1.3- Dipolar cycloadditions nonconcerted

Cycloaddition of stericaUy crowded 2,2,4,4-tetramethyl-3-thioxocyclobutanone (5)-methylide (69) (Scheme 5.25) with extremely electron-poor dipolarophUes are of special interest with respect to the theory of dipolar cycloaddition chemistry. In these reactions, a nonconcerted, two-step mechanism was proposed (96,97). 

Recently, Huisgen and coworkers have reported on the first unequivocal example of a nonconcerted 1,3-dipolar cycloaddition.27 Sustmann s FMO model of concerted cycloadditions envisions two cases in which the stepwise mechanism might compete with the concerted one.21 Two similar HOMO-LUMO interaction energies correspond to a minimum of rate and a diradical mechanism is possible, especially if stabilizing substituents are present. A second case is when the HOMO (l,3-dipole)-LUMO (dipolarophile) is strongly dominant in the transition state. The higher the difference in rr-MO energies of reac-... 

Cycloadditions may be pericyclic reactions or (nonconcerted) stepwise reactions. The term dipolar cycloaddition is used for cycloadditions of 1,3-dipolar compounds. 

To date, the Huisgen 1,3-dipolar cycloaddition is described as nonconcerted when catalysts, processing via metallacycle intermediates, are used, leading to snbstituted heterocycles in excellent selectivity (for specific examples, see Sections 9.2 and 9.5). The outstanding discovery of the Cu(I)-catalyzed azide-aUcyne cycloaddition by Meldal and Sharpless particularly improved the rates as well as the regioselectivity of the reaction (see subsequent chapter) and is now the most commonly used 1,3-dipolar cycloaddition in organic synthesis. ... 

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