Rearrangements
The term “rearrangement” is used to describe two different types of organic chemical
reactions. A rearrangement may involve the one-step migration of an H atom or
of a larger molecular fragment within a relatively short-lived intermediate. On
11
B
the other hand, a rearrangement may be a multistep reaction that includes the
migration of an H atom or of a larger molecular fragment as one of its steps. The
Wagner–Meerwein rearrangement of a carbenium ion (Section 11.3.1)
exemplifies a rearrangement of the first type. Carbenium ions are so short-lived
that neither the starting material nor the primary rearrangement product can be
isolated. The Claisen rearrangement of allyl alkenyl ethers also is a one-step
rearrangement (Section 11.5). In contrast to the Wagner–Meerwein
rearrangement, however, both the starting material and the prod- uct of the
Claisen rearrangement are molecules that can be isolated. The ring expan- sion
reaction shown in Figure 11.23 and the alkyne synthesis depicted in Figure 11.29
are examples of multistep rearrangement reactions.
11.1 Nomenclature of Sigmatropic Shifts
In many rearrangements, the migrating group connects to one of the direct
B
neighbors of the atom to which it was originally attached. Rearrangements of
this type are the so-called [1,2]-rearrangements or [1,2]-shifts. These
rearrangements can be considered as sigmatropic processes, the numbers “1” and “2”
characterizing the subclass to which they belong. The adjective “sigmatropic”
emphasizes that a s-bond migrates in these reactions. How far it migrates is
described by specifying the positions of the atoms be- tween which the bond is
shifted. The atoms that are initially bonded are assigned po- sitions 1 and 1 The
subsequent atoms in the direction of the s-bond migration are
labeled 2, 3, and so forth, on the side of center 1 and labeled 2 , 3 , and so forth,
on the side of center 1 After the rearrangement, the s bond connects two atoms
in po-
sitions n and m The rearrangement can now be characterized by the positional num-
bers n and m in the following way: the numbers are written between brackets,
sepa- rated by a comma, and the primed number is given without the prime.
Hence, an [n,m]-rearrangement is the most general description of a sigmatropic
process. A [1,2]- rearrangement is the special case with n 1 and m 2 (Figure
11.1). [3,3]- Rearrangements occur when n m 3 (Figure 11.2). Many other types
of rearrangements are known, including [1,3]-, [1,4]-, [1,5]-, [1,7]-, [2,3]-, and
,[5,5]- rearrangements.
In this chapter we will be dealing primarily with [1,2]-rearrangements. In addition,
the most important [3,3]-rearrangements, namely, the Claisen and the Claisen–
Ireland rearrangements, will be discussed.
,436 11 Rearrangements
Fig. 11.1. The three 1
R(H) 1
R(H)
reactions on top show 1 2
C C
[1,2]-rearrangements to a 1
C C 2
sextet carbon. The two 1
R(H)
reactions at the bottom
1
R(H) 1 2
show [1,2]- 1 2
O C C
rearrangements to a O C C
neighboring atom that is 1 2 1
R(H) C C R(H)
coordinatively saturated
1
but in the process of losing C C2 1
X
a leaving group.
1X
1 2 1 2
a b Y a b
ba –Y
1
X
X
1a b2 Y
ba a b
–Y
2
Fig. 11.2. A Claisen 2
3 3
rearrangement as O
1 1 O
an example of a 1 1 3
3
[3,3]-
rearrangement. 2 2
11.2 Molecular Origins for the
Occurrence of [1,2]-Rearrangements
Figure 11.1 shows the structures of the immediate precursors of one-step [1,2]-
B
sigmatropic rearrangements. These formulas reveal two different reasons for the
occurrence of re- arrangements in organic chemistry. Rows 1–3 of Figure 11.1
reveal the first reason for [1,2]-rearrangements to take place, namely, the
occurrence of a valence electron sex- tet at one of the C atoms of the substrate.
This sextet may be located at the C of a carbenium ion or at the C: of a carbene.
Carbenium ions are extremely reactive species. If there exists no good opportunity
for an intermolecular reaction (i.e., no good pos- sibility for stabilization),
carbenium ions often undergo an intramolecular reaction. This intramolecular
reaction in many cases is a [1,2]-rearrangement.
Suppose a valence electron sextet occurs at a carbon atom and the possibility
exists for a [1,2]-rearrangement to occur. The thermodynamic driving force for
, the potential [1,2]-rearrangement will be significant if the rearrangement leads
to a structure with octets on all atoms. It is for this reason that acylcarbenes
rearrange quantitatively to give ketenes (row 2 in Figure 11.1) and that
vinylcarbenes rearrange quantitatively to give acetylenes (row 3 in Figure 11.1).
