Theoretical study of the electrocyclic ring closure of hydroxypentadienyl cations

At the 6-311G* level of theory, DFT methods predict that the rearrangement of 1,4-dihydroxy-5-methylpentadienyl cation I (R = Me) to protonated trans-3-hydroxy-2-methylcyclopent-4-en-1-one 2, an intermediate step in the Piancatelli reaction or rearrangement of furfuryl carbinols to trans-2-alkyl(aryl)-3-hydroxycyclopent-4-en-1-one, is a concerted electrocyclic process. Energetic, magnetic, and stereochemical criteria are consistent with a conrotatory electrocyclic ring closure of the most stable out,out-1 isomer to afford trans-2. Although the out,in-1 isomer is thermodynamically destabilized by 6.84 kcal mol(-1), the activation energy for its cyclization is slightly native conrotatory electrocyclization of this isomer to provide trans-2. Geometric scrambling by isomerization of the terminal C1-C2 bond of 1 is also unlikely to compete with electrocyclization. The possibility to interpret the 1-->2 reaction as a nonpericyclic cationic cyclization was also examined through NBO analysis, and the study of bond lengths and atomic charges. It was found that the 1-->2 concerted rearrangement benefits from charge separation at the cyclization termini, an effect not observed in related concerted electrocyclic processes, such as the classical Nazarov reaction 3-->4 or the cyclization of the isomeric 2-hydroxy-pentadienyl cation 5.