(Received: August 6, 2007; Accepted for publication: October 5, 2007; Published on Web: December 6, 2007)
A Frontier Molecular Orbital (FMO) analysis is not so effective for the singlet peri-, site-, regio- and stereoselectivities of singlet photodimerization (anti-3,6-[4+4] cycloaddition for 3) of 2-pyridone (1) and for the cross additions (mainly endo-3,4-ht-[2+2] cycloaddition for 4) with acrylates (2). Their Origins were analysed by transition state (TS) analysis of whole reaction (TS1, TS2, TS3) using MOPAC-PM5 and UCIS/6-31+G(d)//PM5 levels of calculation, and the major factors of the selectivities were inferred quantitatively. The singlet reactions may be concerted, controlled by TS1, and may be competed by orbital overlapping and ionic factors. The Origin for a novel adduct, azocinone (5) between excited singlet 1 and 2 was also estimated by PM5 simulation. The triplet and regioselective cross additions (mainly exo-5,6-hh-[2+2] cycloaddition for 7) between 1 and 2 were inferred to be two-step reactions, and to pass through twisted biradicals, with the first-step transition TS1 energy smaller than the second-step one. The energy of TS2 (ring- closure) was lower than that of TS3 (bond- cleavage).
Keywords: Photocycloaddition reaction, 2-Pyridone, Site- and regioselectivities, Transition state analysis, PM5, UCIS
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