Also contains definition of: dipolar cycloaddition
https://doi.org/10.1351/goldbook.C01496
A reaction in which two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic @[email protected] in which there is a net reduction of the bond multiplicity. The following two systems of notations have been used for the more detailed specification of cycloadditions, of which the second, more recent system [described under (2)] is preferred:
1. A(i + j + ...) cycloaddition is a reaction in which two or more molecules (or parts of the same molecule), respectively, provide units of i, j, ... linearly connected atoms: these units become joined at their respective termini by new σ-bonds so as to form a cycle containing (i + j + ...) atoms. In this notation, (a) a Diels–Alder reaction is a (4+2) cycloaddition, (b) the initial reaction of ozone with an alkene is a (3+2) cycloaddition, and (c) the reaction shown below is a (2+2+2) cycloaddition. (N.B.: parentheses (...) are used in the description based on numbers of atoms.)
C01496-1.png
2. The symbolism [i + j + ...] for a cycloaddition identifies the numbers i, j, ... of electrons in the interacting units that participate in the @[email protected] of reactants to products. In this notation the reaction (a) and (b) of the preceding paragraph would both be described as [2+4] cycloadditions, and (c) as a [2+2+2] cycloaddition. The symbol a or s (a = @[email protected], s = @[email protected]) is often added (usually as a subscript after the number to designate the stereochemistry of addition to each fragment. A subscript specifying the orbitals, viz. σ, π (sigma, pi) with their usual significance) or n (for an orbital associated with a single atom only), may be added as a subscript before the number. Thus the normal Diels–Alder reaction is a $$\left [4_{\text{a}}\: +\: 2_{\text{s}} \right ]$$ or $$\left [_{\pi }4_{\text{a}}\: +\: _{\pi }2_{\text{s}} \right ]$$ cycloaddition, whilst the reaction:
C01496-2.png
would be a $$\left [14_{\text{a}}\: +\: 2_{\text{s}} \right ]$$ or $$\left [_{\pi }14_{\text{a}}\: +\: _{\pi }2_{\text{s}} \right ]$$ cycloaddition. (N.B. Square brackets [...] are used in the descriptions based on numbers of electrons.) Cycloadditions may be @[email protected] or ([email protected]@ stepwise @[email protected] . The term 'dipolar cycloaddition' is used for cycloadditions of 1,[email protected]@.