A reaction in which two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic @A00138@ 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.)
  2. The symbolism [i + j + ...] for a cycloaddition identifies the numbers i, j, ... of electrons in the interacting units that participate in the @T06446@ 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 = @A00377-1@, s = @A00377-2@) 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:
    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 @P04491@ or (non-@C01234@ stepwise @S05970@. The term 'dipolar cycloaddition' is used for cycloadditions of 1,3-@D01753@.
See also:
cheletropic reactions
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1103 [Terms] [Paper]