5-exo or 6-endol Exploring Transition State Structures in Cyclizations of 4-Penten-l-oxyl Radicals

摘要

A recent study on stereoselective ring closures of substituted 4-penten-l-oxyl radicals indicates that transformations involving alkoxyl radicals may indeed be free radical reactions with notable synthetic utility. Despite the high reactivity (k~(exo)= (5 ± 2) X 10~8 s~(-1)') the 4-penten-l-oxyl radical la cyclizes regioselectivcly; derivatives of la even react stereoselectively. To uncover the origin of the high exo selectivity of the cyclization, the isoelectronic, all-carbon derivative 1b of the4-penten-l-oxyl radical la has been the subject of several semiempirical and low-level ab initio calculations. However, the prediction of radical selectivities in cyclization reactions remained a difficult task, because treatment of open-shell moleculeswith unrestricted Hartree-Fock (UHF) methods led in most of the cases to "spin contamination" in the transition states. To account for these problems and to obtain more reliable energy values in general, the computed heats of formation were often corrected by additional time-consuming configuration interaction calculations (CI calculations).