PerspectiveOrganic Chemistry

As simple as [2+2]

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Science  28 Aug 2015:
Vol. 349, Issue 6251, pp. 925-926
DOI: 10.1126/science.aac9883

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Olefin cycloadditions, pericyclic reactions in which the relatively weak π bonds of these unsaturated species are swapped out for stronger σ bonds, are among the most widely exploited transformations in organic synthesis (1). Indeed, the Diels-Alder reaction—a [4π+2π] cycloaddition (a [4+2] cycloaddition for short) that forms six-membered rings from a diene (the 4π component) and an olefin (the 2π component)—has a storied history in the many branches of synthetic chemistry and proceeds readily under thermal conditions (see the figure, panel A) (2). However, a similarly fundamental pericyclic reaction—the corresponding [2+2] cycloaddition of two olefins, leading to cyclobutane rings—usually requires photochemical conditions to proceed efficiently, a fact enforced by the constraints of orbital symmetry (3). On page 960 of this issue, Hoyt et al. (4) report the development of catalysts based on iron that transform simple feedstock olefins into cyclobutanes through [2+2] cycloadditions under mild, thermal conditions.