Isocyanates react by a concerted mechanism for [2+2] cycloaddition reactions when the alkene is electron-deficient. Alkenes that are sufficiently electron-rich release a pi electron to the isocyanate which leads to a 1,4-diradical intermediate via the single electron transfer (SET) pathway. The triplet 1,4-diradical intermediate is in equilibrium with the singlet form. At temperatures around 15 ^oC and above the 1,4-diradical is in the triplet state; but it rearranges to the singlet form below 15 ^oC. The 1,4-diradical intermediate can close to give beta-lactam products, or it can be intercepted with a second isocyanate to give uracil products.