SYMMETRY CONTROLLED REACTIONS

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SYMMETRY CONTROLLED REACTIONS
Dr. ABRAHAM GEORGE READER IN CHEMISTRY MAR
IVANIOS COLLEGE TRIVANDRUM
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CHEMISTRY IS THE MOTHER SCIENCE. IT IS THE
SCIENCE OF MAKING MATTER. CHEMISTS SYNTHESISE
THOUSANDS OF COMPOUNDS EVERY YEAR. WHATEVER MAY
BE THE METHOD EMPLOYED, ORGANIC REACTIONS INVOLVE
THE MAKING AND BREAKING OF BONDS. THIS CAN TAKE
PLACE IN THREE WAYS.
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• FREE RADICAL REACTION
• IONIC REACTIONS
• PERICYCLIC REACTIONS.

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PERICYCLIC REACTIONS
CONCERTED, SINGLE STEP REACTION INVOLVES A
CYCLIC TRANSITION STATE OF PERIPHERAL ELECTRONS
NO INTERMEDIATE INITIATED BY HEAT OR LIGHT
UNAFFECTED BY SOLVENTS AND CATALYSTS HIGHLY
SREREOSPECIFIC
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CONTRIBUTION OF WOODWARD AND HOFFMANN
ROALD HOFFMANN AND KENICHI FUKUI NOBEL PRIZE
1981 for their theories, developed
independently, concerning the course of chemical
reactions"
ROBERT B. WOODWARD NOBEL PRIZE 1965 for his
outstanding achievements in the art of organic
synthesis"
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THEY CONSIDERED THE SYMMETRY OF THE FRONTIER
ORBITALS THE C2 AXIS OF SYMMETRY AND PLANE OF
SYMMETRY OF THE ORBITALS ARE TAKEN INTO ACCOUNT
?ODD - MIRROR PLANE SYMMETRY ?even -C2 SYMMETRY.
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• THREE TYPES OF SYMMETRY CONTROLLED REACTIONS.
• ELECTROCYCLIC
• CYCLOADDITION
• SIGMATROPIC REARRANGEMENT

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ELECTROCYCLIC
CONVERSION OF A CONJUGATED POLYENE TO A CYCLIC
ENE OR A POLYENE UNDER THE INFLUENCE OF HEAT OR
LIGHT. IF THE OPEN CHAIN PARTNER HAS n p THEN THE
CYCLIC PARTNER WILL HAVE (n-2) p ELECTRONS AND 2
ELECTRONS IN A s BOND.
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CYCLOADDITION
TWO ADDENDAS ADDING TO GIVE RISE TO A CYCLIC
COMPOUND.
2 2
4 2
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SIGMATROPICREARRANGEMENT
MIGRATION OF AN ATOM WITH ITS s ELECTRON
INTRAMOLECULARLY WITHIN A p FRAMEWORK. MIGRATION
IS ACCOMPANIED BY A SHIFT IN THE POSITION OF THE
DOUBLE BOND.
1,5
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• STEREOCHEMISTRY OF PRODUCTS CAN BE PREDICTED BY
• FMO METHOD
• CORRELATION DIAGRAM METHOD
• MOBIUS HUCKEL METHOD

ALL THESE APPROACHES ARE BASED ON THE SYMMETRY
PROPERTIES OF THE FRONTIER MOLECULAR ORBITALS
HOMO AND LUMO.
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PERICYCLIC REACTIONS IN BIOLOGICAL SYSTEMS.
BIOSYNTHESIS OF VITAMIN D 7-Dehydrocholesterol,
is converted into vitamin D3 by two pericyclic
reactions. The first is an electrocyclic ring
opening by conrotatory process to form provitamin
D3. This reaction occurs under photochemical
condition. The provitamin then undergoes a 1,7
sigmatropic rearrangement to form vitamin D3.
The sigmatropic rearrangement takes place under
thermal conditions and is slower than the
electrocyclic reaction.
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ULTRAVIOLET LIGHT AND SKIN CANCER Exposure to
ultraviolet light causes skin cancer. One cause
of skin cancer is the formation of thymine
dimers. At any point in the DNA where there are
two adjacent thymine residues, a 22
cycloaddition reaction can occur, resulting in
the formation of thymine dimer. Because 22
cycloaddition reactions take place only under
photochemical conditions, the reaction take place
only in the presence of ultraviolet light. The
ozone layer absorbs ultraviolet radiation,
protecting organisms in the Earths surface.
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CHEMILUMINESCENCE Fireflies emit light as a
result of reverse 22 cycloaddition reaction.
Fireflies have an enzyme (luciferase) that
catalyses the reaction between luciferin, ATP,
and molecular oxygen that forms a compound with
an unstable four membered ring. When the four
membered ring breaks, an electron in oxyluciferin
is in an excited state because suprafacial
overlap can occur only under photochemical
conditions. When the electron in the excited
state drops down to the ground state, a photon of
light is released.