitrile oxides (Section
12.5.4), in con- nection with the decomposition of primary ozonides to carbonyl
oxides (Section 12.5.5) and the decomposition of phenylpentazole to phenyl azide
(Section 12.5.6).
4.1.2 The Terms syn- and anti-Elimination
In various eliminations the mechanism implies a well-defined stereorelationship
B
be- tween the eliminated atoms or groups X and Y and the plane of the
resulting C“C double bond (Figure 4.2). For example, X and Y may leave into
one and the same half-space flanking this double bond. Their removal is then
called a syn- or cis- elimination. (Be careful when using the second term: cis-
eliminations can also give trans- or E-olefins!) There are other eliminations
where group X leaves the substrate in the direction of one half-space and group Y
leaves in the direction of the other half- space, both flanking the C“C double
bond produced. These are so-called anti- or
R1 R3 R1 R3
X Y syn-Elimination
R2 R4 R2 R4
is identical with
