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

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

Hua Ji, Li Li, Xiaolian Xu, Sihyun Ham, Loubna A Hammad, David M Birney

J Am Chem Soc. 2009 Jan 21;131(2):528-37.

PMID: 19140791

Abstract:

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 beta-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 beta-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 delta-elimination, which is argued to be a pseudopericyclic reaction. Although beta-eliminations have been suggested to be pericyclic, B3LYP/6-31G(d,p), MP2 and MP4 calculations suggest that both beta- and delta-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.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP1574410	cis-1,3-Pentadiene		1574-41-0	Price

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