In contrast, another valence electron sextet species is formed if the [1,2]-
rearrangement of a carbenium ion leads to another car- benium ion. Accordingly,
the driving force of a [1,2]-rearrangement of a carbenium ion
The term “rearrangement” is used to describe two different types of organic chemical
reactions. A rearrangement may involve the one-step migration of an H atom or
of a larger molecular fragment within a relatively short-lived intermediate. On
11
B
the other hand, a rearrangement may be a multistep reaction that includes the
migration of an H atom or of a larger molecular fragment as one of its steps. The
Wagner–Meerwein rearrangement of a carbenium ion (Section 11.3.1)
exemplifies a rearrangement of the first type. Carbenium ions are so short-lived
that neither the starting material nor the primary rearrangement product can be
isolated. The Claisen rearrangement of allyl alkenyl ethers also is a one-step
rearrangement (Section 11.5). In contrast to the Wagner–Meerwein
rearrangement, however, both the starting material and the prod- uct of the
Claisen rearrangement are molecules that can be isolated. The ring expan- sion
reaction shown in Figure 11.23 and the alkyne synthesis depicted in Figure 11.29
are examples of multistep rearrangement reactions.
11.1 Nomenclature of Sigmatropic Shifts
In many rearrangements, the migrating group connects to one of the direct
B
neighbors of the atom to which it was originally attached. Rearrangements of
this type are the so-called [1,2]-rearrangements or [1,2]-shifts. These
rearrangements can be considered as sigmatropic processes, the numbers “1” and “2”
characterizing the subclass to which they belong. The adjective “sigmatropic”
emphasizes that a s-bond migrates in these reactions. How far it migrates is
described by specifying the positions of the atoms be- tween which the bond is
shifted. The atoms that are initially bonded are assigned po- sitions 1 and 1 The
subsequent atoms in the direction of the s-bond migration are
labeled 2, 3, and so forth, on the side of center 1 and labeled 2 , 3 , and so forth,
on the side of center 1 After the rearrangement, the s bond connects two atoms
in po-
sitions n and m The rearrangement can now be characterized by the positional num-
bers n and m in the following way: the numbers are written between brackets,
sepa- rated by a comma, and the primed number is given without the prime.
Hence, an [n,m]-rearrangement is the most general description of a sigmatropic
process. A [1,2]- rearrangement is the special case with n 1 and m 2 (Figure
11.1). [3,3]- Rearrangements occur when n m 3 (Figure 11.2). Many other types
of rearrangements are known, including [1,3]-, [1,4]-, [1,5]-, [1,7]-, [2,3]-, and
,[5,5]- rearrangements.
In this chapter we will be dealing primarily with [1,2]-rearrangements. In addition,
the most important [3,3]-rearrangements, namely, the Claisen and the Claisen–
Ireland rearrangements, will be discussed.
,436 11 Rearrangements
Fig. 11.1. The three 1
R(H) 1
R(H)
reactions on top show 1 2
C C
[1,2]-rearrangements to a 1
C C 2
sextet carbon. The two 1
R(H)
reactions at the bottom
1
R(H) 1 2
show [1,2]- 1 2
O C C
rearrangements to a O C C
neighboring atom that is 1 2 1
R(H) C C R(H)
coordinatively saturated
1
but in the process of losing C C2 1
X
a leaving group.
1X
1 2 1 2
a b Y a b
ba –Y
1
X
X
1a b2 Y
ba a b
–Y
2
Fig. 11.2. A Claisen 2
3 3
rearrangement as O
1 1 O
an example of a 1 1 3
3
[3,3]-
rearrangement. 2 2
11.2 Molecular Origins for the
Occurrence of [1,2]-Rearrangements
Figure 11.1 shows the structures of the immediate precursors of one-step [1,2]-
B
sigmatropic rearrangements. These formulas reveal two different reasons for the
occurrence of re- arrangements in organic chemistry. Rows 1–3 of Figure 11.1
reveal the first reason for [1,2]-rearrangements to take place, namely, the
occurrence of a valence electron sex- tet at one of the C atoms of the substrate.
This sextet may be located at the C of a carbenium ion or at the C: of a carbene.
Carbenium ions are extremely reactive species. If there exists no good opportunity
for an intermolecular reaction (i.e., no good pos- sibility for stabilization),
carbenium ions often undergo an intramolecular reaction. This intramolecular
reaction in many cases is a [1,2]-rearrangement.
Suppose a valence electron sextet occurs at a carbon atom and the possibility
exists for a [1,2]-rearrangement to occur. The thermodynamic driving force for
, the potential [1,2]-rearrangement will be significant if the rearrangement leads
to a structure with octets on all atoms. It is for this reason that acylcarbenes
rearrange quantitatively to give ketenes (row 2 in Figure 11.1) and that
vinylcarbenes rearrange quantitatively to give acetylenes (row 3 in Figure 11.1).
In contrast, another valence electron sextet species is formed if the [1,2]-
rearrangement of a carbenium ion leads to another car- benium ion. Accordingly,
the driving force of a [1,2]-rearrangement of a carbenium ion
