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cycloadditions are not always pericyclic. IUPAC: "[cycloaddtions are pericyclic] provided they are concerted" |
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Cycloadditions can be described using two systems of notation. An older but still common notation is based on the size of linear arrangements of atoms in the reactants. It uses [[parentheses]]: (''i'' + ''j'' + …) where the variables are the numbers of linear atoms in each reactant. The product is a cycle of size (''i'' + ''j'' + …). In this system, the standard [[Diels-Alder reaction]] is a (4 + 2)-cycloaddition, the [[1,3-dipolar cycloaddition]] is a (3 + 2)-cycloaddition and [[cyclopropanation]] of a carbene with an alkene a (2 + 1)-cycloaddition.<ref name=goldbook />
A more recent, IUPAC-preferred notation, first introduced by [[Robert Burns Woodward|Woodward]] and [[Roald Hoffmann|Hoffmann]], uses [[square brackets]] to indicate the number of ''electrons'', rather than carbon atoms, involved in the formation of the product. In the [''i'' + ''j'' + …] notation, the standard Diels-Alder reaction is a [4 + 2]-cycloaddition, while the 1,3-dipolar cycloaddition is also a [4 + 2]-cycloaddition.<ref name=goldbook />
==Thermal cycloadditions and their stereochemistry==
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