TY - JOUR
T1 - Multiphoton infrared initiated thermal reactions of esters
T2 - Pseudopericyclic eight-centered cis-elimination
AU - Ji, Hua
AU - Li, Li
AU - Xu, Xiaolian
AU - Ham, Sihyun
AU - Hammad, Loubna A.
AU - Birney, David M.
PY - 2009/1/21
Y1 - 2009/1/21
N2 - Multiphoton infrared absorption from a focused, pulsed CO 2 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 suggest that both β- and δ-eliminations, as well as [3,3]-sigmatropic rearrangements of esters are primarily pseudopericyclic in character, as judged by both geometrical, energetic and transition state aromaticity (NICS) criteria. Small distortions from the ideal pseudopericyclic geometries are argued to reflect small pericyclic contributions. It is further argued that when both pericyclic and pseudopericyclic orbital topologies are allowed and geometrically feasible, the calculated transition state may be the result of proportional mixing of the two states; this offers an explanation of the range of pseudopericyclic and pericyclic characters found in related reactions.
AB - Multiphoton infrared absorption from a focused, pulsed CO 2 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 suggest that both β- and δ-eliminations, as well as [3,3]-sigmatropic rearrangements of esters are primarily pseudopericyclic in character, as judged by both geometrical, energetic and transition state aromaticity (NICS) criteria. Small distortions from the ideal pseudopericyclic geometries are argued to reflect small pericyclic contributions. It is further argued that when both pericyclic and pseudopericyclic orbital topologies are allowed and geometrically feasible, the calculated transition state may be the result of proportional mixing of the two states; this offers an explanation of the range of pseudopericyclic and pericyclic characters found in related reactions.
UR - http://www.scopus.com/inward/record.url?scp=67849115726&partnerID=8YFLogxK
U2 - 10.1021/ja804812c
DO - 10.1021/ja804812c
M3 - Article
C2 - 19140791
AN - SCOPUS:67849115726
SN - 0002-7863
VL - 131
SP - 528
EP - 537
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 2
ER -