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 1 (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 lower (5.29 kcal mol −1 versus 5.95 kcal mol −1 ). The cyclization of the isomers of 1 with the C1‐hydroxy group inwards showed considerably higher activation energies than their outwards counterparts. in,out ‐ 1 , although close in energy to out,out ‐ 1 (difference of 1.57 kcal mol −1 ) required about 10 kcal mol −1 more to reach the corresponding transition structure. The value measured for the activation energy of in,in ‐ 1 (17.32 kcal mol −1 ) eliminates the alternative conrotatory electrocyclization of this isomer to provide trans ‐ 2 . Geometric scrambling by isomerization of the terminal C1C2 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‐hydroxypentadienyl cation 5 .