6.5 [3,3]Sigmatropic Rearrangements
The principles of orbial symmetry established that concerted [3,3]
sigmatropic rearrangements are allowed processes.
Stereochemical predictions and analyses are based on the cyclic transition
state implied by a concerted reaction mechanism.
6.5.1. Cope Rearrangements
The cope rearrangement is the conversion of a 1,5-hexadiene derivatives
to an isomeric 1,5-hexadiene by the [3,3] sigmatropic mechanism.
When a chair transition state is favored, the E,E- and Z,Z-dienes lead to
anti-3,4-diastereomers whereas the E,Z and Z.E-isomers give the 3,4-syn
product.
Transition state B is less favorable than A because of the axial placement
of the larger phenyl substituent.
favorable
The products corresponding to boatlike transition states are usually not observed
for acyclic dienes. However, the boatlike transition state is allowed, and if steric
factors make a boat transition state preferable to a chair, reaction will proceed
through a boat.
The position of the final equilibrium is governed by the relative stability of
the starting material and the product.
The equilibrium is favorable for product formation because the product is
stabilized by conjugation of the alkene with the phenyl ring or the double
bonds in the product are more highly substituted, and therefore more stable.
(Scheme 6.11, entries 1 & 2)
In the ring strained molecules, the Cope rearrangements can occur at
much lower temperatures and with complete conversion to ring-opened
products.
-40oC
With transition metal catalysts, such as PdCl2(CH3CN)2
The rearrangements occurs at r.t., as contrasted to 240oC in its absence.
The electrophilic character of Pd(II) facilitates the reaction.
Oxy-Cope rearrangement: The formation of the carbonyl compound
provides a net driving force for the reaction. The reaction is catalyzed by
base. When the C-3 hydroxyl group is converted to its alkoxide the
reaction is accelerated by factors of 1010-1017, which is called anion
Oxy-Cope rearrangements. The reactivity trend is K+>Na+>Li+.
Catalysis of Claisen rearrangements has been achieved using highly hindered
bis(phenoxy)methylaluminum as a Lewis acid for E/Z control of the products.
Very bulky catalysts tend to favor the Z-isomer by forcing the a-substituent of
the allyl group into an axial conformation.
Several variation of the Claisen rearrangement .
Scheme 6.12. Claisen Rearrangments
The configuration of the new chiral center is that predicted by a chairlike
transition state with the methyl group occupying a pseudoequatorial position.
The stereochemistry of the silyl enol ether Claisen rearrangement is controlled
not only by the stereochemistry of the double bond in the allyl alcohol but also
by the stereochemistry of the silyl enol ether.
If the enolate is prepared in pure THF, the E-enolate is generated. But if
HMPA is included in the solvent, the Z-enolate predominates due to acyclic
transition state.
E-silyl enol ethers rearrange somewhat more slowly than the corresponding
Z-isomers, This is interpreted as resulting from the pseudoaxial placement of the
methyl group in the E-transition state.
The larger R accelerates the reaction rate, because the steric interaction with
R are relieved as the C-O bond stretches. The rate acceleration would reflect
the higher ground state energy resulting from these interactions.
The enolates of a-alkoxy esters give the Z-silyl derivatives because of
chelation by the alkoxy substituent.
The E-isomer gives a syn orientation whereas the Z-isomer gives rise to
anti -stereochemistry.
O-Allyl imidate esters undergo [3,3] sigmatropic rearrangements to
N-allyl amides.
Yields in the reaction are sometimes improved by inclusion of K2CO3 in the
reaction mixture.
Imidates rearrangements are catalyzed by palladium salts.
Aryl allyl ethers can undergo [3,3] sigmatropic rearrangement.
If both ortho-positions are substituted, the allyl group undergoes a second
sigmatropic migration, giving the para-substituted phenol.
6.6. [2,3] Sigmatropic Rearrangements
The rearrangements of allylic sulfoxide, selenoxide, and nitrones are the most
useful examples of the first type whereas rearrangements of carbanions of a
allyl ethers are the major examples of the anionic type.
Phenyl thiolate to cleave
S-O bond
Allylic sulfonium ylides readily undergo [2,3] sigmatropic rearrangement.
Ring expansion sequence for generation of
Medium-sized rings.
X = N and Y = O-
Anilinosulfonium ylides
The Wittig rearrangement in which a strong base converts allylic ethers to
a-allyl alkoxides. Because the deprotonation at the a’carbon must cmpare
with deprotonation of the a carbon in the allyl group.
Cyclic 5-membered ring transition state in which the a substituent prefers
an equatorial orientation.
6.7 Ene Reaction
Certain electrophilic carbon-carbon and carbon-oxygen bonds can undergo
an addition reaction with alkenes in which an allylic hydrogen is transferred
to the electrophile.
Ene reaction have relatively high activation energies and intermolecular
reaction is observed only for strongly electrophilic enophiles.
The thermal ene reaction of carbonyl compounds generally requires electronattracting substituents. The reaction shows a primary kinetic isotopic effect
indicative of C-H bond breaking in the rate determining step. The observations
are consistent with a concerted process.
The ene reaction is strongly catalyzed by Lewis acids such as aluminum chloride
and diethylaluminum chloride. Coordination by the aluminum at the carbonyl
group increases the electrophilicity of the conjugated system and allows
reaction to occur below room temperature, as illustrated in entry 6.
6.8 Unimolecular Thermal Elimination Reactions
6.8.1. Cheletropic Elimination
The atom X is normally bound to other atoms in such a way that elimination
will give rise to a stable molecule.
The stereochemistry is consistent with conservation of orbital symmetry.
6.8.2 Decomposition of Cyclic Azo Compounds
X-Y = -N=N-
[2p + 2p] forbidden
(high energy)
[2p + 4p] allowed
(low energy)
Nonconcerted diradical mechanism
16 decomposes to norbornene and nitrogen only above 100oC. But 17
eliminates nitrogen immediately on preparation, even at -78oC. Because
a C-N bond must be broken without concomitant compensation by carboncarbon bond formation, the activation energy is much higher than for a
concerted process.
photochemically
The stereochemistry varies from case to case.
Heteroaromatic ring
Pyridazine-3,6-dicarboxylate ester react with electron-rich alkenes
1,2,4-triazine and 1,2,4,5-tetrazines
6.8.3. b-elimination involving cyclic transition state
Thermal syn elimination
These reaction is thermally activated unimolecular reactions that normally do
not involve acidic or basic catalysts.
Amine oxide pyrolysis occurs at temperatures of 100-150oC. The reaction
can proceed at room temperature in DMSO.