Interpretation:
The suprafacial or antarafacial stereochemistry of the following sigmatropic reaction by order [x, y] is,
Concept introduction:
A sigmatropic rearrangement is a process in which a sigma-bonded substituent atom or group migrates across a pie electron system from one position to another. So, A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond.
Sigmatropic rearrangements are concisely described by an order term [i, j], which is defined as the migration of a σ-bond adjacent to one or more π systems to a new position (i-1) and (j-1) atoms removed from the original location of the σ-bond. [3] When the sum of i and j is an even number, this is an indication of the involvement of a neutral, all C atom chain. An odd number is an indication of the involvement of a charged C atom or of a heteroatom lone pair replacing a C = C double bond. Thus, [1, 5] and [3, 3] shifts become [1, 4] and [2, 3] shifts with heteroatoms, while preserving symmetry considerations.
If the migrating group remains on the same face of the π system, the shift is known as suprafacial, while if the migrating group transfers to the opposite face is called an antarafacial shift.
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Chapter 30 Solutions
Organic Chemistry
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