6.5 [3,3]Sigmatropic Rearrangements

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6.5 3,3Sigmatropic 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.
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Transition state B is less favorable than A
because of the axial placement of the larger
phenyl substituent.
favorable
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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.
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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
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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.
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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 KgtNagtLi.
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Catalysis of Claisen rearrangements has been
achieved using highly hindered bis(phenoxy)methyla
luminum 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 .
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Scheme 6.12. Claisen Rearrangments
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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.
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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.
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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.
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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.
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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.
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Imidates rearrangements are catalyzed by
palladium salts.
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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.
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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.
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Phenyl thiolate to cleave S-O bond
Allylic sulfonium ylides readily undergo 2,3
sigmatropic rearrangement.
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Ring expansion sequence for generation of
Medium-sized rings.
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X N and Y O-
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Anilinosulfonium ylides
The Wittig rearrangement in which a strong base
converts allylic ethers to a-allyl alkoxides.
Because the deprotonation at the acarbon must
cmpare with deprotonation of the a carbon in the
allyl group.
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Cyclic 5-membered ring transition state in which
the a substituent prefers an equatorial
orientation.
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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.
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Ene reaction have relatively high activation
energies and intermolecular reaction is observed
only for strongly electrophilic enophiles.
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The thermal ene reaction of carbonyl compounds
generally requires electron- attracting
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.
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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.
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-
The stereochemistry is consistent with
conservation of orbital symmetry.
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6.8.2 Decomposition of Cyclic Azo Compounds
X-Y -NN-
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 carbon- carbon bond
formation, the activation energy is much higher
than for a concerted process.
photochemically
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The stereochemistry varies from case to case.
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Heteroaromatic ring
Pyridazine-3,6-dicarboxylate ester react with
electron-rich alkenes
1,2,4-triazine and 1,2,4,5-tetrazines
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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.
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