Multiphoton Infrared Initiated Thermal Reactions of Esters: Pseudopericyclic Eight-Centered cis-Elimination

Abstract. Multiphoton infrared absorption from a focused, pulsed CO2 laser was used to initiate gas-phase thermal reactions of cis- and trans-3-penten-2-yl acetate. By varying the helium buffer gas pressure, it was possible to deduce the product distribution from the initial unimolecular reactions, separate from secondary reactions in a thermal cascade. Thus, trans-3-penten-2-yl acetate gives 54 ± 5 % of β-elimination to give trans-1,3-pentadiene, 40 ± 3% of [3,3]-sigmatropic rearrangement to give cis-3-penten-2-yl acetate and 6 ± 4% of cis-1,3-pentadiene. Similar irradiation of cis-3-penten-2-yl acetate gives 45 ± 1% of β-elimination to give cis-1,3-pentadiene, 32 ± 2% of [3,3]-sigmatropic rearrangement to give trans-3-penten-2-yl acetate and 23 ± 2% of trans-1,3-pentadiene. The latter process is an eight-centered δ-elimination, which is argued to be a pseudopericyclic reaction. Although β-eliminations have been suggested to be pericyclic, B3LYP/6-31G(d,p), MP2 and MP4 calculations s